Is time travel theoretically possible?
Is significant time travel theoretically possible? The original question must be altered by addition of the word significant because time travel, in either direction has already been demonstrated.
There has been much argument that time travel is as simple as the daily act of merely traveling with time. There has also been much made of Einstein’s theory of relativity. Parts of this theory are no longer theory in light of unequivocal proof that time travel is possible. That proof is most clearly demonstrated in the case of people orbiting the earth. These people orbit the same three-hundred and sixty degrees as a spot on earth, yet both these points experience a different passage of time.
In this proof of time travel, the point that traveled the greater distance in the same interval experienced a time dilation and returned to the origin at a relative time earlier than the time of that point that traveled the lesser distance through the same arc.
Which point moved which direction with respect to time? The answer is, given that neither point can be designated as a universal origin, both points have moved with respect to each other. One has moved half the distance forward in time and the other having moved half the distance backward in time. It has been argued that traveling backward in time presents a paradox and that traveling forward in time has a limit of inability to travel forward in time prior to the creation of a mechanism with which makes that travel possible. Both arguments are dispensed by the fact that an entity at a higher latitude on the surface of the earth experiences time dilation relative to a person at a lower latitude. The mechanism if time travel is a part of the Universe.
The question is, is there a principle of balance in the Universe? That is, is there a universal origin where all relative time-space dilations are equated to zero? Is there a finite timekeeper for the Universe? Maybe, maybe not, there does not seem to be a balance in the relationship between mass and energy. Else, why is it suspected that the Universe is expanding at a rate and scale that will preclude it reaching a steady state in which mass balances energy.
If the question is to remain theoretical, the answer is yes. It is yes because it asks for an answer based on current understanding. Orbital excursions have proved that it is possible. It really does not matter where the origin is placed, two points separated in space will always move differently relative to time. Two objects separated by time will always move differently with respect to space. The technology to make time travel significant and practical does not currently exist in our time-space continuum. That is not to say that it will not exist tomorrow or possibly last week. The concept of time travel, in either direction already exists and its proof is merely perspective.