The Dancing Universe: From Creation Myths to the Big Bang

8 | Of Things Small

❍ 8 OF THINGS SMALL Heaven knows what seeming nonsense may not tomorrow be demonstrated truth. —Alfred North Whitehead (1925) ight, or more generally, electromagnetic waves, posed other challenges for classical physics. We have seen that light emitted and absorbed by chemical elements and analyzed by a spectroscope allowed physicists to study the chemical composition of the Sun and distant nebulae. However, as the nineteenth century drew to a close, no one knew why different elements have different spectra , or why spectra existed in the first place. To make things worse, there was no proper explanation as to why certain objects, such as iron pokers or lamp filaments, glow different colors when they are heated to different temperatures. In the end, the answer to this deceptively simple question held the key to a profound revolution in physics. It is a story worth telling, not only because of the wide repercussions of quantum physics to our understanding of the Universe big and small, but also because it serves as an excellent example of how progress in physics often advances in tortuous ways. THE COLOR OF HEAT We all know that if we heat up an iron poker to sufficiently high temperatures, say in our grandmother’s fireplace, it will eventually L start glowing in a reddish tone. If your grandma gets a really powerful fire going, increasing the temperature will make the poker glow yellower, and at even higher temperatures, the poker will emit a bluish glow. (Actually, this will depend on the type of material, since iron melts before turning blue.) An electric range is also a great laboratory for seeing how hot things glow. As you crank up the dial, the invisible heat (infrared radiation) emanating from the metallic spiral becomes visible, gradually changing from a faint to a very strong orange-red glow. Classical physicists could understand this behavior by marrying thermodynamics with Maxwell’s electromagnetism . If the poker is made of electric charges that somehow vibrate (there was no model of the atom yet!), then, as it gets hotter the charges will vibrate faster, emitting radiation of higher frequency . Since blue has higher frequency than red, the hotter the poker, the bluer the glow. So far so good. As more detailed questions were asked, though, classical physics began to flounder. Soon it became clear that new ideas were desperately needed, but no one had a clue where to start. As unexpectedly as the Michelson-Morley experiment, the familiar hot-red poker had turned into a nightmare. Consider the first obstacle to be faced: Objects made of different materials and of different shapes have different thermal properties . During the late 1850s, about the same time he was investigating the chemical composition of the Sun (another hot body that glows!), the German physicist Gustav Kirchhoff suggested a method physicists could use to study the radiation emitted from a hot body without worrying about its composition, shape, or size. Little did he know that a deep conceptual revolution underlay his clever idea. The object Kirchhoff suggested was a closed cavity, like the interior of an oven or a kiln, which he would then heat to some temperature T. Since heat induces motion, the molecules making up the walls of the cavity would move about, collide, and emit electromagnetic radiation into the cavity. In turn, the electromagnetic radiation in the cavity would be reabsorbed by the walls, and a dance of equilibrium between emission and absorption would quickly be established. Kirchhoff showed that since emission and absorption “cancelled each other out,” the spectrum inside the cavity would include no spectral lines (all chemical signatures were erased), and thus it could not depend on the OF THINGS SMALL 213 shape, size, or material the cavity was made of. Since a perfectly absorbing surface is black, while a perfectly reflecting surface is white, Kirchhoff’s cavity, which absorbed all the heat it received without emitting any, became known as a blackbody. In an oven maintained at temperature T, blackbody radiation escapes through a small hole in one of the walls. In order to study what kind of radiation was inside the cavity, Kirchhoff made a tiny hole to allow some of it to leak out. The resulting spectrum, called blackbody spectrum, displays electromagnetic radiation of all frequencies, each carrying a certain amount of energy with it. In a rough sense, the interior of the cavity is like a choppy ocean...