Sorry it took so long to reply to this.
Here's my take on time travel.
To understand time travel we first have to consider time itself. What is it? The answer I give might at first seem implausible, but please bear with me.
Time is the measurement of change.
Just as a metre is a man-made concept that measures space, an hour is a man-made concept that measures change - 1/24 of the change in the earth's aspect with regard to the sun; 1 rotation of an hour hand on a mechanical clock; x amount of beta decays in a sample of carbon-14...
If you suspend ALL change, ALL time will cease. Time is just the measurement of change. For there to be a 'meaningful' time we must compare the rate of change of one thing relative to another: the change of second hand relative to a minute hand relative to an hour hand, relative to the earth's rotation relative to its orbit relative to...
No meaningful time can emerge without relativity. The only way we can meaningfully measure change is to measure it relative to something else, something that changes at a different rate. One cannot measure change without relativity - for all things change at some rate or another, even a stone.
It is no coincidence that we call those things that change slowly "timeless" and those things that change fast "fleeting". Change is the engine of time. Consider this pair of axioms:
1. Space without time (change) is meaningless.
2. Time (change) without space is impossible.
1: if nothing changes, then nothing can be known. No light travels to the eye, no longitudinal waves to the ear, no chemical reactions for taste or smell nor sensory-array discrepancies for touch. Nothing can be known without change.
2: All changes, even at the quantum level, occur in space. If there is no space, no change is possible - therefore no time is possible.
When we consider time as a measurement of change the impenetrable concept of space-time continuity makes perfect sense: all changes in time represent changes in space, so space and change (time) are inextricably linked - one is merely the manipulation of the other, and neither can meaningfully exist in isolation.
Now we are beginning to arrive at why the idea of time travel is impossible. It's not easy to describe without an analogy:
Imagine you're playing pool, and you're breaking. You strike the cue-ball and break the triangle, sending each ball on its various path. Now consider what a task it would be to reverse this process.
Each and every ball would have to be sent back into the triangle at the exact same speed and direction (velocity, in vector terms) and at the same precise moment, otherwise the triangle wouldn't reform and time won't have been reversed. But not only that.
You have to take into account gravity. To make the balls all enter the triangle at the right speed you'd have to hit the balls in the reverse direction at a speed above the one required because gravity/friction would wash off speed as the balls progressed toward their destination. This means you aren't really 'reversing' time - you're not reversing the events, but making new events that culminate in a semblance of the initial events.
To truly reverse time you need to reverse gravity itself, so that the balls speed up in the reverse of the way they slowed down. You need to have the balls gain speed from stationary and accelerate to their end-point. But you'd have to reverse not only gravity - you need to reverse all laws of physics as well as all direction, acceleration and speed.
Just as sending a single pool ball back toward the triangle won't restore the triangle to what it was, sending a single person in the reverse of their life won't restore them to the past.
The thing is: change is timeless. Whether you hit the pool ball one way or the exact reverse doesn't matter: change is change, and it always goes forward unless all of it goes back.
Imagine a battle ground. All the pieces are moving toward each other. Will causing one person to take the exact opposite of their movements cause the entire war to rewind? No. To make the war rewind you'd have to make all pieces rewind simultaneously. This is the issue of time-travel:
Given the big bang singularity, the reversal of time would necessitate the reversal of all things in existence simultaneously together with the complete reversal of all applicable laws. It's like the pool table analogy where restoring the triangle after a break requires the reversal of all the balls, not just one - the singularity prior to the big bang can be thought of as the triangle prior to breaking.
As for time travel into the future: it would necessitate a simultaneous increase in the rate of change across the entire universe. Unlikely to say the least.
Edit: Forgot to mention entanglement. I was thinking entanglement might operate outside of time because the change is perfectly reversed: the change in one entangled particle is reversed in the other, and there is no net alteration in space (assuming the entangled pair form some sort of unit). The two particles are behaving as one, and reversing the spin in one half while simultaneously reversing the spin in the identical other half will result in exactly the same thing you began with. No change. No time.
Having said that, entanglement is a thing that defies all logic. I tend to think it's more an issue involved in space than with time (change). Usually to change something at a distance requires time, but entangled pairs act as if space is non existent.
Last edited by eripiomundus; 12-01-2017 at 03:57 PM..