Mercury doesn’t play by Newton’s rules. For over half a century, scientists searched for a hidden planet — a planet they called Vulcan — whose gravitational influence would explain Mercury’s odd orbit while preserving the classical Newtonian order of the universe. Unfortunately, in all that time Mercury continued to defy Newton while Vulcan (which, after all, doesn’t exist) refused to be found. The mismatch between theory and observation persisted until one of the greatest minds of the 20th century developed a new theory to explain it all. In “The Hunt for Vulcan,” Thomas Levenson tells the story of the false planet Vulcan — from the seeds of its creation in Newtonian mechanics to its death at the hands of Einstein — and the people who pursued it.
An Orbital Mystery
The story begins with Sir Issac Newton. After he laid out his laws of gravitation in Principia in 1687, astronomers and mathematicians carefully measured and mapped the paths of all the planets known to them, and nearly all of them fell in line with the predictions given by Newton’s equations. Indeed, Newton’s laws were so powerful — so universal — that they could even be used to predict where an as-yet-unobserved planet must exist, based on perturbations in the orbits of the known planets. It was by this method that, in 1846, a French mathematician was able to tell a German astronomer exactly where to look to find Neptune — and lo and behold, there it was.
However, Mercury didn’t fall in line. Its orbit doesn’t follow the Newtonian path, and no known planet accounted for the difference. In order to explain this deviation (and perhaps hoping to repeat the victory of the discovery of Neptune), scientists hypothesized a new planet, Vulcan, which orbited between the sun and Mercury and whose gravitational pull would account for Mercury’s oddities. Surely, they thought, there must be a planet there to explain Mercury’s motion. For if there wasn’t, then that would imply that Newton’s laws — a triumph of modern science — were not quite correct.
For decades, astronomers scoured the skies searching for Vulcan. Some swore they had found it, only to have their evidence crumble under deeper scrutiny. The only problem was one fact that every schoolchild knows today — Vulcan doesn’t exist. And without it, nobody could explain Mercury’s orbit. That is, until a young professor in Berlin presented a new theory called “general relativity” that changed the rules of the game.
The Thrill of the Hunt
In a tale spanning centuries, Levenson takes the reader through the development of mathematical theory and the extensive astronomical observations that formed a major part of the Scientific Revolution. It’s a complex subject material, but Leveson distills it down to the key concepts in language that anyone who remembers high school physics can follow, and couches it in page-turning prose. No advanced degrees required. (For those who prefer a little more detail and rigor, Leveson provides brief footnotes that offer a deeper description and/or point to external sources.)
In fact, despite its scientific density, “The Hunt for Vulcan” flows more like a mystery novel. In a kind of cosmic C.S.I., mathematicians put forward hypotheses for where Vulcan might be and astronomers take to their telescopes to try to spot it. For decades, the scientists uncover promising leads that turn into red herrings. The reader can empathize with the Vulcan-hunters, feeling their frustration as each newly-sighted planet turns out to be a dud. After all, Newtonian mechanics form the basis of their universe — everything else seems to follow these laws. The last time a planet refused to behave, another planet was discovered exactly where it needed to be to bring the universe back in line. Why should it be any different for Mercury?
What makes “The Hunt for Vulcan” so engrossing is that it is not just the story of scientific progress — it’s the story of the people behind that progress and their historical context. The cast of characters is a who’s-who of Western science, including Halley, Laplace, Herschel, and Planck. Edison even makes an appearance at a Vulcan search party. And of course, the two titans Newton and Einstein stand as bookends to the story of the planet that wasn’t there. These are more than just caricatures of mythical heroes of science, however. These are human, fallible characters which sometimes have an excess of pride and stubbornness.
There are wrong turns and dead ends, as well as worldly disruptions — not the least of which being the First World War, which broke just as an expedition set out to observe a solar eclipse in order to test Einstein’s theories. Leveson’s writing reminds the reader that our historical hindsight is not available to those who lived through it: “What possible concern could it be for stargazers from Germany, bound for Russia, that on the 28th of June, 1914, in the Serbian city of Sarajevo, an Austrian archduke had got himself shot?”
Theory and Practice
In the end, “The Hunt for Vulcan” stands both as a curious and entertaining tale from the history of science as well as an example of how human stubbornness can sometimes short-circuit the scientific method. We sometimes like to believe that we are better at putting aside old ideas when new evidence comes to light. However, the story of Vulcan shows how even the most scientific among us can cling to old ideas. As Leveson puts it:
We hold this truth to be self-evident: the hard test of nature trumps even the most beloved, battle-tested, long-standing idea.
Does history behave like that? Do human beings?
No. Real life and cherished fables routinely diverge.
Note: This post is adapted from a review I wrote for a journalism course last semester, used with permission of the instructor.