Time travel

Time travel started off as a plot device used in science fiction to set a story in a particular time, and explore the possible ramifications of the character's interaction with the people and technology of that time. It is also used to explore ideas of change, and reactions to it, also to explore the ideas of Parallel Universes where some little event took place or didn't take place. Famous fictional time machines include the Tardis and H G Wells's Time Machine. In physics, it is useful to study time travel to the past and other consequences of physics theories that push the envelope (such as the possibilty of us living in a Multiverse), as it may force a rethinking of them. It would be unlikely that we would be able to travel backwards in time, only forwards.

Physics

Currently held beliefs in the scientific community espouse 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 (i.e. what happens if you try to go back in time and kill your grandfather; see Grandfather paradox), and in the absence of any experimental evidence that time travel exists, it is theoretically simpler to assume that it does not happen. In fact, Stephen Hawking once suggested that the absence of tourists from the future constitutes a strong argument against the existence of time travel.

There exist solutions to Albert Einstein's general theory of relativity that permit time travel (most famously that of Kurt Goedel), however some of these solutions require the universe to have physical characteristics that it does not appear to have. If one is able to move from one point to another faster than light, then according to relativity time travel may be possible.

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 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 to time travel 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. 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 Forward has noted that a naive application of general relativity to quantum mechanics suggests another way to build a time machine. A proton 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 phenomenon 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 instantaneous in order to maintain coorelations between particles.

Nevertheless, the rules of quantum mechanics curiously appear to prevent an outsiders 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.

Einstein's special theory of relativity allows travel forward in time by means of time dilation (simply accelerate close to the speed of light, or spend time in a deep gravity well); but once one has gone forward in time one cannot go back. As a body approaches the speed of light, relative time speeds. This would lead one to posit that exceeding the speed of light would result in a reversal of time. While this is mathematically sound, as one approaches the speed of light, exponentially more force must be applied to accelerate, until finally an infinite amount of energy must be applied to reach the speed of light exactly. The only people who suggest that we must only develop technology to exceed the speed of light to develop time travel are people who do not understand the concept of mathematical limits.

Time Travel Theme in Science Fiction.

Where can I even start to describe time travel themes in Science Fiction Literature?

Basically you can group the science fiction treatment of time travel (based on effect - methods are extremely varied and numerous) into two types, each of which is further subdivided.

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.

1.2 The Novikov self-consistency principle applies. (named after Dr. Igor D. Novikov, Professor of Astrophysics at Copenhagen University)

1.3 Any event that appears to have changed a time line has instead created a new one.

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.

2.2 The time line is easily changed.

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 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 L. 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.

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.

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.

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 portray's time travel as an ability some are born with.