Quantum mechanics is weird. So weird in fact, that we weren’t even sure how correct it could be for decades after the theory was developed. The two leading theories were that it’s just weird, or our theories are incomplete. One of the strangest quantum phenomenons is called “quantum entanglement,” where systems of particles can become entangled, meaning you cannot describe one without the other. Think of it like tying your shoes together, except you can have infinitely long imaginary shoelaces. The consequence of entanglement is that measuring one immediately gives you information about the other, no matter how far away they are. If you see that one is a left shoe, you immediately know the other one is a right shoe. This should not be possible according to classical physics, information cannot travel faster than light. It’s kinda like making a phone call, there’s always some delay between you and the person you’re calling. Einstein famously called this “spooky action at a distance,” and believed that there must be variables that we hadn’t been looking for.
What makes entanglement so strange? Imagine you order a mummy costume and a skeleton costume, one is for you, and one is for a family member who lives somewhere else, but you don’t know where each one is going to be delivered. When the packages arrive, you open yours and find it to be the skeleton, which means that the one sent to your family member must be the mummy. So you immediately “measured” your costume to be the skeleton, revealing information about the costume sent elsewhere. But the costume sent to you must have been the skeleton the whole time, right? Yes, but only because we’re talking about costumes and not entangled particles. If they behaved as entangled particles, the packages would be both skeletons and mummies at the same time, until one of them is opened. That’d be a pretty scary costume.
But how do we know that one particle wasn’t just a skeleton or a mummy from the beginning? The answer to this question is known as Bell’s inequalities, and the scientists that broke them were the winners of the 2022 Nobel Prize. They put a limit on what can happen if particles can only be affected by things that happen around them. By breaking them, these scientists experimentally showed that we're not missing anything in our theory of quantum mechanics, it’s just weird. Quantum mechanics is key to modern science, it’s how we have lasers, MRIs, even our phones, and now quantum computers.