Physics

Is Time Travel possible



Tweet
Thomas Blunsdon's image for:
"Is Time Travel possible"
Caption: 
Location: 
Image by: 
©  

Time Travel
____________

Time travel is the concept of moving backwards or forwards to different points in time, in a manner analogous to moving through space. Additionally, some interpretations of time travel suggest the possibility of travel between parallel realities or universes.

Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime, or specific types of motion in space, may allow time travel into the past and future if these geometries or motions are possible.[3] 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 worldlines 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.

Physicists take for granted that if one were to move away from the Earth at relativistic velocities and return, more time would have passed on Earth than for the traveler, so in this sense it is accepted that relativity allows "travel into the future" (although according to relativity there is no single objective answer to how much time has 'really' passed between the departure and the return). On the other hand, many in the scientific community believe that backwards time travel is highly unlikely. Any theory which would allow time travel would require that issues of causality be resolved. What if one were to go back in time and kill one's own grandfather? (see grandfather paradox) Additionally, Stephen Hawking once suggested that the absence of tourists from the future constitutes an argument against the existence of time travela variant of the Fermi paradox. Of course this would not show time travel is physically impossible, only that it is never in fact developed; and even if it is developed, Hawking notes elsewhere that time travel may 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.'

However, 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.

Wormholes:

Wormholes are a type of warped spacetime which are also permitted by the Einstein field equations of general relativity, although it would be impossible to travel through a wormhole unless it was what is known as a traversable wormhole.

A proposed time-travel machine using a traversable wormhole would (hypothetically) work something like this. One end of the wormhole is accelerated to nearly the speed of light, perhaps with an advanced spaceship, and then brought back to the point of origin. Due to time dilation, the accelerated end of the wormhole has now aged less than the stationary end, as seen by an external observer; however, time connects differently through the wormhole than outside it, so that synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, no matter how the two ends move around. This means that an observer entering the accelerated end would exit the stationary end when the stationary end was the same age that the accelerated end had been at the moment before entry; for example, if prior to entering the wormhole the observer noted that a clock at the accelerated end read a date of 2005 while a clock at the stationary end read 2010, then the observer would exit the stationary end when its clock also read 2005, a trip backwards in time as seen by other observers outside. One significant limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of the machine;in essence, it is more of a path through time than it is a device that itself moves through time, and it would not allow the technology itself to be moved backwards in time. This could provide an alternative explanation for Hawking's observation: a time machine will be built someday, but has not yet been built, so the tourists from the future cannot reach this far back in time.

According to current theories on the nature of wormholes, construction of a traversable wormhole would require the existence of a substance known as "exotic matter" with negative energy. Many physicists believe this may actually be possible due to the Casimir effect in quantum physics. Although early calculations suggested a very large amount of negative energy would be required, later calculations showed that the amount of negative energy can be made arbitrarily small.

In 1993, Matt Visser argued that the two mouths of a wormhole with such an induced clock difference could not be brought together without inducing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other. Because of this, the two mouths could not be brought close enough for causality violation to take place. However, in a 1997 paper, Visser hypothesized that a complex "Roman ring" (named after Tom Roman) configuration of an N number of wormholes arranged in a symmetric polygon could still act as a time machine, although he concludes that this is more likely a flaw in classical quantum gravity theory rather than proof that causality violation is possible

In conclusion there are various ways in which a person could "travel into the future" in a limited sense: the person could set things up so that in a small amount of their own subjective time, a large amount of subjective time has passed for other people on Earth. For example, an observer might take a trip away from the Earth and back at relativistic velocities, with the trip only lasting a few years according to the observer's own clocks, and return to find that thousands of years had passed on Earth. It should be noted, though, that according to relativity there is no objective answer to the question of how much time "really" passed during the trip; it would be equally valid to say that the trip had lasted only a few years or that the trip had lasted thousands of years, depending on your choice of reference frame.

This form of "travel into the future" is theoretically allowed using the following methods: Using time dilation under the Theory of Special Relativity, for instance:
Traveling at almost the speed of light to a distant star, then slowing down, turning around, and traveling at almost the speed of light back to Earth[21] (see the Twin paradox)
Orbiting Earth for long periods of time (practical, but insignificant);
Using time dilation under the Theory of General Relativity, for instance:
Residing inside of a hollow, high-mass object;
Residing just outside of the event horizon of a black hole
Additionally, it might be possible to see the distant future of the Earth using methods which do not involve relativity at all, although it is even more debatable whether these should be deemed a form of "time travel":

Hibernation
Suspended animation

Tweet
More about this author: Thomas Blunsdon

From Around the Web




ARTICLE SOURCES AND CITATIONS