9 ms·

Physics facts not always taught in school

90 points by monort 8y ago

chasing 8y ago
This is interesting, but I'm going to be one of those people for a second... I hate headlines that assume what I do and don't know. Just call 'em 10 physics facts that most people aren't aware of and are pretty interesting. Because I actually did know a bunch of those facts from school and I was a liberal arts major...
- deleted 8y ago
  [deleted]
- dang 8y ago
  Yes, it's annoying and, as other users pointed out, broke the site guidelines. There's the magic number thing but also the linkbait 'you'. We've reworded it to follow them instead.
  - dustingetz 8y ago
    please embed the faq on the submit page and the reply page and these problems will go away and this conversation can stop happening
throwaway2048 8y ago
Please keep click-bait title crap off HN.
craftyguy 8y ago
> If the original title begins with a number or number + gratuitous adjective, we'd appreciate it if you'd crop it. E.g. translate "10 Ways To Do X" to "How To Do X," and "14 Amazing Ys" to "Ys." Exception: when the number is meaningful, e.g. "The 5 Platonic Solids." https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html
- bbcbasic 8y ago
  These 5 Platonic Solids will make your head spin!
wodenokoto 8y ago
What kind of school? I have a masters degree, and I fail to see where these facts would fit into to my education. Quantum mechanics and Feynman diagrams in first year of highschool?
- Jonnax 8y ago
  I'm presuming that they're using school to denote a University's department. Like "School of Engineering and Mathematical Sciences" etc.
- dboreham 8y ago
  American English. "School" can mean university.
- golergka 8y ago
  Don't they teach basic quantum mechanics in American high schools?
  - colejohnson66 8y ago
    No
  - saagarjha 8y ago
    Some do. Usually it’s not part of the standard curriculum, so it’s not very common.
  - com2kid 8y ago
    American education varies a lot. I did all biology and chemistry in highschool, I didn't have my first physics class until college.
  - pytyper2 8y ago
    It is offered but the student must elect to take the course, it is possible to pass through an American public school system without taking any advanced topics. Someone who does not enroll in courses of at least college prep level will arrive at university and be required to take some remedial credits to catch up with their peers.
platz 8y ago
> the idea that any system tries to minimize its energy is just nonsense. Very interesting, I feel somewhat misled if this is true.
- ktrask 8y ago
  In a closed physical system energy is conserved. For example, if you have container that has Helium on one side and Oxygen on the other. Both gases will mix, i.e. maximize entropy. Minimizing energy would mean, the gas would reach absolute zero.
- philipkglass 8y ago
  I don’t know if it’s always true in the context he mentions - particle physics - but I’m pretty sure it is not always true in e.g. chemical physics. Both entropy and enthalpy matter. The reaction of potassium vapor with bromine vapor is spontaneous even though the solid crystalline potassium bromide product has lower entropy than the gas phase reactants. In the comments on the article Milkshake also gives an example of an entropically driven spontaneous reaction: ammonium nitrate dissolving in water. You need enthalpic and entropic terms to predict if a reaction is thermodynamically spontaneous. And if you want to calculate if a thermodynamically spontaneous reaction will appreciably take place, you also need to understand the actual intermediates between A + B -> C, and find reaction barrier heights for all of them, and weight them appropriately. This is still very hard to do so information of this sort is mostly determined by experiment rather than calculation.
- lostmyoldone 8y ago
  I'll try to answer as best as I can. While it is true that a system doesn't minimize the energy of the entire system, as energy IS conserved, except apparently for a miniscule amount due to the universe expanding, it is also true that locally it often looks like minimizing energy in some sense. If you have a vibrating spring on a table, after a while it will have stopped because it has created motions in the air and in the table, spreading out at the speed of sound in whatever material it encounters, and some of it also at the speed of light in the form of thermal radiation. So, looking only at the spring, it is minimizing it's energy by returning to a stable quiscent state. In the same way, if we look at a sphere with radius as "the speed of light times the time from the start of the experiment", all energy can still be accounted for within that volume. One system, several perspectives. I think the minimization model is often used because the statistical model of entropy can be hard to explain, as it simultaneously allows complex relatively ordered systems like us humans to exist, while at the same time explains the almost complete disorder of thermal energy and how it disperses. Thus, since the energy minimization of the local state can be used as a relatively truthful proxy, we learn that model first. With the possible bonus that it also allows one to temporarily avoiding some rather (afaict) gnarly math until you actually have the tools you need to understand it.
mehrdadn 8y ago
> You can fall into a black hole in finite time. It just looks like it takes forever. This one is really interesting! I've definitely never seen anyone emphasize this before.
- darkmighty 8y ago
  There are variations on this that are more problematic (which I've never seen solutions to). 1) It looks like it takes forever, to an outside observer, for anything to reach the event horizon. So if no matter appears to reach event horizons, how can they (event horizons) form in the first place (i.e. exist in our universe)? 2) While it takes forever for matter to fall in (again to outside observers), it takes finite time for black holes to evaporate via Hawking radiation. So not only do black holes would take forever to form, they seem to have to extinguish before being able to exist (again in the sense of event horizons). I'm not a physicist, but my speculation is that Black Holes don't really exist, they're just a limit of a process that approaches but doesn't converge to a singularity, which I think is unphysical.
  - mehrdadn 8y ago
    I'm also not a physicist but I think I can guess a plausible answer to these? 1. Because if you don't have a black hole yet then there is no event horizon to prevent you from falling in and creating one. I think the event horizon wouldn't form at exactly the same point whose crossing would finally increase density enough to create it, so this shouldn't be a problem. (Although, again, since the event horizon doesn't exist yet, I think it might still not be a problem even in that case?) I would also expect that quantum fluctuations can also inject matter into a black hole, just as they can remove matter from it. 2. Again, they wouldn't take forever to form -- see above. I'm pretty sure black holes have been observed indirectly, so the idea that they actually don't exist would require a pretty rock solid alternative explanation!
    - pdonis 8y ago
      > I'm pretty sure black holes have been observed indirectly What we have observed indirectly are compact objects that emit no light, but contain a large enough mass in a small enough volume that they can't be anything else but a black hole, if we only take classical GR into account. As I responded in another post just now, it is possible that black holes, in the sense of objects with actual event horizons (boundaries of regions from which light will never escape, even in the infinite future), cannot exist when quantum gravity effects are taken into account. If that is the case (and it is not clear whether it is--as I said, this is an open area of research), then the objects we call black holes based on our current observations won't have actual event horizons--eventually, in the very far future, light will escape from those compact regions. But they will still have apparent horizons, i.e., surfaces from which outgoing light is not escaping now (or for a very long time in the future). And that, in itself, is sufficient to show that the issues raised by darkmighty are not valid.
asimpletune 8y ago
Wait, there’s no conservation of energy?
- dschuetz 8y ago
  Right, according to this blogspot post, one could figure that what we've learned about entropy is completely wrong! But no! The net amount of energy inside an isolated system does not change. It dissipates evenly through the system, and the system as a whole reaches it's maximum entropy/equilibrium. That means, each part of the system which can exchange energy, has the same amount of energy.
- nabla9 8y ago
  That's true. Conservation of energy is not a physical law. It's a consequence of mathematical theorem called Noether's theorem. All systems with certain differentiable symmetries have a corresponding conservation laws. In a system that is not time translation invariant, energy don't have to be conserved. Expansion of universe breaks the time symmetry.
  - noobermin 8y ago
    I'll say that most systems are time invariant, so conserve energy. One should always know the system one works with. For example, first year undergrad problems near the surface of the earth for example since they ignore the earth do not conserve momentum.
  - cygx 8y ago
    Note that Noether's second theorem still applies, so any timelike vector field will yield a conservation law due to diffeomorphism invariance. In that sense, energy conservation still holds even in the absence of Killing fields, though some people argue this doesn't count as a 'proper' law of energy conservation as it includes contributions by the gravitational field, for which an energy-momentum tensor cannot be defined...
  - sunstone 8y ago
    "Noether's" theorem. Is that a joke or what?
dschuetz 8y ago
To evaluate the validity of those facts, whom can I ask for advice? Because, I cannot afford years of study in physics to understand and assess the relevance of those facts to graduate school. Besides that, when we talk about facts, why is that I cannot find any solid scientific sources to those claims in that article?
- pdonis 8y ago
  > To evaluate the validity of those facts, whom can I ask for advice? You could ask other physicists. But you would not know whether to believe them any more than you know whether to believe the author of this article. > when we talk about facts, why is that I cannot find any solid scientific sources to those claims in that article? Where have you looked?
  - dschuetz 8y ago
    I looked for footnotes, haven't found any, besides some other blogspot links and an aeon article. The title is misleading, there are little to none sources, the whole thing smells like an opinion and pop-science. I might not be a physicist, but I'm not lacking competency in reading.
    - pdonis 8y ago
      > I looked for footnotes, haven't found any It's a blog post, not a scientific paper, and the statements it makes are very general, of the sort you will find somewhere in many textbooks, but not in one specific place that can be referenced. What you're asking for is kind of like asking for a specific footnote for the definition of a prime number in a blog post about number theory. If what you are looking for is a quick way to know whether what the article says is true without having to take any time to actually study science, there isn't one. There is no royal road to learning.
TheCoelacanth 8y ago
> Wormholes are science fiction It would be more accurate to say the wormholes are hypothetical than to say that they are fictional. They are an idea someone came up with of something that might really exist, we just don't have any evidence of them.
- akvadrako 8y ago
  Wormholes could potentially exist; there is even a well-known non-crackpot theory called ER=EPR that roughly suggests entangled particles are connected by wormholes. What is fiction is wormholes you can send information or people though.
dschuetz 8y ago
The title is misleading. It's an opinion from some blogspot blog without sources. "Aspects of theoretical physics not always taught in school" would be more appropriate.
- saagarjha 8y ago
  These are commonly accepted theories. It would be like citing Darwin every time you talked about evolution–nobody does this because you can easily do this yourself if you were in the unlikely event of being unaware of the science behind it.
  - dschuetz 8y ago
    I'd like to see quantum gravity as a "commonly accepted theory". But it's not. It's a hypothesis.
    - lostmyoldone 8y ago
      If you are saying that the issue apperently most contentious to you is a hypothesis, why did you call the entire article opinion to begin with? Seems contradictory to me. As far as I can tell, she's really only saying is that we would have easily seen QG effects of some sort if it wasn't because it's only relevant at high curvatures, for some measure of high. Hence all of the relevant theories in the QG fields can with high probability only be relevant at high curvatures, because we should have seen them otherwise. Which part of that do you consider to be incorrect?
- robinhouston 8y ago
  I think it’s misleading to characterise Sabine Hossenfelder’s blog as “some blogspot blog”. She is a serious and respected theoretical physicist, not some random blogger.
  - dschuetz 8y ago
    She's a pop-science writer. She is a random blogger. She has a job as a physicist. It doesn't make her opinion a fact, just because she's a theoretical physicist with a job and a blog.
Xcelerate 8y ago
I think the coolest fact that I didn't grok until undergrad is that gravity relates to the stress-energy tensor and is not just due to rest mass alone. Which means that anything that has energy associated with it (i.e., everything we've ever observed) affects the gravitational field. Examples: A compressed spring weighs more than the same spring in an uncompressed state. A box of light (e.g., a box with perfectly reflective mirrored walls) weighs more than the same box without light, despite the fact that photons have zero rest mass. The bulk of a proton's "effective mass" is due to the kinetic energy of the quarks that comprise it. The joint earth + moon system weighs less than if you added up each component weighed in isolation. The other interesting fact is that the Heisenberg Uncertainty Principle is commonly misunderstood to mean that you can't simultaneously measure the position and momentum of a particle. In fact, you can indeed obtain partial information on both properties at the same time, and there's quite a few papers out there describing how to perform joint measurements of incompatible observables. What HUP more accurately entails is that, for a quantum state corresponding to a specific system, you cannot obtain complete information about the two properties — no matter what you do. This is because performing a position measurement destroys your ability to perform a subsequent momentum measurement (on the same system) and vice versa. One of the postulates of quantum mechanics is that measuring a system collapses it into an eigenstate of the observable corresponding to the type of measurement that was performed. Since the position and momentum operators don't commute, it's not possible to put the two into a joint eigenbasis (they can't be simultaneously diagonalized).
- sls 8y ago
  Since you obviously like this sort of thing, let me point out that quantum uncertainty isn't simply a factor of measurement/observer effects[1][2]. (The only part of what you said that I am addressing by this is the "this is because" section.) [1] https://www.nature.com/news/quantum-uncertainty-not-all-in-the-measurement-1.11394 https://www.nature.com/news/quantum-uncertainty-not-all-in-t... [2] https://en.wikipedia.org/wiki/Uncertainty_principle https://en.wikipedia.org/wiki/Uncertainty_principle
  - Xcelerate 8y ago
    Yeah, you may also find interaction free measurements interesting (https://en.wikipedia.org/wiki/Interaction-free_measurement https://en.wikipedia.org/wiki/Interaction-free_measurement).
noobermin 8y ago
I like the first part, entropy is essentially a measure of degeneracy for a state (the elementary definition is the log of the number of states multiplied by a unitful constant). Technically, if you're a supposed super-intelligence, that is good at remembering detail, every state of a system can be distinguishable and thus have small entropy. For example, consider a finite number of legos in a room. A computer could potentially remember where every lego is placed, while person can't do as well but could distinguish between a state where the legos are strewn about or built into a castle. So, the person would lump all the disordered states into just one state (the "mess" state) and give it a high entropy compared to the number of organized states (castles or ships made out of the legos). I guess I always knew this, and this is sort of what we mean when we say "high entropy" but it's kind of fun to say it out explicitly like this. Most of the others seem like conflating the strict applicability of a theory vs. it's practical limits, like that QM doesn't really mean "small" or (equivalently) high energy, it just means when you're near the Heisenberg uncertainty limit for the observations you're making, which in most cases means small. Allow me to add another one: "Special Relativity means when you go faster, time slows down for you!" And another (controversial may be): "In Schrodinger's cat, the cat is both dead and alive at the same time!"
- Xcelerate 8y ago
  > every state of a system can be distinguishable and thus have small entropy Which corresponds quite well to the fact that the entropy of a pure state is zero (since S=−tr[ρlnρ]). Entropy is better described as a property that arises between systems.
- kgwgk 8y ago
  One way to look at it is that the entropy is not a property of the system but a property of the knowledge an observer has about the system. A macroscopic state is not a physical object. The same microscopic state may correspond to different macroscopic states for different observers.
- PopePompus 8y ago
  > For example, consider a finite number of legos in a room. This works with legos, but not subatomic particles. No matter how smart you are, you can't distinguish between two states which differ only by having two electrons swapped.
avmich 8y ago
The very first item - disorder vs. likelihood. Can somebody explain the crucial difference? Why the dough is of higher entropy than the (random) distribution of dough components?
kgwgk 8y ago
> Which state is more orderly, the broken egg on flour with butter over it, or the final dough? > I’d go for the dough. Strange.
- avmich 8y ago
  Thanks, you're answering my question. It's not a random distribution - it's separate substances (butter separate from flour separate from egg), so their order is actually higher. Then I agree - it's strange the author goes for the dough.
  - kgwgk 8y ago
    I don’t understand her point either. And for number 3, entropy for a single particle (as far as such a thing can be defined) is quite different from entropy for a macroscopic system. I’m not sure it makes sense to say that particles decay “to reach a state of highest entropy”.
  - stan_rogers 8y ago
    I think that's meant to be an illustration of the naïve tendency to think of something purposeful - an intended state - as being more ordered than something broken or scattered. A dough is a thing you made for a reason, a mess on the floor is just a mess on the floor. Don't take it as a physicist thinking in physical terms, but as the immediate impression of an ordinary person in ordinary circumstances - you need to reflect and understand to see the truth.
    - cygx 8y ago
      Not only that: Think about a canvas with some random splatters of paint, compared to a canvas uniformly coloured with a homogeneous mixture of that paint. What's more 'orderly'?
sunstone 8y ago
I, for one, thought that conservation of energy was iron clad. It turns out that "except for the expansion of the universe" proviso, it is. As a religious adherent of the conservation laws this makes me unhappy. And brings up new questions. I'll have to sleep on this one.

90 points by monort 8y ago

This is interesting, but I'm going to be one of those people for a second... I hate headlines that assume what I do and don't know. Just call 'em 10 physics facts that most people aren't aware of and are pretty interesting. Because I actually did know a bunch of those facts from school and I was a liberal arts major...

deleted 8y ago
[deleted]
dang 8y ago
Yes, it's annoying and, as other users pointed out, broke the site guidelines. There's the magic number thing but also the linkbait 'you'. We've reworded it to follow them instead.
- dustingetz 8y ago
  please embed the faq on the submit page and the reply page and these problems will go away and this conversation can stop happening

throwaway2048 8y ago

Please keep click-bait title crap off HN.

craftyguy 8y ago

> If the original title begins with a number or number + gratuitous adjective, we'd appreciate it if you'd crop it. E.g. translate "10 Ways To Do X" to "How To Do X," and "14 Amazing Ys" to "Ys." Exception: when the number is meaningful, e.g. "The 5 Platonic Solids." https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html

bbcbasic 8y ago
These 5 Platonic Solids will make your head spin!

wodenokoto 8y ago

What kind of school? I have a masters degree, and I fail to see where these facts would fit into to my education. Quantum mechanics and Feynman diagrams in first year of highschool?

Jonnax 8y ago
I'm presuming that they're using school to denote a University's department. Like "School of Engineering and Mathematical Sciences" etc.
dboreham 8y ago
American English. "School" can mean university.
golergka 8y ago
Don't they teach basic quantum mechanics in American high schools?
- colejohnson66 8y ago
  No
- saagarjha 8y ago
  Some do. Usually it’s not part of the standard curriculum, so it’s not very common.
- com2kid 8y ago
  American education varies a lot. I did all biology and chemistry in highschool, I didn't have my first physics class until college.
- pytyper2 8y ago
  It is offered but the student must elect to take the course, it is possible to pass through an American public school system without taking any advanced topics. Someone who does not enroll in courses of at least college prep level will arrive at university and be required to take some remedial credits to catch up with their peers.

platz 8y ago

> the idea that any system tries to minimize its energy is just nonsense. Very interesting, I feel somewhat misled if this is true.

ktrask 8y ago
In a closed physical system energy is conserved. For example, if you have container that has Helium on one side and Oxygen on the other. Both gases will mix, i.e. maximize entropy. Minimizing energy would mean, the gas would reach absolute zero.
philipkglass 8y ago
I don’t know if it’s always true in the context he mentions - particle physics - but I’m pretty sure it is not always true in e.g. chemical physics. Both entropy and enthalpy matter. The reaction of potassium vapor with bromine vapor is spontaneous even though the solid crystalline potassium bromide product has lower entropy than the gas phase reactants. In the comments on the article Milkshake also gives an example of an entropically driven spontaneous reaction: ammonium nitrate dissolving in water. You need enthalpic and entropic terms to predict if a reaction is thermodynamically spontaneous. And if you want to calculate if a thermodynamically spontaneous reaction will appreciably take place, you also need to understand the actual intermediates between A + B -> C, and find reaction barrier heights for all of them, and weight them appropriately. This is still very hard to do so information of this sort is mostly determined by experiment rather than calculation.
lostmyoldone 8y ago
I'll try to answer as best as I can. While it is true that a system doesn't minimize the energy of the entire system, as energy IS conserved, except apparently for a miniscule amount due to the universe expanding, it is also true that locally it often looks like minimizing energy in some sense. If you have a vibrating spring on a table, after a while it will have stopped because it has created motions in the air and in the table, spreading out at the speed of sound in whatever material it encounters, and some of it also at the speed of light in the form of thermal radiation. So, looking only at the spring, it is minimizing it's energy by returning to a stable quiscent state. In the same way, if we look at a sphere with radius as "the speed of light times the time from the start of the experiment", all energy can still be accounted for within that volume. One system, several perspectives. I think the minimization model is often used because the statistical model of entropy can be hard to explain, as it simultaneously allows complex relatively ordered systems like us humans to exist, while at the same time explains the almost complete disorder of thermal energy and how it disperses. Thus, since the energy minimization of the local state can be used as a relatively truthful proxy, we learn that model first. With the possible bonus that it also allows one to temporarily avoiding some rather (afaict) gnarly math until you actually have the tools you need to understand it.

mehrdadn 8y ago

> You can fall into a black hole in finite time. It just looks like it takes forever. This one is really interesting! I've definitely never seen anyone emphasize this before.

darkmighty 8y ago
There are variations on this that are more problematic (which I've never seen solutions to). 1) It looks like it takes forever, to an outside observer, for anything to reach the event horizon. So if no matter appears to reach event horizons, how can they (event horizons) form in the first place (i.e. exist in our universe)? 2) While it takes forever for matter to fall in (again to outside observers), it takes finite time for black holes to evaporate via Hawking radiation. So not only do black holes would take forever to form, they seem to have to extinguish before being able to exist (again in the sense of event horizons). I'm not a physicist, but my speculation is that Black Holes don't really exist, they're just a limit of a process that approaches but doesn't converge to a singularity, which I think is unphysical.
- mehrdadn 8y ago
  I'm also not a physicist but I think I can guess a plausible answer to these? 1. Because if you don't have a black hole yet then there is no event horizon to prevent you from falling in and creating one. I think the event horizon wouldn't form at exactly the same point whose crossing would finally increase density enough to create it, so this shouldn't be a problem. (Although, again, since the event horizon doesn't exist yet, I think it might still not be a problem even in that case?) I would also expect that quantum fluctuations can also inject matter into a black hole, just as they can remove matter from it. 2. Again, they wouldn't take forever to form -- see above. I'm pretty sure black holes have been observed indirectly, so the idea that they actually don't exist would require a pretty rock solid alternative explanation!
  - pdonis 8y ago
    > I'm pretty sure black holes have been observed indirectly What we have observed indirectly are compact objects that emit no light, but contain a large enough mass in a small enough volume that they can't be anything else but a black hole, if we only take classical GR into account. As I responded in another post just now, it is possible that black holes, in the sense of objects with actual event horizons (boundaries of regions from which light will never escape, even in the infinite future), cannot exist when quantum gravity effects are taken into account. If that is the case (and it is not clear whether it is--as I said, this is an open area of research), then the objects we call black holes based on our current observations won't have actual event horizons--eventually, in the very far future, light will escape from those compact regions. But they will still have apparent horizons, i.e., surfaces from which outgoing light is not escaping now (or for a very long time in the future). And that, in itself, is sufficient to show that the issues raised by darkmighty are not valid.

asimpletune 8y ago

Wait, there’s no conservation of energy?

dschuetz 8y ago
Right, according to this blogspot post, one could figure that what we've learned about entropy is completely wrong! But no! The net amount of energy inside an isolated system does not change. It dissipates evenly through the system, and the system as a whole reaches it's maximum entropy/equilibrium. That means, each part of the system which can exchange energy, has the same amount of energy.
nabla9 8y ago
That's true. Conservation of energy is not a physical law. It's a consequence of mathematical theorem called Noether's theorem. All systems with certain differentiable symmetries have a corresponding conservation laws. In a system that is not time translation invariant, energy don't have to be conserved. Expansion of universe breaks the time symmetry.
- noobermin 8y ago
  I'll say that most systems are time invariant, so conserve energy. One should always know the system one works with. For example, first year undergrad problems near the surface of the earth for example since they ignore the earth do not conserve momentum.
- cygx 8y ago
  Note that Noether's second theorem still applies, so any timelike vector field will yield a conservation law due to diffeomorphism invariance. In that sense, energy conservation still holds even in the absence of Killing fields, though some people argue this doesn't count as a 'proper' law of energy conservation as it includes contributions by the gravitational field, for which an energy-momentum tensor cannot be defined...
- sunstone 8y ago
  "Noether's" theorem. Is that a joke or what?

dschuetz 8y ago

To evaluate the validity of those facts, whom can I ask for advice? Because, I cannot afford years of study in physics to understand and assess the relevance of those facts to graduate school. Besides that, when we talk about facts, why is that I cannot find any solid scientific sources to those claims in that article?

pdonis 8y ago
> To evaluate the validity of those facts, whom can I ask for advice? You could ask other physicists. But you would not know whether to believe them any more than you know whether to believe the author of this article. > when we talk about facts, why is that I cannot find any solid scientific sources to those claims in that article? Where have you looked?
- dschuetz 8y ago
  I looked for footnotes, haven't found any, besides some other blogspot links and an aeon article. The title is misleading, there are little to none sources, the whole thing smells like an opinion and pop-science. I might not be a physicist, but I'm not lacking competency in reading.
  - pdonis 8y ago
    > I looked for footnotes, haven't found any It's a blog post, not a scientific paper, and the statements it makes are very general, of the sort you will find somewhere in many textbooks, but not in one specific place that can be referenced. What you're asking for is kind of like asking for a specific footnote for the definition of a prime number in a blog post about number theory. If what you are looking for is a quick way to know whether what the article says is true without having to take any time to actually study science, there isn't one. There is no royal road to learning.

TheCoelacanth 8y ago

> Wormholes are science fiction It would be more accurate to say the wormholes are hypothetical than to say that they are fictional. They are an idea someone came up with of something that might really exist, we just don't have any evidence of them.

akvadrako 8y ago
Wormholes could potentially exist; there is even a well-known non-crackpot theory called ER=EPR that roughly suggests entangled particles are connected by wormholes. What is fiction is wormholes you can send information or people though.

dschuetz 8y ago

The title is misleading. It's an opinion from some blogspot blog without sources. "Aspects of theoretical physics not always taught in school" would be more appropriate.

saagarjha 8y ago
These are commonly accepted theories. It would be like citing Darwin every time you talked about evolution–nobody does this because you can easily do this yourself if you were in the unlikely event of being unaware of the science behind it.
- dschuetz 8y ago
  I'd like to see quantum gravity as a "commonly accepted theory". But it's not. It's a hypothesis.
  - lostmyoldone 8y ago
    If you are saying that the issue apperently most contentious to you is a hypothesis, why did you call the entire article opinion to begin with? Seems contradictory to me. As far as I can tell, she's really only saying is that we would have easily seen QG effects of some sort if it wasn't because it's only relevant at high curvatures, for some measure of high. Hence all of the relevant theories in the QG fields can with high probability only be relevant at high curvatures, because we should have seen them otherwise. Which part of that do you consider to be incorrect?
robinhouston 8y ago
I think it’s misleading to characterise Sabine Hossenfelder’s blog as “some blogspot blog”. She is a serious and respected theoretical physicist, not some random blogger.
- dschuetz 8y ago
  She's a pop-science writer. She is a random blogger. She has a job as a physicist. It doesn't make her opinion a fact, just because she's a theoretical physicist with a job and a blog.

Xcelerate 8y ago

I think the coolest fact that I didn't grok until undergrad is that gravity relates to the stress-energy tensor and is not just due to rest mass alone. Which means that anything that has energy associated with it (i.e., everything we've ever observed) affects the gravitational field. Examples: A compressed spring weighs more than the same spring in an uncompressed state. A box of light (e.g., a box with perfectly reflective mirrored walls) weighs more than the same box without light, despite the fact that photons have zero rest mass. The bulk of a proton's "effective mass" is due to the kinetic energy of the quarks that comprise it. The joint earth + moon system weighs less than if you added up each component weighed in isolation. The other interesting fact is that the Heisenberg Uncertainty Principle is commonly misunderstood to mean that you can't simultaneously measure the position and momentum of a particle. In fact, you can indeed obtain partial information on both properties at the same time, and there's quite a few papers out there describing how to perform joint measurements of incompatible observables. What HUP more accurately entails is that, for a quantum state corresponding to a specific system, you cannot obtain complete information about the two properties — no matter what you do. This is because performing a position measurement destroys your ability to perform a subsequent momentum measurement (on the same system) and vice versa. One of the postulates of quantum mechanics is that measuring a system collapses it into an eigenstate of the observable corresponding to the type of measurement that was performed. Since the position and momentum operators don't commute, it's not possible to put the two into a joint eigenbasis (they can't be simultaneously diagonalized).

sls 8y ago
Since you obviously like this sort of thing, let me point out that quantum uncertainty isn't simply a factor of measurement/observer effects[1][2]. (The only part of what you said that I am addressing by this is the "this is because" section.) [1] https://www.nature.com/news/quantum-uncertainty-not-all-in-the-measurement-1.11394 https://www.nature.com/news/quantum-uncertainty-not-all-in-t... [2] https://en.wikipedia.org/wiki/Uncertainty_principle https://en.wikipedia.org/wiki/Uncertainty_principle
- Xcelerate 8y ago
  Yeah, you may also find interaction free measurements interesting (https://en.wikipedia.org/wiki/Interaction-free_measurement https://en.wikipedia.org/wiki/Interaction-free_measurement).

noobermin 8y ago

I like the first part, entropy is essentially a measure of degeneracy for a state (the elementary definition is the log of the number of states multiplied by a unitful constant). Technically, if you're a supposed super-intelligence, that is good at remembering detail, every state of a system can be distinguishable and thus have small entropy. For example, consider a finite number of legos in a room. A computer could potentially remember where every lego is placed, while person can't do as well but could distinguish between a state where the legos are strewn about or built into a castle. So, the person would lump all the disordered states into just one state (the "mess" state) and give it a high entropy compared to the number of organized states (castles or ships made out of the legos). I guess I always knew this, and this is sort of what we mean when we say "high entropy" but it's kind of fun to say it out explicitly like this. Most of the others seem like conflating the strict applicability of a theory vs. it's practical limits, like that QM doesn't really mean "small" or (equivalently) high energy, it just means when you're near the Heisenberg uncertainty limit for the observations you're making, which in most cases means small. Allow me to add another one: "Special Relativity means when you go faster, time slows down for you!" And another (controversial may be): "In Schrodinger's cat, the cat is both dead and alive at the same time!"

Xcelerate 8y ago
> every state of a system can be distinguishable and thus have small entropy Which corresponds quite well to the fact that the entropy of a pure state is zero (since S=−tr[ρlnρ]). Entropy is better described as a property that arises between systems.
kgwgk 8y ago
One way to look at it is that the entropy is not a property of the system but a property of the knowledge an observer has about the system. A macroscopic state is not a physical object. The same microscopic state may correspond to different macroscopic states for different observers.
PopePompus 8y ago
> For example, consider a finite number of legos in a room. This works with legos, but not subatomic particles. No matter how smart you are, you can't distinguish between two states which differ only by having two electrons swapped.

avmich 8y ago

The very first item - disorder vs. likelihood. Can somebody explain the crucial difference? Why the dough is of higher entropy than the (random) distribution of dough components?

kgwgk 8y ago

> Which state is more orderly, the broken egg on flour with butter over it, or the final dough? > I’d go for the dough. Strange.

avmich 8y ago
Thanks, you're answering my question. It's not a random distribution - it's separate substances (butter separate from flour separate from egg), so their order is actually higher. Then I agree - it's strange the author goes for the dough.
- kgwgk 8y ago
  I don’t understand her point either. And for number 3, entropy for a single particle (as far as such a thing can be defined) is quite different from entropy for a macroscopic system. I’m not sure it makes sense to say that particles decay “to reach a state of highest entropy”.
- stan_rogers 8y ago
  I think that's meant to be an illustration of the naïve tendency to think of something purposeful - an intended state - as being more ordered than something broken or scattered. A dough is a thing you made for a reason, a mess on the floor is just a mess on the floor. Don't take it as a physicist thinking in physical terms, but as the immediate impression of an ordinary person in ordinary circumstances - you need to reflect and understand to see the truth.
  - cygx 8y ago
    Not only that: Think about a canvas with some random splatters of paint, compared to a canvas uniformly coloured with a homogeneous mixture of that paint. What's more 'orderly'?

sunstone 8y ago

I, for one, thought that conservation of energy was iron clad. It turns out that "except for the expansion of the universe" proviso, it is. As a religious adherent of the conservation laws this makes me unhappy. And brings up new questions. I'll have to sleep on this one.