Q9. Planck’s Quantum Not Yet A Photon

Max Planck properly deserves credit for introducing the quantum idea—fittingly, in 1900, at the dawn of the century in which special and general relativity and quantum mechanics were to be discovered and would flower. He was led to the idea and to what we now call Planck’s constant by studying the way in which energy is distributed in a closed box within which electromagnetic waves are bouncing back and forth—a cavity, or, in Planck’s original German, a hohlraum (hollow space). The waves themselves carry energy that is continually being exchanged with the energy of atoms in the walls, and the problem Planck set out to solve was how to explain the way in which all the available energy was distributed—some of it in the walls, some of it in the radiation, with that in the radiation divided among waves of different frequencies. This is a complex system with a vast number of atoms and infinitely many wavelengths. Planck found that he could account for the observed behavior of this system and successfully explain how the available energy is shared among the various parts of the system only if he postulated that energy could be transferred from matter to radiation in bundles of a certain size, given by the equation

E = hf ,

where E is the energy in one bundle of radiation—or one photon, as we would say now—and f is the frequency of the radiation. Actually Planck thought only in terms of quantized energy exchange, not in terms of quantized radiation. He continued to believe in a continuous pool of radiant energy.¹ By adjusting the numerical value of his new constant h to get the best agreement between the calculated and measured distribution of energy among different wavelengths, Planck was able to determine h to an accuracy of about 2%.

Einstein took the second radical step. He assumed that the quantum bundle of energy, after being transferred from matter to radiation, retained its quantum identity, behaving as an identifiable “particle” of light (later to be called a photon) that could be reabsorbed by matter only as a whole unit or not at all. Einstein realized that this granular concept of electromagnetic radiation could account beautifully for a different and very much simpler phenomenon than the one studied by Planck. Planck had considered a system composed of a vast number of atoms and, as we know now, a vast number of photons; Einstein considered an experiment involving the elementary event of the absorption of a single photon. In this so-called photoelectric effect, a single electron within a solid substance absorbs all the energy of a single photon and springs forth from the surface of the substance, revealing by its kinetic energy how much energy it has absorbed.²

---

---

⇐ PREVIOUS ESSAY | NEXT ESSAY ⇒