Reading Discovery

One of the things that scientific theories are supposed to do is describe reality. If theory A says things should happen a certain way and they don't, then something about theory A is off. The theory might be mistaken, or the measurements and calculations on which it's based overlooked something important. If checking and more re-checking, followed up by new measurements or even new experiments, doesn't show the results, then it may be time to scrap theory A even though it's been accepted for hundreds of years and is the basis for most of the way people understand things.

All of these things happened when astronomers in the middle 19th century noticed that Mercury's orbit around the sun shifted a little bit each time. The story is described in Thomas Levenson's 2015 The Hunt for Vulcan.

The perihelion precession seen when Mercury transited the sun during an eclipse should not happen if Mercury was following Isaac Newton's laws of motion. Well, everything followed Newton's laws of motion, so obviously some other factor had to be affecting Mercury's orbit. French mathematician Urbain Le Verrier had a pretty good idea what it was: Another planet inside Mercury's orbit, whose own mass goofed up that orbit. Since the same theory had led to Le Verrier's discovery of Neptune, it seemed to make sense. He worked out how big the planet would have to be and about where it would have to be found in order to produce Mercury's precession. It even had a name: Vulcan. The only problem was that decades of observation failed to find any evidence Vulcan existed. Although some astronomers claimed to see an intra-Mercury planet transit the sun during several eclipses during the second half of the 19th century none of them were firm enough to convince scientists such a planet existed. They convinced Le Verrier, though, because Newton's laws had been proven over and over again on earth as well as in space. His death in 1877 did not end the search or the frustration over the unsolvable puzzle of properly measuring Mercury's orbit. The idea that Mercury's orbit just jinked around on its own was plainly illogical.

Not until Albert Einstein's theory of relativity was published in 1915 was the mystery solved. Einstein described gravity completely differently than did the classical Newtonian equations, and when his theory was used to predict Mercury's motion as observed during a 1919 eclipse, it matched precisely. The need for the planet Vulcan disappeared (at least until Gene Roddenberry wanted a name for a race of extraterrestrials 45 years or so later).

Levenson outlines the whole saga clearly and doesn't take the attitude that we modern folks have any reason to look down our noses at Le Verrier and the others who believed they had found Vulcan. He effectively highlights the frustration most observers felt at their inability to see something that they knew had to be there. Trying to see something that close to the sun was hard enough anyway, and having effective observation limited to the brief minutes of solar eclipses made it even harder. Telescopic technology of the time produced irritating false leads that didn't hold up when the next chance to look came around.

The Hunt for Vulcan is a great telling of how something everyone knows to be true is...until it isn't. Einstein wasn't so much trying to explain Mercury's motion as he was working out some aspects of his theory of special relativity that he found incomplete. In fact, that prediction was one of the few real-world applications of general relativity until about 1960, when it became the foundation of astronomical observations of peculiar new phenomena like black holes and neutron stars. Hunt is an excellent example of what happens when scientists get to say what science fiction giant Isaac Asimov says are their favorite words. Not, "Eureka!" but, "Hmm. That's odd."

Vulcan doesn't have a place in the actual map of the solar system since it doesn't exist. But its postulation and the story of the hunt for it serves as an excellent cautionary tale about being too sure that everything about anything is really known for certain.