by Marcus Chown
Le Verrier’s goal was characteristically ambitious: to completely understand the orbits of the inner planets: Mercury, Venus, Earth and Mars. If he could do that, then, perhaps, just perhaps, an anomaly would show up that would lead to a new, headline-grabbing discovery.
Each planet, as pointed out before, is influenced not only by the gravity of the Sun but by the gravity of all the other planets. As a result, it does not actually trace and retrace the same path for all eternity. Instead, over long periods of time, its elliptical orbit precesses, causing the planet to trace out a rosette-like pattern in space. Because precession causes the closest approach of a planet to the Sun, known as its ‘perihelion’, to gradually circle the Sun, astronomers talk of the ‘precession of the perihelion’ of a planet.20
It was in 1843, three years before the discovery of Neptune, that Le Verrier had first focused his attention on the four innermost planets. In order to predict the orbit of each world, he had painstakingly added up the gravitational tugs from all the other planets in the Solar System. Unfortunately, his predicted orbits did not match the observed orbits. He suspected that the discrepancies were due to his imperfect knowledge of the distances and masses of the other planets. And so, in the decade after his Neptune triumph, he set himself the task of refining those planetary vital statistics.
In 1852, the best estimate of the average distance between the Earth and the Sun was 95 million miles. By 1858, Le Verrier had refined the figure to 92.5 million miles, which is within half a per cent of the modern value. The following year, armed with this much-improved figure, Le Verrier set out once more to calculate the orbits of the inner planets.
It was a long and tedious marathon of number crunching. And Le Verrier had no more success than he had had sixteen years earlier. The orbits he calculated for the innermost planets did not match the orbits observed by astronomers. But he had faith in Newton’s law of gravity, he believed in his mathematical intuition, and so he persevered with his calculations. It seemed likely to him that the problem was with the numbers he was using for the masses and distances of the planets. Perhaps they were still in error. He tried adjusting them, one at a time. It took a while to do this. But, eventually, his efforts paid off. All that was needed was a simple change. After slightly increasing the masses of the Earth and Mars, he was able to predict the precise orbits of all of the inner planets.
All the planets, that was – except one.
Mercury is the innermost planet, orbiting closest to the fires of the Sun. It is also the tiniest planet, smaller even than Jupiter’s moon, Ganymede.
According to Le Verrier’s calculations, the pull of Mercury’s closest planetary neighbour, Venus, causes the planet’s perihelion to advance by about 1/5,000th of the way around the Sun every century. Astronomers use an even more esoteric and opaque language than this. They say Venus causes Mercury’s perihelion to advance by 280.6 arc seconds per century, with an arc second being l/60th of an arc minute, and an arc minute l/60th of a degree. Le Verrier’s calculations showed that the tug of the giant planet, Jupiter, contributes another 152.6 arc seconds per century; the Earth 83.6 arc seconds per century; and all the remaining planets combined a mere 9.9 arc seconds per century. Adding together all these numbers, Le Verrier arrived at a figure for the precession of the perihelion of Mercury of 526.7 arc seconds per century.
But this was not right.
Careful observations of the inner planet had shown that Mercury’s perihelion advances by 565 arc seconds per century. This left a discrepancy of about 38 arc seconds per century (the modern value is 43 arc seconds per century).
The mismatch was tiny. But Le Verrier’s calculations were precise enough to show that it was real. Mercury’s perihelion was precessing by 38 arc seconds per century more than it should. In other words, if all the other planets in the Solar System were to vanish suddenly, removing at one fell swoop their long-range gravitational effects, Mercury would still trace out a rosette pattern. A rosette that repeats roughly every 3 million years. A rosette pattern that is utterly inexplicable.
Le Verrier could hardly believe his luck. It was the Uranus anomaly all over again. A hidden mass – something within the orbit of the inner planet – was tugging on Mercury. Le Verrier hardly dared voice the question. But could it be, was it possible that it was, a new planet?
To estimate its mass, Le Verrier assumed that it orbited halfway between Mercury and the Sun. His calculations showed that such a planet could account for Mercury’s anomalous precession if its mass was similar to its neighbour. But that immediately posed a problem. A planet that big should long ago have been spotted by astronomers. Yes, it would be hidden in the glare of the Sun. But it should have shown up during a total eclipse when the Moon blots out the Sun and even faint stars can be seen close to the solar disc.
If it was not a planet, what might it be? Le Verrier wondered whether Mercury’s wayward behaviour might instead be caused by a collection of ‘asteroids’ orbiting between the Sun and Mercury. If this was the case, then some of the objects might be big enough to be seen as they crossed, or ‘transited’, the face of the Sun.
Incredibly, someone already had spotted something transiting the Sun. Edmond Modeste Lescarbault was a French country doctor with a passion for astronomy. He had been thinking about the asteroids which had been discovered orbiting between Mars and Jupiter in the early decades of the nineteenth century, and this had got him wondering where else asteroids might lurk.21With his 4-inch refractor telescope at Orgères-en-Beauce, about 70 miles west of Paris, he had previously observed the tiny black speck of Mercury transiting the Sun. So it was natural for him to wonder whether there might exist asteroids closer to the Sun than Mercury, and whether he might be able to spot them as they crossed the solar disc.
On Saturday 26 March 1858, Lescarbault was running his surgery. But there was a gap between patients. So he took the opportunity to go to his telescope and point it at the Sun. To avoid blinding himself, he projected the image of the solar disc onto a card. As soon as he did so he saw something unusual: a small black dot close to the edge of the Sun. He was of course desperate to follow its progress but another patient had arrived and was demanding his attention. When he finally dashed back to his telescope, he was relieved to see that the dot was still there. Lescarbault tracked it continuously until it vanished off the other edge of the Sun. The total time it had taken to transit, he estimated, was 1 hour 17 minutes and 9 seconds. It was exactly what would be expected for an asteroid in the innermost reaches of the Solar System.
Curiously, Lescarbault did nothing about his discovery. It was only nine months later, when he read an article saying that Le Verrier believed there was a body or bodies between Mercury and the Sun, that he put pen to paper and wrote to the Paris Observatory.
Le Verrier was deeply sceptical of the doctor’s claim. But the possibility that he, Le Verrier, might repeat his Neptune success was simply too tantalising. He had to meet Lescarbault. On 31 December 1859, he caught the train from Paris to Orgères-en-Beauce. Arriving unannounced at Lescarbault’s home, he fully expected to find an unimpressive rural amateur. Instead, he met a first-rate observer who had built precision scientific instruments. After grilling him exhaustively on his observations, the Parisian astronomer became convinced of his discovery.
Unbelievably, Le Verrier had done it again. He had repeated his success with Neptune. He had predicted the existence of a planet between Mercury and the Sun. He was truly a god among men.
Back in Paris, Le Verrier converted Lescarbault’s observations into numbers. Assuming the new planet was travelling in a circular orbit around the Sun, it should complete a circuit once every twenty days. That meant it should transit across the face of the Sun several times a year as seen from the Earth.
Le Verrier announced the discovery of the new planet to an astonished world. By February 1860 it even had a name. Planets are named after ancient gods, and the lord of the forge on Mount Olympus, the home of the Greek g
ods, was Vulcan. It seemed an entirely appropriate name since the new world could never escape the fires of the Sun. So Vulcan it became.
Other astronomers, particularly those who monitored the Sun for sunspots, quickly announced that they had also seen Vulcan transiting the Sun but had not recognised it as a planet.22 There was another opportunity to observe a transit between 29 March and 7 April 1860. In Madras in India, and in the Australian cities of Sydney and Melbourne, astronomers watched the Sun’s disc continuously. But nothing showed up.
The years passed. Some observers claimed to see the new planet. Many others did not. And the observations of those who saw something never seemed to be independently verified by anyone else.
On 7 August 1869 there was a total eclipse. Once again, some observers reported that they saw Vulcan. But, crucially, the total eclipse was observed by an American pioneer of astrophoto-graphy from Burlington, Iowa. Benjamin Apthorp Gould took forty-two photographs of the misty white ‘corona’ that surrounds the Sun, and which is visible only during totality. None showed the new planet.
The clincher was the total eclipse of 29 July 1878. Teams of astronomers took the Union Pacific railway to Rawlings, Wyoming, in the American Midwest. Among them were some of the greatest observers of the day. They included Simon Newcomb, from the Naval Observatory in Washington, DC, who was destined, unfortunately, to be remembered for declaring heavier-than-air flight impossible on the eve of the Wright brothers’ pioneering flight; and Norman Lockyer, who, from his garden in the London suburb of Wimbledon on 20 October 1868, had discovered helium on the Sun, the only element to be discovered in space before it was discovered on Earth. Even the world-famous inventor Thomas Edison tagged along.
From Rawlings, the observers tramped with their equipment to suitable observing spots. They were plagued by cloudy skies and eye-watering dust and sand, whipped up by the incessant wind. But, despite all the meteorological difficulties, not to mention malfunctioning equipment, many saw and even photographed the eclipse. Only one man saw a new planet.
James Craig Watson, director of Michigan’s Ann Arbor Observatory, reported a small ruddy object, orbiting the Sun inside the orbit of Mercury. His discovery was immediately wired around the world. Two decades after Le Verrier had proposed the existence of a new planet, could it be that Vulcan had at last put in an appearance?
The trouble was nobody else saw it. Or, rather, they saw the ruddy speck but recognised it as Theta Cancri, a faint star in the constellation of Cancer the Crab. Watson stuck to his guns even when it seemed overwhelmingly likely he was wrong and everyone else was right. In fact, when he went to his grave in 1880, having contracted a fatal infection at only forty-two, he was still utterly convinced he had discovered the planet Vulcan.
But the balance had now tipped. The consensus was that Vulcan did not exist, had never existed. It was a figment of the fevered imagination. A testament to the power of human delusion, of scientific wishful thinking. It lived on only as a half-forgotten historical footnote and, of course, the fictional birthplace of Star Trek’s Mr Spock.
Unsolved puzzle
The idea of a planet like Vulcan turns out to be not so mad after all. Thousands of planets are now known to orbit other stars in our Milky Way, and many of them have Vulcan-like planets.
One of the most unexpected discoveries in modern astronomy is of gas giant planets orbiting their parent stars closer in than Mercury orbits the Sun. Such ‘hot jupiters’ could not possibly have formed where we see them. The gas would have been so hot, its constituent gas atoms flying about so fast, that gravity could not have held onto it. Instead, astronomers believe hot jupiters are born much further out. Friction with the debris disc out of which planets form causes them to spiral inwards. Such planetary ‘migration’ is now believed to have also been a feature of our Solar System’s pre-history, with worlds like Jupiter and Saturn having participated in a game of interplanetary musical chairs before taking up their current locations.
Planetary systems around other stars appear to be telling us that our Solar System is unusually stretched out. More than half the planets in ‘exoplanetary’ systems orbit closer to their parent star than Mercury is to the Sun. Elsewhere in our galaxy, Vulcans abound. It is still possible that this is an illusion caused by ‘observational biases’. Astronomers detect exoplanets by the wobble they create in their parent star or by the dimming they produce in their parent star’s light. Close-in planets are quicker and easier for astronomers to spot since there is less time to wait for them to complete an orbit.
Our planetary system may not always have been so unusual. According to computer simulations of the birth of the Solar System, initially there may have been a number of worlds orbiting close in to the Sun. Collision between them left Mercury as the sole survivor. If this scenario is correct, then Vulcan did indeed exist. Unfortunately, the human race missed it by 4.55 billion years.
Le Verrier died on 23 September 1877. He had solved the problem of the anomalous motion of Uranus, discovering Neptune and in the process expanding the size of the Solar System. But, with Vulcan slipping inexorably from his grasp, he knew that the problem of the anomalous motion of Mercury had defeated him.
The twentieth century arrived and there were marvels galore to attract attention: X-rays and radioactivity and human-powered flight. The anomalous motion of Mercury was a curious puzzle but it was almost certainly not an important one. Nobody lost much sleep worrying about it. In fact, nobody gave it much thought at all. And nobody suspected what it was really telling us: that, incredibly, impossibly, Newton was wrong about gravity.
The man who recognised this and devised a better theory of gravity to supplant Newton’s was Albert Einstein. But, before it dawned on Einstein that his predecessor had been wrong about gravity, he realised that Newton was wrong about something apparently even more fundamental that had a bearing on gravity: the very nature of space and time.
Further reading
Aw, Tash, Map of the Invisible World, Fourth Estate, London, 2010.
Levenson, Thomas, The Hunt for Vulcan . . . And how Albert Einstein destroyed a planet, discovered relativity and deciphered the Universe, Head of Zeus, London, 2015.
Schilling, Govert, The Hunt for Planet X, Copernicus Books, New York, 2009.
PART TWO
Einstein
5
Catch me if you can
How Einstein realised that nothing can travel faster than light and that this is incompatible with Newton’s law of gravity
For Mr Newton, space and time did not talk to each other, never married, and lived separate lives.
Roberto Trotta1
The velocity of light in our theory plays the part, physically, of an infinitely great velocity.
Albert Einstein2
‘What would it be like to catch up a light beam?’ Einstein was just sixteen when he asked the question that would set him on the road to greatness. Frustratingly, he never told anyone the exact circumstances which prompted him to ask this critical question. Instead we can only speculate. We know that he formulated the question in early 1896 while at school in the Swiss town of Aarau, 30 miles west of Zurich. At the time, he was boarding with the Winteler family.
I imagine him waking to sunlight streaming through the window of the attic room he rents. The swaying branches of a linden tree splinter the light into a myriad glowing fragments, which dance kaleidoscopically on the wall beside his bed. He reaches up with his hand and, like a child, tries to grab a jittering lozenge of light. So utterly transfixed is he by the shifting shapes on the wallpaper that he makes no move to push back his covers until the spell is broken by a rap on the door. ‘Herr Einstein!’ It is Marie Winteler, the pretty eighteen-year-old daughter of his landlord, who has taken a fancy to him. ‘Papa says to tell you breakfast is ready.’
I picture Einstein, later that day, sitting at a desk in a high-ceilinged classroom at the Aargau Cantonal School, staring idly out at the River Aare. The rain, which h
as been spattering the window, stops as abruptly as it started. As the thick clouds part, the night-time gloom which has descended on the little Swiss town is pierced by a biblical shaft of light, stabbing down as if from Heaven. Where it strikes the black river, the water sparkles like a nest of diamonds. So mesmerised is he by the sight that he hears nothing of the lecture – on wiring patterns of AC dynamos – until his reverie is interrupted by the roar of Doktor August Tuchschmid, the school’s principal. ‘Herr Einstein! My sincerest apologies for boring you. Perhaps, at some time in the half hour that remains to us, you might deign to honour us with your attention.’
That night I see Einstein and Marie Winteler running hand in hand through the narrow lanes of Aarau, splashing through puddles and laughing hysterically like the teenagers they are. Soaked through but not caring, they stop abruptly and he pulls her towards him and kisses her. Over her shoulder, he sees the eerie-green globes of the gas lamps, marching down the street, growing steadily smaller and fainter as they converge in the distance. In the oily black puddles he sees their reflections, and also the reflection of the full Moon, like some rogue lamp that has broken free of the Earth and floated up to the sky. He stops kissing her and looks upwards.
‘Albert?’
All day he has been mesmerised by light. All day he has been wondering about light. All day he has been asking himself the same pressing question: what is wrong with our understanding of light? In the question lies the answer. But his question is too woolly, too imprecise, to make any progress.
His girlfriend has spoken to him but, in his mind, he is a quarter of a million miles away. The light of the Moon has travelled that far across space to reach his eyes. He pictures the journey it takes — barrelling along through the cold vacuum at a billion kilometres an hour – and his heart misses a beat. Suddenly, he knows the question he should be asking – the one that has the potential to open the doors to a new world of understanding. It is so obvious he cannot believe it has never occurred to him before.