Time travel - the alleged movement of a person or any object from the present to the past or future , in particular, using a technical device called a “ time machine ”.
Ways to Travel the Future
Modern science allows several theoretically possible ways to travel to the future (strictly speaking, any object naturally travels to the future over time, so we are talking about traveling "bypassing" the passage of time):
- Physical (based on the consequences of the theory of relativity ):
- Movement at a speed close to the speed of light . The travel time, measured by the clock of the one who moved at such a speed, is always less than the measured by the clock of the one who remained motionless (more precisely: the one who did not experience acceleration is the “ twin paradox ”).
- Being in the region of ultrahigh gravity, for example, near the event horizon of a black hole .
- Biological - stop the metabolism of the body with subsequent recovery (for example: cryopreservation ).
- Quantum - theoretically, a deeper knowledge of quantum physics will not only teleport information, but also transfer physical objects in time and space.
Ways to Travel Past
There are several hypothetical ways to move into the past:
- Through the so-called " wormholes " ( English wormhole - wormhole ), hypothetically admitted by the General Theory of Relativity , some tunnels (possibly very short) connecting remote areas in space, through violation of the topology of space. Developing the theory of wormholes, C. Thorne and M. Morris noticed that if you move one end of (A) the wormhole with great speed, and then bring it closer to the other end (B), then, due to the twin paradox , the object that got into the moment of time T to the entrance A, can (see below) leave B at the moment preceding T (however, in this way it is impossible to get into the time preceding the creation of the time machine).
- It follows from Einstein's equations that the wormhole will close before the traveler can pass through it (as, for example, in the case of the “ Einstein-Rosen bridge ” - the first described wormhole), if the so-called “ exotic matter ” does not deter it - matter with a negative energy density. [ clarify ]   .
- In 1936, Van Stockum discovered that a body rotating around a massive and infinitely long cylinder could fall into the past ( F. Tipler later suggested that this is also possible in the case of a cylinder of finite length  , but the later result of S. Hawking shows that in this case exotic matter would again be necessary  ). Such a cylinder could be a so-called cosmic string , but there is no evidence that cosmic strings exist, and there is hardly any way to create new ones.
- The solution of the Einstein equation, which was deduced in 2017 by Ben Tippett and David Tsang , allows the existence of a closed time-like curve outside the event horizon of a black hole   . Although the existence of this time machine, according to researchers, is theoretically possible, exotic matter is necessary for this model, as well as for Bubble Alcubierre  .
- You can finally do nothing at all, but just wait until the time machine is formed by itself. There is no reason to expect this to happen, but it is important that if it does form, it will not conflict with any known laws of nature. The simplest model of this situation is the Deutsch-Politzer time machine .
Time Travel Paradoxes
There are several commonly cited arguments against travels in the past:
- Violation of causal relationships .
- " Paradoxes ." Suppose someone at 11 am loads a gun, at 11:30 he creates a time machine and at noon (12:00) enters it. Then, using the properties of a time machine, he returns to 11:50, waits until his younger version approaches the entrance, and tries to kill her. Of course, he cannot succeed in this - a person is able to shoot only under the condition that he survived the assassination attempt that took place an hour ago (according to his watch). However, the question arises: what exactly will prevent him (and all his followers)? Are we in some conflict with the usual notions of free will?
- Sometimes another paradox is also called a paradox, which is formulated, for example, as follows (“the paradox of a murdered grandfather ”): if a grandson returns to the past and kills his own grandfather, his birth will be impossible; but if he is not born, then no one will kill his grandfather, and his birth will be possible. What will really happen? Here, however, there is no paradox, as well as no uncertainty. The words “man” (or “grandson”) and “man whose grandfather was not killed in the cradle” are synonyms.
- The lack of documented publicly available facts of the stay in our time of newcomers from the future.
In science, the first problem is not considered now, believing that the time machine and the violation of cause-effect relationships are simply synonyms and there is no topic for discussion (an alternative opinion was expressed in the book  ).
A solution to the second paradox was proposed in the work of S. Krasnikov  , the essence of which is that when creating a time machine there is an extremely atypical uncertainty for classical physics: no matter how well we know the initial data, we cannot unambiguously predict the evolution of space-time. And among the infinitely large number of possible options there is always one in which the time machine does not appear. Thus, if we see a person trying to build a time machine, the fact that he is armed and determined to shoot in an hour does not mean that this person’s free will will soon be limited by something. In the best case, it means only that one possibility (from an infinite number) is excluded - within an hour the time machine will not appear in this place  .
When developing - not yet existing - quantum mechanics in spaces with causality violations, the principle of Novikov’s self-consistency could be significant  .
In cultural works
In the literature
Description of time travel is a common technique used in science fiction literature . In science fiction, the idea of such travels and a special device (“ machine ”) for this purpose gained popularity largely due to the well-known novel by Herbert Wells “ Time Machine ” (1895).
However, this idea was used in literature earlier, for example, in the play Anno 7603 , written by Johan Wessel in 1781. The journey into the future is described by Thaddeus Bulgarin in his essay “Credible Fables, or Travels around the World in the Twenty-Ninth Century,” 1824. The idea was continued by Alexander Veltman , in the novel “ Alexander Filippovich of Macedon. Ancestors of Kalimeros »(1836), who described a journey into the past in a magical hippogriff .
Traveling the protagonist in time in his own body without the help of technical devices is described in the “classic” works of Mark Twain “ Yankees at the Court of King Arthur ”, Jack London “ Interstellar Wanderer ”, Swatopluck Čech “Travel of Pan Brouchek to the 15th Century”, Lazar Lagin “ Blue man ”and many others. This is connected with the concept of “ traitor ” - a character who, against his will, fell into another world or another time and, having no opportunity to return, builds his life there.
Clifford Saymak , Arthur Clark , Philip Dick , Isaac Asimov ( End of Eternity ), Harry Harrison , Anton Granovsky , Kir Bulychev , Stanislav Lem , Joan Kathleen Rowling , Jack Finney , Mikhail Bulgakov , Ilya Varshavsky , Boris Akunin wrote about time travel and many others.
A classic description of a causal relationship disorder is Robert Heinlein ’s short story , “You Are All Zombies.” His main character is a girl who was seduced by a stranger. Nine months later, she will give birth to a child, and doctors find out that she is hermaphroditic and remove the female organ kit. After that, the main character - now a man, is recruited into the time patrol service. First, he goes to the past, where he seduces a certain girl (who is himself in the past). Having gone nine months ahead, he abducts the child and goes to the distant past, where he leaves the kidnapped child in the shelter, from which he will later grow. Thus, it turns out that man created himself out of nothing. In other words, the question arises of the initial appearance of matter. For it remains unclear where the main character came from in order to create himself.
Also, time travel is present in the tale of Edith Nesbit " The story with the amulet ." Time travel plays an important role in the science fiction novels by Audrey Niffenegger , Diana Gabledon , Kerstin Gere .
|A memorial plaque dedicated to S.V. Saveliev|
One of the above paradoxes gave the name to the story "Grandfather's Paradox"  of the writer Sergei Ushenin . A kind of response to the story was a memorial plaque hung in Voronezh on the wall of house number 45 on Karl Marx Street , the text on which reads: “ In this house from 2063 to 2065. lived S. V. Saveliev, inventor of the time machine ”(we are talking about the main character of the story)   .
The paradoxes mentioned above are "solved" in fiction in a huge number of ways. Here are a few (a detailed study on this subject with hundreds of links can be found in P. Nahin's “Time machines”):
The sequence of events is unchanged.
- Journeys into the past are controllable, but no action can change the course of history. "If a fact exists in time, no matter how you try to change it, the result of all your efforts is this fact." This phenomenon, science fiction writer John Wyndham called " chronoclasm . For example, in Lazar Lagin ’s novel “The Blue Man” (1964), the interests of the old hero, an alumnus of a Soviet orphanage, were influenced by the old Bolshevik teacher in 1959; the hero arrives in Moscow in 1894 and, in turn, educates and determines the fate of a nine-year-old girl in revolutionary interests; she becomes a revolutionary and then educates him in an orphanage. A similar "chronoclasm" can be considered the " Fry Paradox", in which a person, having gone to the past, becomes a biological grandfather to himself. Also in Harry Harrison’s book “ Fantastic Saga ”, Vikings in the 11th century discover America only because in the 20th century a film studio on the verge of bankruptcy decides to urgently remove a film about the discovery of America by the Vikings “from nature”. Another telling example, using this approach, is the film Twelve Monkeys directed by Terry Gilliam.
- Journeys into the past are formally controllable, but in practice, impact on the past is deadly, since the emergence of any time paradox will lead to an immediate disaster on a global scale (up to the death of the Universe).
- Traveling to the past is uncontrollable. For example, in the cycle “End of Time” by Michael Moorcock, when trying to break a causal relationship, the traveler returns in due time. In Sergei Lukyanenko ’s novel “The Pier of the Yellow Ships, ” the result of time travel is temporary faults , which suddenly and unpredictably throw an area of space into the past or the future.
- When moving in time, the traveler also moves in space. For example, when moving 1 year ago, it moves 1 light year (exactly the distance from which it cannot affect the events of the point of departure). From these arguments it follows that one can travel only through the curvatures of space-time, that is, through wormholes .
The sequence of events is editable.
- Each journey into the past creates a new reality , so there are no paradoxes. In the old reality, nothing changes. So, killing grandfather will lead to the fact that a new reality will arise where the time traveler was not born and his grandfather was killed; parallel to it, the old reality will remain, where nothing happened to grandfather. In this case, the fate of a time traveler has two options: either, having created a new reality, he will remain in it forever, having disappeared without a trace from his reality (then, when he tries to return to the future, he will discover the changes made by him in the history and may well meet a new version of himself there ), or when returning to the future, he returns to his own reality and discovers that nothing has changed.
- Time travel creates a reality in which the traveler, as it were, has no place, that is, he is erased from his time. So, in the film "Project Nostradamus" detective Michael Nostrand, returning in due time, discovers that no one knows him.
- Each version of the development of events already exists and a change in the past simply sends the traveler to another world line (to another universe) corresponding to the given development of events. If the past changes without a physical time travel, no one will know about the change in the past, with the exception of those people who can retain memories when the world lines shift. This approach was used in the Japanese visual novel Steins; Gate .
- A variant of the previous one: a new reality appears during a change, but after some time the events naturally bring the changed reality into line with the unchanged. Thus, it is not the “arrow” that appears in history, but the “parallel segment”, which at some point again interferes with the main path. A good example is the death of the heroine in the film “Time machine”. However, from the point of view of modern physics, the availability of the ability to combine several past into one future is very doubtful.
- Each journey into the past instantly rewrites the old reality into a new one. People and objects from the old reality completely disappear (if they do not exist in the new reality) or change (if they exist in it). The time traveler himself does not change. Examples of this approach are The End of Eternity by Isaac Asimov , The Patrol of Time by Paul Anderson, The Palimpsest by Charles Strauss , the Butterfly Effect series, Vincent Van Gogh novel by S. Gansovsky, the novel and the film with the same name “ And Thunder struck ” , a series of " South Park " " Forward, God, Forward XII ." The science fiction writer Larry Niven suggested that in this case reality will change until it reaches a state in which time travel will never be open. This state is stable and is achieved in the "End of Eternity", forming the basic history of the universe of the Foundation and the Galactic Empire . Strauss, in turn, notes that within the framework of time-travel technology using “ wormholes ”, a traveler can be considered as a Hawking radiation wave packet. In the frame of reference associated with the rewritten version of reality, this package arises from a short-lived singularity and then disappears in it, and all the information contained in it does not have to satisfy the causality principle for the universe subjected to “corrections”. As a result, the traveler (or any recording device that he has) is, in principle, able to preserve the memories of the destroyed version of reality. Strauss calls such a set of testimonies “ unhistory ” ( unhistory ), and the set of sets forms the Last Library ( Final Library ) and its subordinates, available for editing Branch Libraries . The latter Library in a sense is the Feynman sum over world lines of all possible variants of human documented history.
- Rewriting can also affect time travelers themselves, as occurs in the story of Yuri Nesterenko, "The Hippocratic Oath." As their memories also change, they themselves do not notice either changes in the outside world or their own changes.
- Overwriting may not be instantaneous, but may take some time. This option is shown in the movie " Back to the Future " and the game " Chrono Trigger ". In this theory, a person who has set off into the past and made his birth impossible will disappear after some time, and not his double, but he himself .
The sequence of events is limited.
Only as long as the events do not affect the subjective past of the traveler in time. For example, a time traveler cannot kill his grandfather, or it turns out that in reality his grandfather was another person. He also cannot change the events that he knows have occurred. Robert Heinlein in his novel “The Door to Summer” developed this idea. The law of causality works for him, in relation to the sequence of actions of the person himself, regardless of his movements in time: the traveler is free in his actions, but in principle, nothing can happen to him that will make impossible the sequence of events that has already taken place for him, independently whether these events occurred in the past or in the future relative to the current moment in the “global” time. As a result, having returned to the past, it is possible to change events, but only those that the traveler did not know about before, and influence the future, but only that which has not yet occurred for the traveler. For example, a robber cannot return to the past to prevent the robbers (since this robbery has already happened to him), but nothing prevents him from capturing the stolen, hiding him in a secluded place and returning him after his return.
Here are some ways to solve the third problem (however, one can always object to this “paradox” by the fact that we cannot know for sure about the non-existence of something).
- It is assumed that in the future travel to the past is prohibited, and those people who nevertheless fall into our time try not to give out their presence in any way (Ass, Begemotov “Forward to the past”).
- According to another hypothesis, travel to the past is possible only after the time of the invention of the time machine, but not earlier. And the fact that our time is not filled with newcomers from the future only indicates that the time machine has not yet been invented, and not that travel to the past is impossible.
- Traveling to the past is not prohibited and there are many travelers from the future in our time, but they cannot or rather do not want to change the past, since the only consequence of this will be the multiplication of realities, which will not allow travelers to return to their original reality in the future. Thus, making changes to the past is simply meaningless, except in cases of special designing of the necessary reality. This option is considered, for example, in A. Makhrov’s science fiction novel “In the Whirlwind of Time”.
The Question of Morality
The moral aspect of the problem of time travel (into the future, by falling asleep under the influence of a special gas and waking up after many years in almost the same physical condition as at the time of going into a dream) raises the film “ Mr. McKinley 's Flight ” according to the script of Leonid Leonov . The song by Vladimir Vysotsky, “The Ballad of Going to Paradise,” used in this film, is addressed to the protagonist of the work: “A certain type will wake you up and let you into a world where war, stench and cancer last. Where is the Hong Kong flu defeated. Everything is ready ... Are you happy, fool? ”
In the movie
List: Time Travel Films and TV Shows
- concept album " Time " (1981) by Electric Light Orchestra 
- 's Time Machine concept album (1988)
List: Computer games about time travel
- Unresolved problems of modern physics
- Time Travel Legends
- Mori. Time Traveler Caught in Museum Photo? . forgetomori (April 15, 2010). Date of treatment July 8, 2019.
- Time traveler caught on film. Hey why not? (English) // Christian Science Monitor. - 2010 .-- October 28. - ISSN 0882-7729 .
- MS Morris, KS Thorne, U. Yurtsever, “ Wormholes, Time Machines, and the Weak Energy Condition, ” Phys. Rev. Lett. 61, 1446-1449 (1988)
- G. Bressi, G. Carugno, R. Onofrio, G. Ruoso, “ Measurement of the Casimir force between Parallel Metallic Surfaces ”, Phys. Rev. Lett. 88 041804 (2002)
- FJ Tipler "Rotating cylinders and the possibility of global causality violation", Phys. Rev. D. 9, 2203-2206 (1974)
- Hawking, Stephen. The Future of Spacetime. - WW Norton, 2002. - P. 96. - ISBN 0-393-02022-3 .
- Benjamin K. Tippett, David Tsang. Traversable Achronal Retrograde Domains In Spacetime . - 2017 .-- March 31.
- existence of a time machine was proved mathematically . Date of treatment June 22, 2017.
- Scientists have created a mathematical model of a time machine (Rus.) . Date of treatment June 22, 2017.
- Zeldovich, Novikov, 1975 , p. 679
- S. Krasnikov. The time travel paradox // Physical Review D. - 2002.- T. 65 , no. 6 . - ISSN 1089-4918 0556-2821, 1089-4918 . - DOI : 10.1103 / PhysRevD.65.064013 .
- Krasnikov S. V. Some questions of causality in general relativity: “time machines” and “superlight movements”. M .: Lenand, 2015. ISBN ISBN 978-5-9710-2216-9
- Friedman, John; Michael Morris Igor Novikov; Fernando Echeverria; Gunnar Klinkhammer; Kip Thorne; Ulvi Yurtsever. Cauchy problem in spacetimes with closed timelike curves // Physical Review D : journal. - 1990. - Vol. 42 , no. 6 . - P. 1916-1917 . - DOI : 10.1103 / PhysRevD.42.1915 . - .
- Ushenin S.G. Grandfather's paradox . // Server of modern literature "Samizdat" (November 24, 2009). Date of treatment November 1, 2015.
- The inventor of the time machine lived in Voronezh (June 27, 2013). Date of treatment November 1, 2015.
- Kolyadina, Elena. Hens behind barbed wire, but eggs have fallen in price // Metro Moscow . - 2014. - No. 26 of May 28 . - S. 12 . (Retrieved November 1, 2015)
- From the hits of the 80s: ELO - Ticket To The Moon . Grammota . grammota.com. Date of treatment November 20, 2018.
- Gardner Martin. Time Travel / Translation from English. Yu.A. Danilova. - M .: World, 1990.
- Glick James. Time travel. - M .: Mann, Ivanov and Ferber, 2018 .-- 288 p. - (MYTH. Scientific Pop). - ISBN 978-5-00100-903-0 .
- Zeldovich Ya. B., Novikov I.D.The structure and evolution of the Universe. - M .: Nauka , 1975 .-- 736 p.
- Kaku M. Time Travel // Physics of the Impossible / Translation: Natalya Lisova. - 10th ed .. - M .: ANF, 2018 .-- S. 386-406. - 586 p.
- Krasnikov S. V. Some questions of causality in general relativity: “time machines” and “superlight movements”. - M .: Lenand, 2015 .-- ISBN 978-5-9710-2216-9 .
- John Earman, Christian Wüthrich, John Manchak. Time Machines // The Stanford Encyclopedia of Philosophy / Edward N. Zalta. - Metaphysics Research Lab, Stanford University, 2016.
- Stockum, WJ van (1937). "The gravitational field of a distribution of particles rotating around an axis of symmetry.". Proc. Roy. Soc. Edinburgh A 57: 135.
- Worlds Enough and Time: Explorations of Time in Science Fiction and Fantasy / Gary Westfahl, George Edgar Slusser, David Leiby. - Greenwood Publishing Group, 2002 .-- 198 p. - ISBN 0313317062 .
- Ilyasov F. N. The nature of time and the impossibility of time travel . IC Orion (2017).
- Levin Alexey. Can you travel to the past? Yes, but purely theoretically // Popular mechanics . - No. 11 (109), November 2011 .
- Hawking S. Chapter 5. Protecting the Past // The World in a Nutshell
- Tsvetkov S. Tanks at the court of King Arthur : a list of films in which time travelers try to change history using modern weapons /
- Films about time travel // Magazine "Ekranka.ru"