Why don't dark matter and other such particles hit us?

  1. Hello,
    I'm having trouble understanding why if there are dark matter particles and other such particles (neutrinos) streaming through the earth at all times, why don't they just collide with the atoms in our body or the desk or what have you? I understand that they're probably "weakly interacting," but then again I don't understand it. Don't they have just physical size, and wouldn't they just collide or hit classically, or is there enough "space" around the nuclei of the atoms in our body that they can pass through?

    Another way to ask this is whether WIMPs, if they exist, are passing "around" the atoms of my body or desk or actually through them. Either way the odds seem high that there would be many, many collisions. What am I not getting?

    Thanks in advance!
  2. jcsd
  3. ChrisVer

    ChrisVer 2,403
    Gold Member

    I am not so familiar but...
    Dark matter is not considered "weakly interacting" but "not-interacting" at all... it interacts only gravitationally (to give the needed "missing" mass for our cosmological observations). I think this would mean that it can travel through us without any problem...

    I suspect you considered righthanded neutrinos? I am not sure why not- probably they are not the correct candidates for darkmatter. Also they are pretty massive to see....

    Lefthanded neutrinos are weakly interacting, and they are extremely penetrating... There are chances for them interacting with your surroundings, but these interactions are so uncommon that nothing really changes.... That's why in order to measure neutrino signals, we need to lower the background and the laboratories are deep under the earth or seas...
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  4. Bill_K

    Bill_K 4,157
    Science Advisor

    All elementary particles are pointlike. Which means they have zero size. One should not think of collisions between them as little balls colliding! :wink: The probability that two particles might collide depends on how strongly they interact. Dark matter particles are believed to be everywhere, passing through atoms, through nuclei, through protons and neutrons... Most of the time they simply pass through undisturbed. Sometimes they interact, so if you build a big enough detector, you can see an occasional collision. So far they have not been seen.

    There are two popular ideas what dark matter particles might be. They might be WIMPs. These are rather heavy particles (rest mass in the GeV to TeV range) that interact with ordinary matter with the same strength as the weak interactions. Or they might be sterile neutrinos. These are light particles (rest mass in the keV range). Sterile means they don't even participate in the weak interaction. These would have to be detected indirectly, perhaps by their decay into one of the ordinary kinds of neutrino.
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  5. What you're missing is that the "WI" in WIMP stand for Weakly interacting. They don't collide with the atoms in your body because they interact too weakly with them to affect them in any noticeable way. It's possible that collisions do occur but they are too rare to matter.
  6. No, WIMP's are not required to interact by gravity alone. Other interactions are allowed as long as they are weak enough.
  7. In order to make that statement, you need to know what dark matter IS. Since you seem to know, you should put yourself in for the Nobel prize right away. :smile:
  8. ChrisVer

    ChrisVer 2,403
    Gold Member

    hehe, I find myself more into supersymmetric particles for dark matter, but of course (since we don't know what it is) it's a matter of personal aesthetics. Also in my statement (in which I said "is considered") is not something that doesn't exist (prolly I didn't understand the WIMP), since dark matter was proposed for counting as extra mass and because we didn't know any other trace of it, it would have to interact only gravitationally.... of course some very weak interaction seems plausible too now that I think of it (dauto's comment)
  9. Bill_K

    Bill_K 4,157
    Science Advisor

    Note that the neutralino (lightest supersymmetric particle) is a variety of WIMP. It does interact weakly.
  10. Vanadium 50

    Vanadium 50 18,453
    Staff Emeritus
    Science Advisor
    Education Advisor

    One neutrino interacts with your body every week. One muon goes through your head every second. Maybe Dark Matter is interacting with your body and maybe it's not. But how would you tell?
  11. I would make you feel more somber ?
  12. mfb

    Staff: Mentor

    Well, we know the interaction rate (with comparable energy exchange) is low compared to the neutrino rate, otherwise dark matter would be clearly visible in neutrino detectors.
  13. Bill_K

    Bill_K 4,157
    Science Advisor

    Neutrino detectors don't see WIMPs, and vice versa. Neutrino detectors look for neutrino-specific reactions, which can be transmutations (e.g. Cl - Ar) or Cherenkov radiation from recoiling electrons. WIMP detectors mainly look for UV photons from collisions with nuclei.
  14. Because "collisions" you are familiar with are electromagnetic interactions on a microscopic level: atoms have electrically charged parts (electrons and protons), and when they try to go through each other, they repel.

    Some other collisions aren't electromagnetic - such as neutrons colliding with atomic nucleus or neutrino kicking out an electron from an atom. But in all cases, collisions are the result of some fundamental interaction (in these two examples, strong and weak forces).

    If a particle does not interact with matter, it *cant* "collide" with it. It will go right through.
    If a particle does interact with matter but very, very weakly, it will collide very rarely, going right through 99.999999999999999999% of the time. Neutrinos are like that.
    Think of dark matter particles as "heavy neutrinos". (BTW, it may be what they *are*, in fact. We do not know yet).
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  15. mfb

    Staff: Mentor

    They don't look for the same thing, but they are backgrounds to each other. I didn't see the other direction as it is hard to account for a background you do not know, but I would expect it to be there as well.
  16. Bill_K

    Bill_K 4,157
    Science Advisor

    Right you are. The latest Résonaances blog has a nice illustration of this.
  17. A friend of mine working on dark matter once said that if you take the current dark matter bounds from direct detection and convert this into an interaction rate with a human, you expect a handful of events in a lifetime.

    So maybe that time you thought your phone was buzzing in your pocket but it wasn't there at all... It was on the desk. (That's a joke by the way)

    But the calculation is possible if you make some general assumptions.
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