Special Spacetime Geometries

The general theory of relativity extends the special theory to cover gravity, illustrating it in terms of curvature in spacetime cause by mass-energy and the flow of momentum. General relativity describes the universe under a system of field equations, and there exist solutions to these equations that permit what are called "closed time-like curves", and hence time travel into the past. The first of these was proposed by Kurt Godel, a solution known as the Godel metric, but his (and many others) example requires the universe to have physical characteristics that it does not appear to have. Whether general relativity forbids closed time-like curves for all realistic conditions is unknown.

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Time Travel To The Past In Physics

Time travel to the past is theoretically allowed using the following methods:

• Traveling faster than the speed of light
• The use of cosmic strings and black holes

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The "present" view

Presentism holds that neither the future nor the past exist - that the only things that exist are present things, and there are no non-present objects. Some have taken presentism to indicate that time travel is impossible for there is no future or past to travel to; however, recently some presentists have argued that although past and future objects do not exist, there can still be definite truths about past and future events, and that it is possible that a future truth about the time traveler deciding to return to the present date could explain the time traveler's actual presence is the present. This view is contested by another contemporary advocate of presentism, Craig Bourne, in his recent book " A Future for Presentism" although for substantially different (and more complex) reasons. In any case, the relativity of simultaneity in modern physics is generally understood to cast serious doubt on presentism and to favor the view known as four dimensionalism (closely related to the idea of block time) in which past, present and future events all coexist in a single spacetime. (Taken from: Wikipedia.org)

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General Relativity

The theory of general relativity does suggest scientific grounds for thinking backwards time travel could be possible in certain unusual scenarios, although arguments from semiclassical gravity suggest that when quantum effects are incorporated into general relativity, these loopholes may be closed. These semiclassical arguments led Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel, but physicists cannot come to a definite judgment on the issue without a theory of quantum gravity to join quantum mechanics and general relativity into a completely unified theory.

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Tourism In Time

Stephen Hawking once suggested that the absence of tourists from the future constitutes an argument against the existence of time travel (a variant of the Fermi paradox). Of course this would not prove that time physically impossible, since it might be that time travel is physically possible but that it is never in fact developed (or was cautiously never used); and even if it is developed, Hawking notes elsewhere that time travel might only be possible in a region of spacetime that is warped in the right way, and that if we cannot create such a region until the future, then time travelers would not be able to travel back before that date, so "This picture would explain why we haven't been over run by tourists from the future. Carl Sagan also once suggested the possibility that time travelers could be here, but are disguising their existence or are not recognized as time travelers.

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Time Travel In Theory

Some theories most notably special and general relativity, suggest that suitable geometries of spacetime, or specific types of motion in space, might allow time travel into the past and future if these geometries or motions are possible. In technical papers physicists generally avoid the commonplace language of "moving" or "traveling" through time ("movement" normally refers only to a change in spatial position as the time coordinate is varied), and instead discuss the possibility of closed timelike curves, which are wordlines that form closed loops in spacetime, allowing objects to return to their own past. There are known to be solutions to the equations of general relativity that describe spacetimes which contain closed timelike curves, but the physical plausibility of these solutions is uncertain.

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Time Travel

Time Travel is the concept of moving between different moments in time in a manner analogous to moving between different points in space, either sending objects (or in some cases just information) backwards in time to a moment before the present, or sending objects forward from the present ti the future without the need to experience the intervening period ( at least not at the normal rate). Some interpretations of time travel also suggest that traveling backwards in time might take one to a parallel universe whose history could begin to diverge from the traveler's history after the moment the traveler arrived in the past.

Although time travel has been a common plot device in fiction since the 19th century, and one-way travel into the future is arguably possible given the phenomenon of time dilation based on velocity in the theory of special relativity (exemplified by the twin paradox) as well as gravitational time dilation in the theory of general relativity, it is currently unknown whether the laws of physics would allow backwards time travel. Any technological device, whether fictional or hypothetical, that is used to achieve two-way time travel is known as a time machine. (Taken from: wikipedia.org)

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