The quantum world is notoriously weird. Single particles can be in two places at once, for example. Only by making an observation do we force it to 'choose'. Before an observation we can only assign probabilities to the likely outcomes. Such a picture cannot be reconciled with a smooth, continuous fabric of space-time. According to Einstein, space-time is warped by matter and energy, but quantum physics says matter and energy exist in multiple states simultaneously — they can be both here and over there. According to Einstein, space-time is like a stage that remains in place whether actors are treading...