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Time taken to absorb neutrons,photons

  1. Aug 21, 2004 #1
    How long do particles take to change their mometum when they "bounce"
    off a surface and when they are absorbed by a surface?
    Do photons have a noticeably different "bounce" and absorption time to particles with rest mass?
    Can a neutron have a rest mass and be at rest in reality rather than just in theory?
  2. jcsd
  3. Aug 21, 2004 #2


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    Photons cannot ever change their speed, so they do not bounce. They are absorbed by an atom, then the atom emits a new photon in another direction.

    Neutrons have zero residual (external) electronic charge, but their component quarks can interact either via the electromagnetic or strong forces with other particles. Depending upon the angles involved in the collision, I'd expect a neutron to be either be scattered by a nucleus ("bouncing"), or captured by a nucleus.

    Neutrons can certainly be at rest in reality.

    - Warren
  4. Aug 21, 2004 #3
    I meant to ask: can a neutrino be at rest in reality and not just in theory?
  5. Aug 21, 2004 #4


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    If the neutrinos have even a small mass, the answer is yes. Nobody has trapped one, but someday it may be done.
  6. Aug 21, 2004 #5
    Over what sort of timescale does a photon reflect off a mirror back along the direction it came from, and the same for an electron "reflected" by a solid surface.
  7. Aug 22, 2004 #6
    As has been said, the photon doesn't reflect. It's absorbed, then a new photon is emitted. I suppose the time scale is comparable to the lifetime of an excited atomic state, something on the order of nanoseconds or less I believe. As for an electron, this can be treated clasically, and I think it depends on the properties of the reflecting medium.
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