How Big Can Schrödinger’s Kittens Get?

It’s time we thought again about quantum theory. There’s nothing actually wrong with the theory itself—it works fantastically well for understanding how atoms and subatomic particles behave.

The problem is how we talk about quantum theory. We keep insisting that it’s weird: waves becoming particles, things being in two places (or two states) at once, spooky action at a distance, that sort of thing. Isn’t it perverse to clothe in mystery a theory that scientists use routinely to understand the world?

Part of the issue is that everyday objects are discrete, localized, and unambiguous, and so, very different to quantum objects. But why is that the case? Why is our everyday world always “this or that” and never “this and that”? Why, as things get bigger, does quantum physics turn into classical physics, governed by laws like those that Isaac Newton wrote down over three centuries ago?

This switch is called the quantum-classical transition, and it has puzzled scientists for many decades. We still don’t completely understand it. But over the past two or so decades, new experimental techniques have pushed the transition to ever-larger sizes. Most scientists agree that technical difficulties will prevent us from ever putting a basketball, or even a human, in two places at once. But an emerging understanding of the quantum-classical transition also suggests that there is nothing in principle that prohibits it—no cosmic censorship separates our “normal” world from the “weird” world that lurks beneath it. In other words, the quantum world may not be so weird after all.

magine a broken drying machine that spits out pairs of unmatched socks. They come in complementary contrasts: if one is red, the other is green. Or, if one is white, the other is black, and so on. We don’t know which of these options we’ll get until we look—but we know that if we find one is red, we can be sure the other is green. Whatever the actual colors are, they are correlated with one another.