By three quantum physicists

Jan 23, 2006 14:28 GMT  ·  By

The idea of time traveling into the past has numerous conceptual difficulties. The following paradox is well known: How about going back into the past and killing your own father before he even meets your mother? Then, you wouldn't be born. But if you're out of existence, who killed your would-be father?

Such hypothetic scenarios prevent many people from believing time travel is possible. However, the general theory of relativity allows various space-time deformations to take place and Kurt G?del, the famous mathematician, managed to prove that, according to this theory, time travel into the past can in principle exist (see the "worm hole" in the picture below).

Time travel isn't explicitly made impossible by quantum mechanics either. On the contrary, one's sense of causality is deeply challenged by it.

Some science-fiction writers tried to get around the paradoxes of time travel by using the idea of parallel universes. When one would go into the past and change something, the universe would continue on a different path. But the original universe would still continue to exist in parallel. This Oedipus time traveler comes from one universe and kills his father in a different universe.

By studying this science-fiction idea with the help of quantum mechanics, three physicists, Daniel M. Greenberger and Karl Svozil on one hand and David T. Pegg on the other, have arrived to the same conclusion independently.

The idea in quantum mechanics is that all the possibilities do exist simultaneously in some abstract way, but they are not parallel, they influence each other. What is actually seen to happen is the result of a certain averaging process over all these possibilities - all the possible universes influence each other and blend together giving rise to the real universe. In this way, one can take into consideration even universes where time flows from future to the past - this is why quantum mechanics isn't explicitly and a priori anti-time-travel.

However, what Greenberger, Svozil and Pegg have found is that as time flows, the alternative universes that could have happened but haven't happened, get erased - these past alternatives cancel each other. Thus, if someone would manage somehow to go back into the past, one wouldn't see the universe as it was back then, one would see a simplified universe which is compatible solely with what has actually happened.

Mark Buchanan, from New Scientist, explains the theory using the wave picture (which is maybe clearer in this case): "The constraint arises from a quantum object's ability to behave like a wave. Quantum objects split their existence into multiple component waves, each following a distinct path through space-time. Ultimately, an object is usually most likely to end up in places where its component waves recombine, or 'interfere', constructively, with the peaks and troughs of the waves lined up, say. The object is unlikely to be in places where the components interfere destructively, and cancel each other out. Quantum theory allows time travel because nothing prevents the waves from going back in time. When Greenberger and Svozil analysed what happens when these component waves flow into the past, they found that the paradoxes implied by Einstein's equations never arise. Waves that travel back in time interfere destructively, thus preventing anything from happening differently from that which has already taken place."

"According to our model, if you travel into the past quantum mechanically, you would only see those alternatives consistent with the world you left behind you. In other words, while you are aware of the past, you cannot change it. No matter how unlikely the events are that could have led to your present circumstances, once they have actually occurred, they cannot be changed. Your trip would set up resonances that are consistent with the future that has already unfolded", Greenberger and Svozil write.

"The model also has consequences concerning the many-worlds interpretation of quantum theory. The world may appear to keep splitting so far as the future is concerned. However, once a measurement is made, only those histories consistent with that measurement are possible. In other words, with time travel, other alternative worlds do not exist, as once a measurement has been made confirming the world we live in, the other worlds would be impossible to reach from the original one", the authors add.

These theoretical studies of time travel also have wider implications for questions such as "Why does time flow?" and "what is the nature of time?". David T. Pegg has argued that basically this is a consequence of quantum mechanics. He argued that all possible processes, in both directions in time, interfere with one another in such a way that, macroscopically, only the processes in one direction are left probable.

Picture credits: C. Andree Davidt, 'Time Travel'; Illustration by Chad Baker/Getty Images

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