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, in the form of a vehicle or of a portal connecting distant
points in spacetime, either to an earlier time or to a later time,
without the need for the time-traveling body to experience the
intervening period in the usual sense.
Time travel is a
widely-recognized concept in philosophy and fiction. It was
popularized by H. G. Wells' 1895 novel The
Time Machine, which moved
the concept of time travel into the public imagination. However, 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
possible according to special relativity and general relativity,
although 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
1.1 Shift to science fiction
Time travel in physics
2.1 General relativity
2.1.1 Different spacetime geometries
2.1.3 Other approaches based on general relativity
2.2 Quantum physics
2.2.1 No-communication theorem
2.2.2 Interacting many-worlds interpretation
2.3 Experimental results
2.4 Absence of time travelers from the future
3 Forward time travel in physics
4.1 Presentism vs. eternalism
4.2 The grandfather paradox
4.3 Ontological paradox
4.3.2 Self-consistency principle
Time travel in fiction
6 See also
8 External links
History of the time travel concept
Rip Van Winkle
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.
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 Taro 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
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 he is who he claims to be.
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
Rip Van Winkle (1819) by
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
The earliest work about backwards time travel is uncertain. Samuel
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.:95–96 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.":85 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.":95–96 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.:3 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.:11–38
Another early work about time travel is The Forebears of Kalimeros:
Alexander, son of Philip of Macedon by
Alexander Veltman published in
Mr. and Mrs. Fezziwig dance in a vision shown to Scrooge by the Ghost
of Christmas Past.
A Christmas Carol
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 itself 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.:54
Early time machines
One of the first stories to feature time travel by means of a machine
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.:55 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
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.:499 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
Novikov self-consistency principle or to a
variation of the many-worlds interpretation with interacting
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.:33–130 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 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
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 which is akin to 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,:499
such as rotation and lack of Hubble expansion. Whether general
relativity forbids closed time-like curves for all realistic
conditions is still being researched.
Main article: Wormhole
Wormholes are a hypothetical warped spacetime which are permitted by
Einstein field equations
Einstein field equations of general relativity.:100 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.:502 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;:503 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,:101 and many physicists
believe that the required negative energy may actually be possible due
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.
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:21 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.
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,:169 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.":96 This result comes from
Hawking's 1992 paper on the chronology protection conjecture, where he
examines "the case that the causality violations appear in a finite
region of spacetime without curvature singularities" and proves that
"[t]here will be a
Cauchy horizon that is compactly generated and that
in general contains one or more closed null geodesics which will be
incomplete. One can define geometrical quantities that measure the
Lorentz boost and area increase on going round these closed null
geodesics. If the causality violation developed from a noncompact
initial surface, the averaged weak energy condition must be violated
on the Cauchy horizon." 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 mechanics of time travel
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
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.
Daniel Greenberger and
Karl Svozil proposed that quantum theory gives
a model for time travel without paradoxes. The 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.
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
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.
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
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
The absence of time travelers from the future is a variation of the
Fermi paradox, and like the absence of extraterrestrial visitors, the
absence of time travelers does not 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
Day (2005) or MIT's
Time Traveler Convention heavily
publicized permanent "advertisements" of a meeting time and place for
future time travelers to meet. Back 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
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
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
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
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.:33–130
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.:76–140 With
current technologies, it is only possible to cause a human traveler to
age less than companions on Earth by a very small fraction of a
second, the current record being about 20 milliseconds for the
cosmonaut Sergei Avdeyev.
Main article: Philosophy of space and time
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, Newton supported the idea of
absolute time, while his contemporary 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
Main article: 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.
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.
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".
Novikov self-consistency principle
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
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
Time travel in fiction
Time travel 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.
Claims of time travel
Time travel claims and urban legends
Time travel in fiction
List of time travel works of fiction
List of games containing time travel
List of television series that include time travel
Wheeler–Feynman absorber theory
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Wikimedia Commons has media related to
Look up time travel in Wiktionary, the free dictionary.
Black holes, Wormholes and
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Time Travel and Modern Physics at the Stanford Encyclopedia of
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Chronology protection conjecture
Closed timelike curve
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Quantum mechanics of time travel
Time travel in fiction
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