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.
RiskableMost photons are emitted and never absorbed by anything…
Yet
Eventually all photons will hit something. Even if it’s a trillion trillion trillion years in the future when nearly everything in the universe has decayed into irony.
Someone correct me if I’m wrong, but eventually the universe is going to be expanding faster than the speed of light. At that point all interaction ceases, and any photons that didn’t get absorbed by something yet never would.
Sort of. The expansion of space causes (and is measured by) redshirt. The photon that doesn’t get absorbed “exists” wavelength is not measurable (as its wavelength approaches infinity).
The cool thing about this is that it is identical to what happens in a black hole. Spaghettification. This also has the fun consequence of us possibly existing inside of a black hole, and black holes themselves are entire universes. Because of the breakdown of physics beyond the event horizon its not exactly easy to confirm or deny this either.