1900

Quantum Mechanics

Max Planck (1858–1947), Albert Einstein (1879–1955)

What is light? The question has perplexed philosophers and physicists for millennia. Aristotle and his followers thought it to be a wavelike disturbance propagating through the air, while followers of Democritus subscribed to a so-called atomist theory, which posited that light existed as particles. Debates about light’s wave-particle duality permeated Renaissance physics as well: Isaac Newton believed that only corpuscles (particles) of light could explain its behavior in optics; Christiaan Huygens held just as firmly that light must be wavelike, because it required a medium to travel through and refract. It was upon this confusing stage that late-nineteenth-century physicists began to propose a fundamental paradigm shift in our scientific understanding of the very nature of matter.

The revolution began with a mathematical trick of sorts by the German physicist Max Planck. Planck was trying to understand why objects of a given temperature, whether atoms or molecules or stars, radiate and absorb energy, and why they sometimes produce distinct bright emission lines or dark absorption lines in their spectra. Planck’s turn-of-the-century trick was to assume that light (or, equally, radiation or energy) could only be emitted or absorbed by matter in discrete packets called quanta, whose energies depend only on the frequency or wavelength of the light.

Planck’s quantization of energy led to a detailed “energy level” theory of electrons orbiting an atomic nucleus, put forth by physicists such as Niels Bohr (1885–1962). In the Bohr model of energy levels in an atom (such as the sodium atom depicted here), electrons gaining and losing energy between levels explain the bright and dark lines in atomic spectra (lower right) at discrete wavelengths.

To Planck, the quantization of energy was a simple mathematical assumption needed to solve an equation (like the physics joke “Assume a spherical cow . . . ”), not necessarily an expression of any physical reality. But his contemporaries, including the physicist Albert Einstein, saw a deeper truth in Planck’s work, proposing that light consists of quanta of energy called photons, whose interactions with matter follow wavelike equations. Rather than a conundrum, the wave-particle duality of light became a fundamental tenet of an entirely new branch of physics called quantum mechanics.

Max Planck at his desk in an undated photograph.