{"id":83110,"date":"2022-06-13T17:22:12","date_gmt":"2022-06-13T21:22:12","guid":{"rendered":"https:\/\/newsroom.carleton.ca\/?post_type=cu_story&p=83110"},"modified":"2025-08-19T09:37:09","modified_gmt":"2025-08-19T13:37:09","slug":"can-we-time-travel","status":"publish","type":"cu_story","link":"https:\/\/carleton.ca\/news\/story\/can-we-time-travel\/","title":{"rendered":"Can we time travel? A theoretical physicist provides some answers"},"content":{"rendered":"\n
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\n Can we time travel? A theoretical physicist provides some answers\n <\/h1>\n \n <\/header>\n <\/div>\n\n \n <\/path>\n <\/path>\n <\/svg>\n <\/div>\n\n \n\n <\/div>\n<\/section>\n\n

This article is republished<\/a> from The Conversation under a Creative Commons licence. All photos provided by The Conversation<\/a> from various sources.<\/p>\n\n\n\n

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Time travel makes regular appearances in popular culture, with innumerable time travel storylines in movies, television and literature. But it is a surprisingly old idea: one can argue that the Greek tragedy Oedipus Rex<\/em>, written by Sophocles over 2,500 years ago, is the first time travel story<\/a>. <\/p>\n\n\n\n

But is time travel in fact 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. <\/p>\n\n\n\n

The simplest answer is that time travel cannot be possible because if it was, we would already be doing it. One can argue that it is forbidden by the laws of physics, like the second law of thermodynamics<\/a> or relativity<\/a>. There are also technical challenges: it might be possible but would involve vast amounts of energy. <\/p>\n\n\n\n

There is also the matter of time-travel paradoxes; we can \u2014 hypothetically \u2014 resolve these if free will is an illusion, if many worlds exist or if the past can only be witnessed but not experienced. 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.<\/p>\n\n\n\n

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\n 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.<\/span>
\n (Rodrigo Gonzales\/Unsplash)<\/span><\/span>
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Time Travel & the Laws of Physics<\/h2>\n\n\n\n

Since Albert Einstein’s theory of relativity \u2014 which describes the nature of time, space and gravity \u2014 is our most profound theory of time, we would like to think that time travel is forbidden by relativity. Unfortunately, one of his colleagues from the Institute of Advanced Study, Kurt G\u00f6del, invented a universe<\/a> in which time travel was not just possible, but the past and future were inextricably tangled. <\/p>\n\n\n\n

We can actually design time machines<\/a>, but most of these (in principle) successful proposals require negative energy<\/a>, or negative mass, which does not seem to exist in our universe. If you drop a tennis ball of negative mass, it will fall upwards. This argument is rather unsatisfactory, since it explains why we cannot time travel in practice only by involving another idea \u2014 that of negative energy or mass \u2014 that we do not really understand. <\/p>\n\n\n\n

Mathematical physicist Frank Tipler conceptualized a time machine that does not involve negative mass, but requires more energy than exists in the universe<\/a>.<\/p>\n\n\n\n

Time travel also violates the second law of thermodynamics<\/a>, which states that entropy or randomness must always increase. Time can only move in one direction \u2014 in other words, you cannot unscramble an egg. More specifically, by travelling into the past we are going from now (a high entropy state) into the past, which must have lower entropy. <\/p>\n\n\n\n

This argument originated with the English cosmologist Arthur Eddington<\/a>, and is at best incomplete. Perhaps it stops you travelling into the past, but it says nothing about time travel into the future. In practice, it is just as hard for me to travel to next Thursday as it is to travel to last Thursday.<\/p>\n\n\n\n

Resolving Paradoxes<\/h2>\n\n\n\n

There is no doubt that if we could time travel freely, we run into the paradoxes. The best known is the “grandfather paradox<\/a>“: one could hypothetically use a time machine to travel to the past and murder their grandfather before their father’s conception, thereby eliminating the possibility of their own birth. Logically, you cannot both exist and not exist.<\/p>\n\n\n\n

Kurt Vonnegut’s anti-war novel Slaughterhouse-Five<\/em><\/a>, published in 1969, describes how to evade the grandfather paradox. If free will simply does not exist, it is not possible to kill one’s grandfather in the past, since he was not killed in the past. The novel’s protagonist, Billy Pilgrim, can only travel to other points on his world line (the timeline he exists in), but not to any other point in space-time, so he could not even contemplate killing his grandfather.<\/p>\n\n\n\n

The universe in Slaughterhouse-Five<\/em> is consistent with everything we know. The second law of thermodynamics works perfectly well within it and there is no conflict with relativity. But it is inconsistent with some things we believe in, like free will \u2014 you can observe the past, like watching a movie, but you cannot interfere with the actions of people in it.<\/p>\n\n\n\n

Could we allow for actual modifications of the past, so that we could go back and murder our grandfather \u2014 or Hitler<\/a>? There are several multiverse theories that suppose that there are many timelines for different universes. This is also an old idea: in Charles Dickens’ A Christmas Carol<\/em>, Ebeneezer Scrooge experiences two alternative timelines, one of which leads to a shameful death and the other to happiness. <\/p>\n\n\n\n

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