OPTION 3: Split the Universe

If all these time machines seem contrived and wildly impractical, well, they are. But that’s not the point. The point is that time travel is possible in principle. And that permits nightmare paradoxes to raise their heads. For instance, you could use a time machine to go back in time to murder your grandfather before your mother was born. The question would then be: how could you have murdered your grandfather if you had never been born? To avoid the grandfather paradox, the late Stephen Hawking proposed the ‘chronology protection conjecture’. This is just a fancy way of saying: time travel is impossible. Hawking was convinced that some undiscovered law of physics must intervene to prevent it ever happening.

But there’s another way out of the grandfather paradox. Quantum theory is our best description of the microscopic world of atoms and their constituents. But it implies that fundamental particles can do many things at once, the equivalent of you doing shopping and mowing the lawn at the same time. According to the many worlds interpretation of quantum theory, every time a quantum event occurs – for instance, a photon is emitted by an atom or not emitted by an atom – both things happen but in parallel realities. The Universe is constantly splitting into versions that play out all possible histories and there are infinite parallel realities stacked like the pages of a neverending book. In the many worlds scenario, if you go back in time and kill your grandfather, you kill a parallel grandfather in a parallel universe, not your one. Hey presto, time travel without the paradoxes.

If the Universe contains parallel realities, then tricky time travel paradoxes are avoided.