Science › Quantum world
Decoherence: why big things look normal
1970· Idea
ProvenTested many times; part of working physics.
The process by which a quantum system's blend of possibilities leaks into its surroundings, making it look ordinary and classical.
See it on the Science map →How it works
Any object constantly bumps into air molecules, light and heat radiation. Each collision entangles it with the surroundings, spreading its superposition across countless particles. The different possibilities can then no longer interfere in any detectable way. For a dust grain this happens in far less than a billionth of a second. H. Dieter Zeh described it in 1970; Wojciech Zurek and others developed it in the 1980s.
Why it matters
It explains why everyday objects never look blurred between states. It is also the main enemy of quantum computers, which must be shielded and cooled near absolute zero.
Think of it like
Like a confidential deal term: once it has been shared with a dozen brokers, it is effectively public, and there is no putting it back.
How we know
Watched directly in 1996 by Serge Haroche's group, which saw superpositions of light lose coherence at rates matching theory. Error rates in quantum computers follow decoherence predictions.
Still a mystery
Decoherence explains why blends become invisible, but most physicists agree it does not by itself explain why just one outcome is experienced.
Connections (3)
▸The measurement problem is partly explained by Decoherence: why big things look normalProven
The relationship
Decoherence shows that when a quantum system interacts with its environment, its different possibilities quickly lose the ability to interfere. To any observer, the system then looks as if it is in one definite state, with standard odds. This explains why measurements look classical, without saying why one particular outcome is picked.
How the link was found
H. Dieter Zeh (1970), Wojciech Zurek (early 1980s), and Erich Joos with Zeh (1985).
Why it's proven
Decoherence rates have been measured, for example by Haroche's group in 1996 and in Vienna molecule-interference experiments in 2003–2004, matching theory.
Open questions and mysteries
It handles the 'why do things look classical' half. The 'why this outcome' half still needs an interpretation.
▸Decoherence: why big things look normal explains why we never see a live-and-dead Schrödinger's catProven
The relationship
A real cat constantly exchanges heat, air molecules and light with its surroundings. Any alive-and-dead blend would spread into the environment within a tiny fraction of a second, after which the two possibilities could never interfere again. The box effectively holds either a live cat or a dead one, never an observable mix.
How the link was found
Developed by Zeh, Zurek, Joos and others from 1970 through the 1990s; Zurek's 1991 Physics Today article brought it to a wide audience.
Why it's proven
Experiments with deliberately built 'cat states' of light show their blends degrade faster the larger they are, as decoherence predicts (Haroche's group, 1996).
Open questions and mysteries
Decoherence says what we will see, not why the cat is definitely one or the other. That last step still depends on interpretation.
▸Quantum entanglement with the environment causes Decoherence: why big things look normalProven
The relationship
When a quantum system bumps into air molecules or light, it becomes entangled with them, sharing its superposition with countless environmental particles. Since nobody can track all of them, the system on its own no longer shows interference. Decoherence is entanglement with the environment, seen from the system's side.
How the link was found
H. Dieter Zeh (1970) and Wojciech Zurek (1981–1982) recognized that entanglement with the environment explains the loss of quantum behavior.
Why it's proven
Experiments that control environmental contact, such as gas pressure around interfering molecules (Vienna, 2003), show interference fading as predicted.
Open questions and mysteries
Is decoherence enough to explain the classical world entirely? Zurek's 'quantum Darwinism' proposes how shared records arise, but it is still being tested.