Interview with Diane Harley 

2002 Oppenheimer Lecture and questions of emergence 

Within my lifetime physical science has given us two astonishing examples of exactness from inexactness - the Josephson effect and the quantum Hall effect. Both are quantum-mechanical in nature (together they measure Planck's constant and the electron charge) yet both are accurate to 10 significant figures and are among the most certain things known to science. How can this be? The answer is that both are caused by, and evidence for, powerful high-level physical organizational principles that are collective in nature and emergent - meaning that they are exact in the thermodynamic limit (but only there), encoded only indirectly by the underlying laws of quantum mechanics, and in a deep sense independent of microscopics. The principles at work in these effects are fairly abstruse - continuous symmetry breaking and localization - but they are merely the latest additions to a family of old friends that includes the laws of hydrodynamics, the laws of elasticity, the laws of plasticity, the laws of magnetism, the laws of electrochemistry, and the laws of thermodynamics. The exactness of these effects, which cannot be deduced from first principles, has deep epistemological implications for physical science. It tells us that our mastery of the physical world is not due to our technological prowess or mathematical brilliance but rather because nature allows understanding by organizing itself according to laws. Indeed one can argue that everything we measure with great accuracy (i.e. that we "know" to be true) rests ultimately on a principle of organization, including such apparently reductionist quantities as the Rydberg constant and masses of elementary particles. This being the case, the central task of physics in our time is not the search for the ultimate equations at all but rather the identification and enumeration of collective organizing principles of nature, including potentially the principles of life.

I do not know whether to continue in science, especially since I can see strong arguments to support both staying and leaving. Although I think that I still love science, my reasons for pursuing a career in research do not seem to match those of most scientists. I surmise that in order to really love scientific research, one has to love the "how" questions and the problem-solving opportunities associated with the day-to-day realities of research life. Although I do like some of these "how" questions and the fun of problem-solving, I tend to think of science more in terms of such things as its epistemology, the inter-relationships among the various branches of sciences (is it a unity or independent multitudes? questions of reductionism), how various disciplines study the same things but describe them differently, and the range and limits of mathematization. The one question relating to science that most intrigues me is the issue of "levels of description"-how do descriptions at various levels (e.g., physical, chemical, biological) relate to one another and indeed, why are there levels in the first place. What in our reality gives rise to these levels? Is it something inherent about the physical world that gives different levels or is it merely a convenient epistemological construct? Since these burning questions of mine are traditionally relegated to the "philosophy of science" (because, I guess, most working scientists do not care about them), I often do not feel at home intellectually in scientific research.