As chilinot said, it’s the speed of any massless particle in a vacuum. A massless particle has nothing slowing it down, so it moves at the maximum possible speed. It’s actually the propagation speed of cause and effect, or put another way, how long it takes for a quantum event to affect whatever is in the adjacent point one Planck length away. It’s simply how quickly these things ripple forward when there is nothing slowing these ripples down.
Every other massless particle moves at the same speed. “Sound” is not a massless particle, it’s propagation of the compression of matter, so therefore moves much, much slower and would not replace c. It has nothing to do with what we can observe and rather to do with its properties. Personally, I find referring it to as “the speed of light” is confusing since it rarely has anything to do with light/photons other than that light happens to move at that speed.
I watched this recently[1] and the whole relativity thing made me think that its kinda like how in games often physics is processed as local clusters (for parallelism) and it made me think that reality appears to be simulated in this local clusters too. Relativity exists because that way each local cluster is independent and can be simulated in parallel, sharded across the servers! :)
Hell, why not take it a step further and say that the simulation is relative to an observer (which can be an inanimate thing, of course) as an optimisation because why bother simulating what isn’t seen or interacted with by an observer?
Every other massless particle moves at the same speed. “Sound” is not a massless particle, it’s propagation of the compression of matter, so therefore moves much, much slower and would not replace c. It has nothing to do with what we can observe and rather to do with its properties. Personally, I find referring it to as “the speed of light” is confusing since it rarely has anything to do with light/photons other than that light happens to move at that speed.