The world takes note. Decoherence

Each bounced molecule becomes entangled with the speck. Like two coins that each land at random but always agree, the two can now only be described together, so the "both" is shared with every molecule flying off. H. Dieter Zeh first described this in 1970, and Wojciech Zurek, Erich Joos and others developed it in the 1980s. In 1996 Serge Haroche's group in Paris watched it happen. A microwave field of a few photons, put into two states at once, lost its quantum signature, and faster when the two states were more different.

If the two ways a molecule can fly off overlap by a fraction s, each bounce multiplies the stripe contrast (the visibility V) by s:

With s = 0.9, ten bounces leave 0.9¹⁰ ≈ 0.35. If bounces arrive at random at a rate Γ, the interference term shrinks smoothly in time:

Nothing in the equations forbids undoing it all. Every molecule's motion would have to be reversed exactly, and so would every molecule it has bumped into since. In air a molecule hits another in less than a billionth of a second, so the secret keeps spreading. That is why the book says the superposition leaks rather than stops. The eraser on page 14 works because there is a single record to destroy, and it leaves the other question open: why each photon lands in one place.

With full records (s = 0), undoing k of N records leaves V = 0^(N − k), which is 1 only when k = N. One second in air holds about 10¹⁸ full records:

The speck is barely visible, yet it holds tens of trillions of atoms. The book's numbers are rough estimates for the speck in two places one speck width apart. At that distance a single air molecule, or a single photon of sunlight, records where it is, so the both-ways wave lasts about as long as the wait between hits. About a billion billion air molecules hit it each second, and a few hundred billion photons of sunlight. Standard tables give a far smaller figure for this speck in air, about 10⁻³¹ seconds, but that comes from a formula meant for hits that each reveal only a little. The stripes cannot go faster than the hits arrive, so for two places one speck-width apart the book's billionth of a billionth is the sounder figure.

When the separation Δx is much larger than the wavelength of what hits the speck, every hit is a full record, and τ_D is simply the time between hits:

The afterglow's waves, about a millimetre long, are far longer than Δx = 10 µm, so each photon tells very little and the rate grows as Δx²: 1/τ_D = ΛΔx², about 1 s⁻¹. In air, light crosses c·τ_D ≈ 3 × 10⁸ m/s × 10⁻¹⁸ s = 0.3 nm.

Decoherence is not one of the ways out. It is ordinary quantum mechanics applied to a thing together with its surroundings, which is why all four accept it. Specialists broadly agree with the book's verdict. After decoherence the possibilities can no longer interfere, yet the mathematics still holds both, now shared with the surroundings. John Bell objected that this settles matters only "for all practical purposes". Every way out uses decoherence. None is finished by it.

On its own, the speck is described by a small table of four numbers. The corners are the chances, and they never change. The cross terms carry the stripes, and decoherence shrinks them:

Decoherence turns the both-ways wave into something that looks like a fair coin. It never turns ½ and ½ into 1 and 0.

A thing can even give itself away. In 2004 a group in Vienna heated large molecules with lasers to above 3,000 kelvin before sending them through gratings. The stripes faded as the molecules' own glow began to carry a record of their path. Shield something well enough and quantum behaviour returns in larger things, as Act Three of the book shows. In a quantum computer the struggle never ends: every qubit slowly leaks, so the machine must finish its work, or correct its errors, before the world takes note.

The rates of every kind of hit simply add, and each shield removes one term:

Room air: about 10¹⁸ s⁻¹. A perfect vacuum in sunlight: about 3 × 10¹¹ s⁻¹. Dark and as cold as deep space: about 1 s⁻¹. Any gas left in a real vacuum adds its own hits.

The line between quantum and classical is not a fixed size. It depends on isolation, which is why quantum experiments happen in vacuum chambers and in fridges colder than deep space. Decoherence still leaves two things open: where the line between a thing and its surroundings comes from, and why one result appears. Those questions run through the rest of the book.