Wednesday, September 29, 2010

Reductionism, emergence, and Sean Carroll

In the last couple of weeks, Sean Carroll has made two separate posts (here and here) on his widely read Cosmic Variance blog at Discover magazine, in which he points out, in a celebratory tone, that we fully understand the laws of physics that govern the everyday world.  In a reductionist sense, he's right, in that nonrelativistic quantum mechanics + electricity and magnetism (+ quantum electrodynamics and a little special relativity) are the basic rules underlying chemistry, biology, solid state physics, etc.  This is not a particularly new observation.  Fifteen years ago, when I was a grad student at Stanford, Bob Laughlin was making the same comments, but for a different reason:  to point out that this reductionist picture is in many ways hollow.  I think that Sean gets this, but the way he has addressed this topic, twice, really makes me wonder whether he believes it, since beneath all of the talk about how impressive it is that humanity has this much understanding, lurks the implication that all the rest of non-high energy physics (or non-cosmology) is somehow just detail work that isn't getting at profound, fundamental questions.   The emergence of rich, complex, often genuinely "new" physics out of systems that obey comparatively simple underlying rules is the whole point of condensed matter these days.  For example, the emergence, in 2d electronic systems in semiconductors, of low energy excitations that have fractional charge and obey non-Abelian statistics, is not just a detail - it's really wild stuff, and has profound connections to fundamental physics.  So while Sean is right, and we should be proud as a species of how much we've learned, not everything deep comes out of reductionism, and some fraction of physicists need to stop acting like it does. 


Sarang said...

I'm not sure to what extent SC really doesn't believe that emergent phenomena are important -- he would almost certainly quibble with your calling it "new physics" -- and to what extent he's addressing people who use emergence in the mushy philosophical sense. I think partly the former, but, like, one must make allowances related to the professional hazards of being a public intellectual.

Doug Natelson said...

Fair enough, though there are certainly many examples of high energy borrowing concepts that emerged (pun intended) from condensed matter. The idea that you can start out with whole electrons and end up with fractionalized quasiparticles that obey exotic statistics was conceptually new. One can argue about whether that's a different kind of new physics than finding a previously undiscovered meson, or observing a quark-gluon plasma. I don't think they're that different.

Joe Renes said...

Excellent post -- I had precisely the same reaction. I also agree that understanding fractionalization and all the cool stuff going on with topological order is of the same type of new physics as what particle physicists do.

And more to the point, who knows whether today's "fundamental" degrees of freedom won't be tomorrow's emergent phenomena? Wen makes this point very elegantly in his book on condensed matter field theory.

In this light, I think Sean's point comes into sharper relief: it's that even if the standard model and gravity are not fundamental, we nevertheless have, for the first time, a good enough description of more elementary phenomena to understand everything "above" them. Once we've done the hard work of understanding all the emergent phenomena we encounter along the way of course.

Peter Armitage said...

Laughlin of that era's point was also that typically the underlying "fundamental" equations of motion are not even essential to the phenomena in question. They are largely independent of the microscopic details.

For instance, it is quite reasonable to imagine that in some universe where Coulomb's law went like 1/r^{1 + \eta}, phonons in crystals would still exist. If \eta is a small number then in that universe NaCl would probably still crystallize in an fcc structure and its phonon dispersion relations etc. would be more or less indistingishable from the one in our universe. So in what sense is the exact form of Coulomb's law important for the existence of phonons or even their explicit form in table salt?

Sean Carroll said...

Well, I did go out of my way to say on multiple occasions that we don't know how the laws play out in complicated systems, and "I certainly wasn’t making any claims at all about priority or importance or interestingness." I like to think it is simultaneously possible to recognize what we have accomplished while still appreciating that there's a long way to go.

Doug Natelson said...

Hi Sean - Thanks for your comment. I know that you weren't intentionally trying to comment on importance or relevance, and perhaps I'm reading too much into things. I agree with all of the factual points you made.

Still, you are privileged to have the public eye and ear (at least to a much larger extent than I do). When you say that we understand the basic laws underpinning everything in our everyday experience, I think you need to recognize that many casual readers will come away with a skewed impression that there are no scientific frontiers (at least, no deep, meaningful ones) in the "everyday" anymore. The whole "more is different" issue is actually pretty profound, and I think it routinely gets short shrift. I'm not suggesting that emergent phenomena are magic, but we don't understand some general organizing principles in collective physical responses, and that lack of understanding is important.