[MaxikCZ]

The fact that at speed of light time stops is just so bonkers to me.

A photon is, from its point of reference, at the point of creation and at the point of destination at the same "time". Its literally seeing both parts of the universe at the same time, and since its traveled some distance over that time it cannot perceive, its essentially connecting 2 points in spacetime.

If I understand it correctly, every photon exists, from its point of view, for only infinitely small amount of time (similar to how virtual particles do exist from our point of reference), but for us its so easy to "play" with the photon along its path, giving us plenty of time to even decide what we want to do with it after it has already been created.

Its just so bonkers that time can be perceived such differently depending on frame of reference.

[flowerthoughts]

Layman thoughts: The photon cannot experience the universe as it passes through it instantly. It seems to me the universal speed limit creates an observability barrier that is really fascinating. The question is what are we missing, because we're zipping through _something_ at the speed of light relative to it.

Lately, I've been wondering what evidence we have that the speed of the photon/light is really the universal speed limit, and not a very close fraction of it. I could find the argument that a photon must be massless, otherwise photons of different wavelengths would travel at different speeds. But that says nothing of the speed of a massless photon relative to maximum causality propagation speeds.

[OkayPhysicist]

It does, though. Because it's massless, it either needs to be going at max speed or zero speed. And a zero-mass, zero-energy object is a pretty good working definition for "nothing", so photons must travel at the speed of causality, thus making it "the speed of light".

[gyomu]

> A photon is, from its point of reference, at the point of creation and at the point of destination at the same "time"

> every photon exists, from its point of view, for only infinitely small amount of time

Why is that amount “infinitely small” and not 0 since photons travel exactly at the speed of light?

[RobotToaster]

Existing for zero time would imply it never existed.

[kburman]

Have a question related to this, if a photon has zero proper time between emission and absorption, how should I think about the influence of later-created photons or fields on it?

In our frame, we can interact with a photon long after it's emitted send it through a filter, bounce it off a mirror, measure it, etc. But from the photon's own “no proper time” perspective, does it make sense to ask how something created after its emission could affect its path?

[mr_mitm]

The photon doesn't have an inertial frame of reference precisely because it's moving at the speed of light, so it doesn't have a perspective. It's a (quantized) wave in the electromagnetic field. The closer you get to the speed of light, the closer the proper time of the journey goes to zero, but actually taking the limit does not make sense physically.

[hhjinks]

>does it make sense to ask how something created after its emission could affect its path

The problem here is likely the concept of "after". It's relativity; what's "after" in our frame of reference isn't after in all frames of reference.

[nrhrjrjrjtntbt]

It sort of makes sense to me. It means effectively there is no speed limit. At least from the reference point of who is moving.