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General relativity

  1. Apr 28, 2004 #1
    If two spin 1 photons are put together so that the tips of their magnetic field vectors just touch one another ( the vector lines are all in the same plane), a particle results which looks the same when it is rotated through 180 degrees - a spin 2 particle.This is the spin a graviton is supposed to have.
    Is this a graviton?
  2. jcsd
  3. Apr 28, 2004 #2
    You can construct systems of n photons in which the total spin angular momentum is any integer between -n and +n. This does not mean that single particle states with helicity n and -n are formed. A single particle state is defined as an irreducible representation of the little group of the Poincare group in whatever spacetime dimension you're considering. In 4 dimensions, the little group is ISO(2). A helicty +/- 1 states form an irreducible representation. A helicty +/- 2 states form an irreducible representation. Etc. Therefore, two photons together do not a graviton make.
    The n photon system forms a reducible representation under ISO(2): n copies of helicty 1 states.

    While we're here, a situation where we can construct single particle states from other single particle states can occur when you go between massless and massive representations. For example, the little group for massive states in 4D is SO(3). A massive spin 1 particle forms the 3 dimensional (irreducible) representation of that group. Under the little group for the massless states, ISO(2), the 3 rep breaks into a 2 and a 1. These are interpreted as a massless spin 1 particle and a scalar particle. This type of decomposition (in reverse) happens when we say a massless vector particle "eats" a scalar particle to become massive.
  4. Apr 29, 2004 #3
    What do you think a graviton would be like then - I think we should be able to visualise what it is - we can visualise photons and atoms - so why should a graviton be so different? I agree that two photons do not a graviton make because two photons in a beam of light would then be a graviton! If the photons travelled faster than light, that would make them different - perhaps gravitons.

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