In several previous posts, I have discussed various paradoxes that time travel could cause if it were possible. Travels to the future controlled from the past do not cause paradoxes. That they are possible is obvious, since we all travel to the future at a rate of twenty-four hours each day.
It is also possible to travel to the future if one is in a coma and later awakens; or through hibernation, if it were possible, which it is not at the moment, but it may become so someday; or by traveling through space at relativistic speeds, which would result in a time dilation that, upon returning to the starting point, translates into a journey to the future. This last procedure is not possible right now, but it could become so someday, apart from the technical difficulties of achieving it.
All journeys into the past, or into the future
controlled from the future, can produce paradoxes, many of them highly
destructive, as they would render inconsistent our universe. Let's look at
these paradoxes:
1.
The suicide paradox: the time traveler goes back a few years and kills
himself as a child. Since he is dead, he will not reach adulthood, and
therefore will not be able to travel back in time to kill himself. One of its
variations is the grandfather paradox, where the traveler kills his grandfather when he
was a child, which means that neither he nor his father (or mother) could have
been born. A version of this paradox also applies to journeys into the future
controlled from the future.
2.
The predestination paradox: the time traveler goes back in time to prevent a
catastrophe, for example, a train collision or Christ’s
crucifixion. Let's suppose that he succeeds. But if the catastrophe didn't
happen, there would be no reason to travel back in time to prevent it, so the
catastrophe wouldn't be averted.
3.
Objects without cause: the clearest example, which I've discussed in other
posts, is given by the short story Find the Sculptor by science fiction writer Sam Mimes.
4.
The paradox of unsourced information: See the description of this paradox in the
post of the same title, and an
example of its use by Woody Allen in the film Midnight in
Paris (2011).
5.
Time travel to the past and human freedom are incompatible. I touched on this topic in a recent post in this
blog: A
new time travel paradox.
6.
Time travel would violate the principle of conservation of
energy: This
observation was made by C.S. Lewis in his unfinished novel The Dark
Tower. I discussed
this in another
post.
7.
The Fermi Paradox: If time travel were possible, why don’t we have
evidence of anyone having traveled to our past? Actually, Fermi formulated
another version of this paradox to deny the existence of extraterrestrial
intelligence in our galaxy: if it existed, it would already be here. The
version of the paradox that applies to time travel is even more powerful.
As physicists often get carried away with strange
ideas and try to find solutions to their equations that would make possible
events that are almost certainly impossible, such as faster-than-light travel
or time travel, some of them have bothered to look for solutions to the
paradoxes. I discussed this in another
post. The most significant solution is that proposed by David Deutsch,
which relies on the supposed existence of Everett's quantum multiverse—the most
absurd version of the multiverse ever conceived by physicists—and asserts that time travel would always occur
between two different universes. This assumption would resolve some of the
paradoxes, but not all. In particular, the last two would still arise.
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| Causal past and future of an event |
We know that there are two
philosophical theories about time: block time or B-theory (all instants of
time exist simultaneously, and time would be just another dimension of the
universe, similar to the three spatial dimensions) and the passage of time or
A-theory (the past no longer exists; the
future does not yet exist and is not written; only the present exists). The
first theory would, in principle, make time travel possible; with the second,
it would be impossible. However much Kant, Einstein, and other physicists and
philosophers may insist, I think the passage of time is vastly confirmed, and
B-theory contradicts many things we know about time.
Thematic Thread about Time: Previous Next
Manuel Alfonseca
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