Originally posted by lemon lime
My question has to do with whether or not light can have a half-life if photons are locked into a state of zero time. Einstein said at the speed of light time stops, so that's my basis for wondering if light itself is frozen in time. If anything is frozen in time, then it will not change or experience entropy or be affected by any other force other than s ...[text shortened]... ppen so rapidly it can't be measured or tested so they can't be sure if this is true or not.
If the photon has a mass it has a half-life if there is something lighter for it to decay into. If it does not have a mass then it does not have a half-life, as there is nothing lighter for it to decay into. Electrons and protons are stable because they have quantum numbers which prevent decay - there's nothing for them to decay into. Neutrons are more massive than protons, so they are unstable against beta decay into protons, there's enough extra mass to make an electron anti-neutrino pair and charge is conserved. I'll talk a bit more about how the photon can interact with anything if "time is frozen" below.
In the Standard Model of particle physics the photon is massless. If photons have a mass then the U(1) symmetry that QED is based on would be broken. The way the electro-weak model has it's symmetry broken you'd expect a residual unbroken U(1) symmetry and the electro-weak model is basically confirmed with the discovery of a candidate Higgs at LHC. Also, if it did have a mass there is also a fine tuning problem as to why it is so small; if it is around (say) 1eV, visible light would travel fairly slowly ~3/4 c, and probably render life difficult to impossible as the force would be short ranged on the order of the size of organic molecules.
However QED is inconsistent anyway due to something called the Landau pole - which predicts that the coupling strength becomes infinite at a finite, but huge, energy scale. This would be a problem for the theory, but we expect new physics and some kind of unification with the other forces on this side of the Planck scale. Since we don't know the correct way to unify all the forces we can't rule out the photon having a mass.
So the question of whether the photon has a mass or not is experimental. There is an upper bound on it's mass of 10^-18 eV which is tiny.
Regarding your question about "what the photon sees". The "on board flight time" is measured in something called proper time
. If there is a mass and it is 10^-18 eV, then photons would take a small but finite amount of proper time. In that case there isn't a problem. So what about if the photon is massless? We have to be more careful about the concept of proper time. More formally it is the elapsed time measured in the reference frame of a co-moving observer, i.e. one who keeps up with the photon. If the photon is massless then it travels at the speed of light and there cannot
be a co-moving observer. The frame of reference doesn't exist. This means that the question you are asking is mal-formed. But, with that in mind, as far as the photon is concerned it is emitted and absorbed at the same instant.
As Kazet mentioned earlier it is possible for them to produce positron electron pairs. If a gamma ray has energy >~1MeV then it can split into a positron electron pair which can either annihilate and give the gamma back, or if one of them emits (or absorbs) a photon they can continue to exist as real particles and travel their separate ways. This is a mechanism for supernovae (see pair-instability supernova on Wikipedia) and has been observed in cloud chamber pictures.