* This exploratory tradition may account for the fact that Holland has, to this day, produced far more than its per capita share of distinguished astronomers, among them Gerard Peter Kuiper, who in the 1940’s and 1950’s was the world’s only full-time planetary astrophysicist. The subject was then considered by most professional astronomers to be at least slightly disreputable, tainted with Lowellian excesses. I am grateful to have been Kuiper’s student.
In Italy, Galileo had announced other worlds, and Giordano Bruno had speculated on other lifeforms. For this they had been made to suffer brutally. But in Holland, the astronomer Christiaan Huygens, who believed in both, was showered with honors. His father was Constantijn Huygens, a master diplomat of the age, a litterateur, poet, composer, musician, close friend and translator of the English poet John Donne, and the head of an archetypical great family. Constantijn admired the painter Rubens, and ‘discovered’ a young artist named Rembrandt van Rijn, in several of whose works he subsequently appears. After their first meeting, Descartes wrote of him: ‘I could not believe that a single mind could occupy itself with so many things, and equip itself so well in all of them.’ The Huygens home was filled with goods from all over the world. Distinguished thinkers from other nations were frequent guests. Growing up in this environment, the young Christiaan Huygens became simultaneously adept in languages, drawing, law, science, engineering, mathematics and music. His interests and allegiances were broad. ‘The world is my country,’ he said, ‘science my religion.’
Light was a motif of the age: the symbolic enlightenment of freedom of thought and religion, of geographical discovery; the light that permeated the paintings of the time, particularly the exquisite work of Vermeer; and light as an object of scientific inquiry, as in Snell’s study of refraction, Leeuwenhoek’s invention of the microscope and Huygens’ own wave theory of light.* These were all connected activities, and their practitioners mingled freely. Vermeer’s interiors are characteristically filled with nautical artifacts and wall maps. Microscopes were drawing-room curiosities. Leeuwenhoek was the executor of Vermeer’s estate and a frequent visitor at the Huygens home in Hofwijck.
* Isaac Newton admired Christiaan Huygens and thought him ‘the most elegant mathematician’ of their time, and the truest follower of the mathematical tradition of the ancient Greeks – then, as now, a great compliment. Newton believed, in part because shadows had sharp edges, that light behaved as if it were a stream of tiny particles. He thought that red light was composed of the largest particles and violet the smallest. Huygens argued that instead light behaved as if it were a wave propagating in a vacuum, as an ocean wave does in the sea – which is why we talk about the wavelength and frequency of light. Many properties of light, including diffraction, are naturally explained by the wave theory, and in subsequent years Huygens’ view carried the day. But in 1905, Einstein showed that the particle theory of light could explain the photoelectric effect, the ejection of electrons from a metal upon exposure to a beam of light. Modern quantum mechanics combines both ideas, and it is customary today to think of light as behaving in some circumstances as a beam of particles and in others as a wave. This wave-particle dualism may not correspond readily to our common-sense notions, but it is in excellent accord with what experiments have shown light really does. There is something mysterious and stirring in this marriage of opposites, and it is fitting that Newton and Huygens, bachelors both, were the parents of our modern understanding of the nature of light.
Leeuwenhoek’s microscope evolved from the magnifying glasses employed by drapers to examine the quality of cloth. With it he discovered a universe in a drop of water: the microbes, which he described as ‘animalcules’ and thought ‘cute’. Huygens had contributed to the design of the first microscopes and himself made many discoveries with them. Leeuwenhoek and Huygens were among the first people ever to see human sperm cells, a prerequisite for understanding human reproduction. To explain how microorganisms slowly develop in water previously sterilized by boiling, Huygens proposed that they were small enough to float through the air and reproduced on alighting in water. Thus he established an alternative to spontaneous generation – the notion that life could arise, in fermenting grape juice or rotting meat, entirely independent of preexisting life. It was not until the time of Louis Pasteur, two centuries later, that Huygens’ speculation was proved correct. The Viking search for life on Mars can be traced in more ways than one back to Leeuwenhoek and Huygens. They are also the grandfathers of the germ theory of disease, and therefore of much of modern medicine. But they had no practical motives in mind. They were merely tinkering in a technological society.
The microscope and telescope, both developed in early seventeenth-century Holland, represent an extension of human vision to the realms of the very small and the very large. Our observations of atoms and galaxies were launched in this time and place. Christiaan Huygens loved to grind and polish lenses for astronomical telescopes and constructed one five meters long. His discoveries with the telescope would by themselves have ensured his place in the history of human accomplishment. In the footsteps of Eratosthenes, he was the first person to measure the size of another planet. He was also the first to speculate that Venus is completely covered with clouds; the first to draw a surface feature on the planet Mars (a vast dark windswept slope called Syrtis Major); and by observing the appearance and disappearance of such features as the planet rotated, the first to determine that the Martian day was, like ours, roughly twenty-four hours long. He was the first to recognize that Saturn was surrounded by a system of rings which nowhere touches the planet.* And he was the discoverer of Titan, the largest moon of Saturn and, as we now know, the largest moon in the solar system – a world of extraordinary interest and promise. Most of these discoveries he made in his twenties. He also thought astrology was nonsense.
* Galileo discovered the rings, but had no idea what to make of them. Through his early astronomical telescope, they seemed to be two projections symmetrically attached to Saturn, resembling, he said in some bafflement, ears.
Huygens did much more. A key problem for marine navigation in this age was the determination of longitude. Latitude could easily be determined by the stars – the farther south you were, the more southern constellations you could see. But longitude required precise timekeeping. An accurate shipboard clock would tell the time in your home port; the rising and setting of the Sun and stars would specify the local shipboard time; and the difference between the two would yield your longitude. Huygens invented the pendulum clock (its principle had been discovered earlier by Galileo), which was then employed, although not fully successfully, to calculate position in the midst of the great ocean. His efforts introduced an unprecedented accuracy in astronomical and other scientific observations and stimulated further advances in nautical clocks. He invented the spiral balance spring still used in some watches today; made fundamental contributions to mechanics – e.g., the calculation of centrifugal force – and, from a study of the game of dice, to the theory of probability. He improved the air pump, which was later to revolutionize the mining industry, and the ‘magic lantern,’ the ancestor of the slide projector. He also invented something called the ‘gunpowder engine,’ which influenced the development of another machine, the steam engine.
Huygens was delighted that the Copernican view of the Earth as a planet in motion around the Sun was widely accepted even by the ordinary people in Holland. Indeed, he said, Copernicus was acknowledged by all astronomers except those who ‘were a bit slow-witted or under the superstitions imposed by merely human authority.’ In the Middle Ages, Christian philosophers were fond of arguing that, since the heavens circle the Earth once every day, they can hardly be infinite in extent; and therefore an infinite number of worlds, or even a large number of them (or even one other of them), is impossible. The discovery that the Earth is turning rather than the sky moving had important implications for the uniqueness of the Earth and the possibility of life elsewhere. Copernicus held that not just the solar system but the entire universe was heliocentric, and Kepler denied that the stars have planetary systems. The first person to make explicit the idea of a large – indeed, an infinite – number of other worlds in orbit about other suns seems to have been Giordano Bruno. But others thought that the plurality of worlds followed immediately from the ideas of Copernicus and Kepler and found themselves aghast. In the early seventeenth century, Robert Merton contended that the heliocentric hypothesis implied a multitude of other planetary systems, and that this was an argument of the sort called reductio ad absurdum (Appendix 1), demonstrating the error of the initial assumption. He wrote, in an argument which may once have seemed withering,