The hunt for a hypothetical planet called Vulcan near Mercury fascinated author Tom Levenson for years. It took some tough talk from Ta-Nehisi Coates to get him to finally write a book about it.
It was just a day or two, but sometime between the twelfth and eighteenth of November 1915, Albert Einstein lost his mind. He felt his heart shudder, he told one friend, actual palpitations in his chest. It was as if, he said, something had broken within him. Such a derailment, though, was a happy madness: six weeks after the fit had passed, he told another colleague that he had been "beside himself with joy."
The catalyst for such sudden, transcendent euphoria? Einstein had just managed to calculate the correct orbit for the planet Mercury, using his still-not-quite-finished mathematical description of the geometry of time and space, what we now call the General Theory of Relativity.
I first encountered Einstein's brief dalliance with Mercury twenty years ago, and when I did, I made a classic writer's mistake: I never stopped to wonder why the greatest scientist of the last century would flip on working out such a seemingly minor calculation on the sort of problem, that, after all, astronomers had been solving for centuries. I finished the Einstein project and moved on.
But the image of Einstein unhinged stuck with me. Why Mercury? Why such joy? Over time, in the cracks between other "important" work, I dug around a bit. The short(ish) answer is that it turned out that Mercury is the sneaky youngest sibling in the solar system. Hard to observe, and subject to a tangle of gravitational influences from its fellow planets, it wasn't until 1859 that anyone managed to work out a satisfactory table of the planet's trajectory. As Urbain Jean-Joseph Le Verrier worked it out, he found a tiny excess of motion — no more than one part in 10,000 — that couldn't be accounted for by the gravitational tug of any other known body in solar system.
There was an obvious solution to the puzzle if you believed in Isaac Newton's system of the world — as Le Verrier and all his colleagues did, for very good reasons. If nothing known was hauling on Mercury, then something yet to be found was out there — which was exactly the reasoning (and the maths) that Le Verrier had used to discover the planet Neptune "at the tip of his pen" in the previous decade. So when he published his new results, he told people to look for a planet or a flock of asteroids between the sun and Mercury.
The first sighting came in December 1859, just three months after Le Verrier went public — an observation by a country doctor turned amateur astronomer who caught what he thought was a transit of what was almost immediately given the obvious name: Vulcan, after the Roman god of the forge.
No one replicated that finding, but no matter: another intra-Mercurian body was soon found. Then another, and another — over the next twenty years there were at least a dozen claimed sightings by professional and amateur star-gazers alike. There was only one slight issue: it didn't exist. That left another problem: Mercury's orbit really did (and does) wobble … and there were no satisfactory Newtonian explanations available other than Vulcan. What to do?
For decades: nothing. Astronomers for the most part treated Vulcan and Mercury like the crazy uncle hooting in the attic. Polite visitors had the kindness not to notice the distant commotion. There were other, more pressing, more obvious questions to ask. No need to dwell on such minor unpleasantness….
That's what captivated me: a tale of how hard it is to see what isn't there, when what should exist makes so much damn sense. I told my friends about it over beers. It only became a book because one of those friends staged an accidental intervention.
I had the good fortune to have Ta-Nehisi Coates as a colleague for a couple of years and we talked writing pretty much every week. I was always working on something else, but I told him about Vulcan once, and then again, and then again…until, one afternoon, outside an ice cream shop, he just stopped me mid-paragraph. He gave me an almost pitying stare. He laid down my orders: Write, he said. Get the story onto the page. Stop talking and get on with it. Which is, of course, at once the most obvious, the best, and the only advice a writer should give, or need.
So I did, ultimately coming to realise that Vulcan's biography was no simple tale of error and folly. Through it runs an account of how change in science really happens, which is quite different from the way the myth of science would have it. Brute facts don't destroy the most lovely theories, not on their own, not all at once. Observations, experiments only gain meaning from the framework in which they're interpreted. It takes a better idea to banish the older one that keeps Vulcan in view, and hides any deeper vision of reality.
Enter Einstein: he had pursued what became general relativity for reasons that had nothing to do with a minor unexplained solar system anomaly — but when he neared the end of almost a decade of labour, Mercury's orbit offered him the first real test of his conception of gravity as simply a measure of the way matter and energy deform space and time. When he got that right, fifty years after Vulcan had first been "seen," he had resolved both a historical curiosity…and a achieved fundamental shift in how we conceive of the cosmos as a whole.
In other words, in that November hour one hundred years ago when Mercury dropped out of his maths, he alone of all humankind knew the universe was not what it had seemed for centuries. That's the reason his moorings snapped, however briefly. As he later wrote, to do such work, one feels "akin to…the religious worshipper or the lover; the daily effort does not originate from a deliberate intention or program, but straight from the heart."
Thomas Levenson is a professor at MIT and director of the Graduate Program in Science Writing. His latest book, out this week, is The Hunt for Vulcan: And How Albert Einstein Destroyed a Planet, Discovered Relativity, and Deciphered the Universe.
Top image: Surface of fictional planet Vulcan from J.J. Abrams' 2009 film Star Trek.