Although the language of science strives for precision, there are terms that are used solely in their historical context even when other meanings may be more familiar. Because of this, it is easy to introduce an ambiguity into discussion. Take the term law, for example. In a legal sense, a law is a body of rules governing the affairs of a community. When applied to the natural world, the word law has the connotation of unwavering fact: a law of science tells you that this is the way that the world works. In the 17th century, when Newton devised his laws of motion and gravitation, the predictive success of this work was unprecedented. As far as could be practically determined at that time, Newton’s laws always held. In fact, in 1846 these laws were responsible for the discovery of one of the planets—two hundred years after Newton’s time. By analyzing the inconsistencies in the orbit of the planet Uranus, the Parisian astronomer Leverrier was able to predict the position of the as yet unseen eighth planet from the Sun, Neptune. Ultimately, however, unresolvable inconsistencies did turn up between Newton’s laws and observed phenomena in the solar system. For example, the orbit of Mercury deviates slightly from that predicted by Newton’s laws. By historical precedent, however, Newton’s laws retained their names. New theories were put forward to account for these inconsistencies, as well as other problems in physics. The most successful theories to date have been Einstein’s Theories of Special and General Relativity. These works account for the discrepancies in Mercury’s orbit; they also predict many other new phenomena not even imagined in Newton’s time. Thus, we have the curious state of affairs where Newton’s constructions are referred to as laws and Einstein’s constructions are referred to as theories, even though Einstein’s theories have enjoyed far more predictive success than Newton’s laws.
