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Gravitational forces between subatomic particles

  1. May 13, 2014 #1
    I know that normally we can ignore gravitational effects when considering interactions between subatomic particles. As I understand it the reason for this is that either they are interacting electromagnetically in which case the gravitational interaction is negligible or they are both fermions and therefore can't get too close by the Paulo exclusion principle, or they are particle anti particle pair and annihilate when they get to close.

    It seems however that if we were to have an election neutrino and say a tau neutrino, there would be no limit as to how close they can get, in which case they should be able to get close enough for gravity to have some obvious effects.

    Am I wrong about this? If not, are there experiments than can measure this?
  2. jcsd
  3. May 13, 2014 #2
    im not very well versed in these matters, but i think neutrinos have a flavor basis and a mass basis in QM. so if you know the flavor, then the mass is in a superposition of states... which i think would be difficult to sort in GR. But if the overall energy is the same, then i guess you would be able to define the stress-energy tensor and determine the metric.
  4. May 13, 2014 #3


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    Neutrinos are fast and gravity is extremely weak. Their interaction time is tiny, and the probability that they come extremely close is extremely tiny. And even if that would happen (as in: more than once per 10whatever years) - how would you see it? There is no way to detect individual neutrinos with any reasonable probability.

    Neutrinos can interact via the weak interaction, and this is significantly stronger than gravity (as in, something like 30-40 orders of magnitude) - and still completely beyond anything measurable.
  5. May 13, 2014 #4


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    Might point out...

    This idea only applies when the spins of the two fermions are parallel ("triplet state"). When the spins are antiparallel ("singlet state") there is no such restriction.

    A particle-antiparticle pair can get close, or even coincide, without instantly annihilating. The electron-positron pair in positronium are in a singlet state (1S), and yet they take ≈ 10-10 sec to annihilate. In the subatomic world, this is a pretty long time!
  6. May 15, 2014 #5
  7. May 15, 2014 #6


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    You can always check to find at which energy scale the gravitational force coupling constant becomes important in comparison to the rest- at this scale the gravitational interactions cannot be negligible anymore.
    That energy is around the Planck's Mass scale (around 1018-19 GeV). So, as long as you are way bellow that energy scale, the gravity can be "forgotten".

    Apart from that, by today's standards, gravity cannot be quantized in a consistent theory, so we don't even know how things are supposed to work there- you cannot easily make conclusions... For theories questioning these things, we have supergravity, strings, loop quantum gravity etc... I don't know much about the last example-but the efforts done on this approach haven't been fruitful yet (String Theory and M-Theory by Becker,Becker and Schwarz)-, but the 1st two are to be verified by their low energy behavior (eg string phenomenology) and how well they could reconstruct the known results. So far these models just belong to the theoretical studies of physics/mathematics...
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