Why do people in the comments
always ignore that dark matter has way more evidence for it than just galaxy rotation curves (something very well explained in the article)? It's quite tiresome to read the same objections as if physicists somehow hadn't thought of them.
Probably because it's the only piece of evidence you can understand with just high school physics. Most laypeople have trouble understanding that the universe is not expanding from a single point, so how would they be able to understand what the cosmic background radiation is, much less why its angular power spectrum has peaks at particular positions?
It's a common pattern with crackpots as well, that they focus on "big" but easy to understand things like special relativity, which doesn't involve more math than you learn in high school. No crackpot ever revolutionizes condensed matter physics or statistical mechanics.
Although the bullet cluster should be a clear indicator as well, the rest of the evidence of DM appears much more subtle to a layperson.
Anyway, cosmology is a weird topic which not only HN reveals tons of arrogance over. Lots of misplaced skepticism. (Probably similar to the Covid topic.)
Yes exactly. We've all experienced reading an account on our particular domain of expertise by a journalist and realising they have absolutely no clue what they're talking about. I find it amazing how presumably otherwise intelligent people reading Hacker News don't see the issue with offering lay opinions on Cosmology of all topics.
I think that's just a factor of the open nature of HN meaning there's always someone ready with a hot take, and plenty of social incentive to provide one. None of us is as dumb as all of us.
Well there is a connection. Often I read news articles about science where I think "Wait, that doesn’t make sense." or "Hey, this assumption is not explained at all!" and without the original paper to read (or the knowledge to understand it) the only place to take this confusion is the comments section, with the hope that someone might explain it.
Crackpots emerge when a conceptual model is weak. They don't care how strong a mathematical model is, which is why they ignore SM and other areas where "theory" literally means a mathematical model, and nothing else. Having a lot of crackpots proposing bad theories is a sign that your conceptual model is garbage, just a bad attempt to hand wave a bunch of math that happens to predict reality, and you should stop pretending.
There are infinitely many mathematical systems that could describe "reality" all with divergent behavior in the parts of the system we haven't observed yet. Trying to make authoritative claims by analogy to coincidentally correspondent mathematical systems is foolhardy and vain.
you seem to believe that there is some distinction between coneptual models and mathematical ones. Mathematics is the entirety of what we can conceive about a thing. It is not the gold standard for conceptualization, it is conceptualization itself.
> Anyway, cosmology is a weird topic which not only HN reveals tons of arrogance over. Lots of misplaced skepticism. (Probably similar to the Covid topic.)
It's the curse of being clever. People believe that since they're clever and were able to be successful at what they do, it must be that all other topics are similarly easy. And with easy access (via the internet) to the most basic introductions for lay people to any topic, the Dunning Kruger effect really kicks in.
The more clever you are, the easier it is to fall into this trap. example: "Buy twitter and solve all its problems myself".
> No crackpot ever revolutionizes condensed matter physics or statistical mechanics
Well, the memory of water still has a lot of support, as the scientific explanation for homeopathy. Lots and lots of crackpots here.
I would not call this crackpot, but it took an awful amount of time for some to let go cold fusion.
As for statistics - my personal hell is when I have to read some biology or biology-like document and there are numbers that are tortured to make them sing the way they are expected to. I do not even talk about economy, psychology, sociology and other similar ologies.
Because it is the only evidence that is presented, and lay-people can easily come up with objections. These people then express their frustration or confusion why some of these objections seem completely unaddressed.
These people are effectively asking about these objections. Perhaps many of them are phrasing it differently. But, with exceptions, I think people tend not to be angry or vitriolic. At best they are frustrated. Just treat these remarks as questions.
It might be annoying to get the same questions repeatedly, but that is a fault of wider communication about dark-matter.
Everything we see in the Universe outside our own galaxy is static. We don't have enough parallax to actually perceive objects (stars) moving outside our galaxy. Ok, there's an exception, people were able to perceive the movement of one star in one of our satellite galaxies in 2012. That's about it. So, when people talk about galaxy rotation curves, that's based on a sample of one, our own galaxy. Actually, we can plot galaxy rotation curves for other galaxies, but based on indirect evidence, such as the redshift. How accurate is that? Come to think of it, we can't estimate directly the distance to any galaxy beyond and including the Andromeda galaxy. But again, we can estimate indirectly. Only that estimation is based on a stack of estimations each with its own uncertainties.
Now, if you work in the area, stop me right here and tell me you went through that stack of estimates, and check all the uncertainties, and are confident there's no potential error anywhere.
Because, as an outsider, if I do a bit of Bayesian estimation, and I have to choose between the dark matter and the possibility that everything that can be explained with dark matter is actually a result of estimation errors, I'm somewhat on the fence. Every year that the hunt for dark matter brings nothing I'm more inclined to think there's a different explanation out there. Like the frame dragging explanation discussed in [1].
I'm sorry, but misplaced confidence like yours is the tiresome part. I think you should also apply Bayesian thinking to the whole debate as well.
I'm not a cosmologist, but I have a degree in Physics. I won't be able to fill in the details, but all your objections sound pretty basic and unfounded even to my semi-amateur ears. About the cosmic distance ladder (as if parallax were the only way. I guess we don't see Earth's curvature either), about how to propagate errors (literally learnt in the 1st week of Physics. Those bars around the scatter points are literally the error bars, and we propagate them from start to finish). And even about the scientific method itself. No one can claim "there's no potential error anywhere" in anything.
And most importantly, which was my original main point, galaxy rotation curves are not our only evidence.
BTW, I would encourage you to look at how fossils are dated. To me it sounds kind of similar in spirit to how distances are measured in astronomical scales.
I'm not that confident actually. Overall, I think it's more likely than not that dark matter exists. But I'm not 100% sure it exists either, for the reasons I listed, and more.
For example, here's a quote from the book "An Introduction to Modern Cosmology" by Andrew Little (third edition, published in 2015, page 70):
It is just about possible given present observations that this matter can be entirely baryonic, since this is marginally consistent with Equation (9.3). However, many models based on low-mass stars and/or brown dwarfs have been excluded adn it is probably difficult to make up all of the halo with them. A popular alternative is to suggest that this density is in some new form of matter, which is non-baryonic and only interacts extremely weakly with conventional matter.
It looks like you are entirely convinced that dark matter exists beyond any reasonable doubt. Good for you. In the end, this doesn't affect your life or my life all that much.
You do realize it is still about missing mass, don't you? What it is indeed up to debate. The whole distance measuring can be tuned as well, but suggesting "we don't measure it through parallax therefore we can't even put error bars to the distance and velocity of stars" is totally unjustified.
Again, think of your area of expertise and apply Bayesian thinking to an amateur coming from outside and claiming things like you're doing here.
Actually, I'm familiar with the cosmic distance ladder [1]. The thing is, you stack up some models on top of some other models. You can propagate the errors if your models are correct, but not if the models themselves are incorrect.
But this is a bit orthogonal. That was not the gist of my argument.
My argument was that lots of things in astrophysics are based on models. The rotation curves themselves, as a sibling comment mentions, are fairly directly observable, via redshift. But that's not fully true. Doing spectral observations is not entirely trivial, see for example [2]. Take a look at the sample profiles [3] and see how noisy they are. Of course, you can remove all sorts of noise with enough observations, and since (as I said before) the universe is static, and not going anywhere, you can point your telescope at the same galaxy and take lots and lots of pictures and do lots and lots of long slit spectroscopy from all sorts of angles, until you are left with very little noise (aside from the "thermal" noise due to the stars having some random velocity component).
How difficult, or easy is the math of eliminating that noise. Most likely it's not super difficult. Not trivial either. Probably somewhat similar to the math used for doing computerized tomography.
But here's the crucial difference. Between CT scans and astrophysics. And between my area of expertise that you encouraged me to consider and astrophysics. If the math for CT scans is wrong, people die. If the math I do for a living is wrong, people lose money (and I lose my job). This math with immediate consequences is self-correcting. If the math to denoise the galaxy rotation curves turns out to be wrong, someone just publishes a paper, and then gets to do a lot of talks at a lot of conferences.
Let's move on to the other evidence. For example the relative abundance of hydrogen, deuterium, helium, lithium. That is based on some models of nuclear interactions. Which agree with our experiments (in particular the crucial part is that neutrons have a half life of 600 seconds, and not, say 100 seconds). But nucleosynthesis happened only for the first 3 minutes after the big bang or so. Can we simply assume that general relativity effects were negligible? Let's put it differently: the whole universe was like a neutron star. Astrophysicists state that modeling neutral stars is difficult because quantum chromodynamics and general relativity happen at the same time, and we have nothing similar that we can study. But then how can we be so confident in our modeling of the early seconds of the universe, when, if anything, conditions were even more extreme.
The bullet cluster. I'm not going to point you to Sabine's blog post of how the bullet cluster is, if anything, a proof against dark matter. You can find it elsewhere in this thread.
Gravitational lensing. That's an impressive thing, but the claim that there can't be small black holes because we'd be able to detect them via gravitation lensing sounds to me quite silly. The space is just too large.
Now take our interstellar visitor Oumaouma. It just zipped by. Can the dark matter be just a large collection of such asteroids? Maybe not. But maybe at least a part of it can be explained by it.
Galaxy-wide lensing. The claim is that the effect of lensing is consitent with the mass of a galaxy being much higher than the one explained by its luminosity. But we know that all galaxies have a giant black hole in the middle. Can that black hole be bigger than we thought? No, you say, because of the rotation curve. But what if the rotation curve is erroneous?
Back to the rotation curve. Is there actually such a thing? Are the stars in the spirals moving at the same angular velocity as their cousins between the spirals? Most likely not. Is it possible that the galaxy rotation curve is a result of the non-homogeneity of the galaxy and the finite speed of the propagation of gravity (the speed of light) ? Here's how this would go: the attraction a start feels towards the center of the galaxy is governed not by what is between the star and the center now, but what it was a few thousand of years ago, when the spiral was a bit back, and so more mass was there. Conversely, the gravitational attraction away is governed by what it was there thousands of years back, when fewer starts were there.
You could say astrophysicists have thought of all these things, and about a thousand more. But do you see how everything depends on models? Models that can't be easily validated.
Is it plausible that there is dark matter? Absolutely. I don't see anything outrageous around the proposals that there are particles out there that interact only via gravity. But is it possible that there could be a different explanation that to various questions that does not require dark matter? That does not sound all that absurd to me.
I'm encouraged to reply by your reference to a (undergraduate cosmology) textbook in your more recent comment in this thread. I'd suggest borrowing (or taking advantage of a free-as-in-beer Internet copy of) the most recent edition of Binney & Tremaine's _Galactic Dynamics_, a standard textbook for graduates (and a good reference for researchers) that explores in detail how galaxy mass distributions are calculated. FWIW Binney has from time to time explored and even embraced MONDian ideas, and this is captured in his publication record.
In their textbook you'll find that galaxy rotation curve studies are spectroscopic. That is they are keenly interested in the relative redshift in the 21 cm neutral atomic hydrogen lines (among others like H2 and CO molecular lines) at the limbs of edge-on disc galaxies, and adapting that to spirals and other disc galaxies that are tilted away from edge on. [Binney 2e sec 6.1]. Practically invariably the relevant lines are relatively redshifted and relatively blueshifted at the limbs, leading to the interpretation that generically there is equatorial spin in disc galaxies.
In elliptical galaxies there is practically no equatorial spin to speak of; instead the spread of relative redshift across the face of the elliptical is interpreted as blobs of hydrogen gas moving radially, that is sinking deeper into the galaxy or rising out of the galaxy's depths. It is also useful to study a wide range of absorption lines given that the clouds are backlit by a galaxy's worth of starlight (and sometimes a quasar), in a process which grinds out surface densities.
These relative redshifts do not depend on cosmological redshift (the whole galaxy, or its whole cluster, is cosmologically redshifted identically for all practical purposes, so the opposite limbs in discs are affected similarly). It is also not sensitive to an isolated galaxy's peculiar motion within a cluster. It may matter for merging galaxies.
We can also look deeper than disc limbs and ellptical surfaces. Optical interferometry is highly sensitive in this application, and provides direct evidence of the motion of the various sources of emission and absorption lines from various gas clouds, dusts, and even starlight. The Large Binocular Telescope does some work in this area. Radio interferometry is useful for looking into the bulk motions within the more central regions of galaxies and clusters; dust obscures optical signals but millimetre signals cut through.
The investigated starlight is bulk and while the interpretation depends on assumptions about the bulk stellar chemistry of an observational target (maybe metallicity varies slightly in different parts of an elliptical which might have a history of galaxy mergers) there is in no way a dependence upon any single star and its meanderings through its galaxy. We're interested in the light generated by ~billions of stars, not the positions or momenta of single stars, mostly because we just cannot resolve the latter with current technology.
> as an outsider
You seem interested in the topic. Learning how observations are made (and the history of them) is probably not inaccessible to you given your comments here. Whether that leads you into any sort of conclusions about the structure and evolution of galaxies is up to you, but I think textbooks will be better for you than whatever ultimately led you to the stackexchange link in your comment. (I did notice however that the Ciotti preprint discussed later in your link cites Binney & Tremaine multiple times, as does the Ludwig paper in the stackexchange question).
I don't particularly buy that a gravity-only interacting particle "is not physics". If that is what exists, that is what exists. And we've already detected it via dark matter observations. So it is physics. What we can't do is ever really prove it other than by exclusion, but if we racked up a thousand years of failure, we'd have to accept that it was real (hopefully by then not dogmatically and we'd let any kid who wanted to tilt at windmill to go for it).
And Woit links to a paper on dark matter being only a right handed majorana neutrino, requiring no real physics beyond the standard model (plus three right handed non-interacting neutrinos): https://arxiv.org/abs/1803.08930
And links to a comment on that in John Baez's blog:
Another big problem is that all LCDM predictions (MACHO/WIMP) were disproven but it still remains a theory. Now we need energy too high to use in universe.
It's eerily similar to Phlogiston theory that at one point was considered to have negative mass.
Reading this I realized that dark matter is starting to sound somewhat like the idea of the "ether". It's everywhere, it affects everything, but we can't see it or interact with it easily. It's somewhat different than the idea of the ether in that it is not thought to be a homogenous, pervasive field; instead it clumps around gravity. But it has some very ether-like aspects, maybe some we have not discovered yet. I always thought we abandoned the idea of an ether too soon. Why does it have to be evenly spread? Why does it have to respond to things like direction or EM waves? Maybe there are some simple experiments that could explore the idea of an ether/dark matter further. Pure speculation but that is my intuition.
Dark matter and dark energy are like what if there was no Einstein to discover relativity in early 1900s. We later continue to discover that gravity can't explain planetary motions correctly and therefore search for any explanation.
There are more laws of Physics waiting to be discovered. At least two big ones from what we currently observe.
You are cherry-picking one particular example. The neutrino and the planet Neptune were "dark matter" for decades until better detectors became available.
There is certainly new physics involved, the issue is more that for dark matter, the evidence points to matter that hardly, or perhaps not at all, interacts* with the stuff we are familiar with (standard model). It might be that the correct theory of dark matter is already found and debated, we just have no way to tell it from the false ones, as for all that we know, dark matter might be a shadow world of spaghetti monsters of which we only know its existence via their gravitation.
> the evidence points to matter that hardly, or perhaps not at all, interacts* with the stuff we are familiar with
I think this is why people get frustrated with the DM story. It's not the evidence that points to that. It's the lack of evidence that points to that. Specifically speaking, we keep hypothesizing limits on interaction in the hopes of finding it below some threshold but keep pushing those limits back. A multimodal observation would be clutch, but we keep confirming that a multimodal observation can't happen.
My intuitive objection against dark matter is that it's basically an infinite amount of additional parameters to be fitted to the observations. Galaxy behaves weirdly? Add dark matter! It behaves as expected? It obviously has lost its dark matter! It stands to reason that for every possible observation, a particular DM configuration can be calculated.
I fail to see how dark matter could ever be falsified. The best potential replacement would be a theory that's simpler, but even then dark matter would probably explain the same observations.
The second question is: Is dark matter even matter if it only interacts via gravity? Wouldn't it be better named a property of space-time? That is, dark matter is a name for a recursive property of gravity?
You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy. Instead you 'just' need to add a weakly interacting particle with the right density (30% of critical). That particle needs to be 'slow enough', i.e. non-relativistic. After that it turns out that the evolution of the universe basically reproduces the vast array of observations.
* The right primordial abundance of different elements
* The amount of structure in the cosmic microwave background.
* The rotation speeds/velocity dispersions in large number of galaxies
* Large scale structure/The amount of clustering of galaxies.
* Gravitational lensing observations.
And much much more.
It is certainly true that there are individual cases that look challenging for Lambda CDM paradigm (CDM stands for Cold Dark Matter), but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of physics at a variety of scales, so I do not think it there is a strong case against CDM. (there are currently some tensions with the Hubble constant measurements and some other cosmological parameters between different techniques, and it remains to to be seen whether those are systematic errors or point towards some modification in the theory).
Comment on the article itself. I think at some point I would be fine with dark matter being essentially a computing trick, i.e. if the behaviour of the universe is well reproduced when we add gravitationally interacting but not interacting otherwise matter. It would be up to theorists to see if somehow that can be written as some sort of extra-term in Einsten's Equations and show we don't really need an actual particle. I think there is a possibility of that, but personally I still think it's more likely there is a particle responsible for dark matter.
> You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy.
No, he's correct that each galaxy does need tuning. Specifically, the amount of DM and it's specific distribution is tuned to match each observation, which is what the OP said.
> but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of physics at a variety of scales, so I do not think it there is a strong case against CDM.
I think this "strong case" for LCDM is often oversold. See this review of the evidence for LCDM compared to modified gravity:
> the amount of DM and it's specific distribution is tuned to match each observation
This is simply not true.
No cosmological theory tries to explain why each star is in its exact observed location. They try to predict the statistics of galaxies, star formation, etc.
If you run a simulation of the universe, beginning with very simple, random initial conditions, and then run physics forward for 13 billion years, you get a universe that statistically looks almost identical to our own. You get the same mix of spiral and elliptical galaxies as observed, clustered together in the same way, with the same star formation history, etc. You don't get exactly our universe, with the Milky Way exactly here and Andromeda exactly there, because you begin with random starting conditions. However, you get a universe that is statistically identical in most ways.
Note that this is not the case for Milgromian theories. So far, no one has managed to create a theory without dark matter that can achieve this. Alternative theories generally get one or a few things right, at the cost of getting a whole host of other things wrong.
It's not that astrophysicists a priori like dark matter. It's just that dark matter is a very simple paradigm that explains a huge number of phenomena with only a handful of free variables, and none of the alternatives work so far.
> No cosmological theory tries to explain why each star is in its exact observed location.
Nice strawman.
> Note that this is not the case for Milgromian theories. So far, no one has managed to create a theory without dark matter that can achieve this.
LCDM also fails at this (see Dwarf galaxy problem, for one example), so this is not a point in your favour.
> It's just that dark matter is a very simple paradigm that explains a huge number of phenomena with only a handful of free variables, and none of the alternatives work so far.
LCDM doesn't work that great either. See the review I linked.
I assume you're referring to the "missing satellites problem." This problem is already essentially solved:
1. Newer, more sensitive surveys have detected more Milky Way satellite galaxies.
2. Reionization and supernova feedback quench star formation in small dark matter halos, meaning that the smallest dark matter halos never form galaxies.
> See the review I linked.
I think we've talked about this elsewhere on HN before, and I pointed out that the paper you're linking to gets a lot of things wrong and is published in an obscure journal (presumably because it wouldn't survive peer review at any of the major journals).
"Essentially solved" is at best a euphemism for "we speculate that this will solve it". Hardly conclusive.
> I think we've talked about this elsewhere on HN before, and I pointed out that the paper you're linking to gets a lot of things wrong and is published in an obscure journal (presumably because it wouldn't survive peer review at any of the major journals).
A random HN comment claiming that a paper "gets a lot of things wrong" is not more convincing than the published paper itself. If you have an actual published paper or other review that addresses those points, then please point that out.
Speculating as to why they published in an "obscure journal" can be explained by poor quality of the work, but it can also be explained by rampant disinterest in challenging orthodoxy, which also has a documented history of happening.
Edit: to be clear, I'm not agreeing with the authors that MOND should be favoured given the evidence, but I disagree with your apparent position that MOND should be disfavoured and that LCDM is largely unproblematic.
> "Essentially solved" is at best a euphemism for "we speculate that this will solve it". Hardly conclusive.
No, it means that it's actually been solved. If you compare the theoretically predicted number of satellite galaxies to the observed number, there's no discrepancy any more. There was a discrepancy a decade ago. There isn't any more, both because of better observations (which detect more faint galaxies) and because of better understanding of the consequences of LCDM (in particular, how reionization and supernova feedback affect star formation in low-mass galaxies).
> rampant disinterest in challenging orthodoxy
That's just not the case. There's huge interest in challenging orthodoxy. If there were a MOND-like theory that worked, there would be tons of astrophysicists who would be very excited to work on it. The problem is simply that none of the proposed MOND theories match the wide range of observations that LCDM does. LCDM is a simple theory that works remarkably well, much to the chagrin of the astrophysics community. It would be really exciting to find a problem in LCDM, and a lot of astrophysicists spend their time searching for such problems. See, for example, the excitement generated by the possible Hubble constant tension.
> If you run a simulation of the universe, beginning with very simple, random initial conditions, and then run physics forward for 13 billion years,
This is actually exactly wrong. If you run LCDM for 13 billion years you don't get galaxies till much later than what we observe via say JWST. However, with MOND you predict early galaxies due to the higher force of gravity at distances causing earlier collapse.
> If you run LCDM for 13 billion years you don't get galaxies till much later than what we observe via say JWST.
I would not take all the recent claims of extremely high-redshift galaxies, found in initial JWST data, at face value. Several of these claims have already been shown to be false.
A single variable being different for each galaxy isn't a problem per se. If a bunch of other factors work out with just that one adjustment, that's pretty good validation that that one adjustment is meaningful. If, on the other hand, your theory could be adjusted to allow for any possible combination of observations, that's when there's a problem. From what I've read, DM theories do a pretty good job of correlating things like rotational speed, luminosity, and galaxy formation. If you had to change one number for luminosity, one for rotational speed, and another to get the right make up of galaxies for the universe, that's when you start having a "tuning" problem.
> A single variable being different for each galaxy isn't a problem per se. If a bunch of other factors work out with just that one adjustment, that's pretty good validation that that one adjustment is meaningful.
I agree, but isn't that basically what MOND does too? It made one small tweak to gravity, and all of a sudden we can successfully predict a range of galaxy properties with a pretty tight margin for all galaxies, where LCDM can't make such precise predictions because each galaxy's DM distribution can only be tuned after its been observed, and the number of free parameters needed for tuning means the margin is very wide. That's great for MOND but doesn't bode well for LCDM.
From direct observations, MOND is strong where LCDM is weak and vice versa. I think LCDM just gets too much credit and more alternatives need to be tried. There's almost certainly something to MOND because the observations it matches and the successful predictions it made would be too coincidental otherwise.
> It made one small tweak to gravity, and all of a sudden we can successfully predict a range of galaxy properties
MOND gets one or two properties of galaxies correct, at the cost of messing up huge numbers of other observed facts about the universe (such as the abundances of chemical elements or the CMB power spectrum).
Also, you should realize that MOND in itself is not actually a theory of nature. You can't just "hack" Newton's 2nd law, because that law is a consequence of other, more fundamental principles. If you want to modify gravity, you have to modify General Relativity. There have been many attempts to modify GR in order to create a MOND-like theory, but they tend to fail in various ways (again, by predicting things that are empirically false). So far, GR passes every empirical test of gravity physicists have come up with.
> Also, you should realize that MOND in itself is not actually a theory of nature. You can't just "hack" Newton's 2nd law, because that law is a consequence of other, more fundamental principles.
That's not how science works. You absolutely can write a formula that matches observations without justifying it or deriving it from first principles, ie. "hacking" Newton's second law is totally fine if it works. If an argument from principles reaches different conclusions that don't match observations, then the argument is flawed or your understanding of the principles is wrong.
If such a hack is empirically successful for gravity, then it suggests there's likely some kind of underlying property missing from your model that yields this emergent order. For instance, superfluid dark matter models reproduced MOND at galactic scales from a dark matter model. That's the novel kind of thinking we need, not this stubborn protectionism.
> So far, GR passes every empirical test of gravity physicists have come up with.
Don't pretend there aren't huge gaping holes you could drive a dump truck through [1]. GR is well-tested and well-motivated at solar system scales, but there are many open problems at larger scales that ultimately stem from assuming GR continues to hold at those scales.
> You absolutely can write a formula that matches observations without justifying it or deriving it from first principles, ie. "hacking" Newton's second law is totally fine if it works.
Physics is an interconnected whole. You cannot simply hack whatever formula you want. Newton's 2nd law is a consequence of Relativity, in the low-velocity limit. You cannot hack Newton's 2nd law without changing Relativity, which means changing basically everything in physics, including extremely precisely measured quantities like the energy levels of the hydrogen atom.
That's why MOND itself isn't a theory of nature. You have to modify General Relativity to produce MOND-like effects. It's very difficult to do that without messing up other parts of the theory.
> Don't pretend there aren't huge gaping holes you could drive a dump truck through
Every precision test of gravity produces results that match GR to within experimental precision. No, these are not just tests within the Solar system. To give you one example, gravitational waves have now been verified to travel at light speed, which rules out large classes of modified GR theories.
> You cannot hack Newton's 2nd law without changing Relativity
Yes, changing some aspects of relativity is what will ultimately happen. We already know relativity is technically incorrect because of its singularities.
And yes, you absolutely can hack Newton's 2nd law if it's empirically supported. It's frankly bizarre that you keep insisting that you can't do this.
> which means changing basically everything in physics
Don't be ridiculous, GR absolutely does not have an impact on "everything in physics".
> No, these are not just tests within the Solar system.
I didn't we've only tested GR within the solar system, I said GR was only well tested within the solar system. There's a big difference and you seem to have a habit of attacking strawman like this.
> And yes, you absolutely can hack Newton's 2nd law if it's empirically supported. It's frankly bizarre that you keep insisting that you can't do this.
You think it's bizarre that you can't arbitrarily change one prediction in a large, densely interconnected theory? Newton's 2nd law is a consequence of a much larger theory. In order to change it, you have to change the theory that gives rise to it. That will have huge numbers of consequences, but just for Newton's 2nd law, but for pretty much everything else predicted by the theory.
> We already know relativity is technically incorrect because of its singularities.
No, we don't know this. All modern theories are relativistic. We know that General relativity (a.k.a. gravity) has to have some sort of more fundamental quantum analog, but that quantum theory will also be relativistic.
> Don't be ridiculous, GR absolutely does not have an impact on "everything in physics".
You don't need GR to get Newton's 2nd law. You only need Special Relativity. You can't "hack" Special Relativity without doing incredible damage all across physics.
> I said GR was only well tested within the solar system
Wrong. The propagation speed of gravitational waves, the inspiral of binary pulsars, strong gravitational lensing (including around directly imaged black holes), the strong equivalence principle as tested by a triple system with a pulsar, and many other tests.
> A single variable being different for each galaxy isn't a problem per se.
As John von Neumann said:
With four parameters I can fit an elephant to a curve. With five I can make him wiggle his trunk
One parameter can do a lot of work. And whether or not astrophysicists are right about dark matter, they’re certainly very very smart and good at math.
> No, he's correct that each galaxy does need tuning. Specifically, the amount of DM and it's specific distribution is tuned to match each observation, which is what the OP said.
Not true. The LCDM predicts the DM distribution with a typical DM profiles, and then the amount of baryons is heavily dependent on star formation, etc. Therefore each galaxy of the same luminosity will have somewhat different dark matter halo. Those are not tuning parameters at all.
Regarding the linked article, it an article written by proponents of MOND and is not objective. That is fine, but at the moment I don't believe MOND is a serious alternative, and the majority of the astro community would agree. Just the fact that MOND still requires dark matter anyway to reproduce CMB is major problem for it. But if someone develops the version of MOND that has equal predictive power to LCDM, I'll be happy to accept it, but we are not there.
Then explain how MOND is able to make predictions that don't need such considerations. This suggests either that dark matter is superfluous, or that dark matter has some underlying order that reproduces MOND in some limit, thus eliminating the tuning needed. This is the approach taken by some superfluid dark matter models, which at least is not ignoring the problem.
> Regarding the linked article, it an article written by proponents of MOND and is not objective
Right, because anti-MOND or dark matter reviews are totally objective?
> Just the fact that MOND still requires dark matter anyway to reproduce CMB is major problem for it
It's not at all a problem to say we need DM like sterile neutrinos, since DM alone is insufficient to adequately explain all observations anyway.
Well, yes. That's because we have a pretty strong prior about the existence of planets orbiting stars. It was there before we started adding planets to explain the stars wobbles.
With DM the rationale goes the other way around. What means it's not a solid explanation.
That is perfectly fine, as long as proponents don't go saying it's settled knowledge and start fighting the people looking for alternatives. When they start doing that, they stop being scientists, and start being believers.
> With DM the rationale goes the other way around.
No it doesn't. We have strong priors that the movement of bodies is affected by gravity, and gravity comes from matter. When we see movement we cant' explain given the matter we can see, the most obvious explanation is matter we cannot see for some reason.
> ...and? If we look at the way a star wobbles, then tune the amount and distribution of planets around it to fit, isn't that just science?
That is science because gravity has been extensively tested and confirmed in that regime.
Gravity on the scales where dark matter is relevant has not been tested to this extent, so we're just assuming our theory is correct in this new regime and thus inferring the DM distribution based on that assumption and then saying, "this must be the case because we understand gravity". Except as I said, our understanding in that regime has yet to be confirmed, so that's ultimately a circular argument.
By contrast, modified gravity theories can match many of the predictions of DM theories using only direct observations of visible matter. It actually made some successful predictions before observations were made, where those observations required adjusting DM models to fit. Successful prediction with fewer parameters is exactly what you want from a scientific theory, and that's what MOND has done. See the review I linked for more information.
At this time, neither theory is fully satisfactory as discussed in that review. Superfluid dark matter models were compelling for awhile because they combined MOND and dark matter in a compelling way, but the early models were also inconsistent with some other observations. This is far from a solved problem and the LCDM model gets too much credit and attention given its problems.
> Gravity on the scales where dark matter is relevant has not been tested to this extent, so we're just assuming our theory is correct in this new regime
Couldn't we just as easily say the same about EM or any of the other forces?
The reason we think it is some kind of matter is because we know that matter interacts with gravity in ways that would explain our observations, as long as we assume that matter isn't visible for some reason. And it isn't like there aren't candidates even within the current standard model that might have the right properties.
> By contrast, modified gravity theories can match many of the predictions of DM theories using only direct observations of visible matter.
...and curve fitting. They keep having to add parameters to MOND to make it fit what we see and currently attribute to dark matter, like the bullet cluster. That's the physics equivalent of code-smell.
> Couldn't we just as easily say the same about EM or any of the other forces?
We've tested other forces in more regimes than gravity because gravity is so weak, but in principle, if predictions of those theories mismatched observations to the same extent as gravity, then we absolutely should be questioning those theories too.
> They keep having to add parameters to MOND to make it fit what we see and currently attribute to dark matter, like the bullet cluster.
I don't know why people keep saying this like cold dark matter model doesn't do this. LCDM has been refuted many times if this is your criterion. The review paper I cited shows numerous examples of parameterization and curve fitting for LCDM.
There is only a handful of parameters. They describe the probability distribution of finding one particular matter configuration. We are simply observing one realization of that probability distribution, in which we should be able to calculate the percentage of galaxies with an unusual amount of dark matter for any definition of "unusual". We don't consider the position and momentum of every baryon a parameter of the standard model either, so why should we do so with DM?
DM can be falsified perfectly well. If the rotation curves of all observed galaxies behaved as predicted by Newton, there would be no DM and every reasonable physicist would consider DM falsified. DM simply passes a lot of falsification experiments.
If DM interacts purely gravitationally, then yes, considering it either matter or an artifact of space-time is merely a matter of preference. Simply move the DM part of the energy-stress tensor on the other side of the Einstein equation.
Differentiating it from MoND or similar theories will then be based purely on how well the theory fits.
However, while I'm only a cosmologist and not a particle physicist, I find the implied assertion by proponents of MoND, that the existence of non-interacting particles is impossible, questionable. Why shouldn't there be heavy particles that don't interact electromagnetically, like a heavy neutrino? Or particles that don't even interact weakly? Or at least, why should their abundance be low compared to baryons?
Edit: Regarding falsification of DM, it's not like DM isn't challenged anywhere. Like I mentioned above, we should be able to compute the abundance of galaxies of certain sizes (dark or not), and the dwarf galaxies are coming up short. It's called the dwarf galaxy problem and poses indeed a challenge to DM, albeit not as much as DM alternatives in the mind of most cosmologists. They think it will be solved eventually as our understanding of galaxy formation improves, but who knows, maybe it's the first glimpse at something bigger. https://en.wikipedia.org/wiki/Dwarf_galaxy_problem
> However, while I'm only a cosmologist and not a particle physicist, I find the implied assertion by proponents of MoND, that the existence of non-interacting particles is impossible, questionable.
I think this is a strawman. MOND proponents don't discount that some observations are better suited to a particle, they just assert that the particle dark matter sledgehammer is vastly overused, and that modified gravity has been unusually successful in cases where it had no right to be if gravity acts as it does under GR.
For instance, this review [1] of the astrophysical evidence by a MOND proponent makes a good case for MOND + sterile neutrinos. This review also covers numerous falsifications of DM, so if you're correct that DM can be falsified, why did these falsifications fail to convince the majority?
>why did these falsifications fail to convince the majority?
Because they are cherry picked. In the big picture, for every area where particle DM struggles, you will find a dozen datapoints where MOND struggles. Physicists are also wary of approaches like MOND+particle DM, because it just introduces more parameters which you can arbitrarily tune to explain data. It feels like every week someone comes with a new model that fits certain edge cases better, but doesn't actually tell us anything new or could act as a reasonable guide for future work. You can imagine it like fitting a polynomial of degree N to N data points. Yes it will fit perfectly, but that doesn't mean it's a good model. This is just sophisticated curve fitting to generate papers and not good science.
> Physicists are also wary of approaches like MOND+particle DM, because it just introduces more [...] You can imagine it like fitting a polynomial of degree N to N data points. Yes it will fit perfectly, but that doesn't mean it's a good model. This is just sophisticated curve fitting to generate papers and not good science.
That's exactly what's happening with cold dark matter models. I agree the state of things is unsatisfactory for either theory, we need bolder ideas. Superfluid dark matter was promising but doesn't currently match some observations. We need more of that.
This seems like a very nicely detailed response to OPs question. Thanks! Your response brought up some questions and remarks from myself.
> DM can be falsified perfectly well.
I recall hearing that DM was attempted to be falsified based on gravitational lensing. Is that correct?
> If DM interacts purely gravitationally, then yes, considering it either matter or an artifact of space-time is merely a matter of preference. Simply move the DM part of the energy-stress tensor on the other side of the Einstein equation.
Would DM as matter not have inertia, whereas treating it like an artifact of space-time would leave it without inertia?
> Differentiating it from MoND or similar theories will then be based purely on how well the theory fits.
A simple question here would be: do all galaxies of similar size have the same amount of dark matter. If so, then the uniformity of dark-matter would need explanation. If not, then I think something like MoND would have a hard time explaining why these similar galaxies rotate at different speeds.
On the contrary, if dark matter were to interact purely gravitationally, then there would be little friction to slow it down. This would prevent dark matter from clumping, which is in agreement with observations.
I don't think so. Say you had two stationary whatevers of dark matter 1 meter apart alone in the universe. At t=0 they accelerate towards each other. At some time they cross and occupy the exact same point. R becomes 0 and the force becomes infinite. They should be stuck together.
If you haven't yet, you should read raattgift's comment. It's great, and it goes in to more details than I can bother with.
But I will briefly point out the flaws with your thought experiment:
1. Point-like particles don't exist, so the force will never become infinite. In fact, this is the wrong paradigm all together -- if you're talking about two particles "colliding", then what you're really referring to is their interaction cross section (which you can think of as being their probability of interacting). Neutrino (weak interaction) scattering is incredibly rare. Gravity is much weaker still. The particles would indeed pass right through each other then oscillate back and forth (not forever, but practically so).
2. You have assumed that both particles begin with a relative velocity of 0. This is not a good assumption. In the real world, two dark matter particles would have random relative velocities according to some distribution, which means they would have angular momentum relative to each other. Again, that angular momentum can only dissipate through friction. If the particles interact solely gravitationally, then they can only lose energy via gravitational waves (i.e., extremely slowly).
You probably know that the Earth formed due to gravity. But gravity is only half the answer -- you also need something to slow the matter. On Earth, that was the electromagnetic force. Purely gravitational matter has only gravity.
The key weakness in your comment's parent comment was, "occupy the exact same point".
One might ask why shining a flashlight during broad daylight (or say while doing work around the international space station) doesn't produce black holes. Or why there aren't black holes in the first image at <https://en.wikipedia.org/wiki/Caustic_(optics)>.
Particle DM is still quantum particles, with Compton and de Broglie wavelengths. A cold milliectronvolt axion has a wavelength on the order of a kilometre. How do you trap a pair of them in a volume comparable to the Schwarzschild radius of about 10^-50 metres?
Additionally, an appreciable mass-energy of dark matter (one you can expect to be described tractably by the Raychaudhuri equations) will be a many-particle system occupying a noncompact volume.
Newton's constant G in SI units is the force in Newtons between two one-kilogram masses at rest with respect to each other at one metre apart. It's small (< 7e-11). You will wait a long time -- about a full Earth day -- for two kg of anything initial 1m apart to get very much closer to each other because of (only) their mutual gravitational interaction.
However, each of the pair of kg-mass blobs of DM particles is not likely to stick around intact that long because (by virtue of them being weakly interacting particles) nothing around them is strong enough to keep them from running away due to thermal motion. They are only cold in the sense they move slowly compared to hot dark matter in the form of relativistic neutrinos; they can still move about at many m/s, which is a lot faster than m/day. Newton's constant is rather small, and rather too tiny when thinking of coaxing a couple kg of CDM into a volume comparable to the Schwarzschild radius of about 10^-27 metres.
One might compare this with two kg-masses of water ice. In a ball of ice various interactions trap molecules within the ball even against sublimation. The ice balls will come into contact in about a day. Thus we can have things like Saturn's rings or periodic comets, helped by the non-gravitational interactions of water ice keeping the mass-energy-momentum from flying away before gravity can do its work.
Indeed, keeping galaxy and galaxy-cluster dark matter halos from flying away puts an upper bound on their temperature (if you heat up the water ice above to a couple hundred kelvins it can sublimate away in minutes). Halo DM needs to be cold in order for it to stick around.
tl;dr, two sets of non-self-interacting (collisionless) dark matter just slide right through each other. cf. the famous Bullet cluster collision. You'd need at least a galaxy worth of dark matter and at least many millions of years of Chandrasekhar dynamical friction (radiating away very low amplitude gravitational waves as structures ("subhalos") form and move through each other) to make a black hole from DM alone.
Thanks, the comparison with light was illuminating (eh!).
I was going to say something about how we don't really know how gravity works at the infinitesimal distance level as we do not have a quantum gravity theory, but IANAP.
Not sure what you mean at "infinitesimal distance level".
With the caveat to follow, as one takes the wavelength of a photon towards zero (i.e., ultra high energy gammas), its contribution to the stress-energy tensor's expectation value at a point climbs. In turn the average radius of curvature around that point becomes smaller than the gamma's wavelength.
When we are in that regime measuring distances by wavenumber and measuring times by photon frequency gives useless results. Likewise, infrared photons with wavelengths comparable to the diameter of a black hole and finding themselves lurking around black holes, that is hard to understand too.
Now we introduce the foreshadowed clarification.
We can always find some ultraboosted observer who would (in its rest frame) measure that a photon emitted from your computer screen as an ultra high energy gamma ray. This doesn't mean your computer screen is creating small black holes. After all we can always find some other observer who sees a photon from your screen as having a wavelength of thousands of light-seconds.
So let's consider instead a photon scattering inelastically off a charged particle. The incoming photon momentum need not equal the outgoing photon momentum. Some of the momentum can be transferred to the charged particle ("recoil"), but some of the momentum can be absorbed into a composite charged particle's internal degrees of freedom. That might lead to, for example, <https://en.wikipedia.org/wiki/Photodisintegration>.
But as we take the incoming photon energy ever higher, and deposit more of it into the charged particle, we start growing the stress-energy tensor's expectation value in the region of the scattering as above. The radius of curvature shrinks, and in extremis may keep daughter products (low energy outgoing radiation ranging from the outgoing scattered photon to the results of photonuclear and photosubnuclear processes; we can also get pair production from a high-energy charged particle inverse-compton-scattering a photon to sufficient energy) from leaving the immediate region.
What happens then? Semiclassical gravity, which is quantum field theory for matter on a classical curved spacetime background, gives us confusing results because the excitations in the quantum fields don't localize like a classical theory, and our averaging procedure (to obtain the expectation value) gives us a smeared stress-energy that less and less resembles the quantum mechanical system (and consequently the generated curvature looks unphysical).
The radius of curvature and the Compton wavelengths of the particles in the strongly curved region are small, but not infinitesmial.
"Possible Connection Between Gravitation and Fundamental Length ", Mead 1964 <https://doi.org/10.1103/PhysRev.135.B849>, which you can access by pasting a Swedish scy hob prefix in front, takes the priority in the development of this line of argument. He finds that the characteristic length scale is on the order of 10^-35 metres. (cf. <https://en.wikipedia.org/wiki/Planck_units>).
But we can have two 10^-35 m gammas pass right through each other without engaging this argument. The gammas do indeed generate stress-energy, but as noted before one can always find observers who will see these gammas as infrared, and their sky won't be full of black holes in one direction and devoid of any photonuclear interactions in the other. (We can even have extreme accelerators ("Unruh observers") who count multiple gammas where non-accelerated observers see just one -- in principle acceleration can make the observer's particle count climb arbitrarily high, but that doesn't result in the observed system becoming heavy, much less a black hole).
The wavelength (and particle number) in some appropriate inertial frame of reference for the scattering detailed above is what we want when analysing this type of interaction. This is usually the "COM" frame <https://en.wikipedia.org/wiki/Center-of-momentum_frame#Speci...>, which can be calculated by arbitrary observers of our charged particle + photon (even ones who want to count more particles than an inertial observer, or redshift everything to very low energies).
tl;dr: What we're doing above is building a COM frame with a small spatial volume and high energy and asking what is the calculated curvature in that part of spacetime. That gets messy long before we have to bring in infinitesimals or start to worry about making a small black hole.
[My lack of response should not be taken as lack of appreciation for your answer. It is tremendously insightful but as I said, IANAP, so a lot of it yet flies over my head.]
No, I mean they will run through one another and continue in the directions they have flown previously, but now slowing down rather than accelerating, and would repeat that loop endlessly. Particles are point-like, they can't "collide".
Also, if there's 5x as much dark matter in the universe why is our solar system so devoid of it? We should be able to detect the equivalent of dark matter suns or planets around us.
All of the below is according to theory, not claiming it to be true.
Because DM passes right through itself it doesn't clump like the stuff that made our planets do. Instead it remains spread out bound by gravity but not clumping. That makes it incredibly diffuse. But space is also quite big, so it can still contribute a lot of mass.
Dark matter is not a “recursive property of gravity” because it’s not consistent like that, as your own initial objection suggests.
The concept of a particle that only interacts via gravity is straightforward. Consider neutrinos, which only interact via gravity and the weak force. Trillions of them pass through your body every second. Take away the weak force interaction and add some mass, and you have a dark matter candidate particle.
> I fail to see how dark matter could ever be falsified.
You’d have to explain what should be done about that. It could nevertheless be a correct theory, and we might one day verify it more directly.
Naive falsificationism is an almost useless principle at the frontiers of modern physics. We’re long past the days when you could run some simple experiments and come up with a clear yea or nay. That’s living in the 19th century, basically - which admittedly, many people still seem to want to do.
> You’d have to explain what should be done about that.
Well, there should be something that follows from the assumption that there are huge amounts of particles that only interact via gravity. What's the prediction that this model makes? If it doesn't make any useful prediction, it's pretty much worthless.
> That’s living in the 19th century, basically - which admittedly, many people still seem to want to do.
I think this attitude is inappropriate. The scientific progress made with 19th century methods, and that includes most of the stuff discovered until the 1950s, is massive. Maybe scientists should focus more on a clear nay or yea? I mean, the LHC is certainly a significant achievement, but compared to the detection of gravitational waves or even just exoplanets it doesn't seem to deliver as much bang for the buck.
> I mean, the LHC is certainly a significant achievement, but compared to the detection of gravitational waves or even just exoplanets it doesn't seem to deliver as much bang for the buck.
Well, first, LHC was "unlucky": they could have observed a lot of new particles if they would have been there. It feels unfair to say "this experiment that looked at X and had 50% chance of seeing something was a waste of money because they haven't found anything, this experiment that looked at Y and had 50% chance of seeing something was not because they saw something": both experiments are as worth a try. LHC still have observed the Higgs boson, which is still quite a big thing. LHC has also constrained a lot of theories (seeing that a particle predicted by a given theory is not there is very useful scientifically).
But more importantly, I have the feeling that the notion of "bang for the buck" is pretty different from a scientific point of view and a layman point of view.
For example, you mention exoplanets. Scientifically, those are really not a big deal. We already know that planets can exist, there are 8 around the Sun, so finding others around other stars is really not a big deal: everybody was expecting them to exist and knew the only reason they were not seen yet was the limits of the technology. One could even argue that using money to detect things that are pretty trivial is in fact the worst delivery of bang for the buck. Don't get me wrong, this is still nice to see them, but it's the same level as pentaquark (discovered at LHC).
It is sometimes surprising to see the public/media reactions to discoveries. I'm still not sure why layman people were so excited about gravitational waves but not much about the tau neutrino.
Indulge the counterfactual for a second. Suppose that dark matter really does exist, and we can't ascertain that fact with any certainty. What do you want Physics as a whole to do about that? I think everybody agrees a simple answer would be great, we're just unable to provide one in this context
> Suppose that dark matter really does exist, and we can't ascertain that fact with any certainty. What do you want Physics as a whole to do about that?
Be more bold in exploring alternative gravity theories and conceiving of tests we can perform to check how gravity behaves in regimes that are not yet well tested.
> Naive falsificationism is an almost useless principle at the frontiers of modern physics. We’re long past the days when you could run some simple experiments and come up with a clear yea or nay. That’s living in the 19th century, basically - which admittedly, many people still seem to want to do.
To a layman like myself this implies both that the things being studied then are both unprovable and not potentially of practical use or significance. I certainly am not saying that is the case, but rather am looking for more informed opinions to the contrary.
The cynical side of me feels like study of something unfalsifiable is as the study of religion. A poor investment for public funds.
As I mentioned however, I am seeking opinions here to the contrary of my gut reaction as my gut is often dumb and reactionary.
The comment you replied to does not suggest that modern physics is unprovable... no simple experiment to prove something does not mean no experiment at all.
The other, related point is that modern physics' frontier is in the quantum world and thus way outside layman's intuition, common sense, or reasoning. Even experts are struggling.
In science nothing is provable. Things can be falsified but if a hypothesis is true it can actually never be proven. This is because at any point in time and additional observation can be added to your sample that definitively disproves your hypothesis.
All zebras have stripes. is my hypothesis. I observe 100 zebras and I see they have stripes. I see 10 million zebras and I see they have stripes. There is no amount of observations I can make to prove my hypothesis. However if at any point in time I see a zebra with no stripes, I have disproven my hypothesis.
So anything that is in actuality true, is not falsifiable and also un-provable. In short, some statement about our universe can be true, but we can never absolutely know for sure whether that statement is actually true.
Basically your comment isn't logical because you're saying we shouldn't study anything that is true because it's unfalsifiable by definition.
In addition to falsification, Science can make statements that are vague and UN-definitive. Un-definitive claims include causal claims and correlative claims.
Correlation is A and B are casually linked directly or through some other source. A maybe causes B or B maybe causes A or maybe a C is causing A and B.
Causation is A and B are directly linked causally. A causes B.
Science can make the two claims above but such claims are made based off of a sample. If the sample is bad or biased the claim is also bad and biased. Given that we live in an unknown universe of which any sample we extract can only be of finite size out of a potentially infinite size it makes both correlative and causative claims un-definitive. The sample can never fully encompass everything. This is the reason why in science, nothing can be proven.
So in short science can definitively falsify things, and in a vague sense it can make statements about correlations and causation. The later two, though undefinitive can be used to say something about some topic of the universe that is true. But it can't say anything definitive and we can never know for sure.
Because Falsification is limited to things that are false, so much of science doesn't actually use it even though it is a definitive metric.
>As I mentioned however, I am seeking opinions here to the contrary of my gut reaction as my gut is often dumb and reactionary.
The GP is kind of the one that's not too bright. You're definitely a layman but you ask the right questions. GP is not a layman and he's suppose to know certain things and not say what he said. He's very opinionated and makes broad statements without addressing nuance. So don't worry about it! You're not dumb!
GP is utterly wrong. Falsification is still used on the frontiers of modern physics, though much of it nowadays is just correlative results, but if one such correlation is shown not to hold at all, it becomes a falsification. In fact, EVERY single experiment in modern physics is an attempt at falsification. And when the falsification fails... a correlative result is presented as the final answer.
But note, even a falsification is not clear in certain cases. This is because our scientific observation tools have limited accuracy. Most of the time we just assume the observation tools are 100% accurate. But at the frontiers of modern physics the observation tools are so bad that the tools themselves yield probabilistic answers. So while in science falsification is still very much used to clearly discard theories, in modern physics, a negative result is not indicative of a clear cut falsification.
If it is not falsifiable, it is not science nor physics. It is meta-physics, ersatz-religion. Which is exactly the core of the complaint against concepts like this.
Of course same thinking can't easily be applied to things that are not always true, like social behaviour. Part of the reason the replication crisis hit so hard in the social sciences.
And of course this is not really as absolute as I wrote above. No field works like this all the time. But it is a good guideline. If falsifiability is completely ignored, then a concept really is nothing more than a religious belief and should be considered as such.
As far as I understood it if somebody could craft an experiment that would demonstrate, quite clearly that it was something other than a special kind of matter causing this effect then the theory would be falsified.
>which admittedly, many people still seem to want to do.
This occurs because most science is not the "the frontiers of modern physics". But even then, falsification in the frontiers of modern physics is still a useful tool.
Unless you have worked extensively with the data and math of modern cosmology I very much doubt your 'intuition' about 'dark matter' is anywhere close to the intended meaning.
Right? After quantum mechanics and relativity there's absolutely no reason to believe that our intuition is worth squat when it comes to how physics works.
I very much agree with you, and have raised these questions before.
I got some interesting responses that half-mollified my worries.
Originally dark matter was hypothesized because of galaxy rotation mismatches. This allows for mapping where 'dark matter' should be. One falsification of dark matter that was tried is gravitational lensing. If there is indeed extra curvature of space-time in these galaxies, then they should bend light-rays, Potentially showing the same galaxy twice.
This was tested, and confirmed. Hypothesized dark matter, as detected by galaxy rotation, causes the expected gravitational lensing.
I suppose another potential falsification is by 'moving' dark matter through a gravitational tractor (very heavy mass with rocket engines) and then detect the resultant change in gravity. You could attempt that detection by gravitational lensing, orbit velocity, or perhaps something else.
Despite all this, dark matter still feels like our theory of gravity was falsified, and instead of concluding the theory was wrong, it was attempted to add random parameters to the theory. Its not like there is a better theory though (I believe MONS has trouble with explaining gravitational lensing), so perhaps this is a problem of PR much more than a problem of science.
As for the second question, for dark matter to be matter, I think it suffices if it has mass and inertia. My gravity tractor detection idea tries to work with that.
"Dark Matter" defies parsimony. Occam's Razor suggests rejecting the
assumption that spacetime is uniform. The search for "Laws" may be a
parochialism built into human epistemology. Advanced aliens may be
comfortable with the idea of "space weather" and local variations. :)
Where does "infinite amount of additional parameters" come from? It's just one parameter - something's (we call dark matter) is either there or it isn't.
> I fail to see how dark matter could ever be falsified
I don't think science cares. Frankly if you could back up your opinion with some actual reasoned argument, it would be a better post.
In physics, we have laws, rules, models, equations, theories, theorems, postulates, and maybe more. I can't think of one instance in which something has been "promoted" from one to the other after some data came in and couldn't even rank these in terms of certainty.
We still call it Newton's law even though we know it's wrong. We still call it Born rule even though it's one of the most tested things we have. We still call it the standard model even though it's the thing that works better than everything else. We call particular Lagrangians in Quantum Field Theory "theory" even though we know it's just a toy theory which doesn't describe reality. I mean, "toy theory" or "toy model" is a thing.
Physicists aren't nearly as nit-picky as internet commentators when it comes to this topic.
My intuitive objection to your comment is that it ignores the basis of the article: that it’s entirely about learning more about dark matter. This is the complete antithesis to what your comment claims.
I’m sure physicists have already thought about this but…
Has anyone considered modeling the effect currently explained via dark matter as a new kind of force or field? Like a distinct force generated by mass and its relationship to space time?
This wouldn’t be modified gravity but a proposal for an entirely new force whose effect is what we currently explain with the “fudge factor” of dark matter.
From the rough picture I do have of dark matter this force would be one whose relationship to distance is different from the usual inverse square law that governs forces. That would be weird but maybe no weirder than particles that can never be detected or MOND.
"Dark matter" implies a new field. Or rather, a new field or force that affects gravity implies some sort of interaction exactly like a massive particle that interacts only gravitationally. Particles are just localized disturbances in fields (in quantum field theory), and new fields can have such disturbances and thus have particles. In this (trivial) sense, MOND is a type of "dark matter" theory that posits a particle with only gravitational interactions.
Of course we don't have a quantum field theory for gravity, so the above is hand-waving away a HUGE issue of what that new field would be like, how its particles would act, etc, since we don't even know what the quantum gravitational field is like and what its particles are like (assuming such a thing even makes sense).
Am I right in thinking no one has been able to detect dark matter despite decades of attempts and research?
My understanding is that based on observations of what can be seen galaxies move as if there's move stuff in them??
If you hypothesis that this extra stuff is dark matter but can't detect it, how do you know it's real?
How have you ruled out all the normal stuff that doesn't give off light or stuff we can't see at the distances we're looking over? Like planets, rocks, dust, etc. Couldn't that stuff, which to my mind there would be vastly more of than stars, make up the extra gravity?
I'm aware I might be talking complete shit because I'm completely ignorant of the detail here. I would love an understandable explanation as to why that's wrong.
> Am I right in thinking no one has been able to detect dark matter despite decades of attempts and research?
Yup [1]
> My understanding is that based on observations of what can be seen galaxies move as if there's move stuff in them??
Yup [2]
> If you hypothesis that this extra stuff is dark matter but can't detect it, how do you know it's real?
It produces certain effects in various different enviromennts or domains, which lends more support to that hypothesis. By certain different domains, I mean like Galaxy and cluster dynamics, gravitational lensing around galaxies and clusters and the precise fluctuations seen in the cosmic microwave background, these are all very different regimes but all seem to support the hypothesis of extra mass which is not interacting electromagnetically.
> How have you ruled out all the normal stuff that doesn't give off light or stuff we can't see at the distances we're looking over?
There are research efforts to look for exactly this kind of stuff; Massive compact halo object (MACHOS). However they don't see enough of it to account for the observations we see in galaxies and galaxy clusters. [3]
Sort of, but gravitational lensing has been seen in places where there is no visible matter. If dark matter doesn't exist, then you'd have to come up with another explanation for the gravitation lensing:
You're absolutely right and this is the one thing about dark mayer that doesn't sound like "just keep sprinkling it on your problem till it works."
I don't know enough about the found examples of gravitational lensing to know whether all possible arrangements of black holes that could have the same effect, have been considered and found lacking. But I presume so.
Always been interested in physics since I was a kid, so seeing this kind of stuff with dark matter and being able to understand how it'd be used in practical applications is fascinating to me.
Honestly, I hope so. The universe isn't totally amenable to our inspection, and it'd be nice to have an example that just stared us in the face all the time.
The minimum mass for a black hole is about 3 times the size of the sun. So we would definitely detect such an object. In fact it would have pretty catastrophic consequences if it entered the solar system...
You mean stellar black holes, right? Primordial black holes could be much smaller, if they exist.
I just looked up what would happen in that case, and it sounds really bizarre:
> Nevertheless, a PBH would have a notable local impact, Rahvar notes. “The passage of a black hole could melt a cylinder along the interior of the Earth with a radius of almost 10 cm. After a short time, this tunnel would freeze,” he explains. In theory, this would produce a distinct metamorphic fingerprint of the passage in the rock record. But given that we might expect only eight of these to have been produced in the Earth’s lifetime (one on each side of the planet per collision episode), “I would expect that such a geological trace is difficult to detect,” Rahvar concedes.
The issue with that is you're really just talking about another form of dark matter: something else theoretically not impossible but also never observed despite decades of looking...
At least it's never observed in a sense that neutrino was never observed until it was. We just lack powerful enough sensors. The other forms of dark matter were never observed in a sense tachyons were never observed. As in we don't just lack sensors, we lack exact understanding of what we are trying to observe.
I should point out that primordial black holes are one of the candidates for dark matter. The scientific community seems to be flipflopping about the plausibility of this theory.
> This isn't evidence against dark matter's existence,
If the repeated failure to find tangible evidence of something in the physical universe isn't evidence against its existence, then you're no longer doing science, but rather playing some religious game
There's a chance dark matter is black holes created in the aftermath of the Big Bang. Some of these then merged into the supermassive black holes we see in the center of galaxies today.
Yet black holes do interact with ordinary matter--they can consume it--and they emit Hawking radiation. Dark matter does not appear to be doing either of those things.
I'm huge fan of that because in my view it requires least amount of making stuff up. So I probably never be convinced by any other explanation unless this one is solidly falsified for all possible blackhole masses.
The only real nightmare is that a couple of Jesuits, the same guys who imprisoned Galileo for daring to suggest that the Earth revolves around the sun, came up with the Big Bang theory to explain cosmological redshift, thereby carving out another century of confusion and creationist psychosis under the psychological rule of their lawful-evil overlords in the vatican.
I think the first paragraph could be rewritten better as:
"Dark matter puzzles us. Einstein's theory of gravity, plus known matter and radiation in the Universe, including particles and antiparticles described by the Standard Model, cannot explain cosmic observations. We need an additional gravity source - dark matter."
I find it very interesting that every time Dark Matter, a theory that actual physicists largely find very convincing, comes up a bunch of non-physicist HNers come out to explain why it doesn't make sense.
It feels like this is implying that questioning is unscientific? I mean, I get being annoyed with laypeople but if you didn’t question the scientific consensus the historic record and the philosophy of science are against you.
For me it's the fact that so many of these commenters seem to think they've come up with something special, found some secret idea. That's completely false. Most physicists aren't very happy with dark matter as a theory (especially since the failure of the big direct particle searches). So the commenters are trying to convince the professionals of something most of them already believe. It's silly.
(My credentials: my fingerprints are all over one of those (pilot) experiments.)
But the very definition of this phenomenon is something interacting with gravity (like matter) and not with EM... And EM in layman terms equals light, thus dark... It totally makes sense to call it "dark matter"
Me neither, MOND seems entirely reasonable intellectually, scientifically; I don't have a strong view as to whether it's right or not, but I know serious people who do.
I've been wondering for a while if light could be the thing that pushes away the boundaries of the universe and that's the reason for the expansion, if u can push a solar sail... anyways, I don't understand enough about this.
There are no known boundaries of the universe. The universe expands everywhere continuously. I think of it as though a little bit of extra volume is injected in between everything on a regular basis. Also when we look out as far back in time as we can, everything looks very similar, and there's no "boundary". Each year we can see a little bit farther out into the universe because those photons from billions of years ago finally got to us and so far things keep looking the same the farther out we can see. So as far as we know the universe is far larger than what we can see (what has had enough time to send us photons). I also don't understand enough about this.
But at the same time we're also racing away. There is light from objects that will never get to us because we're moving away from it, due to expansion, faster than it'd ever get to us.
Eventually we'll have moved so far away from everything that we'll start seeing less and less (granted, "we" likely won't be around, our planet having long since been incinerated when Sol dies).
We can calculate the force exerted on an object by a photon of light of a given frequency. I'm guessing it is not sufficient to produce the effect you envisage, although I have not done the calculation. Interestingly this photon pressure does put an upper limit on the mass of a star; the Eddington Limit [1]. If a star is too large (and therefore luminous) its photon pressure pushes away other material and stops is from growing larger!
Would recommend reading Chapter 3 "From Eternity to Here" by Sean Carroll (or maybe read the entire book) for a "light weight" introduction. If you don't have the time then TL;DR : universe is expanding means literally that, i.e it is almost as if new space is getting created rather than "cosmological scale objects" hurtling away from each other because they "gained momentum (if one can even call it that)" because of that big bang.
Nightmare for who? How hard it is to admit that their equations are incorrect? Even amateur like myself can see that the dark matter is adhoc value that was calculated in order to patch the standard model. There are several better alternatives but do we need to wait until these stubborn believers have passed away until the world can move forward?
At the moment there are no better alternatives that satisfy all the observations. If you try to replace General Relativity with something like Modified Newtonian Dynamics (MOND) [1], then you can get good fits to galaxy rotation curves. However, there are many sources of astronomical/cosmological data that will not bend to alternative gravity models, e.g the precise fluctuations of the cosmic microwave background put stringent bound on the matter components of the Universe.
I vaguely recall hearing that there are galaxies "without any dark-matter". Or, more accurately, galaxies whose rotational speed is consistent with general relativity without needing to presume dark matter exists in those galaxies.
If that is correct, does it not immediately invalidate MOND?
I think every physicist on the planet would love a nice elegant model of gravity that explains away “dark matter” observations. The next layer after Newton and Einstein. Clean and simple.
… but that’s just proven to be very very difficult. Evidence for dark matter comes from a wide range of observations, and while it’s pretty easy to come up with relativity replacements that could explain things like rotation curves nicely… other observations, from lensing and colliding galaxies like the bullet cluster are, just, well, messy.
All modified gravity theories are falsified by astronomy as far as I know. There are galaxies out there without dark matter halos for example. Or where the dark matter halo is not centered around the core. This alone shuts down modified gravity.
I guess the nightmare is more that it might be undetectable. There’s a real possibility that DM is real, but we hit the neutrino floor at some point without finding anything (maybe couplings are too weak).
Until there’s an actual convincing alternative, that indeed is not as apparent as you make it out to be, that would be pretty terrible for a lot of physics.
Why isn’t it good for physics? It has found DM and can observe it via gravitational discrepancies, gets finally “solved” in this part. Does physics really need infinite modifications?
We can't observe anything "directly", other than the contents of our own minds. The "gravitational discrepancies" are an observation.
It seems to me (non-physicist, non-cosmologist) that DM proponents can't tell us what DM is, despite being able to observe it. The article's "nightmare" is that it might turn out to be impossible to do that in principle. To my mind, that would indeed be a nightmare; we would have to accept that there are limits to what we can learn about the Universe. I don't believe we have encountered any such limits before.
Directly here refers to explicit detection (“here is a signal that is DM and gives us immediate knowledge about its properties”) as opposed to implicit detection through inferring DMs existence from a lack of something.
A lot of people complain about DM being fuzzy, and that’s exactly why: we only have indirect evidence. So what everyone wants is a direct measurement of DM, for example a particle signal if DM is a WIMP.
And to your last point: that’s exactly it, the thought of an unknowable thing in the universe is terrifying, frankly. And this has nothing to do with wanting to be right, whatever DM is, I just want to know, even if it’s fancy MOND without defects somehow, or MACHOs.
Well the overall tone of your comment is pretty crummy but this is absolutely not true. You got this exactly backwards. Dark matter is just missing mass that is being observed. It doesn't patch any model, because physicists don't know what it is or what it is made of. But whatever that mass is, it is there, because all the standard ways that we observe mass, it exhibits. So it would instead be a leap to "patch the standard model"--whatever you mean by that--with these other "better alternatives" (I assume you mean MOND? No dice there, doesn't explain gravitational lensing).
Maybe problem is they do consider Space to be empty. Maybe if they would take into account all those quantum fluctuations in there and add that energy maybe they would land at critical mass of the Universe.
To paint this idea with other example, if you were to figure out mass of a world, but the world was just an image reflecting off a surface of water, then they would focus only on the light particles coming off the surface and ignore the water. At some point they would come to realize that something big is missing. They could find out the value for how much it should be. That’s exactly the case with dark matter I’m quite sure. Space is like water they ignore.
But I don’t know anything, really (sorry to hear if that’s a problem for someone). Why there is possibility to comment on these articles anyways?
This is where I reference Mike McCulloch's theory Quantized Inertia, which predicts Galactic Rotation curves far more accurately than Relativity / Newton, without any fudge factors. (This is also where someone jumps in to inform me that he is "a crackpot", and should be ignored. FWIW QI seems like a promising theory to me, and I think it should be tested experimentally. )
How accurately does it predict the CMB fluctuations? Or weak lensing observations? Rotation curves might have been first, but they're actually not the strongest evidence for DM.
There are many good reasons to think that dark matter is a good solution to certain cosmological problems. There are also a few good reasons to think that dark matter is total BS. It is a genuine open question. No theory in this area is a good fit for all available evidence.
This is not a case of "progress by funeral" like, say, the Alzheimer's situation also on today's front page. The linked article is a good summary.
