This argument originated with the English cosmologist Arthur Eddington (opens in new tab), 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 (opens in new tab), which states that entropy or randomness must always increase. Mathematical physicist Frank Tipler conceptualized a time machine that does not involve negative mass (opens in new tab), 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 (opens in new tab), but most of these (in principle) successful proposals require negative energy (opens in new tab), 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 (opens in new tab) 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.

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