# Time travel

In physics and science fiction, time travel is a movement in time that deviates from the usual directional course of time, or a movement through time. Using the theory of relativity , scenarios can be described in which “journeys” into the future take place due to the effect of time dilation . However, that journeys into the past , as described in many works of science fiction, are physically, logically or metaphysically possible at all , is often doubted and there is no empirical evidence for this .

## Physical possibility of time travel

The theory of relativity Albert Einstein offers several ways for time travel:

### Travel to the future

If you leave the earth with a spaceship that is almost as fast as light (relativistic speeds around 10% of the speed of light may be sufficient) and return after a certain travel time has elapsed, a longer period of time has elapsed on earth than on board the spaceship. The reason for this is the time dilation which, according to Albert Einstein's special theory of relativity, occurs at such high speeds . The exact sequence of such a journey through time is described under the twin paradox. A practical experiment on this is the Hafele-Keating experiment with atomic clocks on board a commercial airliner.

Given a sufficiently high travel speed and acceleration , the traveler would in principle be able to reach an arbitrarily distant future on earth in any short travel time . With an acceleration that is reasonable for humans, however, a time shift of years also requires a travel time of over a year from the point of view of the spacecraft crew (around 347 days each for acceleration and deceleration at 9.81 m / s²).

For an object moving at the speed of light in a vacuum, time would stand still. A photon that moves in a vacuum can theoretically measure the time span from the beginning to the end of the universe "in an instant".

According to the general theory of relativity , the passage of time is also dependent on the gravitational and acceleration conditions to which a system is subjected. For example, time passes slightly faster on a high mountain than at sea ​​level . This phenomenon could be interpreted as a time travel into the future, whereby not only a faster, but also a slower journey is possible.

On a neutron star , the gravitational time dilation can be significant. For example, a hypothetical inhabitant of a neutron star could do a time-consuming task in an orbit around the star to make it easier to meet an appointment on the star's surface. In the even more extreme gravitational potential near the Schwarzschild radius of black holes , time can expand as desired according to the general theory of relativity.

### Travel back in time

According to the current state of science, time travel into the past is in principle not possible. Existing theories according to which such a trip is possible are speculative and controversial. In any case, it is undisputed that the practical implementation of such theories by humans will be impossible in the foreseeable future.

A physically feasible journey into the past is possible as pure observation if one looks into space with a telescope, since the speed of light is finite. The sun can be seen as it was just over 8 minutes. The observation of quasars enables a glimpse into the universe billions of years ago.

Kurt Gödel discovered in 1949 that a solution to general relativity , in which the universe rotates, allows an object to return to its own past. Such a universe is called the Godel universe (R-universe).

Even if it can be proven that our universe does not rotate, the R universe shows that Einstein's field equations allow a universe with closed time-like curves. Consequently, the uniform passing of time is not one of the a priori necessary properties of a universe that satisfies the equations.

According to the general theory of relativity, it is conceivable that two different areas of spacetime could be connected to one another via so-called wormholes . If the two exits of such a wormhole were to connect two areas of different times, it would also be possible to travel back in time. However, calculations show that wormholes are usually not stable and collapse so quickly that passage is impossible. If one had a hypothetical material with negative energy density available, the so-called exotic matter , one could stabilize a wormhole with it. However, according to the current state of knowledge, the amount of exotic matter required for this is not available in the entire currently known universe.

A journey into one's own past might be possible on a special trajectory in the vicinity of a sufficiently fast rotating black hole . It is believed, however, that there are no black holes that rotate so rapidly.

A time travel into the past would also be possible in the vicinity of two cosmic strings that fly past each other sufficiently quickly. However, there is no evidence for the existence of cosmic strings.

According to a very controversial interpretation, a time reversal was achieved through superluminous tunneling for particles or photons, that is, the particles seemed to come out of the experimental arrangement - the "tunnel" - before they were radiated into it. However, a very controversial definition of signal time was used in these experiments.

Should journeys into the past be possible, the question arises of how to avoid the paradoxes that can arise in this context from the violation of causality , such as the grandfather paradox . Everett's many-worlds theory would be a possible answer . After that, the past you are traveling to would be located in a parallel world . The original course of things and a course modified by an intervention in the past would both take place. In particular, it would be impossible for the traveler to return to his original version of the present, but to a parallel world that would be almost identical to it.

Occasionally, when talking about journeys into the past, hypothetical faster-than-light particles , so-called tachyons , are brought into play. If a particle could move from A to B at faster than light speed , an observer could always be found for whom the movement from B to A would take place. Since the observers judge the chronological order of events A and B differently, the tachyon moves from the past into the future for everyone involved. It is not possible to derive the possibility of a journey into the past from a hypothetical movement faster than light.

## Time travel in literature and film

Time travel is an old dream of mankind and is therefore often discussed in science fiction literature and in science fiction films . Usually there is a time travel with a time machine (for example in Back to the Future ), less often with other methods, for example in the Highland saga by Diana Gabaldon by walking through a stone circle or by magic, as in the series Charmed - Magical Witches . But it is different with the series Back to the Past , where Dr. Samuel "Sam" Beckett (played by Scott Bakula ), a Nobel Prize-winning quantum physicist and inventor of the "quantum accelerator", travels uncontrollably through time within his own lifetime as a result of a failed self-experiment, temporarily assuming and with the identities of various people from that time swap bodies for them. In the science fiction television series Star Trek , time travel is a recurring, popular and both methodologically and narrative varied topic.

In the Manga series Dragon Ball , the subject is also themed in the Cell saga, as a visitor from the future changes the present. However, a new timeline is being created here on purpose.

In particular, the problems related to causality in time travel to the past are a popular topic in science fiction literature, for example in the novels The Guardian Angel by Dean Koontz or Making History by Stephen Fry .

The butterfly effect from chaos theory is often discussed. This means that even the smallest changes in the past can have extreme effects on further development (see e.g. Butterfly Effect or Back to the Future ), which may end in the non-existence of the time traveler. This problem is not infrequently solved with the surprising punchline that the journey through time was part of the known past from the start and a circle is closed by the journey through time. This in turn denies the assumption that a journey through time can change or even destroy the “timeline”.

Another recurring idea is that of the time warp in which the protagonist is stuck. This is the case, for example, in the films 12:01 , Retroactive or Groundhog Day , in Supernatural Season 3, Episode 11 ("And daily greetings ...") and the animated series Steins; Gate .

The BBC British series Doctor Who is about a mysterious time traveler known only as 'the Doctor'. He travels with his companions in the TARDIS, which from the outside looks like an old police emergency call cell, through time and space and is involved in various adventures.

Erdoğan Ercivan claims in his books that the Egyptian underworld book Amduat contains an exact travel description of the deceased Pharaoh to the double star Sirius , which is 8.6 light years away from Earth. Among other things, the gods of the Egyptians are said to have used a wormhole southwest of Orion , which they called Wamemti .

Michael Crichton uses the thesis in his book Timeline that different times are just different worlds in the many-worlds interpretation . A journey through time therefore takes place through a journey into a parallel world.

A similar interpretation can be found in the novels The Last Day of Creation and The Cusanus Game by Wolfgang Jeschke , in which people travel into the past and thereby create parallel, divergent futures from which various other people travel into the past.

The problem of grammatical tense formation for time travelers is addressed by Douglas Adams in his novel The Restaurant at the End of the Universe .

The description of a journey through time into the future offers the author the opportunity to point out undesirable developments and social criticism. Of which has, among other HG Wells in 1895 in his book The Time Machine exercised.

Closely related to this is the theme of parallel worlds , such as in the film Isn't life beautiful? or in the Sliders series .

In Audrey Niffenegger's novel The Time Traveler's Wife , the main character (Henry) has to travel through time because of a genetic defect.

In the novel Das Jesus Video by Andreas Eschbach, an involuntary time traveler travels to the scene of the last weeks of Jesus Christ's life and, through a video that has been handed down to the present, also involuntarily creates dramas based on actual events and reports in the Bible about the life of Jesus. With the book The Jesus Deal , published in 2014, the story is continued and settled in the evangelical milieu of the USA.

In the series On the Road to Atlantis , based on Johanna von Koczian and Ota Hofman , a journey through time is compared with what happens in dreams.

In the Stargate franchise ( SG-1 , SGA & SGU ), the Stargate in connection with wormholes, which lead through solar flares that take place precisely at this point in time, is often given as a means for time travel. Travel in both directions, both in the future and in the past, is possible.

The gemstone trilogy Love Goes Through All Times by Kerstin Gier is about a girl who jumps uncontrollably through time.

In Fabian Lenk 's children's book series The Time Detectives , three teenagers and a magical cat travel through time with the help of a tense and solve cases in history.

In the 2009 film Star Trek , the Romulan Nero and old Mr. Spock travel 154 years into the past with the help of a wormhole in order to actively intervene in current events.

In the first season of the youth series Hotel 13 , three best friends travel to 1927 in a metal ball. The smallest change in the past can have big consequences in the present.

Also in the film for the series Hotel 13: Rock 'n' Roll Highschool , the three friends meet a girl from the past. You have to bring the girl back to her present, the 1950s, otherwise the three friends would never get to know each other. In the second season, the three friends experience an adventure in the future.

In the book The Attack, the teacher Jacob Epping travels from the present to the 1950s with the help of a time portal in a diner . There he wants to prevent the assassination attempt on Kennedy after a few attempts and their verification in the present time (the portal is accessible from both sides) . The peculiarity of this story is that the timeline resets itself to the original one with every new journey into the past and changes made by the protagonist disappear again. The portal also always leads to exactly the same place and time (on September 9, 1958). In addition, the harder he tries to influence the timeline, the greater the difficulties that Epping faces. Time seems to be fighting back.

In the illustrated novel Das Other Ufer der Zeit (1970), Jack Finney takes up the idea of ​​a journey through time with the help of autosuggestion . The idea was continued by the author in 1995 in the novel Im Strom der Zeit .

In the last two episodes ( an animal journey through time to the dodos and an animal journey through time to the bag wolves ) of the children's series Go Wild! Mission Wilderness, the Wildlife team travels back in time with the help of a time travel trampoline and observes the extinct dodos and Tasmanian pouch wolves .

The television series Dark is about the fictional city of Winden , in which children disappear one year and appear (sometimes dead) in another year.

In the 2019 movie Avengers: Endgame , the heroes travel through time to undo the episodes of Avengers: Infinity War .

## literature

Theoretical philosophy and physics

• Paul Davies : How to Build a Time Machine: Instructions for Use . Piper, Munich 2004, ISBN 3-492-04527-8
• John Bigelow : Time travel fiction . In: Gerhard Preyer, Frank Siebelt (Eds.): Reality and Humean Supervenience. Rowan & Littlefield, Lanham, MD 2001, pp. 58-91
• John Earman : Outlawing Time Machines . chronology protection theorems. In: Knowledge. 42/2 (1995), pp. 125-139
• John Earman: Recent Work on Time Travel . In: Steven Savitt (Ed.): Time's Arrows Today. Recent Physical and Philosophical Work on the Direction of Time. Cambridge University Press, 1995, pp. 268-310
• Daniel M. Greenberger, Karl Svozil: Quantum Theory Looks at Time Travel . In: Avsalom Elitzur: Quo vadis quantum mechanics? Springer, Berlin 2005, ISBN 3-540-22188-3 , pp. 63-72
• Paul Horwich : Asymmetries in Time. Problems in the Philosophy of Science. MIT Press, Cambridge, MA 1987
• Simon Keller, Michael Nelson: Presentists Should Believe in Time-Travel . In: Australasian Journal of Philosophy. 79 (2001), pp. 333-345
• David K. Lewis : The paradoxes of time travel. (PDF; 69 kB) In: American Philosophical Quarterly . 13 (1976), pp. 145-152
• Ned Markosian : The Open Past. In: Philosophical Studies . 79 (1995), pp. 95-105
• Jack W. Meiland: A Two-Dimensional Passage Model of Time for Time Travel . In: Philosophical Studies. 26, pp. 153-173 (1974)
• Werner Bernhard Sendker: The very different theories of space and time. The transcendental idealism of Kant in relation to Einstein's theory of relativity . Osnabrück 2000, ISBN 3-934366-33-3

Popular representations

• Falko Blask, Ariane Windhorst: Time travel. The fulfillment of a human dream . Rowohlt-Taschenbuch-Verlag, Reinbek near Hamburg 2009, ISBN 978-3-499-62558-9
• David Deutsch, Michael Lockwood: The quantum physics of time travel . In: Spectrum of Science. November 1994, pp. 50-57
• J. Richard Gott: Time Travel in Einstein's Universe . Rowohlt Taschenbuch Verlag, 2003, ISBN 3-499-61577-0
• Henriette Nagel: The future was yesterday. Time travel models in the film . Mühlbeyer Filmbuchverlag, Frankenthal 2014, ISBN 978-3-945378-08-3
• Rüdiger Vaas : Tunnel through space and time . 2nd Edition. Franckh-Kosmos, Stuttgart 2006, ISBN 3-440-09360-3