A bad habit is something you do, without being fully aware of it, that makes life harder than it needs to be. It is a bad habit of physicists to take their most successful abstractions to be real properties of our world. Since the distinction between real and abstract is notoriously problematic, you might wonder what it means to wrongly confer reality on something abstract. I shall illustrate our habit of inappropriately reifying our successful abstractions with several examples.

Perhaps the least controversial examples are provided by quantum mechanics. The quantum state may well be the most powerful abstraction we have ever found. (“Found” is a useful word here, since you can take it to mean “discovered” or “invented,” depending on where you stand along the real-abstract axis.) Are quantum states real?

In considering what that question might mean, recall that in the early days Erwin Schrödinger thought that the...

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