Quantum mechanics, also called quantum physics and quantum theory, is the branch of science responsible for explaining how nature works at the subatomic level. Though it is incompatible with the Theory of Relativity, which explains the relationships between space-time, matter, energy, and gravity, quantum mechanics is the most successful theory humans have of how the subatomic universe functions.
The most mysterious and least understood principle of quantum mechanics is a phenomenon called quantum entanglement. Quantum entanglement involves two particles that are linked, so when the state of one particle is measured, the state of the other changes. The change occurs instantaneously, no matter how far apart the particles are from each other in space, even if they are at opposite ends of the Universe. This seems to defy the very basic laws of physics, which state that nothing can travel faster than light.