Flowery Regions of Algebra

Simon Schaffer

  • Pierre Simon Laplace 1749-1827: A Determined Scientist by Roger Hahn
    Harvard, 310 pp, £21.95, November 2005, ISBN 0 674 01892 3

There were fears of revolutionary violence in Paris in the spring of 1773. The police tried to quell the disturbances and make those responsible account for their actions, but they had no success. The trouble spread, first downriver to Normandy, then elsewhere. Statements in the newspapers designed to assuage fears and explain the source of the trouble had little effect, and the crisis lasted well into the summer. The cause of all the bother was some arcane speculation on the chances of comets crashing into the Earth. Journalists grumbled that future Frenchmen would be amused by the tale of ignorant citizens disordered by a mere astronomical calculation. Voltaire joined in, mocking Parisians who’d fled the city to escape the cometary apocalypse: they ‘aren’t philosophers and, if you believe what they believe, don’t have enough time left to turn into philosophers’. The Académie Royale des Sciences had inadvertently started the panic by announcing a talk on comet collisions and their possible hazards. To restore calm, a committee was organised, the evidence reviewed, and an announcement put out to the effect that the chance of a crash was 1:64,000. The sums did not soothe Parisian nerves. ‘The vulgar, ignorant and timid, having no other reason to reassure themselves about rather unusual phenomena but the example and authority of enlightened people, become alarmed very easily,’ the committee concluded.

When foreigners grope for some way of making sense of what they see as the extreme rationalism of the French, they use the word ‘Cartesian’. But they ought rather to say ‘Laplacian’. Pierre Simon Laplace was born into a prosperous rural family in 1749; he trained for the priesthood; and in 1773 was elected to the Académie. He had recently composed a series of astonishingly assured calculations about the application of probability calculus to the orbits of the planets and the distribution of comets in space and he was chosen to lead the Académie committee on comets’ orbits. More than most other Frenchmen Laplace helped define the ways in which precisely calculable laws seemingly govern both nature and society. He was interior minister under the Consulate, senator, head of the Parisian scientific community, and survived the Bourbon restoration as a newly ennobled marquis and grand old man of the establishment. Roger Hahn has been studying this career for half a century. He has located letters and papers thought to be lost, written on the tough problems of Laplace’s religious beliefs and his relation with Newtonian cosmology, and at last written a new biography, first released under the title Le Système du monde in France a couple of years ago.

Though Laplace’s life spanned Enlightenment, Revolution, Empire and Restoration, it was rarely disturbed and seems to have been devoid of much private crisis. An exception was the public embarrassment the family suffered a couple of years after the comet scare, when Laplace’s father was sued for supplying Carmelite nuns in Dieppe with adulterated cider. This was happily resolved when Laplace’s ally, the chemist Antoine Lavoisier, joined a commission of inquiry into the affair. Hahn gives much detail of the episode, and indicates how academic inquiries increasingly governed public affairs at the end of the Ancien Régime.

Hahn’s aim here is to give the stern mechanisms of Laplacian science a human face. The challenge is considerable, as he acknowledges: ‘He was a consistently rational and deliberate man, almost as steady in his behaviour as in his vision of the stable solar system.’ The vision included a grandiose account of the origins of the sun and the planets, tracing the current order back to its primitive vortex. Hahn does the same with Laplace: his opening chapters dwell at length on the Norman upbringing, the initial training for the priesthood, the fierce doctrinal controversies between his professors at the university of Caen. Even Laplace’s deliberation can somehow be found in his Norman roots, or so it’s claimed here: a ‘stereotypical Norman villager’, whether cider grower or devout student, was ‘openly cautious, but inwardly stubborn’. His decisive move to Paris in 1769, when he set himself on the path to the Académie and mathematical sciences, looks like a radical change of heart, a move from provincial piety to urban reason.

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