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rappatic
pessimist
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.
throwaway27448
> 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.
magicalhippo
> 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,...
cactacea
Because stuff like the Bullet Cluster exists. Dark matter is in fact the simplest answer.
chmod775
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.
Tuna-Fish
We have indirectly observed it a bunch, and our understanding of physics allow the existence of material that flatly cannot be directly observed.
wvenable
> 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.
rdtsc
> 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?
dnautics
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?
Tuna-Fish
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.)
pdonis
> 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.
lukan
"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)
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gwynforthewyn
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.
MattPalmer1086
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?
lukeify
The constituent particles of nature are under no requirement to be apparent or achievable to you.
lubujackson
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.
kstrauser
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.
fragmede
Except (IANotAnAstrophysicist) it's not every galaxy. There are two that stick out as having little to no dark matter.
* NGC 1052-DF2
* NGC 1052-DF4
Centigonal
yes - absent experimental confirmation, the effects attributed to dark matter may in fact be due to an unrelated as-yet-undiscovered phenomenon.
gostsamo
You might be interested in that. We have observations of galaxies where dark matter likely is not present.
GuB-42
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".
PaulHoule
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.
adgjlsfhk1
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.
dnautics
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.
wetpaws
[dead]
jahnu
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.
yxhuvud
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.
fooker
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.
wongarsu
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
fooker
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.
anthonypasq
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?
fooker
If you could get there with minor modifications of the current state of the art, yes.
My point is that you likely can not.
cwmma
my understanding is that there are a few MOND champions who are still holding on to the idea while everyone else has moved on.
stronglikedan
so MOND is the new String Theory...
pfdietz
It never had the institutional imprimatur of string theory.
layer8
String theory is still the leading contender for quantum gravity.
elashri
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.
raverbashing
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
vonneumannstan
Until you definitively rule out all the dark matter candidates or get a direct detection the controversy will remain
htx80nerd
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.
davrosthedalek
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")
pixl97
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.
pfortuny
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.
dnautics
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.
njarboe
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.
_ZeD_
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.
Joker_vD
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.
ReptileMan
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".
sebzim4500
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.
cvoss
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.
fooker
Not necessarily.
You'd be correct given hidden variables.
But we know pretty convincingly that quantum anything does not have hidden variables.
ReptileMan
But only if someone observes it. The act of observation forces reality into existence.
jiggawatts
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…
nathan_compton
Everything we don't understand we conceptualize using the most similar tools which we do have command over.
Lvl999Noob
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)?
magicalhippo
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.
NewEntryHN
Why is the article titled "Newton's law of gravity passes its biggest test" if it doesn't explain the movement more than MOND?
magicalhippo
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.
rhdunn
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..
GuB-42
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).
NewEntryHN
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?
GuB-42
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.
DonaldFisk
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
DoctorOetker
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)
adrian_b
Free variant of this research paper:
ricksunny
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?)
yongjik
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."
ricksunny
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.
DivingForGold
just another nag screen ...
sipsi
[flagged]
tech_hutch
Newton's law of gravity passes its biggest test
Yo momma
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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.