
Originally Posted by
Physicist Brian Greene
"Take the paradoxical example of your having gone back in time and having prevented your parents from meeting. Intuitively, we all know what that's supposed to mean. Before you time-traveled to the past, your parents had met - say, at the stroke of midnight, December 31. 1965, at a New Year's Party - and, in due course, your mother gave birth to you. Then, many years later, you decided to travel to the past - back to December 31, 1965 - and once there, you changed things; in particular, you kept your parents apart, preventing your own conception and birth. But now let's counter this intuitive description with the more fully reasoned spacetime-loaf depiction of time.
At its core, the intuitive description fails to make sense because it assumes moments can change. The intuitive picture envisions the stroke of midnight, December 31, 1965 (using standard earthling time-slicing) as "initially" being the moment of your parents meeting, but envisions further that your interference "subsequently" changes things so that at the stroke of midnight, December 31, 1965, your parents are miles, if not continents, apart. The problem with this recounting of events, though, is that moments don't change; as we've seen, they just are. The spacetime loaf exists, fixed and unchanging. There is no meaning to a moment's "initially" being one way and "subsequently" being another way.
If you time-travelled back to December 31, 1965, then you were there, you were always there, you will always be there, you were never not there.
This realisation leads us to some quirky conclusions, but it avoids paradox".