Experimental research into the photoelectric effect in the early 20th century found that the energy of individual emitted electrons was independent of the intensity of incident light and depended instead on its frequency. Planck had proposed that electromagnetic waves were emitted in packets to explain black body radiation. In 1905 Einstein proposed that energy is carried in ‘light quanta’ to eject individual electrons and that the energy of each quantum of light depends on its frequency. Experimental confirmation of the reality of quanta has continued in the 20th and 21st centuries.
All through the 19th century experiments had confirmed the reality of electromagnetic radiation. In 1887 Heinrich Hertz discovered the photoelectric effect shown in 1905 by Einstein to involve quanta of light. The black body radiation emission spectrum investigated by Kirchhoff, and shown to be only dependent of temperature, was explained by Max Planck in 1900 who suggested that electromagnetic energy could only be emitted in quanta. The confirmation in 1905 that the photoelectric effect involves the emitting of electrons whose energy is only dependent on the frequency of the incident radiation could only be understood in terms of quanta of light that are absorbed.
Experiments continued to confirm the reality of quanta. Rutherford’s demonstrated that the atom has a concentrated positively charged nucleus and Niels Bohr set the electrons in definite energy levels. The Stern Gerlach experiment in 1922 revealed that the atom itself is intrinsically quantised with angular momentum only having discrete values. Thus the atomic nucleus, electron energy levels, electron magnetic dipole moment and atomic angular spin momentum were all experimentally demonstrated to involve energy quantisation.
Experimental research into the reality of quanta was developed further by John Clauser in the 1970s, who demonstrated the correlation of two photons emitted in opposite directions from calcium atoms. In the 1980s Alain Aspect demonstrated this entanglement at greater distances. This concurrence of quanta was investigated further by Anton Zeilinger in the 1990s and demonstrated to be a fundamental physical reality. In the 21st century comprehension of these experimental realities of quanta comes from recognising the energies interchanges of the universe that come together and concentrate through quanta immediately and simultaneously as fundamental physical processes.
