???
In the forward direction, yes, in fact it’s mandatory.
Yes - we’re all traveling forward in time - all the time 
To speed it up, have a few drinks.
Or whatever Einstein said.
Sure, just cross the international date line…Eastward to go back in time, westward to go forward…
hrrmph…ok, to really answer the question (sorry, couldn’t resist) - to first answer your question we have to qualify “possible” between “theoretically possible” and “practically possible”.
Under the “Theoretically possible” category the answer would be “yes”. There have been solutions for Einstein’s theory of relativity that demonstrate that under certain circumstances time travel would be possible.
However, in practice, there appears to be little hope that one could time travel (and survive the experience) given the enormous energies and stresses on the human body (think homo sapiens sapiens puree ) that would necessarily have to occur.
Hopefully a physicist that has actually studied this stuff (unlike me giving the condensed Brief History of Time version )
critter42
er…left half a paragraph out…
Hopefully a physicist that has actually studied this stuff (unlike me giving the condensed * Brief History of Time* version ) will be able to respond more authoritatively. (Yes, I know this question can be readily answered by doing a Google search, but I think there is enough debate on this question to warrant a decent response ).
critter42
IT WORKED! And just to prove it I predict that very soon now I will say something very stupid.
I am sure that it is possible and in order to test this I have fabricated a time-machine (based upon a design stolen from Happy Lendervedder in a distant and remote thread).
I intend to pop back in time ONE WHOLE English hour and prepost myself.
About a year or two ago they did a NOVA show about time travel. One or two of the math guys said that if you could enlarge and enter a wormhole, accelerate it at high speed, and then step out, you could jump to the far future. Then you could enlarge and enter it again, accelerate it to a high speeed, and then step out back near the original time you left. But there were no theories that allowed you to go any further back in time than that.
There are a variety of methods proposed in GR (General Relativity) to produce a time machine, but interestingly, all of them require the use of some sort of matter with negative mass, and there is strong reason to believe that any potential time-travel method would also require negative mass matter. Whether negative matter can exist is unknown, but there’s certainly no evidence for it.
Unfortunately I do not remember the name of the effect I am about to describe (Casimir Effect?) but one method I heard of was to charge two plates sitting very close to each other which formed some sort of connection between the two. Then you take one of your plates on a spaceship ride at near light speed while leaving the other one on earth. The two plates will maintain their connection and the light speed plate will effectively be moving through time more slowly than the eartbound plate. A person jumping from earth to the spaceship would effectively travel backwards in time while a person on the spaceship would jump forward in time.
Of course, the energy required to allow this would be stupendous thus likely obliterating our would be time traveller if they got anywhere near a plate much less jumping through one. Also, IIRC, the time traveller would also likely destabilize the link between the two and cause the link to collapse (whether the traveller could get through prior to destabilization I don’t know). So, while thoroughly impractical and likely not useable does this still count as a recipe for a time machine that does not require negative matter?
[sub]I think I’m remembering the above correctly but it’s Monday so I may not be thinking of all it it just right so try not to kill me if I’m way off base here.[/sub]
If you assume that it is possible to send information back in time, you can produce a contradiction. Say I set up a device that can receive a bit of information and send it back in time. I receive its signal and run it through a not gate and send that signal back. We have a contradiction. I would conclude that it is impossible to travel back in time.
There are ways around your contradictions if you use parallel universes. Of course, such things are quite a stretch in their own right but they can’t, at least with today’s knowledge, be ruled out completely.
I remember a story where Carl Sagan had asked one of our foremost physicists (I can’t remember who but he was and is very well regarded in the physics community) to help him out with some issues while writing Contact. One of the problems dealt with time travel issues. Apparently this physicist thought like you did that time travel should be impossible and he was getting tired of all the questions he kept getting about it. As a result he decided to sit down, do the math, and prove once and for all that time travel is impossible. Much to his surprise he found that time travel is not prohibited in the equations of relativity and/or quantum mechanics.
So, given our current knowledge, time travel is possible. Of course, it may turn out to be practically impossible but it is not absolutely impossible in and of itself. Of course, we may also be way off track with relativity and the like or there are things as yet undiscovered that prohibit time travel. In the end the final answer seems to be it seems possible if very very difficult. However, given the issues surrounding time travel such as what you brought up maybe it’s best to say we really can’t say one way or the other just now.
I’m pretty sure that was Kip Thorne. At least, he’s done most of the good work regarding worm holes. His method of hypothetically stabilizing a wormhole is one of the ones I referred to which requires negative mass.
Whack-a-Mole, you’re confusing two different parts of the same (semi-hypothetical) experiment. When you put the two plates close to each other, then, by the Casimir effect, you can get something resembling a negative energy density between the plates, but it’s still not quite what’s needed (I believe that it’s a distinction between global and local measurements of mass, but I’d have to check up on that. Of course, the published papers are much more precise in their terminology). So, one supposes that maybe there’s something similar to the Casimir effect, which does get you what you need. We don’t know what that something would be, or if it exists, but hey, this is a thought experiment. Now, you use this hypothetical material to create/stabilize a wormhole. The hole has two ends, an arbitrarily large distance apart, and you can travel from one end to the other in an arbitrarily small amount of time. There are a variety of ways of turning this into a time machine, one of them being to carry one of the ends around on a high-speed tour for a while.
If anyone else would like to try out my model for time travel, the necessary componants are detailed here.
I just supply the ingredients. The manner of construct is up to you.
Happy
Not all of them. There is a time-travel solution postulated by Richard Gott involving two Cosmic Strings traveling at relativistic speeds past each other. No negative mass required!
By the by, Flip-Disc, I hope you realize that we are talking mostly of time travel backward in time. Forward in time is really a non-issue. It’s pretty much a matter of frames of reference and taken care of quite nicely by Special Relativity.
Yes, you can travel in time. But when you get to the end of time, with your one way time machine, there’s this asshole with a used time machine lot, and he makes you work for him for half the length of time you want to go backwards. No credit. (buy now pay later only works when there is a later.)
Tris