The trouble with measurement

Welcome to Part 1 of a short ongoing series where I track my evolution of thought on quantum interpretation. This article isn’t meant to be a "quantum mechanics explainer.” Rather, it's a record of updating where I stand several months after the start of my dig.

I think quantum interpretations are fascinating. However, I think most public writing on interpretations presents them as a menu, not as a sequence of actual reasoning failures that force you to move. I’ve actually been moving quite a bit, assessing and reassessing as I collect more information.

My overall tldr; is that interpretations aren't chosen for elegance, they're chosen for what they refuse to explain away. I’ve become entirely convinced that accepting a quantum interpretation requires you to accept at least one point that is highly counter-intuitive from the perspective of traditional physics. Are you accepting faster than light communication? Multiple worlds? Humans (or machine!) observation collapsing a wave function?

Starting position: Many-Worlds is an attractive default!

Many Worlds has been identified by physicist Sean Carroll, and others, as austere quantum mechanics. Meaning, it accepts only the minimum to get quantum mechanics working and no more. There’s no collapse postulate, it's deterministic, and it keeps the cruft to a minimum in favor of a minimum of rules.

When you first dive into quantum mechanics after a lifetime of traditional physics, it’s refreshing to see that you only really need one trade-off (accepting the branching of reality) to get right to the heart of matters. There are some issues I ran into, though, as I dug in.

At top of mind, you really have to wonder if waving away problems into a branching function papers over larger questions of branch-individuation and the question of what this unbounded ontological commitment means. Many of the problems identified with Many Worlds (MWI), like the doubling of energy, seem to be handled with a hand-wavy, pithy, truthy answer. It increasingly feels like an exercise in, “let’s keep this simple, guys."

Meaning, is it possible that we are taking elegance on paper to mean that we get to paper over the explanatory debt we create?

Testing an alternative: Pilot Wave / Bohmian Mechanics

After Many Worlds, I rolled right into Pilot Wave/De Broglian mechanics. Honestly, this interpretation seems to feel more like “home” for me still. Bohm restores realism, there are definite trajectories, and there is no observer-dependence.

The independence from the observer affect is probably the single thing that is a difficult thing to accept. Yet, I completely understand that my own taste does not define reality.

But Bohm creates its own problems. Where it stalls for a practitioner-oriented reader:

• the manifest asymmetry between the wave and the particle. I don’t know that I buy it, but I’m willing to be convinced.
• Nonlocality baked in from the start (meaning, not derived). Is this pill more easily swallowed than branches of reality every time you flip a coin?

As I rolled through explanations to nonlocality in my mind, I considered a thought experiment: if I make a promise with someone, travel across the world, and we both plant a flower on the same day, does that reproduce the same kind of correlation as entangled particles collapsing together, light years apart? Nope.

My flower scheme is a local hidden variable: the correlation was fixed in advance and each of us just carries out the plan independently, no contact required at the moment of planting. Bell proved that any theory built that way (local hidden variables of any stripe) makes different statistical predictions than quantum mechanics does. Aspect's experiments in the 1980s checked, and quantum mechanics won.

So the flower promise is foreclosed. Hidden variables generally are not (Bohm's theory is a hidden-variable theory), and it survives Bell's theorem. It just has to give up locality to do it: the pilot wave acts instantaneously across the whole system without requiring a promise.

A slight jaunt over to Copenhagen, only properly stated (and not the strawman)

I had never reacted to the Copenhagen Interpretation with the revulsion that some do. I had simply gone into things more ready to accept certain trade-offs over others. With that said, I wandered to the orthodox (Copenhagen) interpretation for the first time just a couple months ago.

Note that I explicitly don’t mean the "shut up and calculate” caricature; I mean the actual generally accepted content that makes Copenhagen the dominant interpretation, I admit to one major appealing element:

• Measurement is well defined and clear. There’s no claim to describing reality between measurements

But it all goes back to the measuring device as the triggerer of the wave function. Are you viewing? You’re the collapser. A camera? Camera is collapser. Really, what’s the line on what collapses the wave function? If everything can observe, is action everywhere forestalled?

That's the shifty split Bell himself flagged as the theory's real vulnerability, and it seems to be what the next interpretation tries to fix.

Next: GRW, QBism, and Relational QM.