Quantum theory contradicts common sense. Everyone who has even a modest interest in physics quickly gets this message. The quantum view of reality, we’re often told, is as a madhouse of particles that become waves (and vice versa), and that speak to one another through spooky message that defy normal conceptions of time and space.
We think the world is made from solid, discrete objects – trees and dogs and tables – things that have objective properties that we can all agree on; but in quantum mechanics the whole concept of classical objects with well-defined identities seems not to exist. Sounds ridiculous? The much-lauded physicist Richard Feynman thought so, yet he implored us to learn to live with it. ‘I hope you can accept Nature as She is – absurd,’ he said in 1985.
Except that much of the popular picture is wrong. Quantum theory doesn’t actually say that particles can become waves or communicate in spooky ways, and it certainly does not say that classical objects don’t exist. Not only does it not deny the existence of classical objects, it gives a meaningful account of why they do exist.
In some important respects, the modern formulation of the theory reveals why common sense looks the way it does. You could say that the classical world is simply what quantum mechanics looks like if you are six feet tall. Our world, and our intuition, are quantum all the way up.
Why, then, is it still so common to find talk of quantum mechanics defying logic and generally messing with reality? We might have to put some of the blame on the Danish physicist Niels Bohr. He was probably the deepest thinker about the meaning of quantum theory among its founding pioneers, and his intuitions were usually right. But during the 1920s and ’30s, Bohr drove a lasting wedge between the quantum and classical worlds. They operate according to quite different principles, he said, and we simply have to accept that.
According to Bohr, what quantum mechanics tells us is not how the world is, but what we’ll find when we make measurements. The mathematical machinery of the theory gives us the probabilities of the various possible outcomes. When we make a measurement, we get just one of those possibilities, but there’s no telling which; nature’s selection is random.
The quantum world is probabilistic, whereas the classical world (which is where all of our measurements happen) contains only unique outcomes. Why? That’s just how things are, Bohr answered, and it is fruitless to expect quantum mechanics to supply deeper answers. It tells us (with unflagging reliability) what to expect. What more do you want?
Bohr’s ‘Copenhagen interpretation’ – named after the location of the physics institute he founded in 1921 – didn’t exactly declare a contradiction between classical and quantum physics, but it implied an incompatibility that Bohr patched over with a mantra of what he called ‘complementarity’. The classical and quantum worlds are complementary aspects of reality, he said: there’s common sense and there’s quantum sense, but you can’t have both – at least, not at the same time.
The principle of complementarity seemed a deeply unsatisfying compromise to many physicists, since it not only evaded difficult questions about the nature of reality but essentially forbade them. Still, complementarity had at least the virtue of pinpointing where the problems lay: in understanding what we mean by measurement. It is through measurement that objects become things rather than possibilities – and furthermore, they become things with definite states, positions, velocities and other properties. In other words, that’s how the counterintuitive quantum world gives way to common-sense experience. What we needed to unite the quantum and classical views, then, was a proper theory of measurement. There things languished for a long time.
Now we have that theory. Not a complete one, mind you, and the partial version still doesn’t make the apparent strangeness of quantum rules go away. But it does enable us to see why those rules lead to the world we experience; it allows us to move past the confounding either/or choice of Bohr’s complementarity. The boundary between quantum and classical turns out not to be a chasm after all, but a sensible, traceable path.