How does Time traveling work in practice?

Question

There are multiple theories about time traveling. One is "proven": The time slows down according to your speed. The satellites in space are traveling faster than us, thus their clocks slows down a bit. So by traveling at the speed of light, you would travel without the time. So the time stops when you are traveling with the speed of light. Then you would travel forwards in time, because when you slow down, the time around you will have gone in the "regular" speed, while being still in your time.

Another completely "inverse" theory by Einstein is that extra dimension. The space time. So if you make a wormhole, you would be able to travel past the "bends" in the space time, thus traveling faster than the light. If you arrive before the light, then you would have traveled back in time.

I also want to point out that some physicists have managed to send messages back in time with the help of photons, even though we are only talking about microseconds. (I just saw an interview on Discovery Channel..)

Which of these (or both) is generally believed to be true, if any? And what about the paradoxes? Do you know any paradoxes regarding this? Maybe you are able to solve them? (One I've found is that if you travel into the future and grab a product that you start selling in the presence, the product will be in the future because you got it while time traveling. Who did make the product in the first place?

I'm mostly interested in the paradoxes created while time traveling.

Answer 1

My personal preferred method of time-traveling is enjoying a nice cappuccino at a cafe on a sunny day.

Answer 2

It's true that time slows down for someone travelling near the speed of light, so you could use this to travel into the future i.e. only age one day while the earth ages one year. However this isn't really time travelling in the usual science fiction sense.

For physicists time travel generally means a closed timelike curve (CTC). See http://en.wikipedia.org/wiki/Closed_timelike_curve for a good article on this. Basically it means being able to visit the same point in spacetime more than once. There are solutions of the general relativity equations that allow CTCs, but it isn't clear how physical these are. Einstein's equations allow solutions that don't have anything to do with the real universe. There is a suggestion called the Chronology protection conjecture that the universe does not allow CTCs, but as far as I know no-one has proved this.

As of right now no-one knows for sure if CTCs, and therefore time travel, are possible or not, though I suspect most physicists think not. I didn't see the Discovery Channel programme you mention, but as far as I know no-one has sent anything back in time whether it's a photon or anything else.

Answer 3

The theory here seems to be that going back in time immediately puts you in another dimension. Usually, this dimension will be some kind of wormhole or "time tunnel" composed of flashing lights and cool special effects. You may even see images from famous moments in history fly by as a helpful gauge of when you're going. In less serious versions, the tunnel may be decorated with clocks and calendars or be labeled with years. Depending on the story, the wormhole links the user to a different spot on his own timeline or to a different spot on the next timeline over; the difference is largely academic.