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One reason I don't believe in the possibility of backwards time travel (for objects relative to our timeline, anyway; absolute direction in time I don't as such believe in, either) is I don't think that objects leave copies of themselves in the past, as they progress forward. Even if we could somehow travel in a different direction in time, it seems to me that we would travel into an empty space, as such, if we did so. (This seems directly true in another way, inasmuch as the Earth, the solar system, and even the whole galaxy are moving through space: if Earth is now at point X and was at point Y, then when I travel from time A to B, and B is "when" Earth was at Y, then traveling from point X to time B, seems as if it would leave us at point X in time B, not point Y.)

Corollary thoughts here are: supposing a finite amount of mass/energy at a given time, but supposing that time is continuous, we would then be saying that time holds a continuous number of particles (as copies) even though space holds a discrete number per a given moment. So somehow it would be as if there are both a countable and uncountable amount of mass/energy as such. Or: if the universe at time X has a certain quantity of particles, and if a person at time Y is not in the set of objects at time X, then wouldn't traveling to the past increase the amount of matter in the past, violating the conversation of mass/energy?

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  • It seems obvious to me that there is only the now, and the past and future are mental constructs, but it cannot be proved in physics, I am led to believe.
    – Chris Degnen
    Oct 1, 2020 at 16:01
  • This is a question for Physics SE, and no, a closed timelike curve ("time travel") does not violate the spacetime version of conservation. There are solutions with such curves in GR.
    – Conifold
    Oct 1, 2020 at 17:50
  • You seem to be implicitly assuming the truth of presentism, that only present things exist. Are you familiar with eternalism, which posits that entities in all times have the same ontological status, and the block universe variant which says that what we call the "present" is just a 3D cross-section of a larger 4-dimensional structure?
    – Hypnosifl
    Oct 1, 2020 at 18:23
  • The mass/energy conversation is indeed a deep one.
    – christo183
    Oct 2, 2020 at 6:28
  • We need to separate time rewinding from time traveling. I understand the orbital/rotating diorama picture of 4-dimensionalism (every slice of the universe at a given time is a "slide" on the diorama, which simulates time by rotating "in front of" our consciousness OR the simulation comes from us "orbiting" the static diorama) but aside from eventually needing a "second time in which the first would pass" (Kant's analysis of a concept explored much more by another writer later, I forget his name...), the diorama model implies no real change. People only want time "travel" to change the past...
    – Kristian Berry
    Oct 3, 2020 at 0:51

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There’s something a little strange about your framing. Linear time is a dimension, but that doesn’t mean temporally extended bodies are travelling - we don’t think of a Cube as multiple copies of a square created as squares move through space, but rather it is its own three-dimensional shape that happens to have a square cross-section. What you see as “a particle travelling” is, from an extra-temporal perspective, more like a line than a point, and there’s nothing implicitly objectionable about that.

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  • Geometric analogies can be very explicitly visual when well framed, Cheers!
    – user37981
    Oct 2, 2020 at 4:04
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I don't think this is enough to rule out a block universe where the past, (and future if you desire), are equally real to the present.

We don't know that time is really continuous or not, but I don't see how a infinite (not necessarily continuous) worldline of a finite number number of particles is problematic. There some things we know are discrete and some we think are continuous. Having both isn't a priori a problem.

What would going back in time actually mean? If we take the forward arrow of time to be the second law of thermodynamics, then does an entropy descreasing action mean we've reversed the arrow? Not really because there must be an equal or greater entropy increase somewhere else in the universe and the macroscopic entropic arrow still points forward/toward increasing.

But with perfectly symmetrical physical laws (which we do have, nothing has violated the combined CPT symmetry), why can't I reverse my local entropy to the point I am young again? Well in theory you could. Every trajectory has a CPT inverse, and as long as the overall entropy of the universe keeps increasing, any local decrease is totally fine and happens all the time. The engineering to de-age a human is completely out of reach but not inherently impossible.

But this isn't going back in time I would say. It is reversing the local entropic arrow, but not changing the overall second law applied to the whole universe or undoing anything. The past would have still happened, you just locally engineered a mini-recurrance basically, but you never traveled "back in time".

The overall entropy still increased and the second law is happy. Nothing violated the second law and you 'traveled in the reverse entropic direction' to a configuration you had in the past. This may make time travel less interesting, but it isn't time travel really. The more interesting point to me is that we have totally symmetrical laws. Why there is macroscopic direction (toward overall increasing entropy) is only explainable with an additonal claim, that we started in a incredibly low entropy state, the big bang. At least that is the normal framing. There are other theories that don't require the universe to have been in such a state for the 2nd law to emerge from symmetric laws.

So what did the universe's and your worldines look like during this? The universe never looped back into a previous configuration. We think it is unbounded, so there are no reccurances. You never traveled 'back' time, but you did travel on a 'backwards' entropic arrow while de-aging if that makes sense. Did you loop back to find your old body in space? No. But that's not because the block universe doesn't exist. All of the above happens in either case. The block universe comes from our perception of the flow of time. Maybe every "instant" is just a slice like a frame from a movie, and the universe is just some higher dimensionsal stacking of these "instants" and the feeling of time passing is a conscious illusion. The stack of timeslices is static and unchanging and always existed, and does have an ordering. You didn't time travel to a previous timeslice, you just progressed to a new timeslice yet you lowered your local entropy (the de-aging bit).

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  • The first step in any attempt to answer this question is to begin to separate time into its constituent levels of perception and experience. Time is real and yet has its limits. Planets and stars experience no time, only humans do. So that when someone passes away so does time. Now whether they experience something else in a segment of duration or eternity is a different question. Spinoza maintained that part of the mind can experience 'an aspect of eternity'. See "Ethics" Part Two, on wikisource.
    – user37981
    Oct 1, 2020 at 17:42

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