MV#5 — QBism: The World is Unfinished — Ruediger Schack

Quantum mechanics is a powerful tool. It allows us to manipulate matter and make predictions with unparalleled precision. But we typically ask more of our theories than instrumental capabilities, we’d like them to give us an account of nature. 

In order to extract such an account from quantum mechanics we need to confront the measurement problem. A specific, and famous instance of this problem can be served up like so:

  1. A cat in a box can be in states of being alive and dead simultaneously …
  2. …  until opening the box to observe the cat causes one state to be picked. 

Neither portion of this is palatable. With David Wallace, we have previously discussed the Everett interpretation which disputes the first and emphatically denies the second. According to this view, the world branches at events causing macroscopic things (like cats) to develop into superpositions of very different states (like the superposition of being dead and alive). To Everettians there is not one cat that is alive and dead. The live and dead are plural: the cats exist in different branches of a multiverse, they are counterpart cats inhabiting counterpart boxes. Observation tells us the branch we are in but has no privileged role. 

Our guest this week is Ruediger Schack. With Chris Fuchs and Carlton Caves, Ruediger is one of the originators of a very different framework for understanding quantum mechanics, QBism. He’s a professor of mathematics at Royal Holloway, University of London working in the fields of Quantum Information and cryptography.

Under the QBist view cats can be in quantum states of being alive and dead simultaneously and agents can provoke a collapse in the quantum state. Yet, to the QBist none of this is mysterious for under this interpretation the quantum state does not represent the world it represents our knowledge. To use some philosophical terms of art quantum states are epistemic and not ontic.

The original label for the QBist account was Quantum Bayesianism, which captured the notion that the collapse of the wavefunction is nothing more than Bayesian updating — the wavefunction is a representation of a user’s beliefs, as new evidence emerges it is trivial that this should change discontinuously. 

The QBist project has outgrown its original label of Quantum Bayesianism, it has some fascinating features, at the core of which is that agents cannot be removed from reality. Reality is not, according to the QBists, independent of agents, it is not objective. But neither is it purely ideal — Berkleyan — nor dependent on one individual. Rather it is inter-subjective.

This swims against a strong current in Western science and philosophy within the analytic tradition where a “God’s eye view” (or, another term of art, sub specie aeternitatis) is accepted as conceptually meaningful. But within other philosophical traditions — specifically phenomenology — it does receive support. It was perhaps the French philosopher Maurice Merleau-Ponty who most captured the QBist spirit when he wrote:

“there is no world without a being in the world”

Maurice Merleau-Ponty

Merleau-Ponty implies that there is an external world, but it is dependent on the beings within it, just as they depend on it for their situation. 

Another interesting facet of QBism is its use of a personalist Bayesian interpretation of probability, so all probabilities are degrees of belief — even 0 and 1. This enables their rejection of the EPR criterion of reality introduced by Einstein, Podolosky, and Rosen:

If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of reality corresponding to that quantity.

From Can Quantum-Mechanical Description of Physical Reality be Considered Complete? — Einstein, Podolsky, Rosen

The intuition here is that for cases where we can perfectly predict the position that (for example) a particle will be in when observed, it was in that position — whether observed or not. 

The QBist replies: why should a person’s subjective beliefs imply the existence of any element of reality? 

We dig a little into some of the challenges for the QBist — including the above. Harvey Brown outlines arguments for the reality of the wavefunction which go against the core QBist belief that quantum states are epistemic. The spirit of these arguments is thqt quantum states fundamental to the explanatory power of quantum mechanics, and that the best explanation of this is that they exist. This is the fundamental abductive argument that underpins much scientific thinking — it’s why we think particles are real.

I find it illuminating to contrast the hyper-realist view offered by the Many Worlds (or Everett) interpretation of quantum mechanics where quantum states and their evolution capture all of reality with the QBist for whom they are not an element of nature at all. 

While I still count myself as a card-carrying Everettian this was an eye-opening excursion down another path of thinking. I particularly enjoyed Ruediger’s closing thoughts — because agents cannot be removed from the QBist picture, reality is not fully defined, it is being created by us. The world is not finished. 


Ruediger’s article on QBism in The Conversation

Wikipedia and The Stanford Encyclopedia of Philosophy offer good introductions to QBism.

A QBist reads Merleau-Ponty — Ruediger discusses the parallels between QBism and phenomenology

The Reality of the Wavefunction: Old Arguments and New — Harvey Brown’s paper reasoning that QBism fails to capture the explanatory power of the wavefunction

“Was There a Sun Before Men Existed?”: A. J. Ayer and French Philosophy in the Fifties — an entertaining paper by Andreas Vrahimis that contrasts the thinking of phenomenologists and the analytic philosopher A.J. Ayer.

Chris Timpson is always worth listening to on the subject of the interpretations of QM, a self-described agnostic on this topic his discussion of QBism gives a characteristically thoughtful and even-handed critique. Here he is on YouTube

Chris Fuchs is a pleasure to listen to, even if one disagrees, his characterization of the philosophical critiques of QBism is irresistibly brilliant: “wah, wah, wah you’re not realist enough for us”. From this talk on YouTube.

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