AFTERWORD
Christiaan Huygens was the greatest scientist alive in Europe during the period between Galileo and Newton, whose lifespans are separated by nearly eighty years. Though fewer people may know Huygens’s name today, he was no mere interloper, and this period was no thumb-twiddling interlude, but coincided exactly and completely with what historians sometimes call the Scientific Revolution, whose new ways of understanding the world prepared the ground for the Enlightenment.
Huygens’s discoveries and inventions were central to this shift. His continually improved telescopes brought new aspects of the solar system into focus for the first time, while his unerring ability to analyse what he saw, informed by his visual sensibility and mathematical dexterity, enabled him to resolve these features correctly as the first satellite and the ring around Saturn. His lifelong work in optics led him to a substantially correct wave theory of light which, but for the baleful influence of Newton’s corpuscular model, might have advanced understanding in that field by more than a hundred years. His pendulum clocks were more accurate than their predecessors, even if he, like so many of his contemporaries, was unable in the end to solve the longitude problem. This practical project nevertheless yielded a body of theoretical work of great importance in general mechanics, including the description of centrifugal force, the analysis of the centres of oscillation, percussion and flotation, and the principle of the conservation of kinetic energy, formulated in some of the first scientific equations. Huygens’s work on moving bodies of all kinds also led him to reject the idea that motion was absolute. He realized that the concept of motion made no sense without reference to an environment in which the motion is taking place. He described this for linear motion, such as that experienced by the man in the barge passing by the man on the canal side. From here, he might have gone on to assert more daringly that there is no such thing as absolute space – in the sense of the imaginary mathematical grid imposed on the universe by Descartes and Newton – and that all frames of reference are equivalent.
Newton several times recorded his admiration for Huygens’s inventions, discoveries and investigative methods, as well as gratitude for Huygens’s contribution to his own work. Two centuries later, Albert Einstein acknowledged his debt to Huygens, and praised him for his intimation of relativity.
Huygens was not motivated by the quest for a grand universal theory. His early realization that Descartes was not a god must have disabused him of such notions. A demonstrable social need, or at least a patron’s whim, drove his work on clocks and telescopes, magic lanterns and ornamental fountains. At other times, his inventiveness was the product of a kind of leisured opportunism. He always had financial support – from Colbert, from his father’s allowance, from estate incomes – even if it was not always as liberal as he would have liked. This gave him the freedom to investigate the topics that most greatly interested him. Without such support, he might not have dived so deeply into matters such as the geometry of curves and the rules of probability.
But there is another important aspect to Huygens’s scientific contribution. Where Newton sought seclusion, Huygens sought connection. Whenever he was sequestered in The Hague, he longed for nothing so much as to be back with his friends in Paris or London. He knew that his scientific work depended on the expert knowledge of colleagues. He maintained a lively correspondence with scholars across Europe, and relied on them to be kept apprised of the latest thinking. He worked closely with others on many occasions, making refinements to optical arrangements and conducting mechanical experiments. Huygens’s telescopes drew upon what Galileo and other Italians had done, and were perfected in dialogue with Danish, German, English and Scottish astronomers. His work on the air pump confirmed and furthered the ideas of Irish and English physicists. His mathematical work was stimulated above all by his long engagement with French mathematicians. Through this informal international network, Huygens and his colleagues were more readily able to replicate their experiments and confirm their results, and to accelerate the dissemination of their discoveries to the wider world.
As the first foreign member of the Royal Society of London, and an effective founder of the French Academy of Sciences, Huygens saw the value of formalizing this dialogue within dedicated institutions. The natural philosophers who comprised the membership of these early academies incubated the modern scientific method. Informed by the civility that was the watchword of educated people in the seventeenth century, and guided by patient interlocutors such as Mersenne, Oldenburg and Fatio de Duillier, Huygens and his scientific peers learned that it was in general advantageous to talk, to write and to share and compare results with one another. For most of these scientists, most of the time, their nationality was of little consequence. Indeed, it is remarkable how little the virtually constant wars between the countries most active in science affected this intellectual exchange, even when the subjects under discussion were longitude clocks or telescopes, which might easily be regarded as military secrets. Except for the occasional practical inconvenience of a lost letter or the border seizure of some strange-looking piece of scientific equipment, scientists were generally able to carry on unmolested with their esoteric work.
Why does this story matter now?
Science depends ever more on international connections, and so do our lives. As we peer further out into space and deeper into the heart of matter, the sheer physical dimensions of the telescope arrays and particle accelerators require territory that outruns the capacity of any individual country to accommodate them. The vast numbers of scientists involved in their research teams make internationalism a necessity. This cooperation is essential, too, in more down-to-earth but no less vital projects such as minimizing the risk to global public health from infectious diseases or understanding and reversing the crisis of nature that is increasingly evident from our changing climate and from the gathering pace of extinction of plant and animal species. It requires continental or global organization to coordinate this work, and national academies have now been supplemented by international agencies to reflect this.
Networks of science have blossomed since they began with a few personal friendships and convenient collaborations in the seventeenth century. Every step of the journey has demanded patience, tolerance and willingness to reach out across the divide. Yet how easy it is, in moments of tribal wantonness, to endanger what has been painstakingly built by figures such as Huygens for the greater good of all.
And what of Huygens the man?
What must a scientist do to be remembered for himself? By any measure, his are major achievements: new laws of physics, new heavenly bodies, new devices. Yet the man fades from view.
The burials of Constantijn and Christiaan Huygens, father and son, are commemorated on a modest plaque in the east end ambulatory of Sint-Jacobskerk in The Hague; the church is usually locked to visitors. A notice placed nearby explains: ‘They never received a tombstone, and the location of their grave had already been forgotten 80 years after the funeral.’ It was only identified during the course of restoration work in 2007.
There have been fitful and belated attempts to commemorate Christiaan Huygens in other ways. Historians of science received satisfaction when his treatises, papers and correspondence were finally collected and published in a massive international editorial project lasting more than sixty years, coordinated by the Dutch Society of Sciences. The last of the twenty-two volumes of these Oeuvres Complètes, running to more than 12,000 pages in French, Latin and Dutch (and occasionally English, German or Italian), appeared as recently as 1950. Several universities in the Netherlands have a Huygens Laboratory or a Huygens Library. The European Geosciences Union awards an annual Christiaan Huygens medal. The Royal Dutch Academy of Sciences gives out a Huygens award, rotating each year among Huygens’s several fields of interest.
Older Dutch citizens will recall that Huygens’s periwigged face once gazed out from the twenty-five-guilder banknote, with Hofwijck in the background and Saturn hovering overhead. Here and there in Dutch cities is the odd Huygensstraat or Huygensweg, but it is usually the father Constantijn who is intended to be commemorated.
The most ambitious, if not the most visible, memorial to Christiaan Huygens is probably the sandstone monument that stands crumbling gently in a neglected corner of the garden of the Dutch Society of Sciences in Haarlem. The sculpture – a steeply tiered affair like a multistage rocket – is a clumsy melange of classical and Gothic styles. From a niche on one side of the lower section emerges a statue of Huygens, elaborately draped and clutching a staff. Directly above his head is a relief of a seated man demonstrating a cycloidal pendulum. Four robed figures (Archimedes and other great Ancients?) stand at each corner of this lower part, from which rises an octagonal column crowned with a circular frieze of the zodiac (an ill-chosen device to celebrate the rationalist Huygens). Above this, four winged angels clad in copper hold aloft a bronze armillary sphere. The whole work is some four metres tall.
The sculpture, unveiled in 1909 in the presence of Hendrik, Queen Wilhelmina’s prince consort, is in fact a one-third-scale maquette for a much grander monument. A few years earlier, the Society of Sciences had celebrated the completion of the first ten volumes – those containing his complete correspondence – of Huygens’s Oeuvres Complètes. During the formalities, a legacy was read out from one of the society’s recently deceased members, who had left a sum of 40,000 guilders for ‘the raising in The Hague of a statue of Christiaan Huygens, the famous Dutch physicist of the seventeenth century’. The legacy stipulated that the monument should be erected on ‘The Plaats, The Vijverberg, Lange or Korte Voorhout or near surroundings’, in other words, in the ceremonial heart of the city, close to the seat of the national government, the Binnenhof, and the spot where Johan de Witt and his brother were so bloodily slaughtered.
However, when a model of the design was temporarily put up facing the Hotel des Indes on the Lange Voorhout, the burghers of The Hague showed themselves to be less than keen on the project. Their principal objection was that the design was more suited to a graveyard than a public space. Surely, said others, a monument to such a versatile and progressive scientist as Huygens should be executed in a more modern style. It is hard to disagree with their judgement. The Hague city council voted against proceeding, and the legacy was spent instead on the acquisition of new property for the Society of Sciences. In 1999, the Society made enquiries to see if the town of Voorburg would like to take the sculpture off its hands and position it in front of the Hofwijck house, but even this desperate last bid to honour Huygens in the public arena failed when the town turned down the offer.
Compare this fate with that of his English colleague.
Perhaps nobody was ever more smitten by Newton, or by the idea of Newton, than the Parisian neoclassical architect Etienne-Louis Boullée. He summons up his god in Architecture, essai sur l’art: ‘Sublime mind! Prodigious and profound genius! Divine Being! Newton! Deign to accept the homage of my feeble talents!’ In this textbook-cum-confessional, unpublished in his lifetime, he continues like a celebrity stalker: ‘you have defined the shape of the earth; I have conceived the idea of enveloping you with your discovery’.
Boullée’s architectural projects typically displayed the highest degree of symmetry and made copious use of perfect geometric solids – sphere, drum, cone, cube – which he claimed were derived from nature and were symbolic of transcendence. His cenotaph for Newton, designed in 1784, was conceived as a massive hollow sphere 150 metres high, resting on a circular fortress-like base girded by belts of cypress trees. Taller than the pyramids or St Peter’s in Rome, it was intended to be expressive of the whole of nature and the cosmos and its fundamental indivisibility and oneness. Inside, it was to be utterly empty but for Newton’s tomb, and the vault would be pierced with holes to give an illusion of the stars and planets. Visitors (worshippers?) would be transported ‘as if by magic’ through this vast airy space to be brought ultimately into the presence of the tomb, placed at the centre of gravity. ‘I wanted to give Newton that immortal resting place, the Heavens.’
Unlike his colleague, Christopher Wren, who did in the end truly succeed in surrounding himself with his own project of St Paul’s Cathedral (Si monumentum requiris circumspice, reads his epitaph there), Newton never got the monument that his ardent admirer designed for him. In 1727 Voltaire exulted to find himself present in Westminster Abbey for the funeral of the man ‘who destroyed the Cartesian system’. Newton, he wrote, ‘was buried like a king who had done his subjects much good’. Four years later, the architect William Kent and the Flemish sculptor Jan Michiel Rijsbrack produced a more conventional, though still extravagant, memorial in grey-and-white marble, in which Newton reclines majestically on a pile of books and gestures offhandedly towards a geometric pattern drawn on a scroll held out by two putti. Behind him – Boullée would have been pleased – stands a pure pyramid supporting a globe with the female figure of Astronomy lying somewhat uncomfortably across the top. Another neoclassical statue adorns the antechapel of Newton’s Cambridge college, Trinity. Here, the French sculptor Louis-François Roubiliac presents a robed Newton standing, prism in hand, emphatically making his point.
Thereafter, as the historian of science Patricia Fara has revealed, Newton swiftly ‘passed into mythical realms’ to complete his transformation into the ‘legendary figure’ that he is still held to be today, a ‘secular saint endowed with supra-human capacities’. A man who, unlike Huygens, had not been widely known even in his own country during his lifetime, became an icon of rationality and progress, the embodiment of Enlightenment ideals and a symbol of Britannia’s intellectual spirit. The most famous poet of the day, Alexander Pope, produced a suitable (though not to the Abbey) epitaph: ‘Nature and Nature’s Laws lay hid in Night: / God said, “Let Newton be!” and all was Light.’ Rijsbrack and Roubiliac both capitalized on their commissions by knocking out replica Newton busts to adorn the salons of Britain’s country estates, available in marble, terracotta and plaster versions to suit all pockets.
Although he was slower to gain acceptance in France, which remained faithful to the scientific ideas of Descartes, Newton’s reputation there grew even more lustrous than in England. After returning from his British exile, Voltaire produced a popular gloss on Newton’s ideas, while his lover Emilie du Châtelet translated the Principia with explanatory notes, thereby creating a work that many said was superior to the original English translation (from Newton’s Latin). Later, Pierre-Simon Laplace was able to provide a mathematical explanation for certain observed anomalies to do with Newton’s law of gravitation, such as periodic irregularities in the planets’ orbits, that Newton had been happy to leave to divine arbitration. Laplace even went so far as to style himself as the ‘French Newton’. The world was ready – almost – for Boullée’s crazy hommage.
The construction of Newton as the first genius of science continued apace throughout the nineteenth century. Casts of his death mask used by the sculptors of his statues were widely traded. The cast of his skull was subjected to phrenological analysis, which, needless to say, confirmed his unique talents. ‘In the whole course of his life he never exhibited extraordinary aptitude for anything save mathematics,’ opined a correspondent in the Phrenological Journal; ‘in later days, his mind appeared to be singularly devoid of passion and pride; and he had none of that eager and restless hungering for popularity which rendered almost abortive the wonderful acuteness of his contemporary, Hooke.’ All this might have gone on even longer but for the arrival on the scene in the following century of Einstein, whose achievements in the same field as Newton at last challenged his primacy in the popular imagination.
For Newton to attain this exalted status in the first place it was necessary that others be forgotten. Newton himself did what he could to demolish the reputation of his principal rival in English physics, Robert Hooke, after the latter’s death in 1703. The cult of Newton also effectively extinguished Descartes’s once considerable reputation as a scientist, even in France. The effect was even more devastating on Huygens. He makes no appearance at all in Bill Bryson’s A Short History of Nearly Everything, William Bynum’s A Little History of Science, or Patricia Fara’s revisionist Science: A Four Thousand Year History. (Newton, of course, features abundantly in all of these works.) In The Character of Physical Law, Richard Feynman glides from Copernicus to Brahe, Kepler, Galileo and Newton without mentioning Huygens once, either in his coverage of forces or in his discussion of the relation of mathematics to physics.
The omission is not quite universal, it must be said: Huygens is adequately covered by both John Gribbin and David Wootton in their general histories, although Wootton – or his indexer – seems perplexed, and perhaps even a little revolted, by Huygens’s internationalism, and includes incredulous subentries for him: ‘French by choice’, and ‘moves country’. Nevertheless, the imbalance is clear. A search in the library catalogue at Cambridge (Newton’s university) turns up 656 results for ‘Isaac Newton’ and 53 for ‘Christiaan Huygens’. At Leiden (Huygens’s university), the situation is only partially redressed, with 612 results for Huygens and 455 for Newton.
There is no question that Huygens and Newton were effectively equals in their own time. Although Newton often resisted the opportunity to correspond directly with many of his peers, he was eager to compare notes with Huygens. It was Huygens who offered the most sustained and informed critique of Newton’s theories, placing himself as one of very few natural philosophers whose opinions Newton felt bound to take seriously. It was Huygens, thirteen years Newton’s senior, who then enjoyed the greater established reputation. And it was Huygens who, in 1666, while Newton secretly experienced his annus mirabilis, was already being wooed by Louis XIV. As Newton’s biographer Richard Westfall has observed:
The parallel between Newton and Huygens in natural philosophy is remarkable. Working within the same tradition, they saw the same problems in many cases and pursued them to similar conclusions. Beyond mechanics, there were also parallel investigations in optics. At nearly the same time and stimulated by the same book, Hooke’s Micrographia, they thought of identical methods to measure the thickness of thin coloured films. No other natural philosopher even approached their level.
And there lies the problem. In popular perception, it seems, the science of any era can only accommodate a single ‘genius’. In the generation before Newton and Huygens, it is today Galileo who shuts out Kepler, who in fact made the greater contribution to astronomy and physics. By the end of Huygens’s life, it was no longer Padua or Prague, nor even Paris, that was the major centre of science in Europe, but London, driven on by the ferment of activity of the Royal Society and England’s economic expansion. From there, despite the critical comments of continentals such as Huygens and Leibniz, the Principia ‘took Britain by storm’, provoking gushing admiration even from those who couldn’t understand it.
After that, there was then nothing to sustain widespread estimation for Huygens, even though there was still much that would later prove to be correct about his ideas to do with force and light. Huygens never spawned an adjective analogous with cartésien and Newtonian (the former coined just fifteen years after Descartes’s death, the latter while Newton was still relatively young). ‘Huygenian’ was used only in a technical context and as a last resort: Titan was the ‘Huygenian satellite’ before it was named in its own right, for example.
The Système Internationale unit of force is the newton; there is no ‘huygen’.
Huygens leaves something else behind him, though, something more in keeping with his commitment to science and suspicion of honours. This is the extraordinary Dutch contribution to astronomy. Though the Netherlands is famously flat, with big skies, it is still a surprise to find that this often cloud-covered land has produced so many notable astronomers. When yet another Dutchman appeared at his door looking for work, Harlow Shapley, the twentieth-century director of the Harvard College Observatory who calculated the size of the Milky Way, is said to have called it ‘the place where they grow tulips and astronomers for export’. Jan Oort, Willem de Sitter and Gerard Kuiper are among many who have given their names to features of the solar system and universe.
Of course, Christiaan Huygens is not directly responsible for this comparatively recent pre-eminence. In the view of Carl Sagan, who once sat with Kuiper observing the planets and speculating about extraterrestrial life much as Christiaan and Constantijn Huygens had done, it is, rather, its history as an outward-looking, exploratory nation that accounts for ‘the fact that Holland has, to this day, produced far more than its per capita share of distinguished astronomers’. But there again, Huygens’s discoveries do surely have an intoxicating appeal. As another American science writer has suggested, haloed Saturn is, for many amateurs, the ‘gateway drug’ to their astronomical habit.
On 14 January 2005, after a seven-year journey, the European Space Agency Huygens spacecraft parachuted through the thick nitrogenous atmosphere of Titan and settled onto the satellite’s oozy surface. It might have pleased Christiaan that the craft was French-built. It might have pleased him, too, that many space scientists have felt, ever since the two NASA Voyager space probes passed by Saturn and its moons in 1980 and 1981, that it might be his moon, Titan, of all the bodies in the solar system, that would reveal itself to be the most suited to the formation of biogenic molecules and perhaps even capable of supporting life.
During its two-hour descent and in the few minutes left before it expired on the surface, the probe beamed measurements of temperature, density, electrical activity and chemical composition up to the Cassini orbiter for relaying back to Earth. Visual images showed river channels, probably generated by flows of liquid methane, cutting through jagged mountains of water ice. Isotopic analysis of the carbon found on Titan indicated, however, that it was not compatible with a biological source. Huygens’s moon is far too cold to support life now, and there is no sign that it ever has done. But there is a chance that it could do so in billions of years’ time, when the sun has swollen so that its heat causes the methane to evaporate and the ice mountains to melt, forming oceans of water rich in organic compounds and minerals. Then, Titan might enter the ‘Goldilocks zone’ presently occupied by the Earth, where it is neither too hot nor too cold but just right for life. For this reason, Titan is now regarded as a potential ‘simulation’ of the very early Earth. In 2026 NASA is planning to launch a new probe, which will drop a drone into Titan’s atmosphere, able to fly from place to place to search out conceivable habitable environments.
Dozens of fly-bys of the Cassini orbiter during the months before and after the Huygens spacecraft touched down took it close to many of Saturn’s sixty-two major moons and even through some of the gaps between the planet’s rings. Data sent back confirmed that the rings are composed chiefly of ice, in pieces varying in size between snowflakes and bergs, spread in a layer just ten metres thick. The orbiter also resolved a mystery as to why the rings were bright enough for Christiaan Huygens to discern them with his primitive telescope at all. The quantities of space dust detected during the mission suggested that the rings should be dirty and much less reflective. The fact that they are bright suggests that they are much younger than the planet itself, perhaps as little as ten million years old. It was found, too, that the rings are being drawn slowly into the planet itself and may be gone again in as little as a hundred million years.
When it flew past Titan, Cassini revealed lakes of methane near the pole the size of some of the inland seas on Earth, which confirmed that the rugged terrain of the satellite was due to methane rivers that once flowed there. Cassini also flew close by the smaller, ice-covered moon Enceladus on several occasions. Here, it found geysers of ice particles thrust up high into the atmosphere from underground oceans of salty water. This water was found to contain silica and organic molecules, strongly suggesting that it was issuing from thermal vents under the moon’s seas, and that the core of the moon was therefore hot. Such warming is not achievable so far from the sun by solar radiation; on Titan and Saturn, surface temperatures are a chilly −180° Celsius. Instead, it is thought that Enceladus is heated by friction generated by the constant movement of its solid core produced by the fluctuating gravitational field of its massive parent planet, much as a squash ball becomes warm in the hand with repeated squeezing. By chance, this vigorous massage has raised temperatures to levels conducive to the emergence of life.
In a touching remark at the beginning of Cosmotheoros concerning the mysteries of the solar system, directed at the beloved older brother whom he had asked to see the work into print after his death, Christiaan lamented of their days spent at the telescope together: ‘we were always apt to conclude, that ’twas in vain to enquire after what Nature had been pleased to do there, seeing there was no likelihood of ever coming to the end of the Enquiry’. It would surely have delighted Huygens more than any stony memorial to know that a space probe bearing his name is now proving him wrong.