Tuesday, August 7, 2018

Philosophy and emergence in condensed matter

Condensed matter physics is a source of a multitude of beautiful examples of emergence.  On the other hand, for more than a century philosophers have thought seriously about emergence, partly motivated by profound and difficult questions concerning human consciousness and free will.
Prior to the past decade, there appear to have been no substantial interactions between physicists and philosophers about the subject. A few years ago I posted about some recent work by philosophers of science on quasi-particles.

One of the big issues that philosophers wrestle with is the relative merits of weak emergence and strong emergence, which are sometimes distinguished as epistemological and ontological emergence.

I am very happy that in the past year or so that philosophy journals have published more than half a dozen papers about emergence in condensed matter. One of the papers, by Stephen Blundell, I blogged about earlier. Here I will mention two others and discuss one. All the papers are a result of the Durham Emergence Project.

Strong emergence and downward causation in biological physics
Tom C. B. McLeish

Reduction and emergence in the fractional quantum Hall state 
Tom Lancaster and Mark Pexton

McLeish begins with a helpful and succinct summary of the argument by Jaegwon Kim about "the causal completeness of the physical" [or the argument against non-reductive physicalism] that leads to the conclusion that mental events cannot have physical consequences. This argument has attracted significant attention from philosophers and has been used against strong emergence, and particularly to argue that consciousness is reducible.

McLeish rightly points out that the problem of consciousness is a "can of worms" [my phrase] and instead it might be valuable to consider the issue of "downward causation" by considering three important examples in biological physics.

"Downward causation" means "there are high-level entities, carrying unique information about the system essential for its future evolution, and whose form and evolution are not determined entirely by the low level entities."
He gives a nice introduction to soft matter physics and its applications to biological systems, considering the following examples.
  • Membrane and intra-membrane self-assembly
  • Allosteric Signalling in Gene Expression
  • Entangled DNA and Topoisomerases  
He points out how in these systems there is "top down causation" and that the emergent entities such as protein elasticity are not just a result of "coarse graining" but new "long-range physics" that arises from many microscopic realisations.

McLeish considers these examples reflect what Bishop and Silberstein (2016) defines as ``‘epistemological contextual emergence’ (ECE) as applying to systems whose ...description at a particular descriptive level (including its laws) offers some necessary but no sufficient conditions to derive the description of properties at a higher level.''

I thank Stephen Blundell and Tom McLeish for helpful discussions about their papers.

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