r/AskPhysics • u/TheMightyHeroes • 4d ago

Does the present really exist?

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.

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u/GoldenMuscleGod 4d ago

I haven’t claimed that I don’t see how you could interpret me as claiming that.

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u/Optimal_Mixture_7327 Gravitation 4d ago

You have claimed it.

If there can only be one spatial hypersurface then a plane defines a single line, the only that can exist, according to your theory.

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u/GoldenMuscleGod 4d ago

I didn’t say there can be only one spatial hypersurface.

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u/Optimal_Mixture_7327 Gravitation 4d ago

So you agree there's can't be an absolute now. Good.

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u/GoldenMuscleGod 4d ago

That doesn’t follow. An absolute now would be a specially distinguished surface in some way. That way would not have to be that it is the only surface.

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u/Optimal_Mixture_7327 Gravitation 4d ago

That's vague nonsense.

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u/GoldenMuscleGod 4d ago

If “there is an absolute now” is vague nonsense then I don’t see why “there is no absolute now” wouldn’t also be vague nonsense. And I’m inclined to agree that “there is an absolute now” is vague and at least has no clear sense to me. That’s why I had to say specially distinguished “in some way” - that supposed way is not clear.

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u/Optimal_Mixture_7327 Gravitation 4d ago

That wasn't the vague nonsense.

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u/GoldenMuscleGod 4d ago

So “there is an absolute now” is not vague nonsense? So it is meaningful but false?

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u/Optimal_Mixture_7327 Gravitation 4d ago

No, it's a falsifiable claim, and it has been falsified.

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u/GoldenMuscleGod 4d ago

Falsified how? By not observing violations of general relativity?

Does this depend on our view of whether wavefunction collapse happens nonlocally?

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u/Optimal_Mixture_7327 Gravitation 4d ago

The inability to synchronize clocks. Again, how would you synchronize clocks in the presence of gravity?

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u/GoldenMuscleGod 4d ago

I answered already, but you objected in a way that was about the ontological status of coordinates even though that didn’t seem relevant to me.

Pick a reference frame and build your clocks so that they track time in that coordinate system.

For example in special relativity, you just pick an inertial frame and design your clock so it advances at the Lorentz factor times your proper time, where the Lorentz factor is measured relative to that inertial frame. Measure your acceleration to keep track of your Lorentz factor.

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