Experimental Facts Supporting the Quantum Idea
There are many experimental facts underlying the quantum mechanics; and, of course, many were found by experiments intended for testing the quantum idea. Let us see some of the most famous.
(1) Compton Scattering
X-ray is an electromagnetic wave with a very small wavelength. When a monochromatic X-ray (i.e. with a definite wavelength and a frequency) is scattered by an object, we can find a scattered X-ray with a smaller frequency and a longer wavelength than the original ray. This is quite unexpected from the classical electromagnetism. Arthur H. Compton (1892-1962) clarified the reason for this, employing the quantum idea (1923). He essentially argued that if we assume the particulate nature of the X-ray, this fact can be explained quite similarly with the elastic collision of particles. As Einstein already clarified, a definite relationship between the frequency and the momentum (via Planck's constant), and the conservation of momentum are assumed. Compton's discovery was further confirmed and strengthened by other experimental researches, e.g. researches with Wilson's cloud chamber. As a matter of fact, the Nobel physics prize of 1927 was given to Compton and C.T.R. Wilson (1869-1959) . The following figure is adapted from Tomonaga (1969), Fig. 18 on p. 61.
(2) Franck-Hertz Experiment
Bohr's theory postulated stationary states; but what is the justification for this, aside from the theoretical merits produced by that postulate? Franck and Hertz tried to obtain experimental evidence. That is, if stationary states existed, such-and-such phenomena would obtain, and James Franck (1882-1964) and Gustav Hertz (1887-1975) devised an experiment which may show these "such-and-such".
Now, if an atom (via electrons around the nucleus) obtains extra energy by some means, e.g. by the collision of electrons (coming from other sources), its energy level would be raised; and correspondingly, such electrons would lose its energy. Moreover, since the transition between stationary states has a quantum jump, electrons should show a peculiar behavior around the threshold value which is necessary for such quantum jumps. That is, since (E2-E1) is the extra amount of energy for raising the level, if we can find an appropriate method for measuring the energy of electrons (e.g., by measuring the strength of current, which is nothing but a stream of electrons), this energy will show a cyclic up-and-down behavior, and this cycle is determined by (E2-E1); and the latter can be inferred by the study of spectral lines. Franck and Hertz performed this experiment, beginning with mercury gas, with various materials. In the case of mercury, (E2-E1) corresponds to 4.9 volt, and peaks of electric current appear beautifully around 4.9, 9.8, 14.7 volt, etc., as in the following figure.
For details, see Tomonaga (1969), 131-133. Franck and Hertz were awarded the Nobel Prize (physics, 1925) for "their discovery of the laws governing the impact of an electron upon an atom" (from presentation speech). Franck is the same person as the chair of the "Franck Report" (1945) in the history of nuclear weapons.
(3) Stark Effect
If an atom is placed in a uniform electric field (in contrast to a magnetic field, the case of the Zeeman effect), its energy level is changed (due to the field), and the spectral lines also change. Johannes Stark (1874-1957) found this effect with the hydrogen in 1913. Bohr refers to this on p. 38.
(4) Stern-Gerlach's Experiment
Bohr's theory of atomic structure has suggested that azimuthal quantization will occur (see Shells around the Nucleus). Otto Stern (1888-1969) and W. Gerlach conducted an experiment for confirming this (1922). They used a carefully prepared beam of silver atoms, and checked what will happen if this beam is placed in a strong magnetic field. According to Bohr's theory, the magnetic field will affect the direction of the plane of trajectories of the silver atom. The following figure is adapted from Rom Harre's Figure 125 in Harre (1981), and extensively modified.
The result exhibited that the silver beam splits into two different directions, thus showing the expected azimuthal quantization. However, this experiment was performed before the notion of spin was proposed; and later it turned out that the two split lines are due to the spin of the outermost electron of silver. The silver has only one loose electron (its magnetic moment remains without being cancelled by other electrons) in 5s shell, and this electron, if placed in a strong magnetic field, can split its spin either UP or DOWN. The image shows this.
Rom Harre (1981) Great Scientific Experiments, Phaidon Press （邦訳、『世界を変えた２０の科学実験』小出昭一郎・竹内敬人・八杉貞雄訳、産業図書、1984）
Tomonaga, S. (1969) 量子力学 I、みすず書房。
See also a useful site on Microphysics, at Kyushu University: http://www2.kutl.kyushu-u.ac.jp/seminar/MicroWorld/MicroWorld.html
