[Followup to: Building Intuitions On Non-Empirical Arguments In Science]
In your travels, you arrive at a distant land. The chemists there believe that when you mix an acid and a base, you get salt and water, and a star beyond the cosmological event horizon goes supernova. This is taught to every schoolchild as an important chemical fact.
You approach their chemists and protest: why include the part about the star going supernova? Why not just say an acid and a base make salt and water? The chemists find your question annoying: your new “supernova-less” chemistry makes exactly the same predictions as the standard model! You’re just splitting hairs! Angels dancing on pins! Stop wasting their time!
“But the part about supernovas doesn’t constrain expectation!” Yes, say the chemists, but removing it doesn’t constrain expectation either. You’re just spouting random armchair speculation that can never be proven one way or the other. What part of “stop wasting our time” did you not understand?
Moral of the story: It’s too glib to say “There is no difference between theories that produce identical predictions”. You actually care a lot about which of two theories that produce identical predictions is considered true.
Later in your travels, you come to another land. The paleontologists here believe the Devil planted dinosaur fossils to trick humans into doubting Creation.
You approach the paleontologists and argue the same point you argued with the chemists on your last stop – that if two theories make identical predictions, it’s still important to go with the simpler one.
To your surprise, the paleontologists know and agree. “Of course!” they tell you. “And in the dinosaur theory, there must have been, like, millions or even billions of dinosaurs. But the Devil theory explains everything with just one Devil.”
You argue that it doesn’t work that way, but the paleontologists insist that it does. After all, Occam says not to multiply entities beyond necessity. And if the dinosaur theory posits a billion dinosaurs, that’s 999,999,999 more entities than are necessary to explain all those bones.
Moral of the story: “Choose the simpler of two theories that make identical predictions” isn’t trivial. You actually have to understand some philosophy in order to figure out which of two theories is simpler.
You return home and curl up in front of the fire with a good book on quantum mechanics.
Renowned physicist Sean Carroll jumps out from behind you, and exclaims: “Don’t you realize that single-world interpretations of quantum mechanics make both the errors that you fought against abroad?”
You are startled. “This room is locked,” you tell him. “And how did you know what I was doing abroad? Wait a second. Are you secretly the Devil?”
“Untestable, therefore irrelevant!” says Carroll. You wonder if he has always had bright orange eyes. “But being indifferent between ‘wavefunction branches’ and ‘wavefunction branches, and then somewhere we can’t see it one branch mysteriously collapses’ is the same kind of error as being indifferent between ‘acid and base make salt’ and ‘acid and base make salt and water, and then somewhere we can’t see it a star mysteriously goes supernova’.”
He stomps his foot for emphasis, and something falls out of his pocket. Is that a dinosaur bone? He quickly reaches down and pockets it again.
“And,” he adds “preferring collapse interpretations to many-worlds because there are fewer universes – that’s like preferring the Devil theory to dinosaurs because it involves fewer entities. It’s optimizing over the wrong thing! You’re not literally trying to come up with a theory with as few entities as possible! You’re trying to come up with one that has as few extra moving parts as possible. The process that makes wavefunctions collapse is an extra assumption! Now if you’ll excuse me, I’ve got to go plant this” – he taps the bone “in a sedimentary rock formation in China”. He vanishes in a puff of smoke. Can all quantum physicists do that?
Moral of the story: Applying the two previous morals consistently lets you prefer the many worlds interpretation of quantum mechanics without having to worry about this being “untestable”.