Here, too, orderly wave interference (like that from double slits) happens only if there’s coherence in the oscillations of the interfering waves. The concept comes from the science of ordinary waves. These behaviors become possible when there is a well-defined relationship between the quantum “waves”: in effect, when they are in step. It is this waviness that gives rise to distinctly quantum phenomena like interference, superposition, and entanglement. 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. 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. 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.
Why should we accept Bohr’s insistence that they’re fundamentally different things unless we can specify what that difference is? Schrödinger’s cat forces us to rethink the question of what distinguishes quantum from classical behavior. 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. 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. 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. 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. Schrödinger dreamed up his “diabolical” (his word) thought experiment in 1935. 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. 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 question, then, is not why the quantum world is “weird,” but why ours doesn’t look like that, too. You might say that it is quantum all the way up.
Our everyday, commonsense reality is, in this view, simply what quantum mechanics looks like when you’re six feet tall. It actually gives rise to classical physics. The answer is both elegant and rather astonishing.įor quantum physics is not replaced by another sort of physics at large scales. Things have moved on, and we can state much more precisely than Schrödinger and his contemporaries could why and how quantum becomes classical. The fact is, however, that this so-called quantum-classical transition is now largely understood. The cat is still hauled out today as if to imply that we’re as puzzled as ever by the mere fact that the quantum world at small scales turns into the world of classical physics at human scales. One might even say that he was too successful.
This post is adapted from Ball’s new book.
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