Decoherence
The loss of visible quantum interference when a system entangles with its environment — how branches become effectively separate without a collapse rule.
Lukasz Szramuk ·
Decoherence happens when a quantum system becomes entangled with degrees of freedom that are not tracked: air molecules, photons, a measuring device, the wider environment. Phase relationships that allowed alternatives to interfere spread into that environment. For observers who cannot control all those correlations, the alternatives stop producing visible interference and behave like an ordinary statistical mixture.
This explains why macroscopic superpositions are not seen in daily life even if the underlying equations remain quantum. It also supplies the branching structure used by modern many-worlds accounts: after decoherence, different records no longer interact for practical purposes, so each can be treated as a world with a definite outcome. No bubble nucleates and no constants change; the branches are components of one universal quantum state.
Decoherence does not by itself settle every measurement problem. It explains the disappearance of interference and the emergence of a preferred, stable basis of records. It does not automatically explain why an observer experiences one outcome, or derive the Born-rule probabilities without additional argument. Interpretations disagree about what the resulting branches mean.
That distinction is why a quantum many-worlds interpretation should not be casually equated with an inflationary multiverse. One branches outcomes inside quantum theory; the other populates causally separated spacetime regions. Some proposals try to connect them, but the mechanisms answer different questions before any unification is attempted.