Time Travel

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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.[1]

Origins of the concept

There is no widespread agreement on what should qualify as the first time travel story, since a number of early stories feature elements suggestive of time travel but are nevertheless somewhat ambiguous. For example, Memoirs of the Twentieth Century (1733) by Samuel Madden is mainly a series of letters from English ambassadors in various countries to the British "Lord High Treasurer", along with a few replies from the British foreign office, all purportedly written in 1997 and 1998 and describing the conditions of that era. However, the framing story is that these letters were actual documents given to the narrator by his guardian angel one night in 1728; for this reason, Paul Alkon suggests in his book Origins of Futuristic Fiction that "the first time-traveler in English literature is a guardian angel who returns with state documents from 1998 to the year 1728", although the book does not explicitly show how the angel obtained these documents. Alkon later qualifies this by writing "It would be stretching our generosity to praise Madden for being the first to show a traveler arriving from the future", but also says that Madden "deserves recognition as the first to toy with the rich idea of time-travel in the form of an artifact sent backwards from the future to be discovered in the present."

In the science fiction anthology Far Boundaries (1951), the editor August Derleth identifies the short story "Missing One's Coach: An Anachronism", written for the Dublin Literary Magazine by an anonymous author in 1838, as a very early time travel story. In this story, the narrator is waiting under a tree to be picked up by a coach which will take him out of Newcastle, when he suddenly finds himself transported back over a thousand years, where he encounters the Venerable Bede in a monastery, and gives him somewhat ironic explanations of the developments of the coming centuries. It is never entirely clear whether these events actually occurred or were merely a dream—the narrator says that when he initially found a comfortable-looking spot in the roots of the tree, he sat down, "and as my sceptical reader will tell me, nodded and slept", but then says that he is "resolved not to admit" this explanation. A number of dreamlike elements of the story may suggest otherwise to the reader, such as the fact that none of the members of the monastery seem to be able to see him at first, and the abrupt ending where Bede has been delayed talking to the narrator and so the other monks burst in thinking that some harm has come to him, and suddenly the narrator finds himself back under the tree in the present (August of 1837), with his coach having just passed his spot on the road, leaving him stranded in Newcastle for another night.

Charles Dickens' 1843 book A Christmas Carol is considered by some[2] to be one of the first depictions of time travel, as the main character, Ebenezer Scrooge, is transported to Christmases past, present and yet to come. These might be considered mere visions rather than actual time travel, though, since Scrooge only viewed each time period passively, unable to interact with them.

A clearer example of time travel is found in the popular 1861 book Paris avant les hommes (Paris before Men), published posthumously by the French botanist and geologist Pierre Boitard. In this story the main character is transported into the prehistoric past by the magic of a "lame demon", where he encounters such extinct animals as a Plesiosaur, as well as Boitard's imagined version of an apelike human ancestor, and is able to actively interact with some of them. Another clear early example of time travel in fiction is the short story The Clock That Went Backward by Edward Page Mitchell, which appeared in the New York Sun in 1881.

The first time travel story to feature time travel by means of a time machine was Enrique Gaspar y Rimbau's 1887 book El Anacronópete. This idea gained popularity with the H. G. Wells story The Time Machine, published in 1895 (preceded by a less influential story of time travel Wells wrote in 1888, titled The Chronic Argonauts), which also featured a time machine and which is often seen as an inspiration for all later science fiction stories featuring time travel.

Since that time, both science and fiction (see Time travel in fiction) have expanded on the concept of time travel, but whether it could be possible in reality is still an open question.



 

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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, 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 travel—a variant of the Fermi paradox, with time travelers instead of alien visitors. (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 can't 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.'[4]) 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.

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Presentism (philosophy of time)

In the philosophy of time, presentism is the belief that neither the future nor the past exists. The opposite of presentism is 'eternalism', which is a belief in things that are past and things that are yet to come exist eternally. One other view (that has not been held by very many philosophers) is sometimes called the growing block theory of time, which is a theory that takes the past and present to exist but the future to be nonexistent.[1] Presentism is compatible with Galilean relativity, in which time is independent of space but is probably incompatible with Lorentzian/Einsteinian relativity. Presentism can also be used more loosely to refer to a narrow focus on the conditions of the moment.

Saint Augustine proposed that the present is a knife edge between the past and the future and could not contain any extended period of time. This seems evident because, if the present is extended, it must have separate parts - but these must be simultaneous if they are truly part of the present. According to early philosophers time cannot be both past and simultaneously present, so it is not extended. Contrary to Saint Augustine, some philosophers propose that conscious experience is extended in time. For instance, William James said that time is "the short duration of which we are immediately and incessantly sensible". Augustine proposed that God is outside of time and present for all times, in eternity. Other early philosophers who were presentists include the Buddhists (in the tradition of Indian Buddhism). A leading scholar from the modern era on Buddhist philosophy is Stcherbatsky, who has written extensively on Buddhist presentism: "Everything past is unreal, everything future is unreal, everything imagined, absent, mental... is unreal... Ultimately real is only the present moment of physical efficiency [i.e., causation]."[2]

According to J. M. E. McTaggart's The Unreality of Time there are two ways of referring to events: the 'A Series' (also known as 'tensed time': yesterday, today, tomorrow) and the 'B Series' (or 'untensed time': Monday, Tuesday, Wednesday). Later philosophers regard presentism as a belief that only the 'A Series' exists, and Presentists usually maintain that it only makes sense to refer to events with statements that have a definite tense.

In the modern theory of relativity, the conceptual observer is a geometric point in both space and time at the apex of the 'light cone' which observes events laid out in time as well as space. Different observers can disagree on whether two events at different locations occurred simultaneously depending if the observers are in relative motion (see relativity of simultaneity). This theory depends upon the idea of time as an extended thing and has been confirmed by experiment and has given rise to a philosophical viewpoint known as four dimensionalism. However, although the contents of an observation are time-extended, the conceptual observer, being a geometric point at the origin of the light cone, is not extended in time or space. This analysis contains a paradox in which the conceptual observer contains nothing, even though any real observer would need to be the extended contents of an observation to exist. This paradox is partially resolved in Relativity theory by defining a 'frame of reference' to encompass the measuring instruments used by an observer. This reduces the time separation between instruments to a set of constant intervals.[3]

http://en.wikipedia.org/wiki/Presentism_%28philosophy_of_time%29

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Time travel to the past in physics
Time travel to the past is theoretically allowed using the following methods[6]:

Traveling faster than the speed of light
The use of cosmic strings and black holes
Wormholes and Alcubierre 'warp' drive

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The equivalence of time travel and faster-than-light travel

If one were able to move information or matter from one point to another faster than light, then according to special relativity, there would be some inertial frame of reference in which the signal or object was moving backwards in time. This is a consequence of the relativity of simultaneity in special relativity, which says that in some cases different reference frames will disagree on whether two events at different locations happened "at the same time" or not, and they can also disagree on the order of the two events (technically, these disagreements occur when spacetime interval between the events is 'space-like', meaning that neither event lies in the future light cone of the other).[7] If one of the two events represents the sending of a signal from one location and the second event represents the reception of the same signal at another location, then as long as the signal is moving at the speed of light or slower, the mathematics of simultaneity ensures that all reference frames agree that the transmission-event happened before the reception-event.[7] However, in the case of a hypothetical signal moving faster than light, there would always be some frames in which the signal was received before it was sent, so that the signal could be said to have moved backwards in time. And since one of the two fundamental postulates of special relativity says that the laws of physics should work the same way in every inertial frame, then if it is possible for signals to move backwards in time in any one frame, it must be possible in all frames. This means that if observer A sends a signal to observer B which moves FTL (faster than light) in A's frame but backwards in time in B's frame, and then B sends a reply which moves FTL in B's frame but backwards in time in A's frame, it could work out that A receives the reply before sending the original signal, a clear violation of causality in every frame. An illustration of such a scenario using spacetime diagrams can be found here.

It should be noted that according to relativity it would take an infinite amount of energy to accelerate a slower-than-light object to faster-than-light speeds, and although relativity does not forbid the theoretical possibility of tachyons which move faster than light at all times, when analyzed using quantum field theory it seems that it would not actually be possible to use them to transmit information faster than light[8], and there is no evidence for their existence.

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Special spacetime geometries

The general theory of relativity extends the special theory to cover gravity, illustrating it in terms of curvature in spacetime caused 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. [3]The first of these was proposed by Kurt Gödel, a solution known as the Gödel metric, but his (and many others') example require the universe to have physical characteristics that it does not appear to have.[3] Whether general relativity forbids closed time-like curves for all realistic conditions is unknown.


Using wormholes
 
A wormholeWormholes 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. A wormhole is created somehow. 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[9]; 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.[10] 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.[11]

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. [12] 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 than not a flaw in classical quantum gravity theory rather than proof that causality violation is possible. [13]

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Other approaches based on general relativity

Another approach involves a dense spinning cylinder usually referred to as a Tipler cylinder, a GR solution discovered by Willem Jacob van Stockum [14] in 1936 and Kornel Lanczos [15] in 1924, but not recognized as allowing closed timelike curves[16] until an analysis by Frank Tipler[17] in 1974. If a cylinder is long, and dense, and spins fast enough about its long axis, then a spaceship flying around the cylinder on a spiral path could travel back in time (or forward, depending on the direction of its spiral). However, the density and speed required is so great that ordinary matter is not strong enough to construct it. A similar device might be built from a cosmic string, but none are known to exist, and it does not seem to be possible to create a new cosmic string.

Physicist Robert Forward noted that a naïve application of general relativity to quantum mechanics suggests another way to build a time machine. A heavy atomic nucleus in a strong magnetic field would elongate into a cylinder, whose density and "spin" are enough to build a time machine. Gamma rays projected at it might allow information (not matter) to be sent back in time. However, he pointed out that until we have a single theory combining relativity and quantum mechanics, we will have no idea whether such speculations are nonsense.[citation needed]

A more fundamental objection to time travel schemes based on rotating cylinders or cosmic strings has been put forward up by Stephen Hawking, who argues that it is impossible to build a time machine in any finite region that satisfies the weak energy condition, meaning that the region contains no exotic matter with negative energy. Solutions such as Tipler's assume cylinders of infinite length, which are easier to analyze mathematically, and although Tipler suggested that a finite cylinder might produce closed timelike curves if the rotation rate were fast enough,[18] he did not prove this. Hawking, on the other hand, claims that "it can't be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy."[19] This result comes from Hawking's 1992 paper on the chronology protection conjecture, where he examines "the case that the causality violations appear in a finite region of spacetime without curvature singularities" and proves that "[t]here will be a Cauchy horizon that is compactly generated and that in general contains one or more closed null geodesics which will be incomplete. One can define geometrical quantities that measure the Lorentz boost and area increase on going round these closed null geodesics. If the causality violation developed from a noncompact initial surface, the averaged weak energy condition must be violated on the Cauchy horizon."[20]


Time travel and the anthropic principle

It has been suggested by physicists such as Max Tegmark that the absence of time travel and the existence of causality may be due to the anthropic principle. The argument is that a universe which allows for time travel and closed time-like loops is one in which intelligence could not evolve because it would be impossible for a being to sort events into a past and future or to make predictions or comprehend the world around them (at least, not if the time travel occurs in such a way that it disrupts that evolutionary process).[citation needed]

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Time travel to the future in physics



Twin paradox diagramThere 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:[6]

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

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Time dilation

Time dilation is the phenomenon whereby an observer finds that another's clock which is physically identical to their own is ticking at a slower rate as measured by their own clock. This is often taken to mean that time has "slowed down" for the other clock, but that is only true in the context of the observer's frame of reference. Locally, time is always passing at the same rate. The time dilation phenomenon applies to any process that manifests change over time.

In Albert Einstein's theories of relativity time dilation is manifested in two circumstances:

In special relativity, clocks that are moving with respect to an inertial system of observation (the putatively stationary observer) are found to be running slower. This effect is described precisely by the Lorentz transformations.
In general relativity, clocks at lower potentials in a gravitational field-- such as in close proximity to a planet --are found to be running slower. This gravitational time dilation is only briefly mentioned in this article but is described elsewhere (see also gravitational red shift).
In special relativity, the time dilation effect is reciprocal: as observed from the point of view of any two clocks which are in motion with respect to each other, it will be the other party's clocks that is time dilated. (This presumes that the relative motion of both parties is uniform; that is, they do not accelerate with respect to one another during the course of the observations.)

In contrast, gravitational time dilation (as treated in General Relativity) is not reciprocal: an observer at the top of a tower will observe that clocks at ground level tick slower, and observers on the ground will agree. Thus gravitational time dilation is agreed upon by all stationary observers, independent of their altitude.

Overview
The formula for determining time dilation in special relativity is:



where

 is the time interval between two colocal events for an observer in some inertial frame (e.g. ticks on his clock),
 is the time interval between those same events, as measured by another observer, inertially moving with velocity v w.r.t. the former observer,
 is the relative velocity between the observer and the moving clock,
 is the speed of light, and
 is the so-called Lorentz factor.
Thus the duration of the clock cycle of a moving clock is found to be increased: it is "running slow." As indicated, the Lorentz transforms can be used for more general cases.

As shown, the effect increases in an exponential manner with respect to relative speed or gravitational differences. The range of such variances in ordinary life, even considering space travel, are not great enough to produce easily detectable time dilation effects, and such vanishingly small effects can be safely ignored. It is only when an object approaches speeds on the order of 30,000 km/s (1/10 of the speed of light), or lies deep within the gravitational "well" of massive stellar objects, that it becomes important.

Time dilation by the Lorentz factor was predicted by Joseph Larmor (1897), at least for electrons orbiting a nucleus. Thus "... individual electrons describe corresponding parts of their orbits in times shorter for the [rest] system in the ratio :" (Larmor 1897). Time dilation of magnitude corresponding to this (Lorentz) factor has been experimentally confirmed, as described below.

Experimental confirmation
Time dilation has been tested a number of times. The routine work carried on in particle accelerators since the 1950s, such as those at CERN, is a continuously running test of the time dilation of special relativity. The specific experiments include:


http://en.wikipedia.org/wiki/Time_dilation

http://en.wikipedia.org/wiki/Time_travel