Quantum entanglement
Quantum entanglement is one of the concepts that led Albert Einstein to dislike the theory of quantum mechanics. With his co-workers, Boris Podolsky and Nathan Rosen, Einstein used entanglement to try to show weaknesses in quantum mechanics. Einstein called entanglement "spooky action at a distance". He said this because he did not believe that quantum particles could affect one another faster than the speed of light. He tried to show that this weird effect means that quantum mechanics gives an incomplete picture of what really goes on and that in the future it will be taken care of with extra "hidden" variables.