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Ask HN: What scientific phenomenon do you wish someone would explain better?
I've been studying viruses lately, and have found that the line between virus/exosome/self is much more blurry than I realized. But, given the niche interest in the subject, most articles are not written with an overview in mind.
What sorts of topics make you feel this way?
- redlock 6y agoI wish I could get a clear explanation of why faster than light travel breaks causality. I have seen the math but intuitively I am having trouble grasping it especially when you have only two reference frames (most explanations will use three reference frames to show the violation)
- OkayPhysicist 6y agoThe simplest answer is that events that happen further apart in space than in time can be observed in different orders depending on the frame of reference. If an effect can happen before its cause, causality is violated. The three-reference frame example is the easiest, because you can start with a frame where two events, A and B, happen simulateously. A reference frame (say, a spaceship), flying along a line in the A to B direction will observe A happen, then B. A ship flying the opposite direction will experience the opposite, B then A. So whose observations were correct? All them are perfectly valid. The problem is if we allow A to cause B, in which case the B then A frame has the effect happen before the cause.
- redlock 6y agoIsn’t cause and effect mediated by forces that travel at the speed of light? Simultaneous events cannot be in a causal relationship, no?
- qqqqquinnnnn 6y agoAnother frustrating one - what is heredity? If it's possible to inherit something due to a shift in behavior (i.e. it's a cultural change that leads to a biochemical change), how does that connect neatly to mendalian inheritance?
- epmaybe 6y agoAre you talking epigenetics? It's relatively closely related to mendelian inheritance. At a high level view, if genes are present in your genome that aren't "on" in a previous generation, but due to changes in behavior become more active, then in a future generation that gene could be expressed more. It's not that the genes have changed their code, they are just flipped on or off.
- qqqqquinnnnn 6y agoNo, more things that are modifications without any genetic basis - like myopia. Doesn't seem to have any genetic/epigenetic basis. It's purely a physical use case, where the eye matures improperly if it's only exposed to near-field work.
- phkahler 6y agoOne hypothesis: kids learn to do what their parents do. If near field work or hobbies cause the problem then it follows that some kids will "inherit" it through mimicry.
- epmaybe 6y agoah, I see what you're saying. Luckily, I've looked into this specific topic! Myopia is definitely hereditary, especially the pathologic variants that can lead to retinal tears and the like. That being said, there is the process of refractive development that occurs early on in life. The eye develops at a frighteningly fast pace, and you achieve near-adult globe size after about 18 months. The genes that drive this refractive development could be hereditary, if that is what you're trying to figure out. Now, we can make a claim that this adaptation during infancy could eventually affect our genome, but I have not delved into the epigenetic literature to determine if that has been borne out or not.
- qqqqquinnnnn 6y ago90% saturation in a few generations suggests that it is something other than purely hereditary mutations accumulating: https://www.nature.com/news/the-myopia-boom-1.17120 https://www.nature.com/news/the-myopia-boom-1.17120
- crazypython 6y agoQuantum Mechanics, and why we need interpretations of it.
- qqqqquinnnnn 6y agoLike.... why are sub-atomic phenomena important in general?
- Cheyana 6y agoTo add to all this, the double-slit experiment. What exactly does it mean that light moves as a wave or a particle?
- Rury 6y ago>What exactly does it mean that light moves as a wave or a particle? One idea is known as the Copenhagen interpretation. It basically says that the wave-like effects we associate with matter is merely a wave of probabilities. Or in terms of the double-slit experiment and in other words, light behaves like a particle, but the wave-like effects you see is just the result of probabilities where the particles end up. Dark areas are areas of low probability, and lighter areas high probability. One might imagine the light particles streaming through the slit end up having slight variation in trajectory from one particle to another (for various reasons such as interference with other particles), which results in areas where most particles end up and others where few end up... representing a wave.
- ars 6y agoWhy Quantum Mechanics I have no idea. Why interpretations: There is an experiment you can do that is hard to explain: Either particles are able to somehow influence each other faster than light (non-local), or the particle somehow doesn't exist except when interacting with some other particle (non-real). Try this video: https://www.youtube.com/watch?v=zcqZHYo7ONs https://www.youtube.com/watch?v=zcqZHYo7ONs the AHA moment in the video comes when you realize you can entangle the light and that adding a filter by one stream of light somehow causes the other stream of light to also be influenced.
- ars 6y agoHow can metric expansion of space redshift light? Doesn't that violate conservation of momentum, since the redder light has lower momentum? And adding on to that: Will light inside a box redshift? If I weigh the box (i.e. weigh the light inside the box), then wait a bit for the light to redshift, then weigh the box again?
- antognini 6y agoYou are correct! Cosmological redshift does violate conservation of momentum (and energy as well [1]). But conservation of energy and momentum does not actually apply if spacetime itself is globally changing. The underlying reason for this is that Noether's theorem tells us that every physical symmetry implies a conservation law for some physical quantity. Conservation of energy and momentum comes from the fact that the physical laws are the same throughout time and space. However, cosmological expansion violates that assumption, so there is no reason that energy and momentum should still be conserved. [2] [1]: One side note here is that relativistically, energy and momentum are not really separate physical quantities, but instead two components of the same underlying physical quantity. Unfortuantely, this quantity does not really have a good name (despite Taylor & Wheeler's attempt to call it "momenergy"). It ends up being called the momentum 4-vector, but the temporal component of this 4-vector is energy. [2]: This is only true globally. Locally, the laws are approximately the same from one moment to the next, so conservation of energy and momentum hold for small distances and short times.
- aaronblohowiak 6y agoOrbital mechanics
- antognini 6y agoI've been meaning to do a bit of blogging about this since my thesis work was on gravitational dynamics. Is there anything in particular you would be interested in seeing explained?
- ghostpepper 6y agoHave you heard of / played the game Kerbal Space Program? I believe even folks at NASA have even said it helped cement their mathematical knowledge with a better intuitive understanding.
- raverbashing 6y agoObligatory video, even if not very detailed https://www.youtube.com/watch?v=tJiAkBxuqfs https://www.youtube.com/watch?v=tJiAkBxuqfs
- geocrasher 6y agoTwo words: Scott Manley: https://www.youtube.com/results?search_query=scott+manley+orbital+mechanics https://www.youtube.com/results?search_query=scott+manley+or...
- OkayPhysicist 6y agoFall and miss the ground forever.
- autonomousErwin 6y agoProtein Folding
- qqqqquinnnnn 6y agoProtein folding is amazing! Seems like it happens in realtime as it's synthesized by the ribosome.. I'll dig around and see if I can find a better resource for you.
- bobosha 6y agoIsn't protein folding considered one of the biggest problems ever?
- qqqqquinnnnn 6y agosure is.. but somehow biology manages it with few issues. It seems like it has a lot to do with the presence of chaperone proteins that babysit the protein as it's coming off the ribosome, preventing pieces of it from sticking together that shouldn't touch, etc. https://en.wikipedia.org/wiki/Protein#/media/File:Chaperonin_1AON.png https://en.wikipedia.org/wiki/Protein#/media/File:Chaperonin... I'm working on making a model of this chaperone complex relative to a folded protein to get a sense of how it might be interacting with the amino acid chain before it becomes globular
- ramraj07 6y agoWhat would you like to know about protein folding? How it even happens, or how we are trying to "figure it out"?
- ykevinator 6y agoQuantum stuff
- mynegation 6y agoWhy tardigrades are so hardy, how their biology is so different? How immune system and medications work. Why some plastics are recyclable and others are not.
- ramraj07 6y agoI've been mulling making a youtube channel with ten minute videos on immunology; what would be a good starter video that might interest someone like you? I thought I'll do something about antibodies as drugs!
- mynegation 6y agoThank you and please please do it and post a link here or send me an email (in profile)! For me the most interesting is the recognition/pattern matching aspect: how antibodies find what to attack and what to leave alone.
- ramraj07 6y agoMost definitely one of the hardest questions to answer :) I'll take it up as a challenge!
- busyant 6y agoIf you're taking requests... distinguishing Self vs nonself...what we know and what we don't.
- darksaints 6y agoAll plastics are recyclable in the sense that they can be repurposed, but thermoset polymers cannot be repurposed to be used the same way as it was originally formed. Typically they are chopped up and used as matrix materials for asphalt or other composites. In some cases they can use solvents to break down chains into smaller building blocks. However, colloquially thermoset polymers are considered non-recyclable, while thermoplastic polymers are considered recyclable. The difference between thermoset and thermoplastic polymers has to do with irreversible chemical bonding during curing. With thermosets, you have chemical bonds between molecules preventing deformation, whereas with thermoplastics, you just have a viscous friction between molecules that varies with temperature. If you heat up a thermoplastic, that viscous friction goes away and the plastic can be remolded.
- foreigner 6y agoThe Coriolis Force
- cygx 6y agoDrag a marker with a constant radial velocity (from your perspective) across a spinning disk. It'll trace out a curved line, so from the perspective of an observer on the disk, an acceleration must have been present. The full 3d Coriolis force is more complicated than that (eg accounting for the Eötvös effect): The spinning disk example only gets you to the -2vω term (where v denotes radial velocity and ω angular velocity).
- davidmanheim 6y agoNon-interactive zero knowledge proofs. ZK proofs have a number of good explainers, mostly using graph colorings. Non-interactive versions, however, require quite a bit more than that explanation allows - and despite asking experts, I still haven't found a good, basic explanation.
- itcrowd 6y agoMaybe the blog of Prof. Matthew Green of JHU is of use. Specifically, the two-part series about zero-knowledge proofs. Part II discusses non-interactive ZK proofs. Part I is really required to grasp the extension to non-interactive ZKP's, so you may need to read that first. https://blog.cryptographyengineering.com/2017/01/21/zero-knowledge-proofs-an-illustrated-primer-part-2/ https://blog.cryptographyengineering.com/2017/01/21/zero-kno...
- krcz 6y agoI liked this PDF that starts with using modular arithmetic to prove knowledge of polynomial, using bilinear EC pairings to make it self and then, finally, encoding computations as polynomials: https://arxiv.org/pdf/1906.07221.pdf https://arxiv.org/pdf/1906.07221.pdf
- airstrike 6y agoThe one-electron universe is always a personal favorite. Though more a far-fetched theory than a proper "scientific phenomena", I'd be eager to learn more about it in layman's terms https://en.wikipedia.org/wiki/One-electron_universe https://en.wikipedia.org/wiki/One-electron_universe https://www.youtube.com/watch?v=9dqtW9MslFk https://www.youtube.com/watch?v=9dqtW9MslFk
- GnarfGnarf 6y agoQuantum entanglement. How do they know it's happening?
- monktastic1 6y agoHave you studied Bell's Inequalities at all? There are various YT videos: https://www.youtube.com/watch?v=zcqZHYo7ONs https://www.youtube.com/watch?v=zcqZHYo7ONs https://www.youtube.com/watch?v=f72whGQ31Wg https://www.youtube.com/watch?v=f72whGQ31Wg https://www.youtube.com/watch?v=qd-tKr0LJTM https://www.youtube.com/watch?v=qd-tKr0LJTM
- mbo 6y agoThe Quantum Computing for Computer Scientists video by Microsoft Research (https://www.youtube.com/watch?v=F_Riqjdh2oM https://www.youtube.com/watch?v=F_Riqjdh2oM) really hammered it home for me. It goes slowly and the presenter, in his own words "shuts up and does the math".
- anton_tarasenko 6y agoSubreddit /r/askscience does a good job at explaining science in plain words. I usually google "site:reddit.com/r/askscience/ __QUESTION__". The StackExchange sites have less coverage and answers tend to be more technical. University websites return reliable answers, but often neither short nor accessible.
- bjourne 6y agoFermat's theorem: a^n + b^n = c^n For n > 2, why are there no integers a, b, and c that satisfy the equality?
- ken 6y agoOne of my advisors in college said "The proof takes about 10 years of graduate mathematics to understand".
- wesammikhail 6y agoIs there really no intuitive way to communicate the answer to that question without needing 10 years of grad math? I find that to be somewhat hard to believe
- evgen 6y agoIf there were a simple and intuitive way to communicate the answer then I would suggest that we probably would have figured it out in our 300+ years in which this was one of the most famous unanswered questions in mathematics.
- itcrowd 6y agoNo, there is no intuitive way to communicate it. The theorem has taken >350 years to prove, which makes it clear that the proof is not some intuition that was somehow missed by hundreds of people for centuries. Fermat's Last Theorem (book) by Simon Singh is the source to check out if you're interested in the details of how it eluded mathematicians and a general idea of how the problem was solved, without getting too technical. It's a great story well told.
- wesammikhail 6y agoThanks for the recommendation good sir!
- raverbashing 6y agoWell, the answer is "simple": It's because the modularity theorem was proven (or better, the Tanyiama-Shimura conjecture was proven) But why that solves the problem? Because it connects two branches of mathematics (modular forms and elliptic equations) in a way that proves that equations of that form cannot exist (where the exponent is > 2) Though there probably is an easier way of explaining it, it is strongly suspected that Fermat got the wrong idea there.
- VygmraMGVl 6y agoI had a very mathy explanation of Spinodal Decomposition in my graduate work. I wonder if there's a more intuitive explanation than just "that's how the energy landscape works".
- elcritch 6y agoHmmm, you might look up how dendrites form. They’re roughly analogous in how the thermodynamics are favorable to forming more complex structures, IIRC. But it’s easier/more intuit to see how dendrites form. Dendrite formation is also a huge problem in many fields, like electronics manufacturing (eg tin whiskers).
- dvdkhlng 6y agoWave-particle duality (of e.g. light)
- gus_massa 6y agoThe usual misunderstanding is that light is sometimes a wave and sometimes a particle, but that is wrong. Light is always a weird thing, that you have never seen before in macroscopic objects, and you need to use some math to describe. In some experiments the weird mathematical thing can be approximated as an almost classical particle. That approximation simplifies the calculation a lot, and sometime you can get some result intuitively. But it is never true, it is only a very good approximation. In some experiments the weird mathematical thing can be approximated as an almost classical wave. That approximation simplifies the calculation a lot, and sometime you can get some result intuitively. But it is never true, it is only a very good approximation. Try to read again everything you have read about the subject, but every time the text says "here light is a wave/particle" use a red marker to rewrite that sentence as "here light can be approximated as a wave/particle".
- raverbashing 6y agoThat is "less of an issue" when considering https://en.wikipedia.org/wiki/Quantum_field_theory https://en.wikipedia.org/wiki/Quantum_field_theory and Second Quantization Basically: particle are the quanta of waves. So it's not really a duality in the end.
- plurinshael 6y agoSpin aka intrinsic angular momentum
- OkayPhysicist 6y agoBasocally, we can think of momentum in two parts: the momentum of an object orbitting another, and the momentum of the object spinning on an axis. At the subatomic level, we observe that electrons have some extra angular momentum, beyond what we'd expect from their "orbits". We call that spin, because it's intrinsic, like the spinning of a macroscale object.
- pgt 6y agoGravity wells. I only realised in my 20s that the only reason satellites can orbit the Earth without crashing into the ground is by going sideways really, really fast. So as they inch closer to the ground, they also travel parallel to the ground fast enough so that they stay approximately the same height from the ground.
- freeone3000 6y agoThis is Newton's Cannonball. Honestly, I've found the best way to learn more about orbital mechanics is with a simulator - Kerbal Space Program is a fun version.
- DavidSJ 6y agoWhat other subjects would simulation enlighten people on?
- CamperBob2 6y agoTuring equivalence comes to mind.
- lpellis 6y agoBell's theorem. It somehow proves that quantum physics is incompatible with local hidden variables, but I could never see an understandable explanation (for me at least) of just how it works.
- sciolizer 6y agoYudkowsky's explanation[1] is the first one that worked for me. I later found Quantum mysteries for anyone[2] helpful. The latter has less soap-boxing. 1: https://www.lesswrong.com/posts/AnHJX42C6r6deohTG/bell-s-theorem-no-epr-reality https://www.lesswrong.com/posts/AnHJX42C6r6deohTG/bell-s-the... 2: https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/files/makale/mermin.pdf https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/...
- ShamelessC 6y agoI started to read the first one but his insistence that Many Worlds is true was too frustrating. Many Worlds Theorem seems specifically useful at saying "the variables aren't hidden because everything before wavefunction collapses actually plays out in different worlds. But, we specifically have no way of proving that theory. So now we're back to the essence of the original question - if these things seem random why do we know that they're in fact deterministic without any hidden variables?
- kubanczyk 6y agoWell, I'd recommend to read the whole series. It's not so bad as it sounds. There are so many steps from where you are to appreciating the utter weirdness of Bell's experimental result. Not the weirdness of any theory (or an interpretation, which Many Worlds actually is) but of the basic experimental result. If you are properly amazed by it, rejecting MWI or any crazy-ish borderline-conspiracy theory seems suddenly a lot harder. I feel the whole Yudkowsky's QM series in fact served to deliver that one post.
- ShamelessC 6y ago
- zarkov99 6y agoCoroutines in C++.
- ncmncm 6y agoCoroutines are most easily understood as a way to write a state machine in a way that looks like a function. I.e., it's just a notational trick to make one function do different things according to when it's called. To see it, imagine you have a struct with a data member for each local variable of your function, and replace your function with a member function that has no local variables, but uses "this" to get at what was local data. Add one more data member, a number that is set differently right before each place the function returns. Finally, insert some code at the start of the function that, according to the number, jumps to just after the last return statement executed. Then, each time you call the function, what happens depends on what happened last time. There are more details, but that is the gist. You can write that yourself in C++98, with the body of the function inside a switch statement. Getting it past code review would be the real challenge.
- zarkov99 6y agoI think I get coroutines in theory. It's the C++ specific complexity that is the hardest barrier. Thanks for the explanation though
- ncmncm 6y agoYes, a lot of extra junk is needed to make them actually work, but the extra junk goes a long way toward obscuring what you actually need to know. Ultimately, though, you are right that you have to understand it all, once, even if you can't remember it all a month later. The explanations I find online are not good at presenting just the details you need when you need them, and building up to the full picture.
- Rerarom 6y agoTry to understand them in Simula first, since that's where C++ drew its original inspiration from.
- ncmncm 6y agoI like that for a full decade, people discussed measurements at reputable labs indicating that certain radioactive species decayed at rates that varied >0.1% depending on the season, and explored possible neutrino flux influences. The measurements were finally shown to be effects of the immediate environment on the measurement apparatus. That detectors used in labs may vary with time by >0.1%, unknown to their users, seems pretty important. How did everybody involved not know?
- MichailP 6y agoBecause people live in a busy world, where knowledge is not transferred with enough love and integrity. And also people are afraid to say "I don't know" and what little they know they tend not to share. To make things more specific, those labs had uncertainty budget with something like 20 terms for the things they measured. Each of those terms had associated probability distribution etc. They had uncertainty budgets for all the methods they did etc., and some of those where probably dated, done by someone else, etc. etc. Who checks that? Is the check rigorous enough? Are some assumptions made that don't hold to scrutiny? So it is actually very easy for error to creep in, I would say actually very likely.
- CamperBob2 6y agoI've always had trouble following Searle's Chinese Room argument as it applies to the nature and identification of intelligent action. I've never understood how the Chinese Room shows what its adherents say it shows, so that would be one topic I'd like to see more perspectives on.
- ShamelessC 6y ago"Searle understands nothing of Chinese, and yet, by following the program for manipulating symbols and numerals just as a computer does, he sends appropriate strings of Chinese characters back out under the door, and this leads those outside to mistakenly suppose there is a Chinese speaker in the room. " I have trouble with this too. I think it's actually incorrect, or at least misleading. I think what it's _trying_ to say is that even if an entity can perform a complex task doesn't mean it can understand a complex task. I think the more important result of this argument is that certain complex tasks can be "pre-baked" into rulesets _by an existing intelligence_. To me this just means that intelligent entities can sort of copy parts of their intelligence into other entities which are not intelligent i.e. computer programming. I think with this argument they're trying to say "a series of sufficiently complex if statements isn't necessarily intelligent" by choosing something we know computers are good at - string manipulations and applying it to something we consider intelligence - language translation. The argument holds that the computer is obviously not intelligent because it's just a function that takes a character and outputs another character. But it needs to be a convincing translation, right? The computer would then be able to spit out not just accurate translations but also properly converted cultural idioms and new combinations of words where one didn't exist in the other language. That requires context of surrounding characters, memory of common language use, statistical analysis and creativity. One implication that arises from this argument is actually about humans. How do we know that we aren't all just incredibly detailed rulesets ourselves without any actual understanding? Well, first off - we technically can't prove it for anyone other than ourselves. More pragmatically, it's obvious that we, unlike the computer translator, can probe ourselves and be probed by others on whether or not we understand the subject. It's not like we're a bunch of Boltzmann's Brains that just happened into existence. We evolved intelligence in order to survive, not to "trick" other intelligent beings into thinking we're more intelligent than we are. There's no need for that. There's no one smarter around that we need to "trick".
- ramboldio 6y agoFourier Transforms. I'd wish I had a intuitive understanding of how they work. Until then I'm stuck with just believing that the magic works out.
- TheodolphusRose 6y ago3blue1brown has a great video on the topic: https://youtu.be/spUNpyF58BY https://youtu.be/spUNpyF58BY
- supernova87a 6y agoI have to say that video completely changed the level of my understanding about it. Especially the bit of visually intuitively understanding why the imaginary terms are the integration of the wrapping of the frequency component. Well worth watching.
- HenryKissinger 6y agoEvery analytical function, like f(x)=x^2-log(x+1), or signal, like a radio signal, can be rewritten as as infinite sum of sines and cosines. The Fourier transform helps you break down these components for you
- ivan_ah 6y agoThe best way to understand the Fourier transformations is to think of them as change-of-basis operations, like we do in linear algebra. Specifically a change from the "time basis" (normal functions) to the "frequency basis" (consisting of a family of orthonormal functions). Here is the chapter on Fourier transforms from my linear algebra book that goes into more details: https://minireference.com/static/excerpts/fourier_transformations.pdf https://minireference.com/static/excerpts/fourier_transforma... As for the math, there really is no other way to convince yourself that sin(x) and sin(2x) are orthogonal with respect to the product int(f,g,[0,2pi]) other than to try it out https://live.sympy.org/?evaluate=integrate(%20sin(x)*sin(2*x)%2C%20(x%2C0%2C2*pi)%20)%0A%23--%0A https://live.sympy.org/?evaluate=integrate(%20sin(x)*sin(2*x... Try also with sin(3x) etc. and cos(n*x) etc.
- qubex 6y agoI find most explanations of the Equivalence Principle that lies at the foundation of General Relativity to be very lax. To wit, the idea is that you cannot distinguish whether you are in an accelerated frame or in a gravitational field; alternatively stated, if you’re floating around in an elevator you don’t know whether you’re freefalling to your doom or in deep sideral space far from any gravitational source (though of course, since you’re in an elevator car and apparently freefalling... I think we’d all agree on what’s most likely, but I digress). Anyway, what irks me that this is most definitely not true at the “thought experiment” level of theoretical thinking: if you had two baseballs with you in that freefalling lift, you could suspend them in front of you. If you were in deep space, they’d stay equidistant; if you were freefalling down a shaft, you’d see them move closer because of tidal effects dictated by the fact that they’re each falling towards the earth’s centre of gravity, and therefore at (very slightly) different angles. Of course, they’d be moving slightly toward each other in both cases (because they attract gravitationally) but the tidal effect presents is additional and present in only one scenario, allowing one to (theoretically) distinguish, apparently violating the bedrock Equivalence Principle. I never see this point raised anywhere and I find it quite distressing, because I’m sure there’s a very simple explanation and that General Relativity is sound under such trivial constructions, but I haven’t been able to find a decent explanation.
- ben0x539 6y agoI'm gonna assume that for purposes of the thought experiment you're supposed to envision a point-shaped elevator, not one where you can place two baseballs next to each other.
- ironmagma 6y agoI think the elevator scenario is imagining that the earth is a point source, and you are neglecting the (much smaller) gravitational forces for the sake of illustrating a more general phenomenon.
- knzhou 6y agoYou're right that this is glossed over in popular explanations, but the point you make is exactly the starting point for all formal courses and textbooks. The first part of the argument is that for single point particles falling, the effect of gravity is the same for all particles. This suggests that we should model gravity as something intrinsic to spacetime itself, rather than as a field living on top of spacetime, which could couple to different particles with different strengths. The second part of the argument, which is what you point out, is that gravity can have nontrivial tidal effects. (This had better be true, because if all gravitational effects were just equivalent to a trivial uniform acceleration, then it would be so boring that we wouldn't need a theory of gravity at all!) This suggests that whatever property of spacetime we use to model gravity, it should reduce in the Newtonian limit to something that looks like a tidal effect, i.e. a gradient of the Newtonian gravitational field. That leads directly to the idea of describing gravity as the curvature of spacetime. So both parts of the argument give important information (both historically and pedagogically). Both parts are typically presented in good courses, but only the first half makes it to the popular explanations, probably out of simplification.
- yanovskishai 6y agoHow airplane wing really works.
- nnd 6y agoI found this article useful: https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/ https://www.scientificamerican.com/article/no-one-can-explai...
- yanovskishai 6y agothanks!
- deleted 6y ago[deleted]
- na85 6y agohttps://news.ycombinator.com/item?id=22991172 https://news.ycombinator.com/item?id=22991172
- yanovskishai 6y agothanks!
- npr11 6y agoAutomatic differentiation. It's useful to so much computational work, but most people only get a cursory introduction to the topic (a rough intro to the minimum they need to know), whereas really understanding it seems to open up a lot of research.
- OkayPhysicist 6y agoOh man, I read a super cool article about that about a year ago. It provided an algorithm for automatic differentiation using an imaginary number such that it times itself equaled 0, but wasn't equal to zero itself. I'll try to find the link. I don't know if this was it, but an explanation nonetheless https://medium.com/@omaraflak/automatic-differentiation-4d26d03b7508 https://medium.com/@omaraflak/automatic-differentiation-4d26...
- curiousgal 6y agoMeasure theory.
- ivan_ah 6y agoThis is a very good playlist: https://www.youtube.com/playlist?list=PLBh2i93oe2qvMVqAzsX1Kuv6-4fjazZ8j https://www.youtube.com/playlist?list=PLBh2i93oe2qvMVqAzsX1K... Starts from basic concepts and builds up a nice overview.
- pjungwir 6y agoQuantum spin. Electrons aren't really spinning, right? But why do we call it spin? I know it has something to do with angular momentum. What are the possible values? Is it a magnitude or a vector? Is there a reason we call it "spin" instead of "taste" or some other arbitrary name? How do you change it? What happens to it when particles interact?
- leni536 6y agoElectrons have both angular momentum and magnetic momentum, these have fixed magnitudes and they are always parallel. The situation is somewhat similar to a classical spinning charged sphare, although this similarity easily breaks down.
- abdullahkhalids 6y agoIf you take some ball of charge and actually spin it, and then place in inside a magnetic field, it moves in a certain way. If you take a single electron and place it in a magnetic field, it moves in that same way as the ball of charge. Ergo, its natural to call the relevant intrinsic property of electrons "spin".
- contravariant 6y agoAnnoyingly I've always struggled with the precise definition of spin, but to my understanding angular momentum is only conserved if you take spin into account as well. Being quantum mechanical there is some ambiguity in the direction, but in principle it is a vector (except it's a superposition of several vectors, and this superposition is also equal to superpositions of different vectors, I think there is usually one 'pure' vector, but that might just be after measurement).
- galimaufry 6y ago> angular momentum is only conserved if you take spin into account as well. Do you know an example of a process that moves angular momentum from one kind of spin to the other?
- 6y ago
- crosser 6y agoNon-linear optic explanation from quantum standpoint (classical explanation is quite clear).
- Dutchie85 6y agoWhy does time slow down/go faster with movement compared to another object. The well known example that if you travel into space you'd gain let's say 5 years and people on earth 25 in the same time or so. I just don't get it and I can't find any logic explanation. For instance: Two twins who came to live exactly at the same moment in the year 2000 and both die on their 75th birthday at the same time. One travels into space, the other stays on earth. Earth-brother dies on earthyear 2075,space-brother dies in earthyear 3050 or so... I know its Einstein's point but that just doesn't instantly make it correct to me.
- joshvm 6y agoThere are two effects here. One is special relativistic time dilation, and you can derive these effects with very little mathematics (high school is enough). The Wikipedia page has a simple proof, but it's important to realise that this is a result of the postulate that the speed of light is constant. If you travel at 0.99c and emit a photon, it still travels at c, not 1.99c. It's absolutely not intuitive why this should be the case. The other effect is that time in a strong gravitational field runs slowly.
- Dutchie85 6y agoI am sorry but I don't get it. I read the formula and get the formula but that doesn't make it logical. If you move away from a clock, time seems to slow down as your distance to the clock gets larger and the time between a change on the clock reaches you over a longer period. But if you carry a clock in your rocket it will just tick at the same pace as on earth (minus the gravitational impact, which is measured but why does gravity have an impact on time...?)
- joshvm 6y agoA core point of relativity is reference frames. From your perspective light in a vacuum always travels at c. The problem is that by special relativity, an observer moving at some speed relative to you will also see light traveling at c. This is a simple idea, but it causes a lot of very unintuitive effects. If we're being pedantic, relativity is very logical mathematically, but it's conceptually difficult because it flies in the face of how you think the world works. Have a look at the simple inference example here: https://en.m.wikipedia.org/wiki/Time_dilation https://en.m.wikipedia.org/wiki/Time_dilation Time doesn't necessarily slow down the further away you get from a clock. If you and a clock are both stationary (ie you're in the same inertial frame), you will observe it ticking in "normal" time, albeit delayed due to the distance. If the clock is moving relative to you however, you will measure its ticks to be slightly slower. You may be confusing general relativistic effects which are distance dependent (as gravity weakens the further away you get). If you carry a clock in your rocket, you will (in the rocket) measure it to tick once a second. When you get back to Earth, you'll find that it's lagged behind a clock that was started at the same time but was left on Earth. Maybe have a look at simple wiki too https://simple.m.wikipedia.org/wiki/Special_relativity https://simple.m.wikipedia.org/wiki/Special_relativity though it doesn't actually derive the Lorentz transforms unfortunately. Ignore the gravity bit for now, that's general relativity and it's more complicated to explain.
- pjungwir 6y agoEntropy. Sometimes you read that it's a measure of randomness; sometimes, information. Aren't randomness and information opposites?
- baron_harkonnen 6y agoEntropy is measured in bits (they exact same bits we talk about in computing), this can help unify the ideas of information and randomness: Which file can contain more information: a 1.44 MB floppy disk or 1 TB hard disk? Which password is more random (i.e. harder to guess): one that can be stored only 1 byte of memory or one that is stored in 64 bytes? Information theory deals with determining exactly how many bits of it would take to encode a given problem.
- deleted 6y ago[deleted]
- itcrowd 6y agoLet me give an analogy and then a solution to your paradox. Temperature. Sometimes you read that it's a measure of warmth; sometimes cold. Aren't hot and cold opposites? Yes, hot and cold are opposites, but in a way they give the same kind of information. That's also true for information and randomness. Specifically, little randomness means more (certain) information.
- deleted 6y ago[deleted]
- krcz 6y agoThe more randomness, the more information bits you need to encode the observed outcome. But I can see where your dissonance comes from: you probably parsed "information" as "information I already have about the system", not "information I need to describe the system state".
- deleted 6y ago[deleted]
- yters 6y ago
- vvoyer 6y agovertical alignment in CSS
- JJseiko 6y agothis made me laugh!
- arkanciscan 6y agovertical-align: center; /s
- phkahler 6y agoWave function collapse. As far as I can tell there is no discernable difference between a particle whose wave function collapsed (perhaps via measurement with another entangled one) and one that hasn't.
- rkagerer 6y agoThis is super naive but I just think of it as you (or whatever other particle is interacting with the one of interest) have now become part of its system. Like a causal event cone reaching you. Nothing about the particle changed, just your relationship with it (hence locking down the uncertain parameters). I'm sure you or another physicist could point out the flaws in my mental model.
- oxymoron 6y agoYou can always subscribe to the many worlds interpretation and stop worrying about it.
- daenz 6y agoWhy a flat earth is impossible.
- tomp 6y agoYou'd see Mount Everest.
- cygx 6y agoGravity. Gather enough stuff, and you end up with balls (unless you spin things really fast).
- Caylio 6y agoCan you prove that earth is enough stuff? Or that it's not spinning fast enough?
- HenryKissinger 6y agoIf you could make a disc shaped planet appear with a magic wand, and imparted it an initial angular momentum, over time the gravity differential between the middle and the edge would either tear your flat planet apart, or shape it into a blob, and eventually a sphere.
- tomp 6y agoFlight. Apparently "air flows faster on the top side of the wing, lowering the pressure" is an incomplete explanation; I even heard we don't completely understand why it works (?!?).
- thinkloop 6y agoFlight is EXACTLY like swimming except in air and without any component of floating. You're welcome.
- deleted 6y ago[deleted]
- geocrasher 6y agoI've been informed that I'm part of the problem. Comment removed, sorry for the trouble, folks!
- stevebmark 6y agoThat's not how lift works, don't be part of the problem! http://cospilot.com/documents/Lift.pdf http://cospilot.com/documents/Lift.pdf
- na85 6y ago>The top of a wing is curved, making it longer than the bottom of the wing. This means that air takes longer to go over it, meaning it has to spread out further to go the same distance as the air under the wing. As a result, the air going over the top of the wing is less dense, (aka lower pressure). The wing tries to equalize the pressure by moving in the direction of the low pressure, which is Up. We call this Lift. 100% completely false. Imagine you have two particles of air, and they are immediately adjacent to each other. Suppose now that one goes above the wing, and one goes underneath. In your example, the particle going upward goes further in the same amount of time. But ask yourself this: Why do the particles of air have to arrive at the same time? What mechanism from physics requires that they meet up again at the far end of the wing? Then ask yourself this: If what you described is true, then how do aircraft fly upside down?
- sam 6y agoMach's principle. Why is there a "preferred" rotational frame of reference in the universe? Or as stated in this Wikipedia article, "You are standing in a field looking at the stars. Your arms are resting freely at your side, and you see that the distant stars are not moving. Now start spinning. The stars are whirling around you and your arms are pulled away from your body. Why should your arms be pulled away when the stars are whirling? Why should they be dangling freely when the stars don't move?" https://en.wikipedia.org/wiki/Mach%27s_principle https://en.wikipedia.org/wiki/Mach%27s_principle
- contravariant 6y agoThe unsatisfying mathematical answer is that it is impossible to have a uniform distribution of rotational speeds, therefore there must be a preferred one. It's the same reason the universe has an average speed (unlike what you might expect from special relativity), although it is unclear if this is true for the entire universe or just the portion we can see. We can measure how fast we're moving w.r.t the cosmic microwave background radiation though (it is red-/blue-shifted in a particular direction).
- panic 6y agoThis is an interesting argument! Wouldn't it also work for positions, though? That is, either the universe is finite, or, since there can't be a uniform distribution over an infinite space of positions, there must be some preferred "center" of the universe?
- contravariant 6y agoYou'd think, but of course we know that not to be the case. It's hard to pinpoint the exact reason though. Sure we know time and space are rather special, but its hard to say exactly why. In the end though I reckon the most obvious reason is that speed is a property that directly corresponds to energy, therefore for each region of space to have a well defined energy (which is required for e.g. general relativity) every region of space needs to have a well defined distribution of speeds. I suppose this does leave open a small loophole, as you can easily correlate speed with position in order to get a distribution that is uniform in both (but correlated). But this goes against our assumption that the universe is uniform everywhere (which might turn out to be false, but so far it's holding up well).
- kosmodrom 6y agoFirst principle reasoning in real life examples.
- stevebmark 6y agoThe LIGO detector. I've never heard a logical explanation for it. If your explanation is that gravitational waves stretch and squash spacetime so light takes different amount of time to bounce from an emitter back to a measurer, then you don't have even a slight understanding of how it works. If your explanation doesn't involve higher spatial dimensions (not time) then you don't understand it. If you haven't even considered higher spatial dimensions when explaining LIGO then you shouldn't even try for the incorrect explanation above because you don't have any of the pieces of the puzzle.
- knzhou 6y ago> If your explanation is that gravitational waves stretch and squash spacetime so light takes different amount of time to bounce from an emitter back to a measurer, then you don't have even a slight understanding of how it works. Well, I'll bite. I'm a physicist and I understand LIGO. What's your alternative explanation?
- stevebmark 6y agoI have no idea how it works, I don't have an explanation, I just know the one spouted by people doesn't make sense
- knzhou 6y agoWell, if you're uncomfortable with LIGO, you're in good company. Once a whole decade went by in physics where people couldn't agree if it would work in principle or not. And it is true that a lot of common explanations are bad (e.g. "it's just a ruler" is not complete by itself because "why doesn't the light get stretched too?"). Nonetheless, today we have a variety of independent explanations. Maybe you'll find this paper helpful: https://aapt.scitation.org/doi/10.1119/1.18578 https://aapt.scitation.org/doi/10.1119/1.18578
- rrss 6y agoWould you say it's accurate to say LIGO is a big interferometer measuring changes in lengths of the arms due to gravitational waves, with some extra analysis to figure why it still works even though the light waves get stretched? Or is OP correct that such an explanation means I "don't have even a slight understanding of how it works" and a correct explanation must "involve higher spatial dimensions (not time)?"
- geocrasher 6y agoReactance.
- itcrowd 6y agoIf you have a specific question about electrical reactance, I can help. If not electrical, I probably can't help since I don't know what you are referring to.
- Rerarom 6y agoThe connection between elementary particles and group representations. I get the math, I just don't see in what way it corresponds to reality (i.e. in addition to being a bijection).
- neilk 6y agoWhen I hear explanations like “space is expanding like the surface of a balloon” it’s always confusing. Because a surface is an object, separate from anything on it, but space is the thing we’re all embedded in, so we’re like drawings on the balloon. If space is expanding why aren’t the radii of fundamental particles and their orbits and molecules also expanding? And if that were the case we couldn’t notice space expanding.
- knzhou 6y ago> If space is expanding why aren’t the radii of fundamental particles and their orbits and molecules also expanding? And if that we’re the case we couldn’t notice space expanding. > Does space only expand somewhere else? Only between me and the Andromeda galaxy, and not _within_ me and the Andromeda galaxy? How would it know to do that? If you start with expanding space in general relativity, and then carefully take the limit where you get back to Newtonian gravity, then it just corresponds to a classical force, specifically a very tiny force that weakly pulls everything apart, growing with distance. This doesn't expand small objects, because they're rigid. It's the same reason that I can't make my laptop get bigger by gently pulling on the ends. On the other hand, it would pull apart two independent, noninteracting objects (such as the Milky Way and Andromeda).
- bhk 6y agoWhen two particles get closer, their mutual gravitational attraction increases. As the distance approaches zero, the force approaches infinity. In the limit of d -> 0, the energy released -> infinity. Obviously at some scale the notion of a point mass breaks down, but even quantum theory would be problematic if we think of a wave function as describing a probability distribution, wouldn't it? What's the "official" story on this?
- knzhou 6y agoThe official way this is handled is called renormalization. Basically, we just declare that we have no idea what is going on at such short distances, and put in some regulator by hand to get rid of the infinity. One very crude regulator (which nobody uses, but which is suitable for demonstration) would just be to say that particles are simply not allowed to get any closer than some fixed tiny distance. But what about the effects that occur when particles actually do get that close? Well, in most theories, whatever is happening can be parametrized in terms of a few numbers (e.g. it could shift the observed mass of the particles, or their charge, etc.). Our ignorance of what is actually happening prevents us from computing these numbers from first principles. But we can still make scientific progress, because we can treat them as free parameters and measure them -- and after that measurement, we can use the values to crank out perfectly well-defined predictions. Repeating this process through several layers was crucial to building the Standard Model, which currently has about 20 free parameters.
- abernard1 6y agoWhat follows is very hand-wavy, and the renormalization sibling post may touch on it. An answer is that the d->0 approaches infinity presumes a nice, continuous analytic function. If d->epsilon, you can't get to that singularity. There was an equivalent problem in the E/M space with "The Ultraviolet Catastrophe" [1], which turned out to go away if you assumed quantization. I'm not going to claim this is a perfect analog to the gravity problem, only that a lot of physics doesn't quite work right when you assume continuity. (The Dirac delta is a humorous exception that proves the rule here, in that doing the mathematically weird thing actually is closer to how physics works, and it required "distribution theory" as a discipline to prove it sound.) [1] https://en.wikipedia.org/wiki/Ultraviolet_catastrophe https://en.wikipedia.org/wiki/Ultraviolet_catastrophe
- umvi 6y agoI would like to understand how cellular biology processes actually work. Like, how do all the right modules and proteins line up in the right orientation every time? Every time I watch animations, it seems like the proteins and such just magically appear when needed and disappear when not needed [0]. Sometimes it's an ultra-complex looking protein and it just magically flys over to the DNA, attaches to the correct spot, does it's thing, detaches, and flies away. Yeah right! As if the protein is being flown by a pilot. How does it really work? [0] https://youtu.be/5VefaI0LrgE https://youtu.be/5VefaI0LrgE
- knzhou 6y agoThe issue with these animations is that they're getting rid of all the thermal noise. In reality, single proteins are flying around the whole length of the cell many times a second, just from their thermal motion. And when processes like DNA transcription happen, they're not like a regular conveyor belt -- a fraction of the time the machine will even accidentally run steps in reverse! However, if any of this were shown, the animations would become impossible to understand.
- changoplatanero 6y agojust once i would like to see the realistic animation though even if it's impossible to understand
- InDemoVeritas 6y agoIt would be pretty boring, proteins bouncing around randomly and occasionally honking up, substrates flying around like rifle bullets sometimes hitting the target, and everything smooshing around in random directions. If you’ve seen Brownian motion, you’ve seen what is happening to all the molecules but at 1/100 the length scale. Nothing stays put. Everything is moving fast and far on the scale of proteins and small molecules.
- wrycoder 6y agoFast, yes. Far, well, the mean free path in a cell is very short.
- wegymoo 6y agoionization. The resources I can find on it are mostly conspiracy theory types or the same basics explained over and over again.
- yters 6y agoA non handwavy explanation of how does evolution result in such complicated and carefully orchestrated mechanisms, way beyond our engineering capability. The computer analogy of genetic algorithms certainly doesn't explain much, they are not very effective, and if they were, we should be able to generate marvels of engineering with GAs and current computational power.
- sloaken 6y agoA whole LOT of time, a whole lot of space. So estimate 1 minor good random mutation per 10,000 population. Assume 1 major mutation per 1000 years, per 10,000 population.
- yters 6y agoThat's essentially the same kind of explanation as 'god did it'.
- cambalache 6y agoI dont know if you are being serious, but if you are, Dawkins books are pretty good (The blind watchmaker,or the Ancestors tale).
- yters 6y agoI will have to check them out, but I remember being unimpressed with the table of contents. Looks like it will be more handwaving.
- cambalache 6y agoAh OK, you are not being serious, fair enough. Magic god did it all.
- yters 6y agoI am being serious. I just purchased Dawkin's book "Blind Watchmaker". My point is no explanation should amount to "magic X did it" whether X is god, evolution, the earth spirit, aliens, etc. All the above are bad explanations. It is not scientific to substitute one bad explanation for another. The scientific approach is to say we don't know, and then look for a good explanation.
- inopinatus 6y agoWhy bicycles stay upright. For every authoritative-sounding, in-depth explanation, there is an equally plausible, yet conflicting and contradictory alternative.
- itcrowd 6y agoMy hunch is that the science is not quite out on it yet and that the intuitive explanations are not accurate. For example, see "Some recent developments in bicycle dynamics" (2007). Especially the folklore section: "The world of bicycle dynamics is filled with folklore. For instance, some publications persist in the necessity of positive trail or gyroscopic effect of the wheels for the existence of a forward speed range with uncontrolled stable operation. Here we will show, by means of a counter example,that this is not necessarily the case. https://pdfs.semanticscholar.org/bb70/d679c5a2ff67dd2a1a51f265d68160f18860.pdf https://pdfs.semanticscholar.org/bb70/d679c5a2ff67dd2a1a51f2...
- Liron 6y agoMaybe when you stop to think, the balance of explanations tips over one way
- arkanciscan 6y agoThey don't, they fall down. Know why? Cause they are two tired! I'll downvote myself
- strbean 6y agoMy theory: self-correcting action on the front tire when moving forward. When the wheel pitches to the either side, the road under the bicycle pushes the wheel back to center. When the bicycle leans to the side, the wheel pitches as well. Now the road under the bicycle pushes the wheel back towards center, but the angle of the tire to the road is also skewed, so the some of the force also gets translated into pushing the bike upright. It's a little reminiscent of the self-centering action you get when you have a double-cone (or cylinder tapered on both ends) rolling down two rails. I think if you fixed the front tire so there was no steering, you wouldn't get any stability from speed.
- CamperBob2 6y ago
- Crazyontap 6y agoWhat happens when you actually fall inside a black hole and what is the singularity. I never really understood what happened really when the guy fell inside it in Interstellar and how come he started seeing all those photos. I just accepted it as Hollywood bs. I know my question is based on a movie but would still like to know what will someone witness (assuming of course they somehow live)
- guerrilla 6y agoYou should check out PBS Space Time (on youtube), there are several episodes explaining this in different ways.
- hartator 6y agoSingularity in AI?
- Crazyontap 6y agoNo they kept saying that the center of it is called singularity IIRC.
- guerrilla 6y agoNo, https://en.m.wikipedia.org/wiki/Gravitational_singularity https://en.m.wikipedia.org/wiki/Gravitational_singularity
- thegabriele 6y agoProblem is, we have no physics yet that explains what happens "inside" the event horizon.
- fyp 6y agoI have not read it myself but the answer can probably be found in the book "The Science of Interstellar" [1] Kip Thorne, a Nobel prize-winning physicist, worked as the science advisor for Interstellar so the hollywood bs is pretty good! [1]: https://www.amazon.com/Science-Interstellar-Kip-Thorne/dp/0393351378 https://www.amazon.com/Science-Interstellar-Kip-Thorne/dp/03...
- hartator 6y agoQuantum Physics. Still can’t wrap my mind around multiverse being a simpler explanation then the future holds data we won’t have in the now.
- ivan_ah 6y agoIf you remember your linear algebra basics, you could easily pick up QM --- the parts that deal with finite size systems. Here is an excerpt from my book I posted a while back in another thread, and refreshed today since you asked: https://minireference.com/static/excerpts/noBSLA_quantum_chapter.pdf https://minireference.com/static/excerpts/noBSLA_quantum_cha... As for the multiverse, I don't know enough to talk about it. I just know it's one of the possible interpretations of quantum mechanics. Note that the various interpretations are generally considered more philosophy than science, and have no (or very little) practical implications. I would suggest ignoring all analogies and not looking too deeply for interpretations, and instead focus on basic concepts like "What is a quantum state?" and "How do I compute measurement outcomes?" which are super well understood and the same under all interpretations. You can think of the various interpretations of QM as different software methodologies, scrum, agile, waterfall, etc. just stuff people like talk about endlessly, but ultimately irrelevant to the code that will run in the end.
- tprice7 6y ago1. Carbon dating. Sure, I get that carbon decays over time and this changes the proportion of isotopes. But why does this give you any information? That carbon didn’t come into existence just to be in that bone, it was made in the sun billions of years before that, so why does the age of the carbon tell us anything about organic matter? The key fact, which I think is not emphasized enough, is that the ratio of isotopes in atmospheric carbon is kept at a constant equilibrium by cosmic rays. So you can use carbon dating to tell roughly when the carbon was pulled out of the atmosphere. Without this additional fact, the concept of carbon dating makes absolutely no sense. 2. The tides. The explanation I was given is roughly something like “the tides happen because the moon’s gravity pulls the water toward it, so you have high tide facing the moon. There’s also a high tide on the opposite side of the earth, for subtle reasons that are too complicated for you to understand right now and I don’t have time to get into that.” The first problem with this explanation is this: gravitational acceleration affects everything equally right? So it’s not just pulling on the water, it’s also pulling on the earth. So why does the water pull away from the earth? Shouldn’t everything be accelerating at the same rate and staying in the same relative positions? The second problem is that, when viewed correctly, the explanation for why there is a high tide on the opposite side of the earth as the moon is equally simple to why there is a high tide on the same side as the moon. The resolution to both these problem is this: tides aren’t actually caused by the pull of the moon’s gravity per se, but are actually caused by the difference in the strength of the pull of the moon’s gravity between near and far sides of the earth, since the strength of the moon’s gravitational pull decreases with distance from the moon. The pull on the near water is stronger than the average pull on the earth, which again is stronger than the pull on the far water. So everything becomes stretched out along the earth-moon axis. 3. This one isn’t so much a problem with the explanation itself, more about how it’s framed. I remember hearing about why the sky is blue, and wondering, “ok, more blue light bounces off it than other colours. But isn’t that essentially the same reason why any other blue thing is blue? Why are we making such a big fuss about the sky in particular? ” A much superior motivating question is “why is the sky blue during midday, but red at sunrise / sunset”? I was relieved when I saw this XKCD that I’m not the only one who felt this way: https://xkcd.com/1818/ https://xkcd.com/1818/
- nabogh 6y ago
- arkanciscan 6y agoQuantum Computers. Not like I'm five, but like I'm a software engineer who has a pretty decent understanding of how a classical turing machine works. I can't tell you how many times I've heard someone say "qubits are like bits except they don't have to be just 1 or 0" without providing any coherent explanation of how that's useful. I've also heard that they can try every possible solution to a problem. What I don't understand is how a programmer is supposed to determine the correct solution when their computer is out in some crazy multiverse. I guess what I want is some pseudo code for quantum software.
- KhoomeiK 6y agoI had an aha moment with quantum computers a few months ago when reading an article that explained it as probability distributions. I don't think I have the complete understanding in my mind anymore and I wish I had saved the article, but looking into how quantum computers essentially serve as probability distribution crunching machines might help with your understanding.
- arkanciscan 6y agoSo can they still do traditional deterministic(?) calculations? Or would that be somewhat akin to using machine learning to do your taxes; possible but just overkill? I've often heard it said that Quantum Computers can crack cryptographic keys by trying all the possible inputs for a hashing algorithm or something handwavey like that. Are they just spitting out "probable" solutions then? Do you still have to try a handful of the solutions manually just to see which one works?
- augustt 6y agoHere's what Scott Aaronson says in regard to "trying all the possible inputs": https://news.ycombinator.com/item?id=17425474 https://news.ycombinator.com/item?id=17425474
- deleted 6y ago[deleted]
- andrewflnr 6y agoHawking Radiation. I know just enough to know that the story of "anti-particle of virtual pairs happens to always fall in the hole" is a Lie Told To Children, but the explanations seem to go straight from there to rather dense math and I've never wrapped my head around it.
- guerrilla 6y agoI hope this helps: https://www.youtube.com/watch?v=qPKj0YnKANw https://www.youtube.com/watch?v=qPKj0YnKANw
- andrewflnr 6y agoActually... yes, which just makes it worse that something about that presenter's style really rubs me the wrong way. Do they have their content in text format somewhere? Anyway, thanks!
- guerrilla 6y agoI'm sure somone's made transcripts but I have't come across them yet
- gliese1337 6y agoThat's not even a lies-to-children explanation--it's just straight-up wrong, a result of generations of "science journalists" misunderstanding stuff and spreading their misunderstandings around. Hawking radiation does not discriminate between matter and anti-matter. Either form of any type of particle can be emitted.
- elgfare 6y agoThe mathematics of training a neural network. I understand how they work once trained, but that you can train them almost seems too good to be true.
- supernova87a 6y agoAlso, for me, how all these advances in ML/computing are alleged to be on the horizon, when I hear that A/D/C(NN)s are actually so slow in learning. How can something that has to be trained with 1M of <xyz> be smart? What's the next thing?
- OkayPhysicist 6y agoYou and I probably took 4 or 5 years to recognize the alphabet. Cut the machines some slack.
- plun9 6y agoYou likely understand minimizing a continuously differentiable function. Now, you are minimizing a continuously differentiable error function (which measures the difference between the output of your hypothesis function and actual data), with respect to adjustable weights and biases that determine the value for the neurons going from one layer to the next. The complexity is in that the hypothesis function is a composition of many functions due to the layering, and there usually are a large number of neurons. However, you are basically doing the same thing many times.
- galimaufry 6y agoA partial answer is that big fully connected neural networks _are_ pretty much untrainable. Neural networks only became successful once programmers started constraining the space they were optimizing over pretty radically (like requiring convolutional layers if you know you are trying to detect something local).
- cambalache 6y agoOn the contrary, it is pretty simple, it is just the same process of "a little bit to the left, a little bit to the right" when you are trying to hang a poster.
- elgfare 6y agoThe twin paradox. All explanations seem to be just "something something one twin has to accelerate"
- gliese1337 6y agoThe amount of time you experience is directly related to the length of your worldline--the path you trace out in 4D spacetime as you move through space and simultaneously move forward in time. It should be easy to see that a non-accelerating object (or person) will trace out a straight worldline. If you ever change your velocity, though, either through smooth acceleration or instantaneous rotation of your velocity vector, you will trace out some non-straight curve in spacetime. If you leave your friend behind and then, at some later time, meet back up again, if you did not undergo exactly the same amount of acceleration throughout your journeys (i.e., because one of you stayed behind and hardly accelerated at all, tracing out a boring straight line path), then you will have different world-line lengths (different "path integrals") between the starting and ending points, and thus will have experienced different amounts of subjective time. Now, in a Euclidean spacetime, the traveling twin would end up older, because a straight line is the shortest distance between two points. But our spacetime is not Euclidean--it is a Minkowski space, in which acceleration is equivalent to a hyperbolic rather than Euclidean rotation of your velocity vector, so it turns out that straight line is actually the longest distance between any two points, and the twin who leaves and comes back will have a shorter worldline, and thus will have aged less.
- vmception 6y agoHas someone that thought they were taking LSD ever turned into a permanent schizophrenic zombie or in a mental institution, or is it all urban legend. If someone that didn't know they were predisposed to mental illness, is it applicable to dismiss their experience in order to maintain how safe LSD is? If any of this is true, are there any sources aside from "my friend's friend's brother took too much and now he is....", and what is the scientific explanation and do we know enough about the mind at all? I feel like LSD has a lot of contradictory information out there, and the proponents feel the need to hand waive concerns away because it is 'completely harmless and leaves your system in 10 hours'. But when nobody knows what they're actually getting because it doesn't exist in a legal framework, then it muddies the whole experience. People say certain doses can't do more effect than lower doses after a certain threshold. It seems like the same people say "omg man 1000ug you are going to fry your brain!" What is the truth? If it "just" had an FDA warning like "people with a family history of schizophrenia should not take it", that would be wildly better than what we have today. Please no explanation about shrooms. Just LSD the 'research chems' distributed as LSD.
- knzhou 6y agoThis entertaining article lists what happened to the early LSD researchers: https://slatestarcodex.com/2016/04/28/why-were-early-psychedelicists-so-weird/ https://slatestarcodex.com/2016/04/28/why-were-early-psyched...
- vmception 6y agoentertaining, the comments are a good addendum but are just as speculative. maybe there just isn't good information, its just more of the contradictory information. "lasting permanent changes, obviously" "I’ve personally seen several people experience total amnesia after tripping on high doses." No further information. "Not lasting permanent changes" what. Names, sources, medical records, news reports, court cases, there has got to be something out there!
- abacadaba 6y agoHead over to the phantasytour forums, place is full of acid casualties.
- LVB 6y agoGenetics. I still have no intuition for how combining my wife’s and my DNA has resulted in children with traits from both of us. My brain always tries to imagine interleaving two binaries and hoping for a resulting program that works a bit like the two sources, which it of course wouldn’t.
- vmception 6y agoI mean that's close, but not compiled binaries, just merged in very similar source code in a pull request, where sometimes an && is added to a conditional statement to express a gene that had skipped a generation. Both sexes have haploid gametes, which form a zygote when combined. I think this can steer your research when you look into what gametes contain and chromosome combination.
- vikramkr 6y agoDon't think if it as a monolith. It's more like thousands of microservices and thousands of different binaries all interacting in a whole bunch of different and convoluted ways. Sexual reproduction is jumbling together two different people's thousands of microservices and seeing what happens. Or, each program is relying one library, imagine just randomly mixing around different libraries. Genes and proteins are discrete parts that can be swapped out and in.
- refurb 6y agoThe analogy works if each binary way laid out the same - the first MB is eye color, the 2nd MB was hair color, etc. Each segment of code is also duplicated. When you produce sex cells, you body splits the code randomly. The cell might contain the first copy for hair color, but the 2nd copy for eye color. This happens in both the sperm and eggs. Then when they combine you have a full set of genetic data again, but it’s a random selection of 50% of you DNA. The fun part is that the code itself determines which copy is dominant. Your offspring has a copy of your eye color data, and your wife’s. Added on that is that the two copies combine to produce the outcome. Depending on the code, the dad’s copy might dominate, or the mother’s, or the both of them can produce yet a 3rd outcome.
- druml 6y agoEscape velocity
- deleted 6y ago[deleted]
- sloaken 6y agoMy dad was an aero space engineer, and I always had trouble with this. It is the speed you need to be going at, to not be pulled back by gravity. As a kid, I always thought, well big deal, if I am in a plane and just keep going up, eventually I will be out. The key I always missed was the velocity at that point was with no added power. So when the engine cuts off, what is your velocity? Is it enough to escape? Or do you need more power. Of course there is a good chance I still do not understand it.
- nabla9 6y agoGauge theory.
- skanga 6y agoCan anyone explain disc loading and solidity in a propeller and if there are any equations governing them?
- na85 6y agoDisc Loading is just the force on the actuator disc (such as a rotor, in which case the force is the weight of the vehicle the rotor holds aloft) divided by the swept area (pi times r squared). It's used when discussing propulsive efficiency, as it's a proxy measurement for how much "work" each blade is doing. Because propeller/rotor blades are just high-aspect wings, if you have high disc loading your blades are at a high lift coefficient which means they'll be incurring lots of lift-induced drag which increases your power requirements. Solidity in the same context refers to the amount of volume within an actuator disk that's occupied by actual solid material. If you have a 4-bladed rotor and you move to a 5-bladed rotor, all else equal, you've increased your solidity. There are many many equations, and as most things in fluid mechanics you can get as deep into the weeds as you want. As a starting point, have a look at the wiki article for Blade Momentum Theory[0] [0] - https://en.wikipedia.org/wiki/Momentum_theory https://en.wikipedia.org/wiki/Momentum_theory
- skanga 6y agoThanks na85 Let's take a contrived example. I have a 4 engine high wing airplane with 2 bladed 55 inch props on each engine with 100kg force (1 kg-f is equal to 9.8 N) thrust per engine. Now, I need to make that a low wing airplane so need to change to 22 inch blades with the same thrust and I don't want to change engines. So I want to add more prop blades. How many more blades do I need to add?
- na85 6y agoCouple of points: 1) 100kg is a measure of mass, not force. Thrust is a force, not a mass. But let's say you have 100N of thrust per engine. 2) Why would changing from a high-wing to a low wing monoplane require you to add prop blades?
- asfarley 6y agoWhy are clouds flat on the bottom?
- lllr_finger 6y agoBecause that's (roughly) the Lifted Condensation Level: https://en.wikipedia.org/wiki/Lifted_condensation_level https://en.wikipedia.org/wiki/Lifted_condensation_level Water vapor around the LCL starts condensing and turning from a gas into liquid cloud droplets. This process happens considerably faster once it begins for a variety of reasons, so once you can have cloud droplets, you get a ton of cloud droplets - not a gradual transition from water vapor to cloud. It's almost like a light switch. Most air masses are relatively homogenous anyways, so unless there are underlying processes causing things like undulatus asperatus, it will certainly appear very, very flat over a large area.
- ARandomerDude 6y agoSame reason a loaf of bread is. Clouds rest on the "surface" of the higher-density air below them, and flatten on the bottom because of their own weight.
- aazaa 6y agoSort of meta, but I always shudder when someone says that science has "proven" something. What sets science apart from most other methods of seeking answers is its focus on disproof. Your goal as a scientist is to devise experiments that can disprove a claim about the natural world. This misconception rears its head most prominently in discussions at the intersection between science and public policy. Climate change. How to handle a pandemic. Evolution. Abortion. But I've even talked to scientists themselves who from time to time get confused about what science can and can't do. The problem with believing that science proves things is that it blinds its adherents to new evidence paving the way to better explanations. It also leads to the absurd conclusion that a scientific question can ever really be "settled."
- richardbrevig 6y agoIt kind of floors me that we're taught science the way it is. Much simpler: Karl Popper's conjecture and refutation. So I tell people that science mandates "I believe something, so I should try to prove it wrong." I think understanding that is significantly more beneficial than repeating the arbitrary n- steps of a scientific method. It's two steps. Keep it simple.
- xyzzy99 6y agoNot to be rude, but given current daily attacks on science and the scientific method, I can't let this stand - I think your meta intuition represents a fundamental misunderstanding of how science works. It is simply wrong to think that scientific questions can never be definitively settled. Clearly there are some hypotheses that have been difficult (and may be impossible) to prove, for example, Darwin's idea that natural selection is the basis of evolution. There's ample correlative evidence in support of natural selection, but little of the causal data necessary for "proof" (until perhaps recently). In the case of evolution the experiments required to prove that natural selection could lead to systematic genetic change were technically challenging for a variety of reasons. In the case of climate change, the problem again is that the evidence is correlative and not causal. Demonstrating a causal link between human behavior or CO2 levels and climate change (the gold standard for "proof") is technically challenging, so we are forced to rely on correlations, which is the next best thing. But, you are right, it is not "proof". Establishing causality can be difficult but not impossible - the standard is "necessary and sufficient". You must show necessity: CO2 increase (for example) is necessary for global warming; if CO2 remains constant, no matter what else happens to the system global temperatures remain constant. And you must also demonstrate sufficiency: temperatures will increase if you increase CO2 while holding everything else constant. Those are experiments that can't be done. As a result, we are forced to rely on correlation - the historical correlation between CO2 and temperature change is compelling evidence that CO2 increases cause global warming, but it is not proof. It then becomes a statistical argument, giving room for some to argue the question remains "unsettled". My point is that there are plenty of examples in science where things have been proven -- DNA carries genetic information, DNA (usually) has a double stranded helical structure, V=IR, F=Ma, etc. And there are things that are highly likely, but not "proven", e.g., human activity causes of climate change. While some of the issues you bring are remain unproven, what's really absurd is to think that no scientific questions can be settled.
- sloaken 6y agoAvogadro constant - I can accept the number, if someone can show me how, over a hundred years ago, without having every really seen a molecule, this number could be derived. Obviously no one ever sat there and counted them but I find it hard to believe, they could have decided this without being able to see it.
- lbblack 6y ago"The charge on a mole of electrons had been known for some time and is the constant called the Faraday. The best estimate of the value of a Faraday, according to the National Institute of Standards and Technology (NIST), is 96,485.3383 coulombs per mole of electrons. The best estimate of the charge on an electron based on modern experiments is 1.60217653 x 10-19 coulombs per electron. If you divide the charge on a mole of electrons by the charge on a single electron you obtain a value of Avogadro’s number of 6.02214154 x 1023 particles per mole." https://www.scientificamerican.com/article/how-was-avogadros-number/ https://www.scientificamerican.com/article/how-was-avogadros...
- Balgair 6y agoHere's how to estimate the size of an oil molecule just using water and an oil drop: https://spark.iop.org/estimating-size-molecule-using-oil-film https://spark.iop.org/estimating-size-molecule-using-oil-fil... (They use some other stuff, but you get the idea) You can back out Avogadro constant starting with this experiment.
- sloaken 6y agoThank you. I do get the idea. And now I can go back and finish High School chemistry. I appreciate you help in understanding this.
- fegu 6y agoYoneda lemma
- aesthesia 6y agoAs with most things in category theory, the way to really understand the Yoneda lemma is to sit down and prove it yourself. Break the statement down into the underlying definitions, draw lots of diagrams, and convince yourself that it’s true. The other thing you can do is think about what it means for particular types of categories. For a posetal category, it says that an element of a poset is uniquely determined by the set of all elements that come before it in the ordering. For a group, it says that every element is uniquely determined by its action on the group. (This is basically Cayley’s theorem.) See this MSE post for more intuition: https://math.stackexchange.com/questions/37165/can-someone-explain-the-yoneda-lemma-to-an-applied-mathematician https://math.stackexchange.com/questions/37165/can-someone-e...
- harimau777 6y agoI don't know if this would be my "one question" if I could ask the most brilliant minds in science, but something that always bothered me: When I took physics they basically said "at first scientists were disturbed by the fact that magnets imply that two objects are interacting without any physical contact, but then Faraday came along and said 'the magnets are actually connected by invisible magnetic field lines' and that resolved everything." How does saying "but what if there's invisible lines connecting them" resolve anything? To be clear, I'm not objecting to any of the actual electromagnetic laws or using field lines to visualize magnetic fields. It's just that I don't get how invoking invisible lines actually explains anything about how objects are able to react without physical contact. (Also, it is not lost on me I that this question boils down to "fraking magnets, how do they work?")
- eranation 6y agoThis, gravity, and quantum mechanics, I think are things that people just accept as is, I don't think anyone really knows how or why it works, it just works. It could be that our brains are not wired to understand how two things can be pulling each other without anything physically connecting them. My knee jerk explanation is that we live in a simulation, and that the simulation is not in anything like a physical world, and we are just not wired to grasp it, just like an ant won't be able to learn learn calculus.
- McTossOut 6y agoYeah I'm not sure what you're after here. Finding something descriptive and a physical phenomena is exactly physics, though. Field lines are tangent to fields, and the fields provide a predicting power over forces and motion experienced by macro objects undergoing electromagnetic phenomena. Maybe you should look into the hypotheses and techniques that were used by those scientists in developing predictive power over electromagnetism.
- JabavuAdams 6y agoIt changes the problem from one of action at a distance to local interaction. Instead of "field lines", I would say "field". Field lines are just one way of visualizing a field. So, if we don't have the notion of fields, then we have a kind of situation of how does object A know about remote object B. Like how does one object know about the motions of literally every other object in the Universe. Perplexing. Once you come up with the idea of a field, okay you have to at some level accept that there are fields that permeate all of space. But what this intellectual cost buys you is that now an object only has to sense the field local to it to respond to all objects in the universe. Think of objects bobbing on the ocean. One way to conceptualize that is that any object anywhere could cause this object here to bob in some way. How does this object know about all the other objects? Instead we could say that there is ocean everywhere. Locally, objects bobbing put ripples into the ocean. Locally, ripples cause objects to bob. Each object no longer needs to "know about" every other object it just needs to react to the ripples at its location, and the ripples get sent out from its location. Does this help?
- eranation 6y agoLot's of quantum related phenomenon here, but what keeps bothering me is that while I get it that light is both a wave and a particle, but I have no clue what that means. I mean, a wave of sound, is made from air particles, a wave of ripples in a pond is made of movement of water molecules. In the double slit experiment, it's explained that the single photon has to be "interfering with itself", so I don't get it if by being a "wave" it means that the single photon is basically a bunch of "magic" photon ghosts that behave like a wave, but once it is measured or any other reason to "collapse" these ghosts "disappear". I just don't get what the "wave" of light/radio wave is. Is it just an abstract concept of something that behaves like a wave but not the same as sound waves / ripples since we simply don't know? Or is it just a wave of these "not yet collapsed" probabilities of the photons locations that are interfering with each other right until we ask them to choose a location, then they just collapse magically into a single "real" photon. Another thing I don't get is in the double slit experiment, a LOT of the measure before, measure after, etc, are told to be thought experiments, but it's also claimed that someone managed to actually replicate them. Why isn't there a video showing it? I obviously believe they happened, and understand why more or less (e.g. in the one photon at a time experiment, it's spooky that over time you get the same pattern that indicates interference as if you shoot many) but the more spooky result is that thought experiment, that if you measure which slit the photon actually traveled through, you'll see 2 slits on the screen vs the famous pattern. e.g. you'll cause the wave to collapse back to particles. So any video of reproducing of that thought experiment or explanation why it's so hard to reproduce, will be super helpful.
- plun9 6y agoI think this is more about the terminology than the actual concepts. This is like in biology, where you classify things as living and non-living, but then you encounter things like viruses, which would depend on the criteria for a "living thing." Or, is a gel a solid or a liquid? Again, it has properties of both states of matter. Similarly, in order to classify phenomena as waves or particles, you look for common attributes between the things you observe. But light exhibits properties of both.
- JabavuAdams 6y agoSorry, don't have time to unpack all this, as some are things that scientists understand, but some are so deep that ... not. When talking about light or radio waves, you're probably talking about Classical Electrodynamics, which does not include the notion of a photon. In Classical EM, light is an EM wave in the sense that if you look at the electric field E or magnetic field B (of a plane wave) at a fixed point in space it is varying sinusoidally. So the waviness is in the amplitude of the E and B fields. Once you talk about photons, you're in the realm of Quantum Mechanics (QM), and yes things are harder to understand. It's actually all just fields according to the Standard Model (particle physics), a quantum field theory (QFT). In QFT there's a field for each fundamental particle that permeates the whole universe. E.g. an electron field, a photon field, etc. Disturbances in these fields are what one would call particles in non-relativistic QM. So Classical -> QM (quantum system, classical observer/apparatus) -> QFT (quantum everything) In classical EM, light is a wave. In QM, light is particles. In QFT particles are just disturbances in the all-pervading fields. Binney has said that QM is just measurement for grownups, or some such. What is a measurement? It's when the system you're observing becomes entangled with the measuring device. We don't know the exact state of every atom in our measuring device, but these could all perturb the system we're measuring. So QM is a hack where you treat the system as quantum but the observer/measuring device as classical which is why you need this confusing wave-function collapse. It was a conscious choice in the development of the theory. This last bit might give some insight into why trying to sense the photon at one of the double-slits ruins the interference pattern.
- gkolli 6y agoAntenna Design/RF Theory. I would love a simple text/YouTube series to learn this stuff...would definitely be good learning for the summer :)
- kccqzy 6y agoI second this. I once worked with an assistant professor to do some of that, and in the end a lot of what we did boiled down to trial and error: do a design, run simulations in some software, and if it doesn't look good try again. It makes me sad that I didn't really encounter anyone capable of explaining things beyond trial-and-error.
- sloaken 6y agoReminds me of a story: A guy was walking along the beach and found a lamp. Of course he rubs the lamp, and sure enough a genie appears. Genie: master of the lamp I can grant you a wish, you may wish for anything. Guy: Wait, isn't it supposed to be 3 wishes? Genie: One or nothing, and do not wish for more wishes. Guy thinks for a while .... You know I have pretty much everything I need. But I have always wanted to travel to Hawaii. But I get sea sick and am afraid to fly. Genie: Very well I will take you there. Guy: No no, if you take me there, I wont be able to come back. And what about next year? Since I only get one wish, I want a bridge built to Hawaii. Genie: That does not make any sense. Please make a different wish. One that does not involve so much construction. Guy: hmmmm you know I know, can you explain women? Genie: So do you want the bridge to be a suspension bridge or truss? and how many lanes ....
- buboard 6y agoelectromagnetic waves
- throwphoton 6y agoAbsorption spectra baffle me and I've never seen an explanation that helps me understand. It seems conceptually simple, except the requirement that the energy of an photon exactly match some required energy in order to be absorbed seems really unlikely, since photon energy not a discrete quantity, and varies according to doppler effects and other things. It seems like the vast majority of photons would just fly through the universe without interacting with anything, unless there are other ways for photons to interact with matter besides being absorbed. (If there are other ways, they are seemingly never mentioned as a potential alternative fate for the photon).
- galimaufry 6y ago> It seems like the vast majority of photons would just fly through the universe without interacting with anything, unless there are other ways for photons to interact with matter besides being absorbed. Maybe someone who actually knows will chime in, but afaik: - Most light doesn't have a fixed frequency. If it did, it would have a fixed momentum, but then you would have no idea where it is! Instead it is some superposition of many frequencies. That could be part of the story. - Light could be stopped by something other than absorption into an individual atom. Metals don't have discrete spectra.
- rom16384 6y agoThe match between the energy of the photon and the atom orbits doesn't have to be perfect. Part of the energy may be added or removed as vibrations (phonons).
- abetusk 6y ago* Lie algebras and Lie groups - I still don't understand this, what they're used for or how to use them in any practical sense. * Galois Theory - I have a basic understanding of abstract algebra but for some reason Galois theory confounds me, especially as it relates to the inability of radical solutions to fifth and higher degree polynomials * "State-of-the-art" Quantum Entanglement experiments and their purported success in closing all loopholes * Babai's proof on graph isomorphism being (almost/effectively) in P - specifically how it might relate to other areas of group actions etc. * Low density parity checks and other algorithms for reaching the Shannon entropy limit for communication over noisy channels * Hash functions and their success as one-way(ish)/trapdoor(ish) functions - is SHA-2 believed to be secure because a lot of people threw stuff at the wall to see what stuck or is there a theoretical backpinning that allows people to design these hashes with some degree of certainty that they are irreversible?
- galimaufry 6y agoI think the best motivation for the theory of Lie groups and Lie algebras is representation theory. Like you don't need anything besides linear algebra to know what SO_n is, but if you want to know about how it can act on a vector space then you need to think about the Lie algebra. The other great thing about Lie groups is you can discover new and valuable groups just from pretty basic topology. Like the Spin group, which you know has to be out there as soon as you know the fundamental group of SO_n, but otherwise would be very hard to think of. The fancypants but I think most intuitive way to think about Galois theory is also with topology. It's an algebraic version of a much more geometric, visible story, the correspondence between {subgroups of the fundamental group} and {normal covering spaces}.
- aesthesia 6y agoProbably the best place to get started with Lie groups is to wrap your head around SO(n), which has both a nice geometric interpretation as rotations of n-dimensional space as well as a concrete representation as the space of orthogonal (determinant 1) nxn matrices. With a little matrix calculus you can work out what the tangent directions are at the identity matrix: they’re precisely the skew-symmetric matrices. This is the Lie algebra so(n). Where the Lie group consists of rotations, the Lie algebra consists of directions in which you can rotate something, or really velocities of rotation. This is why classical angular momentum is actually an element of a Lie algebra.
- invisibIe 6y agoAll the credible theories of why there are 3 generations of particles.
- deleted 6y ago[deleted]
- spodek 6y agoConsciousness. How is it possible that the thread is up 5 hours and ctrl-f consciousness returns nothing?
- JabavuAdams 6y agoBecause no one knows what it is or even agrees on a definition. Consciousness is a pre-scientific concept at this point. There are attempts to rigorously define it. I'm currently reading this paper, but not really convinced: https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1003588 https://journals.plos.org/ploscompbiol/article?id=10.1371/jo...
- exabyte 6y agoHere are things that I consider to be defining properties of human consciousness: a) The ability to observe an instance of a multidimensional universe, each instance a mapping of a point moving along a continuous trajectory whose direction is constrained by the arrow of time (i.e., increasing entropy) b) the ability to impart a force to change the direction that trajectory follows
- carapace 6y agoConsciousness has no properties, it can't be studied scientifically.
- RocketSyntax 6y agostring theory
- memset 6y agoCrypto and practical security. I get tired of the circular “don’t roll your own crypto unless you’re qualified”. How does one become qualified? I don’t feel like I know how to evaluate many of the arguments people make for or against technologies people argue about on HN, such as Signal or different password managers. I feel like “security through obscurity” is a bad thing, and “layers of security” are a good thing, but isn’t all security obscuring something, and how does one evaluate whether a layer is adequate? “Just use bcrypt” - okay, help me understand!
- azernik 6y agoIf you have a good CS background, I highly recommend the lecture notes for the security class I took in undergrad: https://inst.eecs.berkeley.edu/~cs161/sp10/ https://inst.eecs.berkeley.edu/~cs161/sp10/ That's from 10 years ago, so you might be able to find video of a more recent version; try to find a year when Wagner taught, he's great.
- Eridrus 6y agoThe reason people say not to roll your own crypto is that there is no secret answer to making things secure, we just have smart and creative people bash their heads against a crypto protocol/implement for a long time and hope we found all the problems. So unless you have a good reason to do something else, and the budget to pay experienced people to bash their heads against it, you should stick to an implementation that has had this effort expended on it. If you want an intro about common problems in custom cryptosystems, go look at cryptopals or something, but don't get too cocky that you know everything.
- regularfry 6y agoIt's also easy to dramatically underestimate the order of magnitude of effort involved in "the budget to pay experienced people to bash their heads against it".
- kccqzy 6y agoI recommend Serious Cryptography by Jean-Philippe Aumasson. After reading it, you will gain enough understanding to compose cryptographic primitives and build your own secure system based on well-known best practices, as long as you don't deviate too much from the golden paths. Although with that, you still won't know how to design or implement these primitives yourself. It's like having a nice toolkit of screwdrivers, hammers, spanners etc to build your thing, but you can't build those tools themselves.
- superbaconman 6y agoI'd love to see shows like 'Nature' and 'Planet Earth' but focused exclusively on single celled organisms; I find that whole world very interesting. Journey to the Microcosmos on youtube is the closest thing I've found.
- willhoyle 6y agoCells at work! https://en.m.wikipedia.org/wiki/Cells_at_Work https://en.m.wikipedia.org/wiki/Cells_at_Work! It’s an animated series that takes place inside the human body. I’ve been meaning to watch it myself. It’s supposed to be pretty accurate.
- memset 6y agoLaw. How much of it rests on technicalities, and how much do judges care about the essence of the facts of a case? You only hear about the weird outcomes on the news. For example, I was once working with an attorney because my landlord didn’t supply heat in the apartment. I started keeping a temperature log, but how would a judge know that the log was accurate? Do I need to prove that my thermometer is accurate... etc. In practice, no, but how can I better reason about what is “likely” vs not?
- fernly 6y agoIANAL, but if your log was introduced in evidence in your suit, you would be attesting under oath it was correct to the best of your knowledge. If it later turned out you faked the data, that would be perjury. So it's the threat of punishment for perjury that motivates honesty in court.
- philipkiely 6y agoYou may be interested in the idea of legal realism [0] [0] https://en.wikipedia.org/wiki/Legal_realism https://en.wikipedia.org/wiki/Legal_realism
- Waterluvian 6y agoI've begun to grok so much about how law _practically_ works by listening to Popehat's All the President's Lawyers podcast. Basically you get an educated layperson asking a veteran criminal lawyer questions, usually around the First Amendment, always related to current events. The lawyer (Ken White) explains in practical terms what is likely to happen and why.
- pwned1 6y agoI am a lawyer. In litigation there are two considerations: law and facts. The judge determines questions of law, the jury (or judge in bench trials) determines the facts (often termed the "trier of fact"). How does that work practically? A very simple example: Let's say someone is sued for the tort of battery. It's the first tort you learn in law school. The mnemonic we learn is "IHOC" which stands for intentional harmful or offensive contact. So we now have what we call the "elements" of the tort, which must be proven individually. 1. Intentional 2. Harmful or offensive 3. Contact This is where the question of law comes in. What does "intentional" mean? Did I intend to take the action, or did I intend to cause harm? That's a question of law that courts have determined to be that you intended harm. Did I actually have to know the harm that would result? Or maybe I just need to know that it's likely to cause some harm. Questions of law. Another potential question of law -- what does "harm" or "offensive" mean? What if the contact is with my backpack and not my body? These are issues that judges decide as questions of law. Once we decide on the correct legal standards to apply (which is where lawyers do their most arguing and briefing), then the "trier of fact," generally a jury in the US, determines how the facts of the case fit the law. Did the defendant "intend" the harm? Did the defendant engage in "harmful or offensive" contact? Did the defendant "contact" the plaintiff? The jury decides those things based on a preponderance of the evidence in a civil case, meaning more likely than not. In your case, you have an issue of evidence. When someone testifies, both sides get to question the witness so that the trier of fact can judge the issue and make determinations of credibility. You can testify that you kept a lot of temperatures, and you'll explain how you did it and that you have personal knowledge of what you wrote. Then the other side gets to try and poke holes in your log. Did you actually do it contemporaneous to your observations? Did you write it down the next day? How do you know your thermometer is accurate? Did you open the window just before taking the temperature? Maybe you have a history of lying to people. Then the jury gets to take your testimony in consideration. They can do with it what they like. I hope that helps!
- rockmeamedee 6y agoKalman filters. The explanations always start easy and then get too confusing.
- AmericanOP 6y agoWhy is there no evidence of black matter annihilation in astronomy?
- knzhou 6y agoYou mean dark matter annihilation? There are some hints: an excess of gamma rays detected by Fermi-LAT, an excess of X-rays detected by Chandra, excess ionization detected by EDGES, and excess positrons detected by AMS-02. However, all of these anomalies are contentious and could well be explained by systematic effects or unknown astrophysical sources. If none of these hold up, it could be that dark matter just doesn't annihilate. That's not weird or anything -- I mean, your room is full of matter right now, and it's not annihilating itself either.
- achenatx 6y agoI consider that there are 4 levels of scientific writing. 1) news articles/lay press - basically terrible and typically get things wrong 2) scientific lay press (scientific american, discover, science news) - get things right, but generally no data/citations or nuance 3) journal summaries - get things right, citations and data for everything. Good summary of the latest scientific thought on a topic. Tend to push a point of view, which generally will be right, but that educated people can debate. Dont always show the data, but at least refer to it. These help you to get up to speed with the primary experiments that were used to establish current thinking. 4) first source articles - typically make claims too broad for the actual results. But has all data. Often times the claims don't follow from the data at all. Generally have to work in the field to understand strengths and weaknesses of methods and you cant just take the conclusions at face value. As a PhD student, I used #3 a lot to get centered on a space. To understand 4, I typically had to learn directly from my research advisor or other grad students that specialized in an area. My point here is that you can find these summary articles in journals (microbiology, immunology,virology etc). They are published infrequently so can be hard to find, but they exist and you should look for them.
- Denzel 6y agoI was shoulder deep in primary source material (#4) on real-time fuzzy search and wasn’t able to make heads or tails of where everything fit. Until I found a #3 overview that referenced most of the primary sources and put them in context. That paper was worth its weight in gold! Do you have any tips for how to quickly find these #3 materials in other spaces?
- Thriptic 6y agoThe name for what you and the parent described is a review paper. I can't speak for Math or CS, but in life sciences the following techniques would work: 1. Search X and sort by citation count. High quality review papers get cited A LOT, typically in introduction sections of primary research papers. Alternatively, google "[X] review" or "best review paper on X". 2. Look for review journals. Many fields will have journals who only publish reviews. Nature has several such publications for example. 3. Look for the top journals in the space (start by sorting by impact factor) and see if they have review sections. If they do, try to search those sections. Most journals will reach out to top labs in a space and request that they write a review on a subject if the journal editors feel one is needed. 4. Ask someone in the field. Any researcher should be able to immediately point you to canonical reviews in their space.
- nightfly 6y agoEddy currents. Why does a magnet move slowly down a copper tube?
- OkayPhysicist 6y agoA changing magnetic field induces a current in a material, which creates an opposing magnetic field which slows the moving magnet.
- carapace 6y agoThe kinetic energy turns into heat due to resistance to the induced current?
- myth2018 6y agoHaskell's Monads
- gjvnq 6y agoHow depression, autism and gender dysphoria work on a molecular level.
- smnthermes 6y agoAs for autism, it's related with a kind of general reduction of gene expression: https://www.researchgate.net/publication/328734204_A_theory_of_evolution-biased_reduction_of_gene-expression_manifesting_as_autism_the_antiinnatia_theory_Improvedupdated_presentation_of_A_theory_of_general_impairment_of_gene-expression_manifesting_as_ https://www.researchgate.net/publication/328734204_A_theory_...
- karatestomp 6y agoThe Oberth Effect. I’ve seen a bunch of awful attempts at the “why” and some “well it’s just in the math, so there” but nothing satisfying.
- knzhou 6y agoI answered this here: https://physics.stackexchange.com/questions/287101/where-does-the-extra-kinetic-energy-of-the-rocket-come-from/287102#287102 https://physics.stackexchange.com/questions/287101/where-doe... The basic answer is that the extra energy that goes to the rocket comes from harvesting the kinetic energy that the fuel itself had by virtue of being in the moving rocket.
- est 6y agoHow to design compact antenna without those math voodoo.
- robertakarobin 6y agoIf I buy a stock, does the price at which I agreed to buy it become the new share price on the stock exchange? Every article on "Where do stock prices come from?" seems to just talk at a high level about supply and demand. But where does the price come from at a nitty-gritty level? Is it an average of all existing offers or something? Do different exchanges and stock-ticker websites have different formula for calculating share price? If a very low-volume stock is listed at $4, and then I offer to buy a share for $100, does the NYSE suddenly start listing its price at $100?
- jyap 6y agoIt’s not dissimilar to a commodity like Gold with some extra parameters like number of stocks. Comes down to an open market and buy/sell demand. There are other parts to your example such as depth of order book (orders on the buy sell side) which provide liquidity. So if a low volume stock is at $4 and you offer to buy at $100 you would probably end up with 100/4=25 shares. If was more illiquid you could possibly move the price up depending on order quantity.
- timerol 6y agoThe price is generally the last price at which a transaction happened, modulo people intending to cheat the system. There are people who put limit orders on the exchanges. Say that the price of TSLA is $500. I think it's overpriced, and its likely to go down, but then grow in the future. I can say, "I'm willing to buy 100 shares of TSLA at $420." Someone else holds TSLA and thinks its likely to go up, but not hold it's value, so they say, "I'm willing to sell 100 shares of TSLA at $690." The sum of all of these limit orders forms the market depth chart. The more common way to interact with the market is to say, "I want to buy a share of TSLA at the current market price." In the above example, the only option is to buy TSLA for $690, even though the last transaction was $500! This is a example with very little market depth. In the normal case, you'd buy your share for $500.02 or something like that. (Same, but reversed, for selling at market price.) For more information, but with a crypto focus, see https://hackernoon.com/depth-chart-and-its-significance-in-trading-bdbfbbd23d33 https://hackernoon.com/depth-chart-and-its-significance-in-t... For your example, you would put in a market order, and buy the stock at the lowest price that someone was willing to sell it at. If the last price was $4, but the lowest limit order that currently existed was for $100, and you bought it for $100, then yes, the price would go up to $100. (In real life, those sharp upticks don't happen much. It's more likely that a sharp downtick happens, where suddenly everyone wants to sell oil futures at the same time, but almost no one is willing to buy them, so the price ends up negative.) Note that whenever people defend high-frequency trading for "providing liquidity to the market," this action of setting buy and sell limit orders that are close to each other is what they are talking about. There are algorithms that will see TSLA at $500, and offer to sell TSLA at $500.02 and buy TSLA at 499.98. If both orders go through, they make $0.04. If you operate fast enough to get out ahead of any big market moves, you can make a lot of money. But if you ever accidentally buy a bunch of TSLA for $499.98 right before the price plummets to $420, then you just lost a lot of money. This is why HFT and other trades with similar risk profiles are sometimes referred to as "picking up nickels in front of a steamroller."
- jppope 6y agoWhy is it that the smartest people in various fields constantly need reference materials, but medical doctors never look anything up?
- abecedarius 6y agoI've read that med school is really heavy on memorization. Cynical answer: the culture of medicine was set during a 2000+-year period when physicians harmed more than helped; the way they made a living was by appearing authoritative.
- Balgair 6y agoThey look things up constantly, you just don't see it because it's typically very boring and not a good use of time for you or the MD. They get symptoms from you and then will go off to research. Also, a broken arm or strep isn't very difficult to figure out. ~80% of visitations are fairly simple. The site most MDs use is here: https://www.uptodate.com/home https://www.uptodate.com/home
- jppope 6y agothanks for the reference. I'll taking a peek.
- Thriptic 6y agoThey look stuff up all the time. Most physicians try to stay on top of developments in their field by reading scientific journals. Most also have access to subscription services which will provide background and lay out current best practice guidelines / treatment recommendations for almost every disease. Another poster mentioned uptodate which is one of the most popular examples. It's not uncommon for a physician to reference this with the patient still in the room.
- Waterluvian 6y agoWhy does my bread last for a while and then go bad? Or is it constantly going bad and I just haven't hit critical mass of mold for me to notice?
- Balgair 6y agoCorrect. Mold will grow where there is food for it to grow, like bread. It will divide and grow exponentially, up to a point. That fact of exponents where they seem slow at first and then really rocket away? That's why mold seems to just appear overnight.
- Waterluvian 6y agoSo it might be a fair statement to say that when I eat fresh bread there's already a molecular amount of mold already on it?
- carapace 6y agoThere's a molecular amount of mold already on everything. Mold spores are ubiquitous.
- jryb 6y agoAlthough your question was about mold, you may be thrilled to learn that the process of bread going stale is not fully understood. I found this to be a somewhat jarring claim in a review I read recently: https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1541-4337.2003.tb00011.x https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1541-4337...
- tjpnz 6y agoDouble slit experiment. There's a lot out there detailing what happens but relatively few explanations go into why observing the state of a photon results in it "choosing" one slit over the other. I suspect many come away with the impression that it's magic despite there being a reasonably simple explanation for what's going on.
- Balgair 6y agoThis may help: https://www.youtube.com/watch?v=kTXTPe3wahc https://www.youtube.com/watch?v=kTXTPe3wahc
- aryc19 6y agoThis video explains the double slit experiment using probabilities and it's the only one that made sense to me without bringing in quantum mystery: https://www.youtube.com/watch?v=iyN27R7UDnI&feature=share https://www.youtube.com/watch?v=iyN27R7UDnI&feature=share It also goes on to explain the delayed choice quantum eraser experiment but I don't think that's quite convincing.
- somezero 6y agoWhat's the point of defining summation methods for divergent series? Do they have any connections to any other area of mathematics or physics? Is analytic continuation of complex analysis relevant to these things? How about p-adics?
- abetusk 6y agoThis is not an answer as this is something I've only just started wondering myself, but, if I understand correctly, perturbation theory [1] uses divergent series (and wants divergent series in certain conditions over convergent series(?)) in it's methodology. I've started but haven't finished the physics lectures by Card Bender on mathematical physics, where he features perturbation theory prominently [2]. If someone could chime in on this, I would also be appreciative. Also if someone has better resources to learn about perturbation theory, I would also be appreciative. [1] https://en.wikipedia.org/wiki/Perturbation_theory https://en.wikipedia.org/wiki/Perturbation_theory [2] https://youtu.be/LYNOGk3ZjFM https://youtu.be/LYNOGk3ZjFM
- dreamcompiler 6y agoFrame dragging. How does space know a body is rotating if the body is "smooth"?
- ffhhj 6y ago"Know" in which way, what kind of interaction? What about magnetic field due to poles rotation?
- yamrzou 6y agoEvolution. How does large scale randomness result in such complicated and intelligent systems, while after decades of research and all the computing power we have today, we still struggle to model and reproduce the intelligence of an insect.
- dreamcompiler 6y ago"The Selfish Gene" was the book that finally made this click for me.
- yamrzou 6y agoUnfortunately, The Selfish Gene leaves it an open question of how the brain developed its capacity to represent and model the world, which is the very notion of intelligence. From the book: > The evolution of the capacity to simulate seems to have culminated in subjective consciousness. Why this should have happened is, to me, the most profound mystery facing modern biology. There is no reason to suppose that electronic computers are conscious when they simulate, although we have to admit that in the future they may become so. Perhaps consciousness arises when the brain's simulation of the world becomes so complete that it must include a model of itself.
- pajaroide 6y agoFour billion years is a lot of time for that randomness (via selective pressure of the environment or of life itself) to produce tons of crazy results.
- xabotage 6y agoAgreed. I find it difficult to even ask for a deeper understanding of evolution without first explaining yes, I know basically how DNA works; yes, I understand natural selection; no, I'm not a young-earth creationist. I have a solid grasp of basic biology and genetics and I'd just like to comprehend how randomness can lead to such incredible complexity of organisms within billion-year timescales (and not, say, million-trillion-trillion year timescales). I feel as though it's simply Occam's Razor to assume that evolutionary complexity is the result of randomness because I know of no better explanation. Is there a self-reinforcing process at play? (Natural selection partially counts as reinforcing, I just feel like randomness is still the engine that powers it).
- machawinka 6y ago- Hegel. I haven't found any resource that can explain it clearly.
- efficax 6y agoI’d suggest Kojeve’s Introduction to the Reading of Hegel (a classic expository work on the Phenomenology of Spirit)
- rsync 6y agoWater pressure (like PSI) and how it relates to water flow (like GPM). It's difficult to relate the two together and even after hearing every heuristic and every cutesy analogy, I still can't quite wrap my head around what happens to one when I increase the other (and so on).
- jbanfill 6y agoI have wondered if heat and photons impart a partial charge on atoms and molecules which causes several phenomena. Faster Brownian motion due to the increased repelling action of stronger charges which cause pressure/volume changes in gasses. Also are these charges responsible for some weather effects such as the jet stream. In a tornado is the negative charges on the dry side of the dry line interacting with the moist air on the wet side really just a local intense acceleration of the dry air trying to "get to" the oppositely charged moist air? Are the rotation of low and high pressure systems basically due to the same condition? Is lightning also just basically a flood situation of the charges?
- OkayPhysicist 6y agoCharge can be pretty easily shown to be a conserved, quantized value. So chargeless particals imaprting a partial charge would be a big no-no.
- jbanfill 6y agoI didn't mean it was equivalent to the charge of an electron. What I was wondering is how do heat and light impart more kinetic energy to physical objects? Light may be the quantity of a photon but heat seems to be more of an analog property. And what are some ramifications of these energized particles in small spaces like enclosed gasses or large unenclosed spaces with effects like the weather.
- nojvek 6y agoQuantum computers. I still don’t get it. What are they useful for, what are it’s computing bounds and why it’s a big deal. When will the future actually come ?
- kf6nux 6y agoSuperconductive flux pinning. Specifically, I'm curious what behavior it'd exhibit if used as a core for superconducting inductors.
- chrisMyzel 6y agoMath!
- henearkr 6y ago[Sorry I have misinterpreted the subject, I thought it could be some phenomenon still not well understood by science.] Placebo. There are biological bases of it (I don't believe in soul). Find these bases, study them, make a model of them. Then use proxy variables to measure it instead of trying to eliminate it statistically. Predict it in studies to avoid the need of placebo groups (and possibly of double blind methodology). Also, after it is completely measurable and its mechanisms are understood, if (very hypothetical) it has a really substantial effect, just use it to help treat patients.
- diehunde 6y ago- How computers translate bits into electrical signals - What does it mean the universe is expanding - Bayesian statistics - How information is stored in magnetic tapes
- OkayPhysicist 6y agoFirst one's easy. You've got a bunch of electronically controlled switches (transistors), and you encode a higher voltage as 1, and something near ground as 0. You can build logic gates out of 2 or 3 transistors, and combine those logic gates into more complex gates until you've got a computer. But how does a transistor work? Basically, you've got semiconducting materials of two types (Phosphorus or Boron doped silicon), one of which wants a few more electrons to be a conductor, and the other wants a few less electrons to be a conductor. If you stick the two types next to each other, the electron-wanting (N) one snatches up the electrons from the electron-offering (P) one, and you get an Electric field going from the P to N. Now, that alone makes a diode. Already cool, nonlinear electronics. But what if we go N-P-N? Now we've got two electric fields, going opposite directions. With three leads, you can adjust the strength of those electric fields with one, creating a variable resistor, a transistor.
- abiogenesis_123 6y agoAbiogenesis. I understand that this is not considered understood but I'd be interested in hearing a qualified scientist talk about how we get from "dumb" matter to self-replicating, goal-driven matter. The closest I've ever heard anyone get (in a personal conversation) is that chemistry is about transformation, so "dumb" matter isn't really dumb in the sense that it's static. Still lots of hand waving to get from baking soda and vinegar volcanoes to me typing this question, however. What's the playing field look like for proto-life? How "smart" are the simplest molecular interactions? What does almost-replication look like? Could we use a computational model for this? Not sure how much of this is known, but I'd love to hear an expert paint a picture of their mental model of the subject.
- outlace 6y agoDid you read Nick Lane's "The Vital Question: Why Is Life The Way It Is?" It's all about this and it's great. Also take a look at https://www.quantamagazine.org/first-support-for-a-physics-theory-of-life-20170726/ https://www.quantamagazine.org/first-support-for-a-physics-t...
- kashyapc 6y agoOn Nick Lane's "The Vital Question": I gifted myself The Vital Question in 2015 December. While Lane writes effectively without any mind-numbing jargon, the book still has quite a bit of technical chemistry (understandably). After the excellent first 80 pages, it took me a lot more will power to plough through. (I paused at page 112 to get back later.) Once when I was reading the book on a plane, a seasoned biologist happened to be sitting next to me. When I told that it's the first book of Nick Lane that I picked up, he said: "I'd rather suggest you to pick up Laine's other book, Life Ascending, and only then get back to The Vital Question." PS: FWIW, I've previously mentioned the above in an older thread, where an ex-biochemist chimed in to confirm the above advice: https://news.ycombinator.com/item?id=18714115 https://news.ycombinator.com/item?id=18714115
- javajosh 6y agoThis question interests me as well, and I've done some thinking about it over the years. In particular, what I'm interested in is a hypothetically simplest object that can reproduce in a solution of simpler components, along with some differentiating characteristic. For example, maybe you have toroids that pick up particules, grow the torus until its too big, and then splits - and the ends of both halves click together, forming a total of two toroids. Another characteristic that very simple life must have (I believe) is some level of circularity in the sense that the element is an "accumulation of experience" - we might say a reduction of its environment. In the same way Schordinger was interested in life thermodynamically[1] I am interested in speculating about the simplest possible mechanisms in the beginning. (NB I'd expect none of these very simple machines to survive to present day - in fact, I'd imagine there to be several generations of early life, each obliterating/consuming/sublimating the ones before.) 1 - https://en.wikipedia.org/wiki/What_Is_Life%3F https://en.wikipedia.org/wiki/What_Is_Life%3F
- deleted 6y ago[deleted]
- vijay_nair 6y ago• Magnetism. There are plenty of videos out there calling it the result of a relativistic charge imbalance. But I've never been able to use this point-of-view to practical use cases like understanding how permanent magnets work or how increasing the number of windings in inductors boosts the magnetic field strength. There were more situations I tried to put this POV into use but I can't remember them off the top of my head. • Qualia. What is this subjective experience that I know as consciousness? I've gone through Wiki, SEP and a fair number of books on philosophy and a few on neuroscience but I still don't understand what it is that I experience as the color "red" when in reality it's just a bunch of electric fields (photons). Why can't I get the same experience — i.e., color — when I look at UV or IR photons? These too are the very same electric fields as the red, blue, green I see all the time. • Photographic composition. I'm a designer. I know them. I use them. But only empirically. I just do not understand them at a neuroscientific level. Why does rule-of-thirds feel pleasing? Is the golden ration bullshit? My gut says yes but I'm unable to come up with a watertight rebuttal. Why do anamorphic ultra-widescreen shoots feel so dramatic/cinematic? Yet to see an online exposition on the fundamental reasons underlying the experience. Any questions to artists are deflected with the standard "It's art, not science" reply. • Wave-Particle duality. "It's a probability wave that determines when a particle will pop into existence out of nothingness." okay, where exactly does this particle come from? If enough energy accumulates in a region of empty space, a particle pops into existence? What is this "energy"? What is it made of? What even is an electron, really? I've followed quite a few rabbit holes and come out none the wiser for it. • Convolution. It's disappointing how little I understand it given how wide its applications are. Convolution of two gaussians is a gaussian? Convolution in time domain is multiplication in frequency domain and vice-versa? How do these come out of the definition which is "convolution is sliding a flipped kernel over a signal"?
- TrackerFF 6y agoRE: Convolution I think this was a pretty neat explanation: https://sites.google.com/site/butwhymath/m/convolution https://sites.google.com/site/butwhymath/m/convolution The problem with convolutions, like many things in science, is that how you learn it, depends on what you're studying. Same theory, but with N different explanations, which can cause confusion if some of them are very different and tough to connect (i.e learning convolutions in a physics class vs leaning one in a statistics class)
- unixhero 6y agoHuman counsciousness.
- zander312 6y agoWhy the earth is a sphere.
- OkayPhysicist 6y agoSystems tend towards low-energy states (because small perturbations keep things from getting stuck at local minima or saddle points. The lowest potential energy configuration for a bunch of particles pulled together by gravity is a sphere, because every point is as close to to the middle as possible.
- Caylio 6y agoThat decreases potential energy, but doesn't all that extreme pressure heavily increase the thermal or kinetic energy towards the center? So while this configuration decreases the potential energy, doesn't it also increase the thermal/kinetic energy? Is it obvious that the decrease of potential energy "wins" over the increase in thermal/kinetic?
- mech422 6y agoBlackHoles - where does the 'extra' mass/gravity come from? I've been watching a lot of documentaries lately, and I can't figure out how a star that _radiates_ light, collapses and suddenly light can't escape? Doesn't that mean the blackhole has more mass/gravity then the star that created it?
- filmor 6y agoIt doesn't, it's just a lot denser. A star has a lot of inner pressure from the ongoing fusions inside of it. When these stop or reduce, the star collapses from its own gravity, as soon as its radius is smaller than the event horizon (i.e. the radius at which the escape velocity exceeds the speed of light) you have a black hole.
- mech422 6y agoThat's basically what I keep seeing in the documentaries, but it's still not sinking in. For instance, being denser I could see it making a 'deeper' 'dimple' in the rubber membrane demos they use...but it would also be a much smaller diameter dimple, so only things really close and really small could 'fall' into it ?
- emblaegh 6y agoWhat teachers tell you in school is that as you get close to a uniform sphere the gravitational force increases with the square of the distance to the center of such sphere. What they don't tell is that once inside the sphere the force decreases linearly, because the planetary mass ahead of you is partially balanced by the mass behind you. With this you can see that the gravitational pull of a planet/start/spheroid is largest at its surface. So, if something happens to make a start shrink by some factor, the gravity pull at its surface is increased by the square of this factor, even if the mass of the star remains the same. Actually I believe starts eject a lot of mass when they become black holes, and this is just a Newtonian argument for a phenomenon intrinsically relativistic, but I hope you get the gist of it.
- mech422 6y ago
- Buttons840 6y agoFlight. How can a plane fly when it's thrust to weight ratio is less than one? It's like, if you can produce 10 pounds of thrust, who would look at that and say "ah ha, we can use this to keep a 100 pound machine miles in the air indefinitely"? I understand flight from a mathematical point of view. I've actually read a few books on the subject, and I could explain how flight works to someone. However, I'm still fishing for an explanation that "feels" more satisfying though. Per the question, I still want it explained better. EDIT: There's already a thread about flight. I asked the same question there, but phrased a bit differently: https://news.ycombinator.com/item?id=22993460 https://news.ycombinator.com/item?id=22993460
- handedness 6y agoConsider the vectors of the four forces (thrust, drag, lift, weight). The thrust only needs to equal the drag at a given airspeed, not the weight of the vehicle. (Unless it's climbing vertically.) The weight is countered by the lift generated by the wings, which, given the efficiency of airfoils, is very more or less why it all works: they produce a positive lift-to-drag ratio. Put another way, weight pulls it down, thrust moves it forward, the resultant lift keeps it up, and drag limits its speed. Only rocketeers and fighter/aerobatic pilots need to really worry about the thrust to weight ratio as a constraining factor, because the vertical flight regime matters to them. From your average bugsmasher to your commercial airliner, it's not a factor (to the disappointment of pilots everywhere). Consider that a Cessna 172 has a glide ratio of about 9:1, so it can go 9 units forward for every 9 of altitude it gives up. If that's hard to intuitively grasp, consider that it's traveling through a fluid. Surfing, even. The interaction with that fluid is why it works. That any more satisfying?
- rocqua 6y agoSo the 'glide ratio' is essentially a multiplier from 'thrust' to 'lift'?
- handedness 6y agoSorry, I was unclear. That glide ratio is unpowered (and approximate). Any thrust will only increase that ratio.
- unexaminedlife 6y agoSort of trying to piggy-back on this thread in hopes someone with knowledge will be able to share it. I've been having conversations about viruses recently and in those conversations / thought experiments I keep coming back to a point someone made to me. Someone this person knows, with extensive medical expertise, explained that the "membrane" of the cell contains a ridiculously large number of unique types of proteins. Understanding, in vague terms, how viruses penetrate cells the question I pose is "is this true because each of those proteins has a unique and distinct function in the cell membrane? Or is it more a matter of scale and utility?" In other words, does the observation simply indicate that our bodies are not as perfect as we'd like to think they are and the body's process for creating / repairing cells is more a utilitarian function where the "rules" of cell construction are extremely flexible such that these molecules are constructed in various ways where our cells are using materials available to them at the time? If this is the case it starts to make a lot of sense to me at a molecular level why certain people tend to be more susceptible to contracting certain diseases. Could a lot of it really just come down to diet, along with probably a hint (or more) of DNA's interaction with those proteins we're providing to our bodies? And to what extent does each of those play a role? DNA and the proteins.
- rambojazz 6y agoHow EM radiations propagate through the air and what is an EM field.
- 0xfaded 6y agoThe unification of electo-magnetism. I know the classical equation well, but never formally studied physics and have never been able to get my head around Maxwell's equations.
- adipasquale 6y agoThe Coandă effect [1] and how it applies to plane wings and sailboats. [1] https://en.wikipedia.org/wiki/Coand%C4%83_effect https://en.wikipedia.org/wiki/Coand%C4%83_effect
- billfruit 6y agoLagrangian and Hamiltonian mechanics. That is usually presented in a very mysterious manner. Also I think most engineers do not get taught these.
- billfruit 6y agoWavelets: Usually given explanations are hard to understand.
- partyboat1586 6y agoBasic probability. How come when I flip a coin it is just as likely to come up heads or tails even if it has just been heads 5 times in a row? I understand the physical properties of the coin make it so it is an independent event but if I were to run the experiment multiple times the number of times it would be heads after 5 heads would not be an even probability, it would be unlikely since 6 heads in a row is a rare event.
- Rerarom 6y agoYeah, but 5 heads followed by 1 tail is also a rare event. Basically, after drawing 5 heads you have a choice between two rare events which are equally probable. The same after drawing anything else, not just 5 heads.
- ausbah 6y agothis 3blue1brown video might answer your questions https://m.youtube.com/watch?v=8idr1WZ1A7Q https://m.youtube.com/watch?v=8idr1WZ1A7Q
- shadowprofile77 6y agothe fundamentals of computer science. Unlike many who comment here regularly, I am not a programmer or developer and though it might seem silly, the way in which a bunch of code written in a programming language of any sort translates to a computer physically or electrically causing things to "happen" because of it has always been a bit fuzzy. I know that electronic systems and machines translate all instructions to binary code but still, from there how does the rest happen? such as an OS on my laptops working the way it does, or more specialized: an unmanned spacecraft autonomously navigating its way through the solar system and doing complex physical tasks. Anyone have a suggestion for a good starting point on learning through these fundamentals and on upwards?
- ur-whale 6y agoIt's a long road to the top (i.e. something like a python program), but you can start at the bottom and try to understand how basic hardware gates (and/or/xor/register) work and can perform extremely elementary yet useful tasks. Then it's just a "simple" matter of stacking up a billion of the things to get them to do complex programs.
- omnibrain 6y agoRead "Charles Petzold - Code: The Hidden Language of Computer Hardware and Software", it is a good starter. https://www.nand2tetris.org/ https://www.nand2tetris.org/ may also be insightful, but I did not look further into to.
- grisha 6y ago“Code: The Hidden Language of Computer Hardware and Software” by Charles Petzold should be what you are looking for. Author describes computing from the very ground up, and in clean, approachable manner. https://en.wikipedia.org/wiki/Code:_The_Hidden_Language_of_Computer_Hardware_and_Software https://en.wikipedia.org/wiki/Code:_The_Hidden_Language_of_C...
- air7 6y agoThere's a great course called NAND to Tetris, which teaches, by example, at increasing levels of complexity starting from simple logic gates up to an actual program. It's very good. https://www.nand2tetris.org/ https://www.nand2tetris.org/
- ur-whale 6y agoWhy does quantum mechanics need complex numbers ? Every time I read an introductory QM book / article, the complex numbers just come out of nowhere and no one bothers to explain how that makes any kind of physical sense.
- OkayPhysicist 6y agoImaginary numbers, pretty much everywhere in physics, represent an extra dimension. e^ix results in a unit vector spinning around.
- ur-whale 6y agoThanks for your answer, but that doesn't really cut it for me. For example, if complex numbers are so "natural" at modeling rotations, why don't they show up e.g. in Maxwell's equations ?
- sarthakjshetty 6y agoWhat actually happens to the money that gets put into the stock market? I mean, I understand that there are people holding stocks and then people offer to buy/sell them at a given price; but, how does the company plan to benefit if the stock just keeps moving back and forth multiple third parties? Doesn't a company go public to raise money? If so, how does the company benefit from the daily changes in the stock and when someone buys stock in the company? Does the company have a fixed number of stock that they trade as well on the market?
- drran 6y agoCompany raises money from market by giving away part of the company. If company shares are cheaper than other source of money, then company can buy back them, or other potential owner can buy them. It also sends strong signal to owners and investors about future of the company. If company shares are costing more than other sources of money, then company can sell more of them to return costly money back or to expand business. For example, bank rent rate is 10% per year. Company worth is $1 million. Company created 1 million of shares. 1 share is $1 of company worth. Company pure profit is 10% per year. Company needs $1 million in circulation to operate. If company will rent money from bank, it will have 10%-10%=0% of profit. If company will sell 100% of shares, it will have 10%-0%=10% of profit, but this profit will go to someone else. If company will sell 50% of shares and will rent $0.5 million from bank, it will have 10%-5%=5% of profit, 2.5% will go to company, 2.5% will go to other owners. By reinvesting this profit, company can reduce bank money or expand company to worth more, increasing company profit share.
- dandare 6y agoHow did Mendeleev and his peers knew something was the final indivisible element and not just another molecule?
- OkayPhysicist 6y agoIt was pretty wild wild west, really. The idea that some some compounds didn't just contain some fire was still common when the first list of elements was put together. A big leap forward was realizing air had two principle components, burn-y air and not burn-y air. They figured out water wasn't an element when the burn-y air and some mystery gas burned to make it. Basically, everything was maybe an element until they either broke it into pieces, or made it out of other stuff.
- wsieroci 6y agoI didn't see good explanation of Einstein's twin paradox. Have you seen one? I have seen only clear explanation based on fact that observer will feel acceleration when he will come back and this is what breaks symmetry, but it appears that explanation based on acceleration are wrong, because it seems that acceleration is not necessary to explain this paradox.
- CJefferson 6y agoHuman Biology, particularly the "bigger scale" things like joints & ligaments. I have arthrtic knees, and I'd like a better understanding of how joints work, and where the various clicks, pops and swellings come from. It's easy to find really simple things, but harder to understand "how things go wrong".
- petargyurov 6y agoThis won't help with everything you asked for but there is neat piece of software called Complete Anatomy that helped me with this sort of thing. It renders an interactive model of the human body and you can toggle different layers, from bones to nerves, to various layers of muscles and ligaments. It also contains animations of treatment exercises/stretches, surgeries and highly detailed models of various biological components. It helped me understand my injury and why certain exercises help. It's paid software but it comes with a free trial.
- dandare 6y agoThe cosmic inflation. Yes, the universe is smooth but how is that enough to justify this unimaginable expansion? There seem to be no proof or possible experiment yet the scientific community apparently accepted this theory with its abrupt start and abrupt end, only secondary to the big bang itself in its scale and energy.
- deleted 6y ago[deleted]
- Madblenderuser 6y agowhy does blender become slower at times for no apparent reason
- Madblenderuser 6y agowhy does blender at times become slower for no fucking apparent reason
- dandare 6y agoHow a single hormone can regulate so many different things. Looking at Wikipedia, a single hormone like progesterone which is apparently one molecule regulates a plethora of effects. How is this implemented? As a coder, if I had to use one variable to simultaneously regulate 10 different things I would go crazy.
- mihaaly 6y agoThe rising cost and decreasing productivity of scientific research.
- pvaldes 6y agoWe aren't short of interesting problems. To pick a couple fo them randomly, understanding amyotrophic lateral sclerosis or alzheimer would be terrific starts.
- ben_w 6y agoWormhole time machines. The idea is, you can transform a normal [0] wormhole that isn’t a time machine into one which is by: 1) accelerating one end to high speed relative to the other 2) keeping on end in a lower gravitational potential than the other Why are either of these considered meaningful statements, never mind correct? In the case of 2 in particular, isn’t GR supposed to require smooth values? So any time dilation effect would be almost identical on a pair of points +δ and -δ from the throat? Making it similar to the case of a gravitational potential without a wormhole? And in the case of 1, the more I think about it, the less I understand the concept. What is being moved? An imaginary clock that would’ve been in the part of the wormhole at the far end? The apparent speed as measured going through the throat will be zero regardless of the apparent speed of the same as measured when going the long way around. [0] yes, I know
- c1ccccc1 6y agoI'll answer 2. You are correct that clocks just to either side of the throat will stay synchronized. But it's important to remember that time dilation isn't some kind of absolute effect. The easiest way to talk about it is in terms of clocks following paths. Imagine you have two spaceships in London. At the fire of a starting gun, they both take off, and fly around, each one taking a different path. Each ship has a clock that records the time elapsed since it took off. Eventually, they both return to the same landing pad in New York, landing at the same time. Thanks to time dilation, the readings on the clocks of the two ships might be different, even though they started and landed at the same places and times. Imagine we start both ships from a space station in deep space. One ship doesn't leave the station at all, it just stays in its docking bay, with its clock ticking along. The other flies down to the surface of Earth, sits there for a few years, and then flies back up to the space station. Thanks to the gravitational field of Earth, the ship that stayed home has more time elapsed on its clock than the ship that went to Earth. Now suppose that each ship is carrying one end of a wormhole. The clocks on either end of the wormhole must stay synchronized. Someone sitting in the middle of the wormhole would be able to see the inside of one ship by looking to their left, and the inside of other ship by looking to their right. The clocks start out synchronized. As you point out, no matter how the ships move about, this does not change as the ships fly around. Anyone standing in the middle of the wormhole always sees that the clocks on the wall of each ship are in sync. So: Entering the wormhole from ship A when ship A's clock reads X means you exit at ship B, at the time when ship B's clock reads X. And vice-versa for going from B to A. Now thanks to time dilation, ship B might arrive back at the station when its clock reads X, while ship A, which stayed behind, has a clock reading of (X + 20 minutes). If you are the station master, you can go into each ship to look at the clocks, and you will find that ship A's clock is always 20 minutes ahead of ship B's. But suppose that instead of walking between the ships through the station, you use the handy wormhole that connects them directly. Suppose you enter ship B when its clock reads Y, and walk through the wormhole. You exit at ship A when its clock also reads Y. Then you step out of ship A, and walk through the station to ship B. It's clock reads (Y - 20 minutes), since according to people on the station, ship A's clock is still 20 minutes ahead of ship B's. When you originally entered ship B, its clock read Y. It now reads (Y - 20 minutes). Time travel. By retracing your path in the opposite direction, you can also travel 20 minutes into the future.
- rsp1984 6y agoMacroeconomics. Central banks are "creating" a trillion here, a trillion there, like nobody's business. But what are the consequences? What is the thought process that central bankers have gone through to make these decisions? Also why, exactly, are they buying the exact assets that they are buying (govt. debt, high-yield bonds, etc..) and why not others (e.g. stocks or put money into startups)? And then, what happens if a debtor pays back its debt? Is that money consequently getting "erased" again (just like it's been created)? What happens if a debtor defaults on its debt? Does that money then just stay in the economy, impossible to drain out? What is the general expectation of the central banks? What percentage of the debt is expected to default and how much is expected to be paid back? And specifically in the case of central banks buying govt. debt: Are central banks considered "easier" creditors than the public? What would happen if a country defaults on a loan given by a central bank? Would the central bank then go ahead and seize and liquidate assets of the country under a bankruptcy procedure to pay off the debt (like it would be standard procedure for individuals and companies)?
- sideshowb 6y agoNot sure I'd class it as a science, but here's my take... though I'm no expert and certainly agree this stuff could be better explained! > Macroeconomics. Central banks are "creating" a trillion here, a trillion there, like nobody's business. But what are the consequences? Nobody quite knows; it's still hotly contested between left wing lovers of Keynes and right wing believers in austerity. > What is the thought process that central bankers have gone through to make these decisions? Probably largely a political one. Central banks may be trying to fulfill a remit set by law (e.g. bank of england: keep inflation below x%) and are trying to deliver on that. (why? too much or too little inflation both cause problems, I guess we somehow reached consensus on a "sane" amount that keeps pace with genuine growth of wealth within the economy) > Also why, exactly, are they buying the exact assets that they are buying (govt. debt, high-yield bonds, etc..) and why not others (e.g. stocks or put money into startups)? I think this is about distributed decision making. The central bank does not have the expertise to decide which stocks or startups represent the best investments. The examples here involve lending money to government, presumably the idea being the latter is better placed to decide what to do with the money. Another example is buying assets from other banks, which are again better placed to decide which businesses/homeowners/etc represent a more sound investment as they do it on a daily basis (from a profit/loss point of view ... of course we debate whether or not that's the case on a societal level). > What would happen if a country defaults on a loan given by a central bank? Internally it would depend on laws/balance of political power within the country. Between countries, depending on the currency the country could do crazy stuff like print excessive amounts of money to repay the loan (Germany did this in the 1930s leading to hyperinflation) or they could just as you say, default. The country's credit rating would then be downgraded, making it harder for them to raise credit in future. > Would the central bank then go ahead and seize and liquidate assets of the country under a bankruptcy procedure to pay off the debt (like it would be standard procedure for individuals and companies)? Not the bank, but the country making the loan, may first negotiate some debt relief with strings attached e.g. preferential trade agreements. Beyond that, I have no idea what precedent exists.
- egberts1 6y agoPlasma ball, and how to create one. I’ve seen two of those with my own eyes: One in nature after a lightening strike to a lone maple tree in a pasture and one accidentally man-made via 50,000 VDC. And none of the literature shows how how such a plasma ball can travel in a fairly straight line at steady level from the ground.
- econcon 6y agoAxial flux motors
- barrenko 6y agoConciousness.
- wiz21c 6y agoElectricity. Electricity is always explained by its effects, but never by its actual nature. I'd like better explanation :-)
- bunya017 6y agoAsynchronous programming With the addition of async to django core, I felt its time to finally learn the concept. I first took interest in async early last year when I re-read a medium post on Japronto; an async python web framework that claims to be faster than Go and Node. Since then, I've been on the lookout for introductory posts about async but all I see is snippets from the docs with little or no modifications and a lame (or maybe I'm too dumb) attempt at explaining it. I picked up multi threaded programming few weeks ago and I understand (correct me if I'm wrong) it does have similarities with asynchronous programming, but I just don't see where async fits in the puzzle.
- pixelmonkey 6y agoThe first couple of paragraphs of the documentation for asyncore, the module in Python's standard library that implemented the machinery for async IO all the way back in 2000, has a great description of what async programming is all about. Here it is: https://python.readthedocs.io/en/latest/library/asyncore.html https://python.readthedocs.io/en/latest/library/asyncore.htm... 'There are only two ways to have a program on a single processor do “more than one thing at a time.” Multi-threaded programming is the simplest and most popular way to do it, but there is another very different technique, that lets you have nearly all the advantages of multi-threading, without actually using multiple threads. It’s really only practical if your program is largely I/O bound. If your program is processor bound, then pre-emptive scheduled threads are probably what you really need. Network servers are rarely processor bound, however.' 'If your operating system supports the select() system call in its I/O library (and nearly all do), then you can use it to juggle multiple communication channels at once; doing other work while your I/O is taking place in the “background.” ...'
- FridgeSeal 6y agoSomeone probably much better than me can correct me where I go wrong here, but I'll have a stab at this. The way I think about it, is asynchronous programming gives you the tools to write programs that don't stop doing useful work while they're waiting for something to happen. If parallelism gives you more effective use of your CPU, asynchronous programming gives you more effective use of your time. Let's presume you have a program that does some things, makes several requests to the network or requests several things from the file system, collects the results and carries on. In a synchronous program, you would make each request, wait for it to come back (the program would block at this point), then when it's complete, proceed with the next request. If each request takes ~2 seconds to complete, and you've got 15 to make, you've spent most of that 30 seconds just idling, not actually doing anything. In an asynchronous program, you could submit those requests all at once, and then process them as they came back, which means you only spend about ~2 seconds waiting before you start doing useful work processing the results. Even if your program is single threaded and you can only actually process one item at a time, you've made more efficient use of your time. Some murkiness comes in the intersection of the 2 and how it's implemented in various languages. For example, you could also dispatch each of those requests out to a thread, and if you returned all the results to the main thread before processing them you'd have the same result and near the same performance as the async example (+- thread dispatch overhead etc etc). The power and advantage comes when you can use both to their advantage: you can't necessarily dispatch threads forever, because the overhead will impact you, and you can saturate your CPU. On the flip side, making something asynchronous that actually requires CPU work won't net any benefits because the work still has to be done at some point. Asynchronous programming gives you a way to move things around to maximise your efficiency, it doesn't actually make you go faster. JS and Python are single threaded with event-loops, Rust organises chains/graphs of async code into state machines at compile time and then lets the user decide exactly how it should be run (I'm fairly this is correct, but if I'm wrong someone let me know). Dotnet to the best of my knowledge lets your write "tasks" which are usually threads behind the scenes (someone please correct me here). I don't know what Java uses, but I imagine there's a few options to choose from. Haskell performs magic as far as I can tell. I don't know how it's model works, but I did once come across a library that appeared to let you write code and it would automatically figure out when something could be async, rearrange calls to make use of batching, automatically cache and reuse similar requests and just generally perform all kinds of Haskell wizardry.
- keithyjohnson 6y agoAgile, an empirical survey of whatever the hell it actually is.
- Davidbrcz 6y agoAgile is a short-lived V cycle repeated for ever. Legacy V-cycles (needs - spec - code - test - integration - product) were such that everything was written down and planned in advance for months/years. So, if the customer had made an error, his/her needs had changed... you were basically screwed. Agile advocates for Short V cycles while getting often user feedback. But it's a V cycle. - PO speaks to the customer = get needs - PO writes tickets, UX design something = specification - And then it follows the classical cycle : develop, test, integrate, deliver. What's remain around agile (ceremonies & co) feel much more like bullshit to me, and people follow it religiously without understanding the core idea of agile as they think V cycle is an insult.
- BiteCode_dev 6y agoRelativity. I just can't grok it. I can't understand how time would flow differently depending of your speed. I don't get why C is a constant no matter the referential, for any other object the speed is relative to your referential. I just don't see how those 2 are compatible.
- danesparza 6y agoEinstein's (summarized) quote, "Time is a persistent illusion"
- jccalhoun 6y agoElectricity. What is it? Is it related to electrons? I thought it was? If the plug in my house is AC then why is one wire "hot" and one "neutral?" (on plugs in North America at least) if there is a ground wire then why are plugs polarized? Why do some things come with a three prong plug and others don't?
- enriquto 6y agoThe periodic table. Can the properties of the elements be computed from the first principles of particle physics, or do you need to observe the atoms in real life to figure them out? For example, some isotopes are stable and others have a finite half-life. Can you know beforehand or you have to observe the decay? Can you compute exactly the mass of each atom without measuring it? Can you know compute its electronegativity? Etc.
- scld 6y agoTime dilation with regards to the "no absolute rest frame" in physics. The famous twin thought experiment where one gets in a spaceship, accelerates away from the planet, turns around, and comes back. The twin that stayed on earth is old and the traveling twin is young still. On one hand, I know that time will "pass differently" for each twin....but why is it the twin in the spaceship that ages less? Why isn't it true that the entire universe accelerated away from the spaceship and then returned, leaving the entire earth young?
- strgcmc 6y agoYou're in luck my friend, the Wikipedia article does a rather nice job of describing just this "paradox": https://en.wikipedia.org/wiki/Twin_paradox https://en.wikipedia.org/wiki/Twin_paradox > In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the twin who remained on Earth has aged more. This result appears puzzling because each twin sees the other twin as moving, and so, according to an incorrect[1][2] and naive[3][4] application of time dilation and the principle of relativity, each should paradoxically find the other to have aged less. However, this scenario can be resolved within the standard framework of special relativity: the travelling twin's trajectory involves two different inertial frames, one for the outbound journey and one for the inbound journey, and so there is no symmetry between the spacetime paths of the twins. Therefore, the twin paradox is not a paradox in the sense of a logical contradiction. There's multiple explanations included to resolve the "paradox" from different lines of argument; I particularly like this one: https://en.wikipedia.org/wiki/Twin_paradox#A_non_space-time_approach https://en.wikipedia.org/wiki/Twin_paradox#A_non_space-time_...
- scld 6y agoHahah 2 separate sources each for why I'm incorrect and naive. Thanks!
- Rerarom 6y agoI think we should have this thread more often.
- dennis_jeeves 6y agoInteresting that you mentioned exosome, and no one else did. That in itself should give a hint on why people cannot do a better job at explaining stuff. It's complicated, , not well understood, extremely messy and does not fit well in a textbook.
- jhylands 6y agoLight, a mixture of why a thing is the colour that it is? As well as why reflection would be angle of incidence is angle of exit. My current understanding of colour is that the colour of an object is defined by the ability of the electrons in the compound jump different energy levels. I don't know if that in itself is enough to result in all the colour we see. My current understanding of reflection is that because of wavyness of light when lots of light gets absorbed (to my understanding a single photon exciting a single electron to jump some amount) and reemited (the electron falling back down) together the light ends up forming that angle pattern. Under than understanding single photons don't bounce in the same way rays of light do? I don't know how correct either of those understandings are, but my understanding has been put together from so many places and I've never heard any source explain either like that so I don't trust they are correct.
- rom16384 6y agoYou are mixing a lot of different things: * The reflection angle laws are due to the laws of conservation, see https://en.wikipedia.org/wiki/Snell%27s_law https://en.wikipedia.org/wiki/Snell%27s_law * For a pure colour, the colour is simply the energy of the photons. Atoms have discrete stable electron orbits, and electrons moving between these levels will absorb or emit discrete levels of energy in the form of photons, which is why we have spectral lines. Reality is more complicated because part of the energy may be converted to vibrations of the atom itself (phonons). * Another factor is the perception of colour. In physics to characterize light one measures its spectra, the intensity of the light versus its wavelength (wavelength = speed of light in vacuum / frequency). The perception of colour of these distributions isn't always always what one would expect.
- yfiapo 6y agoI'm sure there are great explanations out there but I haven't had time to read up on them, but a few of the space things that always bother me when I watch pop-sci space tv: - "as soon as iron starts to be produced in the core of a star it instantly collapses" - I get that fusing iron costs energy rather than produces it and this causes a collapse.. but can it really be that quick? There are other fusion reactions that are still producing energy, right? - dark matter / energy - I understand we have observations that indicate there is some type of matter we can't see but it feels a lot like saying "magic" or "the ether". - how different size stars form - if there is a critical mass where a star "ignites" and after igniting starts pushing away from itself with the energy being produced, how do we get stars of such varying masses? Like, why didn't this 100x solar mass star start fusing and pushing the gases away before they were caught in its gravity? Do the more massive stars ignite on the same schedule but continue to suck in additional matter anyway, gravity overcoming the solar wind?
- c1ccccc1 6y agoNot an expert, so take these with a grain of salt: The core of the star is the hottest and most dense part. Greater heat and density make it easier for fusion reactions to run. If suddenly the core is made mostly of iron, then the amount of energy it produces rapidly drops. Even if there are nice, easily fusible hydrogen atoms farther out from the core, they will not be fusing at a very high rate, because the temperature and pressure is lower where they are. Also, the more easily fusible atoms remaining outside the core can't diffuse into the core fast enough to refuel it. The only possible outcome is collapse. In some sense "dark matter" and "dark energy" are just placeholder words for "whatever thing is causing all this weird stuff to happen". This is actually very analogous to how "the ether" was a placeholder term for "whatever thing that radio waves are waves in". (Now we refer to it as "the electromagnetic field". The "ether" terminology was associated with some incorrect assumptions, such as a privileged reference frame, which is why people sometimes say it was an incorrect hypothesis. But the electromagnetic field is certainly real, it just didn't turn out to work like some people thought it did.) Scientists have observed so far the dark matter seems to behave pretty much like ordinary matter, except that it just happens to ignore the electromagnetic and strong nuclear forces. Not only does it hold galaxies together, but its gravity also bends the paths of light rays, just as we expect of anything massive. So calling it "matter" isn't too much of a stretch. It's still very mysterious, though. Radiation pressure actually does limit the mass of stars, to something on the order of 100 to 200 solar masses, see this stack exchange question: https://astronomy.stackexchange.com/questions/328/is-there-a-theoretical-maximum-size-limit-for-a-star https://astronomy.stackexchange.com/questions/328/is-there-a... That doesn't stop smaller clouds of gas from collapsing to form smaller stars, though.
- Razengan 6y agoAmperes, Voltage, Watts, without using a liquid analogy.
- 2Trips 6y agoWomen!
- 8bitsrule 6y agoLow, medium, and high-frequency radio waves (below 30MHz, wavelengths longer than 10 meters) are a form of EM radiation. Which means that an antenna is 'transmitting' low-energy, long-wavelength photons. (Which gets us into the whole wavicle question.) But the signal from a radio station appears to be continuous. So: how to picture this? Is the signal made of discrete 'photons' overlapping, or combined somehow? Or is it that the 'wave-like' aspect of these photons is so predominant at these frequencies? (I've grappled with this one for a long time.)
- vlasev 6y agoA radio station in the 1MHz range is transmitting in the 10s of kW range. The energy of a single photon is tremendously small. So if you divide out the large transmitting power by the energy of a single photon, you'll see that an unimaginable number of photons is being released, giving the impression of a continuous process. More detail: Say you have an AM station that transmits at frequency f = 650kHz and uses power P = 50kW. The equation for a photon's energy is E = hf, where E is the energy, h is Planck's constant and f is the frequency. Here h = 6.6310^(-34) Js and f = 6.510^5 Hz. Thus the photon's energy is E = 4.3110^(-28) J. This is very tiny number. The number of photons per second is n = P/E = 1.1610^32. Let's try to visualize this. Avogadro's number is 6.022*10^23 for each mol of something, so if we divide it out from n, we see that there are almost 200 million mol of photons being released every second! Water is 18g/mol, which takes up about 16 cm^3. 200 million moles of water is about a million gallons. If a photon was like a water molecule, a "water AM station" would be releasing about a million gallons of water per second.
- 8bitsrule 6y agoThanks, Vlasev, helpful ... easy to scale down to my hometown's measly 2,000 gallon AM station. I guess I'll need to go back to the radio physics math and puzzle it out more (directional arrays, groundwaves, and the like.) The idea of a photon with such a miniscule energy yet a wavelength of a thousand meters is a tad non-intuitive to me.
- dimitristi 6y agocoriolis force
- kbradero 6y agocelullar replication and virus code insertion
- JanisL 6y agoI'd really love an explanation of what specific impulse is. I've looked it up a few times but the units seconds^-1 confuses me, what does this represent?
- TheFlyingFish 6y agoI think the unit is just one of those weird dimensional equivalences that pop up from time to time. E.g. fuel efficiency in cars is measured as distance / volume (of fuel consumed), so it's dimensionally equivalent to area^-2. But we don't use this because "1 mile per gallon" makes a lot more sense than "42.5 cm^-2".
- yigithan 6y agoMorphological development of organisms. Mainly, how do cells form the shape of the organism and how do they specialize to different type of tissues? How do cells know where they are? What is the mechanism that puts the constraints on the shape of the body? I can understand mitosis but the rest from there is magic.