Our curiosity about time travel is thousands of years old. Credit: Shutterstock
Time travel appears regularly in popular culture, with countless stories of time travel in film, television, and literature. But it is a surprisingly old idea: it can be argued that the Greek tragedy Oedipus the King, written by Sophocles more than 2,500 years ago, is the first time travel story.
But is it possible to travel back in time? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I find that there are several possible answers to this question, not all of them contradictory.
The simplest answer is that time travel cannot be possible because if it were, we would already be. It can be argued that it is forbidden by the laws of physics, such as the second law of thermodynamics or relativity. There are also technical challenges: it could be possible but it would involve large amounts of energy.
There is also the question of the paradoxes of time travel; we can solve them — hypothetically — if free will is an illusion, if there are many worlds, or if the past can only be witnessed but not experienced. Maybe time travel is impossible simply because time has to flow linearly and we have no control over it, or maybe time is an illusion and time travel is irrelevant.
Laws of physics
Since Albert Einstein’s theory of relativity — which describes the nature of time, space, and gravity — is our deepest theory of time, we would like to think that time travel is forbidden by relativity. Unfortunately, one of his colleagues at the Institute for Advanced Study, Kurt Gödel, invented a universe in which time travel was not only possible, but the past and the future were inextricably intertwined.
In fact, we can design time machines, but most of these (in principle) successful proposals require negative energy, or too much negative energy, that does not seem to exist in our universe. If you drop a tennis ball of negative mass, it will fall upwards. This argument is quite unsatisfactory, as it explains why we cannot travel back in time to practice only by implying another idea — that of energy or too negative — that we do not really understand.
Mathematician Frank Tipler conceptualized a time machine that does not involve too much negativity, but requires more energy than exists in the universe.
Time travel also violates the second law of thermodynamics, which states that entropy or randomness must always increase. Time can only move in one direction, that is, an egg cannot be deciphered. More specifically, traveling to the past we go from now (a state of high entropy) to the past, which must have a lower entropy.
This argument originated with the English cosmologist Arthur Eddington, and at best is incomplete. It may prevent you from traveling to the past, but it says nothing about time travel to the future. In practice, it costs me as much to travel next Thursday as it does to travel last Thursday.
Paradox resolution
There is no doubt that if we could travel in time freely, we would encounter paradoxes. The best known is the “grandfather paradox”: hypothetically, a time machine could be used to travel to the past and kill his grandfather before his father’s conception, thus eliminating the possibility of his own birth. Of course, you can’t exist and you can’t exist.
Kurt Vonnegut’s anti-war novel “Slaughterhouse-Five”, published in 1969, describes how to escape his grandfather’s paradox. If free will simply does not exist, it is not possible to kill your grandfather in the past, as he was not killed in the past. The protagonist of the novel, Billy Pilgrim, can only travel to other points in his line of the world (the timeline in which it exists), but not to any other point in space-time, so he could only contemplate killing his grandfather.
The universe of “Slaughterhouse-Five” is consistent with everything we know. The second law of thermodynamics works perfectly within it and there is no conflict with relativity. But it is not consistent with some things we believe in, such as free will; you can look at the past, like watching a movie, but you can’t interfere with the actions of the people involved.
Could we allow real modifications of the past, to be able to go back and assassinate our grandfather or Hitler? There are several multiverse theories that suggest that there are many timelines for different universes. This is also an old idea: in “A Christmas Carol” by Charles Dickens, Ebeneezer Scrooge experiences two alternate timelines, one of which leads to shameful death and the other to happiness.
Time is a river
Greek Emperor Marcus Aurelius wrote: “Time is like a river formed by events and a violent torrent; , and this one will also be taken away “.
We can imagine that time passes through all parts of the universe, like a river around a rock. But it is difficult to pinpoint the idea. A flow is a rate of change: the flow of a river is the amount of water that passes a certain length in a given time. Therefore, if time is a flow, it is at a rate of one second per second, which is not a very useful view.
Theoretical physicist Stephen Hawking suggested that there must be a “chronological protection conjecture,” a still unknown physical principle that forbids time travel. Hawking’s concept stems from the idea that we can’t know what’s going on inside a black hole, because we can’t get information out of it. But this argument is redundant: we cannot travel in time because we cannot travel in time!
Researchers are investigating a more fundamental theory, where time and space “arise” from something else. This is known as quantum gravity, but unfortunately it does not yet exist.
So is it possible to travel back in time? Probably not, but we don’t know for sure!
Time travel might be possible, but only with parallel timelines provided by The Conversation
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