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November 23, 2006

Unreasonable effectiveness (part 1)

Einstein apparently once remarked that "The most incomprehensible thing about the universe is that it is comprehensible." In a famous paper in Pure Mathematics (13 (1), 1960), the physicist Eugene Wigner (Nobel in 1963 for atomic theory) discussed "The Unreasonable Effectiveness of Mathematics in the Natural Sciences". The essay is not too long (for an academic piece), but I think this example of the the application of mathematics gives the best taste of what Wigner is trying to point out.

The second example is that of ordinary, elementary quantum mechanics. This originated when Max Born noticed that some rules of computation, given by Heisenberg, were formally identical with the rules of computation with matrices, established a long time before by mathematicians. Born, Jordan, and Heisenberg then proposed to replace by matrices the position and momentum variables of the equations of classical mechanics. They applied the rules of matrix mechanics to a few highly idealized problems and the results were quite satisfactory.

However, there was, at that time, no rational evidence that their matrix mechanics would prove correct under more realistic conditions. Indeed, they say "if the mechanics as here proposed should already be correct in its essential traits." As a matter of fact, the first application of their mechanics to a realistic problem, that of the hydrogen atom, was given several months later, by Pauli. This application gave results in agreement with experience. This was satisfactory but still understandable because Heisenberg's rules of calculation were abstracted from problems which included the old theory of the hydrogen atom.

The miracle occurred only when matrix mechanics, or a mathematically equivalent theory, was applied to problems for which Heisenberg's calculating rules were meaningless. Heisenberg's rules presupposed that the classical equations of motion had solutions with certain periodicity properties; and the equations of motion of the two electrons of the helium atom, or of the even greater number of electrons of heavier atoms, simply do not have these properties, so that Heisenberg's rules cannot be applied to these cases.

Nevertheless, the calculation of the lowest energy level of helium, as carried out a few months ago by Kinoshita at Cornell and by Bazley at the Bureau of Standards, agrees with the experimental data within the accuracy of the observations, which is one part in ten million. Surely in this case we "got something out" of the equations that we did not put in.

As someone (apparently) involved in the construction of "a physics of complex systems", I have to wonder whether mathematics is still unreasonably effective at capturing these kind of inherent patterns in nature. Formally, the kind of mathematics that physics has historically used is equivalent to a memoryless computational machine (if there is some kind of memory, it has to be explicitly encoded into the current state); but, the algorithm is a more general form of computation that can express ideas that are significantly more complex, at least partially because it inherently utilizes history. This suggests to me that a physics of complex systems will be intimately connected to the mechanics of computation itself, and that select tools from computer science may ultimately let us express the structure and behavior of complex, e.g., social and biological, systems more effectively than the mathematics used by physics.

One difficulty in this endeavor, of course, is that the mathematics of physics is already well-developed, while the algorithms of complex systems are not. There have been some wonderful successes with algorithms already, e.g., cellular automata, but it seems to me that there's a significant amount of cultural inertia here, perhaps at least partially because there are so many more physicists than computer scientists working on complex systems.

Note: See also part 2 and part 3 of this series of posts.

posted November 23, 2006 11:31 AM in Things to Read | permalink

Comments

Posted by: Suresh at November 24, 2006 03:00 PM

Posted by: Alexandre Borovik at November 24, 2006 04:49 PM