Convincing evidence of the particle nature of electromagnetic radiation was found in 1922 by the American physicist Arthur Holly Compton. While investigating the scattering of X-rays, he observed that such rays lose some of their energy in the scattering process and emerge with slightly decreased frequency. This energy loss increases with the scattering angle, θ, measured from the direction of an unscattered X-ray. This so-called Compton effect can be explained, according to classical mechanics, as an elastic collision of two particles comparable to the collision of two billiard balls. In this case, an X-ray photon of energy hν and momentum hν/c collides with an electron at rest. The recoiling electron was observed and measured by Compton and Alfred W. Simon in a Wilson cloud chamber. If one calculates the result of such an elastic collision by using the relativistic formulas for the energy and momentum of the scattered electron, one finds that the wavelength of an X-ray after (λ′) and before (λ) the scattering event differs by λ′ − λ = (h/mc)(1 − cos θ). Here m is the rest mass of the electron and h/mc is called Compton wavelength. It has the value 0.0243 angstrom. The energy hν of a photon of this wavelength is equal to the rest mass energy mc² of an electron. One might argue that electrons in atoms are not at rest, but their kinetic energy is very small compared with that of energetic X-rays and can be disregarded in deriving Compton’s equation.

Resonance absorption and recoil

During the mid-1800s the German physicist Gustav Robert Kirchhoff observed that atoms and molecules emit and absorb electromagnetic radiation at characteristic frequencies and that the emission and absorption frequencies are the same for a given substance. Such resonance absorption should, strictly speaking, not occur if one applies the photon picture due to the following argument. Since energy and momentum have to be conserved in the emission process, the atom recoils to the left as the photon is emitted to the right, just as a cannon recoils backward when a shot is fired. Because the recoiling atom carries off some kinetic recoil energy E_R, the emitted photon energy is less than the energy difference of the atomic energy states by the amount E_R. When a photon is absorbed by an atom, the momentum of the photon is likewise transmitted to the atom, thereby giving it a kinetic recoil energy E_R. The absorbing photon must therefore supply not only the energy difference of the atomic energy states but the additional amount E_R as well. Accordingly, resonance absorption should not occur because the emitted photon is missing 2E_R to accomplish it.

Nevertheless, ever since Kirchhoff’s finding, investigators have observed resonance absorption for electronic transitions in atoms and molecules. This is because for visible light the recoil energy E_R is very small compared with the natural energy uncertainty of atomic emission and absorption processes. The situation is, however, quite different for the emission and absorption of gamma-ray photons by nuclei. The recoil energy E_R is more than 10,000 times as large for gamma-ray photons as for photons of visible light, and the nuclear energy transitions are much more sharply defined because their lifetime can be one million times longer than for electronic energy transitions. The particle nature of photons therefore prevents resonance absorption of gamma-ray photons by free nuclei.

In 1958 the German physicist Rudolf Ludwig Mössbauer discovered that recoilless gamma-ray resonance absorption is, nevertheless, possible if the emitting as well as the absorbing nuclei are embedded in a solid. In this case, there is a strong probability that the recoil momentum during absorption and emission of the gamma photon is taken up by the whole solid (or more precisely by its entire lattice). This then reduces the recoil energy to nearly zero and thus allows resonance absorption to occur even for gamma rays.

Wave-particle duality

How can electromagnetic radiation behave like a particle in some cases while exhibiting wavelike properties that produce the interference and diffraction phenomena in others? This paradoxical behaviour came to be known as the wave-particle duality. Bohr rejected the idea of light quanta, and he searched for ways to explain the Compton effect and the photoelectric effect by arguing that the momentum and energy conservation laws need to be satisfied only statistically in the time average. In 1923 he stated that the hypothesis of light quanta excludes, in principle, the possibility of a rational definition of the concepts of frequency and wavelength that are essential for explaining interference.

The following year the conceptual foundations of physics were shaken by the French physicist Louis de Broglie, who suggested in his doctoral dissertation that the wave-particle duality applies not only to light but to a particle as well. De Broglie proposed that any object has wavelike properties. In particular, he showed that the orbits and energies of the hydrogen atom, as described by Bohr’s atomic model, correspond to the condition that the circumference of any orbit precisely matches an integral number of wavelengths λ of the matter waves of electrons. Any particle such as an electron moving with a momentum p has, according to de Broglie, a wavelength λ = h/p. This idea required a conceptual revolution of mechanics, which led to the wave and quantum mechanics of Erwin Schrödinger, Werner Heisenberg, and Max Born.

De Broglie’s idea of the wavelike behaviour of particles was quickly verified experimentally. In 1927 Clinton Joseph Davisson and Lester Germer of the United States observed diffraction and hence interference of electron waves by the regular arrangement of atoms in a crystal of nickel. That same year S. Kikuchi of Japan obtained an electron diffraction pattern by shooting electrons with an energy of 68 keV through a thin mica plate and recording the resultant diffraction pattern on a photographic plate. The observed pattern corresponded to electron waves having the wavelength predicted by de Broglie. The diffraction effects of helium atoms were found in 1930, and neutron diffraction has today become an indispensable tool for determining the magnetic and atomic structure of materials.

The interference pattern that results when a radiation front hits two slits in an opaque screen is often cited to explain the conceptual difficulty of the wave-particle duality. Consider an opaque screen with two openings A and B, called a double slit, and a photographic plate or a projection screen, as shown in Figure 9. A parallel wave with a wavelength λ passing through the double slit will produce the intensity pattern on the plate or screen as shown at the right of the figure. The intensity is greatest at the centre. It falls to zero at all locations x₀, where the distances to the openings A and B differ by odd-number multiples of a half wavelength, as, for instance, λ/2, 3λ/2, and 5λ/2. The condition for such destructive interference is the same as for Michelson’s interferometer illustrated in Figure 4. Whereas a half-transparent mirror in Figure 4 divides the amplitude of each wave train in half, the division in Figure 9 through openings A and B is spatial. The latter is called division of wave front. Constructive interference or intensity maxima are observed on the screen at all positions whose distances from A and B differ by zero or an integer multiple of λ. This is the wave interpretation of the observed double-slit interference pattern.

The description of photons is necessarily different because a particle can obviously only pass through opening A or alternatively through opening B. Yet, no interference pattern is observed when either A or B is closed. Both A and B must be open simultaneously. It was thought for a time that one photon passing through A might interfere with another photon passing through B. That possibility was ruled out after the British physicist Geoffrey Taylor demonstrated in 1909 that the same interference pattern can be recorded on a photographic plate even when the light intensity is so feeble that only one photon is present in the apparatus at any one time.

Another attempt to understand the dual nature of electromagnetic radiation was to identify the photon with a wave train whose length is equal to its coherence length cτ, where τ is the coherence time, or the lifetime of an atomic transition from a higher to a lower internal atomic energy state, and c is the light velocity. This is the same as envisioning the photon to be an elongated wave packet, or “needle radiation.” Again, the term “photon” had a different meaning for different scientists, and wave nature and quantum structure remained incompatible. It was time to find a theory of electromagnetic radiation that would fuse the wave theory and the particle theory. Such a fusion was accomplished by quantum electrodynamics (QED).