You’ve no doubt heard of quantum entanglement before: the idea that if you create a mixed quantum state that consists of two particles, you can then know the properties of one by measuring the properties of the other. The odd — and counterintuitive — thing about this is that once these particles are entangled, you can move them an arbitrary distance apart from one another, measure the properties of one, and instantly know about the properties of the other!
Does this violate the law of special relativity, which says the speed limit of everything in the Universe? As it turns out, the answer is no, but Einstein nevertheless thought this shouldn’t be allowed to happen! Despite being the most brilliant mind of his generation, this is one of those times that Einstein got it completely wrong.