Time travel is a concept that has long fascinated humanity -- whether it is Merlin experiencing time backwards, or Mohammed's alleged trip to Jerusalem and ascent to heaven returning before a glass knocked over had spilt its contents. Often nowadays it is a plot device used in science fiction to set a character in a particular time not his/her own, and explore the possible ramifications of the character's interaction with the people and technology of that time - a spin on the "country bumpkin comes to the big city" plot (or vice versa). It evolved to explore ideas of change, and reactions to it, and also to explore the ideas of a parallel universes or alternate history where some little event took place or didn't take place, but causes large changes in the future.
Famous fictional time machines include the TARDIS and H. G. Wells' The Time Machine. Wells' novel is meant to predict the likely future of humanity itself, starting with world wars and ending with humans reverting back to a Garden of Eden existence - with a terrifying twist.
In physics, the "thought experiment" of time travel has been often used to examine the consequences of physical theories such as special relativity, general relativity and quantum mechanics. There is no experimental evidence of time travel, and it is not even well understood whether (let alone how) the current physical theories permit any kind of time travel.
| Table of contents |
|
2 Time Travel Theme in Science Fiction 3 Time Travel, or Spacetime Travel? |
It should be noted that 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 time passes more slowly for faster-moving bodies: for example, a moving clock will run slow; as a clock approaches the speed of light its hands will nearly stop moving. However, this effect allows "time travel" only toward the future: only forward, never backward. It is not the most interesting kind, nor the kind typical of science fiction: hereafter "time travel" will refer to travel with some degree of freedom into the past or future.
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 can happen opens up the question of why and what physical laws exist to prevent time travel from occurring, and how they can be gotten around.
First of all, if one is able to move from one point to another faster than light, then according to Special Relativity time travel may be possible. Einstein predicted that the passage of time slows as one approaches the speed of light, and implies that if one were to pass the speed of light, then time would actually reverse. However, the very notion of "passing the speed of light" is all but a contradiction within Relativity theory.
The General Theory of Relativity extends the Special Theory to cover gravity.
It does this by postulating that matter "curves" the space in its vicinity. But under relativity, properties of space are fairly interchangeable with properties of time, depending on one's perspective, so that a curved path through space can wind up being a curved path through time. In moderate degrees, this allows two straight lines of different length to connect the same points in space; in extreme degees, theoretically, it could allow timelines to curve around in a circle and reconnect with their own past. GTR describes the universe under a complex sytem 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 Gödel), but nearly all of them require the universe to have physical characteristics that it does not appear to have.
A proposed time-travel machine using a wormhole would (hypothetically) work something like this:
A wormhole is created somehow.
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 hasn't been built yet, so the tourists from the future can't reach this far back in time.
Creating a wormhole of a size useful for macroscopic spacecraft, keeping it stable, and moving one end if 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', or 'negative 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 be ever constructed, even with highly advanced technology.
Another approach, developed by Frank Tipler, 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 doesn't seem to be possible to create a new cosmic string.
Physicist Robert Lull Forward has 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 points out that until we have a single theory combining relativity and quantum mechanics, we will have no idea whether such speculations are nonsense.
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.
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.
This misunderstanding seems to be widespread 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, and how the rules of quantum mechanics work to preserve causality is an active area of research.
Recent calculations by Kip S. Thorne indicate that simple masses passing through time travel wormholes could never engender paradoxes--there are no initial conditions that lead to paradox once time travel is introduced. If his results can be generalized they would suggest, curiously, that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, that any situation you can set up in a time travel story turns out to permit of many consistent solutions.
Things might, however, turn out to be almost unbelievably strange.
H. G. Wells' "The Time Machine" is considered the literary masterpiece of the genre. Mark Twain's A Connecticut Yankee in King Arthur's Court is another early time travel classic. Probably the most elaborate demonstration of supposed time travel paradoxes is Robert Anson Heinlein's "All You Zombies."
Time travel themes in science fiction can generally be grouped into two types (based on effect--methods are extremely varied and numerous), each of which is further subdivided.
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 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 PM, then no bits for thirty seconds after that. If you send a bit back to 10:00:00 PM, everything works fine. However, if you try to send a bit to 10:00:15 PM (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 PM. Etc, etc. An excellent example of this kind of universe is found in Timemaster, a novel by Dr. Robert Forward.
In a universe that allows retrograde time travel but no paradoxes, any present moment is the past for a future observer, thus all history/events are fixed. History can be thought of as a filmstrip where everything is already fixed.
See block time for a detailed examination of this way of considering the nature of time.
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 traveler or information sent simply having vanished, never to return. A difficulty with this explanation, however, is that conservation of mass-energy would be violated, unless the mechanics of time travel required that mass-energy be exchanged in precise balance between past and future at the moment of travel. An example of this kind of time travel can be found in David Gerrold's The Man Who Folded Himself.
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 do all 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. However, you could infer that you were by knowing that
a) communication with the past was possible and 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 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.
In The Restaurant at the End of the Universe Douglas Adams does not see a big problem in becoming his own father, since this is nothing a well adjusted family can't deal with. The big problem is grammar - the tense formation for time travellers.
Robert Heinlein's story All You Zombies shows the possible results of taking this concept to its logical conclusion ad absurdam: the time travelling protagonist is/was/becomes his/her own father, son, mother and daughter.
In many science fiction books about time travel, there is a physical machine for transporting people through time but there is a minority which involve time travel through mental disciplines. Jack Finney's Time And Again is one such book. Poul Anderson's There Will Be Time portrays time travel as an ability some are born with. Some people affiliated with the UFO movement say that the ability to time-travel lies latent in everybody's brain, and that that ability is "turned on" in the minds of the Greys, who supposedly have the ability to unlock it in human brains too. Other people believe that both time travel and teleportation can be learned through practice in a similar manner.
In 1992 Harry Turtledove published the novel The Guns of the South which became popular with its story about South African white supremacists using a time travel machine to go back to the days of the American Civil War and equip the dispirited rebel army with 20th century weapons such as the AK-47. They soon win every battle and gleefully march into Washington, D.C. to capture Lincoln. The limits of his time travel machine are ludicrous, however, because it can only take people back a set number of years. This allows him to prevent the white supremacists from making another trip to cure the ills of the first, which (ahem) goes wrong at the end.
It can be argued that the Book of Revelation, describes a form of "spiritual time travel". In contrast to most science fiction conceptualizations of time travel, the Revelation states that John (while on the Greek island of Patmos) had a vision that took him, in spirit, to the future end times in world history and that future events were revealed to him by an angel sent by Jesus Christ.
The classic problem with the concept of "time travel ships" in science fiction is that it invariably treats the earth like it is stationary in absolute space. The idea that you can go into a machine that sends you to "1865 A.D.", and you exit from a door that leaves you in the same spot in Poughkeepsie that the time machine was when you entered it ignores the issue that the earth is moving through space around the sun, which is moving in the galaxy, etc. So, if you think of spacetime as 4 dimensions, and "time travel" is just "moving" along one of them, you couldn't stay in the same place with respect to the surface of the earth, because the earth is a moving platform with a highly complicated trajectory! If you only moved "ahead" 5 seconds, you might materialize in the air, or inside solid rock, depending on where the earth was "before" and "after". If you moved "behind" one year, you'd end up in cold outer space, where the earth was a year earlier - in the same part of the sun's orbit, yes, but where has the sun gone over that year? So, to really do what they make look so easy in films like Back to the Future and The Time Machine, your time machine would have to be a very powerful spaceship that could move you large distances and that kept track of the earth's motion through space as part of the solar system, galaxy, etc. But how can you decouple the ship from momentum? If you try to move forward in time, is your ship automatically going to be propelled by the momentum gained by riding the earth? Or does it decouple? But doesn't that bring back the idea of absolute space? Again, even to move one millisecond forward or backward in time, the ship would have to be far beyond anything humans can build, not to mention the acceleration-deceleration problems and what that might do to your blood pressure. You might even use this to argue, Zeno-style, for the impossibility of time machines. In 1980 Robert Heinlein published an amusing novel The Number of the Beast about a ship that lets you dial-in the 6 (not 4!) coordinates of space and time and it instantly moves you there - without explaining how such a device might work.
Scientific references include:
Literary references:
See also: Anachronism and time travel, anachronism, grandfather paradox.Physics
Time Travel Theme in Science Fiction
Time Travel in a type 1 universe does not allow any paradoxes, although in 1.3, events can appear to be paradoxical.Time Travel, or Spacetime Travel?
Paul Davies, About Time
Paul J. Nahin, Time Machines
Clifford A. Pickover, Time: A Traveler's Guide
The Time Machine (full text), by H. G. Wells
A Connecticut Yankee in King Arthur's Court (full text), by Mark Twain