The Magazine

Heaven and Earth

Their conflict and harmony in the life of Galileo.

Jan 31, 2011, Vol. 16, No. 19 • By DAVID GUASPARI
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Heilbron’s Galileo is an artist. His genius as an observer lay in the ability to interpret what he saw, and it was skill in drawing and perspective and chiaroscuro that helped him recognize changing contrasts visible on the surface of the moon as plays of light and shadow on a mountainous landscape. His first known public lecture discussed the geometry of Dante’s hell. He loved Ariosto’s Orlando Furioso and his critical essay on Ariosto and Tasso is still read. Heilbron finds significance in what Galileo criticizes about Tasso (an alleged poverty of invention) and admires in Ariosto (“the realistic treatment of the marvelous”). He suggests that Galileo could not respond to Tasso’s psychological depth, preferring to judge fictional characters (as he judged real-life friends and opponents) in black and white terms​—​and that this preference for surfaces is one psychological root of his physics of “accidents” rather than “essences,” of laws rather than causes.

Heilbron traces the evolution of Galileo’s early scientific views from notes and drafts for an abandoned work on motion. These ridicule many Aristotelian propositions but argue primarily within a framework of Aristotelian (or Aristotle-friendly) categories such as natural motion. Heilbron imagines a dialogue between Galileo and an alter ego to illustrate how Galileo might have arrived at his radically different physics, rejecting not only Aristotle’s answers but also his questions. Galileo developed not a dynamics that explains what causes motion but a quantitative kinematics that relates different measurable properties of motion: time, distance, speed.

 A central concern is what we would now call free fall. He produces different deductions that begin from different (and incompatible) hypotheses, but conclude with the same law relating time and distance of fall. From this, according to Heilbron’s dialogue, he draws a moral: Mathematical exploration shows its power by leading to a precisely formulated kinematic law; that law is verified by experiment; and the fact that it could be deduced from different hypotheses, corresponding to different underlying causal mechanisms, shows that consideration of causes can be deferred. The role of experiment is merely illustrative, “conclusive when it confirms a quantitative rule, but only an inconvenience when it does not.”  Laws describe ideal, artificial, situations. Disagreement with experiment may simply show that ideal conditions are too difficult to reproduce.

I am grateful that Heilbron lays out these arguments in detail but found some of them difficult to follow. Pulling rank as a professional mathematician, I’d say that he sometimes gives us the words without the music, allowing those details to obscure essentials. (An unsound argument of Galileo’s is sometimes identified as such only in an endnote.) Wootton says his account is novel in emphasizing Galileo’s “reluctant empiricism,” “early Copernicanism,” and “private irreligion.” The reluctance, also illustrated in Heilbron’s dialogue, comes from a deep-seated preference for deductive reasoning over empirical tests. Wootton argues that the never‑resolved tension between that gut feeling and Galileo’s wish to ground knowledge in sensory experience was a fruitful one.

Galileo’s Copernican views were formed not merely early in his life but well before the empirical evidence was compelling, when all available astronomical data were consistent with Tycho Brahe’s geo‑heliocentric model: The moon and sun circle Earth, and the planets circle the sun. Tycho’s system had the scientific advantage that a stationary Earth did not require a radically new physics to explain everyday motion on Earth and the political advantage of being theologically orthodox. Yet something other than science must account for Galileo’s adamant insistence that Copernicus provided not merely the best astronomical model but the truth. Here, says Wootton, his science and irreligion were mutually supportive, each a rejection of the view “that the world was made for man, and that man was made in the image of God.” (He finds the psychological roots of that rejection in Galileo’s relations with his Gorgon-like mother.)

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