My answer to What is preventing us from time traveling both to the past and future?
Answer by Desmond Last:
It is not possible to travel ‘back or forward’ in time. It is only possible to travel parallel to time.
If I travel back or forwards in actual time, then as soon as I arrive in any fixed moment in order for me arrive, I would then change the future of the time phase I am in for all its future phases.
I would alter everything around me. This would then alter the future of every person on the Planet.
Example. I arrive on the x day of x month and x year at x time.
I stand in front of a person – he steps around me and the man next to him then walks around both of us. This makes him catch a different taxi. The one he was going to get is involved in a car accident. So he never meets his wife who gave birth to Barack Obama who became the President of the U.SA.
Time travel in the same time phase or time-tunnel that we are in is not possible.
Billions upon billions of events would be changed. So too the past of the futures would have been altered.
However by entering a parallel system of time which allows us to be part of the time phase we are with but not in then we are not changing anything.
Think of it as two trains that are exactly the same going in opposite directions at the same speed.
At one point you are exactly parallel with the other train.
That is the Principle of Time Travel.