Some solutions to Einstein's equations for
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 were possible. In technical papers,
physicists discuss the possibility of
closed timelike curves, which are
world lines 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, such as
Gödel spacetime, but the physical plausibility of these solutions is uncertain. Any theory that would allow backward time travel would introduce potential problems of
causality. The classic example of a problem involving causality is the "
grandfather paradox," which postulates travelling to the past and intervening in the conception of one's ancestors (causing the death of an ancestor before conception being frequently cited). Some physicists, such as Novikov and Deutsch, suggested that these sorts of
temporal paradoxes can be avoided through the
Novikov self-consistency principle or a variation of the
many-worlds interpretation with interacting worlds. The theory of
general relativity does suggest a scientific basis for the possibility of backward time travel 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
Stephen Hawking to formulate the
chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel, but physicists cannot come to a definitive judgment on the issue without a theory of
quantum gravity to join quantum mechanics and general relativity into a completely unified theory.
Different spacetime geometries The theory of
general relativity describes the universe under a system of
field equations that determine the
metric, or distance function, of spacetime. There exist exact solutions to these equations that include
closed time-like curves, which are
world lines that intersect themselves; some point in the causal future of the world line is also in its causal past, a situation that can be described as time travel. Such a solution was first proposed by
Kurt Gödel, a solution known as the
Gödel metric, but his (and others') solution requires the universe to have physical characteristics that it does not appear to have,
Wormholes Wormholes are a hypothetical warped spacetime permitted by the
Einstein field equations of general relativity. 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 these methods,
time dilation causes the end of the wormhole that has been moved to have aged less, or become "younger", 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. Although early calculations suggested that 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.
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 in 1936 and
Kornel Lanczos in 1924, but not recognized as allowing closed timelike curves until an analysis by
Frank Tipler in 1974. If a cylinder is infinitely long 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 more fundamental objection to time travel schemes based on rotating cylinders or cosmic strings has been put forward by Stephen Hawking, who proved a theorem showing that according to general relativity it is impossible to build a time machine of a special type (a "time machine with the compactly generated Cauchy horizon") in a region where the
weak energy condition is satisfied, meaning that the region contains no matter with negative energy density (
exotic matter). 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, The signal could be said to have moved backward in time. This hypothetical scenario is sometimes referred to as a
tachyonic antitelephone. Quantum-mechanical phenomena such as
quantum teleportation, the
EPR paradox, or
quantum entanglement might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the
Bohm interpretation presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles. This effect was referred to as "
spooky action at a distance" by Einstein. Nevertheless, the fact that causality is preserved in quantum mechanics is a rigorous result in modern
quantum field theories, and therefore modern theories do not allow for time travel or
FTL communication. In any specific instance where FTL has been claimed, more detailed analysis has proven that to get a signal, some form of classical communication must also be used. The
no-communication theorem also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals.
Interacting many-worlds interpretation A variation of
Hugh Everett's
many-worlds interpretation (MWI) of quantum mechanics provides a resolution to the grandfather paradox that involves the time traveler arriving in a different universe than the one they came from; it's been argued that since the traveler arrives in a different universe's history and not their own history, this is not "genuine" time travel. The accepted many-worlds interpretation suggests that all possible quantum events can occur in mutually exclusive histories. However, some variations allow different universes to interact. This concept is most often used in science-fiction, but some physicists such as
David Deutsch have suggested that a time traveler should end up in a different history than the one he started from. On the other hand, Stephen Hawking has argued that even if the MWI is correct, we should expect each time traveler to experience a single self-consistent history, so that time travelers remain within their own world rather than traveling to a different one.
Experimental results Certain experiments carried out give the impression of reversed
causality, but fail to show it under closer examination. The
delayed-choice quantum eraser experiment performed by
Marlan Scully involves pairs of
entangled photons that 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. 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 be observed only 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, only by gathering classical information from the entire system; thus causality is preserved. The experiment of Lijun Wang might also show 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 backward 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 faster than light, so this experiment is understood not to violate causality either. The physicists
Günter Nimtz 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 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 M. 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 speed of the center of the train exceeds the speed of any of the individual cars.
Shengwang Du claims in a peer-reviewed journal to have observed single photons'
precursors, saying that they travel no faster than
c in a vacuum. His experiment involved
slow light as well as passing light through a vacuum. He generated two single
photons, passing one through
rubidium atoms that had been cooled with a laser (thus slowing the light) and passing one through a vacuum. Both times, apparently, the precursors preceded the photons' main bodies, and the precursor traveled at
c in a vacuum. According to Du, this implies that there is no possibility of light traveling faster than
c and, thus, no possibility of violating causality.
Absence of time travelers from the future Many have argued that the absence of time travelers from the future demonstrates that such technology will never be developed, suggesting that it is impossible. This is analogous to the
Fermi paradox related to the absence of evidence of extraterrestrial life. As the absence of extraterrestrial visitors does not categorically
prove they do not exist, so the absence of time travelers fails to prove time travel is physically impossible; it might be that time travel is physically possible but is never developed or is cautiously used.
Carl Sagan once suggested the possibility that time travelers could be here but are disguising their existence or are not recognized as time travelers. '', advertising the Krononauts event Several experiments have been carried out to try to entice future humans, who might invent time travel technology, to come back and demonstrate it to people of the present time. Events such as Perth's
Destination Day,
MIT's
Time Traveler Convention and Stephen Hawking's
Reception For Time Travellers heavily publicized permanent "advertisements" of a meeting time and place for future time travelers to meet. In 1982, a group in
Baltimore,
Maryland, identifying itself as the Krononauts, hosted an event of this type welcoming visitors from the future. These experiments only stood the possibility of generating a positive result demonstrating the existence of time travel, but have failed so far—no time travelers are known to have attended either event. Some versions of the
many-worlds interpretation can be used to suggest that future humans have traveled back in time, but have traveled back to the meeting time and place in a
parallel universe. == Time dilation ==