Fascinating Bohr

This past August 15 marked the 75th anniversary of the lecture "Light and Life," by the physicist and (some would say) philosopher, Niels Bohr. Bohr was roughly as well known, and probably more influential among physicists, than his contemporary and intellectual sparring partner, Albert Einstein. However, in comparison to Einstein and Einstein's ideas, Bohr and Bohr's ideas receive relatively modest levels of attention. I think that's unfortunate. "Light and Life" is perhaps best known for its focus on Bohr's concept of complementarity. According to this concept, some natural phenomena can only be completely understood by combining two or more experimental approaches that cannot be simultaneously implemented. More generally, complementarity asserts that apparently incompatible ideas or perspectives can both be necessary to achieve a fuller understanding of an entity or process.Bohr's original inspiration for the concept of complementarity was that ultimate source of illumination, light. He was impressed by the fact that some phenomena involving light (e.g. diffraction patterns) were best explained by regarding light as waves. However, other phenomena involving light (e.g., the photoelectric effect) were best explained by regarding light as particles. Thus, it may not be justified to claim unqualifiedly that light is waves or that light is particles. Instead, light may be better understood as behaving as if it were particles or waves, depending on the precise methods of detection or measurement.In "Light and Life," delivered in Copenhagen to the International Congress on Light Therapy, Bohr specifically proposed that there is a complementarity relevant to biology, as well, arguing that it may not prove possible to fully explain living processes in physical terms. This notion, depending on precisely how it is interpreted, is generally dismissed, and appropriately so to the extent that it might be confused with nebulous concepts of a uniquely life-associated "vital force" (to which Bohr did not subscribe). But even if Bohr was wrong in his literal thesis, he may nevertheless have been onto a biologically useful insight.Bohr's student in theoretical physics, Max Delbrück, eventually left physics for biology and became the strongest exponent of the relevance of complementarity to biology. He called attention to the fact that, as he put it, molecular structure and integrated biological function may not be compatible observables. Structure determination typically requires molecular homogeneity, while determination of integrated biological function necessarily requires the molecular heterogeneity of the cellular or organismal environment. In other words, to understand molecular structure you need chemical simplicity, while insights into function (at least in one sense) require the full chemical complexity of the whole biological system. The late, great evolutionist (and historian and philosopher of biology) Ernst Mayr offered another example of complementary perspectives in biology. He emphasized two broad types of causation in biology: ultimate (i.e., evolutionary) and proximate (i.e., physiological). In Mayr's view these two strands in the biological fabric of explanation addressed distinct questions: evolutionary biology focuses on why structures and processes are as they are and how they came to be that way, whereas physiological biology is centered on how the structures within organisms actually perform their various functions. For example, experimentally determining why a bird has evolved to migrate south in the fall or winter is a somewhat distinct task from experimentally characterizing the environmental signals that trigger, and the physiological processes that implement, the bird's actual migration.Complementarity also has a place in social sciences, Bohr argued. One can, for example, try to understand the behavior of an individual human being as the result of a unique constellation of genetic and environmental factors, and their interactions. Alternatively, one can seek to account for the behavior of an individual via population-based trends in statistically-defined patterns of behavior. Both types of explanation have their uses, but not necessarily for addressing the same questions.So, while I have appropriate respect for Einstein, I also have high regard for his less fully-appreciated contemporary, Niels Bohr. I have no problem seeing value in their respective views on many subjects, including where they disagreed, as on certain aspects of the meaning and ultimate validity of quantum theory. Appropriately enough, the ideas of Einstein and Bohr, the doubter and the proponent of quantum theory's status as the pinnacle of physical explanation, are often complementary. Neil Greenspan mail@the-scientist.comNeil Greenspan is an immunologist and clinical pathologist at Case Western Reserve University. He has written about the role of semantics in science, the shortcomings of intelligent design, and the challenging path from genetic knowledge to the development of medical therapies.Greenspan thanks Gino Segrè for critical comments on this manuscript.Links within this article:N. Mott, "Working with Bohr," The Scientist, October 20, 1986. http://www.the-scientist.com/article/display/7121/L. Pray, "Ernst Mayr dies," The Scientist, February 4, 2005. http://www.the-scientist.com/article/display/22589/Neil Greenspan http://path-www.path.cwru.edu/N. Greenspan, "Wishful thinking and semantic specificity," The Scientist, August 19, 2002. http://www.the-scientist.com/article/display/13194/N. Greenspan, "Not-so-intelligent design," The Scientist, March 4, 2002. http://www.the-scientist.com/article/display/12895/N. Greenspan, "Beware of direct lines," The Scientist, September 17, 2001. http://www.the-scientist.com/article/display/12614/Gino Segrè http://www.physics.upenn.edu/facultyinfo/segre.html

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