Why abandon “probability is in the mind” when it comes to quantum dynamics?
post by Maxwell Peterson (maxwell-peterson) · 2025-01-14T15:53:55.548Z · LW · GW · 6 commentsContents
7 comments
A core tenet of Bayesianism is that probability is in the mind. But it seems to me that even hardcore Bayesians can waffle a bit when it comes to the possibility that quantum probabilities are irreducible physical probabilities.
I don’t know enough about quantum physics to lay things out in any detailed disagreement, but it seems to me that if one finds a system that one cannot consistently make predictions for, it means we lack the knowledge to predict the systems, not that the system involves physical, outside-the-mind probabilities. For example, I could never predict the exact pattern of raindrops the next time it rains, but no one argues that that means those probabilities are therefore physical.
What is the Bayesian argument, if one exists, for why quantum dynamics breaks the “probability is in the mind” philosophy?
6 comments
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comment by Rafael Harth (sil-ver) · 2025-01-14T20:25:50.461Z · LW(p) · GW(p)
I dispute the premise. Weights of quantum configurations are not probabilities, they just share some superficial similarities. (They're modeled with complex numbers!) Iirc Eliezer was very clear about this point in the quantum sequence.
comment by Yair Halberstadt (yair-halberstadt) · 2025-01-14T19:58:56.186Z · LW(p) · GW(p)
See Bell's theorem. Basically we know that quantum mechanics is truly random, not just pseudorandom, unless you posit non-locality.
comment by Cole Wyeth (Amyr) · 2025-01-14T21:21:50.209Z · LW(p) · GW(p)
Jaynes has argued (jn his book on probability theory) that nearly all progress in physics stopped once quantum mechanics was accepted to be "truly random," and that this anti-scientific attitude has to be abandoned for further progress to be possible.
From the little I know about QM he was probably wrong - it seems you can't remove the randomness without breaking other stuff we like such as locality.
Still, I wouldn't be shocked if he (and perhaps you?) were right after all.
Personally, I can't see any way to rule out the possibility that some things in the universe are "truly random" for all practical purposes - in the sense that, say, a sequence of spin measurements can't be compressed. In order for science to be possible, there have to be some discoverable underlying laws, but Solomonoff induction still works if those laws are probabilistic. If you insist on describing that situation as also not objectively random but only uncertain, fine, but I think it matches our intuitions about objective randomness perfectly and I'm inclined to use that term. This deserves a full post though.
comment by Noosphere89 (sharmake-farah) · 2025-01-14T16:50:12.459Z · LW(p) · GW(p)
I must admit, I think the "probability is in the mind, not the territory" either vacuously true or possibly false if you think that maps aren't the same things as low-resolution territories.
One example of where probability is in the mind, not the territory is false for non-trivial definitions of maps is Wigner functions, which turn out to be basically equivalent to a wavefunction, where it behaves basically like classical Bayesian probability theory, but with the caveat that for quantum physics, negative probabilities are allowed, and the most important impact here is you can't treat the uncertainty as just ignorance anymore (though they can include our ignorance).
More here:
https://www.lesswrong.com/posts/Y6LhXdGfwsAStMuhr/ackshually-many-worlds-is-wrong#nsebEbJbxqkekTbsK [LW(p) · GW(p)]
Replies from: maxwell-peterson↑ comment by Maxwell Peterson (maxwell-peterson) · 2025-01-14T17:21:41.890Z · LW(p) · GW(p)
Hm - reading Ben’s linked comment, it seems to me that the thrust is that negative probabilities must be admitted. But I don’t understand how that is related to the map vs. territory / probability-in-the-mind-or-physical distinction?
Like, “one must modify the relevant functions to allow negative probabilities” seems consistent with “probability is in the mind”, since functions are a part of the map, but it seems you consider it a counterexample! So I find myself confused.
↑ comment by Noosphere89 (sharmake-farah) · 2025-01-14T18:29:24.897Z · LW(p) · GW(p)
The main point here is that it can no longer be just our uncertainty in our map, something else must be added, which was the point.
Another way to say it is that probability can't just be in the mind, so while the probabilities encode our ignorance, it can't be all of the story (according to Wigner functions).
It was way down in the last comment, so maybe you should go to the end of the comment I linked here for more information.
Also, a difference here that doesn't matter for this discussion, but might matter for the general approach, might ultimately be that I disagree with this statement "since functions are a part of the map", because I think the map-territory distinction can often be blurry or fully dissolved in some cases, and also functions can have results when you evaluate them using an algorithm, making them part of the territory (for that specific function).