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.
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
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.
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.
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.
Until you definitively rule out all the dark matter candidates or get a direct detection the controversy will remain
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
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.
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