Merged Dark Matter vs. Modified Newtonian Dynamics
- Thread starter Fudbucker
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For some of the relevant astronomical data, MOND (or rather, some version which is consistent with Relativity, MOND itself is not) is a good match, e.g. some galaxy rotation curves.
For other data, e.g. the CMB, it's way off, and there's nothing to suggest that it could be made consistent, sans awful kludges.
MOND is not being taken any more seriously now than it has been for the last decade or so.
BTW, it's not that DM "continues to evade detection"; rather that certain kinds of DM (models) are being ruled out by new experimental and observational results.
The point is, whatever kind of stuff DM is, efforts to detect it have all come up short. What happens in, say, another 20 years, if we still haven't directly observed it? If it's not WIMP's or primordial black holes, or sterile neutrinos, or XYZ, do you postulate weirder and weirder stuff that it could be? At what point do you start to wonder if it exists at all?
theprestige
Penultimate Amazing
It's not a question of postulating weirder and weirder stuff. It's a question of postulating stuff that a) explains the observations and b) can be tested. Then you rule out postulates by testing them. Obviously "dark matter" exists, in the sense that we've observed its effects. We're not going to stop wondering about that.
If there's a MOND postulate that explains the observations and can be tested, it's no more or less weird than any other postulate.
If the experiments to detect it keep failing, DM adherents will eventually be forced to abandon mainstream DM candidates (e.g., WIMPs) and start postulating more and more "fringe" DM candidates. After enough failures of detecting DM, eventually you'd have to claim it's a type of undetectable stuff (or not detectable by us), which would be pretty weird.
We assume it exists, because of the observations we've made. That doesn't mean that DM actually exists. That assumption could be wrong. There are other explanations for the observations that don't postulate DM. MOND is the leading alternative.
My question is, if the failures to detect DM keep piling up, how long before MOND (or some other non-DM theory) becomes mainstream? Or does it ever move from its fringe status? Do we, at some point, just throw up our hands and say DM exists, but we can't detect it?
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Despite what you might read, that's not how it works.
The various experiments aimed at detecting DM particles are based on certain assumptions about the properties of those particles, and they in turn are based on theories of particle physics.
The unsearched search space for such postulated particles is huge, and experiments so far have explored just a really tiny part of it.
The same is true for astronomical searches for such particles, e.g. via gamma ray lines.
Actually, as I said above, MOND doesn't work, because it is inconsistent with Relativity. Maybe you'd like to pick one of the many relativity-consistent models?
Also, to repeat, MOND-like models are inconsistent with, e.g., CMB observations. And there's no obvious fix for that.
This is a false dichotomy.
An alternative model/idea/whatever will become accepted once it does a good job of accounting for all relevant experimental and observational results. Irrespective of any failures to detect DM particles.
Again, that's not how it works.
Take the question "what powers the Sun?"
That question has been around since forever, certainly since the start of the scientific revolution. And no "mainstream theory" could provide an answer, until the relatively new nuclear physics was applied to it. So, for several centuries it was a mystery, and no Fudbucker ancestor could have predicted when it would be solved.
Oh, and DM can most certainly be detected!
If it couldn't, how would we know it exists?
Cue questions about existence and detection, especially in astronomy, and about direct vs indirect ...
I'm sure that's all true. My point is about down the road (maybe way down the road). Enough failures in experiments to detect DM will cause the "unsearched search space" to shrink. For example, primordial black holes are being talked about in the popscience magazines as a possible candidate. Let's say that gets conclusively ruled out as a candidate. Then the "search space" has shrunk because a possible DM candidate has been ruled out.
What I'm curious about is, how much does it have to shrink before DM starts to be questioned? Suppose we rule out WIMPs, and neutrinos, and all the other DM candidates that are being talked about. What would happen then? Would scientists begin to question whether DM actually exists?
I'm not a proponent of MOND. I just know it's a competing theory. There are others. Competing theories benefit when the theories they compete with suffer. The existence of DM explains a lot of observations. Great. The problem is, what is it? Why can't we detect it like we have other particles? Perhaps the experimental failures now don't constitute much of a problem. But imagine 100 years from now and we still haven't figured out what it is or directly detected it? That would be a huge problem, wouldn't you agree?
The alternative to postulating DM to explain observations is NOT postulating DM to explain observations. Is your claim that the existence of DM will always be assumed? A thousand years from now, if DM still isn't detected, you think people will still think DM exists, in some weird undetectable form?
I don't see how that's relevant. Dark matter either exists, or it doesn't. At what point does the failure to figure out what it is become a problem for asserting its existence? Because at SOME point, if every DM candidate gets conclusively ruled out, the existence of DM itself would be questioned.
Oh, and DM can most certainly be detected!
If it couldn't, how would we know it exists?
Do we know that? Ethan Siegel claims "There are many people working on extensions of MOND". Why would there be many people working on non-DM alternatives, if it's known that DM exists?
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Here's an interesting article about the question I was raising: http://www.npr.org/sections/13.7/2016/09/13/493725999/if-dark-matter-cant-be-seen-what-about-ghosts
"It's still early in the game but, at some point, if nothing is found, scientists may have to re-evaluate their "belief" in dark matter. In that case, they will have to come up with other explanations for the bumps we know we're hearing in the night."
"It's still early in the game but, at some point, if nothing is found, scientists may have to re-evaluate their "belief" in dark matter. In that case, they will have to come up with other explanations for the bumps we know we're hearing in the night."
This recent Résonaances blog post is interesting, and relevant to this thread: Weekend Plot: update on WIMPs. Be sure to read the comments. If you're not familiar with the relevant astrophysics and particle physics literature, it may be rather hard to follow.
And this recent BackReaction blog rather nicely explains "quantum gravity", exploration of which may turn up an alternative to DM: Dear Dr B: What do physicists mean by “quantum gravity”? The comments here are, I think, less interesting (on the whole), but familiarity with the relevant literature is much less an issue.
And this recent BackReaction blog rather nicely explains "quantum gravity", exploration of which may turn up an alternative to DM: Dear Dr B: What do physicists mean by “quantum gravity”? The comments here are, I think, less interesting (on the whole), but familiarity with the relevant literature is much less an issue.
Indeed.
However, that's not really how science works. Despite how often you read such things in popularizations.
The author of the article I linked is an astrophysics professor. He doesn't know how science works?
Pixel42
Schrödinger's cat
I'm reminded of the saying "just because your doctor has a name for your condition doesn't mean he knows what it is". Dark matter is the name for the explanation for certain observations. The observations exist so the explanation exists. We just don't currently know what that explanation is.
Some explanation exists, of course. That goes without saying. The explanation might not include dark matter, though. Dark matter posits there's this mysterious stuff that's causing the observations we see. There are other theories that try to explain our observations that don't posit any new kinds of stuff.
I think you're too fixated on a dichotomy that may not exist.
For example, a perfectly valid assumption about a DM particle is that it is indeed sterile ... it interacts with other forms of mass via the "gravitational force" only. Such a particle is rather difficult to detect, in an Earth-bound lab, wouldn't you say?
And what about the idea that the DM particle is sterile?
No, I do not agree. Like I said, a sterile DM particle is a perfectly OK idea; how can you test for such a particle's existence? Especially if a key criterion is direct detection?
And a sterile DM particle is not "weird" (though it most certainly is "undetectable").
1) must a thing be a particle OR a wave?
2) what if the DM particle(s) is (are) sterile?
OK, three ... what if DM is something like a condensate, some sort of analog to a Bose-Einstein condensateWP?
Do neutron stars exist? How about black holes? No one has found either in any lab experiment, conducted here on Earth (thank goodness!).
Many people are working on alternatives to "DM as a particle" ideas, and have been for several decades. They do so for all kinds of reasons, no doubt one of which is - for some of them - the possibility of making a breakthrough discovery that'll win them a Nobel.
Science doesn't work like that (or at least astrophysics and (high energy) particle physics doesn't).
For example, a perfectly valid assumption about a DM particle is that it is indeed sterile ... it interacts with other forms of mass via the "gravitational force" only. Such a particle is rather difficult to detect, in an Earth-bound lab, wouldn't you say?
Well, despite what's in popsci articles, DM was questioned from the very beginning (in the 1930s).
And what about the idea that the DM particle is sterile?
I think you know the answer to that, right?
No, I do not agree. Like I said, a sterile DM particle is a perfectly OK idea; how can you test for such a particle's existence? Especially if a key criterion is direct detection?
GR is barely a century old. Ditto quantum theory. The two are mutually incompatible, at a quite fundamental level. Yet both are extraordinarily successful, in terms of accounting for all relevant observations and experiments.
And a sterile DM particle is not "weird" (though it most certainly is "undetectable").
Two things:
1) must a thing be a particle OR a wave?
2) what if the DM particle(s) is (are) sterile?
OK, three ... what if DM is something like a condensate, some sort of analog to a Bose-Einstein condensateWP?
Missed the point.
Do neutron stars exist? How about black holes? No one has found either in any lab experiment, conducted here on Earth (thank goodness!).
Many people are working on alternatives to "DM as a particle" ideas, and have been for several decades. They do so for all kinds of reasons, no doubt one of which is - for some of them - the possibility of making a breakthrough discovery that'll win them a Nobel.
Don't really have time for a long reply, but we seem to be in the "is it science or philosophy" debate that's going on in (e.g., is String Theory scientific?). If you're postulating a particle that is, in principle, undetectable, is that scientific?
I'm sure he does.
And in his papers - published in relevant, peer-reviewed journals - he clearly shows science at work.
However, it is rather common, sadly, for many scientists to, um, write for non-peers in a way that distorts what it is they do in their 'day job'. Most of what they do is surely rather boring to outsiders, and boring does not generate headlines or clicks.
Anyway, that article ends with this (link omitted):
"It's still early in the game but, at some point, if nothing is found, scientists may have to re-evaluate their "belief" in dark matter. In that case, they will have to come up with other explanations for the bumps we know we're hearing in the night."
Adam Frank certainly knows that "other explanations" have been sought, and ideas published, for many years now. So why did he write this para the way he did? He's setting up a false dichotomy, one that you seem to have bought into; why?
It pays to try to be accurate.
DM does not posit "there's this mysterious stuff that's causing the observations we see". It posits that there's a form of matter (not "stuff") which has a low, possibly zero, cross-section for strong, weak, and electromagnetic interactions. Astronomical observations constrain this form of matter to have a physical dimension ~tens of pc (I'm going by memory, it may be much larger), but of course it could be very much smaller.
I hope that you do get time, sometime.
Not at all.
Merely trying to reach common ground re what astrophysics (and HEP) are, and can do (and what they aren't, and can't).
Also introduce a bit of historical perspective.
Not.
Earlier I wrote (something you omitted to quote, why?) "Cue questions about existence and detection, especially in astronomy, and about direct vs indirect ..."
In my rather-too-long time spent in fora like this, I've been surprised, and saddened, by how many "hot issues" come down to questions about existence and detection in astronomy.
Example: galaxy rotation curves. The astronomical observations which are used to produce them are relatively easy to describe. As is the standard, textbook, explanation (which involves CDM; "C" is for "cold"). MOND-like explanations are rather more challenging to describe, but not too difficult.
What is "detected"?
Replace "galaxy rotation curves" with "neutron stars"; what's the difference?
Two reasons: he's an authority in the relevant field and seems to know what he's talking about; his conclusion matches up with my own intuition. When you search for something long enough, and don't find it, your confidence that it's there drops with each failed search attempt. Positing an unprovable explanation for an observation isn't intellectually satisfying. It's a dead end.
stuff
stəf/Submit
noun
1.
matter, material, articles, or activities of a specified or indeterminate kind that are being referred to, indicated, or implied.
I don't know why you have a problem with "stuff". It seems to fit what we're talking about quite well.