I don't think that either time travel or eternalism make any sense. Or rather, at least not outside of fantasizing where we just ignore a lot of details (because not ignoring them stresses that the ideas don't make sense), and we're basically just assuming that we can do some magical things per rather superficial whims. So it just becomes a matter of what we want to loosely fantasize.

Re eternalism, how would we move or change anything? Don't we have non-eternalism as soon as we do that?

At the moment we can only look at the facts that exist in general relativity, special relativity and quantum mechanics. Though, incompatible with each other, these are all we have as a foundation for thinking about time travel.

Causality has to do with energy and motion and time seem to be the result of this distribution of energy. Time travel is essentially traveling between different points of energy distribution which represents itself as different times.

The thing is that A and B-theory, most of the philosophy of time does not take into account theoretical physics as much as it should. Causality is too solid a concept for there to be any solution to the grandfather paradox. X causes Y because of the distribution of energy, not time itself. If you change the distribution of energy, changing the trajectory of particles, mass etc. because you kill your own grandfather before your parent was conceived, you have changed the possibility of energy to be distributed into your existence.

But the thing is that all of this is so hard to conceptualize without a unification theory. The most logical conclusions out of the actual physics about time makes for the moment a hard case for causality and relativity for observers.


One interesting thought experiment is the one about communication through large distances of space. In the game series Mass Effect, humanity has expanded throughout the galaxy through ancient found devices which enables FTL travel to fixed points around the galaxy. But this would mean that if you travel to the other side of the galaxy, you essentially travel back in time relative to the position you came from. If looking back at the location you came from, it would probably be millions of light years ago. Essentially you would be able to find a spot in the galaxy in which, when traveled to, you will arrive at the exact moment when light from the position you came from shows you starting your travel to where you are. - But the interesting thing happens when we take their means of communication into consideration. They use quantum entanglement communication, meaning that they entangle a particle at both ends of the communication and communicate through interacting with that particle. This means that they can exist in vastly relative positions in spacetime, but they have a fixed point for their communication.

This means that everyone in the galaxy experience a specific position in spacetime as their "present", but no one can observe that time from the position they are in. Communication is instant, FTL travel is instant, so the arrow of time for all the inhabitants of the galaxy is specific, not relative. It is only relative when trying to observe from a specific position.

How do all the inhabitants of the galaxy then relate to general relativity and time travel? They should all be relative to each other but their experience is not. What happens when they experience time dilation through gravitational variations at their specific location? Can you quantum entangle communicate between two points in vast distance from each other, while still be able to exist within gravitational pull that dilate time?

Essentially, can you entangle a particle at a place that is vastly slower in time relative to our own? What happens if you do?

Time travel is essentially traveling between different points of energy distribution — Christoffer

What does that amount to in practical terms, exactly? "Different points of energy distribution"?

What does that amount to in practical terms, exactly? "Different points of energy distribution"? — Terrapin Station

Essentially I mean that we think we travel in time, but we are generally traveling between different levels of energy distribution under entropy. If I heat up water, let it cool down and then travel back to when it was hot, my initial concept is that of traveling to another time, but essentially I traveled to when the energy was high in that water. If energy and mass create time as it's "byproduct" then we are generally traveling along the causality of energy distribution.

I'm no physicist so how this relates to other aspects like quantum particles, fields (like the Higgs field) etc. I don't know at this time, but if the concept of time is that its the byproduct of mass, energy and gravity we don't really have time as a concept by its own, but as the resulted effects of the causality of all those things. If we view time instead as how mass, energy and gravity works, there might be clues to what that means for the idea of time travel.

If I heat up water, let it cool down and then travel back to when it was hot, my initial concept is that of traveling to another time, but essentially I traveled to when the energy was high in that water. If energy and mass create time as it's "byproduct" then we are generally traveling along the causality of energy distribution. — Christoffer

But how would we "travel back to when it was hot" rather than just heating it up again?

Do you add energy to the water and heat it up again? No, you travel back along the line of energy, backward thermodynamics. The water starts to heat itself up when the energy is coming back into the water. That is what happens if you turn the arrow of entropy.

My point is essentially to think of time in terms of energy distribution and entropy instead of time as the concept of events. It might help thinking about the topic of time and time travel.

Do you add energy to the water and heat it up again? No, you travel back along the line of energy, backward thermodynamics. The water starts to heat itself up when the energy is coming back into the water. That is what happens if you turn the arrow of entropy. — Christoffer

If the water starts to heat itself up when the energy is coming back into the water, how is that not adding energy to the water to heat it up again?

Because adding energy require you to add energy with something, converting energy into new less form of energy along the line of thermodynamics and entropy. Going backward, the less form of energy reverse entropy and there is nothing applied to the water, it heats up by the reversal of energy which left it.

Just like a glass has many more sides after it breaks and going backwards with that entropy, the increased information decrease into a higher, larger form as a single glass.

I'm not sure why you confuse this with adding energy? You know about thermodynamics and entropy?

Yes, I know about thermodynamics and entropy.

If you're saying something specifically about thermodynamics and entropy, you'd need to characterize it that way. (And then we can check if the better qualified characterization would work.)

Saying "the reversal of energy which left it" means that energy is being brought back to it, which is another way of saying that energy is being added to heat it up.

Sorry, reversal of entropy so that the arrow of thermodynamics is turned around, making the energy go backwards, into the water from the high entropy state it had before. The energy leaves the water as it distributes into a lesser form of energy, if entropy is going backwards, that distributed energy is going back into the water.

So if a system can work so that its thermal energy can be converted into mechanical work, then in what sense is the entropy "going backwards"--wouldn't that just be a "forwards" way that energy behaves, simply negating what we thought was the third law of thermodynamics?

If causality, as understood, holds, then it would be impossible to kill your grandfather.

Another thing that can happen is the universe may enter a different time and space, if you succeed in killing your grandfather, in which you don't exist.