Science Fiction presents distant spaces, unknown galaxies, fantastic sceneries, but also scientific achievements that seem impossible.
This is, without doubt, the case of time travel.
A topic treated multiple times in science fiction. Time travel is a brainteaser, not just for authors of science fiction, but also and especially for physicists.
A decade ago, I read a book meant for a wider public by astrophysicist J. Richard Gott, a professor at Princeton University, about time travel.
For an entertaining purpose, J. Richard Gott talks about the checkability of time travels in Sci-fi films, series, and books.
H. G. Wells reflected on the notion of time by introducing it as the fourth dimension in his novel The Time Machine, published in 1895, ten years before Albert Einstein made the space-time a key point of his theory of relativity!
Beyond evil Morlocks, sort of cannibal caricatures that represent all that is detestable in human nature, which the hero of Wells visits in the year 802 701 (just that!), the British author put his finger on a troubling aspect of existence by asking the following question - it's not the exact words, but I summarize it that way:
Can an object have a material existence if it has no duration in time?
Gott ( by the way „Gott“ means „God“ in German), who was already amused to read popular-science books at the age of twelve, describes the universe in four dimensions, based on a model by the physicist George Gamow from the 1940s.
If you imagine the earth revolving around the sun, not only as an object in three dimensions but four (since otherwise, it would not even have the opportunity to move), the earth would look like a spaghetti wriggling around the sun, being itself a tube in space-time.
The shape of a spaghetti or a tube representing the continuity of an object in time, the so-called world line in physics. By that, it's meant, that you're yourself an object in space-time as well, which has a world line from birth to death.
A model of time travel based on that principle would allow you to drink a beer with a younger version of yourself, but you‘d maybe be stuck in a time loop that would be repeated ad infinitum.
Your destiny would be different if you believe in the theory of multiple universes, like the famous physicist with Asperger syndrome, Dr. Sheldon Cooper, of "The Big Bang Theory“ series.
"Back to the Future" by Robert Zemeckis released in 1985 - and one of the most comforting memories of my childhood - shows in a very amusing way the grandfather paradox through the story of Marty McFly who travels from 1985, his present, to 1955, where he meets his parents who don't know each other yet.
The problem is that his mother falls in love with him and this puts the meeting of his parents in danger; and as a result, his own existence.
The rest, almost everyone knows...
The paradox arises through the obstacle that the protagonist is to his parents.
He makes his own birth impossible.
If is parents can't get to know each other, it erases his world line from his birth to the point where he would undertake this journey through time.
The temporal paradox is that there wouldn't be anybody to prevent his parents to meet and his birth would happen anyway.
Let‘s return to the theory of multiple universes, developed by Hugh Everett, American physicist, and respected by one of the most entertaining fictional characters, Dr. Sheldon Cooper.
This concept is based on quantum mechanics. Quantum Mechanics was born in the 1920s and tries to explain the behavior of atoms and molecules.
An important aspect of Quantum Mechanics is the uncertainty principle, developed by the German physicist Werner Heisenberg in 1927. It states that one can't define the exact speed and location of a particle simultaneously.
With the wave equation resolution (see Schrödinger equation, but I 'm far away from beeing a physicist!) on the other hand, it is possible to establish a probability, that a particle will be findable in a specific place. This allows an interpretation of a theory of multiple worlds in quantum mechanics, which assumes that there are several parallel worlds.
Theoretically, every possible place where a particle could be found is a potential parallel world. Parallel worlds involve parallel stories.
The series "Fringe" is based on this concept.
J. Richard Gott quotes the novel by Gregory Benford, „Timescape“, published in 1980 (I'll put it on my long list of books to read), which tells a story from 1998 where the hero uses a tachyon beam (tachyons are hypothetical particles that move faster than light) to send a message in 1963 to prevent scientists from the time of an ecological disaster, that will destroy the world in 1998.
The end of the story is probably as you might suspect. The scientists of that time, who already know the theory of parallel worlds, come to prevent the world from a sad end, but of course in a parallel world.
This means that in the world where the hero sent the signal, the ecological disaster still takes its course.
Outside of science fiction, the physicist Richard Feynman showed that if we want to know the probability of a particular event, we should take into account all possible story patterns (in our global history, for example), which could lead to this particular event,
It will imply, that maybe, all possible patterns of history, really exist.
If we return to the grandfather paradox, we must talk about the principle of consistency of physicists Igor Novikov and Kip Thorne, which contradicts it.
Kip Thorne, by the way, was the scientific mastermind behind "Interstellar" by Christopher Nolan.
Check out "The Science of Interstellar".
According to this principle, time travel can't change the course of events and so there is no paradox possible, since just the fact that the present moment is what it is, it's not possible that a journey into the past has managed to change it.
On this premise, there is only one timeline.
So no matter what different events have happened in the past - for example killing your own grandfather-, it does not alter the course of history.
This implies that there's no free will.
In "The Excellent Adventure Bill and Ted", science fiction comedy released in 1988, the authors used the principle of consistency to make their protagonists travel.
Two friends, who aren't particularly hard-working students at their high school, are threatened to be separated. Ted would be sent to a military school in Alaska if they don‘t both get a good grade for the next presentation of history.
This threatens the creation of their rock band, which according to a messenger from the future, will have a major impact on society, as well. The messenger gives them access to a time travel machine.
They choose to travel back in time to take a few historical personalities into the 20th century. Those personalities, who are obviously not adapted to the world of 1988, will go to jail for vandalism, among other things.
It leaves the two protagonists only a few minutes before the History's presentation. The prison's key, in which the historical personalities are enclosed, were in the possession of Ted‘s father, who is a sheriff.
Two days before the presentation.
Unfortunately, they don‘t have enough time to access the machine before the presentation and they come up with the idea of going back to the past, after their History lesson, to get the keys and put them behind a panel... just right now!
They are behind the panel and the story takes its course. They succeed in their presentation and become rock stars.
Bill and Ted use their free will throughout History, but it doesn't change their destiny!
In all cases, parallel universes or not, the principle of consistency makes it impossible to change the past.
Also a time paradox, a jinn is an object or information around which the world line is looped.
This means that it is an object that has no beginning or end.
Gott had fun portraying himself traveling back to 1905 to explain Einstein's theory of relativity. After this, Einstein would publish an essay on this theory in 1905.
But Gott would actually have acquired his knowledge of the theory of relativity from Einstein‘s paper, published in 1905. So the loop is closed.
The paradox is unavoidable.
According to quantum mechanics, it's possible, in theory, to make a macroscopic object appear spontaneously if enough energy is used.
But in practice, it's very unlikely.
Even more confusing is the concept of self-mothering, through time travel, as described in the short story "All You Zombies" from 1959 by Robert A. Heinlein.
Gott quotes in the same registry "Orion" by Ben Bova published in 1984 in which humanity causes its own existence by traveling through time.
In the novel "Contact“, written in 1985, which will be filmed by Robert Zemeckis in 1997, with Jodie Foster in the lead role, Carl Sagan describes the principle of the wormhole (Wurmloch in German, I love that word!).
The story of "Contact" is about a SETI Scientist (SETI = Search for Extraterrestrial Intelligence) who captures a radio signal while observing the star Vega.
One of the signals alarms the government. It contains a coded message that is emanating from Earth.
This message was received and sent back by the inhabitants of the Vega system in response to a television transmission from 1936!
Small problem: A signal takes approximately 26 years to travel between from Earth and to Vega. It explains why the Earth's signal from 1936 goes back to Earth in 1988.
In this message, a method of construction and employment for a spacecraft is included. And it allows interstellar travels.
From there a global dispute is born, turning around moral issues, and who is has the right to travel in the capsule - which can only accommodate one person.
Due to her lack of religious belief, the SETI-scientist isn't a favorite on the list of potential travelers.
As the "chosen-one" dies in the first experiment, she's accepted as a guinea pig.
The capsule causes a wormhole and lands in the area of Vega.
While she has the privilege of admiring the Vega system for a while, the capsule is sucked back into a wormhole and ends on a beach, where she finally meets an alien who's token the appearance of her father.
Reabsorbed into a wormhole, she's back exactly at the time of her departure. As she exits the capsule, everybody thinks it hasn't worked.
The truth is, that nobody noticed she was gone.
Everyone thinks she's lying, but as the presidential advisor remarks that the contentless videotapes last 18 hours, doubts start to arise.
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