There are processes in physics that make it improbable (and likely impossible) to reverse or go back in time, such as entropy, and the changing of
energy states from one form to another, in which energy is "lost" and there are not (at present) any known ways to reverse. (ie - solid to liquid to
gas) Entropy is the most familiar to everyone. Simply put, it is the change from order to chaos. During that change, in most cases, the original state
Think of an egg.
Can you take an egg, cook it into an omelette, and ever, in any way whatsoever, turn the omelette back into an egg? No. This is because of entropy.
Through heating, and exciting the orderly pattern of particles, energy escapes/transitions and the pattern changes form into something else. We do not
know of any way to put the energy back into an ordered form to make the omelette back into an egg.
Some other things that make it improbable is the current theory of the expanding universe.
This would mean that in order to go back in time, you'd have to be able to take a "snapshot" or picture of, or somehow record, the placement of
EVERY particle and quanta of energy in the universe at that time that you want to go back to, and then when you were ready to go back to that point in
time, you'd have to be able to contract the universe back into its state at the time, and re-arrange EVERY particle and quanta back to it's original
place. Good luck with that one.
Here in lies the paradox, and the rub thanks to our trusted (and measurable) friend, entropy.
You, the human being that you are in the future, are made up of a different amount of matter and energy than you were in the past. If you wanted to go
back in time, you'd have to restore yourself to that state that you existed in at that time as well, which means you'd essentially have no memory of
any of the events leading up to your trip back in time. You would not be able to make the universe "bend" around you and your newly-included matter
and energy from the "future" that is conserved by the known laws of physics. ALL matter and energy would have to be returned to its previous state,
including that which is in your brain, making your trip back in time possible, but then nobody would ever know it existed, including yourself.
Entropy keeps energy changing at all times, it does not stop (yet) in the universe, because it is not at a state of equilibrium, and entropy does not
move in reverse because it follows our perceived arrow of time. Cause NEVER comes after effect. Your thought processes in your brain, your heart
beating, everything that makes you alive, happens because entropy is happening. If the arrow of time could move in two directions, you'd have some
weird stuff that starts to happen when present "cause" can have past "effect" / consequences. If that were possible, we'd likely already have
witnessed the repercussions.
So when does entropy really stop? When equilibrium is reached. And if you really want to bake your noodle... what happens when we reach maximum
entropy, ie - equilibrium - in the universe? Does it stop expanding (if you believe in the expanding universe theory)? Does time travel become
possible? Do we all cease to exist?
Great thread OP... S&F for the ride down quantum lane...