Bayesian Quantum Theory I

A quantum particle can be in a range of possible states. When an observer makes a measurement, she instantaneously “collapses” the wave function into one possible state. QBism argues that this collapse isn’t mysterious. It just reflects the updated knowledge of the observer. She didn’t know where the particle was before the measurement. Now she does.

Why do ordinary folks like me, who know nothing about advanced physics and only a smidgen of calculus, ever write about quantum mechanics?

Heck, why do people who DO know the physics and all the necessary math, people like Fritjof Capra or Lee Smolin, persist in targeting writings on the subject to folks like me?

The journalist Gary Zukav wrote The Dancing Wu Li Masters in 1979. This work has been much ridiculed as "New Age" stuff pretending to be physics. But Zukav wrote it after talking to a lot of real working physicists, participating in a 1976 physics conference as Esalen Institute, California, etc. Physicists don't just "talk among themselves" on these points. They make efforts to talk to the rest of us, explaining what it means.

So they have to expect that the rest of us will talk back once in a while, or discuss with one another what we think we're learning from them (humbly subject to correction, of course).

So, here's what I get. Quantum physics involves the "collapse of wave functions." At a given moment there will be a multiplicity of states incorporated into a single wave, and -- as a consequence, it appears, of someone's observation of the system, the function will collapse into a single spike. To use the cliched image -- once the lid of the box is lifted, either the cat is dead or it isn't.

There are a variety of different ways to understand this. I want to talk about one of them that seems to be fairly new, but first I'll talk about two older onces:

1. The Copenhagen interpretation.

This is the Orthodoxy, insofar as any one view is. The Copenhagen "interpretation" is in essence a determined refusal to "interpret" at all. Mathematical formalism and wave/particle duality at the level of metaphor. This dates to about 90 years ago, the work of Bohr and Heisenberg. The Copenhagen view is that uncertainty is not a subjective fact, but a fact about the world, about the subatomic particles/waves themselves. It holds that when we call them particles, or waves, we are using metaphors drawn from our macro-cosmic experience and they are ill-fitting ones.

So ... what is the electron or the photon really? All we can or need to say is that it is really the reality that produces certain experimental results which fit certain equations which in turn incorporate that uncertainty. The Copenhagen interpretation rests content with this mathematical formalism.

In Heisenberg's words, "Since the statistical nature of quantum theory is so closely [linked] to the uncertainty in all observations or perceptions, one could be tempted to conclude that behind the observed, statistical world a 'real' world is hidden, in which the law of causality is applicable. We want to state explicitly that we believe such speculations to be both fruitless and pointless. The only task of physics is to describe the relation between observations."

2. The many-worlds interpretation

But there is a natural tendency to go beyond formalism and metaphor. The only ways t do so get counter-intuitive but, hey, wave-function duality is counter-intuitive. So are a lot of the experimental results. So let's go there!

The many-worlds interpretation got its start with Hugh Everett in the 1950s. The idea is that the function as shown above would remain uncollapsed if we could observe it in a broad enough multiverse. It is collapsed only because we can't -- we observe it only within a particular time line. So, in one time line the cat lives, in the other the cat is dead when the lid is opened, and the two time lines never thereafter interact, representing difference universes.

Everett himself called this the "relative state" interpretation, which sounds less dramatic. It became the "many worlds" theory in the 1960s at the hands of Bryce DeWitt. That new name helped with the (later) process of popularization.

There are lots of other interpretations. The one I want to discuss tomorrow is called the "Quantum Bayesian Theory," because it combines quantum physics with another long-time concern of this blog, Bayesianism in probability theory.

For now, hear's a link.

https://www.quantamagazine.org/quantum-bayesianism-explained-by-its-founder-20150604

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