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If a photon were to be contained and left alone with no other interference like magnetic fields, radiation, etc. Would it decay? If it would, why? (Dont photons move at the speed of light?)
Answer 1:

You cannot contain a photon without any interference. Anything that contains a photon uses the magnetic/electric fields of a material to contain it. It is those interferences that you use to trap a photon in the first place or slow down its group velocity to a very slow speed.

There is no indication of photon decay in a vacuum. Except for very high-energy photons there's the possibility that it will particle-pair create (i.e. create positron-electron pairs) but otherwise there isn't a natural state for it to go into and in a vacuum they can propagate for a very long time. Its why we can see galaxies and stars that are bordering on a billion light years away... it takes a billion years for them to reach us but we can still see them. Moving at the speed of light only deals with decay in the fact that there's no rest mass and so it can't naturally decay into smaller particles. we think all fundamental particles in the universe are mass less and only acquire a mass through a process called symmetry breaking with the higgs boson (I don't know if you've heard of this with the LHC at CERN), which ventures far too deep into mathematical details of quantum field theory to discuss without you having taken about 10 more years of math =).



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