The Quantum World

Everything in the everyday world behaves itself. A ball is in one place at a time, a coin is either heads or tails, and looking at something does not change what it is. Shrink down to the scale of atoms and all of that stops being true. The rules that govern the very small are called quantum mechanics, and they are, by every measure, the most successful scientific theory ever written, confirmed to a dozen decimal places and responsible for most of modern technology. They are also deeply, irreducibly strange, and a century after their discovery physicists still argue about what they mean.

Lumps

The story begins with the word itself. In 1900 Max Planck found he could only explain the glow of hot objects by assuming that energy is emitted in discrete packets, quanta, rather than continuously. In 1905 Einstein showed that light itself comes in such packets, photons. In 1913 Niels Bohr explained the colours emitted by hydrogen by proposing that electrons can occupy only certain energy levels in an atom, never the values in between, jumping from one to another and emitting a photon of exactly the difference. Nature, at bottom, is grainy.

Waves

In 1924 Louis de Broglie suggested that if light waves could behave as particles, then particles, such as electrons, should behave as waves. They do. Fire electrons at a barrier with two slits and they build up, one at a time, the striped pattern that waves make when they pass through two openings and interfere. Each electron, apparently, goes through both slits. Block one slit and the pattern vanishes. Try to watch which slit the electron uses and the pattern vanishes too. The electron behaves like a wave when you do not look and a particle when you do.

Erwin Schrödinger wrote the equation that describes this wave in 1926. It does not say where a particle is. It gives, for every possible position, the probability of finding the particle there if you look. Until you look, the particle has no definite position; it is spread out in a superposition of all of them. The electron around a hydrogen nucleus is not a dot in orbit but a cloud of possibility with a shape set by its energy.

Uncertainty

Werner Heisenberg showed in 1927 that this is not a failure of our instruments but a property of nature. The more precisely a particle's position is pinned down, the less precisely its momentum can be known, and vice versa. The product of the two uncertainties can never be smaller than a fixed tiny number. This is why electrons do not fall into the nucleus: confining one to a tiny space would give it enormous uncertainty in momentum and so enormous energy. Atoms have size because of the uncertainty principle.

Tunnelling

A quantum particle's wave does not stop at a barrier it lacks the energy to cross; it leaks a little way through. If the barrier is thin enough, the particle sometimes simply appears on the far side. This tunnelling is not a curiosity. It is how the Sun shines: protons in the solar core are not hot enough to overcome their mutual repulsion and fuse, but they tunnel through it. It is how radioactive nuclei decay. And it is how flash memory stores your photographs, electrons tunnelling through an insulating layer to be trapped on the other side.

Entanglement

Two particles that interact can become entangled, so that measuring one instantly determines the state of the other, however far apart they are. Einstein called this "spooky action at a distance" and thought it proved quantum theory incomplete. Experiments since the 1970s, now performed between a satellite and ground stations 1,200 kilometres apart, show that the correlations are real and cannot be explained by any hidden information the particles carried with them. No signal passes between them, and entanglement cannot be used to send messages faster than light, but the universe is connected in a way that classical physics never allowed.

What it means

Physicists agree on the mathematics and on every prediction. They do not agree on what is happening. Does the act of measurement cause the many possibilities to collapse into one, and if so, what counts as a measurement? Do all the possibilities happen, each in its own branch of a vast multiverse? Is the wave function a real thing, or just a summary of what we know? Schrödinger's famous cat, sealed in a box with a quantum trigger and so, by the equations, both alive and dead until someone opens the lid, was his way of insisting that the question cannot be waved away. A century on, it has not been answered.

Why it matters

Whatever it means, it works. Transistors, lasers, LEDs, solar cells, MRI scanners, atomic clocks and the GPS that depends on them are all quantum devices; every computer chip exploits energy levels and tunnelling in billions of transistors at once. Quantum computers, now being built, use superposition and entanglement to attack problems no ordinary computer could finish. And in chemistry, quantum mechanics explains why atoms bond at all, which is to say, why there is anything more complicated than hydrogen gas. The strangest theory in science is also the one you use most.