Everyone has their favourite example of a trick that reliably gets a certain job done, even if they don’t really understand why. Back in the day, it might have been slapping the top of your television set when the picture went fuzzy. Today, it might be turning your computer off and on again.

Quantum mechanics — the most successful and important theory in modern physics — is like that. It works wonderfully, explaining things from lasers and chemistry to the Higgs boson and the stability of matter. But physicists don’t know why. Or at least, if some of us think we know why, most others don’t agree.