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MOND vs DM

  1. Apr 25, 2008 #1
    My question is this:

    Why are we (as a community) so eager to disregard modifying Newtonian Dynamics (MOND)? Of course it's incredibly accurate on macroscopic scales, but isn't science meant to be progressive?

    Since Zwicky's observation of galactic cluster rotation in 1922 and the consequent studies of galactic rotation curves, DM has been the much favoured resolution to the "extra mass" problem. The invocation of a non-baryonic (and not directly observable) form of matter seems just as ludicrous to me as altering Newtonian law. Indeed it sounds like a bit of a botch..

    We even know classical mechanics breaks down on small scales (QM), so what is there to say it doesn't act differently on larger scales also?

    Analogous is the search for the Higgs Boson in PP. If the LHC fails to find evidence for this I doubt they'll propose a new unobservable particle to mediate the Higgs field! The standard model, which has been tried and tested for decades, will collapse.

    What are your thoughts on this?

    Rob
     
    Last edited: Apr 25, 2008
  2. jcsd
  3. Apr 25, 2008 #2
    I think that skepticism is high regarding MOND because the cutoff value re: acceleration is derived empirically, and not from first principles. Of course, so was the initial idea for dark matter, so I think that you're right -- neither method is "better" than the other.

    Hopefully an alternative derived from first principles will appear in the near future.
     
    Last edited: Apr 25, 2008
  4. Apr 25, 2008 #3
    I see what you mean and you're quite right, it just seems equally botched to say that we're missing some amount of mass to fit in with our rotation models so this must be dark unobservable matter!
     
  5. Apr 25, 2008 #4

    Jonathan Scott

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    As far as I know, the cut-off in MOND is there only as a means of explaining why the effect hasn't been detected in laboratory or solar-system experiments. I think that MOND has to cut in below the critical acceleration, but it doesn't actually have to cut out until a much higher acceleration, because the effect it would have if it simply added to the Newtonian (or GR) acceleration would be too small to be relevant for intermediate accelerations.

    I don't understand how MOND can explain how the particles forming a star, which individually have significant accelerations far exceeding the MOND threshold due to the gravity of the star, are affected in such a way that the overall motion of the star is only MOND-like below the overall acceleration cut-off. Some people have told me that relativistic MOND theory claims to address this, but when I've looked into the detail I've so far only found vague assertions that in GR the motion of the whole is not necessarily exactly determined by the motions of the parts.

    Personally I find MOND quite physically plausible apart from the cut-off; if the universe is finite, it makes sense that a region containing significant mass would have a boundary that is not flat but rather "conical" in a solid angle sense, and this would lead to accelerations proportional to the square root of the mass enclosed and inversely proportional to the radius, as in MOND. However, this effect would persist right down to small masses and even individual particles.

    (In fact, the MOND acceleration at the surface of a particle becomes equal to the Newtonian gravitational acceleration at something of the order of 100 times the mass of an electron, which seems quite an interesting coincidence).

    As far as I can tell, if there were no cut-off it should be possible to detect MOND effects at the solar system level (they are well known to be of a similar order of magnitude to the Pioneer anomaly) and even in a laboratory setting, so any MOND effect on that scale should have been spotted by now, even in ordinary Cavendish-type experiments. Certainly, the Pioneer anomaly does not match a MOND prediction very well, but there could be other factors involved.

    I've not seen any specific report of any experiment which specifically rules out MOND effects on this scale, and it's not clear to me whether this would have been detected as a result of other experiments, such as those to measure G, which were not specifically looking for a 1/r acceleration law.

    Most experiments try to maximize the mass and minimize the distance in order to get the maximum Newtonian force, which reduces the relative strength of the MOND effect. Also, although I've seen reports of experiments checking for gravity variation with higher powers of 1/r than 1/r^2, I've not seen any checking for variation with 1/r. There is also the well-known fact that laboratory experiments to measure G have given results which vary far more widely than expected.

    I'd be very interested to know if any constraints on MOND-like 1/r acceleration terms have been established from laboratory experiments.
     
  6. Apr 25, 2008 #5
    Hey astrorob, I would like to make some general remarks regarding your original post here. I say general, as I don't really have any detailed knowledge of the MOND theory.

    First off, any new scientific models proposed are rarely accepted immediately without thorough scrutiny. That would, I hope we can all agree, be very unscientific. Thus it is only to expect that it will take some time for the theory to gain momentum, even if it were to hold up to complete scrutiny at an early stage, which I don't think it has.

    An important point in that regard, which has already been mentioned, is that MOND isn't derived from first principles, but rather constructed to fit some observation. As also pointed out, so was the dark matter hypothesis, but there are some important differences. MOND is based on a modification of an already established theory without any real physical motivation (unlike Einsteins "modification" of the same theory). As far as dark matter goes, it doesn't really change any known theories, but rather has the advantage that the Standard Model of Particle Physics as it stands today, is incomplete and thus actually leaves room for some hitherto unknown particle(s). Newtonian theory doesn't have that same obvious room for modifications beyond that of Einstein.

    Another thing that I've always found somewhat odd about MOND, is why modify Newtonian theory, rather then relativistic theory? I mean, if MOND is ever to be succesful, it would sooner or later have to incorporate realtivistic effects, so why not start with the already known relativistic theory?

    Anyways, to sum up. The reason MOND isn't the talk of the town may be because it hasn't stood up to all scutiny yet, and that it doesn't have any testable predictions which at the end of the day is the alpha and omega for any scientific theory.
     
  7. Apr 25, 2008 #6
    As far as I've seen the thing that really killed a lot of the momentum for MOND was the bullet cluster observation. This observation was widely interpreted as requiring dark matter to explain, and is widely considered to be a direct observation (via gravitational lensing) of dark matter.

    Once you have one example of something in the universe which is basically guaranteed to be dark matter, it gets very hard to convince yourself that that incident was caused by dark matter but others were caused by something else.
     
  8. Apr 26, 2008 #7
    Very true, I do remember reading that also, but gravitational lensing isn't direct observation is it? It implicitly defines there to be something there rather than actually detecting what it is.

    Again, very true. Rereading my original post it seems to perpetuate that I'm pro-MOND, and I didn't mean that to be the case. I'm actually quite impartial at the moment regarding it. What I was trying to ask was why the theory has been so negatively viewed right from the beginning. I'm not saying it should be immediately accepted as gospel, rather than it should've been given more of a fair chance before being completely dismissed for a theory that invokes the existence of unseeable matter.
     
  9. Apr 27, 2008 #8
    Yes, it was very convincing to me, then MOG responded:
    http://www.physorg.com/news113031879.html
    http://arxiv.org/abs/astro-ph/0702146

    MOND was a non-relativistic theory but has since been replaced by the relativistic version, MOG. The cutoff Jonathan speaks of is only a cutoff in the limit, not a cutoff in the sense the effect disappears completely at higher accelerations. I find MOG results very exciting but ultimately I must concur that the structure is ad hoc. Even the interpolation function is by design not well defined. However, even though the structure is ad hoc, this alone does not by itself constitute modeling to fit ALL the data. There are a large variety of rotation curves which MOG fits them all fairly well. Now even the Bullet Cluster data. This variety of fits was not itself modeled. Some people assume it was due to the use of the Tully Fisher relation. However, the Tully Fisher relation alone can be construed as a thorn in the side of Dark Matter.

    I think when you ask why we are so eager to disregard MOND, or the relativistic version MOG, you are mistaking a recognition that MOG is not well defined and lacking a mechanism as outright rejection. If this was the case we couldn't discuss it here as anything more than crackpottery, which it is not. Personally I agree that MOND/MOG is not sufficiently defined well enough to overtake Dark Matter. I would also say that ignoring it is likely at your own peril. If my rejection of imposing MOG onto our standard model is construed as outright rejection so be it. It's just too early in the game.
     
  10. Apr 29, 2008 #9
    to answer why mond doesn't bother with relativistic effects - the galaxy rotation problem involves speeds too small for relativity theory to be involved in the problem. most galaxy rotation is speed <1000km/sec, or <0.01c.
    It seems relativity theory isn't central to understanding the galaxy rotation prob.

    Why bother using relativity theory to calculate how long your icecream will take to melt on a sunny day?

    The only other reason to incorapate relativity theory into mond would be to make mond part of a grand theory of everything. But mond isn't a 'principles' theory, it isn't made to conform with the rest of physics. The principles for MOND are yet to be generally agreed on.
     
  11. Apr 30, 2008 #10
    See 0804.3804 on how gravitational wave detectors could help settle
    the MOND vs DM issue.
     
  12. May 1, 2008 #11
    Right, but the circumstances place some tight constraints on what that matter could be. This is how I understand things (this is partly copied from a post of mine in a previous thread):

    In the bullet cluster, two galaxies were observed colliding. The collision looks pretty normal. But, if you look just past the collision on either side, you see small areas of heavy gravitational lensing in what appears to be otherwise empty space. Astronomers interpret this like so: When the two galaxies collided, the normal matter in the galaxies slowed down as it all collided with each other, but the dark matter in each galaxy just kept going on its original trajectory. This interpretation is basically compelled since the matter in these "past the collision" areas cannot be anything that interacts, or else it would have been caught up in the collision too. And it can't interact with or emit light, because we can't see it. But it must be there, since we can observe its presence by the gravitational lensing it causes. Anything which would behave in such a manner would fit our standard definitions of "dark matter".

    I'm actually not really qualified to interpret the MOND-interpretation-bullet-cluster paper my_wan linked :( so I can't really form a valid opinion on this myself. But it does seem to be my (vague, from reading blogs and articles and stuff) impression though that even if the MOND interpretation of this data is sound, it does not seem to have been widely accepted even by some people in the field who were previously excited about MOND.

    my_wan, I would like to ask:

    I do personally think MOND is much more philosophically satisfying than dark matter. But it seems to me that if we accept MOND it should be because it's ultimately simpler-- because it provides a straightforward theoretical explanation for the failures of our models, rather than forcing us to assume invisible structure to the universe just to force observations to coincide with the model again. The problem is though that MOND seems to have quite a lot of parameters which the modelbuilders are free to set in order to make their curves fit; so at a certain point it starts to seem like there is as much there which is arbitrary, fine-tuned or hidden as there was in the dark matter model, and the thing that attracted us to MOND in the first place is gone. Although it is good the MOND people can find a way in their models to accommodate the bullet cluster data, from what little I understand of the paper you link it does seem to me like some of that is happening here-- like the fit of the data does not happen "naturally" but happens because they massaged the model to make it fit.

    So here is what I wonder:

    The dark matter explanation of what is happening with that bullet cluster data is very simple and straightforward. It has a "story" to it: The red stuff is luminous matter, the blue stuff is dark matter. The MOND explanation, on the other hand, looking through this paper, it appears you have to really delve very deep into the math to even begin to understand what is happening and why! Is this really the case, or is it just that I'm unfamiliar with the theory? Is there a way to explain why MOND predicts that the bullet cluster lensing would look the way it does, in a way that a layperson could understand and visualize it? Is it possible to make a "story" out of all this math?
     
  13. May 12, 2008 #12
    The present problem with MOND is it does not provide a straightforward theoretical explanation for the failures of our models. However, MOND does not have a lot of parameters, it is more constrained than even dark matter in this respect. Many of the predictions of MOND are generally independent of what values you choose for the open parameters. The fine tuning for rotation curves, etc. is limited mainly to assumptions about the mass to light ratio of galaxies.

    You wanted a simple story that helps explain what MOND is. Essentially we have gravity that is a 1/r^2 force. MOND assumes there is a tiny 1/r correction to this. That means that at in something as small as the solar system such effects would be extremely hard to detect. Notice that even though a 1/r correction would decrease with distance it would do so much more slowly than gravity. At some point then it would be as noticeable as the expected acceleration. Only when you measure the acceleration at great distances and low accelerations is the expected gravitational acceleration to MOND correction ratio large enough for a noticeable effect.

    The constraints MOND does have begs the question: If Dark Matter was the source of anomalous acceleration why does Dark Matter always appear distributed in such that it remains within the constraints of MOND/MOG? The cutoff point confuses some people. It's not a cutoff where the effect simply goes away. The MOND/non-MOND regimes are smoothly joined by an interpolation function. This function is not well defined (not theoretically specified) by MOND but as it turns out it doesn't need to be to define constraints and make predictions.

    For more detail ("story"):
    http://www.astro.umd.edu/~ssm/mond/
     
  14. May 26, 2008 #13
    MOND vs DM vs Magnetic Confinement ?

    As someone who came into astrophysics from laboratiry plasma physics (fusion)
    I have a different perspective on this. I've read a few articles explaining flat rotation curves using magnetic confinement. one group is in spain and the other is in los alamos. they seem reasonable to me. the field strength required is in the ballpark of
    measurements made from radio measurements. I've read some of the criticism of this work and it doesnt seem too deadly. the exception is in explaining interactions between galaxies, but that could be a different mechanism anyway.

    So, given the possibilities for magnetic confinement of spiral galaxies, I dont understand why it doesnt get more attention. People would rather consider the possibility of new exotic types of matter or a modification to newtons laws than allow that electromagnetic forces might be responsible, and we know these forces exist. is there some bias here ?

    Could someone explain this to me ?
     
  15. May 27, 2008 #14

    Jonathan Scott

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    My problem with any sort of cutoff is that it means that a star as a whole (which may be subject to a very small overall acceleration and hence experience MOND effects) is subject to different rules from its constituent particles and fields (which undergo much higher accelerations due to the gravitational forces within the star itself and would therefore seem to be outside the MOND regime). Also, the stars in a double star system would each experience a higher acceleration due to each other and hence would apparently be outside the MOND regime and behave differently from a single star. Neither of these effects seems at all plausible. The only explanation I've seen for this is a very vague assertion (which I don't find convincing) that in relativity it is possible for the motion of the whole to be different from the motion of the parts.
     
  16. May 27, 2008 #15

    Chronos

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    Dark matter is a stand alone solution. It can account for all observations. MOND cannot make this claim without invoking some amount of dark matter. The rest is Occam's razor.
     
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