Time travels--- part-one

The idea of time traveling into the past has numerous conceptual difficulties. The following paradox is well known: How about going back into the past and killing your own father before he even meets your mother? Then, you wouldn't be born. But if you're out of existence, who killed your would-be father?

Such hypothetic scenarios prevent many people from believing time travel is possible. However, the general theory of relativity allows various space-time deformations to take place and Kurt G�del, the famous mathematician, managed to prove that, according to this theory, time travel into the past can in principle exist (see the "worm hole" in the picture below).

Time travel isn't explicitly made impossible by quantum mechanics either. On the contrary, one's sense of causality is deeply challenged by it.

Some science-fiction writers tried to get around the paradoxes of time travel by using the idea of parallel universes. When one would go into the past and change something, the universe would continue on a different path. But the original universe would still continue to exist in parallel. This Oedipus time traveler comes from one universe and kills his father in a different universe.

By studying this science-fiction
idea with the help of quantum mechanics, three physicists, Daniel M. Greenberger and Karl Svozil on one hand and David T. Pegg on the other, have arrived to the same conclusion independently.

The idea in quantum mechanics is that all the possibilities do exist simultaneously in some abstract way, but they are not parallel, they influence each other. What is actually seen to happen is the result of a certain averaging process over all these possibilities - all the possible universes influence each other and blend together giving rise to the real universe. In this way, one can take into consideration even universes where time flows from future to the past - this is why quantum mechanics isn't explicitly and a priori anti-time-travel.

However, what Greenberger, Svozil and Pegg have found is that as time flows, the alternative universes that could have happened but haven't happened, get erased - these past alternatives cancel each other. Thus, if someone would manage somehow to go back into the past, one wouldn't see the universe as it was back then, one would see a simplified universe which is compatible solely with what has actually happened.

I thought I would start with Jan's hypothetical question; and will make a 2nd post that we can all tell... whom; when; why; we would like to see; share sometime with; from the past....I had 5 candidates going in...and finally settled on the one, that I think would be the most interesting and fun for me...a virtual trip would be worth the money to meet this person; instead of a complete vacation any where in the world. think about it and we can have fun on part two of this..sorry, but there is so much to talk about..that I felt getting the tech stuff out of the way first..would make the 2nd part a little fun.