Friday, October 8, 2010

Isaac Newton, Biological Physicist?

Arguably the greatest physicist of all time (and probably the greatest scientist of all time) is Isaac Newton (1643–1727). Newton is so famous that the English put him on their one pound note (although I gather nowadays they use a coin instead of paper currency for one pound). Given Newton’s influence, it is fair to ask what his role is in the 4th edition of Intermediate Physics for Medicine and Biology. One way Newton (along with Leibniz) contributes to nearly every page of our book is through the invention of calculus (or, as I prefer, “the calculus”). Russ Hobbie states in the preface of our book that “calculus is used without apology.”

When I search the book for Newton’s name, I find quite a few references to Newton’s laws of motion, and in particular the second law, F = ma. Newton presented his three laws in his masterpiece, the Principia (1687). (Few people have read the Principia, including me, but a good place to learn about it is the book Newton’s Principia for the Common Reader by Subrahmanyan Chandrasekhar) Of course, the unit of force is the newton, so his name pops up often in that context. The only place where we talk about Newton the man is very briefly in the context of light.
A controversy over the nature of light existed for centuries. In the seventeenth century, Sir Isaac Newton explained many properties of light with a particle model. In the early nineteenth century, Thomas Young performed some interference experiments that could be explained only by assuming that light is a wave. By the end of the nineteenth century, nearly all known properties of light, including many of its interactions with matter, could be explained by assuming that light consists of an electromagnetic wave.
Newton’s name also arises when talking about Newtonian fluids (Chapter 1): a fluid in which the shear stress is proportional to the velocity gradient. Not all fluids are Newtonian, with blood being one example. Newton appears again when discussing Newton’s law of cooling (Chapter 3, Problem 45).

Some of Newton’s greatest discoveries are not addressed in our book. For instance, Newton’s universal law of gravity is never mentioned. Except for a few intrepid astronauts, animals live at the surface of the earth where gravity is simply a constant downward force and Newton’s inverse square law is not relevant. I suppose tides influence animals and plants that live near the ocean shore, and the behavior of tides is a classic application of Newtonian gravity, but we never discuss tides in our book. (By the way, harkening back to my vacation in France last summer, the tides at Mont Saint Michel are fascinating to watch. I really must plan a trip to the Bay of Fundy next.) Newton, in his book Optiks, made important contributions to our understanding of color, but Russ and I introduce that subject without referring to him. We don’t discuss telescopes in our book, and thus miss a chance to honor Newton for his invention of the reflecting telescope.

Never at Rest: A Biography of Isaac Newton, by Richard Westfall, superimposed on Intermediate Physics for Medicine and Biology.
Never at Rest:
A Biography of Isaac Newton,
by Richard Westfall.
A wonderful biography of Newton is Never at Rest, by Richard Westfall. I must admit, Newton is a strange man. His argument with Leibniz about the invention of calculus is perhaps the classic example of an ugly priority dispute. He does not seem to be particularly kind or generous, despite his undeniable genius.

Was Newton a biological physicist? Well, that may be a stretch, but Colin Pennycuick has written a book titled Newton Rules Biology, so we cannot deny his influence. I would say that Newton’s contributions are so widespread and fundamental that they play an important role in all subfields of physics.