by Marcus Chown
Le Verrier took the location of the new planet to the director of the Paris Observatory, but François Arago made it clear that he did not think the search for a new planet was a priority. He had good reason. For a start, national observatories such as his in Paris existed principally to make charts of the locations of planets and stars for the purposes of navigation. This involved many people carrying out lengthy and painstaking observations, and Arago understandably did not want to use up their valuable time on a wild goose chase for a planet whose existence seemed to him to be the remotest of long shots. It probably did not help that Le Verrier was a man with a reputation for being arrogant and difficult to deal with.
Capricorn and Aquarius would not be visible from the Northern Hemisphere much later than November, so it was imperative that any search for the new planet begin soon. For a while Le Verrier was patient, but when a start date from Arago was not forthcoming, his frustration grew. As it happened, he had already started trying other avenues and had sent a paper containing his predictions to the editor of the German journal Astronomische Nachrichten. In his accompanying letter to Heinrich Schumacher, he vented his frustration at not being able to get French astronomers to look for his planet. Schumacher was sympathetic and wrote back with a suggestion: why did Le Verrier not contact other astronomers with powerful telescopes? The two that came immediately to his mind were Friedrich Struve in Germany and Lord Rosse at Birr in Ireland, whose ‘Leviathan’, with its 72-inch mirror, was the biggest telescope in the world. Le Verrier would probably have contacted both of them had Schumacher’s suggestion not reminded him of a letter he had received the previous year from a young astronomer at the Berlin Observatory.
The appeal of Johann Galle was that he was a low-ranking assistant astronomer. Le Verrier expected that Johann Encke, director of the Berlin Observatory, would be as reluctant to search for a new planet as his Parisian counterpart, but Galle might be hungry to make his name. Le Verrier might have more success, he reasoned, by bypassing Encke and contacting the younger astronomer directly. Would Galle take him seriously, or would Le Verrier be disappointed yet again? There was only one way to find out.
The only problem was that the French astronomer had ignored a letter from Galle a year earlier, together with a thesis which had been included with it. This was embarrassing now that he needed a favour from him. However, a bit of flattery might get around that, so before he made his request that Galle embark on a search for his planet, Le Verrier penned some pointed and belated praise, congratulating Galle on the ‘perfect clarity’ and ‘complete rigour’ of his thesis. Then, on 18 September 1846, he sent his letter, which contained a rough estimate of the location of the new planet, to Berlin.
Berlin, 24 September 1846
As the clock ticked towards dawn, three men were gathered at the Fraunhofer telescope in the dome of the Berlin Observatory. D’Arrest, who had run all the way to Encke’s home, had returned with the observatory director, who was a little unsteady on his feet after his birthday celebrations. The trio, struggling to stay calm, took turns at the eyepiece until they were absolutely sure. The object seen by Galle and d’Arrest was definitely not on the star map. And the reason was patently clear: it was not a star. Stars, because of their distance from Earth, appear as pinpricks of light no matter how powerful the magnification of a telescope. But this object was no dimensionless pinprick. It was a tiny shimmering disc. They had found it! They had found Le Verrier’s planet!
Galle could scarcely believe the events of the last half-day. He had knifed open what looked like a perfectly ordinary letter from France, not for an instant suspecting that it would change his life forever. He recognised Le Verrier’s name immediately and could easily have exacted his revenge on the Frenchman for ignoring him by losing his letter among the papers on his desk. But the favour Le Verrier had requested piqued his interest.
The letter contained a prediction of the existence and location of a new planet. Galle knew that such a prediction was ridiculous, yet something caused him not to dismiss it out of hand. ‘I would like to find a persistent observer’, wrote Le Verrier, ‘who would be willing to devote some time to an examination of a part of the sky in which there may be a planet to discover.’ Galle decided to be that persistent observer.
If the truth be admitted, Galle had never expected to find anything. It did not seem possible. How could a man sitting at a desk in Paris ‘see’ the universe with the aid of mathematics? It was about as likely as a blindfolded astronomer discovering a comet with the Fraunhofer telescope. But – miracle of miracles – there it was: Le Verrier’s planet, looming out of the inky depths of space, exactly where the man had predicted it would be.
The new world had been trailing around the Sun in the frigid darkness beyond the orbit of Uranus since the birth of the solar system. And until an hour ago no human being had known of its existence. For the moment, they were the only three people on Earth who had seen it, and it had no name. Soon, however, everyone in the world would know it as Neptune.
Paris, 29 September 1846
In Paris, a few days later, Le Verrier tore open a letter from Berlin, dated 24 September 1846. ‘Sir,’ he read. ‘The planet whose position you have pointed out actually exists.’
Galle had found his planet! Le Verrier was dizzy with euphoria, but also relief. He had believed in the new planet – of course he had – but he had also not believed in it. He was human, after all. He had staked his reputation on a piece of arcane mathematics, which the Creator may or may not have decided to respect. He had sounded confident when he made his prediction, but he alone knew how much of that was bravado.
On 1 October, Le Verrier replied to Galle. He thanked the German astronomer profusely for being the only one to take seriously his request, writing, ‘We are, thanks to you, definitely in the possession of a new world.’
The discovery of Uranus had been a sensation: twice as far from the Sun as Saturn, it had overnight doubled the size of the solar system. But the discovery of Neptune was a sensation of an entirely different order. Whereas Herschel had stumbled on Uranus by accident, the existence of Neptune, its location and even its appearance had been predicted by Le Verrier, with nothing more than a pen and paper.
‘Without leaving his study, without even looking at the sky,’ wrote French astronomer Camille Flammarion, ‘Le Verrier had found the unknown planet solely by mathematical calculation, and, as it were, touched it with the tip of his pen!’2
Discovering something in the real world from a desk, as Flammarion recognised, was truly something new under the sun. ‘The entire annals of Observation probably do not elsewhere exhibit so extraordinary a verification of any theoretical conjecture adventured on by the human spirit!’ wrote the Scottish astronomer John Pringle Nichol.3
But the discovery of Neptune was a triumph not only for Le Verrier; it was also a triumph for Isaac Newton and the universal theory of gravity he had devised almost two centuries earlier. Newton’s law explained not only what we could see but predicted what we could not.
Le Verrier had demonstrated in spectacular fashion the central magic of science – its astonishing ability to predict things never before suspected which turn out to exist in the real world. It stretched credulity that mathematical equations scrawled across a page could so perfectly capture reality, but miraculously, they did. Using abstract formulae, Le Verrier had uncloaked a real body in the real world, and nobody in the history of the world had done anything like it. Le Verrier was the first of the magicians.
*
The discovery of Neptune triggered a heated priority dispute between France and England because an English mathematician had also used the anomalous motion of Uranus to predict the location of the new planet. John Couch Adams was an autistic mathematical genius from Cornwall in England. In 1841, while a student at the University of Cambridge, he set out to deduce where in the night sky the new planet must be in order to have the observed effect on Uranus. His calculations took four years
, but in 1845, he took his result to Sir George Biddell Airy, Astronomer Royal and director of the Royal Observatory at Greenwich. Unfortunately, like Le Verrier in France, he was fobbed off. When Airy did eventually take notice of Adams, rather than publicising his prediction and authorising a search with one of the Greenwich telescopes, he chose to pass the information to George Challis, who had succeeded him as director of the Cambridge Observatory.
Challis could immediately see that Adams’ prediction was not a precise location but an extended patch of the sky where the hypothetical planet might be found. A comprehensive search would take almost one hundred sweeps with the Cambridge transit telescope, each of which would last several hours. Estimating that the whole process would take around 300 hours of observing time, Challis held off for a while. When he eventually started the search, he recorded Neptune – twice, in fact – while failing to recognise it. By then it was too late and Galle in Berlin had already found the new planet.
The episode was a great embarrassment for Airy and Challis, since they had received a prediction of the new planet’s location from Adams before Galle received a similar one from Le Verrier. Things were made worse by the fact that they had kept Adams’ prediction a secret, perhaps to make sure that, if it was discovered, Cambridge would have the glory. However, the fact that none of Adams’ calculations had ever been published made the French suspicious that there had ever been an English prediction.
The international dispute over Neptune was prolonged and bitter, but to the credit of Adams and Le Verrier, neither of them took any part in it. Perhaps because they appreciated each other’s mathematical wizardry and had faced similar obstacles in getting mere mortals to take them seriously, as soon as they met in England they became firm, lifelong friends. Nowadays, as often as not, the discovery of Neptune is attributed jointly to Adams and Le Verrier.
*
After his triumphant prediction of the existence of Neptune, Le Verrier’s star rose in the scientific firmament and in 1854 he became director of the Paris Observatory. But nothing he achieved came close to matching the exhilaration he had felt at magically unveiling an unknown world at the edge of the solar system. He had been courted by kings and revered as a god by scientists. Fame and adulation had intoxicated him, and craving that feeling again, he turned his attention from the outer to the inner solar system.
Le Verrier’s goal was to understand thoroughly the orbits of the inner planets: Mercury, Venus, Earth and Mars. If he could do that, then perhaps, just perhaps, a Uranus-type anomaly would show up that would lead to a headline-grabbing discovery. Remarkably, such an anomaly did indeed exist, and it concerned the innermost planet. Even when the gravitational tug of the other planets on Mercury was taken into consideration, it still did not move as expected.
Le Verrier became convinced that there was a planet orbiting even closer to the Sun than Mercury and, by February 1860, it had a name. Planets are named after ancient gods, and the lord of the forge on Mount Olympus, home of the Greek gods, was Vulcan. It seemed an appropriate name, since the new world could never escape the fires of the Sun.
For almost half a century, astronomers searched for Vulcan, but gradually it fell out of favour and all sightings of it turned out to be mirages. The anomalous motion of Mercury remained, and nobody suspected what it was really telling us: that, incredibly, impossibly, Newton was wrong about gravity. Nobody, that was, until Albert Einstein, who in 1915 devised a better theory of gravity – the general theory of relativity – to supplant Newton’s.
But although Vulcan had been a dead end, Neptune very definitely had not been. Le Verrier had shown how it was possible to use Newton’s law of gravity to predict what we could not see – to make a map of the invisible world.
*
In the first decades of the twentieth century, there was a suggestion that the orbit of Neptune, just like that of Uranus, was being perturbed. It turned out to be untrue. Nevertheless, it triggered a search for a ‘Planet X’, even more distant from the Sun. This culminated on 18 February 1930 in the discovery of Pluto, the only planet to be named by a child – eleven-year-old Venetia Burney from Oxford.4
Pluto, which is smaller even than Earth’s moon, turned out to be far too tiny to affect Neptune. In fact, at the end of the twentieth century, it was revealed to be one of tens of thousands of similar bodies circling the Sun beyond the orbit of Neptune. It was the discovery of this ‘Kuiper Belt’ of icy builder’s rubble left over from the formation of the solar system 4.55 billion years ago that led the International Astronomical Union to demote Pluto in August 2006 from a planet to a ‘dwarf planet’.
But Newton’s law of gravity may not yet have exhausted its ability to reveal the invisible in our solar system. At the beginning of 2016, two astronomers at the California Institute of Technology in Pasadena pointed out that at least half a dozen Kuiper Belt Objects are moving oddly. Mike Brown and Konstantin Batygin claim that their motion is due to them being tugged by an unknown planet orbiting the Sun at the periphery of the solar system.5 But rather than a celestial tiddler like Pluto, this planet would have about ten times the mass of the Earth.
Brown and Batygin claim ‘Planet 9’ orbits on average about twenty times as far from the Sun as Neptune. Since planets shine only due to reflected sunlight, it would be extremely faint and hard to find, but many astronomers are keen to be the new Johann Galle and searches for Planet 9 are already underway.
The real success of the technique pioneered by Adams and Le Verrier, however, has been in detecting the anomalous motion of stars caused by the gravitational tug of their invisible planets. In 1995, 51 Pegasi b was the first planet to be discovered around a normal star other than the Sun; now more than four thousand ‘exoplanets’ are known and the total is rising at an ever-quickening rate.
But arguably the most important invisible thing revealed by Newton’s law of gravity is ‘dark matter’. Although its existence was first suspected in the 1930s by Swiss–American Fritz Zwicky and Dutchman Jan Oort, it took the work of two astronomers at the Department of Terrestrial Magnetism of the Carnegie Institution of Washington to confirm its existence. In the late 1970s and 1980s, Vera Rubin and Kent Ford found that stars in the outer regions of spiral galaxies are orbiting the centres far too fast. Like children on a speeded-up merry-go-round, they should be flung into intergalactic space.
Astronomers have explained this anomaly by suggesting that there is much more matter in spiral galaxies than we see in the form of stars, and that it is the extra gravity provided by this invisible dark matter that holds on to the outermost stars. Across the universe, dark matter outweighs the visible stars and galaxies by a factor of about six. Nobody knows what it is made of, although the best bets are undiscovered subatomic particles or Jupiter-mass black holes left over from the Big Bang. If you can figure out the identity of the dark matter, there is a Nobel Prize waiting for you in Stockholm.
Notes
1 Philosophy of the Inductive Sciences, Volume 2 by William Whewell (1847, p. 62).
2 Astronomy for Amateurs (1915) by Camille Flammarion (Kessinger Publishing, Whitefish, 2008, p. 171).
3 The Planet Neptune: An Exposition and History by John Pringle Nichol (Kessinger Publishing, Whitefish, 2010, p. 90).
4 Venetia Burney talking at eighty-five about how, aged eleven, she came to name Pluto: https://www.nasa.gov/mp3/141071main_the_girl_who_named_pluto.mp3.
5 ‘Evidence for a Distant Giant Planet in the Solar System’ by Konstantin Batygin and Mike Brown (Astronomical Journal, vol. 151, 20 January 2016, p. 22).
* In guessing the distance of the hypothetical planet from the Sun, Le Verrier was aided by the Titius–Bode law, although no scientific reason is known why the planets should follow such a rule. See http://demonstrations.wolfram.com/TitiusBodeLaw/.
2
Voices in the sky
This velocity is so nearly that of light, that it seems we have strong reasons to conclude that light itself (including radiant heat
, and other radiations if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws.
JAMES CLERK MAXWELL
From a long view of the history of mankind – seen from, say, ten thousand years from now – there can be little doubt that the most significant event of the nineteenth century will be judged as Maxwell’s discovery of the laws of electrodynamics.
RICHARD FEYNMAN1
Karlsruhe, Germany, 13 November 1887
Today was the day. He was sure of it. Heinrich Hertz bolted down his breakfast, kissed his wife Elisabeth and his baby daughter Johanna goodbye, and hurried through the streets of Karlsruhe to the university campus. On reaching his laboratory, he pulled down the blinds and switched on the ‘oscillator’ circuit that he and his assistant, Julius Amman, had been building these past few days. The current surged through the 20,000-volt induction coil and he heard a faint crackle but could see nothing. Only when his eyes adjusted to the gloom was he sure that a spark was stuttering in the 7.5-millimetre air gap he had left in the circuit. Satisfied that his ‘transmitter’ was working as intended, he turned to his ‘receiver’.
A metre and a half away along the bench, Hertz had propped up a vertical loop of copper wire, which also contained a tiny air gap. He adjusted the gap with a screw to make it as small as possible and squinted at it in the laboratory gloom. Nothing.
He returned to his transmitter. Because the frequency of his oscillator circuit was so high, the spark was leaping back and forth across the air gap too rapidly for him to detect any motion with the naked eye. On each side of the gap there was a 1.5-metre length of conducting wire, terminated by a thirty-centimetre-diameter zinc ball. By moving the zinc balls along the wires, Hertz could change the ‘capacitance’ of the circuit and, with it, the frequency of the leaping spark. He did this several times while peering closely at his receiver, which he had ‘tuned’ so that, if it felt a vibration at a particular frequency, it would oscillate in sympathy. Still nothing.
