This article by Peter Watson, of Carlton University, originally appeared in the interview and was published here with permission.
Time travel regularly appears in popular culture, with countless stories of time travel in movies, television and literature. But this is a surprisingly old idea: it can be argued that the Greek tragedy King Oedipus, written by Sophocles more than 2,500 years ago, is the first time travel story.
But is time travel 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 controversial.
The simplest answer is that time travel cannot be possible, because if it were, we would already be doing it. It can be argued that this is forbidden by the laws of physics, such as the second law of thermodynamics or relativity. There are also technical challenges: this may be possible, but it would involve huge amounts of energy.
There is also the question of the paradoxes of time travel; we can – hypothetically – resolve them if free will is an illusion, if there are many worlds, or if the past can only be observed but not experienced. Maybe time travel is impossible simply 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 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 Research, Kurt Gödel, is inventing a universe in which time travel is not just possible, but the past and the future are inextricably intertwined.
We can actually design time machines, but most of these (generally) successful proposals require negative energy or negative mass that doesn’t seem to exist in our universe. If you drop a tennis ball with a negative mass, it will fall up. This argument is quite unsatisfactory, as it explains why we cannot travel through time in practice simply by including another idea – that of negative energy or mass – that we do not really understand.
Mathematical physicist Frank Tipler conceptualizes a time machine that does not include 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 chance must always increase. Time can only move in one direction – in other words, you can’t stir an egg. In particular, traveling in the past, we go from now (high entropy state) to the past, which must have lower entropy.
This argument comes from the English cosmologist Arthur Eddington and is incomplete at best. It may stop you from traveling in the past, but it says nothing about time travel in the future. In fact, it is just as difficult for me to travel until next Thursday as it is to travel until last Thursday.
Resolving paradoxes
There is no doubt that if we can travel freely in time, we face paradoxes. The most famous is the “grandfather’s paradox”: one can hypothetically use a time machine to travel to the past and kill one’s grandfather before conceiving one’s father, thus eliminating the possibility of one’s own birth. Logically, you cannot exist and not exist at the same time.
Kurt Vonnegut’s anti-war novel Slaughterhouse Five, published in 1969, describes how to avoid the grandfather paradox. If free will simply does not exist, it is not possible to kill your grandfather in the past, because he was not killed in the past. The protagonist of the novel, Billy Pilgrim, can only travel to other points on his world line (the time line in which he exists), but not to any other point in space-time, so he can’t even thinks of killing his grandfather.
The universe in Slaughterhouse-Five is in line with everything we know. The second law of thermodynamics works perfectly in it and there is no conflict with relativity. But this is incompatible with some things we believe in, such as 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 make real modifications to the past so that we can go back and kill our grandfather – or Hitler? There are several theories about the multiverse that suggest that there are many timelines for different universes. It’s also an old idea: in Charles Dickens’s Christmas Carol, Ebony Scrooge experiences two alternate timelines, one leading to a shameful death and the other to happiness.
Time is a river
The Roman emperor Marcus Aurelius wrote that:
“Time is like a river made up of events that happen and a stormy stream; for when something is seen, it relates, and in its place another comes, and that too will be taken away. “
We can imagine that time does run along every point in the universe, like a river around a rock. But it is difficult to clarify the idea. Flow is the rate of change – river flow is the amount of water that travels 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 insight.
Theoretical physicist Stephen Hawking suggests that there must be a “hypothesis for the protection of chronology”, 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 extract information from it. But this argument is superfluous: we cannot travel in time because we cannot travel in time!
Researchers are exploring a more fundamental theory in which time and space “come out” of something else. This is called quantum gravity, but unfortunately it does not yet exist.
So, is time travel possible? Probably not, but we don’t know for sure!
Peter Watson, Honorary Professor of Physics at Carlton University
This article was republished by The Conversation under a Creative Commons license. Read the original article.
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