Thursday, March 17, 2016

Can time travel paradoxes be avoided?

Cover of Fantastic SciFi with a
Porgess story (The Shadowsmith)
In a science fiction story written in 1962 by Arthur Porges, entitled The rescuer, the inventors of a time machine discover that a man has entered the machine to travel back in time. To stop him, they destroy the machine with the man inside. When they are tried for murder and destruction of valuable property, they explain:
This man had taken with him a repeating rifle and five thousand rounds of ammunition. His intention was to arrive at Golgotha in time to rescue Jesus Christ from the Roman soldiers. In short, to prevent the crucifixion. And with a modern rifle, who can say he wouldn’t succeed? And then what?... What of the effect on the future, the entire stream of history, secular as well as religious?
The story is an excellent example of the paradox of predestination mentioned in a previous article, with several more that make us doubt the possibility of time travel. But is there no way to avoid the paradoxes? Is it possible to devise a theory that would remove them making time travel feasible, at least in principle? There have been several attempts to achieve this.

First, the past cannot be changed. If time travel were possible, travelers could reach the past, but not interact with it. In a tale by Charles F. Hall, cited by C.S. Lewis in the preface to his book The Great Divorce, the time traveler discovers that raindrops go through his body, and he cannot eat a sandwich, because his teeth cannot pierce it.
Second, the principle of consistency, proposed by the Russian cosmologist Igor Dmitrievich Novikov, which states that those events caused by time travelers that could lead to paradoxes, have zero probability. In other words, the traveler could try murdering himself as a child, but whatever his efforts, he would not be able to do it. Notice that in Harry Potter and the Prisoner of Azkaban, where Harry and Hermione travel back in time, the author has taken care that the principle of consistency is fulfilled, as all the interactions of the travelers with their own past are compatible with what they themselves had previously experienced.
Third, the problem can be solved by introducing a concept ​​which we have mentioned here before, the quantum multiverse in time, proposed over half a century ago by Hugh Everett III. According to his interpretation of quantum mechanics, whenever there is a collapse of a property of a particle (for instance, its spin), the universe splits in two, differing only in the result of that collapse. As there is a huge number of particles, and every fraction of a second there are many collapses, by now there would be a huge number of copies of our universe, with slightly different histories. Every possible history would have happened in one of these universes.
David Deutsch
How does this prevent the paradoxes? Consider, for instance, the suicide paradox. The inventor of a time machine travels to the past and kills himself as a child. According to David Deutsch, well known for his research on quantum computing, every instance of time travel into the past entails that the inventor would move to one of the innumerable universes in the Everett multiverse, different from ours. After murdering himself, in that universe he will not invent the time machine and travel back in time, but in the original universe nothing changes, the child is not killed and the paradox does not take place.
Now let us consider my time machine problem. It shows that time travel is incompatible with human freedom in decisions as simple as changing chairs or not. In the scenarios presented in the problem, if the traveler changes chairs in a moment of time, he must change also in the other, lest it generates an inconsistency in the universe (an object without a cause, as in the story by San Mimes). Using the Everett multiverse the paradox disappears, since the two changes of chairs, which take place before and after a travel in time, would occur in different universes and would not cause inconsistencies.
In short: Deutsch saves the possibility of time travel at the expense of assuming that the origin and the destination of the trip are in different universes; a somewhat drastic condition. It seems, therefore, that for time travel to be possible, the following things must happen:
1.      The B-theory of time must be true. Otherwise, time travel would be impossible.
2.      Everett’s quantum multiverse must be the correct interpretation of quantum mechanics. This implies that there are an incalculable number of universes, many of them extremely similar to each other, where anything that can happen will happen somewhere.
3.      Time travel must always take place between two different universes.
Enrico Fermi

I find it surprising that prestigious physicists waste their time with these theories, which belong to science fiction, rather than science, and publish them in serious scientific journals (which accept them).
But the worst is that the Fermi paradox would remain in force, despite all these imaginative solutions. If time travel were possible, where are the travelers? Why aren’t they here?
In conclusion, postulating that the A-theory of time is correct and time travel is impossible seems much more true to reality, such as we know it. Anything else would be just moonshine.

The same post in Spanish
Thematic thread on Multiverse and Fine Tuning: Preceding Next
Thematic thread on Time: Preceding Next
Manuel Alfonseca

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