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Dark matter highlights extra dimensions

  1. Sep 5, 2005 #1
  2. jcsd
  3. Sep 5, 2005 #2

    Garth

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    From that Nature article
    Garth
     
  4. Sep 5, 2005 #3
    Any more speculative than WIMPS etc. ? And why the missing dense dark matter halos at galaxy cores ? You can invent 'crumbly' forms of dark matter that could explain it but then you are inventing something purely to fit the data, with no fundamental basis for doing so. On the other hand there are plenty of fundamental mathematical reasons for considering extra dimensions.

    Of course its not by itself "evidence for" extra dimensions. But the current trend of the multiplication of entities approach to physics will keep having to plaster up all the different invented entities to get them to fit the data - which gives you a "standard model" thats less 'speculative' - but is it more realistic ? 'Where' does gravity 'leak' to (heirarchy problem) ? How can a superposition of states collapse instantly across 3D space ? And what about quantum gravity ? ( http://www.calphysics.org/articles/gravity_arxiv.pdf )

    There's no point simplifying what can't be simplified any further. But if there is a chance that a more fundamental look at the basics across all these 'anomalies' can resolve them all at a fundamental level, what a grand prize that would be :)

    Simon
     
  5. Sep 5, 2005 #4

    ohwilleke

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    God forbid that theory could ever follow empirical evidence instead of the other way around!
     
  6. Sep 5, 2005 #5

    Garth

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    My point exactly
    Until it is discovered in a laboratory exotic non-interacting DM is just "pixie dust" invented purely to fit the data
    Check out Self Creation Cosmology - An Alternative Gravitational Theory .

    Garth
     
  7. Sep 5, 2005 #6
    Okay so you notice that the motion of galaxies do not fit Newtonian predictions, because there is not enough mass to hold it all together at that velocity in Newtonian terms. So, following what was missing from Newtons predictions - namely mass - invent a kind of mass that interacts 'weakly' with baryonic (normal) matter. Then you look a little closer and all kinds of things do not match predictions. Then you spend millions to try to detect this invented concept and fail to do so. So in effect the theory of non baryonic matter contained within 3D space has so far failed to find empirical support.

    Science is not always about "follow[ing] empirical evidence". Ideally its about understanding empirical evidence. Einstein opened up a new level of understanding by considering time as a dimension. Surely there is room in your empirical world to consider there may be even more to it all ? According to E=mc^2 mass travelling at c becomes "energy". What does that mean ? If you where 'watching' an electron, specifically in the orbital shell of an atom, and that electron recieved enough energy to jump to another shell, what would you say had happened to that electron ?
     
  8. Sep 5, 2005 #7

    SpaceTiger

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    I would say that it's considerably less ad hoc to postulate a weakly-interacting particle than it is to postulate an alternative theory of gravity and/or extra dimensions. I said it before and I'll say it again. If you're whining about why we haven't detected a particle that could be the dark matter, you need to stop and think about why it is that the particle would be dark and what implications that has for its detectability.
     
  9. Sep 5, 2005 #8

    turbo

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    Why does it need to be a particle? Could it not be a field of particle/antiparticle pairs that is difficult to detect except locally (quantum vacuum field)? This would entail a new model of gravitational interaction (a polarized self attractive vacuuum field), but is this a bad thing? The Newtonian inverse-square relationship works well for simple systems, and Einstein's improvement works well in some domains. Is it not appropriate to explore the question of whether we need an even better theory of gravitation, to explain the flat rotation curves of galaxies, the excess lensing of clusters, and the excess binding energy of clusters?
     
  10. Sep 5, 2005 #9

    SpaceTiger

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    It doesn't, I was responding to the lack of verification of the leading theory.


    No.


    It's perfectly appropriate, but it makes no sense to attack the conventionial theories for "making things up to fit the data" when the alternative gravity models do so in an even more egregious fashion.
     
  11. Sep 5, 2005 #10
    Amusing.

    From Websters;

    ad hoc
    For the special purpose or end at hand; also, by extension, improvised or impromptu. The term, Latin for "to this," is most often used for committees established for a specific purpose, as in The committee was formed ad hoc to address health insurance problems. The term is also used as an adjective (An ad hoc committee was formed), and has given rise to the noun adhocism for the tendency to use temporary, provisional, or improvised methods to deal with a particular problem.


    Errr ... whining ? Why do I need to stop and think about something so inately and implicitly obvious ?
     
  12. Sep 5, 2005 #11

    SpaceTiger

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    Good, now explain to me which part of the definition you don't understand.


    I dunno, you tell me.
     
  13. Sep 5, 2005 #12
    An "even more egregious fashion" ? Strange - I have no idea where you are coming from. Which gravity model are you talking about exactly ? Einsteins stretched-by-mass concept of spacetime ? I think we all can agree to accept that. So is M Theory the "alternative gravity model" you have a problem with ? It does have all those 'messy' extra dimensions that no one claims to understand properly. So I guess you consider the likes of Ed Witten and Lisa Randal to be the kind of people who willingly spend their time on "egregious" theories ?

    What is this "attack the conventional theories" comment referring to ? The idea of science is to question. In fact its the heart of the scientific process.

    So tell me how making things up purely to fit the data in cosmology, things that have no direct evidence whatsoever, let alone any kind of fundamental significant basis for their derivation, compare with a concept based on the broad horizon of cutting edge theoretical physics, derived from well established maths ? How do you weigh such things ?

    Your comments are as dubious as Garth's paper. You say "It makes no sense to attack the conventional theories" ??? Where would science be if we all followed your lead ? Of course you did qualify that, but in terms that where such nonsense my cat could do a better job of defending whatever was in your head when you thought of "standard gravity models".

    If you're such a SpaceTiger then I assume that along with you guttural roar that shakes the ground - as all good tigers do, you will provide us all with a relativisticaly invariant description of youngs slit experiment that accounts for Bell's inequalities within Aspects experiment ?

    And if relativistic principles are important to you, and you feel confident enough to mock theories of gravity when you yourself have not proposed anything significant in what everyone considers a troublesome question, then what are your comments about ? Why this talk of "alternative gravity models" that are "egregious" ? Tell me of this non-alternative gravity model you have confidence in and tell me why you feel confident about it. If you can't - then why on earth did you reply on this thread as you did ?

    Simon
     
    Last edited: Sep 5, 2005
  14. Sep 5, 2005 #13

    SpaceTiger

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    I don't "have a problem" with any of the above theories, I just think they're more ad hoc than the dark matter theory, seeing that there's no observational support for them. Perhaps you've forgotten that this was what we were discussing...


    When did I say one shouldn't question? That's rather nasty of you to put words in my mouth.


    So yeah, that's where I'll stop you. You see, the alternative gravity theory you're referring to in the post up top is not only lacking direct evidence, it's lacking any evidence at all other than that for which it was created to explain. Dark matter has made many successful predictions, including the CMB power spectrum, large scale structure, and lensing results. Meanwhile, alternative theories (like MOND), have done no such thing. I'll grant that we're not yet at the stage where we can rule them out, but it seems to me to be considerably more ad hoc to "invent" a theory that has no basis other than...


    As a scientist (that is, not a philosopher), I would give those things little or no weight. Math and the physical world are two different things and, as much as we'd like to evaluate our theories based on their "beauty", I'm strongly inclined to say that observational support should always be the trump card.


    Now that was just mean. :tongue2:


    Wow, that's one of the most egregious (in italics so that you can plug it into Webster's) examples of taking a quote out of context that I've ever seen. The quote was:

    "...it makes no sense to attack the conventionial theories for "making things up to fit the data" when the alternative gravity models do so in an even more egregious fashion."


    Oh, this is fun. Let me try. You said:

    "Of course...conventionial theories...are as dubious as Garth's paper."

    Mr. SimonA, it's so foolish of you to say that. You must not be very smart. Of course, you did qualify those things...


    I think I pushed a button...
     
  15. Sep 5, 2005 #14

    Not at all. Your original quote was "I would say that it's considerably less ad hoc to postulate a weakly-interacting particle than it is to postulate an alternative theory of gravity and/or extra dimensions". Why are extra dimensions "more ad hoc than the dark matter theory, seeing that there's no observational support for them" ? What is the "observational support" to suggest that the motion of galaxies can be explained by a four dimensional universe ?


    Well I was questioning and you replied;


    Hey I'm no fan of MOND


    ?

    Que ? You tell me - what is the basis for extra dimensions ? Is it all a new concept lacking any kind of scientific rigour ? Did Einstein have the final world by considering a 4D reality ?


    Yes and it rained yesterday and I need rain to survive and so it must be me that causes it to rain... "Observational support" relies on correct interpretation.


    :uhh:


    Yes and thats the one I was replying to. I did say that you qualified it and dealt with that as well...



    Mmm ... it must be that I'm not very smart. But even in my half-witted and troubled with the whole concept of reality state, I can still see that this is fun :)


    Do you have a button that answers the question ? :tongue2:
     
  16. Sep 5, 2005 #15

    SpaceTiger

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    I'm not sure what you're asking for here. Einstein's theory (specifically, GR) is well supported by observation, but I'm sure you know that. If you're asking how we can be sure that there aren't extra dimensions outside of the limits of our experiments, then we can't be...but the burden of proof is not on the negative...


    None of which said or even implied that one shouldn't question convention.


    It is lacking observational/experimental support, which relegates it to the status of an untested possibility. Mathematically, I don't doubt its rigor -- what I doubt is its correspondence to reality. But that's fine, prove me wrong. I have no objection to extra dimensions as a concept and if the observations turn out to support it, then I will as well. Until then, however, I will not lend it support simply based on a philosophical prejudice.


    Absolutely right, but I hope that you can see how the two are intertwined. The beauty of, say, relativity is certainly amazing in of itself, but it was not that alone that made it one of the foundations of modern physics. The important thing was that it survived rigorous experimental testing. If, as in your example, I postulated that you caused it to rain yesterday, what testable predictions could I draw from that? How does that contribute to the general acceptance or rejection of the theory?


    The point is that it makes no sense to argue based on something that was taken out of context. No rational person would conclude that this quote:

    "...it makes no sense to attack the conventionial theories for "making things up to fit the data" when the alternative gravity models do so in an even more egregious fashion."

    means that I think science should be done without challenges to tradition.


    Yeah, I've been copying and pasting this into some of my AIM conversations. I love your bit about the "guttural roar". :rofl:
     
  17. Sep 5, 2005 #16

    Sure - of course the "burden of proof" in science must admit at least the same space to less fundamental issues that don't even provide any hope of providing answers across the broad reach of enquiry - such as "how can we be sure there are weakly interacting particles that exist within 3D space" ?


    Now wait just one second (or more) there. How can you say that the heirarchy issue, the motion of galaxies, the accelerating expansion of the universe, quantum entanglement and inertia are "lacking observational/experimental support" ? I've deliberately turned what you said upside down. Its a question of epistemology in many ways.


    Where is the "philosophical prejudice" ? The observations support extra dimensions far better than the standard 4D cosmology - unless you wish to invent particles like the Higgs Bosun that have never been seen and are unlikely to ever be seen. What gives mass to the Higgs Bosun itself ? Its all nonsense speculation "based on a philosophical prejudice".



    Well it doesn't contribute anything because its nonsense - that was my point.



    So what exactly is the non-alternative gravity model you are implicitly refering to ? Stretched spacetime :confused:

    LOL

    Its well past my bedtime ... :zzz:
     
  18. Sep 6, 2005 #17

    Chronos

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    SpaceTigers reasoning is quite sound, IMO. I have some doubts about this
    There are more than a few reasons to suspect the existence of dark matter. While direct detection is still lacking, there is plenty of 'empirical' evidence favoring DM:

    The Dark Side of the Universe
    http://arxiv.org/abs/astro-ph/0508279

    Cosmological Parameters and the case for Cold Dark Matter
    http://arxiv.org/abs/astro-ph/9906277

    More Evidence that Dark Matter Rules the Universe
    http://www.space.com/scienceastronomy/dark_matter_021023.html
    I disagree. Science is all about following empirical evidence to its logical conclusion.
    That is a gross oversimplification, at best.
    It means you do not understand basic relativity.
    It obeyed the rules of quantum physics.
     
  19. Sep 6, 2005 #18

    SpaceTiger

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    You either ignored the part of my post where I gave you examples of evidence for the dark matter theory or hold much higher standards for those theories that don't align with your beliefs.


    Those things were not predicted by your "extra dimensions" theory. In fact, extra dimensions are instead being used to try to explain those phenomena. You know, that ad hoc thing we were talking about...


    Oh really? Please, link away.


    Apparently, you missed mine. Theories need to make predictions to be useful. The theory that you caused it to rain yesterday might lead to some predictions -- for example, a correlation between rain and your presence -- but they would be very quickly refuted by experiment. If theories are going to be of any scientific value, they must make predictions as well. It's not enough to go back to your unsolved problems and say, "Hey, if we introduce this length scale into the theory, we can explain this old observation!" Theories invoking extra dimensions are notoriously bad in this regard. Oh, and please don't quote me in your next post as having said, "Theories invoking extra dimensions are notoriously bad."


    It's called general relativity.
     
  20. Sep 6, 2005 #19
    Mmmm - "empirical evidence" ??? The three forms of 'evidence' quoted for the existence of dark matter are "(i) rotation curves, (ii) gravitational lensing, and (iii) hot gas in clusters". Where have I said anything that contradicts these anomalies ? This paper is pretty much just a rehash or summary of current standard theories and DOES NOT provide any substantial empirical evidence for WIMPs or MACHOs whatsoever.


    This one is great! Starts of with the assumption;

    Then discusses the possibility that the effects could be caused by baryonic matter, then openly admits the paper is based on the assumption of cold dark matter as a first starting point;

    And surprise surprise the final conclusion is;


    Yes this one has a good start;

    Then goes on to discuss evidence of why MOND is not suitable to explain dark matter. Where on earth do you see me supporting MOND ?

    You mean like the logical conclusion that if you chased a beam of light fast enough you would eventualy catch it up ?

    LOL

    I wasn't trying to prove relativity from first principles! What is the "over.." part of your "..simplification" for ? Was it not one of the many fundamental changes Einstein made to scientific thought ? One of the most significant ?


    Okay so in those particle accelerators you have particles that get heavier and heavier as they get closer to the speed of light. If you could (and we can't get anywhere near the amount of energy needed at present) get that particle to the actual speed of light (in a vacuum), what would happen to it according to "basic relativity" ?


    [​IMG]
     
  21. Sep 6, 2005 #20
    There is no "evidence for the dark matter theory". The "dark matter theory" as you call it is a theory invented to explain anomalies that do not fit established and proven laws governing gravity related dynamics.



    Hey ? What exactly is my "extra dimensions theory" ? I was stating a couple of anomalies that could potentially be explained by extra dimensions. Are you suggesting that WIMPs and MACHOs where derived from first principles rather than being a proposed plaster (come educated guess) for gaping holes that are the anomalies themselves.



    Yes exactly and they make a far better job of doing so than inventing imaginary particles like WIMPs or MACHOs.



    Errr ... http://xxx.arxiv.org/PS_cache/astro-ph/pdf/0508/0508572.pdf ?




    It quite an uphill strugle teaching myself all the different areas that are necessary to build a theory that has direct predictions. However I'm working from the ground upwards and the likes of string theory are as good as useless because of all their confusion related to the 'shape' of extra dimensions - something rediculous in itself. Lisa Randall, Nima Arkani-Hamed and now this new paper from Oxford are heading in a far more promising direction.

    But a significant point is that current dark matter theories don't really make any specific predictions other than the anomalies they where designed to explain. I say 'really' because of course there is 'clumpiness' in the universe and galaxies are 'seeded' in some way etc. But those are models retro-fitted onto dark matter because its the current theory and so many smart people are using it as a basis for their models. That doesn't mean its the accurate description of what causes these anomalies or that extra dimensions could not describe those phenomena just as well. Certainly they are not of the calibre that relativity is when it makes predictions.



    Hey ? When have I disputed GR ???

    Simon
     
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