Time+Travel,+III



Time travel is the moving of different points in [|time] in a manner analogous to moving between different points in [|space], either sending objects (or in some cases just ) have expanded on the concept of time travel. Although time travel has been a common plot device in fiction since the 19th century, and one-way travel into the future is arguably possible given the phenomenon of time dilation based on velocity in the theory of special relativity (exemplified by the twin paradox), as well as gravitational time dilation in the theory of general relativity, it is currently unknown whether the laws of physics would allow backwards time travel. Any technological device, whether fictional or hypothetical, that is used to achieve time travel is commonly known as a time machine. Some interpretations of time travel also suggest that an attempt to travel backwards in time might take one to a parallel universe whose history would begin to diverge from the traveler's original history after the moment the traveler arrived in the past.[1]
 * 1. What is Time Travel** ( well at least what wikipedia says it is)
 * Time travel** is the concept of moving between different points in time in a manner analogous to moving between different points in space, either sending objects (or in some cases just information) backwards in time to some moment before the present, or sending objects forward from the present to the future without the need to experience the intervening period (at least not at the normal rate).

Origins of the concept
 * 700s BCE to 300s CE - //Mahabharata//
 * 200s to 400s CE - Talmud
 * 720 CE - "Urashima Tarō"
 * 1733 - Samuel Madden’s //Memoirs of the Twentieth Century//
 * 1771 - Louis-Sébastien Mercier’s //L'An 2440, rêve s'il en fût jamais//
 * 1819 - Washington Irving's "Rip Van Winkle"
 * 1838 - //Missing One's Coach: An Anachronism//
 * 1843 - Charles Dickens’ //A Christmas Carol//
 * 1861 - Pierre Boitard’s //Paris avant les hommes//
 * 1881 - Edward Page Mitchell’s //The Clock That Went Backward//
 * 1887 - Enrique Gaspar y Rimbau's //El anacronópete//
 * 1888 - H. G. Wells' //The Chronic Argonauts//
 * 1889 - Mark Twain’s //A Connecticut Yankee in King Arthur's Court//
 * 1895 - H. G. Wells’ //The Time Machine//

. According to special relativity it would take an infinite amount of energy to accelerate a slower-than-light object to the speed of light, and although relativity does not forbid the theoretical possibility of tachyons which move faster than light at all times, when analyzed using quantum field theory it seems that it would not actually be possible to use them to transmit information faster than light,[23] and there is no evidence for their existence.

edit] Special spacetime geometries
The general theory of relativity extends the special theory to cover gravity, illustrating 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.[15] The first of these was proposed by Kurt Gödel, a solution known as the Gödel metric, but his (and many others') example requires the universe to have physical characteristics that it does not appear to have.[15] Whether general relativity forbids closed time-like curves for all realistic conditions is unknown. Time travel to the future in physics Twin paradox diagramRelativistic rocket: This cockpit view of a hypothetical spacecraft traveling at eight-tenths the speed of light shows the visual distortions that would be experienced at such high speeds. The star field is actually being wrapped toward the front of the craft in addition to being significantly blue-shifted. Somewhat more rigorous diagrams of what would be seen by such a craft can be found [|here] Credit: [|NASA]; digital art by Les Bossinas (Cortez III Service Corp.), 1998 There are various ways in which a person could "travel into the future" in a limited sense: the person could set things up so that in a small amount of his own subjective time, a large amount of subjective time has passed for other people on Earth. For example, an observer might take a trip away from the Earth and back at relativistic velocities, with the trip only lasting a few years according to the observer's own clocks, and return to find that thousands of years had passed on Earth. It should be noted, though, that according to relativity there is no objective answer to the question of how much time "really" passed during the trip; it would be equally valid to say that the trip had lasted only a few years or that the trip had lasted thousands of years, depending on your choice of reference frame. This form of "travel into the future" is theoretically allowed using the following methods:[21] Additionally, it might be possible to see the distant future of the Earth using methods which do not involve relativity at all, although it is even more debatable whether these should be deemed a form of "time travel":
 * Using velocity-based time dilation under the theory of special relativity, for instance:
 * Traveling at almost the speed of light to a distant star, then slowing down, turning around, and traveling at almost the speed of light back to Earth[46] (see the Twin paradox)
 * Using gravitational time dilation under the theory of general relativity, for instance:
 * Residing inside of a hollow, high-mass object;
 * Residing just outside of the event horizon of a black hole, or sufficiently near an object whose mass or density causes the gravitational time dilation near it to be larger than the time dilation factor on Earth.
 * Hibernation
 * Suspended animation

Time dilation
Transversal Time dilationMain article: Time dilation //Time dilation// is permitted by Albert Einstein's special and general theories of relativity. These theories state that, relative to a given observer, time passes more slowly for bodies moving quickly relative to that observer, or bodies that are deeper within a gravity well.[47] For example, a clock which is moving relative to the observer will be measured to run slow in that observer's rest frame; as a clock approaches the speed of light it will almost slow to a stop, although it can never quite reach light speed so it will never completely stop. For two clocks moving inertially (not accelerating) relative to one another, this effect is reciprocal, with each clock measuring the other to be ticking slower. However, the symmetry is broken if one clock accelerates, as in the twin paradox where one twin stays on Earth while the other travels into space, turns around (which involves acceleration), and returns—in this case both agree the traveling twin has aged less. General relativity states that time dilation effects also occur if one clock is deeper in a gravity well than the other, with the clock deeper in the well ticking more slowly; this effect must be 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 black hole. It has been calculated that, under general relativity, a person could travel forward in time at a rate four times that of distant observers by residing inside a spherical shell with a diameter of 5 meters and the mass of Jupiter.[21] For such a person, every one second of their "personal" time would correspond to four seconds for distant observers. Of course, squeezing the mass of a large planet into such a structure is not expected to be within our technological capabilities in the near future. There is a great deal of experimental evidence supporting the validity of equations for velocity-based time dilation in special relativity[48] and gravitational time dilation in general relativity.[49][50][51] However, with current technologies it is only possible to cause a human traveller 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.

Time perception
Time perception can be apparently sped up for living organisms through hibernation, where the body temperature and metabolic rate of the creature is reduced. A more extreme version of this is suspended animation, where the rates of chemical processes in the subject would be severely reduced. Time dilation and suspended animation only allow "travel" to the future, never the past, so they do not violate causality, and it's debatable whether they should be called time travel. However time dilation can be viewed as a better fit for our understanding of the term "time travel" than suspended animation, since with time dilation less time actually does pass for the traveler than for those who remain behind, so the traveler can be said to have reached the future faster than others, whereas with suspended animation this is not the case.

Other ideas about time travel from mainstream physics The possibility of paradoxes
The Novikov self-consistency principle and calculations by Kip S. Thorne[//citation needed//] 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 //many// consistent solutions. The circumstances might, however, turn out to be almost unbelievably strange.[//citation needed//] 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. If all possibilities exist, any paradoxes could be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction, but some physicists such as David Deutsch have suggested that if time travel is possible and the many-worlds interpretation is correct, then a time traveler should indeed end up in a different history than the one he started from. On the other hand, Stephen Hawking has argued that even if the many-worlds interpretation is correct, we should expect each time traveler to experience a single self-consistent timeline, so that time travelers remain within their own world rather than traveling to a different one. Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel without paradoxes.[53][54] 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.

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 presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles.[55] 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.[56] The no-communication theorem also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals. The fact that these quantum phenomena apparently do //not// allow FTL time travel is often overlooked in popular press coverage of quantum teleportation experiments.[//citation needed//] How the rules of quantum mechanics work to preserve causality is an active area of research.[//citation needed//]

edit] Philosophical understandings of time travel
Main article: Philosophy of space and time Theories of time travel are riddled with questions about causality and paradoxes. Compared to other fundamental concepts in modern physics, time is still not understood very well. Philosophers have been theorizing about the nature of time since the era of the ancient Greek philosophers and earlier. Some philosophers and physicists who study the nature of time also study the possibility of time travel and its logical implications. The probability of paradoxes and their possible solutions are often considered. For more information on the philosophical considerations of time travel, consult the work of David Lewis or [|Ted Sider]. For more information on physics-related theories of time travel, consider the work of Kurt Gödel (especially his theorized universe) and Lawrence Sklar.

Presentism vs. eternalism
The relativity of simultaneity in modern physics favors the philosophical view known as eternalism or four dimensionalism (Sider, 2001), in which physical objects are either temporally extended space-time worms, or space-time worm stages, and this view would be favored further by the possibility of time travel (Sider, 2001). Eternalism, also sometimes known as "block universe theory", builds on a standard method of modeling time as a dimension in physics, to give time a similar ontology to that of space (Sider, 2001). This would mean that time is just another dimension, that future events are "already there", and that there is no objective flow of time. This view is disputed by Tim Maudlin in his //The Metaphysics Within Physics//. Presentism is a school of philosophy that holds that neither the future nor the past exist, and there are no non-present objects. In this view, time travel is impossible because there is no future or past to travel to. However, some 21st century presentists have argued that although 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 appear in the present could explain the time traveler's actual existence in the present.[57][58]

The grandfather paradox
Main article: Grandfather paradox One subject often brought up in philosophical discussion of time is the idea that, if one were to go back in time, paradoxes could ensue if the time traveler were to change things. The best examples of this are the grandfather paradox and the idea of autoinfanticide. The grandfather paradox is a hypothetical situation in which a time traveler goes back in time and attempts to kill his grandfather at a time before his grandfather met his grandmother. If he did so, then his father never would have been born, and neither would the time traveler himself, in which case the time traveler never would have gone back in time to kill his grandfather. Autoinfanticide works the same way, where a traveler goes back and attempts to kill himself as an infant. If he were to do so, he never would have grown up to go back in time to kill himself as an infant. This discussion is important to the philosophy of time travel because philosophers question whether these paradoxes make time travel impossible. Some philosophers answer the paradoxes by arguing that it might be the case that backwards 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.

Theory of compossibility
David Lewis’ analysis of compossibility and the implications of changing the past is meant to account for the possibilities of time travel in a one-dimensional conception of time without creating logical paradoxes. Consider Lewis’ example of Tim. Tim hates his grandfather and would like nothing more than to kill him. The only problem for Tim is that his grandfather died years ago. Tim wants so badly to kill his grandfather himself that he constructs a time machine to travel back to 1955 when his grandfather was young and kill him then. Assuming that Tim can travel to a time when his grandfather is still alive, the question must then be raised; Can Tim kill his grandfather? For Lewis, the answer lies within the context of the usage of the word "can". Lewis explains that the word "can" must be viewed against the context of pertinent facts relating to the situation. Suppose that Tim has a rifle, years of rifle training, a straight shot on a clear day and no outside force to restrain Tim’s trigger finger. Can Tim shoot his grandfather? Considering these facts, it would appear that Tim can in fact kill his grandfather. In other words, all of the contextual facts are compossible with Tim killing his grandfather. However, when reflecting on the compossibility of a given situation, we must gather the most inclusive set of facts that we are able to. Consider now the fact that Tim’s grandfather died in 1993 and //not// in 1955. This new fact about Tim’s situation reveals that him killing his grandfather is not compossible with the current set of facts. Tim cannot kill his grandfather because his grandfather died in 1993 and not when he was young. Thus, Lewis concludes, the statements "Tim doesn’t but can, because he has what it takes," and, "Tim doesn’t, and can’t, because it is logically impossible to change the past," are not contradictions, they are both true given the relevant set of facts. The usage of the word "can" is equivocal: he "can" and "can not" under different relevant facts. So what must happen to Tim as he takes aim? Lewis believes that his gun will jam, a bird will fly in the way, or Tim simply slips on a banana peel. Either way, there will be some logical force of the universe that will prevent Tim every time from killing his grandfather.

Ideas from fiction
Further information: Time travel in fiction

Rules of time travel
Time travel themes in science fiction and the media can generally be grouped into two general categories (based on effect—methods are extremely varied and numerous), each of which can be further subdivided. However, there are no formal names for these two categories, so concepts rather than formal names will be used with notes regarding what categories they are placed under. //Note: These classifications do not address the method of time travel itself, i.e. how to travel through time, but instead call to attention differing rules of what happens to history.// 1. **There is a single fixed history, which is self-consistent and unchangeable.** In this version, everything happens on a single timeline which doesn't contradict itself and can't interact with anything potentially existing outside of it.1.1 This can be simply achieved by applying the Novikov self-consistency principle, named after Dr. Igor Dmitrievich Novikov, Professor of Astrophysics at Copenhagen University. The principle states that the timeline is totally fixed, and any actions taken by a time traveler were part of history all along, so 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 and the predestination paradox; for examples of circular causation, see Robert A. Heinlein's story "By His Bootstraps". The 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 spacetime.1.2 Alternatively, new physical laws take effect regarding time travel that thwarts attempts to change the past (contradicting the assumption mentioned in 1.1 above that the laws that apply to time travelers are the same ones that apply to everyone else). These new physical laws can be as unsubtle as to reject time travelers who travel to the past to change it by pulling them back to the point from when they came as Michael Moorcock's //The Dancers at the End of Time// or where the traveler is rendered an noncorporeal phantom unable to physically interact with the past such as in some Pre-Crisis Superman stories and Michael Garrett's "Brief Encounter" in //Twilight Zone Magazine// May 1981.2. **History is flexible and is subject to change** (//Plastic Time//)2.1 //Changes to history are easy and can impact the traveler, the world, or both//Examples include //Doctor Who//, //Back to the Future// and //Back to the Future II//. In some cases, any resulting paradoxes can be devastating, threatening the very existence of the universe. In other cases the traveler simply cannot return home. The extreme version of this (//Chaotic Time//) is that history is //very// sensitive to changes with even small changes having large impacts such as in Ray Bradbury's "A Sound of Thunder".2.2 //History is change resistant in direct relationship to the importance of the event// ie. small trivial events can be readily changed but large ones take great effort. In the //Twilight Zone// episode "Back There" a traveler tries to prevent the assassination of President Lincoln and fails, but his actions have made subtle changes to the //status quo// in his own time (e.g. a man who had been the butler of his gentleman's club is now a rich tycoon, and a man whose family had long been members of the club is now the butler).In the 2002 remake of //The Time Machine//, it is explained via a vision why Hartdegen could not save his sweetheart Emma — doing so would have resulted in his never developing the time machine he used to try and save her.In //The Saga of Darren Shan//, major events in the past cannot be changed, but minor events can be affected. Under this model, if a time traveler were to go back in time and kill Hitler, another Nazi would simply take his place and commit his same actions, leaving the broader course of history unchanged.3. **Many-worlds interpretation and Parallel universe (fiction)** are often used interchangeably in fiction but mechanically they differ in that the first says time travel creates a coexisting alternate history while the second says the traveler actually goes to an already existing parallel world. In either case the traveler's original home reality continues to exist unaffected. These versions of time travel are sometimes placed under one of the two above categories.James P. Hogan's //The Proteus Operation// fully explains parallel universe time travel in chapter 20 where it has Einstein explaining that all the outcomes already exist and all time travel does is change which already existing branch you will experience.Though //Star Trek// has a long tradition of using the 2.1 mechanic, as seen in "City on the Edge of Forever", "Tomorrow is Yesterday", "Time and Again", "Future's End", "Before and After", "Endgame" and as late as Enterprise's Temporal Cold War, "Parallels" had an example of what Data called "quantum realities." His exact words on the matter were "But there is a theory in quantum physics that all possibilities that can happen do happen in alternate quantum realities," leaving it up the viewer as to the exact nature of quantum realities.Michael Crichton's novel //Timeline// takes the approach that all time travel really is travel to an already existing parallel universe where time passes at a slower rate than our own but changes in any of these parallel universes affect the main timeline making it behave as if it was a type 2 universe.[//clarification needed//]In the Homeline setting of //GURPS Infinite Worlds// there are echos — parallel universes at an early part of Homeline's history but changes to their history do not affect Homeline's history.

Immutable timelines
Time travel in a type 1 universe does not allow paradoxes such as the grandfather paradox to occur, where one deduces both a conclusion and its opposite (in the case of the grandfather paradox, one can start with the premise of the time traveler killing his grandfather, and reach the conclusion that the time traveler will //not// be able to kill his grandfather since he was never born) though it can allow other paradoxes to occur. In 1.1, the Novikov self-consistency principle asserts that the existence of a method of time travel constrains events to remain self-consistent. This will cause any attempt to violate such consistency to fail, even if seemingly 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. Examples of this kind of universe are found in //Timemaster//, a novel by Dr. Robert Forward, the //Twilight Zone// episode "No Time Like the Past", and the 1980 Jeannot Szwarc film //Somewhere In Time// (based on Richard Matheson's novel //Bid Time Return//). In 1.2, time travel is constrained to prevent paradox. How this occurs is dependent on if interaction with the past is possible. If interaction with the past is possible and one attempts to make a paradox, one undergoes involuntary or uncontrolled time travel. 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 with intentions to kill Hitler finds himself on a Montana farm in late April 1945. If interaction with the past is //not// possible then the traveler simply becomes an invisible insubstantial phantom unable to interact with the past as in the case of James Harrigan in Michael Garrett's "Brief Encounter".

While a Type 1 universe will prevent a grandfather paradox it doesn't prevent paradoxes in other aspects of physics such as the predestination paradox and the ontological paradox (//GURPS Infinite Worlds// calls this "Free Lunch Paradox"). The predestination paradox is where the traveler's actions create some type of causal loop, in which some event A in the future helps cause event B in the past via time travel, and the event B in turn is one of the causes of A. For instance, a time traveler might go back to investigate a specific historical event like the Great Fire of London, and their actions in the past could then inadvertently end up being the original cause of that very event. Examples of this kind of causal loop are found in //Timemaster//, a novel by Dr. Robert Forward, the //Twilight Zone// episode "No Time Like the Past", the 1980 Jeannot Szwarc film //Somewhere In Time// (based on Richard Matheson's novel //Bid Time Return//), the Michael Moorcock novel //Behold the Man//, and //Harry Potter and the Prisoner of Azkaban//. It is also featured in 1972's //Doctor Who//, in the three part //The Day of the Daleks//, where three freedom fighters from the future attempt to kill a British diplomat they believe responsible for World War Three, and the subsequent easy conquest of Earth by the Daleks. In the future they were taught an explosion at the diplomat's (Sir Reginald Styles) mansion with foreign delegates inside caused the nations of the world to attack each other. The Doctor (Jon Pertwee), figures out that //they// caused the explosion all along by way of a temporal paradox. This is also seen in the 2006 crime thriller //Déjà Vu//. The Novikov self-consistency principle can also result in an ontological paradox (also known as the knowledge or information paradox)[65] where the very existence of some object or information is a time loop. //GURPS Infinite Worlds// gives the example (from //The Eyre Affair//) of a time traveler going to Shakespeare's time with a book of all his works. Shakespeare pressed for time simply copies the information in the book from the future. The "free lunch" is that //no one// really writes the plays! The philosopher Kelley L. Ross argues in "[|Time Travel Paradoxes]" that in an ontological paradox scenario involving a physical object, there can be a violation of the second law of thermodynamics. Ross uses //Somewhere in Time// as an example where Jane Seymour's character gives Christopher Reeve's character a watch she has owned for many years, and when he travels back in time he gives the same watch to Jane Seymour's character 60 years in the past. As Ross states

"The watch is an impossible object. It violates the Second Law of Thermodynamics, the Law of Entropy. If time travel makes that watch possible, then time travel itself is impossible. The watch, indeed, must be //absolutely identical// to itself in the 19th and 20th centuries, since Reeve carries it with him from the future instantaneously into the past and bestows it on Seymour. The watch, however, cannot be identical to itself, since all the years in which it is in the possession of Seymour and then Reeve it will **wear** in the normal manner. It's [//sic//] entropy will increase. The watch carried back by Reeve will be **more worn** that [//sic//] the watch that would have been acquired by Seymour."

On the other had, the second law of thermodynamics is understood by modern physicists to be a statistical law rather than an absolute one, so spontaneous reversals of entropy or failure to increase in entropy are not impossible, just improbable (see for example the fluctuation theorem). In addition, the second law of thermodynamics only states that entropy should increase in systems which are isolated from interactions with the external world, so Igor Novikov (creator of the Novikov self-consistency principle) has argued that in the case of macroscopic objects like the watch whose worldlines form closed loops, the outside world can expend energy to repair wear/entropy that the object acquires over the course of its history, so that it will be back in its original condition when it closes the loop.

Mutable timelines
Time travel in a Type 2 universe is much more complex. 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. The //Back to the Future// trilogy films also seem to feature a single mutable timeline (see the "[|Back to the Future FAQ]" for details on how the writers imagined time travel worked in the movies' world). By contrast, the short story "Brooklyn Project" by William Tenn provides a sketch of life in a Type 2 world where no one even notices as the timeline changes repeatedly. In type 2.1, attempts are being made at changing the timeline, however, all that is accomplished in the first tries is that the method in which decisive events occur is changed; final conclusions in the bigger scheme cannot be brought to a different outcome. As an example, the movie //Déjà Vu// depicts a paper note sent to the past with vital information to prevent a terrorist attack. However, the vital information results in the killing of an ATF agent, but does not prevent the terrorist attack; the very same agent died in the previous version of the timeline as well, albeit under different circumstances. Finally, the timeline is changed by sending a human into the past, arguably a "stronger" measure than simply sending back a paper note, which results in preventing both a murder and the terrorist attack. As in the //Back to the Future// movie trilogy, there seems to be a ripple effect too as changes from the past "propagate" into the present, and people in the present have altered memory of events that occurred after the changes made to the timeline. The science fiction writer Larry Niven suggests in his essay "The Theory and Practice of Time Travel" 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 might 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 1.1 scenario.[//citation needed//] This is sometimes referred to as the "Time Dilution Effect". Few if any physicists or philosophers have taken seriously the possibility of "changing" the past except in the case of multiple universes, and in fact many have argued that this idea is logically incoherent,[59] so the mutable timeline idea is rarely considered outside of science fiction. Also, deciding whether a given universe is of Type 2.1 or 2.2 can not be done objectively, as the categorization of timeline-invasive measures as "strong" or "weak" is arbitrary, and up to interpretation: An observer can disagree about a measure being "weak", and might, in the lack of context, argue instead that simply a mishap occurred which then led to no effective change. An example would be the paper note sent back to the past in the film //Déjà Vu//, as described above. Was it a "too weak" change, or was it just a local-time alteration which had no extended effect on the larger timeline? As the universe in //Déjà Vu// seems not entirely immune to paradoxes (some arguably minute paradoxes do occur), both versions seem to be equally possible.

Alternate histories
In Type 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 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.[//citation needed//] Some examples of this kind of time travel can be found in David Gerrold's book //The Man Who Folded Himself// and //The Time Ships// by Stephen Baxter, plus several episodes of the TV show //Star Trek: The Next Generation//.

Gradual and instantaneous
In literature, there are two 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, though in most cases, there is a machine of some sort, and some energy expended in order to make this happen (like the time-traveling De Lorean in //Back to the Future// or the phone booth that traveled through the "circuits of history" in //Bill and Ted's Excellent Adventure//). In some cases, 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. The "Universal Remote" used by Adam Sandler in the movie //Click// works in the same manner, although only in one direction, the future. While his character Michael Newman can travel back to a previous point it is merely a playback with which he cannot interact. 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. 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. 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 backward in time, because he was born "at the wrong end of time" and has to live backwards from the front. "Some people call it having second sight", he says. This method of gradual time travel is not as popular in modern science fiction, though a form of it does occur in the film //Primer//.

edit] Time travel, or space-time travel?
An objection that is sometimes raised against the concept of time machines in science fiction is that they ignore the motion of the Earth between the date the time machine departs and the date it returns. The idea that a traveler can go into a machine that sends him or her to 1865 and step out into the exact same spot on Earth might be said to ignore the issue that Earth is moving through space around the Sun, which is moving in the galaxy, and so on, so that advocates of this argument imagine that "realistically" the time machine should actually reappear in space far away from the Earth's position at that date. However, the theory of relativity rejects the idea of absolute time and space; in relativity there can be no universal truth about the spatial distance between events which occurred at different times[67] (such as an event on Earth today and an event on Earth in 1865), and thus no objective truth about which point in space at one time is at the "same position" that the Earth was at another time. In the theory of special relativity, which deals with situations where gravity is negligible, the laws of physics work the same way in every inertial frame of reference and therefore no frame's perspective is physically better than any other frame's, and different frames disagree about whether two events at different times happened at the "same position" or "different positions". In the theory of general relativity, which incorporates the effects of gravity, //all// coordinate systems are on equal footing because of a feature known as "diffeomorphism invariance".[68] Nevertheless, the idea that the Earth moves away from the time traveler when he takes a trip through time has been used in a few science fiction stories, such as the 2000 AD comic //Strontium Dog//, in which 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-appear in space. Other science fiction stories try to anticipate this objection and offer a rationale for the fact that the traveler remains on Earth, such as the 1957 Robert Heinlein novel //The Door into Summer// where Heinlein essentially handwaved the issue with a single sentence: "You stay on the world line you were on." In his 1980 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. In Clifford Simak's 1950s short story "Mastodonia" (later broadcast on the //X Minus One// radio anthology show, and then significantly re-written into a longer novel of the same name) the protagonists are aware of the possibility of changes in ground level while traveling back in time to the same geographical coordinates and mount their time machine in a helicopter so as to not materialize underground. When the helicopter is damaged beyond repair while in the past, they then build a mound of rocks from which to launch their return to the present. The television series //Seven Days// also dealt with this problem; when the chrononaut would be 'rewinding', he would also be propelling himself backwards around the Earth's orbit, with the intention of landing at some chosen spatial location, though seldom hitting the mark precisely.[//citation needed//] In Piers Anthony's //Bearing an Hourglass//, the potent Hourglass of the Incarnation of Time naturally moves the Incarnation in space according to the numerous movements of the globe through the solar system, the solar system through the galaxy, etc.; but by carefully negating some of the movements he can also travel in space within the limits of the planet. The television series //Doctor Who// cleverly avoided this issue by establishing early on in the series that the Doctor's TARDIS is able to move about in space in addition to traveling in time.
 * Is Time Travel Possible?**

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There may be no other concept that captures the imagination more than the idea of [|time] travel -- the ability to travel to any point in the past or future. What could be cooler? You could jump into your time machine to go back and see major events in history and talk to the people who were there! Who would you travel back to see? Julius Caesar? Leonardo da Vinci? [|Elvis] ? You could go back and meet yourself at an earlier age, go forward and see how you look in the future... It's these possibilities that have made ** time travel ** the subject of so many science fiction books and movies.
 * 2. How Time Travel Will Work **



­ It turns out that, in some sense, we are all time travelers. As you sit at your desk, doing nothing more than clicking your [|mouse], time is traveling around you. The future is constantly being transformed into the past with the present only lasting for a fleeting moment. Everything that you are doing right now is quickly moving into the past, which means we continue to move through time. Ideas of time travel have existed for centuries, but when Albert Einstein released his theory of [|special relativity], he laid the foundation for the theoretical possibility of time travel. As we all know, no one has successfully demonstrated time travel, but no one has been able to rule it out either. []

= 3. Time Travel Physics = As we discussed earlier, the theory of relativity states that as the velocity of an object nears the speed of [|light], [|time] slows down. Scientists have discovered that even at the speeds of the [|space shuttle], astronauts can travel a few nanoseconds into the future. To understand this, picture two people, person A and person B. Person A stays on [|Earth], while person B takes off in a [|spacecraft]. At takeoff, their watches are in perfect sync. The closer person B's spacecraft travels to the speed of light, the slower time will pass for person B (relative to person A). If person B travels for just a few hours at 50 percent the speed of light and returns to Earth, it will be obvious to both people that person A has aged much faster than person B. This difference in aging is because time passed much faster for person A than person B, who was traveling closer to the speed of light. Many years might have passed for person A, while person B experienced a time lapse of just a few hours. Find out more about this ** twin paradox ** in [|How Special Relativity Works]. ** Wormholes could allow you to travel into the past and the future. ** If wormholes could be discovered, it might allow us to travel to the past as well as the future. Here's how it would work: Let's say the mouth of the wormhole is portable. Then person B in the example above, who traveled at 50 percent of light speed into space for a few hours, could carry one wormhole mouth into space, while the mouth at the opposite end of the wormhole would stay with person A on Earth. The two people would continue to see one another as person B traveled into space. When person B returned to Earth a few hours later, a few years may have passed for person A. Now, when person A looks through the wormhole that traveled into space, that person will see him or herself at a younger age, the age he or she was when person B launched into space. The cool thing about it is that the older person A would be able to step into the past by entering the wormhole, while the younger person B could step into the future. []

= 4. Problems with Time Travel = If we are ever able to develop a workable theory for [|time] travel, we would open up the ability to create very complicated problems called ** paradoxes **. A paradox is defined as something that contradicts itself. Here are two common examples: Let's say, for the sake of argument, that you could travel back to a time before you were born. The mere fact that you could exist in a time before you were born creates a paradox. If you were born in 1960, how could you exist in 1955? Possibly the most famous paradox is the ** grandfather paradox **. What would happen if a time traveler went back and killed one of his or her ancestors before the traveler was born? If the person killed his or her grandfather, then how could that person be alive to go back and kill his or her grandfather? If we could change the past, it would create an infinite number of paradoxes. Another theory regarding time travel brings up the idea of ** parallel universes **, or alternative histories. Let's say that you do travel back to meet your grandfather when he was a boy. In the theory of parallel universes, you may have traveled to another universe, one that is similar to ours, but has a different succession of events. For instance, if you were to travel back in time and kill one of your ancestors, you've only killed that person in one universe, which is no longer the universe that you exist in. And if you then try to travel back to your own time, you may end up in another parallel universe and never be able to get back to the universe you started in.

The idea here is that every action causes the creation of a new universe, and that there are an infinite number of universes that exist. When you killed your ancestor, you created a new universe, a universe that was identical to your own up until the time you changed the original succession of events. Confused yet? Welcome to the world of time travel. Just imagine how complicated the ticket prices will be. []



Time travel is the concept of traveling forward and backward to different points in time, much as we do through space. Time Travel Unsolved problems in physics: Is time travel theoretically possible? Is it practically possible? If so, what are we to make of the time travel paradoxes, such as going back in time and killing one's own grandfather, etc.? Humans are in fact always traveling in time - in a linear fashion, from the present to the immediate future, inexorably, until death. Some theories are predicated on the fact that we move forward in time, and both forward and backward in space. Since time and space have been shown to be intrinsically linked, travelling forwards and backwards through time is not a theoretical impossibility. Currently, traveling at speeds approaching the speed of light can cause time dilation, the effects of which cause the individual traveling to pass through time more slowly. From the perspective of the traveler, external time would be going much faster, causing the traveler, upon stopping, to arrive at a place farther in the future. Often it is a plot device used in science fiction and many movies and television shows to set a character in a particular time not their own, and explore the character's interaction with the people and technology of that time - as a kind of culture shock. Other ramifications explored are change and reactions to it, parallel universes, and alternative history where some little event took place or didn't take place, but causes large changes in the future. In physics, the concept 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. Although theories do exist about the possibility of folding time to hop from one point to another. ** Physics ** 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. However, this effect allows "time travel" only toward the future: never backward. It is not typical of science fiction, 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. 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. 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 equivalence of time travel and faster-than-light travel ** First of all, if one is able to move 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 degrees, theoretically, it could allow timelines to curve around in a circle and reconnect with their own past. General relativity describes the universe under a complex 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 **
 * 5. More Time Travel**

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 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," 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 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. 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. ** 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 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. 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 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. 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. ** 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. Note that this imposes no restriction on supernatural agents (e.g. God) which are not confined by the bounds of spacetime. See the next section for details. ** Time Travel and Religion ** ** Prophecy and theology ** It is interesting to note that any religion which postulates the existence of fulfilled prophecy requires, at the very least, an agent which can move information from the future into the past. In Christian theology, for example, God is assumed to exist unbound by space or time. Doctrinally, God is held to be omniscient and omnipresent. Statements in the Bible such as Jesus's claim "before Abraham was born, I am" (John 8:58) and Peter's claim "[Jesus] was chosen before the creation of the world" (1 Peter 1:20) (assuming the creation of the world began at t = 0) imply that God does not occupy the same timeline that we do. This is further supported by the assertion "I the LORD do not change" (Malachi 3:6), since change requires movement along, and constrained by, a temporal continuum. Two popular interpretations of these statements are that God exists outside the space-time continuum; or exists at every point in space-time simultaneously. In either case, God can transfer information from one point in space-time to any other point without restriction. ** Transcending time through ancient wisdom ** The Yoga Sutras of Patanjali have been considered by some, such as physicist Fred Alan Wolf in his book, The Yoga of Time Travel to describe an inner process by which we can access knowledge of the past and future in the present. This form of time travel can be acquired by transcending the five Earthly anchors of the ego mind which otherwise leave us locked into the illusory self. ** Time Travel in Fiction ** ** Literature ** 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 demonstrations of supposed time travel paradoxes are in Robert A. Heinlein's //All You Zombies// and //By His Bootstraps.// One very well known time travel fiction writer is Jack Finney. His novels include //Time and Again//, //From Time to Time, The Third Level,// among others. Harry Potter and the //Prisoner of Azkaban// by J. K. Rowling features a time travel paradox as does the film version. Michael Crichton's //Timeline,// in which characters travel to 14th century France, describes time travel in great detail, explaining the science of exactly how the time machine works. The book was made into a movie in 2004, with much of the science explanation missing. []


 * Time Travel With Proof**

[|Embed this Topic] Time travel is a common theme in science fiction and is depicted in a variety of media. It simply means either going forward in time or backward, like seeing the future, or the past. Time travel can form the central theme of a book, or it can be simply a plot device. Time travel in fiction can ignore the possible effects of the time-traveler's actions, as in A Connecticut Yankee in King Arthur's Court, or it can use one resolution or another of t... moreTime travel is a common theme in science fiction and is depicted in a variety of media. It simply means either going forward in time or backward, like seeing the future, or the past. Time travel can form the central theme of a book, or it can be simply a plot device. Time travel in fiction can ignore the possible effects of the time-traveler's actions, as in A Connecticut Yankee in King Arthur's Court, or it can use one resolution or another of the Grandfather paradox. Although The Time Machine by H. G. Wells was instrumental in causing the idea of time travel to enter the public imagination, non-technological forms of time travel had appeared in a number of earlier stories, and some even earlier stories featured elements suggestive of time travel, but remain somewhat ambiguous. A number of themes tend to recur in science fiction time-travel stories, often with enough variation to make them interesting. Stories that involve time travel devices and technologies that take people... less[|Read article at Wikipedia]

Books In This Genre:
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 * The Man Who Folded Himself
 * —All You Zombies—
 * The Time Machine
 * Counting Up, Counting Down
 * Slaughterhouse-Five
 * The Plot To Save Socrates
 * A Sound of Thunder and Other Stories
 * Axis of Time
 * The Didymus Contingency
 * Chronocules

TV programs of this genre:
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 * Life on Mars
 * Seven Days
 * Journeyman
 * Ashes to Ashes
 * The Time Tunnel
 * Life on Mars
 * Moondial
 * The Girl from Tomorrow
 * Odyssey 5
 * Mirror, Mirror

Films of this genre:
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 * The Time Machine
 * Time After Time
 * Back to the Future
 * Back to the Future Part II
 * Conneticut Yankee in King Arhtur's Court
 * Roman Scandals
 * Timerider: The Adventure of Lyle Swan
 * Next One
 * Me Myself I
 * Retroactive

Also known as:

 * Time travel in fiction
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 * Fictional Universes
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Films of this genre

 * ====The Time Machine====
 * ====Time After Time====
 * ====Back to the Future====
 * ====Back to the Future Part II====
 * ====Conneticut Yankee in King Arhtur's Court====

Stories In This Genre:
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 * —All You Zombies—
 * The Chronic Argonauts
 * By His Bootstraps

Characters With This Occupation:
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 * The Doctor
 * Billy Pilgrim

Characters with this power or ability:
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 * Wally West
 * Mister E
 * Legion
 * The Shrike
 * Amalgam
 * Ned Frischman
 * Barry Allen
 * Waverider
 * Death
 * Hiro Nakamura

Works Written About This Topic
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 * ====In The Garden of Iden====
 * ====Mendoza in Hollywood====
 * ====The End of Eternity====
 * ====Thrice Upon a Time====
 * ====The Merchant and the Alchemist's Gate====