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Why would some particles not interact through the strong force?

  1. May 12, 2013 #1
    I've been thinking about dark matter, WIMPs and neutrinos, and why these particles would not interact with matter through the strong force. All massive particles interact gravitationally, all charged particles interact electromagnetically, (I don't fully understand the weak force, so I won't comment on that), so what is it about WIMPS that make them not interact with the universe's most powerful fundamental force?
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  3. May 12, 2013 #2

    Simon Bridge

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    ... they don't because, as Feynman puts it, "Thems the rules."

    Physics is like assembling a jigsaw puzzle - the existing theory has gaps, to advance we need to fill the gaps.
    So we look at the gap and see what sort of piece will fit in there - maybe several are needed together? Whatever, you can get an idea of what sort of bit you need and then go look for it.

    The bit you need has to be the right shape for the hole you have, and have the right bit of picture on it.

    An object is proposed to account for some gap in our existing models.
    It has to fill the gap in order to do its job. For a WIMP (say) to interact strongly would have it overflow the gap it has to fill... therefore, if such things exist, they cannot interact strongly. The next step is to go look for some.

    Neutrinos were proposed in this way but have since been detected.
    If a neutrino were ever to be observed to feel the strong-nuclear interaction, we would have to rewrite a block of physics - more so than discovering that they have mass. Fortunately none have [been observed to feel the strong nuclear interaction] - so far. It is one of the reasons we feel that the object in question is the proposed "neutrino".

    Thus, asking why a WIMP cannot interact by the strong force is like asking why all the puzzle pieces in the sky part of the picture cannot have a picture of a tree - or why the pieces filling triangular holes cannot be square.

    Of course we could ask where the rules came from in the first place but that's a different kind of question and a whole huge topic by itself.
  4. May 13, 2013 #3
    Lots of particles don't interact via the strong force: electrons, muons, taus, neutrinos, photons, W/Z bosons, and the Higgs.

    The only truly universal force we know of is gravity (there are compelling theoretical reasons why everything must interact with gravity). But there's no reason any other force has to be universal, so it's not hard to imagine particles that don't interact via any of the known forces. This makes the idea of WIMPs very plausible.
  5. May 15, 2013 #4


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    Then there are bi-metric theories and various relatives. Though it is true that large categories of those are ruled out by observation. Most such theories would show up as violations of the equivalence principle.
  6. May 15, 2013 #5


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    Presuming that gravity really is general relativity (or some variation or elaboration of GR that is still basically a metric theory), then not just massive particles but all particles interact with it. It's geometry and it's hard not to interact with geometry.

    It comes down to charge. Electrical charge corresponds to the E&M interaction. Electrons (or other electrically charged particles) interact with photons.

    In the standard model, strong force acts through color charge. So anything that does not have a color charge, and isn't a gluon, can ignore the strong force. So you can toss neutrinos at quarks all day and not get any strong force interactions. So neutrinos interact only through the weak force, and so can pass through ordinary matter with extremely few interactions.

    So too with WIMPs.
  7. Jun 4, 2013 #6
    I like the idea that particles can independently pass through matter. The puzzle is all about what really happens so we might ask what forces might affect the wimp?
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