Einstein-Podolsky-Rosen (EPR) paradox

The Einstein-Podolsky-Rosen (EPR) paradox involves pairs of particles that are linked, or entangled, so that measuring one instantly reveals information about the other, no matter how far apart they are. Einstein thought this suggested that quantum theory might be incomplete because it implies "spooky action at a distance." Later experiments showed entanglement is real, indicating particles are connected in ways classical physics can't explain, challenging our notions of locality and reality. The paradox highlights fundamental questions about how the universe works at the quantum level.