"The Hunt for Vulcan": Theory, experiment, and the origin of scientific revolutions

Joseph Urbain La Verrier: The force of his personality
and his spectacular prediction of Neptune solidified
faith in the existence of Vulcan

In his book "The Hunt for Vulcan", MIT science writing professor Thomas Levenson tackles one of the most central questions in all of science - what do you do when a fact of nature disagrees with your theory? In this particular case the fact of nature was an anomaly in the orbit of Mercury around the sun. The theory was Newton's successful theory of gravitation which had reigned supreme for two hundred years in explaining the motion of everything from rocks to the moon. Levenson’s book looks at this question through the lens of an important case study. His writing is clear, often elegant and impressionistic, and he does a good job driving home the nature of science as a human activity with all its human triumphs and follies.

The physical entity invoked to explain the anomalies in Mercury's orbit - a small planet close to the sun which would usually be too small and intensely illuminated by the sun to be seen - was called Vulcan. The idea was that Vulcan's gravitational tug on Mercury would cause its orbit to stray from the expected path. The hypothesis had much merit to it since it was similar theorizing about the anomalies in the predicted orbit of Uranus that had resulted in the discovery of Neptune. The man who proposed the theories of both Neptune and Vulcan was Joseph Urbain Le Verrier, the most important French astronomer of his day and one of the most important of the 19th century. The successful prediction of Neptune and its dazzlingly swift observational validation was a resounding tribute to both Le Verrier’s acumen and to Newton’s understanding of the universe. Not surprisingly Le Verrier's prediction of Vulcan was taken seriously.

The book recounts how partly because of past successes of Newton's theories and partly because of the force of Le Verrier’s personality astronomers spent the next one hundred years unsuccessfully looking for Vulcan. Spectators included a host of well-known astronomers and amateurs, including Thomas Edison. The search was peppered by expeditions to exotic places like Rhodesia and Wyoming. Occasionally the newspapers would ridicule Vulcan-chasers, but none could disprove its evidence conclusively. This fact raises an important point: As far as scientific theories go Vulcan was a good theory since it was testable, but because its existence really strained the limits of astronomical technique as it existed during the time, it did not really satisfy the criteria for being a cleanly falsifiable theory. This led to the Vulcan hypothesis having enough wiggle room for people to get away with explaining away the lack of observation as bad technique or faulty equipment.

As Levenson describes in the latter half of the book, the culmination of the hunt for Vulcan came in the early half of the twentieth century with Einstein’s theory of relativity which did away with Vulcan for good. Levenson spends a good deal of time on Einstein's background and his mathematical preparation; there's a lucid description of the special theory of relativity. Vulcan was almost an afterthought in Einstein's intellectual development, but when he realized that his own theory could explain Mercury's anomalous orbit as an effect of the curvature of spacetime, the realization left him feeling like "something had snapped inside him". When finished his general theory of relativity demonstrated one of the most fascinating features of scientific discoveries – sometimes tiny anomalies in observation point not just to the reworking of an existing theory but a complete overhaul of our understanding of nature. In this case the dramatic change was an appreciation of gravity not as a force but as a curvature of spacetime itself.

It is also instructive to apply lessons from Vulcan to my own fields of drug discovery and biochemistry. Often when a drug does not work it seems convenient to invoke the existence of hitherto unobserved entities (specific proteins, artifacts, side products from organic reactions etc.) to explain the anomalies or failures. Vulcan tells us that while it is prudent to look for these entities experimentally, it's also worth giving a thought to how their existence might be explained by tweaks - or in rare cases significant overhauls - of existing theories of biological signaling or drug action. This might especially be true in case of neurological disorders like Alzheimer's disease where the causes are ill-understood and the underlying theories (the amyloid hypothesis for instance) are constantly being subjected to revision.

Levenson’s book is a tribute to how science actually works as opposed to how it's thought to work. It's also a good instruction manual for how science works when experiment disagrees with theory. In such cases the theory can then be slightly amended, radically amended or replaced. In Vulcan’s case Newtonian gravity was not really replaced, but the amendment required was so drastic that it led to a new epoch in our view of our cosmos. The story of Vulcan is a story for our scientific times.