This post is part of a series of posts originally written for my blog at Forbes.com that I’m copying to my personal site, so I have a (more) stable (-ish) archive of them. This is just the text of the original post, from March 2018, without the images that appeared with it (which were mostly fairly generic photos ).

Over at NPR’s science blog, Adam Becker has a post on “The Puzzle Of Quantum Reality”. It’s a good example of what it is, namely yet another explanation of the measurement problem and interpretations of quantum physics. As you can tell from the tone of the previous sentence, though, while I think interpretations of quantum physics do matter, I’m a little tired of reading these kinds of pieces.

Instead, I’d like to reverse the question a bit, and ask not what’s weird about quantum physics, but what’s weird about us. If the universe is really governed by quantum rules, why do those rules seem so strange to us?

The answer is basically right there in the definition of “weird”: quantum physics seems weird because it runs counter to our everyday intuitions about how the world works. The rules we have discovered for the behavior of quantum systems– the Schrodinger equation and so on– don’t obviously resemble the rules we use to describe everyday objects– Newton’s Laws of motion and the other things you learn in high-school physics. We spend the vast majority of our lives interacting with things that obey Newton’s Laws, and that defines our intuition for how things “ought” to behave. When quantum physics departs from that, it seems weird.

But why is there such a difference between the everyday physics rules that define our intuition and the quantum rules? The Copenhagen approach, which Scott Aaronson amusingly dismissed as “shut-up and calculate except without ever shutting up about it” basically asserts that this is just The Way Things Are and tries to impose an absolute separation between the microscopic scale where quantum rules apply and the macroscopic scale where classical rules hold sway, but that position is pretty clearly untenable. The point of Schrodinger’s infamous cat gedankenexperiment was to show exactly that: the state of the macroscopic cat is entangled with the state of a microscopic atom in a way that crosses the scale boundary that the Copenhagen approach would impose.

A better answer is to say that there isn’t really a difference in the rules that apply for big objects and the rules that apply for small ones– the universe is quantum on every scale. The “classical rules” that we see are just the result of quantum physics when applied to really big things. In a sense, it’s just another application of the principle that “More Is Different,” to borrow the title of an influential 1972 paper by Philip Anderson. As Anderson noted, when you study the behavior of huge numbers of objects whose individual interactions are described by simple rules, you often find that the collective behavior of the large system seems to be described by another set of simple rules, rules that aren’t necessarily obviously related to the original interaction rules. This idea of high-level rules emerging from lower-level ones leads to the hierarchical structure of sciences– chemistry is the physics of too many atoms, and biology is the chemistry of enormous numbers of molecules, and so on.

In a sense, that’s what’s going on: when we apply quantum mechanics to enough particles to make up a visible object, the particles and their interactions are all governed by quantum rules, but the collective effect is to give the appearance of a different set of rules that we call “classical.” Everyday reality is just what happens when all those quantum properties blur together.

In some cases, this transition is relatively easy to see. If you look at the behavior of a single quantum particle, you find that you can’t trace out a classical trajectory with a well-defined position and momentum at all times. This is what leads to some of the signature quantum phenomena like the wave-like interference of material particles.

It’s an undergraduate-level problem to show, though, that the average position and average momentum over a large collection of measurements do exactly what you expect from Newton’s Laws. And in a sense, when we follow the trajectory of a classical object– a golf ball in flight, say– that’s really what we’re looking at: the average position of an uncountably huge number of atoms making up that ball.

The disappearance of the other signature quantum behavior, a kind of discreteness of energy, is a little trickier to understand. If you look at the behavior of a single electron in an atom, it’s very much not classical: you can only change its energy in discrete jumps from one orbit to another. If you look at the behavior of a macroscopic number of electrons in a conductor, though, you don’t see that discreteness– when you apply a voltage to drive a current, the electrons move in a way that looks very classical. The average velocity seems to increase smoothly, without any discontinuous jumps.

So, where do the jumps go? They haven’t really gone anywhere– the individual electrons in a conductor still jump between discrete states of well-defined energy. It’s just that the states become more numerous as you add more particles, and the energy difference between states becomes smaller, until they start to run together and the sharp energy states of atoms become energy bands in solids. They’re not really continuous energy bands, but when you’re working at the coarse scale that defines everyday life, they blur together so thoroughly that they might as well be.

Of course, just because the quantum rules smear out into classical ones doesn’t mean that the quantum rules are completely gone. If you look closely, you can find small behaviors that indicate the underlying quantum nature of things. Electrons in a conductor move about in a very classical way, but insulators do not, and that points toward the existence of band gaps, which in turn are evidence of the wave nature of electrons. You can also set up situations where huge numbers of quantum objects act independently of one another, so the quantum-ness doesn’t smear out, giving you phenomena like the bright spectral lines of excited atoms (pointing to the discrete energy states within) and the photoelectric effect (pointing to the particle nature of light). That’s how we learned about quantum physics in the first place– following a trail of clues leading from the behavior of everyday objects down into the strange world of the quantum.

Again, this is not to say that quantum interpretations aren’t addressing interesting questions (they are), or that quantum physics isn’t weird (it is). Given the vast amount of evidence that our universe is quantum, though, it’s interesting to step back and reflect on why, as quantum creatures in a quantum world, we’re surprised by quantum physics.