Time travel is the concept of movement between certain points in time, analogous to movement between different points in space by an object or a person, typically using a hypothetical device known as a time machine. Time travel is a widely-recognized concept in philosophy and fiction. The idea of a time machine was popularized by H. G. Wells' 1895 novel The Time Machine.
It is uncertain if time travel to the past is physically possible. Forward time travel, outside the usual sense of the perception of time, is an extensively-observed phenomenon and well-understood within the framework of special relativity and general relativity. However, making one body advance or delay more than a few milliseconds compared to another body is not feasible with current technology. As for backwards time travel, it is possible to find solutions in general relativity that allow for it, but the solutions require conditions that may not be physically possible. Traveling to an arbitrary point in spacetime has a very limited support in theoretical physics, and usually only connected with quantum mechanics or wormholes, also known as Einstein-Rosen bridges.
History of the time travel concept
Statue of Rip Van Winkle in Irvington, New York
Some ancient myths depict a character skipping forward in time. In Hindu mythology, the Mahabharata mentions the story of King Raivata Kakudmi, who travels to heaven to meet the creator Brahma and is surprised to learn when he returns to Earth that many ages have passed. The Buddhist Pāli Canon mentions the relativity of time. The Payasi Sutta tells of one of the Buddha's chief disciples, Kumara Kassapa, who explains to the skeptic Payasi that time in the Heavens passes differently than on Earth. The Japanese tale of "Urashima Tarō", first described in the Nihongi
(720) tells of a young fisherman named Urashima Tarō who visits an
undersea palace. After three days, he returns home to his village and
finds himself 300 years in the future, where he has been forgotten, his
house is in ruins, and his family has died. In Jewish tradition, the 1st-century BC scholar Honi ha-M'agel
is said to have fallen asleep and slept for seventy years. When waking
up he returned home but found none of the people he knew, and no one
believed his claims of who he was.
Shift to science fiction
Early science fiction stories feature characters who sleep for years
and awaken in a changed society, or are transported to the past through
supernatural means. Among them L'An 2440, rêve s'il en fût jamais (1770) by Louis-Sébastien Mercier, Rip Van Winkle (1819) by Washington Irving, Looking Backward (1888) by Edward Bellamy, and When the Sleeper Awakes (1899) by H.G. Wells. Prolonged sleep, like the more familiar time machine, is used as a means of time travel in these stories.
The earliest work about backwards time travel is uncertain. Samuel Madden's Memoirs of the Twentieth Century
(1733) is a series of letters from British ambassadors in 1997 and 1998
to diplomats in the past, conveying the political and religious
conditions of the future. Because the narrator receives these letters from his guardian angel, Paul Alkon suggests in his book Origins of Futuristic Fiction that "the first time-traveler in English literature is a guardian angel."
Madden does not explain how the angel obtains these documents, but
Alkon asserts that Madden "deserves recognition as the first to toy with
the rich idea of time-travel in the form of an artifact sent backward
from the future to be discovered in the present." In the science fiction anthology Far Boundaries (1951), editor August Derleth claims that an early short story about time travel is Missing One's Coach: An Anachronism, written for the Dublin Literary Magazine by an anonymous author in 1838. While the narrator waits under a tree for a coach to take him out of Newcastle, he is transported back in time over a thousand years. He encounters the Venerable Bede in a monastery
and explains to him the developments of the coming centuries. However,
the story never makes it clear whether these events are real or a dream. Another early work about time travel is The Forebears of Kalimeros: Alexander, son of Philip of Macedon by Alexander Veltman published in 1836.
Mr. and Mrs. Fezziwig dance in a vision shown to Scrooge by the Ghost of Christmas Past.
Charles Dickens's A Christmas Carol
(1843) has early depictions of time travel in both directions, as the
protagonist, Ebenezer Scrooge, is transported to Christmases past and
future. Other stories employ the same template, where a character
naturally goes to sleep, and upon waking up finds themself in a
different time. A clearer example of backward time travel is found in the popular 1861 book Paris avant les hommes (Paris before Men) by the French botanist and geologist Pierre Boitard,
published posthumously. In this story, the protagonist is transported
to the prehistoric past by the magic of a "lame demon" (a French pun on
Boitard's name), where he encounters a Plesiosaur and an apelike ancestor and is able to interact with ancient creatures. Edward Everett Hale's
"Hands Off" (1881) tells the story of an unnamed being, possibly the
soul of a person who has recently died, who interferes with ancient
Egyptian history by preventing Joseph's enslavement. This may have been the first story to feature an alternate history created as a result of time travel.
Early time machines
One of the first stories to feature time travel by means of a machine is "The Clock that Went Backward" by Edward Page Mitchell, which appeared in the New York Sun
in 1881. However, the mechanism borders on fantasy. An unusual clock,
when wound, runs backwards and transports people nearby back in time.
The author does not explain the origin or properties of the clock. Enrique Gaspar y Rimbau's El Anacronópete (1887) may have been the first story to feature a vessel engineered to travel through time. Andrew Sawyer
has commented that the story "does seem to be the first literary
description of a time machine noted so far", adding that "Edward Page
Mitchell's story 'The Clock That Went Backward' (1881) is usually described as the first time-machine story, but I'm not sure that a clock quite counts." H. G. Wells's The Time Machine (1895) popularized the concept of time travel by mechanical means.
Time travel in physics
Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime or specific types of motion in space might allow time travel into the past and future if these geometries or motions 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.
Many in the scientific community believe that backward time
travel is highly unlikely. Any theory that would allow time travel would
introduce potential problems of causality. The classic example of a problem involving causality is the "grandfather paradox":
what if one were to go back in time and kill one's own grandfather
before one's father was conceived? Some physicists, such as Novikov and
Deutsch, suggested that these sorts of temporal paradoxes can be avoided through the Novikov self-consistency principle or to a variation of the many-worlds interpretation with interacting worlds.
General relativity
Time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling faster than the speed of light, such as cosmic strings, transversable wormholes, and Alcubierre drive. 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 definite 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, such as rotation and lack of Hubble expansion. Whether general relativity forbids closed time-like curves for all realistic conditions is still being researched.
Wormholes
Wormholes are a hypothetical warped spacetime which are 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 of 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.
This means that an observer entering the "younger" end would exit the
"older" end at a time when it was the same age as the "younger" end,
effectively going back in time as seen by an observer from the 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 backward 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". More technically, the wormhole spacetime requires a distribution of energy that violates various energy conditions,
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 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.
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 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 Ronald Mallett
is attempting to recreate the conditions of a rotating black hole with
ring lasers, in order to bend spacetime and allow for time travel.
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,
he did not prove this. But Hawking points out that because of his
theorem, "it can't be done with positive energy density everywhere! I
can prove that to build a finite time machine, you need negative
energy." 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 "there 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."
This theorem does not rule out the possibility of time travel by means
of time machines with the non-compactly generated Cauchy horizons (such
as the Deutsch-Politzer time machine) or in regions which contain exotic
matter, which would be used for traversable wormholes or the Alcubierre drive.
Quantum physics
No-communication theorem
When a signal is sent from one location and received at another
location, then as long as the signal is moving at the speed of light or
slower, the mathematics of simultaneity
in the theory of relativity show that all reference frames agree that
the transmission-event happened before the reception-event. When the
signal travels faster than light, it is received before it is sent, in all reference frames. 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 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.
The physicist Allen Everett argued that Deutsch's approach "involves
modifying fundamental principles of quantum mechanics; it certainly goes
beyond simply adopting the MWI". Everett also argues that even if
Deutsch's approach is correct, it would imply that any macroscopic
object composed of multiple particles would be split apart when
traveling back in time through a wormhole, with different particles
emerging in different worlds.
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 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,
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,
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 3 ft (0.91 m) 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 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
Krononauts
The absence of time travelers from the future is a variation of the Fermi paradox.
As the absence of extraterrestrial visitors does not 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. Some versions of general relativity suggest that time travel might only be possible in a region of spacetime
that is warped a certain way, and hence time travelers would not be
able to travel back to earlier regions in spacetime, before this region
existed. Stephen Hawking stated that this would explain why the world has not already been overrun by "tourists from the future."
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 or MIT's Time Traveler Convention 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.
Forward time travel in physics
Time dilation
Transversal time dilation.
The blue dots represent a pulse of light. Each pair of dots with light
"bouncing" between them is a clock. For each group of clocks, the other
group appears to be ticking more slowly, because the moving clock's
light pulse has to travel a larger distance than the stationary clock's
light pulse. That is so, even though the clocks are identical and their
relative motion is perfectly reciprocal.
There is a great deal of observable evidence for time dilation in special relativity and gravitational time dilation in general relativity, for example in the famous and easy-to-replicate observation of atmospheric muon decay. The theory of relativity states that the speed of light is invariant for all observers in any frame of reference; that is, it is always the same. Time dilation is a direct consequence of the invariance of the speed of light.
Time dilation may be regarded in a limited sense as "time travel into
the future": a person may use time dilation so that a small amount of proper time passes for them, while a large amount of proper time passes elsewhere. This can be achieved by traveling at relativistic speeds or through the effects of gravity.
For two identical clocks moving relative to each other without
accelerating, each clock measures the other to be ticking slower. This
is possible due to the relativity of simultaneity.
However, the symmetry is broken if one clock accelerates, allowing for
less proper time to pass for one clock than the other. The twin paradox describes this: one twin remains on Earth, while the other undergoes acceleration to relativistic speed
as they travel into space, turn around, and travel back to Earth; the
traveling twin ages less than the twin who stayed on Earth, because of
the time dilation experienced during their acceleration. General
relativity treats the effects of acceleration and the effects of gravity
as equivalent, and shows that time dilation also occurs in gravity wells,
with a clock deeper in the well ticking more slowly; this effect is
taken into account when calibrating the clocks on the satellites of the Global Positioning System,
and it could lead to significant differences in rates of aging for
observers at different distances from a large gravity well such as a black hole.
A time machine that utilizes this principle might be, for
instance, a spherical shell with a diameter of 5 meters and the mass of Jupiter.
A person at its center will travel forward in time at a rate four times
that of distant observers. Squeezing the mass of a large planet into
such a small structure is not expected to be within humanity's
technological capabilities in the near future.
With current technologies, it is only possible to cause a human
traveler to age less than companions on Earth by a few milliseconds, the
current record being about 20 milliseconds for the cosmonaut Sergei Krikalev.
Philosophy
Philosophers have discussed the nature of time since at least the time of ancient Greece; for example, Parmenides presented the view that time is an illusion. Centuries later, Isaac Newton supported the idea of absolute time, while his contemporary Gottfried Wilhelm Leibniz
maintained that time is only a relation between events and it cannot be
expressed independently. The latter approach eventually gave rise to
the spacetime of relativity.
Presentism vs. eternalism
Many philosophers have argued that relativity implies eternalism, the idea that the past and future exist in a real sense, not only as changes that occurred or will occur to the present.
Philosopher of science Dean Rickles disagrees with some qualifications,
but notes that "the consensus among philosophers seems to be that
special and general relativity are incompatible with presentism."
Some philosophers view time as a dimension equal to spatial dimensions,
that future events are "already there" in the same sense different
places exist, and that there is no objective flow of time; however, this
view is disputed.
The bar and ring paradox is an example of the relativity of simultaneity.
Both ends of the bar pass through the ring simultaneously in the rest
frame of the ring (left), but the ends of the bar pass one after the
other in the rest frame of the bar (right).
Presentism
is a school of philosophy that holds that the future and the past exist
only as changes that occurred or will occur to the present, and they
have no real existence of their own. In this view, time travel is
impossible because there is no future or past to travel to.
Keller and Nelson have argued that even if past and future objects do
not exist, there can still be definite truths about past and future
events, and thus it is possible that a future truth about a time
traveler deciding to travel back to the present date could explain the
time traveler's actual appearance in the present; these views are contested by some authors.
Presentism in classical spacetime deems that only the present
exists; this is not reconcilable with special relativity, shown in the
following example: Alice and Bob are simultaneous observers of event O. For Alice, some event E is simultaneous with O, but for Bob, event E
is in the past or future. Therefore, Alice and Bob disagree about what
exists in the present, which contradicts classical presentism. "Here-now
presentism" attempts to reconcile this by only acknowledging the time
and space of a single point; this is unsatisfactory because objects
coming and going from the "here-now" alternate between real and unreal,
in addition to the lack of a privileged "here-now"
that would be the "real" present. "Relativized presentism" acknowledges
that there are infinite frames of reference, each of them has a
different set of simultaneous events, which makes it impossible to
distinguish a single "real" present, and hence either all events in time
are real—blurring the difference between presentism and eternalism—or
each frame of reference exists in its own reality. Options for
presentism in special relativity appear to be exhausted, but Gödel and
others suspect presentism may be valid for some forms of general
relativity. Generally, the idea of absolute time and space
is considered incompatible with general relativity; there is no
universal truth about the absolute position of events which occur at
different times, and thus no way to determine which point in space at
one time is at the universal "same position" at another time, and all coordinate systems are on equal footing as given by the principle of diffeomorphism invariance.
The grandfather paradox
A common objection to the idea of traveling back in time is put forth
in the grandfather paradox or the argument of auto-infanticide.
If one were able to go back in time, inconsistencies and contradictions
would ensue if the time traveler were to change anything; there is a
contradiction if the past becomes different from the way it is.
The paradox is commonly described with a person who travels to the past
and kills their own grandfather, prevents the existence of their father
or mother, and therefore their own existence.
Philosophers question whether these paradoxes make time travel
impossible. Some philosophers answer the paradoxes by arguing that it
might be the case that backward time travel could be possible but that
it would be impossible to actually change the past in any way, an idea similar to the proposed Novikov self-consistency principle in physics.
Ontological paradox
Compossibility
According to the philosophical theory of compossibility, what can
happen, for example in the context of time travel, must be weighed
against the context of everything relating to the situation. If the past
is a certain way, it's not possible for it to be any other way. What can happen when a time traveler visits the past is limited to what did happen, in order to prevent logical contradictions.
Self-consistency principle
The Novikov self-consistency principle, named after Igor Dmitrievich Novikov,
states that any actions taken by a time traveler or by an object that
travels back in time were part of history all along, and therefore it is
impossible for the time traveler to "change" history in any way. The
time traveler's actions may be the cause of events in their own past though, which leads to the potential for circular causation, sometimes called a predestination paradox, ontological paradox, or bootstrap paradox. The term bootstrap paradox was popularized by Robert A. Heinlein's story "By His Bootstraps".
The Novikov self-consistency principle proposes that the local laws of
physics in a region of spacetime containing time travelers cannot be any
different from the local laws of physics in any other region of
spacetime.
The philosopher Kelley L. Ross argues in "Time Travel Paradoxes"
that in a scenario involving a physical object whose world-line or
history forms a closed loop in time there can be a violation of the second law of thermodynamics. Ross uses "Somewhere in Time"
as an example of such an ontological paradox, where a watch is given to
a person, and 60 years later the same watch is brought back in time and
given to the same character. Ross states that entropy
of the watch will increase, and the watch carried back in time will be
more worn with each repetition of its history. The second law of
thermodynamics is understood by modern physicists to be a statistical
law, so decreasing entropy or non-increasing entropy
are not impossible, just improbable. Additionally, entropy
statistically increases in systems which are isolated, so non-isolated
systems, such as an object, that interact with the outside world, can
become less worn and decrease in entropy, and it's possible for an
object whose world-line forms a closed loop to be always in the same
condition in the same point of its history.
Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel where the past must be self-consistent.
In fiction
Time travel themes in science fiction
and the media can generally be grouped into three categories: immutable
timeline; mutable timeline; and alternate histories, as in the
interacting-many-worlds interpretation. Frequently in fiction, timeline
is used to refer to all physical events in history, so that in time
travel stories where events can be changed, the time traveler is
described as creating a new or altered timeline. This usage is distinct from the use of the term timeline to refer to a type of chart that illustrates a particular series of events, and the concept is also distinct from a world line, a term from Einstein's theory of relativity which refers to the entire history of a single object.
