Compact Sapcetime and Time Travel

**thehangedman** · Jun18-09, 01:02 PM

Someone here once said to me, via post, that "any compact spacetime must have closed timelike curves". Are there any good references out there on why that is / how that is derived?

As an after thought...

Isn't it true that a particle traveling in one direction in time is equivalent to its anti-particle traveling in the opposite direction in time? So, for a closed geodesic in a compact space-time, a particle traveling in this "loop" would appear the same to us as a particle and anti-particle popping out of the void and then coming back together again and disappearing no? As the particle comes around and travels back in time, we would interpret that (or at least the equations make it equivalent to) as it's anti particle moving forward in time.

I would think that a closed universe, in space and time, would allow for regions of time where energy would be isolated. If we could look at this universe from the outside, as we do say looking at the surface of the earth (a closed 2d example), we would see that on the whole energy is conserved. But if time to those on the surface is like the motions from the south pole to the north (big bang, big crunch), they would see energy being "created" and then "destroyed".

**George Jones** · Jun18-09, 01:32 PM

Originally Posted by thehangedman

Someone here once said to me, via post, that "any compact spacetime must have closed timelike curves". Are there any good references out there on why that is / how that is derived?

Here is a sketch:

http://www.physicsforums.com/showthr...58#post1254758.

**George Jones** · Jun18-09, 01:40 PM

Originally Posted by thehangedman

Someone here once said to me, via post,

Hey, that someone was me,

http://www.physicsforums.com/showthr...17#post2233717,

and I gave some (partial) references

: my sketch; Hawking and Ellis; Naber. I think that it also is proved in Wald.

**thehangedman** · Jun18-09, 01:49 PM

Thank you for that link, and I will look into this more. From my comments after my question though I think this issue can be solved. It seams to me that QM addresses this and that these closed loops can be interpreted as I said. Put simply, any CTC would be the equivalent to the popping in and out of particle anti-particle pairs from the void. Or, put another way, this feature of the void is actually the direct result of a compact space-time.

**thehangedman** · Jun18-09, 01:50 PM

Sorry, but for some reason I don't see any images / sketches showing up. :-(

**benk99nenm312** · Jun18-09, 02:13 PM

Originally Posted by thehangedman

Isn't it true that a particle traveling in one direction in time is equivalent to its anti-particle traveling in the opposite direction in time?

Actually, from what I've been told, antimatter travels forwards in time. Its state mimics that of normal matter traveling backwards in time, but the particle itself travels forwards in time.