[ChuckMcM]

This was perhaps my favorite part of Physics 390 ("modern physics") which was about quantum dynamics and relativity. The speed of light is defined in terms of a velocity (~300,000,000 m/s) but if you were traveling at the speed of light time stops (which keeps the rule that its constant in all frames of reference). That and time passes more quickly at higher altitudes and these days we can actually measure that. Wild stuff.

[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".

[flowerthoughts]

Thanks for the reply. That's still a theoretical reasoning. "Based on our current _models_, it must follow that c=c'." I can accept that. I guess part of a wider theoretical answer is that a photon is just an interaction in quantum fields, and that indicates there's nothing special about a photon that could limit its speed (as you imply.) What you're saying makes me think I should be looking for impediments for attaining speed, and it seems only (inertial) mass is that thing.

My question is if this part of the model has been validated experimentally somehow.

BTW, it seems odd calling a photon a zero-energy object.

[OkayPhysicist]

Photons are zero-mass, some energy. (and thus moving at max speed). I was trying to convey that the only way for something to be massless and not moving at the speed of light would be for the massless object to be stationary (and thus zero energy).

[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.

[skissane]

> Existing for zero time would imply it never existed.

In the mathematics of infinity, something can have exactly zero probability yet still happen, and exactly one probability yet nonetheless fail to happen (hence the standard term “almost surely”).

If something can have literally zero probability of existing yet still exist, why can’t it exist for literally zero time yet still exist?

[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.

[MaxikCZ]

I wonder if Photon doesnt have an inertial frame of reference - from our point of view.

Like, from our point of view, we assume that from photons point of view it has no perspective.

But maybe we are limited by our spacetime, where photon goes trough our universe in an instant, and continues to pierce infinite universes over its non-zero frame of reference.

[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.

[euroderf]

> but if you were traveling at the speed of light, time stops

Are we saying then that it is theoretically possible that (a) photons have a zero or near-zero lifetime, but that also (in our frame of reference) (b) they exist for a finite duration of time and are visible during that time simply because time stops for them ? Or did someone manage to slow down light to some fraction of light speed ?

[tacker2000]

Also see the twin paradox, which is related to this and equally fascinating.

If one twin stays on earth and the other makes an intergalactic trip (with the speed of light), upon return, the one on earth will have aged much more than the one on the trip.

[Tor3]

The paradox isn't actually when the twin is returning, the paradox is seen during the trip away from earth. Both are observing each other through telescopes, and both can see the clock on the wall in each other's home (a house, or for the twin, his space ship. Which has got a big window). And both can observe that the other twin's clock is moving at a slower pace than their own clock. And that's the paradox.

(p.s. the spacefaring twin doesn't have to move at the speed of light, and indeed cannot, it's enough to move at a "relativistic" speed, i.e. fast enough that this is actually measurable. With today's clock that doesn't have to be very fast actually)

[gilbetron]

My favorite explanation: https://www.youtube.com/watch?v=Vitf8YaVXhc