For quantum physics is not replaced by another sort of physics at large scales. It actually gives rise to classical physics. Our everyday, commonsense reality is, in this view, simply what quantum mechanics looks like when you’re six feet tall. You might say that it is quantum all the way up.

The question, then, is not why the quantum world is “weird,” but why ours doesn’t look like that, too.

In Schrödinger’s day, traversing the quantum-classical transition seemed like crossing an ocean between two continents: Drawing a border somewhere in the open sea might be an arbitrary exercise, but the continents are undeniably distinct. The land of the quantum, Schrödinger said, is random and unpredictable, yet the classical realm is orderly and deterministic because it depends only on statistical regularities among that atomic-scale chaos.

Schrödinger dreamed up his “diabolical” (his word) thought experiment in 1935. It was intended as a challenge to Niels Bohr’s interpretation of quantum mechanics, toward which Schrödinger shared a great deal of Albert Einstein’s skepticism.

It was all very well for Bohr to impose a strict separation of quantum and classical, and to make observation the process by which they are distinguished—but what, then, if the quantum and the macroscopic are coupled without any observation taking place? Schrödinger was looking for what he called a “ridiculous case”: a reductio ad absurdum, not to be taken literally, in which we are confronted by a superposition of macroscopic states that seems not just bizarre (such as a large object being in “two places at once”) but logically incompatible. Einstein raised the prospect of a keg of gunpowder being in a superposition of exploded and unexploded states, and Schrödinger upped the ante with his cat, whose life or death is yoked to a quantum event such as the radioactive decay of an atom. If, as Bohr said, the state of the atom is undetermined (in a superposition) until we look, then so must be the state of the cat.

Schrödinger’s cat forces us to rethink the question of what distinguishes quantum from classical behavior. Why should we accept Bohr’s insistence that they’re fundamentally different things unless we can specify what that difference is?

We might then be inclined to point to features that classical objects like coffee cups have but that quantum objects don’t necessarily have: well-defined positions and velocities, say, or characteristics that are localized on the object itself and not spread out mysteriously through space. Or we might say that the classical world is defined by certainties while the quantum world is (until a classical measurement impinges on it) no more than a tapestry of probabilities, with individual measurement outcomes determined by chance. At the root of the distinction, though, lies the fact that quantum objects have a wave nature—which is to say, the equation Schrödinger devised in 1924 to quantify their behavior tells us that they should be described as if they were waves, albeit waves of a peculiar, abstract sort that are indicative only of probabilities.