User:Johnjbarton/sandbox/introduction to quantum mechanics
History
Maxwell's unification of electricity, magnetism, and even light in the 1880s lead to experiments on the interaction of light and matter. Some of these experiments had aspects which could not be explained. Quantum mechanics emerged in the early part of the 20th century from efforts to explain these results.[1]
Evidence of quanta from the photoelectric effect
The seeds of the quantum revolution appear in the discovery by JJ Thomson that cathode rays were actually "corpuscles" or particles now called electrons. Since no solid theory of cathode rays existed, the electron was exciting news, but not a revolution. However, in 1905 Einstein proposed that light was also corpuscular, consisting of "energy quanta", seemingly in contradiction to the established science of light as a continuous wave, stretching back a hundred years to Young's work on diffraction. Light quanta would be revolutionary.
Einstein's evidence was twofold. First he analyzed blackbody radiation. Hot objects radiate heat; very hot objects – red hot, white hot objects – all look similar when heated to the same temperature. This temperature dependent "look" results from a common curve of light intensity at different frequencies (colors). The common curve is called blackbody radiation. The lowest frequencies are invisible heat rays – infrared light. White hot objects have intensity across many colors in the visible range. Continuous wave theories of light and matter cannot explain the blackbody radiation curve. Einstein showed that, by assuming that light energy transferred in discrete "energy quanta", the radiation curve could be explained. Max Planck showed the same result five years earlier, but he did not propose that the light was quantized.
Einstein's second evidence connected to JJ Thomson's electron. In 1902, Philipp Lenard directed light from an arc lamp onto freshly cleaned metal plates housed in an evacuated glass tube. Then he measured the electric current coming off the metal plate, for higher and lower intensity of light and for different metals. This is the photoelectric effect. Lenard showed that amount of current – the number of electrons – depended on the intensity of the light, but that the velocity of these electrons did not depend on intensity.
The continuous wave theories of the time would predict that the electron would be accelerated to higher velocity if the light intensity was increased. Einstein's energy quanta explained the increase: one electron is ejected for each quanta: more quanta means more electrons.
Einstein then predicted that the electron velocity would increase in direct proportion to the light frequency above a fixed value that depended upon the metal. Here the idea is that energy in energy-quanta depends upon the light frequency; the energy transferred to the electron comes in proportion to the light frequency. The type of metal gives a barrier, the fixed value, that the electrons must climb over to exit and be measured.
Ten years elapsed before Einstein's prediction was completely verified. During that time many scientists rejected the revolutionary idea of quanta. But Planck's and Einstein's concept was in the air and soon affected other theories.