Nature is at it again

Last week, Nature ran a piece under the headline Quantum physics: What is really real? I obnoxiously posted this on Facebook (because I’m too old and out of touch to be on any of the hipper social media sites):

I have now read the piece, and I can report that there’s no need for a recantation. As expected, Nature is making grandiose claims about quantum mechanics and the nature of reality that go beyond anything supported by evidence.

Nature writes pretty much the same story every couple of years. The main idea behind all of these articles is the question of whether the quantum mechanical wavefunction describes the way a system really is  or merely our knowledge of the system. In philosophy-of-science circles, these two points of view are sometimes known as the psi-ontic and psi-epistemic stances. More specifically, all three of these articles have to do with a theorem published (in one of the Nature journals) by Pusey et al. that claims to provide an experimental way of distinguishing between these possibilities. After Pusey et al. published this theoretical result, others went ahead and performed the proposed experimental tests, leading to the (claimed) conclusion that the wavefunction describes actual reality, not merely our knowledge.

You should of course be skeptical of any claim that an experimental result reveals something about the deep nature of reality. Sure enough, if you dig down just a little bit, it becomes clear that these results do no such thing. The Pusey et al. theorem proves that a certain class of psi-epistemic theories make predictions that differ from the predictions of standard quantum mechanics. The subsequent experiments confirmed the standard predictions, so they rule out that class of theories.

The problem is that ruling out a specific class of psi-epistemic theories is not the same thing as ruling out the psi-epistemic point of view as a whole. We now know that that class of theories is wrong, but that’s all we know. To make matters worse, the class of theories ruled out by these experiments, as far as I can tell, does not contain any theories that any proponents of psi-epstemicism actually believe in. The theories they tested are straw men.

In particular, the most prominent proponents of the psi-epistemic point of view are the advocates of something called quantum Bayesianism (QBism). QBism is an interpretation of quantum mechanics, as opposed to an alternative theory — that is, it makes predictions that are identical to those of standard quantum mechanics. There is, therefore, no experimental result that would distinguish QBism from psi-ontic versions of quantum mechanics.

Not all psi-epistemicists are QBists, of course, but as far as I can tell even the others never advocated for any theories in the class considered by Pusey et al. If I’m wrong about that, I’d be interested to know.