In the middle of November 1915, Albert Einstein was hard at work. He would later say he never labored more intensely than he did that autumn. Since the beginning of October, he'd been wrestling with what would become his greatest discovery, what we now call the General Theory of Relativity, a theory of gravity that said our universe is one in which space and time bend, warped by all the matter and energy it contains. It was a genuinely revolutionary conception, and it would, on its public confirmation four years later, transform Einstein into the first true celebrity of science.
But in the week between November 11 and 18, the problem that occupied him in that second year of what was already being called "the Great War" seemed simple, even trivial: retracing the orbit of the planet nearest our sun, fleet-footed Mercury. More precisely, could his brand-new mathematical approach account for a wobble in that orbit that had been noticed and unexplained for half a century?
The effect was tiny — on the order of one part in 10,000. But it was — and is — really there. The anomaly had lingered for decades, an embarrassment to astronomers. That ended the moment Einstein worked through the last line of his calculation. There, as he put it in the formal language required of proper scientific communication, "The calculation for the planet Mercury yields a perihelion advance of 43 arc minutes per century, while the astronomers assign 45" +/- 5" per century as the unexplained difference between observations and the Newtonian theory." Belaboring the obvious, he added, "This theory therefore agrees completely with the observations."
In private, he let himself go. As the correct answer appeared, he told a friend, his heart actually shuddered in his chest — genuine palpitations. He wrote once that it was as if something had snapped within him, and told another friend that he was "beside himself with joy."
That leaping heart is the reason The Hunt for Vulcan exists. I'd known the story of the Mercury result since the early 1990s, when I began working on what led to a biography of Einstein for the NOVA series on television and to my book Einstein in Berlin. It had always struck me as odd. Delight in General Relativity I could understand: that theory supplanted Newton's gravitation, the most famous idea in physics, and, arguably, the climax of the scientific revolution itself. But fixing a tiny hole in that seeming backwater of research, solar system dynamics? Why should that cast Einstein's heart loose from its moorings? It took me a while to get back to it — two decades — but eventually I decided to find out.
Digging backwards, I learned that the mystery of Mercury's orbit had first emerged in 1859, the time when the emergence of enough first-rate observations of the notoriously difficult-to-track innermost planet met the one man most perfectly placed to analyze them. Urbain Jean-Joseph Le Verrier is hardly a household name now, but in the mid-nineteenth century he was the most famous astronomer, quite possibly the most famous scientist in the world.
His celebrity rested on the single most romantic incident in the history of the Newtonian cosmos. Analyzing irregularities in the orbit of Uranus in 1846, Le Verrier concluded that everything would work out according to Newton's laws if there were a planet of a particular size to be seen in a well-defined patch of the night sky. It took the observers to whom he sent the coordinates just a couple of hours to find the new planet — Neptune — discovered "at the tip of his pen."
With that triumph of prediction in his pocket, the astronomical world knew what the answer had to be once Le Verrier tackled Mercury, and announced in 1859 that there was an unexplained residue of motion of just one part in 10,000 that couldn't be explained by the gravitational tug of any known member of the solar system. Le Verrier's calculation was perfectly good. Mercury really does move like that (though the number would be revised upward slightly as more precise measurements were made later in the century). And given both the existence of the glitch and the man who found it, the solution seemed obvious: there had to be another planet, Neptune's toastier sibling, lurking inside Mercury's track, unobserved in the glare of the sun.
There's nothing wrong with that logic. It was the correct interpretation based on the best science available at the time. But the story gets trickier with what happens next. The case for an undiscovered planet was so strong that it would be seen almost as soon as Le Verrier announced his result — the first time by a country-doctor-turned-amateur-astronomer. Le Verrier himself endorsed the good doctor's "discovery" and soon, though no one knows by whom, the new planet gained its name: Vulcan.
No one was able to find whatever it was that first stargazer had seen, but no matter. There was no doubt the proposed planet would be hard to find, and its orbit was uncertain, so plenty of observers continued to search... and find. Vulcan would be discovered again and again, by both amateurs and professional astronomers, more than a dozen times in the next two decades.
The only problem: it was, of course, never there.
|"Facts on their own don't have the power that popular notions of science suggest they do. They take on meaning only in the context of the ideas that organize them into pictures of the world."|
Finally, after the eclipse of 1878 brought with it two more glimpses of the seemingly necessary planet that were somehow missed by dozens of other qualified planet-hunters, the astronomical community came to the consensus view that Vulcan was a mirage. For more than 30 years, Mercury's motion remained unexplained. A few researchers proposed ad hoc fixes, none of which worked. And there the matter rested, with the unexplained motion turning into a kind of astronomer's crazy uncle in the attic, hooting away while everyone tried to ignore the noise. No one went the next step, to wonder what if there might something be wrong with Newton's theory of gravity itself.
Eventually, of course, Einstein dealt with Mercury, or rather, found he'd done so while solving a different problem. He'd found a logical contradiction between Newton's theory and his own discoveries about the relativity of space and time. It was that — not the conflict of the fact — Mercury's wandering — with Newtonian math — that led Einstein to construct his own version of gravity, the one he would finally put to the test in mid-November 1915, almost exactly 100 years ago.
From Einstein to Vulcan and back again: there's a wonderful story of ideas and discoveries and ambitious thinkers and Thomas Edison shooting at a stuffed jackrabbit (it makes sense, I promise). That story leads to two useful realizations. First, it's so easy to see ourselves as not just more knowledgeable than our predecessors, but smarter too. We're not. Vulcan shows us how the capacity for self-deception, of believing what ought to be true, even when it isn't, is always with us. None of those folks who thought they saw Vulcan were committing fraud. They had every good reason to be right. They just weren't, and couldn't see it.
And that leads to the second moral of the tale: we remain at risk of sharing the same fate. Facts on their own don't have the power that popular notions of science suggest they do. They take on meaning only in the context of the ideas that organize them into pictures of the world. And it's so very hard to see past such pictures when the need arises…
…Which, to take the long road home, is simply a way of saying that Albert Einstein had a damn good reason to feel almost frightening joy when Mercury dropped out of his mathematics. The internal logic of his theory had long since persuaded him that something like what he was working on would provide a better account of gravity than Newton could. But reality does get the final vote, even if it may take many a while to accept the result.
This time, when nature cast its ballot in the numbers he wrote out on his desk, that was it: the moment at which he knew, first of all humanity, that it was his cosmos, Einstein's universe, that we inhabit.
Thomas Levenson is a professor at MIT and head of its science writing program. He is the author of several books, including Einstein in Berlin and Newton and the Counterfeiter: The Unknown Detective Career of the World’s Greatest Scientist. He has also made 10 feature-length documentaries (including a two-hour Nova program on Einstein) for which he has won numerous awards.