If an electron can be in a superposition of two paths, why can't a chair? Chairs are made of electrons, protons and neutrons, all obeying quantum rules. Yet nobody has seen a chair interfere with itself, tunnel through a door, or sit in a blur of possible places. Explaining how the quantum world turns into the familiar one is a central achievement of modern physics, and the work is not finished.

The correspondence principle

Around 1920 Niels Bohr insisted that any correct quantum theory must agree with classical physics where classical physics works. This is the correspondence principle: for large systems and large energies, quantum results must match classical physics, blending smoothly into the older predictions. Quantum mechanics passes this test. Newton's laws are not wrong; they are what quantum mechanics looks like when the steps are tiny and the waves are short.

Tiny wavelengths

Part of the answer comes from level 2. A thrown baseball has a de Broglie wavelength about a trillion trillion times smaller than an atom. Any interference stripes it could form would be far too fine to see or measure. For everyday objects, the wavelength is so small that wave effects stay hidden. The same is true of tunneling, as level 7 showed, and of the energy steps of level 1.

Decoherence

But small wavelengths are not the whole story. Skilled experimenters can create superpositions of fairly large things, and the theory itself sets no size limit. The deeper reason we do not see superpositions in daily life is a process called decoherence, worked out from 1970 onward by the German physicist H. Dieter Zeh, the Polish American physicist Wojciech Zurek and others.

Any real object is constantly bumped by air molecules, struck by light and warmed by its surroundings. Each collision carries away a little information about where the object is. Recall from level 2 that interference vanishes once information about the path exists anywhere. In the same way, an object's surroundings constantly record its state, and this leaking information destroys the interference between its possibilities. In the language of QUAN 201, the object becomes entangled with its environment. The superposition is not exactly erased; it spreads into the surroundings, where it can no longer be observed.

And it happens fast. The larger and warmer an object, and the more it interacts with its surroundings, the faster decoherence works. For a speck of dust floating in air, a superposition of two places a dust grain's width apart would be destroyed in a vanishingly small fraction of a second. For a cat, there is no hope. In 1996 Serge Haroche and colleagues in Paris watched decoherence happen step by step, using a few photons trapped in a cavity, work honored with a share of the 2012 Nobel Prize.

Decoherence explains why we never see interference between alive and dead cats. Yet many physicists say it does not fully solve the measurement problem, because it does not by itself say why one particular outcome happens. That debate belongs to QUAN 202.

Isolation, not size

The lesson is that what separates quantum from classical behavior is how well a system is isolated from its surroundings, not a fixed size. Isolate something well enough, and quantum behavior survives even in large systems. When certain materials are cooled far enough, enormous numbers of particles act together as one quantum wave. In a superconductor, electrons pair up and flow with no resistance at all, an effect discovered by the Dutch physicist Heike Kamerlingh Onnes in 1911. Superfluid helium flows without friction and creeps up the walls of its container. In 1995 Eric Cornell and Carl Wieman in Colorado, followed soon after by Wolfgang Ketterle, cooled thousands of atoms into a single shared quantum state. These are quantum effects written large.

Worked example: keeping a qubit alive

A quantum computer stores information in qubits, small devices held in superpositions. Its power depends on keeping those superpositions, and the interference between them, intact while it calculates. Decoherence is therefore its chief enemy. Every stray photon, vibration or flicker of heat that reaches a qubit leaks information about its state and turns its delicate superposition into an ordinary, useless mixture.

So engineers fight decoherence in exactly the ways this level predicts. Many machines use tiny superconducting circuits, chilled in large refrigerators to about a hundredth of a degree above absolute zero, shielded from magnetic fields and stray light, and wired with care so that heat cannot creep in. Even so, today's qubits typically keep their superpositions for only small fractions of a second, so calculations must be fast, and much research goes into error correction, which spreads one logical qubit across many physical ones. QUAN 203 tells that story in detail.

The quantum computer turns this level's lesson into an engineering problem. The world looks classical not because quantum rules stop at some size, but because nearly everything is constantly in touch with everything else. Cut those contacts, and the strange rules of the earlier levels come back into view.