Saturday, September 4, 2010

Is emergence the nature of physical reality?

Yesterday I finished my White paper for the conference, "Quantum physics and the nature of reality," to be held in honour of the 80th Birthday of Sir John Polkinghorne, later this month in Oxford.
I welcome any comments on the paper. The abstract is below.

Is emergence the nature of physical reality?

The concept of emergence provides a unifying framework to both characterise and understand the nature of physical reality, which is intrinsically stratified. Emergence raises philosophical questions about what is fundamental, what is real, and the possible limits to our knowledge. Important emergent concepts, particularly predominant in condensed matter physics, are highlighted: spontaneous symmetry breaking, quasi-particles and effective interactions, universality and protectorates. Three alternative emergent perspectives on the quantum measurement problem are discussed. (i) The classical world emerges from the quantum world via decoherence. (ii) Quantum theory is not exact but emerges from the statistical mechanics of underlying classical bosonic and fermionic fields, and (iii) The discontinuity between the quantum and classical worlds is another example of the epistemological (and possibly ontological) barriers between different strata of reality. Addressing the merits and deficiencies of each of these three perspectives from both theoretical, experimental, and philosophical approaches appears to me important, realistic, and exciting.


  1. i'm gonna read it , and after that, i'll try to comment something... it's a very interesting and puzzling subject!

  2. Typographical corrections:
    "Mass is an additive properties of matter." (p.2) [property]
    "get booged down" (p.3) [bogged]
    "``because it on the airlines schedule’’ [it is on]; [airline's]

    Thoughts on the content:
    I think the question "Do electrons exist?" is a very good and rather subtle one. I get the impression that a free electron in a box seems different in some ways to an electron in a material (often treated as a quasi-particle with an effective mass).

    I like the juxtaposition between the idea of emergence and the questions of 'what is understanding' and 'what is fundamental'; to me it strongly implies to me that the 'important' properties of a system depend on the question we are asking. An important part of understanding something is accurately predicting if we change X then Y will happen. The nature of X and Y determine the properties of interest. For example in the "What makes an airplane fly" scenario, if X were the number of baggage handlers and Y was time of departure the answer "Because it is on the airlines schedule" makes good sense. But if X were the geometry of the wing and Y was, say, the weight the airplane could fly with then the answer in terms of air molecules is more useful. Similarly the properties you might measure - the number of staff or the pressures on the wing would vary depending on your question.
    So emergence has partly to do with what questions we are trying to answer? (Which of course are influenced by the time and length scales we are interested in).

    You claim that due to emergence there is a stratification of scientific principles - I'd like to see this argument expanded/justified a little. Can you give a detailed description of some of these emergences, and explain how they are emergence?

    In particular, most of the examples you give of emergence come in some way from a reductionist viewpoint; quasi-particles, orbitals and localised spins from a lattice model Hamiltonian are exact quantum solutions to a slightly different problem from the one of interest, and so are approximate solutions to the problem of interest. Similarly thermodynamics (and hydrodynamics) can be viewed as statistical averages of microscopic properties for systems sufficiently large that the variations in these averages are insignificant.
    It would be interesting to have a clear example somewhere higher up in the scheme where the relationship isn't so clear.

    When you talk about the emergence of the classical world from quantum mechanics I think it would be good to emphasise that, in some non-trivial way, the idea of a particle with a well defined position and momentum is an emergent property.

    I really quite like the paper - it gives a lot of interesting things to ponder!