# Time travel?

Time travel??

I was wondering if its actually theoretically possible to travel back in time. I know its possible to "travel to the future" because of time dilation, but I just dont understand what the deal is with traveling back in time. I've heard many times since I was a little boy, that going faster than the speed of light would make time run backwards. Can anyone tell me why this would happen?

ArmoSkater87 said:
I was wondering if its actually theoretically possible to travel back in time. I know its possible to "travel to the future" because of time dilation, but I just dont understand what the deal is with traveling back in time. I've heard many times since I was a little boy, that going faster than the speed of light would make time run backwards. Can anyone tell me why this would happen?
http://www.geocities.com/zcphysicsms/chap12.htm#BM12_3
http://www.geocities.com/zcphysicsms/chap3.htm#BM26

Von Stokum's Cylinder

If you can get yourself there, and where this is located, what potential lies in its function?

Does such a place exist? Look up Ronalds Mallets experiment. A true patriot of Time Travel.

of you want to travel back intime, you do have to travel with speed exceeding c. This in itseld if an impossibility because no object with and mass can reach the speed of light, let alone surpass it. Also traveling back in time causes a bumch of paradoxes. Lets say you travel back in time and kill yourself when you were younger, what happens to you. Do you die imidiatelly or do you stil live on, time travelling around like Mr. Peabody.

My favourite paradox is brought up in the short story, "All You Zombies" by Robert Heinlein. If time travel back in time was possible, then even if it turns out you can't change to past, only fulfill it, the story points out how someone could end up having no history...they're their own mother, father, daughter, and son!

Why a Cylinder?

http://www.walterzeichner.com/thezfiles/time2.gif [Broken]

A time machine based on an immense cylinder spinning at near-light speed. The physicist W. J. van Stokum realized in 1937 that such an object would effectively stir spacetime as if it were treacle, dragging it along as the cylinder turned. What van Stokum didn't realize is that circumnavigating such a cylinder can lead to closed time-like paths. Anyone orbiting the cylinder in the direction of the spin would be caught in the current and, from the perspective of a distant observer, exceed the speed of light and thus travel back in time. Circling the cylinder in the other direction with just the right trajectory would project the subject into the future. The van Stokum time machine is based on the Lense-Thiring effect and uses ordinary matter but of enormous density - many orders of magnitude greater than that of nuclear matter.

http://www.daviddarling.info/encyclopedia/V/vanStokum_cylinder.html
Weak gravitational field of the electromagnetic radiation

Ronald L. Mallett Department of Physics, 2152 Hillside Road and UniÍersity of Connecticut, Storrs, CT 06269, USA Received 19 January 2000; accepted 3 April 2000 Communicated by P.R. Holland

Abstract
The gravitational field due to the circulating flow of electromagnetic radiation of a unidirectional ring laser is found by solving the linearized Einstein field equations at any interior point of the laser ring. The general relativistic spin equations are then used to study the behavior of a massive spinning neutral particle at the center of the ring laser. It is found that the particle exhibits the phenomenon known as inertial frame-dragging. q2000 Elsevier Science B.V. All rights reserved.
http://temporology.bio.msu.ru/EREPORTS/mallett.pdf [Broken]

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Kip Thorne and Hawkings have a bet on travelling back in time.

What's that conjecture......... um........ the Time Protection Conjecture or something. Basically any time machine would destroy itself the instant you tried to use it to travel back in time. Something like that.

DW said:
Thanks for the links. It was interesting, but unfortunatly talked about time travel assosiated with GR, which was a little over my head.

of you want to travel back intime, you do have to travel with speed exceeding c. This in itseld if an impossibility because no object with and mass can reach the speed of light, let alone surpass it. Also traveling back in time causes a bumch of paradoxes. Lets say you travel back in time and kill yourself when you were younger, what happens to you. Do you die imidiatelly or do you stil live on, time travelling around like Mr. Peabody.
Very true, this is better known as the Grandfather paradox. But quantum mechanics has a theoritical solution to it...if you went back in time and killed your grandfather before he fathered your father, then you would create a parallel universe to the one you traveled from. In one universe your grandfather was murdered by you, in the other, he's alive.

sol said:
http://www.walterzeichner.com/thezfiles/time2.gif [Broken]

Quote:
A time machine based on an immense cylinder spinning at near-light speed. The physicist W. J. van Stokum realized in 1937 that such an object would effectively stir spacetime as if it were treacle, dragging it along as the cylinder turned. What van Stokum didn't realize is that circumnavigating such a cylinder can lead to closed time-like paths. Anyone orbiting the cylinder in the direction of the spin would be caught in the current and, from the perspective of a distant observer, exceed the speed of light and thus travel back in time. Circling the cylinder in the other direction with just the right trajectory would project the subject into the future. The van Stokum time machine is based on the Lense-Thiring effect and uses ordinary matter but of enormous density - many orders of magnitude greater than that of nuclear matter.

http://www.daviddarling.info/encycl...m_cylinder.html [Broken]
Very interesting, to say the least.

whydoyouwanttoknow said:
Kip Thorne and Hawkings have a bet on travelling back in time.

What's that conjecture......... um........ the Time Protection Conjecture or something. Basically any time machine would destroy itself the instant you tried to use it to travel back in time. Something like that.
Oh, I had no idea. Last I remember, Hawking said you can time travel, but not farther than the time your time machine was built. Or in other cases, where you use blackholes, you couldnt go back farther than when the blackhole was formed.

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