Einstein's theory of relativity teaches us that
"Two events, simultaneous for some observer, may not be simultaneous for another observer if the observers are in relative motion."
What this means is that if 2 people are walking down the road opposite each other, each one is in a different reality. Each person will pass by the other and go on to live in worlds with entirely different futures. I know this is hard to believe but it's true.
I thought that Einstein's theory was one of relative perspective. As an example, if I am going faster than a bullet and pass it, than to me the bullet is going backwards. I don't see how this applies to your example of different realities & futures.
Regards.
a bullet is different from a wave of light