Time travel appears regularly in popular culture, with countless stories of time travel appearing in films, television, and literature. But it’s a surprisingly old idea: one could argue that the Greek tragedy Oedipus Rex, written by Sophocles more than 2,500 years ago, is the first time travel story.
But is time travel actually possible? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I find that there are many possible answers to this question, and not all of them are contradictory.
The simplest answer is that time travel can’t be possible because if it was, we’d already be doing it. One could argue that it is prohibited by laws of physics, such as the second law of thermodynamics or relativity. There are also technical challenges: it may be possible but it can involve huge amounts of energy.
There is also the question of the paradoxes of time travel. We can – by default – solve these problems if free will is an illusion, if there are many worlds or if the past can only be seen but not experienced. Maybe time travel is impossible just because time has to flow in a linear way 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 most profound theory of time, we like to think that time travel is forbidden by relativity. Unfortunately, one of his colleagues from the Institute for Advanced Study, Kurt Gödel, invented a universe in which not only time travel was possible, but the past and the future were inextricably intertwined.
We can actually design time machines, but most of these successful proposals (in principle) require negative energy, or negative mass, which doesn’t seem to exist in our universe. If you drop a tennis ball of negative mass, it will fall upwards. This argument is somewhat unsatisfactory, as it explains why we can’t travel through time practically just by involving another idea – the idea of negative energy or mass – that we don’t really understand.
Mathematical physicist Frank Tippler conceptualized a time machine that does not have negative mass, but requires more energy than is present 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 – in other words, you cannot decipher an egg. More specifically, by traveling to the past we are moving from now (high entropy state) to the past, which should have lower entropy.
This argument originated with the English cosmologist Arthur Eddington, and is at best incomplete. It might prevent you from traveling to the past, but it doesn’t say anything about time travel into the future. Practically speaking, it is just as hard for me to travel to next Thursday as it is to travel to last Thursday.
There is no doubt that if we could freely travel through time, we would encounter paradoxes. The most famous of these is the “grandfather paradox”: one could theoretically use a time machine to travel into the past and kill their grandfather before their father had conceived, thus eliminating the possibility of them being born. Logically, you can neither be nor exist.
Kurt Vonnegut’s anti-war novel The Slaughterhouse Five, published in 1969, describes how to avoid the grandfather paradox. If free will simply does not exist, then the ancestor cannot be killed in the past, because he was not killed in the past. The protagonist, Billy Pilgrim, could only travel to other points on his world line (the timeline in which he is), but not to any other point in space-time, so he couldn’t even think of killing his grandfather.
The universe in Slaughterhouse Fifth corresponds to everything we know. The second law of thermodynamics works well within it and there is no conflict with relativity. But it goes against some of the things we believe in, like free will — you can observe the past, like watching a movie, but you can’t interfere with the actions of the people in it.
Can we allow actual alterations to the past, so we can go back and kill our grandfather – or Hitler? There are many multiverse theories that assume that there are many timelines for different universes. This is also an old idea: In Charles Dickens’ “A Christmas Carol,” Ebeneezer Scrooge encounters two alternate timelines, one leading to a shameful death and the other to happiness.
The Greek emperor Marcus Aurelius wrote that: “Time is like a river made up of events that happen, and a violent river; as soon as something is seen, it is carried away, and another comes in its place, and that will also be carried away.”
We can imagine that time passes through every point in the universe, like a river around a rock. But it’s hard to make the idea accurate. Flow is the rate of change – the flow of a river is the amount of water that passes a certain length at a given time. Hence, if time is a flow, it is at a rate of 1 second per second, which is not a very useful idea.
Theoretical physicist Stephen Hawking has suggested that the “chronology protection conjecture” must exist, a hitherto unknown physical principle that prevents time travel. Hawking’s concept arose from the idea that we can’t tell what’s going on inside a black hole, because we can’t get information from it. But this argument is redundant: we cannot time travel because we cannot time travel!
Researchers are investigating a more fundamental theory, where time and space “spring” from something else. This is referred to as quantum gravity, but unfortunately it has not yet been found.
Is time travel possible? Probably not, but we don’t know for sure!
Time travel can be possible, but only with parallel timelines
Introduction of the conversation
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