This thought experiment argues for the existence of hidden variables. The idea is as follows: imagine that you are predicting the flight of baseball pitches and you do not know about the hidden variable of the spin of the ball, and how it affects the trajectory of the ball. Seemingly similar pitches would appear to have random variation between them. This is explained deterministically by spin. However, if you don’t know about spin, you might build a highly probabilistic model for the ball flight that carries a high amount of uncertainty. Such uncertainty is contrived and is just due to the nature of the model and lack of knowledge of all variables. The idea is that this is the case for Quantum Mechanics—that there are hidden variables that deterministically describe the universe that are not included in the theory, making it incomplete. The exact paradox isn’t really important, just the high level reasoning, but it can be found here if needed. The paradox just shows that two particles can be measured simulataneously, when this shouldn’t be the case. Ultimately Quantum Mechanics wins over Einstein’s assumptions under Special Relativity in this case. Einstein assumed that the particles had spin in the first place, and information couldn’t travel infinitely fast. However, particles simply do not have properties until they are measured, and they can collapse their state instantly. This is known as nonlocal behavior/quantum weirdness/spooky action at a distance. However, this collapse doesn’t actually work for sending information due to randomness.