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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.

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Yes, I've read those, but this is generally more about the paradoxes. –  Friend of Kim Apr 23 '12 at 15:19
    
I would guess that most of us believe closed timelike curves are impossible, therefore worrying about the paradoxes seems a bit pointless. –  John Rennie Apr 23 '12 at 15:21
    
Don't physicists believe both of the theories I added in the question? –  Friend of Kim Apr 23 '12 at 15:29
    
In the comments to his answer, John Rennie has discovered the Discovery Channel program you were talking about, and most physicists would disagree with the Cramer that there is any possibility of sending messages back in time. –  Peter Shor Mar 17 '13 at 21:24

3 Answers 3

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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.

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About the Discovery Channel program: He explicitly said that this was sending messages (or rather signals, just a sign that the photon was received) back in time. He said that it was like we were in the stone age, and today we can do time traveling. So, if we are in the stone age when it comes to time traveling, then it's a long way to go.. –  Friend of Kim Apr 23 '12 at 15:22
    
But, do you have any paradoxes that isn't already showed as cliches in movies? –  Friend of Kim Apr 23 '12 at 15:24
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Was it John Cramer on the Discovery Channel? If so, as far as I'm aware he has not demonstrated that a photon can be sent back in time. Physicists generally don't believe paradoxes exist, but instead that If you think you've found a paradox this just means you don't understand the physics properly. –  John Rennie Apr 23 '12 at 16:20
    
Mm, I don't remember. He just showed when he actually did do this. He said that different photons have a connection that is independent of time. –  Friend of Kim Apr 23 '12 at 16:35
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Cramer is trying to demonstrate retrocausality in entangled photons. Even if he succeeds, and so far no-one believes he has, it wouldn't be time travel in the CTC sense. having said that, full marks to him for trying! See seattlepi.com/local/article/… for a non-technical description. –  John Rennie Apr 23 '12 at 16:51

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

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Yes, that is indeed time traveling. But I'm talking about traveling at a different speed than everyone else.... –  Friend of Kim Apr 23 '12 at 15:33

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.

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-1 nice science fiction, but not physics. Welcome to Physics SE. –  Stefan Bischof Mar 17 '13 at 20:02

protected by Qmechanic Mar 17 '13 at 21:42

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