The Myth and the Truth

The Reluctant Revolutionary

Max Planck did not believe in the quantum. He called it an act of desperation, spent a decade trying to take it back, and lost almost everything he had to a century he never wanted.

The myth

The story goes like this. In 1900, a German physicist named Max Planck solved a stubborn problem about how hot objects glow, and in solving it discovered that energy comes in packets. He called them quanta. The old physics ended and the new physics began, and Planck — bold, prescient — had lit the fuse. He got a Nobel Prize for it. The constant that governs the entire quantum world carries his name.

Almost every element of that is true. The conclusion drawn from it is close to backwards.

The problem on the desk

The problem was real and it was embarrassing. Heat something — an iron bar, a kiln, a star — and it glows, and the colour of the glow depends on the temperature. Physicists wanted a formula for how much energy came out at each wavelength. They had two, and each worked beautifully over half the range and catastrophically over the other. One was accurate for long wavelengths and predicted, at short wavelengths, that a warm object should radiate infinite energy — a result later nicknamed the ultraviolet catastrophe. The other fit the short wavelengths and failed at the long ones.

This was not an abstract quarrel. The German national metrology institute, the Physikalisch-Technische Reichsanstalt, was measuring blackbody spectra to unusual precision, partly because the German lighting and steel industries wanted a reliable way to judge temperature by colour. Planck had experimentalists down the hall producing curves that no existing theory could reproduce.

In October 1900 he found a formula that fit all of it. He found it, by his own account, essentially by interpolation — he wrote down an expression that reduced to each of the two known laws at the appropriate extreme, and it matched the data. He had the answer before he had any idea why it was the answer.

Then he spent two months trying to derive it, and could not — unless he assumed something he considered absurd. He had to treat the energy of his oscillators as though it came in discrete lumps proportional to frequency, with the constant of proportionality a small new number he called h. He presented the result to the German Physical Society on 14 December 1900.

What he thought he had done

He did not think he had discovered that energy is quantised. He thought he had found a bookkeeping device.

The clearest statement of this comes from Planck himself, in a letter written thirty-one years later to the American physicist Robert Williams Wood, in which he described the introduction of the quantum as "an act of desperation" — adding that he was by nature peaceable and disinclined to questionable adventures, but that he had been prepared to sacrifice any of his previous convictions about physics to get a result. The phrase is the single most quoted thing Planck ever wrote, and it is nearly always quoted as though it were modest. It was not modest. It was accurate, and it was a confession.

What followed is the part the myth omits entirely. For most of the next decade Planck tried to get rid of the quantum. He worked repeatedly to recover his radiation law from classical electrodynamics, to show that h was an artefact of the derivation rather than a fact about the world. He did not succeed, and the failure was not for want of trying.

It was Einstein, in 1905, who took the quantum seriously as physics — arguing that light itself is quantised, which explained the photoelectric effect. Planck resisted that conclusion for years. In 1913, recommending Einstein for membership of the Prussian Academy of Sciences — a recommendation in the warmest terms, and one that mattered — Planck and his co-signatories asked that Einstein not be held too much against for his speculation on light quanta, on the grounds that no genuinely new idea arrives without some risk. He was apologising, on Einstein's behalf, for the idea that would win Einstein the Nobel Prize.

The founder of quantum theory spent a decade as its most distinguished sceptic.

The professor who said physics was finished

There is a second, smaller myth attached to Planck, and it is worth handling because it is usually told wrong in a way that reverses its point.

The version in circulation: young Planck asked a professor whether he should study physics, and was told not to bother, because everything had been discovered and only a few decimal places remained.

The professor was Philipp von Jolly, at Munich, around 1878. Planck recounted the advice himself in later life, and it does appear he was told the field was essentially complete. But the interesting half is his reply, and it is nearly always cut. Planck said he did not wish to discover new things — only to understand the foundations of the field, and perhaps deepen them. He went into physics knowing he had been told there was nothing left to find, and went anyway, because finding things was not the appeal.

Twenty-two years later, that man broke classical physics. The myth is usually told as a lesson about not listening to pessimists. It is better read as a lesson about what temperament actually does: Planck was not looking for a revolution, which may be exactly why he was the one who noticed that the numbers demanded it. Someone hunting novelty would have had a dozen pet theories to protect. He had only the data and an equation that would not behave.

The cost

Planck's public life was one of accumulating honour: Nobel Prize for 1918 (awarded in 1919), the presidency of the Kaiser Wilhelm Society, the position of the most respected physicist in Germany. His private life over the same span is close to unbearable to summarise.

His first wife, Marie Merck, died in 1909, after twenty-two years of marriage. His eldest son, Karl, was killed at Verdun in 1916. His twin daughters, Grete and Emma, each married and each died in childbirth — Grete in 1917, and Emma in 1919, having married her late sister's widower. Planck was sixty-one and had buried three of his four children by his first marriage.

He stayed. In May 1933 he sought a meeting with Hitler to argue for Jewish scientists — the case he pressed was that of Fritz Haber, the chemist whose nitrogen fixation had fed Germany and whose gas warfare had disgraced him, and who was being driven out of the country he had served. The meeting went badly and achieved nothing; Haber died in exile the following year. Planck remained head of the Kaiser Wilhelm Society into 1937, in a position that required public accommodation with a regime dismantling German science, and his conduct through those years has been argued over ever since — as collaboration, as damage limitation, as the paralysis of a patriot who could not conceive of leaving. He was seventy-five when Hitler took power. He had spent his life believing German science was a thing worth preserving institutionally, and he acted on that belief past the point where it made sense.

In 1944 an air raid destroyed his house in Berlin. His library, his papers and his correspondence burned.

In January 1945 his surviving son, Erwin — the one child of his first marriage still living, who had been a minister in the German government in the 1930s — was executed by the Nazis for his part in the 20 July 1944 plot to kill Hitler. Planck was eighty-six. He said afterwards that Erwin had been his most precious possession, and that his life had lost its meaning.

He lived two and a half more years, into 1947, long enough to see quantum mechanics become the central fact of twentieth-century physics.

Why the truth is the better story

The myth gives us a revolutionary — a man who saw further, moved first, and was right. It is a flattering shape and it is available for almost any scientist, which is part of why it is suspicious.

What the record gives us is stranger and more useful. It gives us a conservative by temperament and conviction, working on an unglamorous industrial calibration problem, who arrived at a result he did not want, published it because the numbers required it, and then spent ten years trying to make it go away. The quantum did not enter physics because someone was bold. It entered because someone was honest, and could not construct a way out.

Planck himself left the sharpest formulation of what that costs. In his scientific autobiography, published the year he died, he observed that a new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die and a new generation grows up familiar with it. It is usually compressed into the joke that science advances one funeral at a time, and it is usually quoted as a cynic's line about other people.

It is not. It is a man in his late eighties, who had watched a generation of physicists — including himself — fail to accept an idea he had personally introduced, writing down what he had learned about how belief actually changes. He was not describing his opponents.

He was describing himself, and he knew it.

On sources. Planck's "act of desperation" is from his 1931 letter to Robert Williams Wood; the Jolly exchange and the remark on how scientific truth prevails are from Planck's own late autobiographical writing, and both are recollections set down decades after the events they describe — the Jolly conversation especially should be read as Planck's memory of 1878, not a transcript. The compressed form "science advances one funeral at a time" is not Planck's phrasing and does not appear in his work. Dates of the family deaths and of Erwin's execution are drawn from the standard biographical record.