What are some proposed theories for time travel by people who think that it may be possible one day?

Question

I know some of them: Black holes/White holes, Wormholes etc. What are the others?

Answer 1

1) into the future....

there are a couple of ways to do this. the idea is you put yourself in a position where your local frame of reference of time passage slows down relative to the universe around you. this is called time dilation and has been both theoretically predicted and experimentally measured. see here http://en.wikipedia.org/wiki/Time_dilation

regardless, you slow your local time rate to near zero, and the universe continues around you.. suddenly you find yourself in the future.

2) into the past.

can't do it.

Answer 2

Traveling in future is quite easy... Which ever is moving will travel little ahead in the time compared to static object. Faster you go, time will slow down and you will be ahead of other in time.

But traveling in past is quite tedious. One proposed theory is to create space time curve using powerful laser. The problem is that the curvature of the space time required to send the particles into past is very high. As per the calculations the curvature required is, some what is there at the black holes. Sending something into past has lot of paradoxes with that...

Suppose you are sent into past using time machine say 100 years or so. Imagine you killed your grandfather. Then what??? without the grandfather you cant be here!!!!! thats problem in going into past....

Answer 3

Erm... Make some area so cold that it's at absolute zero, so there's "no" particle vibration, no moving forward, and maybe no actual "time" either. Then, after finding some way to protect yourself from the scary cold, walk out across the land. Tada! You are now a future person standing in a land that is of the past.

Yep. Sleep deprived here.

Answer 4

Because people believe that if they can imagine it then it is possible. This a fallacy in our thought process and a general misuse of the word 'possible'. Einstein (and countless others since him) has proven that nothing travels faster than light and as anything with mass approaches the speed of light it takes more and more energy to produce a little more speed to where, theoretically, it would take infinite energy to accelerate just one atom to the speed of light. I don't think travel to the past is possible, but if you were to travel fast enough you could travel to the future (twin paradox).

Answer 5

Time Travel - Fact or Fiction?

We define time travel to mean departure from a certain place and time followed (from the traveller's point of view) by arrival at the same place at an earlier (from the sedentary observer's point of view) time. Time travel paradoxes arise from the fact that departure occurs after arrival according to one observer and before arrival according to another. In the terminology of special relativity time travel implies that the timelike ordering of events is not invariant. This violates our intuitive notions of causality. However, intuition is not an infallible guide, so we must be careful. Is time travel really impossible, or is it merely another phenomenon where "impossible" means "nature is weirder than we think?" The answer is more interesting than you might think.

The Science Fiction Paradigm

The B-movie image of the intrepid chrononaut climbing into his time machine and watching the clock outside spin backwards while those outside the time machine watch the him revert to callow youth is, according to current theory, impossible. In current theory, the arrow of time flows in only one direction at any particular place. If this were not true, then one could not impose a 4-dimensional co-ordinate system on space-time, and many nasty consequences would result. Nevertheless, there is a scenario which is not ruled out by present knowledge. This usually requires an unusual spacetime topology (due to wormholes or strings in general relativity) which has not yet seen, but which may be possible. In this scenario the universe is well behaved in every local region; only by exploring the global properties does one discover time travel.

Conservation Laws

It is sometimes argued that time travel violates conservation laws. For example, sending mass back in time increases the amount of energy that exists at that time. Doesn't this violate conservation of energy? This argument uses the concept of a global conservation law, whereas relativistically invariant formulations of the equations of physics only imply local conservation. A local conservation law tells us that the amount of stuff inside a small volume changes only when stuff flows in or out through the surface. A global conservation law is derived from this by integrating over all space and assuming that there is no flow in or out at infinity. If this integral cannot be performed, then global conservation does not follow. So, sending mass back in time might be all right, but it implies that something strange is happening. (Why shouldn't we be able to do the integral?)

General Relativity

The possibility of time travel in GR has been known at least since 1949 (by Kurt Godel, discussed in [1], page 168). The GR spacetime found by Godel has what are now called "closed timelike curves" (CTCs). A CTC is a worldline that a particle or a person can follow which ends at the same spacetime point (the same position and time) as it started. A solution to GR which contains CTCs cannot have a spacelike embedding - space must have "holes" (as in donut holes, not holes punched in a sheet of paper). A would-be time traveller must go around or through the holes in a clever way.

The Godel solution is a curiosity, not useful for constructing a time machine. Two recent proposals, one by Morris, et al. [2] and one by Gott [3], have the possibility of actually leading to practical devices (if you believe this, I have a bridge to sell you). As with Godel, in these schemes nothing is locally strange; time travel results from the unusual topology of spacetime. The first uses a wormhole (the inner part of a black hole, see figure 1 of [2]) which is held open and manipulated by electromagnetic forces. The second uses the conical geometry generated by an infinitely long string of mass. If two strings pass by each other, a clever person can go into the past by travelling a figure-eight path around the strings. In this scenario, if the string has non-zero diameter and finite mass density, there is a CTC without any unusual topology.

Grandfather Paradoxes

With the demonstration that general relativity contains CTCs, people began studying the problem of self-consistency. Basically, the problem is that of the "grandfather paradox": What happens if our time traveller kills her grandmother before her mother was born? In more readily analyzable terms, one can ask what are the implications of the quantum mechanical interference of the particle with its future self. Boulware [5] shows that there is a problem - unitarity is violated. This is related to the question of when one can do the global conservation integral discussed above. It is an example of the "Cauchy problem" [1, chapter 7].

Other Problems (and an escape hatch?)

How does one avoid the paradox that a simple solution to GR has CTCs which QM does not like? This is not a matter of applying a theory in a domain where it is expected to fail. One relevant issue is the construction of the time machine. After all, infinite strings aren't easily obtained. In fact, it has been shown [4] that Gott's scenario implies that the total 4-momentum of spacetime must be spacelike. This seems to imply that one cannot build a time machine from any collection of non-tachyonic objects, whose 4-momentum must be timelike. There are implementation problems with the wormhole method as well.

Tachyons

Finally, a diversion on a possibly related topic.

If tachyons exist as physical objects, causality is no longer invariant. Different observers will see different causal sequences. This effect requires only special relativity (not GR), and follows from the fact that for any spacelike trajectory, reference frames can be found in which the particle moves backward or forward in time. This is illustrated by the pair of spacetime diagrams below. One must be careful about what is actually observed; a particle moving backward in time is observed to be a forward moving anti-particle, so no observer interprets this as time

The unusual situation here arises because conventional causality assumes no superluminal motion. This tachyon example is presented to demonstrate that our intuitive notion of causality may be flawed, so one must be careful when appealing to common sense. See the FAQ article on tachyons, for more about these weird hypothetical particles.

Conclusion

The possible existence of time machines remains an open question. None of the papers criticizing the two proposals are willing to categorically rule out the possibility. Nevertheless, the notion of time machines seems to carry with it a serious set of problems.

Answer 6

Time travel "may not" be physically possible, but it is mentally possible through meditation and deep regression hypnosis.

Answer 7

It's impossible.If it were, the laws of quantum physics fall apart and so does our universe.