Can we time travel? A theoretical physicist gives some answers

Our curiosity about time travel is thousands of years old. Credit: Shutterstock

Time travel is a regular feature in popular culture, with countless time travel storylines appearing in movies, 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 several possible answers to this question, not all of which are contradictory.

The simplest answer is that time travel is not possible, because if it were, we would already be doing it. 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 done, but it would take enormous amounts of energy.

There is also the issue of time travel paradoxes; we can – hypothetically – solve these if free will is an illusion, if many worlds exist or if the past can only be perceived but not experienced. Maybe time travel is impossible simply because time has to be linear 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 would like to think that time travel is forbidden by relativity. Unfortunately, one of his colleagues at the Institute of Advanced Study, Kurt Gödel, invented a universe in which time travel was not only possible, but where past and future were inextricably linked.

We can actually design time machines, but most of these (basically) successful proposals require negative energy, or negative mass, which doesn’t seem to exist in our universe. If you drop a tennis ball with negative mass, it will fall upwards. This argument is rather unsatisfactory, because it explains why in practice we can’t travel in time, only by using another idea – that of negative energy or mass – that we don’t really understand.

Mathematical physicist Frank Tipler invented a time machine that has no negative mass, 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 – in other words, you can’t decipher an egg. More specifically, by traveling to the past, we move from now (a high-entropy state) to the past, which must have a lower entropy.

This argument comes from English cosmologist Arthur Eddington and is incomplete at best. It may keep you from traveling to the past, but it says nothing about time travel to the future. In practice, it is just as difficult for me to travel to next Thursday as it is to travel to last Thursday.

Solving paradoxes

There is no doubt that if we were allowed to travel freely in time, we would run into the paradoxes. The best known is the “grandfather paradox”: hypothetically, one could use a time machine to travel to the past and kill their grandfather before their father’s conception, ruling out the possibility of their own birth. Logically, you cannot both exist and not exist.

Kurt Vonnegut’s anti-war novel “Slaughterhouse-Five”, published in 1969, describes how to get around the grandfather paradox. If free will simply does not exist, then it is not possible to kill one’s grandfather in the past as he has not been killed in the past. The novel’s protagonist, Billy Pilgrim, can only travel to other points on his worldline (the timeline in which he exists), but not to another point in space-time, so he couldn’t even consider killing his grandfather.

The universe in “Slaughterhouse-Five” matches everything we know. The second law of thermodynamics works perfectly in it and there is no conflict with the theory of relativity. But it conflicts with some 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 people in it.

Could we allow actual changes in the past so that we can go back and kill our grandfather – or Hitler -? There are several 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 experiences two alternate timelines, one leading to shameful death and the other to happiness.

Time is a river

The Greek Emperor Marcus Aurelius wrote: “Time is like a river composed of the events that take place, and a violent current; for as soon as something is seen it is carried away and another comes in its place, and this also will be. be taken.”

We can imagine that time does indeed flow past every point in the universe, like a river around a rock. But it is difficult to make the idea precise. 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. So if time is a flow, it’s at a rate of one second per second, which isn’t a very useful insight.

Theoretical physicist Stephen Hawking suggested that there must be a “presumption of chronological protection,” an as-yet-unknown physical principle that prohibits 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 extract information from it. But this argument is superfluous: we can’t travel in time because we can’t travel in time!

Researchers are exploring a more fundamental theory, in which time and space “appear” from something else. This is called quantum gravity, but unfortunately doesn’t exist yet.

So is time travel possible? Probably not, but we’re not sure!

Time travel could be possible, but only with parallel timelines

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