The photon does have a lifespan?

artevolved

it is born.

alexg

It 'lives' until it runs into an electron or a positron, then it 'dies', to be 'reborn' again when the electron (or positron) falls back to a lower energy level and 'gives birth' to the photon.

Bobbywhy

The "oldest" photons we measure have come to us from the most distant edge of the "observable universe". They have traveled for billions of years. (google "observable universe") So far as we can tell those photons are identical to newly generated photons. So, the observational evidence is that there seems to be no limited lifespan of a photon.

mrspeedybob

I'd think a photon could be considered "dead" when it has red shifted to the point that its wavelength is greater then the radius of the observable universe. At that point we could never recieve the entire wave. Granted that the universe is not old enough for that to have happened yet.

sophiecentaur

'As far as the photon is concerned', no time has elapsed, despite the fact that we see it as having traveled for billions of years.

M Quack

Absorption of a photon is a question of energy, not wavelength. The two are related, of course, but I don't see how the size of the universe matters