They must exchange information with each other no faster than the speed of light to confirm. An observer at the faraway particle does not know if the local observer has disturbed the entangled system, and vice versa. However, although entangled systems do not maintain locality (meaning one part of an entangled system can immediately influence a faraway particle), they do respect causality, meaning that effects always have causes. There is no generally accepted resolution to the paradox. But experiments have repeatedly confirmed that entangled particles do influence each other regardless of distance, and quantum mechanics remains verified to this day. This result is known as the EPR paradox (short for Einstein, Podolsky and Rosen), according to the American Physical Society - an effect Einstein dubbed 'spooky action at a distance.' He used the paradox as evidence that quantum theory was incomplete.

In principle, you could place two entangled particles on opposite ends of the galaxy and still have this instantaneous knowledge, which appears to violate the limit of the speed of light. As Einstein, Podolsky and Rosen discovered, entanglement appears instantaneous: Once you have knowledge of one quantum state, you automatically know the quantum state of any entangled particles.

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