WE ARE used to weird things in physics – particles can exist in two places at once, for example – and it is widely accepted that the universe is stranger than we could ever have imagined. But sometimes weirdness is taken as a sign that someone’s theory might be flawed.
Step up, Albert Einstein. According to his general relativity, time travel is possible – so you could go back to kill your grandfather and prevent your own existence. How do we deal with such a paradox? Stephen Hawking conjectures that something will always stop time travel happening. Others invoke the “many worlds”…
