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Can dark matter travel FTL?

  1. May 5, 2009 #1

    ACG

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    Hi!

    I'm sure this has been done before, but is it possible for dark matter to travel faster than light (at least some candidates for dark matter)? We know that ordinary matter is limited by the speed of light, but what about dark matter? For all we know, dark matter may not use c-limited particles to exchange forces and stuff.

    I would expect that it can't travel faster than light because its mass can affect the behavior of ordinary matter. However, I don't know enough about dark matter for certain.

    Thanks in advance,

    ACG

    P.S. If dark matter CAN travel faster than light, it would explain the speed at which my father used to back out of the driveway...
     
  2. jcsd
  3. May 5, 2009 #2
    I'd say no because things can move at c always move at c and so far there is no evidence to show that DM is moving at those sorts of speeds, and it would be quite evident as the dynamics of galaxies would be drastically different.
     
  4. May 6, 2009 #3

    Chalnoth

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    Yeah, dark matter has to be moving pretty slowly for it to stay within gravitational wells, which it does. And particles which move faster than the speed of light are tachyons, and in nearly all situations those are simply impossible, and can therefore be ignored.
     
  5. May 6, 2009 #4

    ACG

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    But is it possible that there are certain laws of physics which dark matter does not obey simply because of the way it works? Perhaps it obeys some of the laws (gravitational, for example), but not others (such as the c limit).

    I'm not even sure we know enough about dark matter to rule out two different types of dark matter running around out there (one which may exceed c and another which gets sucked into gravitational traps). For all we know, the rules of physics for dark matter are such that its velocity can be on either side of c and there's no tachyon/tardyon distinction.
     
  6. May 6, 2009 #5

    Chalnoth

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    Perhaps I didn't explain why we can be so sure it obeys the speed of light limitation. Tachyons that have any interaction at all with normal matter lead to an unstable vacuum. This means that if the theory is correct, it predicts that everything will always explode instantaneously with runaway energy production. As an unstable vacuum is trivially ruled out by the fact that we exist, and that our universe is relatively stable, we can be sure that most any tachyons don't exist.

    There's always the possibility that they exist but exhibit no interactions (not even gravitational ones), but then what's the point in even proposing the idea if it is, by its very nature, untestable? This fact also means that dark matter cannot be made of tachyons because dark matter interacts with normal matter, at the very least through gravity.

    Furthermore, we know from its behavior that dark matter must be relatively "cold". That is, it is made up by relatively slowly-moving particles. If the stuff that made up dark matter were moving anywhere close to the speed of light, then it would be completely incapable of collecting into the clumps we see it in.
     
  7. Jul 18, 2009 #6
    It wouldn't be untestable just because thy can't be detected. What would make it untestable is if they both couldn't interact with anything and couldn't be produced by the four fundamental forces in our universe.

    e.g. - If we tried to produce tachyons and found that there was missing mass/energy, but nothing to explain that missing energy, we could infer the existence of non-interacting tachyons.
     
  8. Jul 18, 2009 #7

    Chalnoth

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    They couldn't be produced by any possible interaction, whether the four forces we are aware of or anything of which we aren't yet aware. If they could, the vacuum would be unstable, and the universe would explode.

    If they were non-interacting, they couldn't be produced. Missing mass in detector collisions comes from weakly-interacting particles.
     
  9. Jul 19, 2009 #8

    Chronos

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    It appears dark matter was created during the big bang. It does not appear any has been created by post BB processes. This is a little weird because you would think things like GRB's, supernaova and quasars would be energetic enough to generate DM in the current universe. Perhaps they do and we just havent figured out how to detect it. Missing energy in some hugely energetic event would be the thing to look for, IMO.
     
  10. Jul 19, 2009 #9

    Chalnoth

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    A difficult thing when we don't know precisely what the energy of these processes is except empirically.
     
  11. Jul 19, 2009 #10
    note to our over active moderators
    this is a question NOT A STATEMENT OF FACT

    IF AND ONLY IF dark stuff is moving at near light speeds [say 99.999%]
    would that multiply the mass and there for the gravity of a small amount
    of dark stuff to equal the assumed 25% of everything and many times
    the amount of normal matter's total mass ??????????
    HOW WOULD VERY HIGH SPEED AND RESULTING MASS INCREASE
    EFFECT GRAVITY
     
  12. Jul 19, 2009 #11

    Chalnoth

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    If dark matter were moving close to the speed of light, it couldn't clump. It clumps.
     
  13. Jul 21, 2009 #12
    Yes, of course. I was simply musing about the possibility of some type of particle being able to be created but not able to be detected by any means after its creation (including weak force interaction). There certainly is no evidence for such a thing (to my knowledge), but I wouldn't be so hasty as to say that if something cannot be detected then it cannot be produced and therefore cannot be falsified. Then again. perhaps I am wrong and you can enlighten me as to why that is so. :)
     
  14. Jul 22, 2009 #13

    Chalnoth

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    Anything that can be produced by some interaction can be detected by the same interaction. Basically, in order to be produced, it has to interact with some force that also interacts with normal matter. In principle we can make use of that force to detect it.

    Now, it may be a practical challenge to perform the detection, but it won't be impossible.
     
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