Are there parallel universes? Universes in which, rather than reading this article, you are still asleep; in which you are happier, unhappier, richer, poorer, or even dead? The answer is "possibly". It's a controversial claim but one that has won more and more followers over the last few decades.
There are many theories in particle physics that cannot be tested in experiments. Does this make it unscientific? This debate, featuring one of our favourite theoretical physicists, David Tong, explores the question.