this post was submitted on 08 Aug 2024
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If I understand that right, gravity also moves in space at the speed of light, therefore Earth will keep on orbiting for 8min around nothing?
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.
But that also doesn't translate. If the moment the photon is created (from whatever reaction that caused the light source), to the moment it hit the person's eyes had no time pass (nothing in the universe moved) then it would be instantly created and observed by the observer. But the moment the switch turns on and the moment the photon hits the observer (as miniscule as this distance is) the eye of the observer has moved from A: switch goes on to B: observed.
Yeah no time passes for the photon I guess, but the universe still moved around the photons travel.
Let's preface this, I'm no astrophysicist. but from my understanding:
That's just the thing, different speed observers do not agree on when things happen, or even the shape of the universe. The faster you go the more the universe compresses in front of you, making distances shorter from your frame of reference.
From the photons perspective it instantly moves through an infinitesimally thin sheet of universe. Everything that "happens around it" from our frame of reference all instantly happens at once if you ask the photon.
Here's a really good explanation from someone far smarter than me https://youtu.be/-NN_m2yKAAk