Knocking on Heaven's Door - Lisa Randall [16]
PHOTONS AND LIGHT
The story of theories of light nicely exemplifies the ways in which effective theories are used as science evolves, with some ideas being discarded while others are retained as approximations appropriate to their specified domains. From the time of the ancient Greeks, people studied light with geometrical optics. It is one of the topics any aspiring physics graduate student is tested on when taking the physics GRE (the exam that is a prerequisite for graduate school). This theory assumes that light travels in rays or lines and tells you how those rays behave as they travel through different media, as well as how instruments use and detect them.
The strange thing is that virtually no one—at least no one at Harvard where I now teach and was once a student—actually studies classical and geometrical optics. Maybe geometrical optics is taught a little bit in high school, but it is certainly no big part of the curriculum.
Geometrical optics is an old-fashioned subject. It hit its heyday several centuries ago with Newton’s famous Opticks, continuing into the 1800s when William Rowan Hamilton made what is perhaps the first real mathematical prediction of a new phenomenon.
The classical theory of optics still applies to areas such as photography, medicine, engineering, and astronomy, and is used to develop new mirrors, telescopes, and microscopes. Classical optical scientists and engineers work out different examples of various physical phenomena. However, they are simply applying optics—not discovering new laws.
In 2009, I was honored to be asked to give the Hamilton lecture at the University of Dublin—a lecture several of my most respected colleagues had given before me. It is named after Sir William Rowan Hamilton, the remarkable nineteenth-century Irish mathematician and physicist. I confess that the name Hamilton is so universally present in physics that I foolishly didn’t initially make the connection with an actual person who was in fact Irish. But I was fascinated by the many areas of math and physics that Hamilton had revolutionized, including, among them, geometrical optics.
The celebration of Hamilton Day is really quite something. The day’s activities include a procession down the Royal Canal in Dublin where everyone stops at the Broom Bridge to watch the youngest member of the party write down the same equations on the bridge that Hamilton, in the excitement of discovery, had many years past carved into the bridge’s side. I visited the College Observatory of Dunsink where Hamilton lived and got to see the pulleys and wooden structure of a telescope from two centuries ago. Hamilton arrived there after his graduation from Trinity College in 1827, when he was made the chair of astronomy and Astronomer Royal of Ireland. Locals joke that despite Hamilton’s prodigious mathematical talent, he had no real knowledge of or interest in astronomy, and that despite his many theoretical advances, he might have set back observational astronomy in Ireland fifty years.
Hamilton Day nonetheless pays homage to this great theorist’s many accomplishments. These included advances in optics and dynamics, the invention of the mathematical theory of quaternions (a generalization of complex numbers), as well as definitive demonstrations of the predictive power of math and science. The development of quaternions