| MadSci Network: Physics |
A photon exists until it interacts with something, that something usually being an electron that is associated with an atom. The detailed theory of that interaction is called quantum electrodynamics (QED). The 2.7 K microwave background that we currently detect pervading the universe is the radiation from about 300,000 years after the universe's "big bang", and these photons are believed to be the original photons (though red shifted) from that time at which they were freed to roam the universe (that event being called "recombination" by astrophysicists). A photon is believed to never stop existing until it does interact with something.
The lifetime of a proton is at least on the order of 1032 years, that being the lower limit calculated from experiments of the past decade or so. You can find some excellent previous answers in our archives by searching on "proton lifetime" using our search engine. Protons may in fact never decay, but all we can say for sure at the moment is that their lifetime is no shorter than what I wrote above.
Free neutrons (those not in an atomic nucleus), however, do have a finite lifetime, which halflife is about 880 seconds, if it is nonrelativistic. Again, you can find some previous answers in our archives by searching on "neutron lifetime".
John Link, MadSci Physicist
Try the links in the MadSci Library for more information on Physics.