There is no doubt that if we could time travel freely, we run into the paradoxes. In practice, it is just as hard for me to travel to next Thursday as it is to travel to last Thursday. Perhaps it stops you travelling into the past, but it says nothing about time travel into the future. This argument originated with the English cosmologist Arthur Eddington, and is at best incomplete. More specifically, by travelling into the past we are going from now (a high entropy state) into the past, which must have lower entropy. Time can only move in one direction - in other words, you cannot unscramble an egg. Time travel also violates the second law of thermodynamics, which states that entropy or randomness must always increase. Mathematical physicist Frank Tipler conceptualized a time machine that does not involve negative mass, but requires more energy than exists in the universe. This argument is rather unsatisfactory, since it explains why we cannot time travel in practice only by involving another idea - that of negative energy or mass - that we do not really understand. If you drop a tennis ball of negative mass, it will fall upwards. 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. Unfortunately, one of his colleagues from the Institute for Advanced Study, Kurt Gödel, invented a universe in which time travel was not just possible, but the past and future were inextricably tangled. 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. (Rodrigo Gonzales/Unsplash) Laws of physics Some time travel theories suggest that one can observe the past like watching a movie, but cannot interfere with the actions of people in it. Perhaps time travel is impossible simply because time must flow in a linear manner and we have no control over it, or perhaps time is an illusion and time travel is irrelevant. There is also the matter of time-travel paradoxes we can - hypothetically - resolve these if free will is an illusion, if many worlds exist or if the past can only be witnessed but not experienced. There are also technical challenges: it might be possible but would involve vast amounts of energy. One can argue that it is forbidden by the laws of physics, like the second law of thermodynamics or relativity. The simplest answer is that time travel cannot be possible because if it was, we would already be doing it. As a theoretical physicist, I find that there are several possible answers to this question, not all of which are contradictory. But it is a surprisingly old idea: one can argue that the Greek tragedy Oedipus Rex, written by Sophocles over 2,500 years ago, is the first time travel story.īut is time travel in fact possible? Given the popularity of the concept, this is a legitimate question. Time travel makes regular appearances in popular culture, with innumerable time travel storylines in movies, television and literature.
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