[lolc]

My understanding is that photons don't have a life the way we do. They move at the speed of light and thus time does not advance for them. They cannot change between emission and absorption, no matter the distance. Always bends my mind to think about it.

[Kranar]

Almost everything you said is correct. Photons do not have a reference frame, so time does not advance because for a photon there is no time coordinate system in the first place. It's not simply that photons don't experience time, it's that time and space don't exist for photons.

>They cannot change between emission and absorption, no matter the distance.

From the point of view of a photon, neither time or space exist. They have no reference frame at all. However, from an outside frame of reference that is travelling less than the speed of light, photons do change for example they get red shifted as they move through stronger gravitational fields.

[lolc]

Not to quibble about definitions of what "change" means, but I thought the red shift depended entirely on the relative speed of the emitting and receiving body? A laser beam for example could not be analyzed for its red shift, unless we knew the original frequency, because there would be no spectral absorption pattern to determine the shift. So we cannot tell how far laser light traveled before it reached our sensor.

[Kranar]

Not a quibble at all. The redshift I'm referring to is the kind of redshift due to gravity, as opposed what you're describing which is redshift due to the Doppler effect.

[lolc]

I'm trying to come up with a scenario to understand the difference: Say I have a triangle of emitter, receiver, and a large body, all at a fixed distance. And the receiver would see emitted light both directly and bent around the body. The bent light would be red shifted?

[rvbissell]

> They move at the speed of light and thus time does not advance for them.

Isn't it more accurate to say that photons move at the speed of causality, when the medium is a pure vacuum? Because in some other medium like glass, the speed of light is slower than the speed of causality.

So my follow-up question is: do slower photons (such as those propagating through a fiber-optic strand, or water) then experience the advancement of time?

[Kranar]

There is no such thing as slow photons, photons always travel at the speed of light.

When light enters a medium there are two mostly (but not entirely) equal ways to think about what happens, one is to view light as a purely electromagnetic wave that interacts with atoms and causes the atoms to oscillate. This oscillation produces its own electromagnetic wave that interferes with the original wave. The result of this interference will be an electromagnetic wave with the same frequency, same amplitude, and travelling in the same direction as the incoming light but shifted backwards and it's that shift backwards that gives the appearance of light slowing down.

That explanation is pretty good and accounts for almost everything except for the latency of light through a medium.

If that's what you want to model, then it's better to think of light as made up of photons instead of being a wave, and then when photons enter a material they no longer exist as independent particles but through a process of absorption and reemission by electrons in the material become particles called polaritons. Polaritons do have mass and hence travel slower than the speed of light.

Neither of these explanations are perfect, but the full explanation is ridiculously complicated and there's no suitable metaphor for it. If you are interested in knowing the edge latency of light through a medium, then the polariton explanation is appropriate. If you want to know the "bandwidth" explanation of light through a medium, then the wave explanation is appropriate.