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Why did dark matter surprise us?

  1. Jan 23, 2010 #1
    It might be a dumb question, but why didn't any quantum model ever predict dark matter? I always hear about how current quantum theory is wildly successful, which might suggest that it does somehow represent the true underlying reality. So it seems that it should have predicted dark matter, and the fact that it didn't seems to suggest something, some sort of clue about a deeper physical law maybe, at least to this layperson. Anyone have any thoughts?
     
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  3. Jan 24, 2010 #2

    Chalnoth

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    Well, in a sense it did. Supersymmetry predicts that if the lightest supersymmetric particle is electrically neutral, it will be stable, and thus be dark matter.

    However, this wasn't an unambiguous prediction of supersymmetry, as even if supersymmetry is correct there are a large number of free parameters remaining that would determine its exact contents.

    In the end, it appears that the main reason that dark matter wasn't explicitly predicted is that it appears that the physics that determines the particles which we see in particle accelerators isn't simply related to the physics that produced dark matter. Another way of putting this is simply to state that the standard model of particle physics is, in a sense, closed: there is no obvious missing particle or set of particles in the theory (not any more).
     
  4. Jan 24, 2010 #3
    For the most part quantum field theory talks about the interactions between different types of particles, but it doesn't make too many predictions about what particles do and don't exist. This is a huge gap in the theory that people have been trying to deal with string theory and supersymmetry, but they haven't had very much success at it.

    There are large gaps in our understanding of the universe. We don't have any theories that can convincingly predict what particles exist and what particles don't.
     
  5. Jan 24, 2010 #4

    Chalnoth

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    Well, in a sense it does: many of the particles in the standard model of quantum mechanics were predicted long before they were detected. But the problem is that there are no more obvious undetected particles in the standard model.
     
  6. Jan 24, 2010 #5

    Nabeshin

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    I'd like to point out that there's a very big difference between predicting the existence of a certain particle and predicting that our universe be filled with them. Even if they were predicted, it's quite a leap to attempt to argue that they should make up ~25% of the energy content of the observable universe!
     
  7. Jan 25, 2010 #6
    *What is dark matter?

    *What is the quantum model?

    *What does quantum mean?
     
  8. Jan 25, 2010 #7

    Chalnoth

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    Well, actually the expected result is that if there are any around at all, they should be the dominant contribution to the dark energy density.

    A very, very basic sketch of the argument is this:
    1. The masses of the particles are expected to be within an order of magnitude of the supersymmetry breaking scale, which is expected to be on the order of 1TeV.
    2. The more massive the individual particles are, the earlier they stop interacting with other matter.
    3. The earlier they stop interacting, the more abundant they are.

    So while there was most definitely no prediction of the precise quantity of dark matter, the fact that they're strongly dominant compared to normal matter is the expected result.
     
  9. Jan 27, 2010 #8

    Nabeshin

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    Hah, interesting! I rather like that cute little argument synopsis, thanks for that.
     
  10. Jan 27, 2010 #9
    From one interested layperson to another, watch these excellent youtube videos. The jargon is at a minimum.

    watch?v=7ImvlS8PLIo ('A Universe From Nothing' by Lawrence Krauss, AAI 2009)
    watch?v=AHzbKrg2HIw (QED and richard feynman--three parts)
    watch?v=ViQoUXu5uK0 (Quantum Physics for Dummies--two parts)

    The gurus out here will give you a much more thorough and therefore complicated answer (or they might tell you that I've oversimplified or just gotten it completely wrong!), but I'll give you a basic layperson answer about what quantum means, and you should accept it skeptically. The basic idea is this question: is the universe continuous and smooth, or is it lumpy? Can space, matter, energy, and time be divided into indefinitely smaller units? The answer is no--the universe is lumpy, "quantized". Everything exists in discrete packets. Obviously matter is composed of discrete particles. But also space, energy, and time are also composed of discrete units. We discovered this about energy at about the same time that we were discovering it about matter. Energy comes in discrete packages called photons, which you probably already know, but also, a given photon can't have just any energy level. There are specific levels that can be taken, and the levels in between that you might think could be reached are simply unreachable. This is somehow related to the electron "shells" around an atom that you may recall from high school chemistry. If you hit an atom with a photon of the right energy, you can make one or more of its electrons jump to a higher shell. But if you hit that atom with slightly more of that energy, nothing will happen. Once you've made an electron jump up, then when the atom feels like it, it will allow the electron to settle, and in the process will release a photon with a specific energy relating to which shell the electron was in and which one it ended up in when it settled. So the basic idea is that energy exists in discrete packets, known as "quanta" (singular "quantum"). Hope this helps you, and I hope that in the simplification I haven't misrepresented the details. If I have, you'll know it soon enough because there will be a flurry of more informed (and probably cranky) responses.
     
  11. Jan 27, 2010 #10

    Chalnoth

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    That's a pretty good description. It's interesting how many things follow from the simple fact that quantities in nature appear to be discretized. I believe the first investigation of this came from looking at the spectrum of thermal radiation: early attempts to derive the spectrum of thermal radiation from first principles yielded decent results for low frequencies, but just went horribly wrong for high frequencies: they predicted that as the wavelength got smaller, the amount of energy emitted at that wavelength just kept increasing towards infinity. This is obviously not what happens (this problem is known as the "Ultraviolet Catastrophe").

    Back in 1900, Max Planck was working on this problem, attempting a variety of solutions. He found that he could get a good answer if he merely proposed that energy existed in discrete packets, instead of continuously. Now, at the time, he didn't think that this had any real-world implications. He thought it was just some mathematical trick that had nothing to do with the real world. Five years later, Einstein decided to take Planck's "mathematical trick" seriously, and proposed that this was genuinely the case (he even went about demonstrating it was true by looking at the photoelectric effect).

    And from this, quantum mechanics, with all of its delightful weirdness, was born.
     
  12. Jan 27, 2010 #11
    Thanks for the illumination. I vaguely remember this about Planck now that you mention it. That was the "blackbody" problem that he was working on, right? The thing that gave an infinite answer?
     
  13. Jan 27, 2010 #12

    Chalnoth

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    Right, blackbody radiation = thermal radiation.
     
  14. Jan 27, 2010 #13
    Funny to see how involved Einstein was in our understanding of quantum physics, given how much he hated the whole thing in the end.
     
  15. Jan 27, 2010 #14

    Chalnoth

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    Oh, he didn't hate it. But he was deeply distrustful of some of its implications.
     
  16. Jan 28, 2010 #15

    Chronos

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    Einstein believed his equations were scalable at all levels. Quantum physics flew in the face of this belief, hence, his distrust. He may still be proven right in some sense. I think there is some kind of cutoff where quantum forces take charge. It is not that GR is wrong, merely impotent at planck scales.
     
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