When I see dark matter in the news I'm always reminded of the story of Vulcan.
In the 1800s, detailed observations of the planet Mercury showed that its orbit was slightly different than Newtonian mechanics predicted-- a difference of about 43 arcseconds per century. The study was rigorous enough to rule out any observation errors.
Le Verrier, the astronomer who made these observations, wondered how to explain the difference. A decade earlier, he had noticed a similar irregularity in the orbit of Uranus, which led to the discovery of Neptune, whose gravity caused these perturbations. So Le Verrier reasoned that something similar must be going on for Mercury, and he posited the existence of Vulcan, a tiny planet close to the Sun.
Many attempts were made for decades to observe Vulcan. It was even included on some maps of the Solar System at the time (https://www.loc.gov/resource/g3180.ct003790). But it was never conclusively observed.
When Einstein published his theory of relativity in 1915, the mystery of Mercury's orbit was finally explained-- Newtonian mechanics were simply incomplete, and the irregularity of Mercury's orbit was due to relativistic effects.
Could it be that something similar is happening today? Observations of gravity on galactic scales doesn't quite align with what relativity would predict, so we use dark matter to fill the gaps. We've tried for decades to detect dark matter, with no dice. Is our theory of gravity simply incomplete?
MOND may not be the solution, but I'm still skeptical about dark matter.
There are multiple independent observations pointing toward dark matter:
1. Galaxy rotation curves.
2. Galaxy cluster mass measurements from gravitational lenses and infrared.
3. Cosmic Microwave Background models (mass measurements from inhomogeneities that correspond to acoustic waves, for eg).
MOND only explains 1.
Dark matter accounts for all 3. Only catch is that it hasnt been directly observed.
> Only catch is that it hasnt been directly observed.
Ok, so... why do people take it seriously as a concept? Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.
> Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.
For some general misunderstanding to explain what we see, ie something which is not dark matter, it means our equations of gravity must be wrong.
People have tried hard to tweak the equations to fix the observations. One such proposal is indeed MOND that the submission is about.
The problem with tweaking the equations is that we have a lot of observations of very different circumstances and scales, and it's so far proved impossible to tweak the equations in a given way so they explain all of the observations. What happens is that you can tweak them so they fit one set of observations, like MOND fitting galaxy rotation curves, but then not fit other sets of observations, like MOND with the motion of galaxies in galaxy clusters as discussed in the submission.
Meanwhile, adding dark matter seems to work much better at explaining all the observations at once. I say much better because there is still some tension, but it's way less than any modifications of gravity.
Having dark matter particle(s) is also not that strange. As far as we know there's no requirement that a particle interacts with any of the known forces beyond gravity. And while the early particle candidates seem to be getting ruled out, axion-like particles, originally proposed to solve entirely different problems, are still a good dark matter candidate.
That said, it's not settled. People are proposing alternatives. One example I know of is Alexandre Deur[1], a QCD physicist which has taken his QCD knowledge into the realm of GR and proposed that perhaps graviton-graviton self-interaction can explain a lot of the dark matter phenomena. It's not mainstream, but he's getting published in peer-reviewed journals.
> Having dark matter particle(s) is also not that strange. As far as we know there's no requirement that a particle interacts with any of the known forces beyond gravity.
This is the key thing. As we still don't have a theory of everything, which explains the fundamental reasons why every particle is the way it is, dark matter is a very straightforward and simple explanation.
The name makes it sound stranger than it is. It's just particles that have mass and are affected normally by gravity, but which are unaffected by the electromagnetic force.
It's not a particularly exotic or implausible or wild idea. In fact it's so simple it's almost boring. It's just obviously inherently difficult to test directly, since gravity is such a weak force.
I feel compelled to point out that (a)ether was the simplest answer until we confirmed that space is a vacuum and that light can travel in the absence of a medium.
aether was not the simplest explanation since it was disproved by Michelson-Morley experiment. Nothing as of now yet has disproved Dark matter as thoroughly.
Aether: observations show lack of aether so we update the theory that makes aether unnecessary.
Dark matter: observations show lack of observable matter so we keep hunting for decades locating it. The aether analogue would have been to continue to look for dark aether. Dark matter is more like god of gaps. We can 'darkify' any theory that does not fit empirical evidence.
It's hard to tell now which leans more towards the more correct theory.
Abandoning aether was NOT a "simple" answer. One you abandon aether, all manner of weirdness suddenly pops in.
Light being the same speed irrespective of observer is weird. Velocity dilating length and time is weird. Not having a preferential observation point is weird. Not needing a medium for transmission is weird. Not being able to agree on simultaneity is weird. etc.
Aether wasn't just something that a bunch of dullards clung to. You have to abandon some very long held common-sense understanding when you give it up.
You are of course very welcome to propose this supposed better model, but please do some due diligence and learn to roughly understand the current flawed models firsts.
For example "angry gods" was never a simple theory, one needs only to read some fan fiction to understand that theology gets complicated fast. Instead "angry gods" is a simplified summary of a very complicated theory about metaphysical hierarchy, creation, agency, the meaning of divinity, and cause and effect, etc etc.
Simplest in this sense does also imply predictability for Dark matter, which cannot be said for angry gods. Angry gods is anything but a simple theory.
We already know that some particles interact extremely weakly with others. Neutrinos were postulated as a "dark" particle in order to preserve conservation of energy and momentum, but it took more than a decade before they were observed directly. There's no reason there couldn't be another particle with an even weaker interaction.
A "general misunderstanding" isn't really a theory whose complexity you can compare to the Dark Matter theory.
One could also argue that on the one hand you need just one concept (dark matter) to explain three phenomena (galaxy rotation curves, gravitational lensing, CMBR), and the "general misunderstanding" might need to have three aspects to make it explain all three things, making it more complex.
Occam's razor is a good guiding principle, but in some cases, it's really hard to compare the complexity of competing theories. My favorite example of this is the Many Worlds interpretation of quantum mechanics. In some sense, it's really simple, the wave function just follows the Schrödinger equation. In another sense, it implicitly postulates the existence of many parallel worlds, which you can count as a quasi-infinite complexity penalty, or you can shrug and say it's not a big deal, the theory says the parallel worlds aren't observable, so there is nothing wrong with postulating them. There's no objective way to decide that.
Occam's razer is not a thing with explanatory or legal or even scientific force. if Occam's razor was really a thing, quantum theory and general relativity would be ruled out as complicated and neither 100 percent explanatory.
> Historically, science has predicted lots of true things about the universe that could not be observed at the time.
Just some examples I could think of:
The neutrino[1], predicted in 1930 due to missing mass in beta decays, discovered in 1956.
Gravitational waves[2], predicted by Einstein in 1916, indirectly detected via pulsars in 1974 and directly by LIGO in 2015.
Higgs boson[3], predicted in 1964 to solve the problem of the Standard Model at the time requiring particles to be massless, which we knew they weren't. Hints from LEP at the turn of the millennium, detection by the LHC in 2012.
Cosmic microwave background[4], predicted in 1948 as a consequence of the expanding universe on the early radiation. Detected by accident in 1964.
Gravitational lensing[5] was first proposed in 1801 as a consequence of Newtonian corpuscular theory of light. Einstein realized in 1915 that GR predicted a larger value. Weak lensing by the sun was observed in 1919. Strong lensing by a distant galaxy of an object behind the galaxy was was predicted in 1937, and first observed in 1979.
I don't know what or why our science education is like this, but it seems like everybody's understanding of science bottomed out at a straw man version of popper & positivism.
And to be clear, falsification and being empirical & skeptical about theoretical claims is great. What I see all too often on the Internet is just pattern matching to the words "observable" and "falsification" without a second thought, without actually looking into how science develops, and any and all narratives are historically rewritten to fit only those two categories.
Which is why it's even more impressive to be a real scientist, to actually be able to navigate the muddy waters properly where it's not just some simple adjective checklists to run through. (As a non scientist)
I feel the same, but then I remind myself of how I used to feel about dark matter (I really disliked it). Having an arena where no scientific question is out of bounds is great.
Because the discrepancy exists and shows consistency. This gap needs to be called something, you can't just pretend it's not real. But the underlying model seems fine and all alternatives are broken.
So we came up with an idea that fills the gap in the current model and allows us to continue predictive science until we find out more.
People get so absurdly touchy about Dark Matter just because it's named and "weird". It's just a placeholder value.
> Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.
Dark matter is the Occam's Razor theory. It explains almost all of the observations while assuming the least.
Why do you assume we have no evidence just because we don't have direct observations? Black holes were a similar phenomenon that we had no direct evidence for a fairly long time even though we had lots of other indirect phenomenon that really couldn't be explained any other way.
William of Ockham objected to his fellow theologians inventing things out of whole cloth (like dark matter question mark). That’s the point, not that a simpler explanation is more likely to be true.
The common understanding would have us believe that creationism, being simpler, outshines evolution, or that there is no such thing as a color revolution because the simplest explanation is that the mass protests are earnestly aggrieved locals.
As a kid in 90s who was into this stuff, it seemed like anyone who knew about dark matter knew it was just theorized and never observed. It was included as a caveat in a lot of things I read to ensure the readers understood this.
Then in sci-fi it started showing up as a material the characters would interact with, and a lot more people were introduced to the term.
Then as a teen in the 2000s the caveat was usually gone and it started being treated as something we had proof of. People I knew started thinking it was something we'd actually found and was now fact instead of theory.
> Ok, so... why do people take it seriously as a concept? Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.
One scientist says "We don't know, really, it's a head scratcher" and the others say something cool like "Dark energy", which ones will get more attention and publicity?
But it really isn't a head scratcher, and it's not a wild guess, and also, dark matter is not the same as dark energy.
Think about some random person born in the 1950s. It will be impossible for you to directly prove that that person has two parents, but with all your knowledge about biology, and your knowledge that humans could not be cloned in the 1950s, it's not a head scratcher, unless our understanding of biology is completely off, it's not just speculation there were two parents. That's the comparable situation with dark matter. It's not a head scratcher. Every indication shows there is a lot of mass around that doesn't interact with light.
Associating dark matter with epicycles is unfair, but it's still a risk (CDM, WDM, SIDM, and probably more by next year). In that light, of course it has more observations (by virtue of creating branches to explain incompatible observations) - but counterpoint is that it has made some predictions.
My stance is that anyone pointing at it in either light probably isn't taking everything into account. It's an incredibly immature theory space - are we going to get 20 more branches of it (making it modern epicycles), or are we going to see one of the current branches pay off?
I mean there are modified gravity candidates other than MOND. I think those are worth more consideration. MOND seems pretty well disproven at this point, yet somehow it's still what people focus on in terms of modified gravity! Where are the tests of other modified gravity theories?
Indeed, in terms of the negatives of MOND, I'll go further than you -- MOND barely explains galaxy rotation curves at all. When you look at the actual rotation curves compared to MOND's predictions, well, yes MOND does better than Newton w/o dark matter, but it's still pretty heavily fudged. If you have to fudge it that much, it seems to me there's no point. So I'd say that really MOND doesn't explain any of the three.
2 is "who can say" because nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND).
3 is subject to questions like "is the CMB really what we think it is" -- if it's early thermalized dust, then that ALSO resolves hubble tension, e.g.
MOND explains several things LCDM cannot:
- why most elliptical galaxies seem to "not have dark matter" (effectively a prediction)
- external field effect (predicted and confirmed)
- renzo's rule
- DM halos that are way too big
- early galaxies (this was a prediction)
HM. people have been downvoting. Anyone care to post a substantive rebuttal?
2 needs a more substantive rebuttal. LCDM correctly predicts where the dark matter is located after a galaxy collision, such as in the bullet cluster. There is no reasonable interpretation of MOND that has the center of mass of the galaxy shifted away from where it's visible matter lies, precisely how LCDM says it should be.
There is a reason why LCDM used to be a lot more disputed before the work of Clowe, Gonzales and others on the bullet cluster, and is now generally treated as settled science by practitioners. We might still be surprised by something, the universe is more wondrous and complex than we can possibly understand, but Occam's razor massively supports LCDM now. If you want to propose any alternative, you need to start by showing how it explains bullet cluster as well or better than LCDM. (And the bullet cluster specifically is not the only place where this is visible, there are others like MACS J0025.4-1222.)
> LCDM correctly predicts where the dark matter is located after a galaxy collision, such as in the bullet cluster. There is no reasonable interpretation of MOND that has the center of mass of the galaxy shifted away from where it's visible matter lies, precisely how LCDM says it should be.
It does not really make that "prediction", its a post hoc assignment of dark matter density based on weak lensing for which you can make a plausible "this is how it started" explanation.
you can counter with lcdm cant explain tons of stuff that MOND can, from tully fisher relation through barred spiral galaxies (n >> thousands) etc.
> nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND).
My take on this is a bit different. The obvious way to get a relativistic theory that looks like MOND is to add a scalar field. (And possibly a vector field, as in TeVeS.) But from a relativistic, quantum field theory perspective, adding more fields is just...adding a new type of "matter". I.e., dark matter!
In other words, when you take relativity into account, "MOND" and "dark matter" don't really look like two different possibilities any more. They just look like two different ways of describing the same math in imprecise ordinary language.
"HM. people have been downvoting. Anyone care to post a substantive rebuttal?"
Just that asking this might get you more downvotes. I upvoted as I found your input interesting. I would suggest to edit that out (then I will delete my comment)
I don't really care (i'm doing just fine fake-internet-points-wise), just if I made a mistake it would be nice to know. Otherwise worth it to know that there are people out there who find "alternative explanations by professional scientists" threatening. for some reason.
Well, I rather meant there are guidelines to keep this space meta debate free and I do think they make sense.
(And apart from that, HN is not free of dogmatic ideologists of all sorts who feel triggered by anything straying off from mainstream theory. That is my guess here and my advice to ignore it, but I would be also curious if there is something clearly wrong with your comment deserving downvotes.)
Of course it can be that this is the case. There are predictions from the dark matter hypothesis, though, that have been testable. For example, there're some of the asymmetries in the cosmic microwave background that are explained presently by the dark matter hypothesis.
Something that's worth bearing in mind is that "dark matter" doesn't actually mean "totally new never before seen thing" it means "we don't know what this matter is". So, for example, a candidate that wouldn't be super novel but could fit the bill is microscopic black holes. In that sense, the hypothesis is more mundane than it might seem.
As far as I know, dark matter hasn't made any testable predictions which were subsequently proved correct. It's an infinitely tunable hypothesis that can be retro fitted to new observations, since it has many tweakable parameters.
Could you point me to a prediction made by dark matter theory which was subsequently confirmed by observation, rather than a dark matter theory which explains it post facto?
Not sure why the downvotes just for asking a question.
Dark matter (well, LCDM) has predicted many things which were proven false, e.g. that big galaxies assembled through merging with smaller ones over long periods of time. JWST shows us this is not in fact the case. There are now lots of papers showing how LCDM can explain that, but these are post facto explanations.
Happy to be proven wrong if someone can point me to a prediction subsequently confirmed by observation.
Not really, Cold Dark Matter is part of LCDM, the main cosmological model. Whether hot or cold, I'd like to see a solid prediction made for it confirmed subsequently by observation.
100%. It's important to realize our understanding of "dark matter" is fuzzy because we only understand it through data anomalies. Dark matter is a classic catch-all concept that we use as a crutch while we try to understand the underlying system better.
Similar to how we used to believe in "aether" to explain how light could travel through empty space. It is important to understand how these crutches help and hinder understanding.
It's kind of weird in this case, though. All the math acts like there's something invisible and heavy everywhere that we find clumps of visible matter. When we look at the motion of galaxies, they behave as if they're much more massive than the count of stars and such in the would have you believe, and in ways that otherwise jibe with our understanding of physics if only that galaxy were heavier.
If you have one galaxy that's acting heavier than you can eyeball, measured by things like light bending around it, then maybe you have some weird phenomenon. When every galaxy calculates out to be about 6x fatter than you'd expect, something else is going on.
The strongest evidence that something like MOND isn't the answer is that in some galaxy collisions, the visible matter and the dark matter appear to separate: the collision disrupts the visible matter and the dark matter appears to pass right through, uninterrupted, and we see galaxy remnants that look like they don't have dark matter. If MOND or some other modification of gravity were the answer we'd never see this kind of sorting.
Astrophysicists have gone all through this. Absent direct observation or at least a good explanation of what it actually is, I don't think anyone really likes dark matter. The main objection is that it feels a little too flexible: you can explain a lot of things by adding mass in different places. But it's not infinitely flexible: you don't need arbitrary and utterly weird distributions of dark matter. And so it's generally emerged as the best explanation of observations so far, and not for lack of trying with tweaks to theories of gravity themselves.
If you follow Sabine Hossenfelder's channel, she has a MONDOmeter. With MOND (modified Newtonian gravity) on one side and dark matter on the other side.
As new papers come out the needle goes back and forth, and I guess that she will make a new video if she hasn't already, with the needle moving one step towards dark matter.
I find it interesting how it doesn't seem to settle. Dark matter is still the favorite, but there is a lot of back and forth between "MOND is dead" and "we found new stuff we couldn't explain with dark matter, but it matches MOND predictions".
MOND does amazingly well at galactic rotation curves, less well at anything else. If you think it started with Vera Rubin in 1966 MOND seems natural, but if you know that it started with Fritz Zwicky in 1933 than dark matter is easier to believe.
MOND only really does well on galactic rotation curves because it has free parameters that are tuned to "predict" the correct answer for galactic rotation curves.
I think you mean LCDM only does well on galactic rotation curves because it has free parameters per galaxy. MOND only has one free parameter, maybe two if you use the MOND+Relativity model that doesn't work.
I don’t think these are free parameters in the same sense.
Like, if one theory says that a hunk of metal actually is made of many microscopic grains of various sizes and orientations, where the sizes and orientations of these grains has an effect on the behavior of the metal, you don’t count the “the sizes and orientations of these grains” as free parameters, do you?
But at least the experimental results disproving these incremental fixes should be exactly the kind of thing the next Einstein should need for coming up with an entirely new way of looking at things
> Some once-in-a-generation scientist has an intuition that turns out to be true and mathematically elegant.
That’s a bit simplistic. There was a lot of research activity around the aether in the late 19th century that was ultimately useful for the foundation of special relativity. Like the Michelson-Morley experiment, which was supposed to measure aether winds, but showed it did not actually seem to exist. Lorentz developed his transform as a theory of the aether, but it became a cornerstone of special relativity. Einstein based his ideas on a lot of things that were done a couple of decades before, some of which were supposed to be part of a theory of the aether.
There was actually quite a lot of activity and vigorous debate between aether and something else, unknown at the time, that turned out to be relativity. Einstein did not just show up and invent everything. There is a very quick overview of this here: https://en.wikipedia.org/wiki/History_of_special_relativity
If you revisit what I wrote, you'll find that I agree with you.
There was vigorous debate about a wrong and an unknown thing (both was basically wrong) and it took Einstein's intuition and the new GR math to turn it into science.
I was drawing a parallel between this and the current MOND, string theory, dark matter debate. More specifically, I'd even say dark anything is our generations aether!
Dude, if you genuinely want to know what happened, you should read some proper history of science. Here take this: https://arxiv.org/pdf/physics/0405066
It shows both how Einstein very much didn't make the theory alone, was inspired to take impotat technical steps by work of other thats that created a theory based on his principles before him, and that actually he first created (in intense collaboration) a failed theory that got Mercury's anomaly all sorts of wrong.
you think the deepest mysteries of reality and the universe should just reveal themselves because we have a couple thousand smart people working on it for... 10 years?
Ok, I'll present my evidence an you can present yours.
In the latest Marcel Grossman meeting (2024) there was no string theory parallel session but there was an explicit Loop Quantum Gravity session: https://indico.icranet.org/event/8/program
So string theory is now pretty officially an "also ran", definitely not a leading theory.
MOND is dead is a true statement if we say MOND is dead as a general theory of gravity. It does not mean is does not have its success with explaining galactic rotation curves but failing at mostly everything else.
MOND is kinda like a dead horse now, that people like to keep flogging
I think it's possible for an alternative gravitational law to work, but not MOND
MOND is stronger at longer distances than Newtonian Gravity. To me that does not pass the sniff test. It could be a step in understanding a more exact law but to me it feels weird
I mean, there are modified gravity candidates other than MOND. I think people should perhaps give more consideration to those and not focus on MOND as their idea of modified gravity. I feel like the evidence is pretty well against MOND at this point. But other ideas of modified gravity, that aren't MOND-like, may still be worth considering. Framing it as "dark matter vs MOND" implicitly excludes these and I think that's a mistake.
This comment and GP are two of the most concise and punchy descriptions I've ever heard of some of the deepest aspects of modern physics. On the one hand we have principles of locality and finite propagation speed, which limit the computational work to a small neighborhood, and on the other hand we have principles of non-locality and superposition, which cause the computation to explode as it swallows up potentially everything and every possible thing.
- Locality is a consequence of the use of a scaled compute cluster with peer-to-peer networking only.
- The speed of light is the speed of information exchange between compute nodes.
- General Relativity is caused by network links with finite bandwidth taking longer to exchange information if there is more of it. (Effectively slowing down time)
- Quantum Mechanics is caused by finite precision of the n-bit numbers at small scales.
- MOND is caused by cumulative summation and rounding errors that show up only in the case of weak fields over large distances.
Dark Matter : supposedly makes up a big amount of the mass of the universe, but cant be seen, does not emit, absorb, or reflect light. Also it can 'pass through' other normal matter, and other dark matter.
It's basically magic aka not actually real, just something in vogue to pretend is real at the present moment.
The "pass through matter" is a consequence of not interacting electromagnetically. That's not that uncommon. Think neutrinos. (Also, "not" might just mean "very much reduced")
It's one thing to be electromagnetically inert, but if it is matter, it has mass, and if it has mass then it must be possible to collide with it. That we can't suggests it does not exist.
Yes, and dark matter will interact with visible matter gravitationally. When we say "doesn't collide with normal matter", it is not exact. It just means "the interaction length is very very long". Same for neutrinos. Their interaction length is huge as well: 1 TeV Neutrinos have an interaction length of 2.5 million kilometers.
> cant be seen, does not emit, absorb, or reflect light. Also it can 'pass through' other normal matter, and other dark matter
Why do you say that like it's obviously ridicule or impossible? Neutrinos do exist and they fit all these criteria. We just know they're not dark matter (or at least not all of it) because they're not heavy enough (and some other things).
Don't try to rely on intuition when thinking about particles, there's no reason for evolution to make what happens at quantum scale or relativistic scale intuitive to us.
I mean it's easy to say it's fake, but to counter this, why can a particle that only interacts with gravity not exist?
The neutrino is a good example of a particle that almost doesn't exist. They are produced in solar reactions in spectacular amounts. Trillions of them are flitting through you right now as if you don't exist. You'd need a light year block of lead to ensure you could stop one. Mind-boggling amounts of them have to pass through our detectors to see even a single interaction.
Simply put, the particle physics does not have to behave nice so you can sleep well at night.
I mean sure nature has no obligation to not have a unfalsifable particle, but you wind up in weird places, like, there exists a distribution of dark matter that explains the poltergeist that knocked over your coffee cup last week.
We measure the matter distribution by its affect on light (strong/weak lensing). We also measure the matter distribution by the amount of light coming from it. The results are not the same. The simplest explanation is that there is matter which does not produce or reflect light via e/m, i.e. it is dark. Dark Matter.
We know of particles which behave the same way. Neutrinos for example.
I mean I don't believe a particle that only reacts with gravity is unfalseifiable , it's that gravity just demands the use of unimaginable energies that we've not accomplished at this time.
You act like we've managed to probe the depths of physics with certainty when in reality you find any means to reject that which offends your sensibilities.
> You act like we've managed to probe the depths of physics with certainty when in reality you find any means to reject that which offends your sensibilities.
At the root of science is "sensibilities", like occam's razor, even "what counts as experimental reproduction", etc.
Dark matter doesn't necessarily have to be a new kind of particle (though there are enough constraints it's a bit hard to explain otherwise): it could be cold dust, gas, diffuse and tiny black holes, or large amounts of cold rocky planets.
Dark Unknown Matter would be a better name for lay people to understand what's going on. I'm no cosmologist but isn't it just a placeholder for something that gravity interacts with (and not much else) and we don't know what it currently is. When we discover what it is the name will change.
Yeah, pretty much, which is why this adherence to dark matter seems even more puzzling: we already had a mysterious substance with nonsensical mechanical properties (perfectly solid, but has zero collision) that turned out to be completely superfluous; the actual answer was the different shape of the physical laws. Now we again have a mysterious substance with nonsensical properties (has gravitational pull, doesn't interact with normal matter in any other way) — could it be that it simply doesn't exist?
And it's not like the concept of aether itself was really all that useful for anything. The physicists wanted the light to have some mechanical medium to propagate through instead of being a thing of itself, that actual itself shaped mechanical media, not the other way around (mechanical properties arise from the E-M interaction, not the other way around), simply because all other known waves phenomena existed in mechanical media.
Of course, it could be that dark matter does not exist. In a very real way, nothing in physics “exists” because like all natural sciences physics does not make statements of objective truth, it makes testable predictions.
Dark matter, string theory, aether, etc., those are models that we, at some point in time, think may help us get better predictions and design further experiments. All models turn out to be wrong in the end, but they can be helpful until we come up with better ones.
If you drop the dark matter model, then you would want to have some other model as for why we observe what we observe. Some people find that other available models are even worse than the dark matter one, but if you don’t think so you can take your pick.
Except that there is nothing nonsensical about a particle that has mass and doesn't participate in any SM interaction. It's inconvenient if such a particle exists, as it's very very hard to detect things precisely by their gravitational effects, but there is nothing nonsensical, or even particularly weird, about the idea. Plenty of particles only interact with a few of the SM forces - e.g. photons are not affected by the strong force, nor are electrons, neutrinos are affected by neither the strong force nor EM, only the weak force, gluons only interact with the strong force, not EM nor the weak force, etc
It's funny how for MOND we cant accept that it has some unknowns yet but we are more than willing to accept the FULL UNKNOWN Dark Matter. it's easy. put "Dark" in front of something and you don't have to explain it at all, no matter that something else explains at least 60-70% instead of 0.
Dark matter is invisible, but it isn't magic. It is not significantly different from neutrinos. No one seriously denies the existence of neutrinos nowadays, even though they are invisible (i.e. they don't interact electromagnetically).
Dark matter is actually a very parsimonious theory. None of the laws of physics have to change to accommodate it, unlike with MOND. We may not see it, but it has to move around and affect normal matter in predictable patterns consistent with our current understanding of physics. If it doesn't, then the theory is wrong and may need some revision (which may be a dark matter + MOND hybrid).
In parallel with the research that attempts to find the properties of dark matter that best describe our observations is research that attempt to find what other properties it may have. It is a new particle? Can it interact in ways other than gravity? We didn't find anything, but the universe is under no obligation to make things easy for us.
One possible idea called the "nightmare scenario" is that dark matter is made of particles that only interacts gravitationally. It is a perfectly fine theory, maybe the cleanest one, but unfortunately, it would mean that we may never be able to detect these particles because gravity is so weak that the required detectors would be way beyond our technological abilities.
We make a similar guess for the stuff that is in the center of Earth. We measure local gravity and speed of sound velocity, and we guess here is liquid, here is solid, here is this rock, here is this another rock [1]. See for example https://www.livescience.com/64943-nobody-understands-the-gia... nobody has seen them, we guess they are there.
Dark matter is another guess. We guess there is more matter in galaxies than what the telescopes show. We can compare the amount of mater guessed from galaxy rotation with other measurements. In this case they compare it with the gravity between a few galaxies.
Nobody is happy that we don't know what dark mater is. There are a few theories, but none of them has enough experimental support. More lack of confirmed details in https://en.wikipedia.org/wiki/Dark_matter#Composition
Dark matter is not another Guess. is no guess at all. its the same as saying the center of the earth is made of Dark Core. and when someone proposed liquid core, there would be papers how a liquid core doesnt explain 100 %, and neither does rock or solid so Dark Core it is because it's sIMpLer to have just one Dark Something than 3 different somethings to explain anything.
The fact that there are tentatives to identify what it might be does not ammegliorate the fact that at it's core (pun intented) Dark matter is something to make equations fit without any other thought behind it or whether there might be several things behind it or god forbid that we juddge the equations themselves. I mean we got relativity because of a minor discord with newtonian Laws. (the orbit of Mercury). just a tiny percentage of obeservable behaviour at that time but it was a different time. a time where you could bring down the existing science of the day for a tiny percentage and now we accept 90% observation disaccordance (dark energy+dark matter) with what the equation require.
> at it's core (pun intented) Dark matter is something to make equations fit without any other thought behind it or whether there might be several things behind it or god forbid that we juddge the equations themselves
Another way to interpret dark matter is that we can observe something using several different ways, but all those ways use gravity. When trying to observe this something using electromagnetism, we see nothing. It doesn't seem so crazy then to hypothesize that this something only interacts with gravity, and not electromagnetism.
> I mean we got relativity because of a minor discord with newtonian Laws. (the orbit of Mercury).
I don't think that's true. One of Einstein's test for General Relativity, using Mercury's orbit, came around 10 years later after special relativity was proposed, which is understood to be motivated by both Maxwell's equations and experimental results suggesting that the speed of light (electromagnetic waves) not depending on the frame of reference. General relativity (explaining gravity) seem to have been motivated by Newton's gravity not playing well with Special relativity (with mass being relative and all).
I understand the frustration with Dark Matter, but my understanding is that Dark Matter is a guess that is known to be incomplete. Scientists are shooting everywhere to try to explain the discrepancy in gravitational effects and some form of undetected matter is currently the best hypothesis (but not the only one). You say that "we accept 90% observation disaccordance", but the source of its effect is being searched for.
The "it looks like there's stuff we didn't previously know about here" class of guesses has historically been fruitful, yielding, among other things, the discovery of several planets. Maybe dark matter isn't the answer, but it's not "no guess at all."
> just one Dark Something than 3 different somethings to explain anything
There are many candidates and I think nobody discarded it's a mix, it looks like a plausible scenario. I made a quick look in Wikipedia, but I didn't find anything relevant to a mix of Dark Mater.
There is(was?) some big discussion about cold vs hot (and warm?) dark mater. IIUC "cold" dark mater won. I'm not sure if there is enough details to make a good guess of the split, or if it's a 100.00% vs 0.00% case.
No, it's not quite "no guess at all". Using gravitational lensing to estimate how much mass there is in a part of a galaxy is pretty smart. It turns out that there sometimes indeed is mass (because the lensing is there) even though it doesn't seem to give off much light.
(But I don't really believe in dark matter, either. It's mostly epicycles and phlogiston.)
Because MOND doesn't actually explain anything, it's basically just fine tuning equations to make them fit with observations. Dark matter is the opposite, it takes all the observations and induces a fitting explanation
I thought newtonian gravity was already proven to be inaccurate with Einstein's Special Relativity (or General Relativity?) giving better results on cosmic scales (basically analogous to an approximation vs an exact formula)?
General Relativity reduces to Newtonian gravity as the curvature goes to zero, that is when you're very far away from objects relative to their masses, for slow non-relativistic objects like stars and galaxies.
Galaxies are typically so far away from another they're almost like point sources to each other, hence Newtonian gravity explains their motion very well.
However, inside galaxies things do not behave as expected, as stars in almost all the galaxies we've measured does not move like Newtonian (nor GR) behaves based on the matter in the galaxy we see. One alternative to the mainstream theories of dark matter is to modify Newtonian gravity, called MOND.
This work tested if MOND fit the motion of galaxies in galaxy clusters. They found it did not.
MOND already does not explain other phenomena that dark matter can so it's not terribly surprising. Here[1] is a nice accessible talk going through all the evidence for dark matter.
But it is technically a possibility that there's two things are going on, something MOND-like as well as dark matter, so worth checking.
Other way around. Newtonian dynamics explains the data very well, MOND did not.
In particular, Newtons law of gravity says the effect of gravity falls off as 1/r^2 where r is the distance from the mass. MOND modifies the standard equations so that gravity starts like 1/r^2 when r is small, and acceleration is large, but for greater distances, when the acceleration is low, instead falls off like 1/r.
MOND explains the movement of the stars in (most) galaxies very well. However this result showed that MOND was not consistent with the motion of the galaxies in the cluster. On the other hand the motion was consistent with plain Newtonian dynamics. Hence Newtons law of gravity (and by extension GR) passed the test.
Special Relativity is an extension of Galilean/Newtonian mechanics (motion of projectiles and other objects) to the case where the object is travelling at speeds that are a fraction of the speed of light. It deals with non-accelerating frames of reference. Satelites need to use this to correct for time dilation effects, but tracking the trajectory of an arrow/etc. or a car/etc. travelling from one location to another then classical mechanics is sufficient.
General Relativity is an extension of Newtonian gravity. It is also an extension of Special Relativity to cover accelerating frames of reference. Satelites need to use this, as does tracking the orbit of Mercury. However, for the orbits of other planets and the moon, using Newtonian gravity is sufficient for a reasonable degree of accuracy, and is used for tracking things like equinoxes/solstices, full moons, etc..
At these scales (entire galaxies, very weak forces), it doesn't make a significant difference.
There are ways of adapting MOND to match general relativity, should it turn to be correct at explaining what it is supposed to explain (like the movement of galaxies).
I think OP's question is more how could Newton's law "pass" a test any more than General Relativity would, considering that it's merely an edge case of GR?
It is an argument against MOND, a theory that says that gravity has to be modified to account for some observations. But for these particular observations, general relatively and Newton's laws gives the same results in practice, the difference is negligible, so showing that these observations can be explained by Newton's laws implicitly mean that they can also be explained by general relativity. No need for modifications.
General Relativity. It explained the anomaly in the precession of Mercury's perihelion, and the bending of starlight by the Sun (double the value predicted by Newton's law).
We measure the matter distribution by its affect on light (strong/weak lensing). We also measure the matter distribution by the amount of light coming from it. The results are not the same. The simplest explanation is that there is matter which does not produce or reflect light via e/m, i.e. it is dark. Dark Matter.
We know of particles which behave the same way. Neutrinos for example.
It's bizarre that Newton's law passes this test, when the linearization of general relativity predicts a gravitational equivalent of a magnetic analog for gravity.
Gravity Probe B.
There exists an even simpler derivation for a magnetic term, but its in my file drawer... (simpler in that it just relies on observations from special relativity)
If it had failed, would we be hearing about it in Science before the researchers continued testing it until it passed? (i.e. is this a blindspot in our collective epistemology?)
If it had failed it would've been a top science news. Every scientist dreams about the day when they're mentioned as "(...)'s experiment puts Newton's law of gravity in jeopardy."
Would it? Or would it be treated as an errant experiment by either the journal or the experimentalists themselves, foregoing publishing until they had a result that was better aligned with established norms? Rightly or wrongly, I expect that stakeholders would tend to invoke Sagan’s ECREE over the course of the publication process.
When I see dark matter in the news I'm always reminded of the story of Vulcan.
In the 1800s, detailed observations of the planet Mercury showed that its orbit was slightly different than Newtonian mechanics predicted-- a difference of about 43 arcseconds per century. The study was rigorous enough to rule out any observation errors.
Le Verrier, the astronomer who made these observations, wondered how to explain the difference. A decade earlier, he had noticed a similar irregularity in the orbit of Uranus, which led to the discovery of Neptune, whose gravity caused these perturbations. So Le Verrier reasoned that something similar must be going on for Mercury, and he posited the existence of Vulcan, a tiny planet close to the Sun.
Many attempts were made for decades to observe Vulcan. It was even included on some maps of the Solar System at the time (https://www.loc.gov/resource/g3180.ct003790). But it was never conclusively observed.
When Einstein published his theory of relativity in 1915, the mystery of Mercury's orbit was finally explained-- Newtonian mechanics were simply incomplete, and the irregularity of Mercury's orbit was due to relativistic effects.
Could it be that something similar is happening today? Observations of gravity on galactic scales doesn't quite align with what relativity would predict, so we use dark matter to fill the gaps. We've tried for decades to detect dark matter, with no dice. Is our theory of gravity simply incomplete?
MOND may not be the solution, but I'm still skeptical about dark matter.
There are multiple independent observations pointing toward dark matter:
1. Galaxy rotation curves. 2. Galaxy cluster mass measurements from gravitational lenses and infrared. 3. Cosmic Microwave Background models (mass measurements from inhomogeneities that correspond to acoustic waves, for eg).
MOND only explains 1.
Dark matter accounts for all 3. Only catch is that it hasnt been directly observed.
> Only catch is that it hasnt been directly observed.
Ok, so... why do people take it seriously as a concept? Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.
> Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.
For some general misunderstanding to explain what we see, ie something which is not dark matter, it means our equations of gravity must be wrong.
People have tried hard to tweak the equations to fix the observations. One such proposal is indeed MOND that the submission is about.
The problem with tweaking the equations is that we have a lot of observations of very different circumstances and scales, and it's so far proved impossible to tweak the equations in a given way so they explain all of the observations. What happens is that you can tweak them so they fit one set of observations, like MOND fitting galaxy rotation curves, but then not fit other sets of observations, like MOND with the motion of galaxies in galaxy clusters as discussed in the submission.
Meanwhile, adding dark matter seems to work much better at explaining all the observations at once. I say much better because there is still some tension, but it's way less than any modifications of gravity.
Having dark matter particle(s) is also not that strange. As far as we know there's no requirement that a particle interacts with any of the known forces beyond gravity. And while the early particle candidates seem to be getting ruled out, axion-like particles, originally proposed to solve entirely different problems, are still a good dark matter candidate.
That said, it's not settled. People are proposing alternatives. One example I know of is Alexandre Deur[1], a QCD physicist which has taken his QCD knowledge into the realm of GR and proposed that perhaps graviton-graviton self-interaction can explain a lot of the dark matter phenomena. It's not mainstream, but he's getting published in peer-reviewed journals.
[1]: https://arxiv.org/search/gr-qc?searchtype=author&query=Deur,...
> Having dark matter particle(s) is also not that strange. As far as we know there's no requirement that a particle interacts with any of the known forces beyond gravity.
This is the key thing. As we still don't have a theory of everything, which explains the fundamental reasons why every particle is the way it is, dark matter is a very straightforward and simple explanation.
The name makes it sound stranger than it is. It's just particles that have mass and are affected normally by gravity, but which are unaffected by the electromagnetic force.
It's not a particularly exotic or implausible or wild idea. In fact it's so simple it's almost boring. It's just obviously inherently difficult to test directly, since gravity is such a weak force.
Because stuff like the Bullet Cluster exists. Dark matter is in fact the simplest answer.
https://en.wikipedia.org/wiki/Bullet_Cluster
I feel compelled to point out that (a)ether was the simplest answer until we confirmed that space is a vacuum and that light can travel in the absence of a medium.
aether was not the simplest explanation since it was disproved by Michelson-Morley experiment. Nothing as of now yet has disproved Dark matter as thoroughly.
The two were dealt with very differently.
Aether: observations show lack of aether so we update the theory that makes aether unnecessary.
Dark matter: observations show lack of observable matter so we keep hunting for decades locating it. The aether analogue would have been to continue to look for dark aether. Dark matter is more like god of gaps. We can 'darkify' any theory that does not fit empirical evidence.
It's hard to tell now which leans more towards the more correct theory.
Abandoning aether was NOT a "simple" answer. One you abandon aether, all manner of weirdness suddenly pops in.
Light being the same speed irrespective of observer is weird. Velocity dilating length and time is weird. Not having a preferential observation point is weird. Not needing a medium for transmission is weird. Not being able to agree on simultaneity is weird. etc.
Aether wasn't just something that a bunch of dullards clung to. You have to abandon some very long held common-sense understanding when you give it up.
That's my point exactly.
Angry gods used to be the simplest answer for a lot of things. Inevitably we find a better model
You are of course very welcome to propose this supposed better model, but please do some due diligence and learn to roughly understand the current flawed models firsts.
For example "angry gods" was never a simple theory, one needs only to read some fan fiction to understand that theology gets complicated fast. Instead "angry gods" is a simplified summary of a very complicated theory about metaphysical hierarchy, creation, agency, the meaning of divinity, and cause and effect, etc etc.
Simplest in this sense does also imply predictability for Dark matter, which cannot be said for angry gods. Angry gods is anything but a simple theory.
We already know that some particles interact extremely weakly with others. Neutrinos were postulated as a "dark" particle in order to preserve conservation of energy and momentum, but it took more than a decade before they were observed directly. There's no reason there couldn't be another particle with an even weaker interaction.
It doesn't even matter whether it actually exists, because it works for modeling the universe and making accurate predictions about it.
That means for now it's useful.
We have indirectly observed it a bunch, and our understanding of physics allow the existence of material that flatly cannot be directly observed.
A "general misunderstanding" isn't really a theory whose complexity you can compare to the Dark Matter theory.
One could also argue that on the one hand you need just one concept (dark matter) to explain three phenomena (galaxy rotation curves, gravitational lensing, CMBR), and the "general misunderstanding" might need to have three aspects to make it explain all three things, making it more complex.
Occam's razor is a good guiding principle, but in some cases, it's really hard to compare the complexity of competing theories. My favorite example of this is the Many Worlds interpretation of quantum mechanics. In some sense, it's really simple, the wave function just follows the Schrödinger equation. In another sense, it implicitly postulates the existence of many parallel worlds, which you can count as a quasi-infinite complexity penalty, or you can shrug and say it's not a big deal, the theory says the parallel worlds aren't observable, so there is nothing wrong with postulating them. There's no objective way to decide that.
>Occam's razor would point toward...
Occam's razer is not a thing with explanatory or legal or even scientific force. if Occam's razor was really a thing, quantum theory and general relativity would be ruled out as complicated and neither 100 percent explanatory.
> Ok, so... why do people take it seriously as a concept?
Historically, science has predicted lots of true things about the universe that could not be observed at the time.
> Historically, science has predicted lots of true things about the universe that could not be observed at the time.
Just some examples I could think of:
The neutrino[1], predicted in 1930 due to missing mass in beta decays, discovered in 1956.
Gravitational waves[2], predicted by Einstein in 1916, indirectly detected via pulsars in 1974 and directly by LIGO in 2015.
Higgs boson[3], predicted in 1964 to solve the problem of the Standard Model at the time requiring particles to be massless, which we knew they weren't. Hints from LEP at the turn of the millennium, detection by the LHC in 2012.
Cosmic microwave background[4], predicted in 1948 as a consequence of the expanding universe on the early radiation. Detected by accident in 1964.
Gravitational lensing[5] was first proposed in 1801 as a consequence of Newtonian corpuscular theory of light. Einstein realized in 1915 that GR predicted a larger value. Weak lensing by the sun was observed in 1919. Strong lensing by a distant galaxy of an object behind the galaxy was was predicted in 1937, and first observed in 1979.
[1]: https://en.wikipedia.org/wiki/Neutrino#Pauli's_proposal
[2]: https://en.wikipedia.org/wiki/Gravitational_wave#History
[3]: https://en.wikipedia.org/wiki/Higgs_boson#History
[4]: https://en.wikipedia.org/wiki/Cosmic_microwave_background#Hi...
[5]: https://en.wikipedia.org/wiki/Gravitational_lens#History
I don't know what or why our science education is like this, but it seems like everybody's understanding of science bottomed out at a straw man version of popper & positivism.
And to be clear, falsification and being empirical & skeptical about theoretical claims is great. What I see all too often on the Internet is just pattern matching to the words "observable" and "falsification" without a second thought, without actually looking into how science develops, and any and all narratives are historically rewritten to fit only those two categories.
Which is why it's even more impressive to be a real scientist, to actually be able to navigate the muddy waters properly where it's not just some simple adjective checklists to run through. (As a non scientist)
I feel the same, but then I remind myself of how I used to feel about dark matter (I really disliked it). Having an arena where no scientific question is out of bounds is great.
Because the discrepancy exists and shows consistency. This gap needs to be called something, you can't just pretend it's not real. But the underlying model seems fine and all alternatives are broken.
So we came up with an idea that fills the gap in the current model and allows us to continue predictive science until we find out more.
People get so absurdly touchy about Dark Matter just because it's named and "weird". It's just a placeholder value.
> Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.
Dark matter is the Occam's Razor theory. It explains almost all of the observations while assuming the least.
Why do you assume we have no evidence just because we don't have direct observations? Black holes were a similar phenomenon that we had no direct evidence for a fairly long time even though we had lots of other indirect phenomenon that really couldn't be explained any other way.
The razor is commonly misunderstood.
William of Ockham objected to his fellow theologians inventing things out of whole cloth (like dark matter question mark). That’s the point, not that a simpler explanation is more likely to be true.
The common understanding would have us believe that creationism, being simpler, outshines evolution, or that there is no such thing as a color revolution because the simplest explanation is that the mass protests are earnestly aggrieved locals.
Yours is a common misunderstanding, as well.
Occam's Razor only kicks in when the hypotheses have equal predictive and explanatory power.
The MOND hypothesis does NOT have equal explanatory power with the dark matter hypothesis. As such, Occam's Razor is not relevant.
Maybe you can point me to the "equal predictive and explanatory power" bit in William's writings.
I blame sci-fi.
As a kid in 90s who was into this stuff, it seemed like anyone who knew about dark matter knew it was just theorized and never observed. It was included as a caveat in a lot of things I read to ensure the readers understood this.
Then in sci-fi it started showing up as a material the characters would interact with, and a lot more people were introduced to the term.
Then as a teen in the 2000s the caveat was usually gone and it started being treated as something we had proof of. People I knew started thinking it was something we'd actually found and was now fact instead of theory.
> Ok, so... why do people take it seriously as a concept? Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.
One scientist says "We don't know, really, it's a head scratcher" and the others say something cool like "Dark energy", which ones will get more attention and publicity?
But it really isn't a head scratcher, and it's not a wild guess, and also, dark matter is not the same as dark energy.
Think about some random person born in the 1950s. It will be impossible for you to directly prove that that person has two parents, but with all your knowledge about biology, and your knowledge that humans could not be cloned in the 1950s, it's not a head scratcher, unless our understanding of biology is completely off, it's not just speculation there were two parents. That's the comparable situation with dark matter. It's not a head scratcher. Every indication shows there is a lot of mass around that doesn't interact with light.
Associating dark matter with epicycles is unfair, but it's still a risk (CDM, WDM, SIDM, and probably more by next year). In that light, of course it has more observations (by virtue of creating branches to explain incompatible observations) - but counterpoint is that it has made some predictions.
My stance is that anyone pointing at it in either light probably isn't taking everything into account. It's an incredibly immature theory space - are we going to get 20 more branches of it (making it modern epicycles), or are we going to see one of the current branches pay off?
I mean there are modified gravity candidates other than MOND. I think those are worth more consideration. MOND seems pretty well disproven at this point, yet somehow it's still what people focus on in terms of modified gravity! Where are the tests of other modified gravity theories?
Indeed, in terms of the negatives of MOND, I'll go further than you -- MOND barely explains galaxy rotation curves at all. When you look at the actual rotation curves compared to MOND's predictions, well, yes MOND does better than Newton w/o dark matter, but it's still pretty heavily fudged. If you have to fudge it that much, it seems to me there's no point. So I'd say that really MOND doesn't explain any of the three.
It can't be observed, but it affects all these things.
This reminds me of arguments on the alt.athiesm newsgroup back in the day.
MOND explains 1.
2 is "who can say" because nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND).
3 is subject to questions like "is the CMB really what we think it is" -- if it's early thermalized dust, then that ALSO resolves hubble tension, e.g.
MOND explains several things LCDM cannot:
- why most elliptical galaxies seem to "not have dark matter" (effectively a prediction)
- external field effect (predicted and confirmed)
- renzo's rule
- DM halos that are way too big
- early galaxies (this was a prediction)
HM. people have been downvoting. Anyone care to post a substantive rebuttal?
2 needs a more substantive rebuttal. LCDM correctly predicts where the dark matter is located after a galaxy collision, such as in the bullet cluster. There is no reasonable interpretation of MOND that has the center of mass of the galaxy shifted away from where it's visible matter lies, precisely how LCDM says it should be.
There is a reason why LCDM used to be a lot more disputed before the work of Clowe, Gonzales and others on the bullet cluster, and is now generally treated as settled science by practitioners. We might still be surprised by something, the universe is more wondrous and complex than we can possibly understand, but Occam's razor massively supports LCDM now. If you want to propose any alternative, you need to start by showing how it explains bullet cluster as well or better than LCDM. (And the bullet cluster specifically is not the only place where this is visible, there are others like MACS J0025.4-1222.)
> LCDM correctly predicts where the dark matter is located after a galaxy collision, such as in the bullet cluster. There is no reasonable interpretation of MOND that has the center of mass of the galaxy shifted away from where it's visible matter lies, precisely how LCDM says it should be.
It does not really make that "prediction", its a post hoc assignment of dark matter density based on weak lensing for which you can make a plausible "this is how it started" explanation.
you can counter with lcdm cant explain tons of stuff that MOND can, from tully fisher relation through barred spiral galaxies (n >> thousands) etc.
> nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND).
My take on this is a bit different. The obvious way to get a relativistic theory that looks like MOND is to add a scalar field. (And possibly a vector field, as in TeVeS.) But from a relativistic, quantum field theory perspective, adding more fields is just...adding a new type of "matter". I.e., dark matter!
In other words, when you take relativity into account, "MOND" and "dark matter" don't really look like two different possibilities any more. They just look like two different ways of describing the same math in imprecise ordinary language.
"HM. people have been downvoting. Anyone care to post a substantive rebuttal?"
Just that asking this might get you more downvotes. I upvoted as I found your input interesting. I would suggest to edit that out (then I will delete my comment)
I don't really care (i'm doing just fine fake-internet-points-wise), just if I made a mistake it would be nice to know. Otherwise worth it to know that there are people out there who find "alternative explanations by professional scientists" threatening. for some reason.
Well, I rather meant there are guidelines to keep this space meta debate free and I do think they make sense.
(And apart from that, HN is not free of dogmatic ideologists of all sorts who feel triggered by anything straying off from mainstream theory. That is my guess here and my advice to ignore it, but I would be also curious if there is something clearly wrong with your comment deserving downvotes.)
2. It's called relativistic MOND. That exists as a category. RelMOND(for short) includes TeVeS, Einstein–Aether variants, BIMOND, and others.
3. I would wager that what we call the CMB is more of a local effect than a big bang remnant.
I lack the karma to downvote, don't blame me. :)
2. not a physicist but iiuc the relativistic MONDs have been disappointing? but not enough to rule out
Of course it can be that this is the case. There are predictions from the dark matter hypothesis, though, that have been testable. For example, there're some of the asymmetries in the cosmic microwave background that are explained presently by the dark matter hypothesis.
Something that's worth bearing in mind is that "dark matter" doesn't actually mean "totally new never before seen thing" it means "we don't know what this matter is". So, for example, a candidate that wouldn't be super novel but could fit the bill is microscopic black holes. In that sense, the hypothesis is more mundane than it might seem.
As far as I know, dark matter hasn't made any testable predictions which were subsequently proved correct. It's an infinitely tunable hypothesis that can be retro fitted to new observations, since it has many tweakable parameters.
Could you point me to a prediction made by dark matter theory which was subsequently confirmed by observation, rather than a dark matter theory which explains it post facto?
Not sure why the downvotes just for asking a question.
Dark matter (well, LCDM) has predicted many things which were proven false, e.g. that big galaxies assembled through merging with smaller ones over long periods of time. JWST shows us this is not in fact the case. There are now lots of papers showing how LCDM can explain that, but these are post facto explanations.
Happy to be proven wrong if someone can point me to a prediction subsequently confirmed by observation.
Is this the "hot" vs "cold" dark mater discussion? (Not my area. I just read that in Wikipedia.)
Not really, Cold Dark Matter is part of LCDM, the main cosmological model. Whether hot or cold, I'd like to see a solid prediction made for it confirmed subsequently by observation.
100%. It's important to realize our understanding of "dark matter" is fuzzy because we only understand it through data anomalies. Dark matter is a classic catch-all concept that we use as a crutch while we try to understand the underlying system better.
Similar to how we used to believe in "aether" to explain how light could travel through empty space. It is important to understand how these crutches help and hinder understanding.
It's kind of weird in this case, though. All the math acts like there's something invisible and heavy everywhere that we find clumps of visible matter. When we look at the motion of galaxies, they behave as if they're much more massive than the count of stars and such in the would have you believe, and in ways that otherwise jibe with our understanding of physics if only that galaxy were heavier.
If you have one galaxy that's acting heavier than you can eyeball, measured by things like light bending around it, then maybe you have some weird phenomenon. When every galaxy calculates out to be about 6x fatter than you'd expect, something else is going on.
Except (IANotAnAstrophysicist) it's not every galaxy. There are two that stick out as having little to no dark matter.
Of course this is hotly contested as it destroys the argument for MOND, but research is ongoing.The constituent particles of nature are under no requirement to be apparent or achievable to you.
The strongest evidence that something like MOND isn't the answer is that in some galaxy collisions, the visible matter and the dark matter appear to separate: the collision disrupts the visible matter and the dark matter appears to pass right through, uninterrupted, and we see galaxy remnants that look like they don't have dark matter. If MOND or some other modification of gravity were the answer we'd never see this kind of sorting.
See, for example, https://www.caltech.edu/about/news/dark-matter-flies-ahead-o...
Astrophysicists have gone all through this. Absent direct observation or at least a good explanation of what it actually is, I don't think anyone really likes dark matter. The main objection is that it feels a little too flexible: you can explain a lot of things by adding mass in different places. But it's not infinitely flexible: you don't need arbitrary and utterly weird distributions of dark matter. And so it's generally emerged as the best explanation of observations so far, and not for lack of trying with tweaks to theories of gravity themselves.
yes - absent experimental confirmation, the effects attributed to dark matter may in fact be due to an unrelated as-yet-undiscovered phenomenon.
You might be interested in that. We have observations of galaxies where dark matter likely is not present.
https://news.ycombinator.com/item?id=47641057
If you follow Sabine Hossenfelder's channel, she has a MONDOmeter. With MOND (modified Newtonian gravity) on one side and dark matter on the other side.
As new papers come out the needle goes back and forth, and I guess that she will make a new video if she hasn't already, with the needle moving one step towards dark matter.
I find it interesting how it doesn't seem to settle. Dark matter is still the favorite, but there is a lot of back and forth between "MOND is dead" and "we found new stuff we couldn't explain with dark matter, but it matches MOND predictions".
MOND does amazingly well at galactic rotation curves, less well at anything else. If you think it started with Vera Rubin in 1966 MOND seems natural, but if you know that it started with Fritz Zwicky in 1933 than dark matter is easier to believe.
MOND only really does well on galactic rotation curves because it has free parameters that are tuned to "predict" the correct answer for galactic rotation curves.
I think you mean LCDM only does well on galactic rotation curves because it has free parameters per galaxy. MOND only has one free parameter, maybe two if you use the MOND+Relativity model that doesn't work.
I don’t think these are free parameters in the same sense.
Like, if one theory says that a hunk of metal actually is made of many microscopic grains of various sizes and orientations, where the sizes and orientations of these grains has an effect on the behavior of the metal, you don’t count the “the sizes and orientations of these grains” as free parameters, do you?
You would if you didn't have any ability to observe those sizes and orientations.
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There are galaxies that appear to be free of dark matter and rotate accordingly. How does MOND account for that?
My understanding is that these observations are a fatal blow to any serious MOND models.
MOND obviously don't have to account for the lack of dark matter, as all galaxies lack it under MOND.
You have to actually do the calculations and compare what MOND output to the observed behaviour of the galaxies in question.
MOND reduces but doesn't eliminate the need for dark matter.
We are likely going to find out that both are unfixably faulty.
It'll take either the next Einstein or some groundbreaking experimental observation to get there in my opinion.
If it was possible to incrementally fix these theories, the army of postdocs working on these would have already done so in the last decade or so.
But at least the experimental results disproving these incremental fixes should be exactly the kind of thing the next Einstein should need for coming up with an entirely new way of looking at things
Interestingly, more often than not it happens the other way.
Some once-in-a-generation scientist has an intuition that turns out to be true and mathematically elegant.
It gets proven experimentally years or decades later.
Relativity was exactly like this.
> Some once-in-a-generation scientist has an intuition that turns out to be true and mathematically elegant.
That’s a bit simplistic. There was a lot of research activity around the aether in the late 19th century that was ultimately useful for the foundation of special relativity. Like the Michelson-Morley experiment, which was supposed to measure aether winds, but showed it did not actually seem to exist. Lorentz developed his transform as a theory of the aether, but it became a cornerstone of special relativity. Einstein based his ideas on a lot of things that were done a couple of decades before, some of which were supposed to be part of a theory of the aether.
There was actually quite a lot of activity and vigorous debate between aether and something else, unknown at the time, that turned out to be relativity. Einstein did not just show up and invent everything. There is a very quick overview of this here: https://en.wikipedia.org/wiki/History_of_special_relativity
Did I write invent?
If you revisit what I wrote, you'll find that I agree with you.
There was vigorous debate about a wrong and an unknown thing (both was basically wrong) and it took Einstein's intuition and the new GR math to turn it into science.
I was drawing a parallel between this and the current MOND, string theory, dark matter debate. More specifically, I'd even say dark anything is our generations aether!
It was far from exactly like that. GR was in part prompted by the precession of the perihelion of mercury for which there was plenty of data.
Look at the timeline here https://en.wikipedia.org/wiki/Tests_of_general_relativity
Dude, if you genuinely want to know what happened, you should read some proper history of science. Here take this: https://arxiv.org/pdf/physics/0405066
It shows both how Einstein very much didn't make the theory alone, was inspired to take impotat technical steps by work of other thats that created a theory based on his principles before him, and that actually he first created (in intense collaboration) a failed theory that got Mercury's anomaly all sorts of wrong.
you think the deepest mysteries of reality and the universe should just reveal themselves because we have a couple thousand smart people working on it for... 10 years?
If you could get there with minor modifications of the current state of the art, yes.
My point is that you likely can not.
my understanding is that there are a few MOND champions who are still holding on to the idea while everyone else has moved on.
so MOND is the new String Theory...
It never had the institutional imprimatur of string theory.
String theory is still the leading contender for quantum gravity.
I don't think "leading" is correct, more like "ill regarded but still standing".
It has fallen far from its glory days, and is more an old reliable tool than a leading physical theory.
It’s correct because there is no competing theory for quantum gravity that most physicists would consider being more likely to be on the right track.
This is different from MOND, which is a distant second contender compared to dark matter.
Ok, I'll present my evidence an you can present yours.
In the latest Marcel Grossman meeting (2024) there was no string theory parallel session but there was an explicit Loop Quantum Gravity session: https://indico.icranet.org/event/8/program
So string theory is now pretty officially an "also ran", definitely not a leading theory.
MOND is dead is a true statement if we say MOND is dead as a general theory of gravity. It does not mean is does not have its success with explaining galactic rotation curves but failing at mostly everything else.
MOND is kinda like a dead horse now, that people like to keep flogging
I think it's possible for an alternative gravitational law to work, but not MOND
MOND is stronger at longer distances than Newtonian Gravity. To me that does not pass the sniff test. It could be a step in understanding a more exact law but to me it feels weird
I mean, there are modified gravity candidates other than MOND. I think people should perhaps give more consideration to those and not focus on MOND as their idea of modified gravity. I feel like the evidence is pretty well against MOND at this point. But other ideas of modified gravity, that aren't MOND-like, may still be worth considering. Framing it as "dark matter vs MOND" implicitly excludes these and I think that's a mistake.
Until you definitively rule out all the dark matter candidates or get a direct detection the controversy will remain
Once I joked that a lot of things in the universe make sense if you view it as a "simulation with optimizations like lazy loading".
Yeah until you get to quantum computing and then it seems as if the universe is doing enormously more work than you would think necessary.
This comment and GP are two of the most concise and punchy descriptions I've ever heard of some of the deepest aspects of modern physics. On the one hand we have principles of locality and finite propagation speed, which limit the computational work to a small neighborhood, and on the other hand we have principles of non-locality and superposition, which cause the computation to explode as it swallows up potentially everything and every possible thing.
It might just be a reflection of the architecture the universe simulation is running on...
See Timmy, this is what happens when you run your universe on a holographic medium at an infinite distance.
Not necessarily.
You'd be correct given hidden variables.
But we know pretty convincingly that quantum anything does not have hidden variables.
https://en.wikipedia.org/wiki/Bell%27s_theorem
It doesn't have local hidden variables. That's an important distinction.
I'm not sure a non-local hidden variable explanation of QM is any distinct from superdeterminism though.
> non-local hidden variable
Like, global variables?
Naked singletons in your locality.
Alright, who's been messing with the universal gravitational constant and making it not universal? No one's in trouble, I just want to know.
There's another tantalizing possibility, that it varies over time rather than across space.
But only if someone observes it. The act of observation forces reality into existence.
The analogy goes surprisingly far:
- Locality is a consequence of the use of a scaled compute cluster with peer-to-peer networking only.
- The speed of light is the speed of information exchange between compute nodes.
- General Relativity is caused by network links with finite bandwidth taking longer to exchange information if there is more of it. (Effectively slowing down time)
- Quantum Mechanics is caused by finite precision of the n-bit numbers at small scales.
- MOND is caused by cumulative summation and rounding errors that show up only in the case of weak fields over large distances.
Etc…
Everything we don't understand we conceptualize using the most similar tools which we do have command over.
Dark Matter : supposedly makes up a big amount of the mass of the universe, but cant be seen, does not emit, absorb, or reflect light. Also it can 'pass through' other normal matter, and other dark matter.
It's basically magic aka not actually real, just something in vogue to pretend is real at the present moment.
The "pass through matter" is a consequence of not interacting electromagnetically. That's not that uncommon. Think neutrinos. (Also, "not" might just mean "very much reduced")
It's one thing to be electromagnetically inert, but if it is matter, it has mass, and if it has mass then it must be possible to collide with it. That we can't suggests it does not exist.
Yes, and dark matter will interact with visible matter gravitationally. When we say "doesn't collide with normal matter", it is not exact. It just means "the interaction length is very very long". Same for neutrinos. Their interaction length is huge as well: 1 TeV Neutrinos have an interaction length of 2.5 million kilometers.
> cant be seen, does not emit, absorb, or reflect light. Also it can 'pass through' other normal matter, and other dark matter
Why do you say that like it's obviously ridicule or impossible? Neutrinos do exist and they fit all these criteria. We just know they're not dark matter (or at least not all of it) because they're not heavy enough (and some other things).
Don't try to rely on intuition when thinking about particles, there's no reason for evolution to make what happens at quantum scale or relativistic scale intuitive to us.
I mean it's easy to say it's fake, but to counter this, why can a particle that only interacts with gravity not exist?
The neutrino is a good example of a particle that almost doesn't exist. They are produced in solar reactions in spectacular amounts. Trillions of them are flitting through you right now as if you don't exist. You'd need a light year block of lead to ensure you could stop one. Mind-boggling amounts of them have to pass through our detectors to see even a single interaction.
Simply put, the particle physics does not have to behave nice so you can sleep well at night.
We are still in the "ether" times of dark matter. We have still not had a Michelson-Morley experiment. That's it.
Not that I am saying it does not exist. Only that we do not have the means of falsifying it if it is false.
I mean sure nature has no obligation to not have a unfalsifable particle, but you wind up in weird places, like, there exists a distribution of dark matter that explains the poltergeist that knocked over your coffee cup last week.
If there would be a distribution of dark matter that explains the poltergeist, we could measure that distribution of dark matter.
We can measure the mass distribution on astronomical scales. We "see" the dark matter. Just not with light.
We don't measure dark matter, we measure some anomaly and then we say "it must've been dark matter".
It's not crazy different from saying the same about that poltergeist.
We measure the matter distribution by its affect on light (strong/weak lensing). We also measure the matter distribution by the amount of light coming from it. The results are not the same. The simplest explanation is that there is matter which does not produce or reflect light via e/m, i.e. it is dark. Dark Matter.
We know of particles which behave the same way. Neutrinos for example.
You're saying the same thing, we see some anomaly in measured light and we say "the simplest explanation is dark matter".
We are not measuring dark matter, we are measuring something that is not what we expect and we decided it's dark matter.
I mean I don't believe a particle that only reacts with gravity is unfalseifiable , it's that gravity just demands the use of unimaginable energies that we've not accomplished at this time.
You act like we've managed to probe the depths of physics with certainty when in reality you find any means to reject that which offends your sensibilities.
> You act like we've managed to probe the depths of physics with certainty when in reality you find any means to reject that which offends your sensibilities.
At the root of science is "sensibilities", like occam's razor, even "what counts as experimental reproduction", etc.
Dark matter doesn't necessarily have to be a new kind of particle (though there are enough constraints it's a bit hard to explain otherwise): it could be cold dust, gas, diffuse and tiny black holes, or large amounts of cold rocky planets.
Dark Unknown Matter would be a better name for lay people to understand what's going on. I'm no cosmologist but isn't it just a placeholder for something that gravity interacts with (and not much else) and we don't know what it currently is. When we discover what it is the name will change.
Or, you know, as aether.
It's a scientific theory. It's the best that we have right now to model the real world and be able to do prediction on its behavior.
Does it seems to be kept together by duct tape? Maybe.
Is it yet useful? Yep.
Will it be discarded if anything more fitting will came up? You can be sure of it.
Yeah, pretty much, which is why this adherence to dark matter seems even more puzzling: we already had a mysterious substance with nonsensical mechanical properties (perfectly solid, but has zero collision) that turned out to be completely superfluous; the actual answer was the different shape of the physical laws. Now we again have a mysterious substance with nonsensical properties (has gravitational pull, doesn't interact with normal matter in any other way) — could it be that it simply doesn't exist?
And it's not like the concept of aether itself was really all that useful for anything. The physicists wanted the light to have some mechanical medium to propagate through instead of being a thing of itself, that actual itself shaped mechanical media, not the other way around (mechanical properties arise from the E-M interaction, not the other way around), simply because all other known waves phenomena existed in mechanical media.
Of course, it could be that dark matter does not exist. In a very real way, nothing in physics “exists” because like all natural sciences physics does not make statements of objective truth, it makes testable predictions.
Dark matter, string theory, aether, etc., those are models that we, at some point in time, think may help us get better predictions and design further experiments. All models turn out to be wrong in the end, but they can be helpful until we come up with better ones.
If you drop the dark matter model, then you would want to have some other model as for why we observe what we observe. Some people find that other available models are even worse than the dark matter one, but if you don’t think so you can take your pick.
Except that there is nothing nonsensical about a particle that has mass and doesn't participate in any SM interaction. It's inconvenient if such a particle exists, as it's very very hard to detect things precisely by their gravitational effects, but there is nothing nonsensical, or even particularly weird, about the idea. Plenty of particles only interact with a few of the SM forces - e.g. photons are not affected by the strong force, nor are electrons, neutrinos are affected by neither the strong force nor EM, only the weak force, gluons only interact with the strong force, not EM nor the weak force, etc
It's funny how for MOND we cant accept that it has some unknowns yet but we are more than willing to accept the FULL UNKNOWN Dark Matter. it's easy. put "Dark" in front of something and you don't have to explain it at all, no matter that something else explains at least 60-70% instead of 0.
Dark matter is invisible, but it isn't magic. It is not significantly different from neutrinos. No one seriously denies the existence of neutrinos nowadays, even though they are invisible (i.e. they don't interact electromagnetically).
Dark matter is actually a very parsimonious theory. None of the laws of physics have to change to accommodate it, unlike with MOND. We may not see it, but it has to move around and affect normal matter in predictable patterns consistent with our current understanding of physics. If it doesn't, then the theory is wrong and may need some revision (which may be a dark matter + MOND hybrid).
In parallel with the research that attempts to find the properties of dark matter that best describe our observations is research that attempt to find what other properties it may have. It is a new particle? Can it interact in ways other than gravity? We didn't find anything, but the universe is under no obligation to make things easy for us.
One possible idea called the "nightmare scenario" is that dark matter is made of particles that only interacts gravitationally. It is a perfectly fine theory, maybe the cleanest one, but unfortunately, it would mean that we may never be able to detect these particles because gravity is so weak that the required detectors would be way beyond our technological abilities.
We make a similar guess for the stuff that is in the center of Earth. We measure local gravity and speed of sound velocity, and we guess here is liquid, here is solid, here is this rock, here is this another rock [1]. See for example https://www.livescience.com/64943-nobody-understands-the-gia... nobody has seen them, we guess they are there.
Dark matter is another guess. We guess there is more matter in galaxies than what the telescopes show. We can compare the amount of mater guessed from galaxy rotation with other measurements. In this case they compare it with the gravity between a few galaxies.
Nobody is happy that we don't know what dark mater is. There are a few theories, but none of them has enough experimental support. More lack of confirmed details in https://en.wikipedia.org/wiki/Dark_matter#Composition
[1] I don't know enough about rocks https://xkcd.com/2501/
Dark matter is not another Guess. is no guess at all. its the same as saying the center of the earth is made of Dark Core. and when someone proposed liquid core, there would be papers how a liquid core doesnt explain 100 %, and neither does rock or solid so Dark Core it is because it's sIMpLer to have just one Dark Something than 3 different somethings to explain anything.
The fact that there are tentatives to identify what it might be does not ammegliorate the fact that at it's core (pun intented) Dark matter is something to make equations fit without any other thought behind it or whether there might be several things behind it or god forbid that we juddge the equations themselves. I mean we got relativity because of a minor discord with newtonian Laws. (the orbit of Mercury). just a tiny percentage of obeservable behaviour at that time but it was a different time. a time where you could bring down the existing science of the day for a tiny percentage and now we accept 90% observation disaccordance (dark energy+dark matter) with what the equation require.
> at it's core (pun intented) Dark matter is something to make equations fit without any other thought behind it or whether there might be several things behind it or god forbid that we juddge the equations themselves
Another way to interpret dark matter is that we can observe something using several different ways, but all those ways use gravity. When trying to observe this something using electromagnetism, we see nothing. It doesn't seem so crazy then to hypothesize that this something only interacts with gravity, and not electromagnetism.
> I mean we got relativity because of a minor discord with newtonian Laws. (the orbit of Mercury).
I don't think that's true. One of Einstein's test for General Relativity, using Mercury's orbit, came around 10 years later after special relativity was proposed, which is understood to be motivated by both Maxwell's equations and experimental results suggesting that the speed of light (electromagnetic waves) not depending on the frame of reference. General relativity (explaining gravity) seem to have been motivated by Newton's gravity not playing well with Special relativity (with mass being relative and all).
I understand the frustration with Dark Matter, but my understanding is that Dark Matter is a guess that is known to be incomplete. Scientists are shooting everywhere to try to explain the discrepancy in gravitational effects and some form of undetected matter is currently the best hypothesis (but not the only one). You say that "we accept 90% observation disaccordance", but the source of its effect is being searched for.
See:
- https://hsm.stackexchange.com/questions/622/what-was-einstei...
- https://en.wikipedia.org/wiki/History_of_gravitational_theor...
- https://en.wikipedia.org/wiki/Tests_of_general_relativity
Edit: formatting, updated links
The "it looks like there's stuff we didn't previously know about here" class of guesses has historically been fruitful, yielding, among other things, the discovery of several planets. Maybe dark matter isn't the answer, but it's not "no guess at all."
> just one Dark Something than 3 different somethings to explain anything
There are many candidates and I think nobody discarded it's a mix, it looks like a plausible scenario. I made a quick look in Wikipedia, but I didn't find anything relevant to a mix of Dark Mater.
There is(was?) some big discussion about cold vs hot (and warm?) dark mater. IIUC "cold" dark mater won. I'm not sure if there is enough details to make a good guess of the split, or if it's a 100.00% vs 0.00% case.
No, it's not quite "no guess at all". Using gravitational lensing to estimate how much mass there is in a part of a galaxy is pretty smart. It turns out that there sometimes indeed is mass (because the lensing is there) even though it doesn't seem to give off much light.
(But I don't really believe in dark matter, either. It's mostly epicycles and phlogiston.)
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Because MOND doesn't actually explain anything, it's basically just fine tuning equations to make them fit with observations. Dark matter is the opposite, it takes all the observations and induces a fitting explanation
> We know it holds incredibly well terrestrially and within individual galaxies
I somehow thought that this was not really holding true and that Mr. Einstein had something to say.
I thought that without Einstein's "corrections" stuff like GPS wouldn't be useful at all.
Maybe I misunderstood last 100+ years in gravitational physics.
I thought newtonian gravity was already proven to be inaccurate with Einstein's Special Relativity (or General Relativity?) giving better results on cosmic scales (basically analogous to an approximation vs an exact formula)?
General Relativity reduces to Newtonian gravity as the curvature goes to zero, that is when you're very far away from objects relative to their masses, for slow non-relativistic objects like stars and galaxies.
Galaxies are typically so far away from another they're almost like point sources to each other, hence Newtonian gravity explains their motion very well.
However, inside galaxies things do not behave as expected, as stars in almost all the galaxies we've measured does not move like Newtonian (nor GR) behaves based on the matter in the galaxy we see. One alternative to the mainstream theories of dark matter is to modify Newtonian gravity, called MOND.
This work tested if MOND fit the motion of galaxies in galaxy clusters. They found it did not.
MOND already does not explain other phenomena that dark matter can so it's not terribly surprising. Here[1] is a nice accessible talk going through all the evidence for dark matter.
But it is technically a possibility that there's two things are going on, something MOND-like as well as dark matter, so worth checking.
[1]: https://pirsa.org/26030070
Why is the article titled "Newton's law of gravity passes its biggest test" if it doesn't explain the movement more than MOND?
Other way around. Newtonian dynamics explains the data very well, MOND did not.
In particular, Newtons law of gravity says the effect of gravity falls off as 1/r^2 where r is the distance from the mass. MOND modifies the standard equations so that gravity starts like 1/r^2 when r is small, and acceleration is large, but for greater distances, when the acceleration is low, instead falls off like 1/r.
MOND explains the movement of the stars in (most) galaxies very well. However this result showed that MOND was not consistent with the motion of the galaxies in the cluster. On the other hand the motion was consistent with plain Newtonian dynamics. Hence Newtons law of gravity (and by extension GR) passed the test.
Special Relativity is an extension of Galilean/Newtonian mechanics (motion of projectiles and other objects) to the case where the object is travelling at speeds that are a fraction of the speed of light. It deals with non-accelerating frames of reference. Satelites need to use this to correct for time dilation effects, but tracking the trajectory of an arrow/etc. or a car/etc. travelling from one location to another then classical mechanics is sufficient.
General Relativity is an extension of Newtonian gravity. It is also an extension of Special Relativity to cover accelerating frames of reference. Satelites need to use this, as does tracking the orbit of Mercury. However, for the orbits of other planets and the moon, using Newtonian gravity is sufficient for a reasonable degree of accuracy, and is used for tracking things like equinoxes/solstices, full moons, etc..
At these scales (entire galaxies, very weak forces), it doesn't make a significant difference.
There are ways of adapting MOND to match general relativity, should it turn to be correct at explaining what it is supposed to explain (like the movement of galaxies).
I think OP's question is more how could Newton's law "pass" a test any more than General Relativity would, considering that it's merely an edge case of GR?
Oh yes, I think the title is a bit misleading.
It is an argument against MOND, a theory that says that gravity has to be modified to account for some observations. But for these particular observations, general relatively and Newton's laws gives the same results in practice, the difference is negligible, so showing that these observations can be explained by Newton's laws implicitly mean that they can also be explained by general relativity. No need for modifications.
General Relativity. It explained the anomaly in the precession of Mercury's perihelion, and the bending of starlight by the Sun (double the value predicted by Newton's law).
The test here is for the inverse square law of gravity. The rival theory in this case isn't GR, but MOND: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
Free variant of this research paper:
https://arxiv.org/abs/2604.14327
We measure the matter distribution by its affect on light (strong/weak lensing). We also measure the matter distribution by the amount of light coming from it. The results are not the same. The simplest explanation is that there is matter which does not produce or reflect light via e/m, i.e. it is dark. Dark Matter. We know of particles which behave the same way. Neutrinos for example.
It's bizarre that Newton's law passes this test, when the linearization of general relativity predicts a gravitational equivalent of a magnetic analog for gravity.
Gravity Probe B.
There exists an even simpler derivation for a magnetic term, but its in my file drawer... (simpler in that it just relies on observations from special relativity)
If it had failed, would we be hearing about it in Science before the researchers continued testing it until it passed? (i.e. is this a blindspot in our collective epistemology?)
If it had failed it would've been a top science news. Every scientist dreams about the day when they're mentioned as "(...)'s experiment puts Newton's law of gravity in jeopardy."
Would it? Or would it be treated as an errant experiment by either the journal or the experimentalists themselves, foregoing publishing until they had a result that was better aligned with established norms? Rightly or wrongly, I expect that stakeholders would tend to invoke Sagan’s ECREE over the course of the publication process.
Yes. Yes, it would. This is so tiring.
It is not my intention to induce fatigue in anyone, though I am impressed that a reply comment in a forum is capable of doing so.
now I'm tired too
I hope you manage to get some rest after the tiring tribulations people pose on forums.
Woow
just another nag screen ...
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