# Time Travel

1. Apr 28, 2006

### kuahji

Ok, I know you all probably get a ton of lame questions about time travel so I thought I'd add another.

If I could travel back in time, would I not be adding extra matter & energy into the universe? What I mean is, my old body & my "future" or "present" body would be added into the universe. I just don't see how that would be possible. Anyway, does anyone have any links to material that addresses this issue? I know time travel is more or less sci-fi but none the less I am curious.

2. Apr 28, 2006

### jns90

I always blow a fuse when I try to think about time travel. I might just assume that time it something that is set (like space) if someone can just travel through it.

And then in the time before you get to when you go back through time you could say that you are predetermined to travel through time, so that your presence would be no problem.

mind = blown

3. Apr 28, 2006

### rbj

since this is a science forum...

time travel to the past is paradoxical. if you could, go back in time and shoot yourself before you step into the machine.

we time travel into the future at the rate of 1 minute per minute. (remember that old psychodelic 70s rock song: "Time keeps on slippin', slippin', slippin', into the future..." ?) it's a one-way trip.

if you want to slip into the future at a faster rate, you need to do suspended-animation, or at least slowed-animation and you can do it if you have a really fast space-ship and more fuel than anyone has ever used before. get in the space-ship and blast off to Alpha Centauri at 99.5% of the speed of light, slow down and come back the way you came. it might seem to folks here that you've been gone for a decade, but it won't seem as long for you. again, it's a one-way trip into the future. ain't no coming back.

4. Apr 29, 2006

### kuahji

I dunno, I'd personally think that this would be my main argument against time travel. I was really just curious as to if Hawkins, Gott, Thorne, or anyone else in the has addressed the issue.

5. Apr 29, 2006

Staff Emeritus
To speculate on the OP question, would the momentumenergy of the time traveller increses the total energy of the past he travelled to, I think that if you had a spacetime manifold with a closed timelike curve in it, that CTC would be a permanent feature of the geometry, so that momentumenergy would "always" have been there. "Always" in this sense refers not to time but to the static fourdimensional Minkowski geometry.

Einstein occasionally blew a fuse when thinking like this too (not when doing physics but in private correspondence).

6. Apr 29, 2006

### nrqed

I have always wondered how to think of those closed timelike curves... If someone follows such a curve, what happens to the proper time?

And what do you mean by Einstein blowing a fuse''? He was upset by this?

And what is the story of those CTC? Who suggested them first? I think Godel showed that there could be solutions to Einstein's eqs that allowed those in a rotating universe (I think), but had people had thought at all about those CTC before Godel?

to the OP: I read once a sci-fi story in which the author avoided the energy issue by saying in the story that whenever a certain amount of energy was sent in the past, an equivalent amount had to be brought from the past to the present. So if someone was sent in the past, the equivalent of its mass had to be brought from the past. I thought it was an interesting twist.

EDIT: well, I guess it is obvious that the proper time simply increases in the usual way. That was a dumb question. But what would happen if a spaceship going on a CTC would send signal (every second in its frame) to a planet as it goes on a CTC from the planet and back to it? And of course, what happens to the planet (it sees a spaceship arriving first while an identical spaceship is being built? But if later the spaceship being built is sent while the one that had arrived is kept on Earth, wouldn't there had been *2* spaceships arriving in the first place...and then why not an infinite number of them?
My point is that all those paradoxes must be resolved in a CTC but it is hard to imagine how.

Last edited: Apr 29, 2006