关于时光旅行的概念大家应该应该不陌生，作为物理学的一个研究方向，牵涉到：时间、空间、光速、黑洞、虫洞等诸多理论知识，全球众多科学家正在孜孜不倦的进行研究和实验。穿越时空是指从一个空间里通过某种途径转移到另外一个时间段不同的空间（即穿越到过去或未来），时光之旅在理论上是可行的，人类可以打开回到过去的大门和通向未来的捷径。为了实现时光旅行，霍金首先建议人们接纳时间作为第四维的观念。他举了一个非常简单的例子：当人们驾驶汽车时，向前直行和向后倒车是第一维，向左或向右转弯是第二维，在山路上爬坡和下坡是第三维，那么时间就是第四维。对于物理学家来说，时光隧道也许就是虫洞。霍金说，虫洞就在我们周围，只是小到肉眼无法看见。宇宙万物都会出现小孔或裂缝，这种基本规律同样适用于时间。时间也有细微的裂缝和空隙，比分子、原子还要小的空隙被称作“量子泡沫”，而虫洞就存在于“量子泡沫”中。

本文是一篇关于物理学时间旅行的研究分析类文章,主要是针对时光旅行理论的研究。

INTODUCTION

Time travel is the concept of moving between different moments in time

Time is a component of the measurement used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects.... in a manner analogous to moving between different points in space

Space is the boundless, three-dimensional extent in which Physical body and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physics usually consider it, with time, to be part of the boundless four-dimensional continuum known as spacetime.... , 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 to 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 an attempt to travel backwards in time might take one to a parallel universe

A time machine is a fictional device that allows time travel to the past or future.The concept derives from:* The Time Machine, an 1895 novel by H....

The multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:... whose history would begin to diverge from the traveler's original history after the moment the traveler arrived in the past. Although time travel has been a common plot device

A plot device is an element introduced into a narrative solely to advance or resolve the Plot of the story. In the hands of a skilled writer, the reader or viewer will not notice that the device is a construction of the author; it will seem to follow naturally from the setting or characters in the story.... in fiction

Fiction is an imaginative form of narrative, one of the four basic rhetorical modes. Although the word fiction is derived from the Latin fingo, fingere, finxi, fictum, "to form, create", works of fiction need not be entirely imaginary and may include real people, places, and events.... since the 19th century, and one-way travel into the future is arguably possible given the phenomenon of 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.... based on velocity in the theory of special relativity

Special relativity is the physical theory of measurement in inertial frames of reference proposed in 1905 by Albert Einstein in the paper "Annus Mirabilis Papers#Special relativity".... (exemplified by the twin paradox In physics, the twin paradox is a thought experiment in special relativity, in which a twin who makes a journey into space in a high-speed rocket will return home to find he has aged less than his identical twin who stayed on Earth.... ) as well as gravitational time dilation

Gravitational time dilation is the effect of time passing at different rates in regions of different gravitational potential; the higher the local distortion of spacetime due to gravity, the more slowly time passes.... in the theory of general relativity

General relativity or the general theory of relativity is the Geometry Theoretical physics of gravitation published by Albert Einstein in 1916.... , it is currently unknown whether the laws of physics would allow backwards time travel. Time travel has not been proven to be impossible nor possible. Any technological device, whether fictional or hypothetical, that is used to achieve time travel is known as a time machine.

ORIGINS OF THE CONCEPTS

There is no widespread agreement as to which written work should be recognized as the earliest example of a time travel story, since a number of early works feature elements ambiguously suggestive of time travel. For example, Memoirs of the Twentieth Century (1733) by Samuel Madden

Samuel Madden was an Ireland author. His works include Themistocles; The Lover of His Country, Reflections and Resolutions Proper for the Gentlemen of Ireland, and Memoirs of the Twentieth Century.... 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."

Louis-Sébastien Mercier

Louis-S?bastien Mercier was a France dramatist and writer.He was born in Paris to a humble family: his father was a skilled artisan who polished swords and metal arms.... 's L'An 2440, rêve s'il en fût jamais ("The Year 2440: A Dream If Ever There Were One") is a utopian novel set in the year 2440. An extremely popular work (it went through twenty-five editions after its first appearance in 1771), the work describes the adventures of an unnamed man, who, after engaging in a heated discussion with a philosopher friend about the injustices of Paris, falls asleep and finds himself in a Paris of the future. Robert Darnton writes that "despite its self-proclaimed character of fantasy...L'An 2440 demanded to be read as a serious guidebook to the future." [Robert Darnton, The Forbidden Best-Sellers of Pre-revolutionary France (New York: W.W. Norton, 1996), 120.]

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

Anonymity is derived from the Greek word a??????a, meaning "without a name" or "namelessness". In colloquial use, the term typically refers to a person, and often means that the Identity , or personally identifiable information of that person is not known.... 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

A stagecoach is a type of four-wheeled closed coach for passengers and goods, strongly sprung and drawn by four horses, usually four-in-hand.... which will take him out of Newcastle

Newcastle upon Tyne is a City status in the United Kingdom and metropolitan borough of Tyne and Wear, in North East England. Situated on the north bank of the River Tyne, the city developed from a Roman Empire settlement called Pons Aelius, though it owes its name to the Newcastle Castle built in 1080, by Robert Curthose, the eldest son of... , when he suddenly finds himself transported back over a thousand years, where he encounters the Venerable Bede

Bede , , was a monasticism at the Northumbrian monastery of Saint Peter at Monkwearmouth, today part of Sunderland, England, and of its companion monastery, Saint Paul's, in modern Jarrow , both in the Kingdom of Northumbria.... in a monastery

Monastery , a term derived from the Greek language word ???ast?????, neut. of ???ast????? - monasterios denotes the building, or complex of buildings, that houses a room reserved for prayer as well as the domestic quarters and workplace of Monk, whether monks or nuns, and whether living in Cenobium or alone .... , 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 1837), with his coach having just passed his spot on the road, leaving him stranded in Newcastle for another night.

The first time travel story to feature time travel by means of a time machine was Enrique Gaspary Rimbau Enrique Lucio Eugenio Gaspar y Rimbau was a Spain diplomat and writer, who authored plays, zarzuelas , and novels.... 's 1887 book El Anacronópete. This idea gained popularity with the Herbert George Wells , known by his pen name H. G. Wells, was an England author, best known for his work in the science fiction genre. Wells and Jules Verne are each sometimes referred to as "The Father of Science Fiction".... story The Time Machine

The Time Machine is a novella by H. G. Wells, first published in 1895 and later directly adapted into at least two feature films of the same name, as well as two television versions, and a large number of comic book adaptations.... , published in 1895 (preceded by a less influential story of time travel Wells wrote in 1888, titled The Chronic Argonauts "The Chronic Argonauts" is a short story written by H. G. Wells. First published by the Royal College of Science in 1888 in literature, it predates Wells's famous novel The Time Machine.... ), which also featured a time machine and which is often seen as an inspiration for all later science fiction stories featuring time travel.

Time travel in theory

Some theories, most notably special

Special relativity is the physical theory of measurement in inertial frames of reference proposed in 1905 by Albert Einstein in the paper "Annus Mirabilis Papers#Special relativity".... and general relativity

General relativity or the general theory of relativity is the Geometry Theoretical physics of gravitation published by Albert Einstein in 1916.... , suggest that suitable geometries of space time

In physics, spacetime is any mathematical model that combines space and Time in physics into a single continuum . Spacetime is usually interpreted with space being Three-dimensional space and time playing the role of a fourth dimension that is of a different sort than the spatial dimensions.... , or specific types of motion in space

Space is the boundless, three-dimensional extent in which Physical body and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physics usually consider it, with time, to be part of the boundless four-dimensional continuum known as spacetime.... , 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 time like curve

In a Lorentzian manifold, a closed timelike curve is a worldline of a material particle in spacetime that is "closed," returning to its starting point.... , which are world lines that form closed loops in space time, allowing objects to return to their own past. There are known to be solutions to the equations of general relativity that describe space times which contain closed time like 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

Special relativity is the physical theory of measurement in inertial frames of reference proposed in 1905 by Albert Einstein in the paper "Annus Mirabilis Papers#Special relativity".... 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 Causality describes the relationship between causes and effects, is fundamental to all natural science, especially physics, and has a basis in logic.... be resolved. The classic example of a problem involving causality is the "grandfather paradox The grandfather paradox is a proposed physical paradox of time travel, first described by the science fiction writer Ren? Barjavel in his 1943 book Le Voyageur Imprudent .... ": what if one were to go back in time and kill one's own grandfather before one's father was conceived? But some scientists believe that paradoxes can be avoided, either by appealing to the Novikov self-consistency principle The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a principle developed by Dr. Igor Dmitriyevich Novikov in the mid-1980s to solve the problem of paradoxes in time travel, which is theoretically permitted in certain solutions of general relativity .... or to the notion of branching parallel universes The multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:...

Tourism in time

Stephen Hawking

Stephen William Hawking Companion of Honour, Commander of the British Empire, Fellow of the Royal Society, Fellow of the Royal Society of Arts, Doctor of Philosophy is a British Theoretical physics.... 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

The Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of Extraterrestrial life and the lack of evidence for, or contact with, such civilizations.... . Of course this would not prove that time travel is physically impossible, since it might be that time travel is physically possible but that it is never in fact developed (or is 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

Carl Edward Sagan, Ph.D. was an United States astronomer, Astrochemistry, author, and highly successful popularizer of astronomy, astrophysics and other natural sciences.... also once suggested the possibility that time travelers could be here, but are disguising their existence or are not recognized as time travelers.

General relativity

However, the theory of general relativity

General relativity or the general theory of relativity is the Geometry Theoretical physics of gravitation published by Albert Einstein in 1916.... does suggest scientific grounds for thinking backwards time travel could be possible in certain unusual scenarios, although arguments from semi classical gravity

Semiclassical gravity is the approximation to the theory of quantum gravity in which one treats matter fields as being quantum and the Gravitation as being classical.... Suggest that when quantum

Quantum mechanics is a set of principles underlying the most fundamental known description of all physical systems at the microscopic scale . Notable amongst these principles are both a dual wave-like and particle-like behavior of matter and radiation, and prediction of probabilities in situations where classical physics predicts certaintie... Effects are incorporated into general relativity, these loopholes may be closed. These semi classical arguments led Hawking to formulate the chronology protection conjecture

The chronology protection conjecture is a conjecture by the physicist Professor Stephen Hawking that the laws of physics are such as to prevent time travel on all but sub-microscopic scales.... , 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

Quantum gravity is the field of theoretical physics attempting to unify quantum mechanics, which describes three of the Fundamental interaction , with general relativity, the theory of the fourth fundamental force: Gravitation.... to join quantum mechanics and general relativity into a completely unified theory.

Time travel to the past in physics

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

Space traveling faster than the speed of light

Faster-than-light Superluminal communication and interstellar travel refer to the propagation of information or matter faster than the speed of light.... The use of cosmic string A cosmic string is a hypothetical 1-dimensional topological defect in various fields. Cosmic strings are hypothesized to form when the field undergoes a phase change in different regions of spacetime, resulting in condensations of energy density at the boundaries between regions.... s and black holes.

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

Special relativity is the physical theory of measurement in inertial frames of reference proposed in 1905 by Albert Einstein in the paper "Annus Mirabilis Papers#Special relativity".... , there would be some inertial frame of reference

In physics, an inertial frame of reference is a frame of reference, tied to the state of motion of an Observer , with the property that each physical law portrays itself in the same form in every inertial frame.... in which the signal or object was moving backwards in time. This is a consequence of the relativity of simultaneity

The relativity of simultaneity is the concept that simultaneity is not absolute, but dependent on the observer. That is, according to the special theory of relativity formulated by Albert Einstein in 1905, it is impossible to say in an absolute sense whether two events occur at the same time if those events are separated in space.... 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

In physics, spacetime is any mathematical model that combines space and Time in physics into a single continuum . Spacetime is usually interpreted with space being Three-dimensional space and time playing the role of a fourth dimension that is of a different sort than the spatial dimensions.... between the events is 'space-like', meaning that neither event lies in the future light cone

In special relativity, a light cone is the surface describing the temporal evolution of a flash of light in Minkowski spacetime. This can be visualized in 3-space if the two horizontal axes are chosen to be spatial dimensions, while the vertical axis is time.... of the other). 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. 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

See also: 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

Causality describes the relationship between causes and effects, is fundamental to all natural science, especially physics, and has a basis in logic.... in every frame. An illustration of such a scenario using spacetime diagrams

The Minkowski diagram was developed in 1908 by Herman Minkowski and provides an illustration of the properties of space and time in the special theory of relativity.... can be found here.

According to special relativity

Special relativity is the physical theory of measurement in inertial frames of reference proposed in 1905 by Albert Einstein in the paper "Annus Mirabilis Papers#Special relativity".... It would take an infinite amount of energy to accelerate a slower-than-light object to the speed of light, 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

Quantum field theory or QFT provides a theoretical framework for constructing quantum mechanics models of systems classically described by field or of Many-body problem.... it seems that it would not actually be possible to use them to transmit information faster than light, and there is no evidence for their existence.

Special spacetime geometries

The general theory of relativity extends the special theory

Special relativity is the physical theory of measurement in inertial frames of reference proposed in 1905 by Albert Einstein in the paper "Annus Mirabilis Papers#Special relativity".... 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

The Einstein field equations or Einstein's equations are a set of ten equations in Einstein's theory of general relativity in which the fundamental force of gravitation is described as a curved spacetime caused by matter and energy.... , 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 Gödel

Kurt G?del was an Austrian-United States logician, mathematician and philosopher. One of the most significant logicians of all time, G?del made an immense impact upon scientific and philosophical thinking in the 20th century, a time when many, such as Bertrand Russell, A.... , a solution known as the Gödel metric

The G?del metric is an Exact solutions in general relativity of the Einstein field equations in which the stress-energy tensor contains two terms, the first representing the matter density of a homogeneous distribution of swirling dust particles, and the second associated with a nonzero cosmological constant .... , 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 .

Using wormholes

Wormholes are a hypothetical warped spacetime which are also permitted by the Einstein field equations

The Einstein field equations or Einstein's equations are a set of ten equations in Einstein's theory of general relativity in which the fundamental force of gravitation is described as a curved spacetime caused by matter and energy.... of general relativity, although it would be impossible to travel through a wormhole unless it was what is known as a traversable wormhole

In physics, a wormhole is a hypothetical topology feature of spacetime that is fundamentally a 'shortcut' through space and time. Spacetime can be viewed as a 2D surface, and when 'folded' over, a wormhole bridge can be formed.... A proposed time-travel machine using a traversable wormhole would (hypothetically) work in the following way: One end of the wormhole is accelerated to some significant fraction of the speed of light, perhaps with some advanced propulsion system, and then brought back to the point of origin. Alternatively, another way is to take one entrance of the wormhole and move it to within the gravitational field of an object that has higher gravity than the other entrance, and then return it to a position near the other entrance. For both of these methods, 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.... causes the end of the wormhole that has been moved to have 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

Synchronized can refer to the following meanings:*synchronization, the coordination of events to operate a system in unison.*Synchronized , a 2002 album by sHeavy.... 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 2007 while a clock at the stationary end read 2012, then the observer would exit the stationary end when its clock also read 2007, 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

Stephen William Hawking Companion of Honour, Commander of the British Empire, Fellow of the Royal Society, Fellow of the Royal Society of Arts, Doctor of Philosophy is a British Theoretical physics.... '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 with negative energy (often referred to as "exotic matter

Exotic matter is a hypothetical concept of particle physics. It covers any material which violates one or more classical conditions or is not made of known Baryon.... ") . More technically, the wormhole spacetime requires a distribution of energy that violates various energy condition

In theory of relativity classical field theory of gravitation, particularly general relativity, an energy condition is one of various alternative conditions which can be applied to the matter content of the theory, when it is either not possible or desirable to specify this content explicitly.... s, such as the null energy condition along with the weak, strong, and dominant energy conditions. However, it is known that quantum effects can lead to small measurable violations of the null energy condition, and many physicists believe that the required negative energy may actually be possible due to the Casimir effect

In physics, the Casimir effect and the Casimir-Polder force are physical force arising from a quantum field theory. The typical example is of two electric charge metallic plates in a vacuum, placed a few micrometers apart, without any external electromagnetic field.... 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.

Experiments carried out

Certain experiments carried out during the last ten years give the impression of reversed causality

Causality denotes a necessary relationship between one event and another event which is the direct consequence of the first.While this informal understanding suffices in everyday use, the Philosophy analysis of how best to characterize causality extends over millennia.... but are interpreted in a different way by the scientific community. For example, in the delayed choice quantum eraser

A delayed choice quantum eraser is a cross between a quantum eraser experiment and Wheeler's delayed choice experiment. This experiment has actually been performed and published by Yoon-Ho Kim, R.... experiment performed by Marlan Scully

Marlan Orvil Scully is a physicist best known for his work in theoretical quantum optics. He is currently a professor at Texas A&M University and Princeton University.... pairs of entangled

Quantum entanglement is a possible property of a quantum state of a system of two or more Physical bodys in which the quantum states of the constituting objects are linked together so that one object can no longer be adequately described without full mention of its counterpart ? even though the individual objects may be nonlocality.... photons are divided into "signal photons" and "idler photons", with the signal photons emerging from one of two locations and their position later measured as in the double slit experiment, and depending on how the idler photon is measured, the experimenter can either learn which of the two locations the signal photon emerged from or "erase" that information. Even though the signal photons can be measured before the choice has been made about the idler photons, the choice seems to retroactively determine whether or not an interference pattern is observed when one correlates measurements of idler photons to the corresponding signal photons. However, since interference can only be observed after the idler photons are measured and they are correlated with the signal photons, there is no way for experimenters to tell what choice will be made in advance just by looking at the signal photons, and under most interpretations of quantum mechanics the results can be explained in a way that does not violate causality.

The experiment of Lijun Wang might also give the appearance of causality violation since it made it possible to send packages of waves through a bulb of caesium gas in such a way that the package appeared to exit the bulb 62 nanoseconds before its entry. But a wave package is not a single well-defined object but rather a sum of multiple waves of different frequencies (see Fourier analysis), and the package can appear to move faster than light or even backwards in time even if none of the pure waves in the sum do so. This effect cannot be used to send any matter, energy, or information backwards in time, so this experiment is understood not to violate causality either.

The physicists Günter Nimtz

G?nter Nimtz is a German physicist. Working at the 2nd Physics Institute at the University of Cologne in Germany, he has been conducting experiments that purport to show that under certain conditions, particles may travel Faster-than-light than the Speed of light.... and Alfons Stahlhofen, of the University of Koblenz, claim to have violated Einstein's theory of relativity by transmitting photons faster than the speed of light. They say they have conducted an experiment in which microwave photons - energetic packets of light - traveled "instantaneously" between a pair of prisms that had been moved up to apart, using a phenomenon known as quantum tunneling. Nimtz told New Scientist magazine: "For the time being, this is the only violation of special relativity that I know of." However, other physicists say that this phenomenon does not allow information to be transmitted faster than light. Aephraim Steinberg, a quantum optics expert at the University of Toronto, Canada, uses the analogy of a train traveling from Chicago to New York, but dropping off train cars at each station along the way, so that the center of the train moves forward at each stop; in this way, the center of the train exceeds the speed of any of the individual cars.

Some physicists have attempted to perform experiments which would show genuine causality violations, but so far without success. The Space-time Twisting by Light (STL) experiment run by physicist Ronald Mallett Ronald L. Mallett, Ph.D. is a professor of physics in the University of Connecticut.... is attempting to observe a violation of causality when a neutron is passed through a circle made up of a laser whose path has been twisted by passing it through a photonic crystal.

Photonic crystals are periodic optical nanostructures that are designed to affect the motion of photons in a similar way that periodicity of a semiconductor crystal affects the motion of electrons.... Mallett has some physical arguments which suggest that closed timelike curves would become possible through the center of a laser which has been twisted into a loop. However, other physicists dispute his arguments.

Time travel to the future in physics

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 his 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

Special relativity is the physical theory of measurement in inertial frames of reference proposed in 1905 by Albert Einstein in the paper "Annus Mirabilis Papers#Special relativity".... 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

A frame of reference in physics, may refer to a coordinate system or Cartesian coordinate system within which to measure the position, orientation , and other properties of objects in it, or it may refer to an observational reference frame tied to the state of motion of an Observer ....

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 The speed of light in an free space is an important physical constant usually written as c, with a value of 299,792,458 metres per second.... to a distant star, then slowing down, turning around, and traveling at almost the speed of light back to Earth 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 In general relativity, a black hole is a region of space in which the gravitational field is so powerful that nothing, including electromagnetic radiation , can escape its pull after having fallen past its event horizon.... , or on the surface of a larger-than-earth mass object. 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":

Suspended animation is the slowing of life processes by external means without termination. Breathing, heartbeat, and other involuntary functions may still occur, but they can only be detected by artificial means.... Time dilation

Time dilation is permitted by Albert Einstein

Albert Einstein was a Germany-born theoretical physics. He is best known for his theory of relativity and specifically mass?energy equivalence, expressed by the equation E=mc2.... 's special

Special relativity is the physical theory of measurement in inertial frames of reference proposed in 1905 by Albert Einstein in the paper "Annus Mirabilis Papers#Special relativity".... and general

General relativity or the general theory of relativity is the Geometry Theoretical physics of gravitation published by Albert Einstein in 1916.... theories of relativity. These theories state that, relative to a given observer, time passes more slowly for bodies moving quickly relative to that observer, or bodies that are deeper within a gravity well

In physics, a gravity well is the gravitational potential field around a massive body . Physical models of gravity wells are sometimes used to illustrate orbital mechanics.... . For example, a clock which is moving relative to the observer will be measured to run slow in that observer's rest frame

In special relativity the rest frame of a particle is the coordinate system in which the particle is at rest.The rest frame of compound objects is taken to be the frame of reference in which the average momentum of the particles which make up the substance is zero .... ; as a clock approaches the speed of light it will almost slow to a stop, although it can never quite reach light speed so it will never completely stop. For two clocks moving inertially

In physics, an inertial frame of reference is a frame of reference, tied to the state of motion of an Observer , with the property that each physical law portrays itself in the same form in every inertial frame.... (not accelerating) relative to one another, this effect is reciprocal, with each clock measuring the other to be ticking slower. However, the symmetry is broken if one clock accelerates, as in the twin paradox

In physics, the twin paradox is a thought experiment in special relativity, in which a twin who makes a journey into space in a high-speed rocket will return home to find he has aged less than his identical twin who stayed on Earth.... where one twin stays on Earth while the other travels into space, turns around (which involves acceleration), and returns-in this case both agree the traveling twin has aged less. General relativity states that time dilation effects also occur if one clock is deeper in a gravity well than the other, with the clock deeper in the well ticking more slowly; this effect must be taken into account when calibrating the clocks on the satellites of the Global Positioning System

The Global Positioning System is a global navigation satellite system developed by the United States Department of Defense and managed by the United States Air Force 50th Space Wing.... , and it could lead to significant differences in rates of aging for observers at different distances from a black hole

In general relativity, a black hole is a region of space in which the gravitational field is so powerful that nothing, including electromagnetic radiation , can escape its pull after having fallen past its event horizon....

It has been calculated that, under general relativity, a person could travel forward in time at a rate four times that of distant observers by residing inside a spherical shell with a diameter of 5 meters and the mass of Jupiter

Jupiter is the fifth planet from the Sun and the Solar system by size planet within the Solar System. It is two and a half times as massive as all of the other planets in our Solar System combined.... . For such a person, every one second of their "personal" time would correspond to four seconds for distant observers. Of course, squeezing the mass of a large planet into such a structure is not expected to be within our technological capabilities in the near future.

Time perception

Time perception can be apparently sped up for living organisms

In biology, an organism is any life thing . In at least some form, all organisms are capable of response to stimulus , reproduction, growth and developmental biology, and maintenance of homeostasis as a stable whole.... through hibernation

Hibernation is a state of inactivity and Metabolism depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate.... , where the body temperature

Thermoregulation is the ability of an organism to keep its core temperature within certain boundaries, even when the surrounding temperature is very different.... and metabolic

Metabolism is the set of chemical reactions that occur in living organisms in order to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments.... rate of the creature is reduced. A more extreme version of this is suspended animation

Suspended animation is the slowing of life processes by external means without termination. Breathing, heartbeat, and other involuntary functions may still occur, but they can only be detected by artificial means.... , where the rates of chemical processes in the subject would be severely reduced.

Time dilation and suspended animation only allow "travel" to the future, never the past, so they do not violate causality

Causality denotes a necessary relationship between one event and another event which is the direct consequence of the first.While this informal understanding suffices in everyday use, the Philosophy analysis of how best to characterize causality extends over millennia.... , and arguably should not be considered time travel. However time dilation should be considered an actual form of time travel, since the person does actually travel into the future at a faster pace than normal, whereas with suspended animation this is not the case.

OTHER IDEAS ABOUT TIME TRAVEL FROM MAINSTREAM PHYSICS

The Novikov self-consistency principle

The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a principle developed by Dr. Igor Dmitriyevich Novikov in the mid-1980s to solve the problem of paradoxes in time travel, which is theoretically permitted in certain solutions of general relativity .... and recent calculations by Kip S. Thorne indicate that simple masses passing through time travel wormholes could never engender paradoxes-there are no initial conditions that lead to paradox once time travel is introduced. If his results can be generalized, they would suggest, curiously, that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, that any situation you can set up in a time travel story turns out to permit many consistent solutions. The circumstances might, however, turn out to be almost unbelievably strange.

Parallel universes might provide a way out of paradoxes. Everett's

Hugh Everett III was an American physicist who first proposed the many-worlds interpretation of quantum physics, which he called his "relative state" formulation.... many-worlds interpretation

The many-worlds interpretation is an interpretation of quantum mechanics.It is also known as MWI, the relative state formulation, theory of the universal wavefunction, parallel universes, many-universes interpretation or just many worlds.... of quantum mechanics suggests that all possible quantum events can occur in mutually exclusive histories. These alternate, or parallel, histories would form a branching tree symbolizing all possible outcomes of any interaction. If all possibilities exist, any paradoxes could be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction, but some physicists such as David Deutsch

David Elieser Deutsch Fellow of the Royal Society#Fellowship is a physicist at the University of Oxford. He is a non-stipendiary Visiting Professor in the Department of Atomic and Laser Physics at the Centre for Quantum Computation, Clarendon Laboratory.... have suggested that if time travel is possible and the many-worlds interpretation is correct, then a time traveler should indeed end up in a different history than the one he started from. On the other hand, Stephen Hawking

Stephen William Hawking Companion of Honour, Commander of the British Empire, Fellow of the Royal Society, Fellow of the Royal Society of Arts, Doctor of Philosophy is a British Theoretical physics.... has argued that even if the many-worlds interpretation is correct, we should expect each time traveler to experience a single self-consistent timeline, so that time travelers remain within their own world rather than traveling to a different one.

Daniel Greenberger and Karl Svozil proposed that quantum theory

Quantum mechanics is a set of principles underlying the most fundamental known description of all physical systems at the microscopic scale . Notable amongst these principles are both a dual wave-like and particle-like behavior of matter and radiation, and prediction of probabilities in situations where classical physics predicts certaintie... gives a model for time travel without paradoxes. In quantum theory observation causes possible states to 'collapse' into one measured state; hence, the past observed from the present is deterministic (it has only one possible state), but the present observed from the past has many possible states until our actions cause it to collapse into one state. Our actions will then be seen to have been inevitable.

The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a principle developed by Dr. Igor Dmitriyevich Novikov in the mid-1980s to solve the problem of paradoxes in time travel, which is theoretically permitted in certain solutions of general relativity .... Theory of compossibility

David Lewis

David Kellogg Lewis was a 20th century philosopher. Lewis taught briefly at UCLA and then at Princeton University from 1970 until his death. He is also closely associated with Australia, whose philosophical community he visited almost annually for more than thirty years.... ' analysis of compossibility and the implications of changing the past is meant to account for the possibilities of time travel in a one-dimensional conception of time without creating logical paradox

A paradox is a Proposition or group of statements that leads to a contradiction or a situation which defies intuition ; or, it can be an apparent contradiction that actually expresses a non-dual truth .... es. Consider Lewis' example of Tim. Tim hates his grandfather and would like nothing more than to kill him. The only problem for Tim is that his grandfather died years ago. Tim wants so badly to kill his grandfather himself that he constructs a time machine to travel back to 1955 when his grandfather was young and kill him then. Assuming that Tim can travel to a time when his grandfather is still alive, the question must then be raised; Can Tim kill his grandfather?

For Lewis, the answer lies within the context of the usage of the word "can". Lewis explains that the word "can" must be viewed against the context of pertinent facts relating to the situation. Suppose that Tim has a rifle, years of rifle training, a straight shot on a clear day and no outside force to restrain Tim's trigger finger. Can Tim shoot his grandfather? Considering all of the facts that I have just listed, it would appear that Tim can in fact kill his grandfather. In other words, all of the contextual facts are compossible with Tim killing his grandfather. However, when reflecting on the compossibility of a given situation, we must gather the most inclusive set of facts that we are able to.

Consider now the fact that Tim's grandfather died in 1993 and not in 1955. This new fact about Tim's situation reveals that him killing his grandfather is not compossible with the current set of facts. Tim cannot kill his grandfather because his grandfather died in 1993 and not when he was young. Thus, Lewis concludes, the statements "Tim doesn't but can, because he has what it takes," and, "Tim doesn't, and can't, because it is logically impossible to change the past," are not contradictions, they are both true given the relevant set of facts. The usage of the word "can" is equivocal: he "can" and "can not" under different relevant facts. So what must happen to Tim as he takes aim? Lewis believes that his gun will jam, a bird will fly in the way, or Tim simply slips on a banana peel. Either way, there will be some logical force of the universe that will prevent Tim every time from killing his grandfather.

REFERENCES

1.     WWW.PHY.EDU/TIME MACHINE.HTML

2.     WWW.PHYSICSLIBRARY/TIME TRAVEL.HTML

3.     WWW.WIKIPEDIA.ORG/TIMEMACHINE.HTML

4.     WWW.PHYFORUM.COM/TIMEMACHINE.HTML