GodricThe life of a photon
xionzuiTo be pedantic, photons never accelerate. They only ever travel at one speed in one direction
And as they’re massless, photons do not experience time. Regardless of how far a photon travels, from its perspective the journey takes no time.
PseuIt also does not experience space, as the entire universe has been length contracted in its direction of motion into a 2d plane. It is simultaneously occupying every point along its path. So it doesn't need to experience time.
I’ve been trying to wrap my head around this part and what it means for cause and effect for a while. I think Feynman said something like a photon is only ever emitted when the source and destination agree to exchange one. Which makes sense if the exchange is instantaneous to the photon. But how can billions of years pass for us in the mean time?
NeatoBecause there isn't perspective of a photon. It doesn't experience because it doesn't change like mass does.
I'm not sure Feynman was right. Most photons are emitted and never absorbed by anything.
Peruvian_SkiesPhotons exist, so there is the perspective of a photon. Most may not be absorbed but that's irrelevant because some are. And when they are, their perspective - like them - ends. Like yours does when you die.
The photon does not experience time, but we do, so from our perspective they can be emitted and absorbed even though from their perspective they are timeless. Again, like us. Before you were born, you didn't experience being not alive. From your own perspective, you've always existed,even though from the perspective of someone older than you, there was a time when you didn't.
I was using the wrong term. Photons don't have a frame of reference.
But even from the colloquial definition, photons don't have perspective. They don't live and die because they never experience time. If you had their point of view, your beginning and end would happen simultaneously, meaning you wouldn't experience anything. They are immutable particles whose only interactions are emission and absorption.
There's a difference between time not passing and not existing. To a photon, space (in the direction of its movement) doesn't exist, as its origin and destination points are the same. But time does not pass - the axis of time is there, but the photon never budges in either direction, like a rock buried in the middle of the desert doesn't move in any spatial direction on a human timescale. The photon's beginning and end aren't simultaneous, quite the opposite. Since it can't move in time, they might as well be infinitely far apart.