Warped Passages - Lisa Randall [61]
Five years later, in 1905, Einstein made a major contribution to quantum theory when he established that light quanta were real things, not merely mathematical abstractions. Einstein was a very busy man that year, developing special relativity, helping to prove that atoms and molecules exist by studying the statistical properties of matter, and providing a validation of quantum theory—all while he was working at the Swiss patent office in Bern.
The particular observation that Einstein interpreted using the hypothesis of light quanta, thereby enhancing its credibility, is known as the photoelectric effect. Experimenters shone a single frequency of radiation onto matter, and that incoming radiation propelled electrons out. Experiments had shown that bombarding material with more light, which carries more total energy, did not change the maximum kinetic energy (energy of motion) of the emitted electrons. This is contrary to what intuition might suggest: larger incident energy should surely produce electrons with larger kinetic energy. The limit on the electron’s kinetic energy was therefore a puzzle. Why didn’t the electron absorb more energy?
Einstein’s interpretation was that radiation consists of individual quanta of light, and only a single quantum will donate its energy to any particular electron. Light is delivered to an individual electron like a single missile, not like a blitzkrieg. Because only one quantum of light ejects the electron, more incident quanta would not change the energy of the emitted electron. Increasing the number of incident quanta makes the light eject more electrons, but it doesn’t influence the maximum energy of any particular electron.
Once Einstein interpreted the results of the photoelectric effect in terms of these definite packets of energy—the quantized units of light—it made sense that the emitted electrons always had the same maximum kinetic energy. The most kinetic energy an electron can have is the fixed energy that it receives from the quantum of light minus the energy required to eject the electron from the atom.
Using this logic, Einstein could deduce the energy of the light quanta. He found that their energy depended on the frequency of the incident light exactly as Planck’s hypothesis predicted. To Einstein, this was clear evidence that light quanta were real. His interpretation gave a very concrete picture of light quanta: a single quantum hit a single electron, which it thereby ejected. It was this observation and not relativity that earned Einstein the Nobel Prize for Physics in 1921.
Oddly enough, however, although Einstein acknowledged the existence of quantized units of light, he was reluctant to accept that these quanta were actually massless particles, which carried energy and momentum but had no mass. The first convincing evidence for the particle nature of the quanta of light came from the 1923 measurement of Compton scattering, in which a quantum of light hits an electron and is deflected (see Figure 41). In general, you can determine a particle’s energy and momentum by measuring its deflection angle after a collision. If photons were massless particles, they would behave in a well-defined manner when they collided with other particles such as electrons. Measurements showed that the quanta of light behaved precisely as if the quanta were massless particles that interacted with the electrons. The inexorable conclusion was