If you "ride" a photon, in your frame of reference, time is still (that is, no time passes with respect to the outside world, you see the outside world still).

However, suppose you are indeed "riding" on a light beam, and are shining from Alpha Centauri, bounce off a mirror on Earth, then bounce back to Alpha Centauri.

For you, time is still, so nothing happens (everything you watch outside is non-moving in time). However, for an observer on Alpha Centauri, your arrival happens after 8 years (if I'm not messing up the distances), so they are 8 years older.

How can this be explained? Am I missing some general relativity concepts, for example, acceleration of a massless particle when bouncing off a mirror? The question is probably trivial for a cosmologist, but I can't find a satisfactory answer.

Thanks.

Similar questions and discussions