(Self-promotion warning.) Alexander Gietelink Oldenziel pointed me toward this post after hearing me describe my physics research and noticing some potential similarities, especially with the Redundant Information Hypothesis. If you’ll forgive me, I’d like to point to a few ideas in my field (many not associated with me!) that might be useful. Sorry in advance if these connections end up being too tenuous.
In short, I work on mathematically formalizing the intuitive idea of wavefunction branches, and a big part of my approach is based on finding variables that are special because they are redundantly recorded in many spatially disjoint systems. The redundancy aspects are inspired by some of the work done by Wojciech Zurek (my advisor) and collaborators on quantumDarwinism. (Don’t read too much into the name; it’s all about redundancy, not mutation.) Although I personally have concentrated on using redundancy to identify quantum variables that behave classically without necessarily being of interest to cognitive systems, the importance of redundancy for intuitively establishing “objectivity” among intelligent beings is a big motivation for Zurek.
Building on work by Brandao et al., Xiao-Liang Qi & Dan Ranard made use of the idea of “quantum Markov blankets” in formalizing certain aspects of quantum Darwinism. I think these are playing a very similar role to the (classical) Markov blankets discussed above.
In the section “Definitions depend on choice of variables” of the current post, the authors argue that Wentworth’s construction depends on a choice of variables, and that without a preferred choice it’s not clear that the ideas are robust. So it’s maybe worth noting that a similar issue arises in the definition of wavefunction branches. The approach severalresearchers (including me) have been taking is to ground the preferred variables in spatial locality, which is about as fundamental a constraint as you can get in physics. More specifically, the idea is that the wavefunction branche decomposition should be invariant under arbitrary local operations (“unitaries”) on each patch of space, but not invariant under operations that mix up different spatial regions.
Another basic physics idea that might be relevant is hydrodynamic variables and the relevant transport phenomena. Indeed, Wentworth brings up several special cases (e.g., temperature, center-of-mass momentum, pressure), and he correctly notes that their important role can be traced back to their local conservation (in time, not just under re-sampling). However, while very-non-exhaustively browsing through his other posts on LW it seemed as if he didn’t bring up what is often considered their most important practical feature: predictability. Basically, the idea is this: Out of the set of all possible variables one might use to describe a system, most of them cannot be used on their own to reliably predict forward time evolution because they depend on the many other variables in a non-Markovian way. But hydro variables have closed equations of motion, which can be deterministic or stochastic but at the least are Markovian. Furthermore, the rest of the variables in the system (i.e., all the chaotic microscopic degrees of freedom) are usually “as random as possible”—and therefore unnecessary to simulate—in the sense that it’s infeasible to distinguish them from being in equilibrium (subject, of course, to the constraints implied by the values of the conserved quantities). This formalism is very broad, extending well beyond fluid dynamics despite the name “hydro”.
On hydrodynamic variables/predictability: I (like probably many others before me) rediscovered what sounds like a similar basic idea in a slightly different context, and my sense is that this is somewhat different from what John has in mind, though I’d guess there are connections. See here for some vague musings. When I talked to John about this, I think he said he’s deliberately doing something different from the predictability-definition (though I might have misunderstood). He’s definitely aware of similar ideas in a causality context, though it sounds like the physics version might contain additional ideas
Out of the set of all possible variables one might use to describe a system, most of them cannot be used on their own to reliably predict forward time evolution because they depend on the many other variables in a non-Markovian way. But hydro variables have closed equations of motion, which can be deterministic or stochastic but at the least are Markovian.
This idea sounds very similar to this—it definitely seems extendable beyond the context of physics:
We argue that they are both; more specifically, that the set of macrostates forms the unique maximal partition of phase space which 1) is consistent with our observations (a subjective fact about our ability to observe the system) and 2) obeys a Markov process (an objective fact about the system’s dynamics).
(Self-promotion warning.) Alexander Gietelink Oldenziel pointed me toward this post after hearing me describe my physics research and noticing some potential similarities, especially with the Redundant Information Hypothesis. If you’ll forgive me, I’d like to point to a few ideas in my field (many not associated with me!) that might be useful. Sorry in advance if these connections end up being too tenuous.
In short, I work on mathematically formalizing the intuitive idea of wavefunction branches, and a big part of my approach is based on finding variables that are special because they are redundantly recorded in many spatially disjoint systems. The redundancy aspects are inspired by some of the work done by Wojciech Zurek (my advisor) and collaborators on quantum Darwinism. (Don’t read too much into the name; it’s all about redundancy, not mutation.) Although I personally have concentrated on using redundancy to identify quantum variables that behave classically without necessarily being of interest to cognitive systems, the importance of redundancy for intuitively establishing “objectivity” among intelligent beings is a big motivation for Zurek.
Building on work by Brandao et al., Xiao-Liang Qi & Dan Ranard made use of the idea of “quantum Markov blankets” in formalizing certain aspects of quantum Darwinism. I think these are playing a very similar role to the (classical) Markov blankets discussed above.
In the section “Definitions depend on choice of variables” of the current post, the authors argue that Wentworth’s construction depends on a choice of variables, and that without a preferred choice it’s not clear that the ideas are robust. So it’s maybe worth noting that a similar issue arises in the definition of wavefunction branches. The approach several researchers (including me) have been taking is to ground the preferred variables in spatial locality, which is about as fundamental a constraint as you can get in physics. More specifically, the idea is that the wavefunction branche decomposition should be invariant under arbitrary local operations (“unitaries”) on each patch of space, but not invariant under operations that mix up different spatial regions.
Another basic physics idea that might be relevant is hydrodynamic variables and the relevant transport phenomena. Indeed, Wentworth brings up several special cases (e.g., temperature, center-of-mass momentum, pressure), and he correctly notes that their important role can be traced back to their local conservation (in time, not just under re-sampling). However, while very-non-exhaustively browsing through his other posts on LW it seemed as if he didn’t bring up what is often considered their most important practical feature: predictability. Basically, the idea is this: Out of the set of all possible variables one might use to describe a system, most of them cannot be used on their own to reliably predict forward time evolution because they depend on the many other variables in a non-Markovian way. But hydro variables have closed equations of motion, which can be deterministic or stochastic but at the least are Markovian. Furthermore, the rest of the variables in the system (i.e., all the chaotic microscopic degrees of freedom) are usually “as random as possible”—and therefore unnecessary to simulate—in the sense that it’s infeasible to distinguish them from being in equilibrium (subject, of course, to the constraints implied by the values of the conserved quantities). This formalism is very broad, extending well beyond fluid dynamics despite the name “hydro”.
Thanks for that overview and the references!
On hydrodynamic variables/predictability: I (like probably many others before me) rediscovered what sounds like a similar basic idea in a slightly different context, and my sense is that this is somewhat different from what John has in mind, though I’d guess there are connections. See here for some vague musings. When I talked to John about this, I think he said he’s deliberately doing something different from the predictability-definition (though I might have misunderstood). He’s definitely aware of similar ideas in a causality context, though it sounds like the physics version might contain additional ideas
John has several lenses on natural abtractions:
natural abstraction as information-at-a-distance
natural abstraction = redundant & latent representation of information
natural abstraction = convergent abstraction for ‘broad’ class of minds
the thing that felt closest to me to the Quantum Darwinism story that Jess was talking about as the ’redudant/ latent story, e.g. https://www.lesswrong.com/posts/N2JcFZ3LCCsnK2Fep/the-minimal-latents-approach-to-natural-abstractions and https://www.lesswrong.com/posts/dWQWzGCSFj6GTZHz7/natural-latents-the-math
Curious if @johnswentworth has any takes on this.
This idea sounds very similar to this—it definitely seems extendable beyond the context of physics: