I agree with this. If a person were able to travel back in time it would create an alternate universe at that point. Your presence at that time would change things going forward.
That solves what is called the "Grandfather Paradox". That is when you travel back in time and kill your grandfather so you would never be born. However, if you killed your grandfather a different universe would not include you but the universe you came from would continue to exist so you could continue to exist as well.
The problem I can't figure out is that if you travel back in time, what would prevent someone from your future arriving just before you killed your grandfather and stopping you from killing him?
It depends on the perspective of the observer. Say a person is aboard a spaceship traveling near the speed of light. By his own perspective a year passes. He ages one year. The cat he has on the ship with him ages one year. The wine he brought is one year older. Now he returns to Earth. Everyone there is many years older, but time SEEMED to pass at the same rate for each of them.
So the aging process was the very same for both of them, but the relative passing of time was slower for the person aboard the ship traveling very fast. Taken individually each would perceive time to have passed at the same rate -- one tick of the clock being a second, 60 minutes being an hour.
