By Hugh Aldersey-Williams
During the 17th century, as knowledge of the Universe and its contents increased, so did speculation about life on other planets. One such source, as Hugh Aldersey-Williams explores, was Dutch astronomer, mathematician, and inventor Christiaan Huygens, whose earlier work on probability paved the way for his very modern evaluation of what alien life might look like.
October 21, 2020
Portrait of Christiaan Huygens by Caspar Netscher, 1671 — Source.
When things look bleak in this world, it is perhaps natural to turn one's mind to conditions on other worlds. This is what the Dutch astronomer Christiaan Huygens did in the late 1680s. He had been ejected from his influential post as a government scientist of Louis XIV in Paris and found himself isolated back home in the provincial town of The Hague, frequently ill with depression and fevers, and missing the companionship of his brother Constantijn, who was away serving as secretary to the Dutch King William III in England.
It was then that Huygens began to write Cosmotheoros, a book-length speculation on the possibility of life on other planets, and the first such work to be based on recent scientific knowledge rather than philosophical conjecture or religious argument. Fearful of censure by “those whose Ignorance or Zeal is too great”, Huygens instructed his brother to publish the work only after his death, which he did in 1698. Originally written in Latin, Cosmotheoros was quickly translated into Dutch and other languages. A lively English translation appeared that same year under the audacious title, The Celestial Worlds Discover’d.
Philosophers had of course always thought about the existence of life beyond the Earth. Aristotle ruled it out, believing that the Earth was unique and that other celestial bodies were pure geometrical entities. But the atomists, among them Democritus and Epicurus, accepted the notion of a plurality of worlds, somewhat on the analogy of particulate matter of various kinds existing with space in between. Medieval thinkers picked up on this debate, but could only add to it their own concerns about the implications of one view or another for church doctrine, which did nothing to advance it.
The revelation that there were yet more bodies in the solar system than the ones that had been known since antiquity, which came with Galileo's discovery of four moons of Jupiter in 1610, added an unexpected new dimension to the discussion. And when Huygens discovered the first satellite of another planet, Saturn, in 1655, the balance of the argument seemed to change again.
Diagram showing the orbiting moons of Earth, Jupiter, and Saturn from Huygens's The Celestial Worlds Discover'd (1722 English edition). As Huygen modestly notes “The outermost but one, and brightest of Saturn's, it chanc'd to be my lot... The rest we may thank the industrious Cassini” — Source.
Huygens achieved fame in the 1650s for that discovery of Saturn's first satellite (later named Titan) and the planet's ring (later seen to be rings) and as the creator of the first accurate pendulum clock. He also invented numerous other devices, including a “magic lantern”, a kind of primitive slide projector, and made important contributions to mathematics, especially the fields of geometry and probability, and introduced mathematical formulas as a means of expressing the relationship between quantities such as speed and mass in physics problems. All of these achievements make him the greatest scientist in the period between Galileo and Newton.
Christiaan Huygens was precocious in his fascination with the physical world. As a child, he made little machines and delighted in solving mathematical puzzles, such that people began to refer to him as the “Dutch Archimedes”. He rejected the life of a courtier and diplomat pursued by his father and brothers, and soon distinguished himself in physics, mathematics, and astronomy. After his breakthroughs with Saturn and clocks, his experiments with moving objects led him to the conclusion that all motion is only relative (which later earned the admiration of Einstein). In the 1670s, he devised a wave-based theory of light, which was substantially correct but was neglected for nearly 150 years until it could be confirmed by experiment.
Unlike some illustrious contemporaries, he maintained a systematic focus on his chosen problems and recognised the joint importance of their practical and theoretical aspects, rejoicing when these were shown to reinforce one another, as they did in his improvements to the pendulum. Although, like any natural philosopher of the seventeenth century, he worked on a range of problems that would seem hopelessly broad to a modern specialist, he did not — as Newton did — become sidetracked into alchemy, occultism, or religion.
Huygens was a true internationalist. He sought to adapt his improved pendulum clocks with the aim of being able to calculate longitude at sea in collaboration with Scottish inventors. He swapped ideas about the air pump used to investigate the properties of the vacuum with the Irish Robert Boyle. He found himself caught in an ugly dispute with the English Robert Hooke over the invention of the balance spring to regulate the timekeeping of portable watches. He compared telescope designs and planetary observations with the Polish Johannes Hevelius and the Italian Giovanni Domenico Cassini, among others. He tutored the young German philosopher Gottfried Leibniz in mathematics (before the student surpassed the master and invented calculus).
In 1663, Huygens became the first foreigner to be elected to the Royal Society. More significantly, he was instrumental in establishing the French Academy of Sciences around the same time, making him “the recognised leader of European science”, according to a biographer.
Huygens' discovery of Saturn's ring in 1656 demanded years of patient observation of the planet using a telescope of his own design (for which Christiaan and his brother Constantijn even ground the lenses themselves). During this time, the apparent shape of the planet changed, leading to many interpretations of its form. It was Huygens' powerful optics together with his mathematically informed sense of what was physically most likely that led him to the correct interpretation.
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