spooky action at a distance
'Spooky action at a distance' is Einstein's wary, slightly mocking phrase for the way entangled particles seem to influence each other instantly across any distance. He used the German spukhafte Fernwirkung in a 1947 letter to Max Born, expressing his deep discomfort with a theory in which measuring one particle appears to settle the state of a far-off partner at once. To Einstein this smelled wrong, and he took it as a sign that quantum mechanics was incomplete.
The phrase captures a genuine tension but can mislead if read too literally. Nothing actually travels between the particles, no force reaches across the gap, and no energy or signal is sent. What looks like instant influence is the appearance of perfect correlation: the moment you learn your own outcome, you also know what your partner's matching measurement must give. The 'action' is in our knowledge and in the joint state, not in a push felt across space.
History gave Einstein's worry an ironic twist. The very correlations he found spooky turned out to be real, confirmed again and again by Bell tests, and they cannot be explained away by the local, predetermined reality he favoured. Yet they also respect his cherished speed limit, since they carry no usable message. The world is stranger than Einstein wanted, but not in the law-breaking way the phrase might suggest.
Einstein's own label for entanglement's instant correlations — a worry, not an endorsement.
The phrase tempts people to picture a faster-than-light push. There is none: no force, energy, or signal crosses the gap, and the no-signaling theorem guarantees the correlations cannot carry a message.