From: Mendham at AOL.COM Date sent: Sat, 20 Mar 1999 23:19:13 EST To: johnny at charm.net Subject: more Newton conference gossip Send reply to: johnny at charm.net This morning in FSK auditorium we heard from alumnus Bill Donahue on Newton's optics. Mr. Donahue is now a partner, with Dana Densmore, in the Green Lion Press in Santa Fe. They publish new editions of old scientific texts. He said he was speaking 'just as a Johnny' and not as a Newton expert. And he noted that Newton was known as an inventor, having dreamed up the Newtonian telescope, universal gravitation, the scientific journal article and the cat door. He showed a video of Newton's double prism experiment, a video made by Donahue and tutor Howard Fisher. It reproduced Newton's demonstration that light of different colors is refracted at different angles. But he said Newton was careful to avoid offering hypotheses about the nature of light, limiting himself to mathematical definition of the ancient law of refraction. In fact his double prism experiment tended to refute one of the current, vague stories about light: that it was somehow changed by the prism, which is why it became divided into colors and refracted at different angles. His own failure to offer a "story" about light was criticized by his contemporaries (Hook, Huygens, etc.) Donahue wonders, however, if Newton's use of the term "ray" to describe light doesn't smuggle in an element of hypothesis despite his effort to stay neutral about causes. "It's hard to get a conversation going between Newton and Aristotle because they're not speaking the same language," Donahue said. "But Newton knew that language is important; that without the 'baggage' of language -- a shared understanding - you can't experiment." Donahue contrasts Newton's approach with Descartes': "Cartesian science is done by finding plausible explanations based as much as possible on mechanical principles. But Newton didn't think he was evaluating hypotheses. Newton hated hypotheses so much he said he'd rather have his ideas dismissed as vain and empty than treated as experimental hypotheses." Newton sought laws, rather than theories, derived from concrete measurements and rendered general by induction. Later in the morning, Curtis Wilson literally raised the curtain on two elaborately engineered pendula set up on stage and demonstrated the double pendulum experiment. This reproduced Newton's showing -- to one part in a thousand -- that weight is proportional to mass. (Newton was the first to distinguish the two.) Because the lights were out for much of the talk I couldn't take notes on much of it. He did remark that in 1972 this relationship was experimentally confirmed to one part in 10 million million, as if to remind us that Newton's disciples are still at it. His description of the pendulum method offered a good look at the practical difficulties of making precise measurements with 17th century equipment. In the afternoon we heard a lecture by George Smith, a professor at Tufts and a practicing engineer specializing in aerodynamically induced metal fatigue in jet turbines. Smith proposed that "Newton did not discover universal gravity in a flash of insight." Rather, Smith thinks he can unravel a trail of "theses" that led to the Principia. He presented 10 theses. Five, he says, (if I understood him correctly) can be found discussed in the European literature of mechanics in the century before the Principia was written. The last five he thinks Newton came to, one by one, in the years just before the Principia pulled them all together. The ten theses are: 1. Orbiting bodies are retained in orbit, rather than moving forward in a straight line, by forces directed toward central bodies. 2. These forces, and hence the resulting "centripetal" accelerations, vary inversely with the square of the distance from the central body. 3. These forces act not only on the principal bodies orbiting the central bodies, but on other bodies as well. 4. In the case of the moon, the force in question is simply terrestrial gravity. 5. In all celestial cases, the force in question is one in kind with terrestrial gravity. 6. There is a force of this same kind on the central body directed toward each body orbiting it, so that the two bodies -- e.g. the sun and Jupiter -- interact. 7. There are mutual forces of this kind between all celestial bodies -- eg between Jupiter and Saturn, as well as between each of these and the sun. 8. the forces in question vary in accord with the law of gravity -- ie the "motive" force on a body directed toward another body is proportional to the product of the masses and inversely proportional to the square of the distance between them. 9. The force of gravity is universal -- i.e. the law of gravity holds between any two particles of matter in the universe. 10. The force of gravity is one of the fundamental forces of nature -- i.e. it is not composed out of forces of other known kinds. None of these, says Smith, is an hypothesis. Nevertheless, Smith believes Newton produced a system that transcended the "mere kinematics" of Galileo and Huygens and offered a deeper understanding of nature. Smith speculated about why Newton published the Principia, given the loose ends of which Newton "was surely aware" and given his chronic refusals to publish earlier work. He believes Newton was compelled by the "scope and majesty" of his reasoning by 1686 to make the Principia public. Later in the afternoon William Harper of the University of Western Ontario proposed that it was Newton's enormous success at matching theory with observation that gave his anti-hypothetical approach its great force and legitimacy. The growing ability of astronomers to make precise measurements made his results all the stronger. Harper presented many examples, particularly the close agreement of Newton's laws with the motions of Jupiter's moons. the moons, Harper said, displayed with great precision the harmonic law and the law of areas. Interestingly, last night's lecturer, Professor de Gandt, objected to Harper's notion that the orbiting of Jupiter's moons was a well-observed phenomenon. He pointed out that an earth-bound observer sees the moons as nothing but specks oscillating back and forth at different rates, since they lie in the same plane as earth. How could one know that their orbits were actually round, let alone whether they obeyed the "same area" law? "the orbital behavior of the moons are not phenomena, they are conjectures," he said. De Grandt's objection started a debate, with Harper insisting that the eclipses of the moons could provide good confirming observations. It seemed to me the question was never resolved, perhaps because I'd had the same objection. Let me warn again that this isn't an effort to recapitulate everything covered in hours of talks -- and I'm sure there are mistranslations! I'll try to make some notes on Ms. Densmore tomorrow. Tom Geyer