In November 1915, Albert Einstein delivered a series of lectures before the Prussian Academy of Sciences in Berlin, in which he laid out his theory of general relativity. When he stepped down from the podium on November 18, he had upended our conception of gravity and dethroned over two centuries of Newtonian physics. He also destroyed an entire planet.
If you have never heard of the planet Vulcan, you are not alone. Thomas Levenson, the head of MIT's science writing program, digs into the past to tell the story of this non-planet and what it reveals about the process of scientific discovery. Vulcan never existed, but during the nineteenth century astronomers convinced themselves that it must. Why? Because Mercury's orbit exhibits a small precession that the gravity of the sun and known planets cannot explain. Something had to be interfering with Mercury's predicted path. If Newton's theory of gravity was correct, that something was almost certainly another planet, hiding somewhere between Mercury and the sun.
Nineteenth-century astronomers were not wrong to assume that there were undiscovered planets in our solar system. After all, they had a precedent: Neptune. In 1846, Neptune's existence was predicted purely mathematically, well before astronomers observed it in their telescopes. How? The problem lay with Uranus, which Levenson calls a planetary “troublemaker.” Since its discovery in 1781, Uranus was never quite where Newton's laws of gravitation predicted it would be; it was prone to “wandering off course.” This led to the disturbing possibility that there were exceptions to Newton's laws. Yet the very thought that Newton was wrong was anathema; Uranus's orbit represented a crisis of 19th-century scientific belief.
Enter the Parisian mathematician Urbain-Jean-Joseph Le Verrier. He had a solution, albeit a difficult one. Churning through a series of calculations—first with 13 variables, and later a more manageable nine (“which is to say,” quips Levenson, “merely a hugely difficult operation, instead of an impossible one”)—Le Verrier demonstrated that the actual orbit of Uranus, as opposed to its predicted one, made sense if it was under the gravitational influence of another, previously unknown, planet. The Frenchman did the math and then told the young German astronomer Johann Gottfried Galle where to point his telescope. It only took Galle one night's observing from Berlin's Royal Observatory to find it. Voilá! A new planet.
Levenson writes that the discovery of Neptune through Le Verrier's mathematical genius (and Galle's diligent observing), “was the climax of what was almost immediately understood to be the popular triumph of Newtonian science.” And yet the puzzle of Mercury's strange precession could not be solved. Le Verrier tried, others tried, but the planet's erratic orbit resisted the standard Newtonian solution.
So, as with Uranus, it was naturally assumed there was another planet whose mass was causing Mercury's wobble. This planet was given a name—Vulcan—and astronomers set out to find it as well, although it was presumed to be orbiting so close to the sun that it was nearly impossible to see under normal conditions.
From the 1850s to the end of the 1870s, occasional reports of Vulcan sightings were published in scientific journals. But none were confirmed. Even Le Verrier, whose fame (and ego) had reached massive heights, could not crack it. It was simply too hard to find a small planet against the blinding surface of the sun. Total solar eclipses, which would temporarily block the sun's overwhelming brightness, offered the best chance for locating Vulcan. But those came infrequently and lasted for an agonizingly short time—mere minutes. And even when they occurred, Vulcan (assuming it did in fact exist) would have to be out to one side of the sun, not in front or behind it. Talk about a needle in a haystack.
Vulcan's big reveal was expected during the total solar eclipse of July 29, 1878, which occurred over the Rocky Mountains. Astronomers came from all over the world to CO, WY, and TX. Not all of them were looking for Vulcan, but for those who were it was a make-or-break event. A few Vulcan sightings were claimed, but they were nearly as short-lived as the eclipse itself. No one could definitively confirm Vulcan's discovery that summer day in the Rockies, and so for nearly four decades the matter was left alone. Mercury maintained its odd precession, while science maintained its unshakable faith in Newton.
But the problem of Mercury's orbit could not be ignored forever. Either Vulcan would have to be found, or Newtonian gravity would have to be revised—or even abandoned altogether.
That November of 1915, Einstein finally broke the deadlock. At the Prussian Academy of Sciences (less than a mile from where Galle first observed Neptune in 1846), he demonstrated that the curvature of spacetime, and not a hidden planet, convincingly accounted for Mercury's odd orbit. “Vulcan was gone, dead, utterly unnecessary,” Levenson concludes. “The sun with its great mass creates its dent in space-time…until, as Einstein finally captured in all the abstract majesty of his mathematics, the orbit of [Mercury] precesses away from the Newtonian ideal.”
The Hunt for Vulcan is a splendid little book—so much is done so well, and in so few pages, that upon finishing the reader is left with the impression of having read a much longer, much denser work. Levenson also resists the easy temptation to make the story about losers and winners. He does not dismiss 19th century Vulcan hunters as foolish or misguided. Instead, he recognizes that they were working with the best theory then available: Newtonian physics.
Those who study scientific revolutions recognize that scientific theories remain viable only as long as they are able to explain observed phenomena and account for new observations. When a theory ceases to be able to do this—as when Newtonian gravity was unable to explain Mercury's orbital precession—alternative theories are sought. Einstein's general theory of relativity not only accounts for everything Newtonian gravity does, it also explains that which Newton could not, and, in the case of Mercury, without the need for hypothetical planets. For Levenson, then, the death of Vulcan is not so much a cautionary tale as an important scientific milestone. “For more than two centuries humankind lived in the cosmos Newton discovered. Vulcan's nonexistence did not demolish that dwelling place; rather it is the marker on which its passing is written.”
Steve Ruskin received his Ph.D. degree in History and Philosophy of Science from the University of Notre Dame. His is the author of one book and numerous articles on the history of astronomy, including “‘Among the Favored Mortals of Earth’: The Press, State Pride, and the Eclipse of 1878.”