Warped Passages - Lisa Randall [62]
Figure 41. In Compton scattering, a photon (γ) scatters off a stationary electron (e-) and emerges with a different energy and momentum.
It’s perplexing that Einstein was so resistant to the quantum theory that he helped to develop. But his reaction is no more remarkable than Planck’s response to Einstein’s quantization proposal—which was disbelief. Planck and several others praised Einstein’s many achievements, but qualified their enthusiasm.* Planck even said, somewhat disparagingly, “That he missed the target in his speculations, as, for example, in his hypothesis of light-quanta, cannot really be held too much against him, for it is not possible to introduce really new ideas even in the most exact sciences without sometimes taking a risk.”† Make no mistake. Einstein’s conjectured light-quanta were right on target. Planck’s comment merely reflects the revolutionary nature of Einstein’s insight and the initial reluctance of scientists to accept it.
Quantization and the Atom
The story of quantization and the old quantum theory didn’t end with light. It turns out that all matter consists of fundamental quanta. Niels Bohr was next in line with a quantization hypothesis. In his case, he applied it to a well-established particle, the electron.
Bohr’s interest in quantum mechanics developed, in part, from attempts at the time to clarify the atom’s mysterious properties. During the nineteenth century, the notion of an atom was unbelievably vague: many scientists didn’t believe that atoms existed other than as heuristic devices that were a useful tool but which had no grounding in reality. Even some of the scientists who did believe in atoms nonetheless confused them with molecules, which we now know to be composites of atoms.
The atom’s true properties and composition were not accepted until the beginning of the last century. Part of the problem was that the Greek word “atom” meant a thing that could not be divided, and the original picture of the atom was indeed one of an unchanging, indivisible object. But as nineteenth-century physicists learned more about how atoms behave, they began to realize that this idea had to be incorrect. By the end of the century, radioactivity and spectral lines, the specific frequencies at which light is emitted and absorbed, were some of the best-measured properties of atoms. Yet both of these phenomena showed that atoms could change. On top of that, in 1897, J.J. Thomson identified electrons and proposed that the electron was an ingredient of the atom, which meant that atoms had to be divisible.
At the beginning of the twentieth century, Thomson synthesized the atomic observations of the time in his “plum pudding” model, named after the British dessert containing isolated pieces of fruit stuck in a bready blob. He suggested that there was a positively charged component spread throughout the atom (the bready part), with negatively charged electrons (the pieces of fruit) embedded inside.
The New Zealander Ernest Rutherford proved this model wrong in 1910, when Hans Geiger and a research student, Ernest Marsden, performed an experiment that Rutherford had suggested. They discovered a hard, compact atomic nucleus, much smaller than the atom itself. Radon-222, a gas produced in the radioactive decay of radium salts, emits alpha particles, which we now know to be helium nuclei. The physicists revealed the existence of the atom’s nucleus by shooting alpha particles at atoms and recording the angles at which the alpha particles scattered. The dramatic scattering they recorded could arise only if there were a hard, compact atomic nucleus. A diffuse, positive charge spread throughout the extent of the atom could never have scattered the particles so widely. In Rutherford’s words, “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”*
Rutherford’s results disproved the