Time travel regularly appears in popular culture, with countless time travel stories in movies, television and literature. But it is a surprisingly old idea: one can argue that the Greek tragedy of Oedipus Rex, written by Sophocles more than 2,500 years ago, is the first time travel story. But is time travel really possible? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I believe that there are many 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 may be possible, but it would require huge amounts of energy. There is also the issue of time travel paradoxes. we can – hypothetically – solve them if free will is an illusion, if there are many worlds or if the past can be witnessed but not. 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 believe that time travel is forbidden by relativity. Unfortunately, one of his colleagues from the Institute for Advanced Study, Kurt Gödel, came up with a universe in which time travel was not only impossible, but the past and the future were inextricably linked. We can actually design time machines, but most of these (in principle) successful proposals require negative energy or negative mass, which does not seem to exist in our universe. If you throw a tennis ball of negative mass, it will fall upwards. This argument is rather unsatisfactory, as it explains why we can not travel through time in practice just by involving another idea – that of negative energy or mass – that we do not really understand. Mathematical physicist Frank Tipler created a time machine that does not contain negative mass, but requires more energy than that 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 not dust an egg. More specifically, traveling in the past we go from the now (state of high entropy) to the past, which must have a lower entropy. This argument was started by the English cosmologist Arthur Eddington and is at best incomplete. It may prevent you from traveling in the past, but it says nothing about time travel in the future. In practice, it is as difficult for me to travel for next Thursday as it was for me last Thursday. Solving paradoxes There is no doubt that if we could travel freely in time, we would face paradoxes. The best known is the “grandfather paradox”: hypothetically one could use a time machine to travel back in time and kill one’s grandfather before his father was arrested, thus eliminating the possibility of being born. Of course you can not exist and not exist. Kurt Vonnegut Slaughterhouse-Five, an anti-war novel published in 1969, describes how to avoid the grandfather’s paradox. If there is simply no free will, it is not possible to kill one’s grandfather in the past, since he was not killed in the past. The protagonist of the novel, Billy Pilgrim, can only travel to other parts of his world line (the time line in which he exists), but not to any other point in space-time, so he could not even think of killing his grandfather. The Slaughterhouse-Five universe 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 in line with certain things we believe in, such as free will – you can observe the past, like watching a movie, but you can not interfere in people’s actions in it. Could we allow real modifications of the past so that we can go back and kill our grandfather – or Hitler? There are many multiverse theories that assume that there are many timetables for different universes. This is also an old idea: in Charles Dickens’s A Christmas Carol, Ebeneezer Scrooge experiences two alternative timelines, one of which leads 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 violent current. for as soon as one thing is seen, it is carried away, and another comes in its place, and this too will be carried away. “
We can imagine time flowing across every part of the universe, like a river around a rock. But it is difficult to get the idea right. A flow is a rate of change – the flow of a river is the amount of water that crosses a certain length at a given time. Therefore, if time is flowing, it is at a rate of one second per second, which is not very useful. Theoretical physicist Stephen Hawking suggested that there must be a “chronology protection conjecture”, a still unknown natural principle that forbids time travel. Hawking’s idea comes from the idea that we can not know what is going on inside a black hole because we can not get information from it. But this argument is unnecessary: we can not travel in time because we can not travel in time! Researchers are exploring a more fundamental theory, where time and space “emerge” from something else. This is referred to as quantum gravity, but unfortunately does not yet exist. So is time travel possible? Probably not, but we do not know for sure! Peter Watson, Emeritus Professor of Physics, Carleton University This article is republished by The Conversation under a Creative Commons license. Read the original article.
