Back to the Future re- presents the epitome of time travel paradoxes, dubbed by physicists the bootstrap paradox in honor of one of the most convoluted and complex tales of loops in all of science fiction literature: By His Bootstraps (1941) –Titled in Spanish By His Own Means – by the always imaginative Robert A. Heinlein. That a train can arrive at the station before it leaves it is a catastrophe in physics , since it implies a break in causality. It is somewhat difficult to handle.
Another example is James Cameron’s script in the film that launched him to stardom as a director: Terminator (1984). Let’s imagine that the cyborg Schwarzenegger manages to kill Sarah Connor, the mother of the future leader of the revolution against Skynet – the artificial intelligence that governs the machines – before she becomes pregnant. And if your child is not born, there is no human leader, then there is no need to send any robosicario from the future. The very existence of the mission highlights its failure. But then who kills Sarah Connor? Murder occurs if no murderer is sent; a real gibberish. Despite the philosophical discomfort it causes, there is no law of physics that prohibits the existence of time machines. Therefore, how do we resolve the paradoxes it causes?
In Back to the Future , Marty McFly dissolves as the likelihood of his parents meeting decreases. The writer Robert Silverberg proposes a similar solution in his novel By Time (1969): we are protected from all the changes we cause during the trip to the past, but when we return they will materialize suddenly. In other words, if I prevent my parents from getting together, I won’t notice anything until I return to the present: then I will disappear.
Another solution is that, whatever we do, we will never change the course of events. Writer Connie Willis has left us in his two best novels, The Book of the Day of the Last Judgment (1992) Not to mention the dog (1997), a solution that protects the history of any interference: the slippage or sliding. If we want to get to a conflictive moment, such as the Battle of Thermopylae, the laws that protect the space-time continuum will cause the time machine to send us to a place far enough away to avoid interference, or to the same place but in a time before or after.
And even if this fails, the laws of chronological protection can roll their dice and cause something highly unlikely to restore what we’ve modified or simply prevent travel from occurring. Precisely, this is the idea that the English physicist Stephen Hawking defends. There must be a kind of censorship that makes it impossible to create time machines. His argument is that if they could be built, we would be invaded by hordes of tourists from the future. With his trademark humor, Hawking claims that conjecture keeps the world safe for historians.
Of course, this proposal leaves scientists who like to push theories to the limit without a toy. For example, the defenders of the multiverse. This radical interpretation is associated with the so-called measurement problem, within quantum theory, and was first formulated in 1957 by Hugh Everett III. As it was established in the 1920s, every subatomic system is defined by what is called the wave function: with it we are able to predict how, for example, an electron will evolve. Now, it tells us by means of probabilities: for example, that there is a 50% chance that a quantum characteristic called spin is worth 1/2 and another 50% that it is worth -1/2. Only when we make the experimental measurement will we know what value it really takes: this is called the collapse of the wave function.
Such indeterminacy has generated different hypotheses, since in our heads it is inconceivable that a system does not have well-defined properties. The first of the solutions is called the Copenhagen interpretation and it says that we must accept the world as it is: nothing has exact values until someone or something measures it. Put more poetically, the moon does not exist until we look at it.
Everett’s proposal denies the older one. There is no such wave function collapse, but in each measurement the universe forks: in one of the branches the electron has spin 1/2, and in the other, -1/2. Thus we have two absolutely identical worlds, except for the difference in the properties of an electron. As the physicist Bryce DeWitt (1923-2004) claimed, “each quantum transition that occurs in each star, each galaxy, each remote corner of the cosmos, is dividing our local world into myriad copies of itself. It’s schizophrenia with desire! ”. Obviously, we are not aware of this multiplication of universes, totally unconnected to each other.
In this hypothesis of the many worlds – or of the many stories, as its defenders prefer to call it – the bootstrap paradox does not exist. The time traveler can kill his grandfather when he is still a teenager: in reality, the murderer will sneak into a branch of the multiverse where, simply, he was not born. And most importantly, the world where the grandfather is alive does not disappear: it is one more branch of the cosmic fabric.
The cleavage ad infinitum does not convince many scientists. So how do you solve the paradoxes? The answer is simple: a time traveler cannot change the past because he is always part of it. You can have coffee with your mother, but you cannot carry her; You can observe a past event, but no matter how hard you try, what happened will always happen. As John Richard Gott III of Princeton University says, “It doesn’t matter how many times you see Casablanca : Ingrid Bergman will always get on the plane .” It is the principle of self-coherence, formulated by Igor Novikov and Kip Thorne.
One of the main drawbacks of this filter is the disappearance of free will, as if something higher prevented us from making decisions with total freedom. However, as the philosopher David K. Lewis (1941-2001) pointed out, this is only appearance , because we cannot do logically impossible things: a watermelon will never end up having a size greater than that of the universe, no matter how much we want it.
Now, can you always find a self-consistent solution to any time travel? This is what Novikov and Thorne demonstrated by setting up thought experiments in which a billiard ball traveled back in time to collide with itself and prevent it from entering a time machine . Well, for each of the scenarios they always found a solution where the shock remained in a slight tap that did not prevent the ball from entering its destination.
