>Hello Steemit! opportunely, I have read again "***The End of Eternity***", a successful science fiction novel that has as its central theme time travel, written by the renowned Isaac Asimov, I wanted to write a post about the possibilities of traveling in time and the main hypotheses that are posed as a solution to the paradoxes that may emerge in these trips.
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## <center>***Time***</center>
<center>This topic has been the subject of heated discussions for a long time, so before you start talking about travel in time, you need to know the various theories of what is known as "***time***". The concept of time has evolved from the ancient Greeks. With the advancement of science, the problem shifted from philosophy to the natural sciences, and physics. Before the eighteenth century it was assured that time was absolute, in this sense Newton's mechanics coincided with that idea, in his ***theory of gravitation***.</center>
<center>It would be from the eighteenth century on when we would begin to debate our idea of reality, and then raise the question of whether space and time, which seem to us as two essential components of reality, really exist or is simply a matter of modes of perception. In a metaphysical exposition ***Immanuel Kant*** defined space and time as the forms of our sensibility, which give to the things we know, its structure.</center>
<center>In 1883, ***Michelson-Morley*** undertook an experiment where they sought to measure the speed with which the earth moved with respect to the ether (hypothetical substance that fills space), turning its results into the experimental basis of ***Einstein's theory of Special Relativity*** , which postulated that there was no ether, no absolute space, nor absolute time.</center>
<center>However, Einstein in postulating his theory of ***General Relativity*** takes a step further, since time would not only be influenced by movement, but also by the matter of the universe. ***Gravitational phenomena were defined as simple alterations of the curvature of space-time produced by the presence of masses***. That is, clocks advance slower in the proximity of great masses.</center>
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<center>This theory has its greater scope of implication for the physics and the philosophy when it applies to the cosmos in general, because its equations imply that there cannot be a static universe. At the beginning Einstein did not agree with this idea, but it is ***Edwin Hubble*** who shows that the universe is expanding, leading to "cosmological time" and ***the time elapsed since the Big Bang (14 billion years), is equal throughout the universe***.</center>
## <center>***Time travel and its paradoxes***</center>
<center>A forward or backward shift in the timeline is what is defined as time travel. Scientists have said that so far these journeys have not been possible in real life, however since Einstein integrated time as the fourth coordinate (dimension) of space-time geometry (Relativity Theory), time travel began to be possible for many researchers, making this a challenge for theories of time and a way to develop and contrast their logic.</center>
<center>Although it is very difficult nowadays that we can construct a machine that allows us to travel in time, to know the future or to return to the past has been part of the plot in multiple novels of science fiction, in the cinema and in the television. We have grown up with ***the time machine*** as an icon within pop culture. However, all fiction on these journeys is often encountered with the inconvenience of temporal paradoxes, this being one of the reasons why some scientists discard the journey in time (In the novel The End of Eternity, there is a time policeman which tries to avoid these paradoxes).</center>
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<center>***The paradox of the grandfather*** is one of the most celebrated paradoxes, was written by Rene Barjavel in his book “***Future Times Three***”. What happens in this paradox is that the person who travels to the past, kills his grandfather, before he has established a love relationship with his grandmother and his father was born. The traveler would be making the mistake of ceasing to exist in a possible future, however this would imply that the traveler is not born and cannot travel to the past, so his grandfather does not die and if he does not die, the traveler can be born, which repeats the cycle giving origin to the paradox of the grandfather. In films like “Back to the Future”, this idea has already been used, when Marty Mcfly almost prevents his parents from falling in love. In a more comic version in “Futurama”, we see how Fry tries to avoid the death of his grandfather, but not getting it, he make loves with his grandmother, so he becomes his own grandfather</center>
## <center>***The invisible hand and the theory of many worlds***</center>
<center>Any solution of these paradoxes must be compatible with relativity and quantum theory. To be compatible with relativity, time must not be stopped, it must be continuous, kill your grandfather and simply disappear violates the laws of physics. So the paradox of the grandfather could be solved by posing the theory of the Russian cosmologist ***Igor Nokinov*** who believes that an undiscovered physical law prevents the story from being changed and that although the time traveler will try to kill his grandmother could not do so because that an ***"invisible hand"*** would prevent it. This would be ***Novikov's principle of self-consistency*** which postulates that any change we make in the past has to be consistent with the present. Another solution might be that by performing an action that changes the future, it would be creating a universe in a different line of time, which would coincide with ***the theory of many worlds of Everett***, where a quantum event takes place where all possible worlds could exist, avoiding most of the paradoxes associated with time travel. This idea is often used in science fiction.</center>
<center>So far we have seen how complicated it can be, defining the time, as well as finding the solution to solve the main paradoxes that occur in temporary trips. Many may wonder: is it possible to go back in time? Why we do not see tourists from the weather? ***Stephen Hawking affirm that "the best proof that it has not been possible to travel in time is that we have not been invaded by future tourists".*** As you see, many controversies and investigations are present in the study of time travel.</center>
<center>Do you think it is possible to travel in time? If you liked the post and want to continue seeing more articles of this type, upvote, follow me and resteem.</center>
>References:
> 1. Asimov, Isaac. The End of Eternity.
> 2. Gott, J. Richard, Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time
> 3. Nahin, Paul J., Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction
> 4. Rindler, Wolfgang “Relativity”, Oxford University Press, 2001.