Time doesn't exist therefore you can't travel in it.
Yes that's a valid point and well-taken.
But there is a big difference between "impossible" and "just really really hard". And that's where modern physics is right now.
And if we were able to get a better handle on the hard problem of consciousness, the riddle of "what is time" might also yield.
So it's back to the metaphysical debate about whether the strict materialists are the ones who are "right" in all of this.
Matter goes through "events" that have cause and effect relationships, but can effect ever precede cause?
I don't see how you can empirically claim to have sufficient knowledge to answer that question.
But that's just me, I'm sure you see it differently.
So to disagree it's enough to proof time existence or link to such proof.
First, if we're realists/utilitarian/pragmatists we don't need some fancy proof for time existing. We can see that the material world changes and we call that time. We even measure that passage with clocks. We even know (and have proven experimentally) that things going faster slow down clocks on them. So, whether time "really exists" or not is a meaningless question when we're concerned with time as we experience and know it.
It's like someone shooting a bullet at you, and you are asking "yes, but does this bullet _really_ exist?". Well, it might not (e.g. we might all live in a simulation, or the nature of physical objects might be just some probability distribution of information, and not "exist" in the plain sense we understand it), but in this life as we know it, a bullet hitting you will still have the consequences we know and expect.
Second, time exists in Relativity equations, so there's that.
>Time is a useful abstraction for calculating physical models, making forecasts. As we have other useful abstractions in math or programming - it doesn't make them existing in real world.
Doesn't matter. It's enough that we can measure it, we don't care for the "thing in itself" or if it's merely some illusion. We live in a universe where whether it's an illusion or not, it has the same effects for us (e.g. we grow old, things change state, our clock dials rotate).
I don't need a fancy proof either - any proof would be enough. What we both can agree is that things or processes are happening. They don't need time for that. Constant change, constant mutation doesn't require past or future, but now only. But as I said, we - intelligent species - need a concept of time to predict how the change will proceed.
What we measure in clocks depends on a clock - a sand falling down in a sand glass, a pendulum or a spring cycle and so on. After this we make a relation from this event and start to measure other events with it. Still it's just things happening, not a prove of a time existence.
If you say that you experience time. Well. That's how neural network works - they are shaped by events/impulses which happened to the network and accessing this information gives you illusion of a past - but this information encoded in your brain exists now.
> Doesn't matter. It's enough that we can measure it, we don't care for the "thing in itself" or if it's merely some illusion. We live in a universe where whether it's an illusion or not, it has the same effects for us (e.g. we grow old, things change state, our clock dials rotate).
So we agree in a part that things change state and this is enough to imply growing old and rotating clocks. I don't agree it doesn't matter - as if the time doesn't exist, we don't need to have a pointless discussion about "travelling in time".
The change itself might not, but to perceive something as having changed, e.g. to understand that what is in N state now was in some N - 1 state before, and N - 2 before that, presupposes time (or rather, can be considered the same thing as time).
>So we agree in a part that things change state and this is enough to imply growing old and rotating clocks. I don't agree it doesn't matter - as if the time doesn't exist, we don't need to have a pointless discussion about "travelling in time".
It wouldn't be pointless as to the actual phenomena we'd experience. It would the same as asking "since we see we can grow old, or see the clock hand rotating, can we somehow manage to go to the now moment when we were young, or where the clock hand was 1 hour before?".
Though I suppose you didn't really travel; you reset everything -- including yourself -- to a previous state/time. If you could isolate yourself, and then return the rest of the universe to some previous state (while finding a coherent place for your personal newer state in the old rest-of-universe state, possibly editing out the parts of the universe's state that contain the old you), then "traveling" would make more sense.
> but only that somehow the same state occur one more time
Assuming that state transitions are deterministic (apart from your meddling that allowed you to return to a prior state, that is), then this is equivalent to rolling back time, so the distinction doesn't really matter. If you pick a quantum state that occurred 10 years ago (by your subjective perception of time), then everyone will be 10 years younger, people who were born in the last 10 years will no longer exist, and people who died in the last 10 years will be alive again.
Also remember that everyone has their own reference frame when determining time. There's no such thing as "absolute time", or even the absolute passage of time. But that's ok; we don't need that to be the case for this to work out.
Yes.
>If the moment is different in this way, can it still count as the past?
Well, several answers here.
a) If it's the same in every other way, then it's good enough as "the past" to me.
b) We understand things as substantially the same all the time even though they've changed in small or big ways (e.g. we consider ourselves the same person as the child we once were, we consider a city to still be the same city even though new buildings have come up/gone down as time passes, etc.). So why not consider the past + that difference as "the past"?
c) What else could it be? It's surely not the future, and it looks a hella lot like the past. At worst, we could say it's a new divergent version of our original past.
(But how would we even know it's a divergent version? Nothing might have changed, our trip might be a closed loop, where we always were to visit the past -- that is, there was never a past at time X without us visiting it from the future).
"This kind of argument" you mean "argument"? Arguments applies to everything, agree. This kind of argument applies first of all to time and possibly to something else by analogy. Beauty of reasoning.
But if in some hypothetical universe, you realise by arguments, that nothing exists - what's the reason of "giving back the word" which in this hypothetical universe has no meaning? I don't follow this logic.
I think you'll find that your argument that time does not exist also applies to literally everything else. Space does not exist. Matter does not exist. People do not exist. There is no clearly defined moment of birth or death, or boundary between a person and the rest of the world; thus "people" don't exist. To use your words:
If you say that you experience seeing distinct people. Well. That's how neural networks work - they categorize the events/impulses which happened to the network into distinct objects that didn't exist in the events/impulses themselves.
> But if in some hypothetical universe, you realise by arguments, that nothing exists - what's the reason of "giving back the word" which in this hypothetical universe has no meaning? I don't follow this logic.
If you define a word such that it never applies, you're communicating badly. By the above arguments, nothing exists. And by the definition of the word "exists" that you're using, that's correct: nothing exists.
But that's a dumb way to define "exists". It's a perfectly useful word. And you should give it a more reasonable definition such that time, space, and people exist.
What the parent says is take your argument for time not existing:
"If you say that you experience time. Well. That's how neural network works - they are shaped by events/impulses which happened to the network and accessing this information gives you illusion of a past - but this information encoded in your brain exists now."
By the same logic, matter doesn't exist either.
Our brains [1] perceive a material world, but it's only a as thoughts an impressions they do so. There doesn't need (nor do we have of there existing) an actual material world. A neural network as complex as our brain that gets the same weights and is fed the same events/impulses as raw information (e.g. digits of input) would see the same "material" world.
No more reason for the material world to exist, than there's for time.
In fact, perhaps it's just my own brain, alone, as perhaps I'm the only actual person in the universe. What proof do I have that anybody else is not a "non playing character"?
About Solipsism - for sure in the universe you're living in you're the only one, as your perception is unique. We all have our own universes of thoughts. There's no proof for the question, that's why you should accept such possibility.
You're traveling at the speed of light. You have your perception of entropical change and an observer has theirs. There is no singular time that "exists". There are multiple perceptions that "exist". Time itself is not existing, as much as it is a perception of process(es). Humans mark states and compare later. This is not time, this is a measure of states. The term time is a shorthand for that, to the detriment of comprehending/understanding what it (the term Time) is describing.
A reconfiguration of the matter on the world on the same state would still be exactly equivalent to a travel back in time (to that "now") in that case.
And such a time travel might not even take active agency to happen:
https://en.wikipedia.org/wiki/Poincar%C3%A9_recurrence_theor...
Occurring the same state is obvious on condition of finite amount of state and infinite amount of mutations. The question is if amount of states are finite.