One aspect of these two that is crucial is the idea of quantum criticality. In this week's issue of Nature (Nature 446, 379, 22 March 2007), Piers Coleman once again has written a wonderful piece in which a bunch of issues were tackled in just one page: emergent behavior, superconductivity, life, quantum criticality, string theory, etc... There are several passages that are worth quoting:
The potential for quantum matter to develop emergent properties is far more startling. Atoms of niobium and gold, individually similar, combine to form crystals that, kept cold, show dramatically different properties. Electrons roam free across gold crystals, forming the conducting fluid that gives gold its lustrous metallic properties. Up to about 30 nanometres, there is little difference between gold and niobium. It's beyond this point that the electrons in niobium start binding together into the coupled electrons known as 'Cooper pairs'. By the time we reach the micrometre scale, these pairs have congregated in their billions to form a single quantum state, transforming the crystal into an entirely new metallic state — that of a superconductor, which conducts without resistance, excludes magnetic fields and has the ability to levitate magnets.
Superconductivity has always been the poster child for collective, emergent phenomena. It is what Bob Laughlin used to demonstrate emergent behavior. And it is what Carver Mead used to proclaim that "Nowhere in natural phenomena do the basic laws of physics manifest themselves with more crystalline clarity."
On the issue of reductionism versus emergent behavior, Coleman has this to say:
Some believe that emergence implies an abandonment of reductionism in favour of a more hierarchical structure of science, with disconnected principles developing at each level. Perhaps. But in almost every branch of physics, from string theory to condensed-matter physics, we find examples of collective, emergent behaviour that share common principles. For example, the mechanism that causes a superconductor to weaken and expel magnetic fields from its interior is also responsible for the weak nuclear force — which plays a central role in making the Sun shine. Superconductors exposed general principles that were used to account for the weak nuclear force.
This is clearly an argument against those who think that ideas and principles that come out of condensed matter are not "fundamental". Such importance is certainly not only the monopoly of elementary particle physics, String, etc.
It is a very good article, and I strongly recommend reading it from start to finish. Piers Coleman, besides being one respected physicist, has also written several terrific articles in various journals. I always look forward to his next one.
Zz.
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