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Reading group: Philip Anderson, “More Is Different” (Science, 1972)

Lars Eriksen Fellow
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Laws of nature & emergenceAnalyticPhilip Anderson#more-is-different#reading-group#reductionism#symmetry-breaking
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Its coordinates: 1.9 Laws of Nature · 1.11 Emergence and Reduction

Anderson's essay is about exactly the phenomena that drew me into physics as a student. It is four pages long, written by a physicist who later won the Nobel Prize, and it has probably done more to shape physicists' attitudes to reductionism than any work of philosophy. I'd like to read it slowly with non-physicists.

The text is freely available in many places. It's short enough to read in one sitting, but I suggest three sessions:

  1. The opening: Anderson accepts reductionism (everything obeys the same fundamental laws) but rejects what he calls the "constructionist" hypothesis, that you can start from those laws and reconstruct the universe.
  2. The middle: symmetry breaking. His examples include the ammonia molecule, crystals and superconductivity. Don't worry about the details; I'll explain what you need.
  3. The end: the hierarchy of sciences, each level needing its own concepts, and his memorable closing lines.

Post your reactions per session. Physicists and non-physicists equally welcome.

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6 replies

  1. Lars Eriksen

    Fellow

    Session 1. The key move is that reductionism and constructionism come apart. Anderson is a reductionist in the sense that nothing in chemistry violates physics. But "can be reduced to" doesn't mean "can be built up from". Each level of complexity, he says, involves entirely new properties, and understanding them requires research "as fundamental in its nature" as particle physics. That last claim annoyed many particle physicists at the time, which was rather the point.

  2. Emeka Anyaoha

    Fellow

    Session 2. I studied symmetry breaking at university and never saw it put this way. The fundamental laws are symmetric (no preferred direction, say), but the ground state of a large system may not be. A crystal picks out particular directions though the laws don't. So the large system has properties that, though consistent with the laws, are not contained in them. You need to know the system is large and in a particular state. That's what "more is different" means.

    Helpful · 1
  3. Abdulrahman Al-Bassam

    Fellow

    Session 2, practical angle. In reservoir modelling we never start from molecules. We use Darcy's law for flow through porous rock, which is an emergent description that ignores almost everything about the molecules. Anderson made me feel less guilty about that. The higher-level law isn't a lazy approximation; it captures structure the molecular description would hide.

    Helpful · 2
  4. Ignacio Cárcamo

    Fellow

    Session 3. Biologists quote the hierarchy section constantly: at each level, new laws, concepts and generalisations are needed. I had read the quotations without the physics. Now I see that his argument is not "biology is too complicated to compute" but "biology has organising principles that are not in the physics, even though nothing in biology violates physics". That's a stronger and more interesting claim.

    Helpful · 3
  5. Priya Raghavan

    Fellow

    Session 3. I'd like to ask Lars whether Anderson is defending weak or strong emergence. It sounds like weak: everything is consistent with the fundamental laws, it's just that the higher-level properties need new concepts. But some people read him as defending something stronger.

    1. Lars Eriksen

      Fellow

      Priya, I read him as weak emergence with a strong methodological claim. Nothing he describes is underivable in principle; but in practice the derivation from the bottom is impossible, and more importantly, the right concepts (order parameter, rigidity, broken symmetry) only appear at the higher level. So: ontologically modest, methodologically radical. And his closing lines, a borrowed quip that quantitative differences become qualitative and a joke about the rich, are the best ending to a physics paper I know.

      Helpful · 1