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Time travel is the concept of moving
backward or forward to different points in time, in a manner
analogous to moving through space. Additionally, some interpretations
of time travel suggest the possibility of travel between parallel
realities or universes.
Time travel in theory
Some theories, most notably special and general
relativity, suggest that suitable geometries of spacetime,
or certain types of motion in space, may allow time travel
into the past and future if these geometries or motions are
possible. Concepts that aid such understanding include the
closed timelike curve.
Albert Einstein's special theory of relativity
(and, by extension, the general theory) very explicitly permits
a kind of time dilation that would ordinarily be called time
travel. The theory holds that, relative to a stationary observer,
time appears to pass more slowly for faster-moving bodies:
for example, a moving clock will appear to run slow; as a
clock approaches the speed of light its hands will appear
to nearly stop moving. The effects of this sort of time dilation
are discussed further in the popular "twin paradox".
A second, similar type of time travel is permitted
by general relativity, where a distant observer sees time
passing more slowly for a clock at the bottom of a deep gravity
well, and a clock lowered into a deep gravity well and pulled
back up will indicate that less time has passed compared to
a stationary clock that stayed with the distant observer.
These effects are to some degree similar to
hibernation or hypothetical suspended animation (which slow
down the rates of chemical processes in the subject), and
only allow "time travel" toward the future: never
backward. They do not violate causality. This is not typical
of the "time travel" featured in science fiction
(where causality is violated at will), and there is little
doubt surrounding its existence. "Time travel" will
hereafter refer to travel with some degree of freedom into
the past or future of proper time.
Many in the scientific community believe that
time travel is highly unlikely. This belief is largely due
to Occam's Razor. Any theory which would allow time travel
would require that issues of causality be resolved. What happens
if you try to go back in time and kill your grandfather?—see
grandfather paradox. Also, in the absence of any experimental
evidence that time travel exists, it is theoretically simpler
to assume that it does not happen. Indeed, Stephen Hawking
once suggested that the absence of tourists from the future
constitutes a strong argument against the existence of time
travel—a variant of the Fermi paradox, with time travelers
instead of alien visitors. However, assuming that time travel
cannot happen is also interesting to physicists because it
opens up the question of why and what physical laws exist
to prevent time travel from occurring.
The "presentist" view
Some theorists have argued that the matter
of the universe only exists in the present moment. Thus, if
one were to travel back from the 'present' to an earlier time,
none of the material universe would be found there, because
it will have remained in the present: the traveller alone
is the only part of the universe to have gone back to the
earlier time. In terms of a 4-dimensional spacetime, the traveller
(or, more generally the atomic particles that comprise the
traveller) would have travelled 'back' to an area of spacetime
corresponding to an earlier value of 't'; but none of the
other particles that form the universe will have done so,
so the traveller finds precisely nothing when arriving back
at the earlier time. This viewpoint eliminates all of the
supposed paradoxes about time travel.
The equivalence of time travel and faster-than-light travel
If one were able to move information or matter
from one point to another faster than light, then according
to special relativity, there would be an observer who sees
this transfer as allowing information or matter to travel
into the past. Additionally, faster than light travel along
suitable paths would correspond to travel backward in time
as seen by all observers. This results simply from the geometry
of spacetime and the role of the speed of light in that geometry.
Special spacetime geometries
The general theory of relativity extends the
special theory to cover gravity, describing it in terms of
curvature in spacetime caused by mass-energy and the flow
of momentum. General relativity describes the universe under
a system of "field equations," and there exist solutions
to these equations that permit what are called "closed
time-like curves," and hence time travel into the past.
The first and most famous of these was proposed by Kurt Godel,
but all known current examples require 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. Most physicists believe
that it does, largely because assuming some principle against
time travel prevents paradoxical situations from occurring.
Using wormholes
A proposed time-travel machine using a wormhole
would (hypothetically) work something like this: A wormhole
is created somehow (one theory is from the time-space dilation
of atomic particles under the heat and sub-pressure akin to
that of an exploding sun). One end of the wormhole is accelerated
to nearly the speed of light, perhaps with an advanced spaceship,
and then brought back to the point of origin. Due to time
dilation, the accelerated end of the wormhole has now experienced
less subjective passage of time than the stationary end. An
object that goes into the stationary end would come out of
the other end in the past relative to the time when it enters.
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'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, creating a wormhole of a size useful for a person
or spacecraft, keeping it stable, and moving one end of it
around would require significant energy, many orders of magnitude
more than the Sun can produce in its lifetime. Construction
of a wormhole would also require the existence of a substance
known as "exotic matter", which, while not known
to be impossible, is also not known to exist in forms useful
for wormhole construction (but see for example the Casimir
effect). Therefore it is unlikely such a device will ever
be constructed, even with highly advanced technology. On the
other hand, microscopic wormholes could still be useful for
sending information back in time.
Matt Visser argued in 1993 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. [1] Because of
this, the two mouths could not be brought close enough for
causality violation to take place. However, in a 1997 paper,
Visser hypothesized that a complex "Roman ring"
(named after Tom Roman) configuration of an N number of wormholes
arranged in a symmetric polygon could still act as a time
machine, although he concludes that this is more likely than
not a flaw in classical quantum gravity theory rather than
proof that causality violation is possible. [2]
Another approach — attributed to Frank
Tipler, but invented independently by Willem Jacob van Stockum
[3] in 1936 and Kornel Lanczos [4] in 1924 — involves
a spinning cylinder. If a cylinder is long, and dense, and
spins fast enough about its long axis, then a spaceship flying
around the cylinder on a spiral path could travel back in
time (or forward, depending on the direction of its spiral).
However, the density and speed required is so great that ordinary
matter is not strong enough to construct it. A similar device
might be built from a cosmic string, but none are known to
exist, and it does not seem to be possible to create a new
cosmic string.
Physicist Robert Forward noted that a naive
application of general relativity to quantum mechanics suggests
another way to build a time machine. A heavy atomic nucleus
in a strong magnetic field would elongate into a cylinder,
whose density and "spin" are enough to build a time
machine. Gamma rays projected at it might allow information
(not matter) to be sent back in time. However, he pointed
out that until we have a single theory combining relativity
and quantum mechanics, we will have no idea whether such speculations
are nonsense.
Using Quantum Entanglement
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 presumes
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 rules of quantum mechanics
curiously appear to prevent an outsider from using these methods
to actually transmit useful information, and therefore do
not appear to allow for time travel or FTL communication.
The fact that these quantum phenomena apparently do not allow
FTL/time travel is often overlooked in popular press coverage
of quantum teleportation experiments. The assumption that
time travel or superluminal communications is impossible allows
one to derive interesting results such as the no cloning theorem.
How the rules of quantum mechanics work to preserve causality
is an active area of research.
The possibility of paradoxes
The Novikov self-consistency principle 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 generalised, 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 many-worlds interpretation 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.
In a 2005 paper, Professor Daniel Greenberger
of City University of New York and Karl Svozil of the Vienna
University of Technology proposed that quantum theory gives
a model for time travel without paradoxes. [5] 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.
Since all possibilities exist, any paradoxes
can be explained by having the paradoxical events happening
in a different universe. This concept is most often used in
science-fiction. However, in actuality, physicists believe
that such interaction or interference between these histories
is not possible (see Chronology protection conjecture).
A further suggestion related to paradoxes
suggests that time travel will never exist, even if theoretically
possible. The reasoning is that as long as time travel exists,
history will change, and will only become static when a timeline
is reached in which no time travel exists and thus no further
changes can be made. Assuming there is only a single dimension
of time, the timeline we perceive must be the one that exists
after all changes (if any) are made, and thus we will never
perceive the invention of time travel, since it will have
already destabilised itself out of the timeline by the time
we would have reached it.
Time travel and the direction of time
The notion of time travel (either toward the
future or toward the past) tacitly assumes that there exists
a direction of time, the direction from the past to the future.
On the other hand, the direction of time (or the arrow of
time) may not be a fundamental intrinsic property of time,
but rather could be viewed as an emergent property traceable
to the fact that we live in a universe in which the entropy
increases with time. In this view, as the direction of time
is not fundamental, the notion of time travel is also not
fundamental. Without a fundamental notion of time travel there
can be no fundamental problems with time travel. Without an
intrinsic direction of time, time can be viewed as a "static"
coordinate similar to other spacetime coordinates. From this
point of view, the Novikov self-consistency principle is a
tautology, a demand that hardly needs to be questioned, which
automatically prevents causal paradoxes.
Time travel and the anthropic principle
It has been suggested by physicists such as
Max Tegmark that the absence of time travel and the existence
of causality may be due to the anthropic principle. The argument
is that a universe which allows for time travel and closed
time-like loops is one in which intelligence could not evolve
because it would be impossible for a being to sort events
into a past and future or to make predictions or comprehend
the world around them.
Time travel in fiction
Types of time travel
Time travel themes in science fiction and
the media can generally be grouped into two main types and
a third, less common type (based on effect—methods are
extremely varied and numerous), each of which is further subdivided.
These type classifications do not address the issue of time
travel itself, i.e. how to travel through time, but instead
call to attention differing rules of the time line.
1. The time line is consistent and can never
be changed.
1.1 One does not have full control of the time travel. One
example of this is The Morphail Effect. This concept of time
can be referred to as circular causation.
1.2 The Novikov self-consistency principle applies (named
after Dr. Igor Dmitrievich Novikov, Professor of Astrophysics
at Copenhagen University). The principle states that if you
travel in time, you cannot act in such a way so as to create
a paradox.
1.3 Any event that appears to have changed a time line has
instead created a new one.
1.3.1 Such an event can be the life line existence of a human
(or other intelligence) such that manipulation of history
ends up with there being more than one of the same individual,
sometimes called time clones.
1.3.2 The new time line may be a copy of the old one with
changes caused by the time traveler. For example there is
the Accumulative Audience Paradox where multitudes of time
traveler tourists wish to attend some event in the life of
Jesus or some other historical figure, where history tells
us there were no such multitudes. Each tourist arrives in
a reality that is a copy of the original with the added people,
and no way for the tourist to travel back to the original
time line.
2. The time line is flexible and is subject to change.
2.1 The time line is extremely change resistant and requires
great effort to change it. Small changes will only alter the
immediate future and events will conspire to maintain constant
events in the far future; only large changes will alter events
in the distant future.
2.2 The time line is easily changed (example: Doctor Who,
where the time line is fluid and changes often naturally).
3. The time line is consistent, but only insofar as its consistency
can be verified.
3.1 The Novikov self-consistency principle applies, but if
and only if it is verified to apply. Attempts to travel into
the past to change events are possible, but provided that:
-They do not interfere with the occurrence of such an attempt
in the present (as would be the case in the Grandfather Paradox),
and
-The change is never ultimately verified to occur by the traveller
(e.g. there is no possibility of returning to the present
to witness the change).
There are also numerous science fiction stories allegedly
about time travel that are not internally consistent, where
the traveler makes all kinds of changes to some historical
time, but we do not get to see any consequences of this in
our present day.
Immutable timelines
Time travel in a type 1 universe does not
allow any paradoxes, although in 1.3, events can appear to
be paradoxical.
In 1.1, time travel is constrained to prevent
paradox. If one attempts to make a paradox, one undergoes
involuntary or uncontrolled time travel. Michael Moorcock
uses a form of this principle and calls it The Morphail Effect.
In the time-travel stories of Connie Willis, time travelers
encounter "slippage" which prevents them from either
reaching the intended time or translates them a sufficient
distance from their destination at the intended time, as to
prevent any paradox from occurring.
Example: A man who travels into the past and
intends to shoot his grandfather as a young boy finds himself
snapped back to the present as he's about to pull the trigger.
In 1.2, the Novikov self-consistency principle asserts that
the existence of a method of time travel constrains events
to remain self-consistent (i.e. no paradoxes). This will cause
any attempt to violate such consistency to fail, even if extremely
improbable events are required.
Example: You have a device that can send a
single bit of information back to itself at a precise moment
in time. You receive a bit at 10:00:00 p.m., then no bits
for thirty seconds after that. If you send a bit back to 10:00:00
p.m., everything works fine. However, if you try to send a
bit to 10:00:15 p.m. (a time at which no bit was received),
your transmitter will mysteriously fail. Or your dog will
distract you for fifteen seconds. Or your transmitter will
appear to work, but as it turns out your receiver failed at
exactly 10:00:15 p.m.. Etc, etc. Two excellent examples of
this kind of universe is found in Timemaster, a novel by Dr.
Robert Forward, and the 1980 Jeannot Szwarc film Somewhere
In Time (based on Richard Matheson's novel Bid Time Return).
An example which could conceivably fall into either 1.1 or
1.2 can be seen in book and film versions of Harry Potter
and the Prisoner of Azkaban. Harry and Hermione go back in
time to change history. As they do so it becomes apparent
that they are simply performing actions that were previously
seen in the story, although neither the characters nor the
reader were aware of the causes of those actions at the time.
This is another example of the predestination paradox. It
is arguable, however, that the mechanics of time travel actually
prevented any paradoxes, firstly, by preventing them from
realizing a priori that time travel was occurring and secondly,
by enabling them to recall the precise action to take at the
precise time and keep history consistent.
In 1.3, any event that appears to have caused a paradox has
instead created a new time line. The old time line remains
unchanged, with the time traveller or information sent simply
having vanished, never to return. A difficulty with this explanation,
however, is that conservation of mass-energy would be violated
for the origin timeline and the destination timeline. A possible
solution to this is to have the mechanics of time travel require
that mass-energy be exchanged in precise balance between past
and future at the moment of travel, or to simply expand the
scope of the conservation law to encompass all timelines.
Some examples of this kind of time travel can be found in
David Gerrold's book The Man Who Folded Himself, the Robert
Zemeckis film Back to the Future Part II (1989), The Time
Ships by Stephen Baxter and the (1994) film Star Trek: Generations.
Spoiler warning: Plot and/or ending details
follow.
Example: In Back to the Future Part II, Marty McFly and Doc
Brown decide (after Doc returns from the 21st century to 1985)
to travel to 2015 to save McFly's future son. While there,
McFly buys an almanac of sporting events from 1950-2000, hoping
to use it for financial gain. The book is stolen by the aged
Biff Tannen, who takes the time-traveling DeLorean back in
time to give the almanac to his 1955 younger self. When McFly
and Doc Brown use the DeLorean to go back to 1985, they soon
discover what Tannen had done; the younger Tannen has used
the almanac for financial gain and changed the timeline. The
alternate 1985 that McFly and Brown have returned to is now
the future of a tangent that started in 1955, where Hill Valley
is now corrupt and its citizens' lives changed because of
Tannen.
Spoilers end here.
Mutable timelines
Time travel in a Type 2 universe is much more
difficult to explain. The biggest problem is how to explain
changes in the past. One method of explanation is that once
the past changes, so too do the memories of all observers.
This would mean that no observer would ever observe the changing
of the past (because they will not remember changing the past).
This would make it hard to tell whether you are in a Type
1 universe or a Type 2 universe. You could, however, infer
such information by knowing if a) communication with the past
were possible or b) it appeared that the time line had never
been changed as a result of an action someone remembers taking,
although evidence exists that other people are changing their
time lines fairly often. An example of this kind of universe
is presented in Thrice Upon a Time, a novel by James P. Hogan.
Larry Niven suggests that in a type 2.1 universe,
the most efficient way for the universe to "correct"
a change is for time travel to never be discovered, and that
in a type 2.2 universe, the very large (or infinite) number
of time travelers from the endless future will cause the timeline
to change wildly until it reaches a history in which time
travel is never discovered. However, many other "stable"
situations may also exist in which time travel occurs but
no paradoxes are created; if the changeable-timeline universe
finds itself in such a state no further changes will occur,
and to the inhabitants of the universe it will appear identical
to the type 1.2 scenario. This is sometimes referred to as
the "Time Dillution Effect."
Gradual and instantaneous
In literature, there are two (commonly used)
methods of time travel:
1. The most commonly used method of time travel
in science fiction is the instantaneous movement from one
point in time to another, like using the controls on a CD
player to skip to a previous or next song. There is not even
the beginning of a scientific explanation for this kind of
time travel; it's popular probably because it is more spectacular
and makes time travel easier.
2. In The Time Machine, H.G. Wells explains
that we are moving through time with a constant speed. Time
travel then is, in Wells' words, "stopping or accelerating
one's drift along the time-dimension, or even turning about
and traveling the other way." To expand on the audio
playback analogy used above, this would be like rewinding
or fast forwarding an analogue audio cassette and playing
the tape at a chosen point. This method of gradual time travel
fits best in quantum physics, but is not popular in modern
science fiction. Perhaps the oldest example of this method
of time travel is in Lewis Carroll's Through the Looking-Glass
(1871): the White Queen is living backwards, hence her memory
is working both ways. Her kind of time travel is uncontrolled:
she moves through time with a constant speed of –1 and
she cannot change it. This would make Lewis Carroll the inventor
of time travel. T.H. White, in the first part of his Arthurian
novel The Once and Future King, The Sword in the Stone (1938)
used the same idea: the wizard Merlyn lives back in time,
because he was born "at the wrong end of time" and
has to live backwards from in front. "Some people call
it having second sight".
Time travel, or space-time travel?
The classic problem with the concept of "time
travel ships" in science fiction is that it invariably
treats Earth as the frame of reference in space. The idea that
a traveller can go into a machine that sends you to "A.D.
1865" and leave through a door into the same spot in Poughkeepsie
ignores the issue that Earth is moving through space around
the Sun, which is moving in the galaxy, etc. So, given space-time
as four dimensions, and "time travel" referring to
just "moving" along one of them, a traveller could
not stay in the same place with respect to the surface of Earth,
because Earth is a moving platform with a highly complicated
trajectory. A vessel that moves "ahead" 5 seconds
might materialize in the air, or inside solid rock, depending
on where Earth was "before" and "after."
In the 2000 AD comic Mutant Bounty Hunter, Johnny Alpha uses
"Time Bombs" to propel an enemy several seconds into
the future, during which time the movement of the Earth causes
the unfortunate victim to re-materialize in space. To really
do what filmmakers make look so easy in films such as the Back
to the Future series and The Time Machine, the device might
have to be a very powerful spacecraft which could move across
large distances in space to compensate for the offset of position
associated with the change in time.
A possible rebuttal to this criticism is the
fact that cars and airplanes manage to move around the surface
of the Earth with it, despite the surface itself moving with
an astronomical speed. One could postulate that a time traveller
experiences a combination of spatial temporal inertia that
makes him move along with the Earth.
In the 1980 Robert Heinlein novel The Number
of the Beast a "continua device" allows the protagonists
to dial in the six (not four!) co-ordinates of space and time
and it instantly moves them there—without explaining
how such a device might work. The television series Seven
Days also dealt with this problem; when the chrononaut would
be 'rewinding', he would also be propelling himself backwards
along the earth's orbit, with the intention of landing in
the same place (in space) that he originated.
References
Popular:
Paul Davies, How to Build a Time Machine
ISBN 0142001864
Clifford A. Pickover, Time: A Traveler's Guide ISBN
0195130960
John Gribbin, In Search of Schrodinger's Cat
Paul J. Nahin, Time Machines: Time Travel in Physics,
Metaphysics, and Science Fiction ISBN 0387985719
Heinz Pagels, Perfect Symmetry, the Search for the
Beginning of Time
Physics:
Kornel Lanczos, On a Stationary Cosmology
in the Sense of Einsteins Theory of Gravitation, 1924, republished
in 1997 by Springer Science+Business Media B.V.
Willem Jacob van Stockum, The Gravitational
Field of a Distribution of Particles Rotating about an Axis
of Symmetry, 1936, Proceedings of the Royal Society of Edinburgh.
Frank J. Tipler, Rotating Cylinders and the Possibility
of Global Causality Violation, Physical Review D 9 (1974),
2203
Graham M. Shore, Constructing Time Machines, Invited
review article prepared for Int. J. Mod. Phys. A, Theoretical
Physics Preprint: http://xxx.arxiv.org/PS_cache/gr-qc/pdf/0210/0210048.pdf
H. Nikolic, Causal paradoxes: a conflict between relativity
and the arrow of time
Paul Davies, About Time ISBN 0684818221
J. Richard Gott, Time Travel in Einstein's Universe:
The Physical Possibilities of Travel Through Time ISBN 0618257357
Philosophy:
Richard M Gale, The Philosophy of Time
Miller, Kristie. Time travel and the open future. Disputatio
Vol 1. Issue 19 (2005): 223-232.
Bizzare:
Fred Alan Wolf, The Yoga of Time Travel
(2004) ISBN 083560828X
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