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Clouds form in the air when a drop in temperature causes water droplets to condense out of water vapour. But this process happens rapidly only if there are things like dust particles in the air that act as “seeds” around which water droplets can grow. Now the seed—and this is the key to the cloud chamber’s operation—need not be as big as a dust grain. In fact, it need be only a single atom that has lost an electron—an ion.

A cloud chamber is a box filled with water vapour with a window in its side to look through. Crucially, the water vapour is ultrapure, so there are no seeds about which the vapour can condense. The vapour is held in a state in which it is absolutely desperate to form droplets, but it is frustrated because there are no seeds. Enter a high-speed subatomic particle. Where it knocks an electron out of an atom, a water droplet will instantly grow around the ion. The droplet is small but big enough to see through the window of the cloud chamber if properly illuminated.

So what would you see if you looked through the window? The answer is of course just one of the possibilities—either a single water droplet or no water droplet. You would never see a superposition of both—a ghostly droplet, hovering half in existence and half out of existence. The question is, what happens in the cloud chamber to prevent it from recording this superposition?

Take the event in which a water droplet forms. It was triggered by a single ionised atom. The same atom exists in the event in which no droplet formed. It just does not get ionised, so no water droplet forms around it. Say, this atom is painted red in both cases to make it stand out (forget the fact that you can’t paint an atom!).

Now, in the event a droplet forms, zoom in on an atom near the red atom. Water is denser than water vapour; the atoms are closer together. Consequently, the atom in question will be closer to the red atom than it is in the event in which no water droplet forms. For this reason, the probability wave representing the atom in the first event only partially overlaps with the probability wave of the same atom in the second event. Say, for example, that their waves only half overlap.

Now take a second atom in the first event. It too will be closer in the first case than in the second. Once again, their probability waves will only half overlap. If we now consider the probability wave representing the two atoms together, it will overlap only one-quarter with the second case, since 1/2 × 1/2 = 1/4.

See where this is going? Say the water droplet contains a million atoms, which actually corresponds to a very small droplet. How much will the probability wave representing a million atoms in the first event overlap with the probability wave representing a million atoms in the second event? The answer is 1/2 × 1/2 × 1/2 ×… a million times. This is an extraordinarily small number. There will therefore be essentially zero overlap.

But if two waves don’t overlap at all, how can they interfere? The answer is, of course, they cannot. Interference, however, is at the root of all quantum phenomena. If interference between the two events is impossible, we see either one event or the other but never the effect of one event mingling with the other, the essence of quantumness.

Probability waves that do not overlap and so cannot interfere are said to have lost coherence, or to have decohered. Decoherence is the ultimate reason why the record of a quantum event in the environment, which always consists of a lot of atoms, is never quantum. In the case of the cloud chamber, the “environment” is the million atoms around the ionised/nonionised atom. In general, however, the environment consists of the countless quadrillions of atoms in the Universe. Decoherence is therefore hugely effective at destroying any overlap between the probability waves of events entangled with the environment. And since that’s the only way we can experience them—what the observer knows is inseparable from what the