Saturday, February 10, 2018

What everyone gets wrong about Wigner's interpretation of QM

There's an interpretation of QM known as the "von Neumann - Wigner interpretation," aka the "consciousness causes collapse" interpretation.

Wikipedia says:
In the 1960s, Eugene Wigner[2] reformulated the "Schrödinger's catthought experiment as "Wigner's friend" and proposed that the consciousness of an observer is the demarcation line which precipitates collapse of the wave function, independent of any realist interpretation. 
There are other possible solutions to the "Wigner's friend" thought experiment, which do not require consciousness to be different from other physical processes.
This is true, but he also suggests the other way out of the paradox: consciousness is not something that other people have:
"It is not necessary to see a contradiction here from the point of view of orthodox quantum mechanics, and there is none if we believe that the alternative is meaningless, whether my friend's consciousness contains either the impression of having seen a flash or of not having seen a flash.

However, to deny the existence of the consciousness of a friend to this extent is surely an unnatural attitude, approaching solipsism, and few people, in their hearts, will go along with it."
(Remarks on the Mind-Body Question, from Symmetries and Reflections, p.180)

But a few pages later, he seems to have switched to the view that only he has it:
This takes place whenever the result of an observation enters the consciousness of the observer - or, to be even more painfully precise, my own consciousness, since I am the only observer, all other people being only subjects of my observations.
(From Two Kinds of Reality, from Symmetries and Reflections p.185)

This is presumably for rhetorical effect, but nonetheless it makes his model so much more sensible (at least, to my eye). It's not that consciousness is some magical property that lives inside human skulls and collapses wave functions. It's that (from your perspective, you might say) you are the only observer.

Even the Stanford Encyclopedia of Philosophy gets it wrong! 
Among these approaches, the one with the longest history was initiated by von Neumann in the 1930s, later taken up by Wigner, and currently championed by Stapp. It can be roughly characterized as the proposal to consider intentional conscious acts as intrinsically correlated with physical state reductions.
Wigner's view is actually a natural result of the Many-Worlds Interpretation. MWI claims to get away from the consciousness problem by explaining that each branch has a copy of you, with its own consciousness. But there remains a meaningful sense in which "I collapse the wave function," and it reduces to Wigner's view.

Specifically, before I come into contact with the world-eating superposition that is the natural evolution of any experiment, I should (in theory, though decoherence makes this impossible "for all practical purposes") model the system as evolving unitarily. In particular, this is because if I undid enough of the evolution, I should be able to run an interference experiment demonstrating that the original observable is in superposition. But once I come into contact with it, and I join the superposition, the copy of me on either branch is now free to model the entire universe with just his branch.

Sometimes people explain that different branches of the multiverse can "communicate" -- in the sense of interference. But it seems that I ought to be able to do this only before the superposition has spread to me. Once I make manifest the branch -- aka, collapse the wave function, or become conscious of the result -- there is no other universe, as far as it concerns me.

This is functionally indistinct from "I collapse the wave function."

Now, what is this "I" that collapses the wave function? I better pin it down if it plays such a crucial role in reality, right?

Well, the Vedantins and (some) Mahayana Buddhists covered the "locating the I" thing. And what do you find at the end? That I, as consciousness itself, am the very fabric from which the world is made. Of course the buck stops here. It's the only place that's real, so to speak.

I am that which does not undergo unitary evolution, like all of so-called "physical reality." In other words, I am not definable from within physical law.

What's particularly unfortunate is how many assume that Wigner's view is radically different from MWI (see, e.g. "Wigner's friend in Many Worlds"). In fact, he was sagely pointing out that any interpretation will run into something fishy with consciousness. He continues the quote above:
Alternatively, one could say that quantum mechanics provides only probability connections between the results of my observations as I perceive them. Whichever formulation one adopts, the consciousness evidently plays an indispensable role.
And in another piece:
it will remain remarkable, in whatever way our future concepts may develop, that the very study of the external world led to the conclusion that the content of the consciousness is an ultimate reality.
As you try to pin down the point at which I am (in principle, not just FAPP) allowed to stop modeling the world as a superposition in MWI, you close in on the definition of "I," or that which seems to be looking out these eyes. I am not my toes, I am not my torso, ....

Hmm he also later says:
Solipsism may be logically consistent with present quantum mechanics, monism in the sense of materialism is not. The case against solipsism was given at the end of the first section. 

No comments: