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this post was submitted on 08 Aug 2024
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Kind of. The concept of simultaneity breaks down at distances where the speed of light matters. If we base it on what we currently observe and call "now" on the Sun the eight minute old state we currently observe then what does "now" on earth look like from the point of view of the Sun at that same moment? You can't reconcile a single "now" for observers in both locations.
An alternative take which is also consistent with observable physics is that the speed of light is infinite but it's causality itself that propagates at c.
Thinking in those terms also makes a number of relativistic effects more intuitive. You need infinite energy to reach the speed of light simply because it's infinitely fast. Time dilates when moving because you're encountering approaching causality earlier than you otherwise would have. Time "stops" for anything traveling at the speed of light because at infinite speed it just experiences literally everything in its line of travel at once and the concept of "after" becomes meaningless, encountering all future oncoming causality in a single instant.
This was a bit of a tangent but it's something that has fascinated me for a long time.
I'm trying to understand how that reference frame works when you just just bounce a photon off a mirror and time how long it takes to come back? Like, light must have a non-infinite speed to the stationary observer, or it wouldn't take time to traverse the distance.
thats the thing, thats from your reference frame. From the photons perspective time stands still and everything happens at once
Well, yes. Sorry, I thought the claim was that photons move at infinite speed, relative to a stationary observer.
The observable effect is the same either way. If light is infinitely fast and causality propagates at c then it's still going to take (distance to the mirror / c) for the fact that you turned on the light to reach the mirror, and that same amount of time for the fact that the light reflected to propagate back to you.
Those two things don't square. If you're moving relative to the mirror when your fire the photon, it would hit in a different place than if you were stationary. The photon can't be moving infinitely fast in your reference frame for that to happen.
Yeah I think infinite is the wrong word for them to use there. Maybe call it maximally fast? Like it goes as fast as possible no matter your reference frame, but that speed is limited by the speed of causality. The photon has 100% of its skill points in speed through space and 0% on speed through time.
Would it? What does "stationary" mean when discussing relative velocities? The mirror being stationary and the person firing the photon moving at a constant velocity is literally an indistinguishable scenario from a stationary person firing the photon at a moving mirror.
If I am moving relative to a mirror when I fire the photon, then the mirror is moving relative to me, and will be in a different relative position by the time the "event" of my firing that photon reaches it.
Also, the photon isn't moving infinitely fast in my (the firer's) reference frame. It's moving infinitely fast in it's own reference frame.
Yeah I eventually picked up that that's what you meant in your original comment, not that photons move instantaneously and that causality somehow catches up later.