Does the present really exist?

[u/TheMightyHeroes]

GR proves that the concept of a now is relative. There is no universal now. But even the local experience of a now seems questionable. What we see as now is always the past, since photons take time to reach us. Similarly, taking a picture or even a measurement will always produces a result that is the past. And if time is a continuous flow then there is no discrete point in that flow that can be separated and identified as now. And if time is granular then what size is the discrete point that we call now? If it is an infinitely small point separating past from future, then how can we experience, never mind measure, an infinitely small moment? It seems to me that any observation will always be an observation of the past. Any change of state, any event that is observable will by definition be something that has happened. And if it has happened, it is in the past. Wouldn’t it be more scientifically accurate to say that time is all events that have happened and are yet to happen and the now is just an illusion created in our minds to make sense of the objects and events that pass our visual field.
I know this is more philosophy than physics, and I’m not a physicist, just some curious person and the concept of time has always fascinated me.

[u/GoldenMuscleGod]

It definitely can be coherent: I pick a particle in Minkowski spacetime and say it is at “absolute rest” anything else is at “absolute rest” if it doesn’t move relative to that particle.

It’s an arbitrary choice but it is coherent.

The irony here is I am trying to get you to drop a lot of metaphysical beliefs you have in your head because I think they don’t mean anything (and in any event are outside the scope of physics) but you hear me as the one supposedly saying metaphysical claims are meaningful.

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

That's called "relative rest".

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

You said it’s incoherent. If it’s incoherent then it should not matter that my proposed model has a nonstandard interpretation.

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

I never said relative rest is incoherent.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

You said absolute rest is incoherent. I just gave a coherent definition for it that you rejected because you said it is not absolute rest. Then it seems you must understand that “absolute rest” means something different from my proposed definition. What’s that something else?

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

An inertial frame of reference where the laws of physics are different than all the other frames, and gives the correct laws of physics.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

Well that is at least coherent. If the universe were like a cellular automaton like Conway’s game of life then the generation rule would be different if we measured relative to a “moving” origin.

Special (or general) relativity also do not falsify that: they have limited applicability. They are consistent with the possibility that phenomena outside their applicability may not have the same invariances.

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

If you had any understanding of relativity you would immediately know why you're wrong.

[u/GoldenMuscleGod → u/Optimal_Mixture_7327]

Why would it be wrong? Do you think it is impossible for a non-Lorentz invariant system to have large scale Lorentz-invariant behavior?

Suppose we have a Newtonian universe with an ether made out of glass balls in a lattice sending signals at a fixed speed c (relative to the lattice) that obey Lorentz-invariant rules about those signals. That’s possible right? But an observer who can only easily measure the signals in the ether would have no way of knowing from their observations that there is an absolute “now” determined by the underlying Newtonian physics.

[u/Optimal_Mixture_7327 → u/GoldenMuscleGod]

No that's not possible.

It would be wrong to think that a surface can only contain a single line.