"His was by nature a conservative mind," inherently suspicious of speculation, wrote Max Born in a 1947 obituary for theoretical physicist Max Karl Ernst Ludwig Planck.
Born 23 April 1858 in the German city of Kiel, Planck was the sixth child in a traditional intellectual family of professors of theology and law. Planck systematically analyzed subjects, mastering his interests rather than jumping to conclusions. He believed that the power of logical reasoning could discover the true nature of physics, and he wanted to do the right thing with his knowledge.
As a student, Planck had broad interests, but physics won out during an era when classical physics was still maturing. Electromagnetism and thermodynamics were young, and doubts remained about atomic theory. Planck chose thermodynamics for his doctoral dissertation, and came to hope that it could help tie together the disparate threads of physics. He saw the second law of thermodynamics, regarding entropy, as central.
In 1889, the University of Berlin hired Planck as a professor of theoretical physics to succeed Gustav Kirchhoff. By the late 1890s, Planck was a full professor, author of a textbook on thermodynamics, and considered the world's premier expert in the field. Still focused on the second law, he wanted to prove that the irreversibility of thermodynamics that drives increasing entropy does not depend explicitly on atomic theory. This line of questioning was not in the mainstream of physics, but Planck thought it could help in unifying chemistry and physics.
Kirchhoff had argued in 1859 that the blackbody spectrum was a fundamental aspect of physics, and Planck hoped that deriving a formula to relate the blackbody spectrum of a body to its temperature could give new insight into the process. He was encouraged in 1896 when his friend and collaborator, Wilhelm Wien of the Physikalisch-Technische Reichsanstalt in Berlin, developed an empirical formula describing blackbody emission across the spectrum as a function of temperature.
As a theorist, Planck wanted to derive the blackbody law rigorously. It took him three years to derive a formula for the entropy of ideal dipole oscillators that were not specific atoms or molecules that seemed to validate Wein's empirical law. However, new measurements of low-frequency emission failed to match calculations using Planck's formula.
In October 1900, Planck went back and tried again, but was unsatisfied with the results. After a month, determined to find a better formula, he tried on an idea of Ludwig Boltzmann's that he did not really like: that entropy was a probabilistic phenomenon rather than deterministic. That gave Planck a new way to calculate the distribution of energy among oscillators, by assuming the energy was "made up of a completely determinate number of finite equal parts," with each of the parts he called quanta having a constant value he calculated at h=6.5510-27 (erg-sec). Multiplying h by the light frequency gave the energy. With that assumption, he derived a formula to calculate the blackbody spectrum as a function of temperature.
At the time, Planck and most other physicists thought his big achievement was solving the troublesome blackbody problem. After publishing two papers, Planck moved onto other things, leaving quanta buried in the details. It would take another troublesome problem and a more daring physicist to put quanta in the spotlight.
Late 19th century experiments revealed that shining light on certain metals caused them to emit light. Electromagnetic theory predicted that the photoelectric effect came from light waves delivering energy gradually until the electron absorbed enough to escape from the atom. Yet experiments by Philipp Lenard found that electrons only escaped when the light waves had frequencies higher than a certain level, no matter how long the metal was illuminated. Doubling the intensity at lower frequencies did nothing, but increasing the frequency could start emission. Something seemed wrong with electromagnetic theory.
When Albert Einstein analyzed the photoelectric effect, he found that quanta were not limited to thermal radiation. Electromagnetic radiation in general delivered energy in discrete quanta. That explained the electromagnetic effect, Stokes' fluorescence, and other previously puzzling observations. Where Planck had viewed quanta as a conceptual tool in his calculations, Einstein saw them as wave packets of energy—what would later become known as "photons."
Einstein published his quantum paper in 1905, the same year as his famous papers on special relativity and Brownian motion. His concept of light quanta stimulated research. Two years later Einstein stimulated more interest by applying quantum theory to the vibrations of atoms and molecules.
Initially, Planck was more interested in Einstein's theory of relativity than in his interpretation of the photoelectric effect. Planck accepted the paradoxes of relativity, and his support helped Einstein and the theory gain wider acceptance. (Einstein's admiration for Planck, on the other hand, took some time to develop.) Yet Planck remained uneasy about applying quantum theory broadly to electromagnetism.
The first Solvay conference in 1911 brought together 21 of the world's leading physicists to discuss quantum theory. Planck found the conference exhausting, and remained hesitant to embrace quantum wholeheartedly. Yet he was excited and gratified by the attention given to his idea, and how it interested new people, most prominently Niels Bohr, who applied quantum theory to atomic structure.
The physics establishment as a whole was slow to accept quantum theory. The 1911 Nobel Prize in Physics went to Wilhelm Wein for his blackbody studies. Only in 1918 did Planck receive the Nobel for "his discovery of energy quanta." Einstein followed in 1921 for his work on the photoelectric effect. In 1922, the Nobel went to Bohr for his quantum work.
Planck continued to work on quantum theory, but Bohr, 27 years younger, became the central figure. Neither Planck nor Einstein ever fully accepted the Copenhagen interpretation of quantum mechanics built on Bohr's work. Einstein drew the line at the idea of quantum entanglement, and Planck kept trying to bridge the gap between quantum and classical mechanics. We may never understand all the weirdness of quantum mechanics, but as Planck predicted in 1911, "The hypothesis of quanta will never vanish from the world."
Jeff Hecht is an SPIE Member and freelancer who writes about science and technology.
