Hacker News Reader

Also what I think is being asked:

...if the photon comes to rest on earth, and we compare clocks...

IIRC, the deceleration of the photon relative to me (as it comes to rest on earth as would a returning space traveler) would "reverse" the time dilation relative to me, so both our clocks would read 12:08.

Please, correct me if I'm wrong. I read this many years ago but never dove all the way into the math.

View on HN

No. This difference in the clocks will remain even if you could somehow slow down the photon without it being absorbed. This is why if you sync two atomic clocks, bring one atomic clock up to orbiting speed (such as on a gps satellite), then decelerate it and return it to earth the clocks will still read different numbers.

Which is why if you could accelerate yourself to say 90% the speed of light, then decelerate and return to earth, you would have essentially traveled into the future and outlived everyone you ever knew.

Reply on news.ycombinator.com