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Stephenson:Neal:Quicksilver:20:…most disorderly…(Alan Sinder)

From the Quicksilver Metaweb.

This is the Quicksilver page for Time Travel — a science fiction favorite.

Stephensonia

The long lived Enoch Root tells Godfrey and Ben that the way he accesses his memory lacks tidyness. This may be a “hint” to his being “unstuck in time” somehow.**

Authored entries

Enoch's possible literary precedents

Billy Pilgrim comes to mind. As does “The Time Tunnel” — the story of Dr. Tony Newman (James Darren) and Dr. Doug Phillips (Robert Colbert), two research scientists who developed the top secret Time Tunnel - only to become lost, tumbling among the infinite corridors of time. And then there's Doctor Who. Americans may be fonder of Marty McFly and Dr. Emmett Brown though. Don't ask me about the Quantum Butterfly though.

Time Travel

From various Wikipedia sources.

Time travel is a concept that has long fascinated humanity -- whether it is Merlin experiencing time backwards, or Christ, descended from the Kingdom of God, which exists out-of-time-and-space, to incarnate a human body, 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 Whovian 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.MWTimeMacCI133.jpg
THE TIME MACHINE

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.

Physics

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 cannot happen is also interesting to physicists before it opens up the question of why and what physical laws exist to prevent time travel from occurring.

The equivalence of time travel and faster-than-light travel

First of all, if one is able to move from information from one point to another faster than light, then according to special relativity, there will be an observer who sees this information transfer as allowing information to travel into the past.

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.

Using wormholes

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.

Using massive spinning cylinders

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.

Using Spooky Action at a Distance

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. 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.

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 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.

Time travel in science fiction

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."

Types of time travel

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. 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 Dmitrievich 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.

1. 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 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 for the origin timeline and the destination timeline. A possible solution to this is to have the mechanics of time travel required 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. An example of this kind of time travel can be found in David Gerrold 's The Man Who Folded Himself.

2. 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 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.

Other approaches to and examples of time travel

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. The Butterfly Effect (Eric Bress and J. Mackye Gruber) displays time travel as an inherited talent, where one's mind and/or spirit travels back into its past, able to change its current present when it returns. 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".

Time travel, or spacetime travel?

The classic problem with the concept of "time travel ships" in science fiction is that it invariably treats Earth as if 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 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 Earth, because 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 Earth was "before" and "after". If you moved "behind" one year, you'd end up in cold outer space, where 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 might have to be a very powerful spaceship that could move you large distances and that kept track of 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 Earth? Or does it decouple? But doesn't that bring back the idea of an absolute reference frame? 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.

"Distance" of time travel

According special relativity, the physical laws may be invariant over Lorentz transformations. This mixes time and space dimensions. In Lorentz-transformable-space-thingies, time can be compared to a distance times the speed of light. So, the second is comparable to a unit of distance equal to 299,792.458 km. Conversely, the distance of 1 m is comparable to about 3,34 nanoseconds. (You can also compare a "year" to a "light-year"). (Since the square of a distance has the opposite sign to the square of a time, time and space aren't actually identical.)

Now, if we suppose that the same distances in space and time present the same level of technical difficulty, then moving in time for just one second, forward or backward, would be like flying to the Moon. Moving for a few years would be like flying to some of the nearest stars. And if you want to go visiting dinosaurs, perhaps it would be like flying to a far-off galaxy. On the basis of the above argument, some people think that time travel will require a lot of http://en.wikipedia.org/wiki/Energy energy] (unless we use something like teleportation).

One may even say that the past is not accessible to us simply because it is too far away. And then we've the — Antikythera mechanism.

Antikythera mechanism

The Antikythera mechanism is an ancient artifact believed to be an early clockwork mechanism. It was discovered in a shipwreck off the Greek island of Antikythera, between Kythera and Crete, and has been dated to about 87 BCE. The wreck was discovered in 1900 at a depth of about 40m (140 feet), and many statues and other works were retrieved from it by sponge divers. On May 17, 1902 archaeologist Spyridon Stais noticed that one of the pieces of rock had a gear wheel embedded in it.

The mechanism is the oldest surviving geared mechanism, made from bronze in a wooden frame, and has puzzled and intrigued historians of science and technology since its discovery. The most commonly accepted theory of its function is that it was an analog computer designed to track the movements of heavenly objects. Recent working reconstructions of the device support this analysis. The device is all the more impressive for its use of a differential gear, which was previously believed to have been invented in the 13th century CE.

The late Professor Derek De Solla Price, a science historian working at Yale University, published an article on the mechanism in Scientific American in June 1959 while the device was still only partially inspected. In 1973 or 1974 he published an analysis based on gamma ray imaging by Greek archaeologists. He claimed that the device had been built by a Greek astronomer, Geminus of Rhodes. His conclusion was not accepted by experts at the time, who believed that the Ancient Greeks had the theoretical knowledge but not the necessary practical skills.

A partial reconstruction was built by Australian computer scientist Allan George Bromley (1947 - 2002) of the University of Sydney and Sydney clockmaker Frank Percival. This project led Bromley to review Price's X-ray analysis and to make new, more accurate X-ray images that were studied by Bromley's student, Bernard Gardner, in 1993.

Later, a British orrery maker named John Gleave constructed a working replica of the mechanism. According to his reconstruction, the front dial shows the annual progress of the Sun and Moon through the zodiac against the Egyptian calendar. The upper rear dial displays a four-year period and has associated dials showing the Metonic cycle of 235 synodic months, which equals 19 solar years. A synodic month is the period between two new moons. The lower rear dial plots the cycle of a single synodic month, with a secondary dial showing the lunar year of 12 synodic months.

Another reconstruction was made in 2002 by Michael Wright, mechanical engineering curator for the Science Museum in London, working with Bernard Gardner of Sydney. The original mechanism is kept in the National Archaeological Museum in Athens. The Antikythera mechanism is occasionally interpreted as an anachronism by those attempting to prove the occurrence of time travel (see anachronism and time travel).