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Sir Isaac Newton: charting the course of modern thought. With the 300th anniversary of the publication of the `Principia,' Newton's genius is being appreciated anew. In this watershed work, he set up the rigorous experimentation that now characterizes physical science.

THE next time you're flying, you might give a thought to Sir Isaac Newton. Engineers need modern quantum theory to design the aircraft's electronics. But Newton's classical mechanics still accounts for the jet engines' thrust, the aerodynamic forces that hold the craft up, and gravity's tug, which tries to pull it down. Furthermore, were your plane forced down in the wilderness or at sea, the satellite that would relay its search and rescue signal travels on an orbit that Newton showed how to calculate 300 years ago. Many people are giving thought to Newton this year in conferences, lectures, and other events that honor this anniversary. His great work - ``Mathematical Principles of Natural Philosophy,'' or ``Philosophiae Naturalis Principia Mathematica,'' to use the proper Latin title - first appeared on July 5, 1687. It's considered one of the brightest beacons in the history of thought.

The ``Principia'' elucidated the action of natural forces and showed how to calculate their effects. It laid out the discovery of gravity as a universal force and showed in precise detail how it guides planets and comets in their courses and raises tides in Earth's seas.

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Natural philosophers, as 17th-century scientists called themselves, had been groping vaguely to do these things. Newton gave new direction to their work. In the ``Principia,'' he established the rigorous strategy that now characterizes physical science. This strategy probes nature in ways guided by theories that are expressed mathematically. It judges the value of theory by the degree to which its calculated results agree with observations and experiments and suggest new phenomena to explore. As science historian Richard S. Westfall of Indiana University observes, with publication of the ``Principia,'' ``the universe of precision had replaced the world of `more or less.'''

In short, Newton opened up one of the main avenues of modern scientific research and showed how to follow it. No other figure in the history of science has done as much except Charles Darwin, whose 19th-century work with evolution established an equally rigorous strategy for the scientific study of the development of organic life. This is why even the greatest scientists of today hold Newton in awe.

Einstein called him ``this brilliant genius, who determined the course of Western thought, research, and practice like no one else before or since.'' He added that ``the figure of Newton has, however, an even greater importance than his genius warrants, because his destiny placed him at a turning point in the history of the human intellect.''

Subrahmanyan Chandrasekhar, a mathematical physicist and Nobel Prize-winner at the University of Chicago, has been studying some of the mathematical proofs in the ``Principia.'' In a recent lecture, he proved some of Newton's theorems using modern mathematics. In each case, Newton's original proof, which usually used geometry, was shorter, simpler, and more elegant than that using modern mathematics. Dr. Chandrasekhar said he felt like a schoolboy sitting at the feet of the master.

Even the master made mistakes, however, as Robert Garisto, a student who graduated this year from the University of Chicago, discovered. Astronomer-historian Noel Swerdlow had been unable to make Newton's numbers agree with one another in Proposition Eight of Book III of the ``Principia,'' where Newton calculated the mass and density of planets. Earth's mass, for example, is 15 percent greater than can be accounted for by Newton's data. Professor Swerdlow suggested Mr. Garisto check it out.

Tracing the relevant material through all available editions of the ``Principia,'' Garisto found that the error sprang from repeated changes in one of the data items - a measure of the Earth-sun distance. Newton used the latest value available. But he failed to revise his calculation of Earth's mass to reflect that new value in the final edition of his work. ``What's really interesting is not that Newton made a small mistake, but that nobody found and explained this [particular] error before,'' Garisto observed.

HE found four other related inconsistencies among ``Principia'' editions. But Garisto notes that some apparent errors turned out to be approximations Newton made to simplify calculations.

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Newton also did not hesitate to bend data and fudge calculations to get the results he wanted, as Richard Westfall has shown. Newton's central purpose was to explain the operation of the universe in terms of the action of natural forces whose effects could be calculated precisely.

Strange as it may seem to us today, many of Newton's contemporaries considered the term natural force an oxymoron. By definition, any action that was forced couldn't be natural to such thinkers. They particularly objected to Newton's concept of gravitational force, which arises in an unknown manner from material mass and acts over large distances by an unexplained mechanism. They accused Newton of introducing an occult factor. So Newton felt compelled to show that his theory could predict the action of such forces with high precision.

But sometimes the precision reached is far higher than the original data justify. In the case of Newton's manipulations to make his calculation of the speed of sound agree with the measured speed, Professor Westfall accuses the master of ``nothing short of deliberate fraud.''

As Westfall explains, ``Newton consolidated and confirmed the quantitative character of modern science.'' But, he adds: ``Successful polemics [arguments] are the necessary condition of every intellectual revolution. ... Newton comprehended perfectly the nature of the polemic he deployed.'' Thus, in Westfall's judgment, among the reasons for the success of the ``Principia'' was ``the fudge factor, manipulated with unparalleled skill by the unsmiling Newton.''

To keep this in perspective, Westfall notes that, in spite of Newton's peccadilloes, publication of the ``Principia'' was ``one of the major events of Western history.'' Indeed, Newton's work transcended science to permeate Western thought in general.

In the colorful phrase of Gale E. Christianson, an Indiana State University historian, ``Newton `democratized' the universe.'' He permanently laid to rest the concept of hierarchical dominance among stars, sun, moon, and planets - a concept that had its counterpart in social, political, and ecclesiastical hierarchies.

Nick Herbert, a consulting physicist and philosopher of science, observes, ``Coincident with the rise of Newtonian physics was the ascent of the modern democracy, which stresses a `rule of laws rather than of men' and which posits a theoretical equality between parts of the social machinery.'' He continues, ``The egalitarian mechanism that Newton discovered in the heavens has insinuated itself into every aspect of ordinary life.''

NEWTON'S eminence in Western culture was recognized in the East as well. When Japan began to make contact with the West in the middle of the last century, the artist Hosai picked Newton as a symbol of Western science. The scene shows the master scientist sitting in a garden watching an apple fall from a tree - an event Newton claimed inspired his concept of gravity as a universal force. The Japanese legend reads: ``Isaac Newton, very great head of school, but not pompous.'' It's a characterization that Newton's contemporaries would have recognized, although they also knew he could be vindictively small-minded in attacking challengers, such as Robert Hooke, who claimed Newton stole their ideas.

In its day, the ``Principia'' made clear so much that had been perplexing that many scholars came to share the enthusiasm of its publisher, the astronomer Edmund Halley, who exclaimed in a preface: ``nearer to the gods no mortal may approach.'' But the work also left its readers with an enduring puzzle - the true nature of gravity. Newton had no idea what gravity's cause might be and refused to speculate about it. ``It is enough that gravity does really exist, and ... abundantly serves to account for all the motions of the celestial bodies, and of our sea,'' Newton wrote.

Scientists are just as mystified three centuries later. Einstein's relativity extends Newton's system to deal with phenomena where objects are moving very fast and where gravity is very strong - far stronger than in the vicinity of Earth. But it says nothing about why material mass causes the effect we call gravity.

While Newton would ``frame no hypotheses'' about gravity's true nature, to use his famous phrase, he did speculate about other mysteries. His ``query'' in the second edition of his treatise on optics about possible electrical forces that bind together ``the small particles of bodies'' and mediate their interaction with light seems prophetic. When that edition appeared in 1717, the only electricity that experimenters knew was the static charge generated by rubbing such substances as amber or glass. And no one knew anything about the nature of atoms. It would be another century before physicists would try to explain atomic action in terms of electrical force.

Newton also sighted a chimera that scientists would chase until the end of the 19th century when he wondered if an all-pervading ether might carry gravity's force. It's fortunate that he refrained from publishing this speculation in the ``Principia.'' Scientists would eventually realize that the concept of an all-pervading ether is scientifically worthless, because the ether would be a body standing still in space. It would provide an absolute frame of reference.

According to Einstein's well-tested relativity theory, such a concept can never lead to correct physical laws. Newton, with his 17th-century convictions about the primacy of absolute time and space, would never have suspected such a thing.

Newton's three laws of motion Law I: Every body continues in its state of rest, or uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it. Law II: The change of motion is proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed. Law III: To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

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