decoherence

Decoherence is the process by which a quantum system loses its delicate interference effects as it becomes entangled with the countless particles of its surroundings. A superposition is only observably 'quantum' as long as its different parts can still interfere with one another. The moment information about which part is which leaks into the environment — into stray photons, air molecules, vibrations — that interference is no longer accessible to anyone looking only at the system itself.

Crucially, the more particles a system contains and the warmer and busier its environment, the faster this leakage happens — often almost unimaginably fast for anything larger than a molecule. This is why we never see a coffee cup in two places at once: any such superposition would be scrambled by its environment long before we could notice it. Decoherence explains, quantitatively, why the strange features of quantum mechanics are so well hidden at everyday scales.

It is important to be careful about what decoherence does and does not claim. It explains why interference effects disappear and why certain stable, classical-looking states are singled out, which goes a long way toward dissolving the puzzle of the quantum-classical boundary. But decoherence by itself does not pick out a single actual outcome from the menu of possibilities; the system plus environment together are still, formally, in a giant superposition. Whether that final step needs collapse, many worlds, or nothing at all is left to interpretation.