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Why dark matter feels like cheating (and why it isn’t)
- college_physics 4y agoMaybe it is not "cheating" but it certainly casts a dark shadow over the otherwise fairly cooky stance of fundamental physics that "we have figured things out" As an explanatory concept is has very low utility: there is not much else you can do with it except plug what you find missing. You can't say, for example, that because of this and this aspect of dark matter I predict this cool effect and then go search for it and either falsify or strengthen the confidence of your thought framework
- lisper 4y agoThat's not true. There have been many hypotheses advanced regarding the nature of dark matter (WIMPs, MACHOs) and the all have made experimental predictions. The problem is that none of those predictions have actually been confirmed by experiment, and we're running out of ideas. But this is not much different than the situation on the eve of the discovery of relativity and quantum mechanics at the beginning of the 20th century. So all of this is just business as usual for cutting-edge physics.
- college_physics 4y agoEvery plausible but failed hypothesis (its been half a century now?) bakes-in an intractability that may become permanent for any relavant timescale. There is still some hope as cosmological observations feel less exhausted than particle physics. But its a very awkward admission of defeat after some fairly triumphant decades last century.
- mr_mitm 4y agoSure you can. Example: you observe wonky rotation curves of galaxies. You reckon there might be some extra invisible matter. From that you predict that you should also see this extra matter in lensing observations. You make the lensing observations and lo and behold, you see the same amount of extra matter that is needed to explain the rotation curves. I wrote a longer comment on this before: https://news.ycombinator.com/item?id=34365591 https://news.ycombinator.com/item?id=34365591
- college_physics 4y agoThis simply says that the gravitational footprint of dark matter is consistent. The opposite would be quite remarkable though.
- naasking 4y ago> You make the lensing observations and lo and behold, you see the same amount of extra matter that is needed to explain the rotation curves. This is so oversimplified that I'd call it incorrect. The galaxy rotation curves cannot be predicted a priori using dark matter models, the distribution has to be tuned a posteriori to fit the curve. This is in contrast to MOND which can make successful a priori predictions. There's clearly something missing in the dark matter picture.
- zinclozenge 4y agoI've posted this before, but never in my entire academic physics career did I ever get the impression, either in coursework or research, except in the courses covering classical theories (ie mechanics, electrodynamics, etc). As soon as you start learning things beyond non-relativistic quantum mechanics, it's consistently drilled into us that all we have are _effective_ theories. For god's sake, we don't even have a Grand Unification Theory, let alone a Theory of Everything.
- college_physics 4y agoUnifying gravity requires insights into energy scales so remote from our own that you cant hold against eg string theorists for not nailing what happens at such extremes where fundamental notions may break down Dark matter, being (likely?) a low energy kind of thing feels like a genuine gap within the realm we claim as understood. In this sense its more fascinating.
- at_a_remove 4y agoThis was originally true of the neutrino. It was just a placeholder. Took quite a while before we found out that it wasn't just a convenient mathematical fudge factor.
- d--b 4y agoWhat about dark energy? Dark matter seems quite solid. But then dark energy does feel like a hack, right? Throwing the whole “dark thing” concept under the bus.
- lisper 4y ago> Dark matter seems quite solid. So to speak :-) >But then dark energy does feel like a hack, right? Throwing the whole “dark thing” concept under the bus. Yes, dark energy really is nothing more than a mathematical hack (at least for now). There's a reason that a certain personality type chooses to go into physics and not marketing, with the result being that the market for physics often leaves something to be desired.
- zinclozenge 4y agoWhy would dark energy be a hack? We know that the universe's expansion is accelerating. Since it's accelerating, and not expanding at a constant rate, that means there has to be something driving that. Dark energy is basically a "placeholder" term to describe the phenomenon.
- lytefm 4y agoI thought the same when reading the article. Dark matter doesn't just seem solid - it has even has mass :) Dark energy is a whole different story and seems more like the epicycles of the heliocentric model: Adding a mathematical hack in order to fit the model into the data. I'm remain curios about stationary universe model proposed by Peter Ostermann [1] which can explain the SNe-Ia data with homogeneously distributed dark matter and without a need for dark energy. Also, it is a lot more elegant and "Einsteinian" than the concordance model. [1] https://www.peter-ostermann.de/ https://www.peter-ostermann.de/
- kloch 4y ago> For dark matter, we keep those standards. The evidence for some kind of dark matter, that there is something that can’t be explained by just the Standard Model and Einstein’s gravity, is at this point very strong. Option 1: Do more research on Gravity to see what we might be missing there. The difficulty in measuring G, and the flyby anomaly would be good places to start. Option 2: Make up fully unconstrained variable 'X' that can take any value you want at any location or time in the universe[1]. This is amazing because it can perfectly solve so many difficult problems! Obviously many people will and should be skeptical of throwing full confidence at option 2 in it's current state. [1] Unlike neutrinos or black holes, there is no theory for what dark matter is made of, and how it is created or destroyed. Thus there are no constraints on how much you can have at any given location or time.
- ajross 4y agoThe point of the article is that dark matter is the result of doing 1, not 2. What's this "fully unconstrained" nonsense? We have outrageous amounts of data that need to be fit. Why is "the flyby anomaly" a useful place to start and not a century's worth of galactic rotation and mass estimates? Dark Matter persists precisely because it's the best theory that fits the available observations. Arguments against it are the bits predicated on squishy stuff like "aesthetics". No one thinks we have all the answers, but demanding we throw out the best model we have is going backwards.
- kloch 4y agobeing "skeptical of throwing full confidence at option 2 " is not the same as "demanding it be thrown out". It just means show us something more than "magic variable 'X' fits all curves."
- ajross 4y agoYou want... more from a theory than that it fit the data? That's not how theories work. I think what you're asking for is a lab-testable prediction from a dark matter theory. And no, we don't have that. It would be nice if we did, but we don't. We don't have it from MOND either, though, so I still don't get your denialism. If you don't want us to do science to figure this out, what are you asking for?
- teddyh 4y agoGod of the gap matter.
- drdeca 4y agoThat’s not even what the “God of the gaps” idea is
- awinter-py 4y ago> we can map dark matter's location this has always felt circular to me -- someone drops an astronomy paper like 'we found a galaxy with no dark matter' and I wish someone would rewrite it as 'dear non physicist, here are ten critical takes you just thought of and why we discarded each' this is mostly a knock on my own knowledge, and slightly a knock on pop science press, but I don't know the steps between 'mass as inferred from light doesn't explain galactic rotation curves' and '80% of mass is ghosts'
- consilient 4y ago> but I don't know the steps between 'mass as inferred from light doesn't explain galactic rotation curves' and '80% of mass is ghosts' This is the root of the disconnect, I think: dark matter isn't ghosts. Or if you're feeling snarky: all matter is ghosts. Any talk of "substances" or "essences" or "intrinsic properties" is entirely outside the scope of physics: the only things we ever see are interactions, and interactions with light don't have any special epistemic status that interactions with gravity lack. If anything, the opposite is true.
- deleted 4y ago[deleted]
- FreeFull 4y agoThere are observations that point towards dark matter which don't have anything to do with rotation curves. In particular, gravitational lensing, and the effects of mass on the cosmic microwave background. https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster The Bullet Cluster is very hard to explain with any kind of MOND theory.
- deleted 4y ago[deleted]
- awinter-py 4y agothe bullet cluster is like 'two clusters collided in the visual plane, and now the lensing peaks are correlated with the respective centers of luminosity' unless I'm missing something, this is mysterious in the exact same way as normal galaxies
- remote_phone 4y agoThe fact that dark matter has never been seen “but it must be there!” is basically the same argument that religious people make. It’s 100% a religious, faith-based argument and non-scientific. There I said it. There’s an explanation for all of this behavior that we are measuring but saying that it’s something we can’t see but it’s there!! is objectively absurd.
- drdeca 4y agoWhy is not interacting with EM absurd?
- pixl97 4y agoThis is flat dumb. Imagine that if when we died that we saw a large electrical impulse leaving our body, or that suddenly some amount of mass disappeared. If this discrepancy existed but was completely unexplainable then the 'is there a soul' or 'does god exist' question would be up for question. It is much the same for dark matter/dark energy. Without the dark matter 'fudge factor' cosmic observations at galactic scales or above are broken. We've also when and by observation removed a number of things that this fudge factor cannot be. And yet in our observations this fudge factor keeps existing and is found groups like the bullet cluster, or in a small number of galaxies that appear to have no dark matter. There is not a single reputable scientist that says they know what 'dark matter' is. Scientists know that there are knowledge holes that exist here. They do say it is measurable and that you the denier cannot handwave it away without bringing forth a better theory.
- lll-o-lll 4y agoDark Matter is unintuitive for the layman (which I am), and extremely intuitive for the physicist, which I suspect is where the origins of this problem lie. This article is another in long line of you just don’t get it man, which is true. Sabine Hossenfelder has some great videos that actually explain the concepts in a way that the layman can grasp, while also pointing out a number of problems with the standard orthodoxy. The problems don’t indicate that Dark Matter (the theory) is wrong necessarily. MOND is much better at explaining a number of observations of large scale (better meaning simpler in this case), but Dark Matter is better for other observations at smaller scale. Unfortunately it seems that the physicist community (outside of a small subset) is unwilling to research in the MOND space; it has the taint. So physicists just pile on a bunch of extra variables to make Dark Matter fit certain observations, when MOND describes those observations very simply. Sabine argues long and hard that modern particle physicists have made no fundamental progress for 50 years due to poor scientific method, and she’s acerbic and popular with the plebs (such as me). I’m glad she’s a voice out there, but I’m sure she has put herself offside with a number (maybe most) working particle physicists. It’s unfortunate. The phenomenon that MOND and Dark Matter seek to explain are really interesting, and the depths have clearly not been plumbed. The continual search and failure to find the Dark Matter particle is not doing physics any favours.
- throwawaymaths 4y agoWhy exactly is dark matter unintuitive to the layperson? (Any more so than say, germ theory, or praying to an interventionist god)?
- lll-o-lll 4y agoUntil you have it properly explained it just looks like a kludge. It’s an obvious extrapolation once a number of fundamental concepts have been understood, but these are concepts you won’t be exposed to outside of a physics university education. That’s all it is I think. Sabine (and probably others) have done a great job of explaining these concepts for the layman audience. This article, and so many like it, don’t do that. So the physicist is left shaking their head, and the layman comes away still thinking “what a giant kludge that dark matter is”.
- nitwit005 4y agoLet us imagine the view of a "normal person". Pop open a science textbook, and it will likely suggest the scientific method requires you to state your hypothesis, and test it as best you can. So this normal person looks to see if this has happened, finds there are several rival theories, and no experiment done on Earth had produced results. Naturally, they start to doubt.
- imiric 4y agoBut doubt what, exactly? All science tells us is that there should be something there, but we don't know what it is. Nobody's asserting the contrary.
- mdorazio 4y agoDoubt that magic pixie dust is the correct answer to "what's there?".
- nitwit005 4y agoI've yet to encounter a speaker or article that just stated there is an observation they can't explain. They all talk about some theory or theories to try to explain things.
- enkid 4y agoThat's how science works. There's a mystery and science purposes a hypothesis resolving that mystery. If a hypothesis is clearly more consistent with the evidence then other hypothesises, it wins out. If not, there continues to be a debate.
- nitwit005 4y agoMy point is that people aren't sticking to tested ideas when communicating with the public. They're throwing untested ideas at them, or sometimes multiple incompatible untested ideas at them.
- thriftwy 4y agoThe photons of CMB has lost near the same order of magnitude of energy as the postulated dark energy, if I'm not mistaken. This happened via red shift. I wonder where that energy went and how they acquired that much energy in the first place.
- bmacho 4y agoThey have some equations that never ever match the observed data. They like their equations, so they call the difference dark matter (which varies in space and time, as we expect). It is quite ridiculous if you think about it, and also is the fact, that they can get away with it. I am waiting eagerly how will this turn out. (Well only the "dark matter exist", or the "Einstein equations are so useful that they create a meaningful dark matter concept with properties, even that they are incorrect" cases are interesting, if say MOND wins, that would be less amusing.)
- jhanschoo 4y agoI have my equations, that tell me that I look straight from where I'm standing, I should see the sun rise in the morning, that never ever matches the observed data. I like my equations, so I call the difference the wall in front of my face.
- bmacho 4y agoYou probably shouldn't call it 'wall', it is easy to mistake it for wall[0] (I did it first too). [0] : https://en.wikipedia.org/wiki/Wall https://en.wikipedia.org/wiki/Wall
- simonh 4y agoWhat do you think they are “getting away with”. Is that something you think they should not be allowed to do? Who would stop them and how?
- teawrecks 4y agoI'm too dumb on this subject to know the difference between your statement and the one directly challenged by the article. It just sounds like you're making the same "dark expenses" argument that it suggests is a mistake.
- Ma8ee 4y agoNo, they have some equations that match the observed data extremely well, up to interplanetary scale, and are the basis for a lot of technology that we use daily. Now there are some discrepancies between what we expect and what we measure at galactic scale. A very popular hypothesis is that these discrepancies are due to some dark material that we haven’t been able to observe otherwise.
- Ekaros 4y agoFor me the main issue with dark matter is that I haven't read that we have produced it or even named properly what is constitute of or is. For us to tell that there must be this amount of something that only affects things gravitationally for observations to work, just seems like pure kludge factor to get everything look right. Then again, I have not studied whole thing deeply.
- nickdrozd 4y agoDark matter skeptics believe, in effect, that everything that interacts with gravity also interacts with electromagnetism. They tend to believe this really strongly, but I've never heard anyone give a reason for it. It's a strangely abstract thing to be so dogmatic about.
- ninkendo 4y agoWell, I would turn this back and say Dark Matter proponents believe that everything that causes gravity must also be matter, which is a strange thing to be dogmatic about. The only thing we know is that spacetime is curving without any visible/detectable thing causing it (at least not in a way our equations predict.) Calling it “matter” is already making a huge set of assumptions.
- Retric 4y agoDark matter was initially just a fudge factor because planets etc didn’t emit light. That kind of dark matter obviously exists, reusing the term may not actually imply the stuff is matter.
- andreareina 4y agoI'm not aware of anyone reputable claiming that e.g. photons are matter, or that they don't gravitate.
- ncallaway 4y ago> Dark Matter proponents believe that everything that causes gravity must also be matter I don’t think that’s correct. Most Dark Matter proponents are ardent adherents to general relativity. General Relativity is pretty clear that massless non-matter particles like the photon contribute to the bending of spacetime. So it seems like most Dark Matter proponents would stridently object to the claim that “everything that causes gravity must be matter”.
- ninkendo 4y ago> most Dark Matter proponents would stridently object to the claim that “everything that causes gravity must be matter” Given that the only thing we’ve observed is a change in the gravitational field (curvature of spacetime), then why call it matter? Why assume that it’s matter in the first place? This is where my criticism comes from. It seems like they’re eager to assume something that affects gravity must be matter, hence why it’s called dark matter. The fact that the leading candidate for dark matter is a “weakly interacting massive particle” further cements this criticism IMO… it’s something we’ve never observed, but it’s just assumed to exist because it must be some sort of matter, right? Because we can’t think of anything else that bend spacetime…
- efitz 4y agoMost people with a high school education have been taught a minimal amount of skepticism about "fill in the blank" type science, like spontaneous generation or the aether. In this case, skepticism means that our STEM education worked as intended and we should celebrate that IMO. I was pretty skeptical about dark matter until I watched this presentation by Sabine Hossenfelder (she's a great explainer of science, IMO): https://www.youtube.com/watch?v=U4sw3-__pGo https://www.youtube.com/watch?v=U4sw3-__pGo I think that she did a better job of explaining why we should be open to the idea of dark matter, than the original article did.
- rolenthedeep 4y agoSabine's explanation is the most compelling I've ever seen, but I still can't get past the very obvious and much simpler explanation that we just don't understand gravity. We very famously don't have a model of gravity that works at quantum scales, and our current model clearly doesn't work at galactic scales. So our solution to this problem is... Invisible pixie dust. We've just reinvented the aether, and we're even still doing the same experiments to detect dark matter as we tried to detect aether. I really just don't buy it. Dark matter would require reworking everything from particle physics up to cosmological interactions. How is this more acceptable than our model of gravity being more wrong than it is now? We're very clearly missing something fundamental in our understanding of gravity. We know that, and we've known it for a long time. The absense of dark matter would mean that our understanding of gravity is still wrong. Having a more incorrect model of gravity involves far fewer variables than an entirely new type of matter, so why is everyone on the dark matter bandwagon?
- x-complexity 4y ago> So our solution to this problem is... Invisible pixie dust. As stated before, it satisfies the following conditions: A) It fills in the holes of existing formulas/models B) It prevents MASSIVE retooling of everything we know Until a better model comes along to explain the area filled in by dark matter, this is the best solution we have right now. It's the physics equivalent of strcpy() but without any available replacement: It works until the edge case. Right now the (big J) Job is finding physics' strlcpy(): Something that can be used in place of strcpy().
- AmericanOP 4y agoFor a lay person with an interest in cosmology, please check out the History of the Universe YouTube channel. It’s really superb: https://m.youtube.com/watch?v=fX_1pM64uUk https://m.youtube.com/watch?v=fX_1pM64uUk
- mensetmanusman 4y agoMaybe reality is more complex than beautiful equations, and that we will never be able to understand the truth of existence to infinite precision.
- flangola7 4y agoThe biggest problem about dark matter: If it is real and supposedly so common, why doesn't it exist here on Earth? What makes our world so uniquely special? Historically scientific models that treat Earth as unique in the universe always turn out to be wrong. Hundreds of years of scientific research of all kinds crawling the planet and to date no one has been able to isolate it.
- pixl97 4y agoI mean the neutrino wasn't physically discovered until 1970 and yet is one of the most numerous particles there is. It just might turn out that a particle that doesn't want to interact is going to be a real pain in the ass to discover.
- bigbacaloa 4y agoDark matter is nonsense but no one has a better idea.