system
Given the following post, respond as if you're commenting on Reddit:
human
Are photons "immortal"?

If a beam of light doesn't hit anything, does it just keep on going forever?
gpt
This question is kind of hard to answer.

To say whether or not a photon is "immortal" implies, that there is some kind of reference frame of the photon in which time passes at a measurable rate. 

It turns out, though, that we cannot do that. You may know, that the speed of light is constant in all frames of reference. Thus, photons move at c relative to **all** valid frames of reference. However, this renders it impossible to define a frame of reference in which the photon is at rest. 

Since the rate at which time passes depends on the relative velocity between object and observer, it really makes little sense to talk about the passage of time of an object that has no rest frame.

Thus, photons have no perspective. Not even theoretically. Since photons have no valid "perspective", it is not possible to define a rate at which time passes from their perspective. Without a rate at which time passes, it is impossible to say how "old" they are.


I suppose they are immortal in the sense that they travel at a straight line until they hit something. Space is not a perfect vacuum though, so they will eventually interact with a hydrogen atom floating randomly through space.