Scientific Autobiography

Overview

Sixty pages, written in 1946 and 1947 when Planck was eighty-eight, and published after his death in October 1947. It is not an autobiography in the ordinary sense. There is almost nothing about his family, his marriages, his politics or the catastrophes he lived through, and a reader who did not know would not learn from it that Germany had fought two world wars.

What it is instead is an account of the problems that occupied him, told in order, by a man reviewing what he had understood. The tone is plain and slightly formal, and the modesty is real rather than performed: Planck describes his central discovery as something forced on him against his inclinations.

The Great Books edition prints it with four lectures, including one on the meaning and limits of exact science and one on the question of free will, and those are worth reading with it because the memoir assumes them.

A little background helps. Planck came from a family of pastors, lawyers and professors, was a good enough pianist to have considered music, and chose physics after being told there was nothing left in it to discover. He became the most respected figure in German science, presided over the Kaiser Wilhelm Society, and used that position through the 1930s in an accommodating way he was criticised for afterwards. His son Erwin was executed in 1945 for involvement in the July plot against Hitler. None of this is in the book.

The argument

The choice of physics. He opens with the advice from Philipp von Jolly at Munich that physics was essentially complete and that a young man should do something else. Planck says this did not trouble him because he had no ambition to discover anything: what drew him was the existence of laws that hold independently of who states them, and understanding those seemed worth a life even if nothing was left to find. The remark sets up everything after it.

Entropy and the absolute. His first real work, and the thing he cared about most, was the second law of thermodynamics. Planck was struck that entropy singles out a direction in time that mechanics does not, and that it therefore points to something absolute rather than conventional.

He recounts, without bitterness but at length, that nobody paid attention. Helmholtz did not read the paper, Kirchhoff disapproved, and Clausius did not answer letters. He later discovered that Josiah Willard Gibbs in America had done much of it already. The passage is a useful corrective to the idea that good work is noticed.

The black-body problem. The central episode. By the late 1890s the distribution of energy in the radiation of a heated cavity was the outstanding problem in physics, and it had the advantage of being independent of any particular material, which is exactly the kind of absolute Planck was drawn to.

Wien’s law fitted the short wavelengths and failed at the long. Rayleigh’s approach fitted the long and diverged at the short. In October 1900, after being shown new infrared measurements, Planck wrote down a formula that interpolated between the two and fitted everything. He had no derivation for it.

He then, as he puts it, set about finding one, and the only way he could get there was to treat the energy of the oscillators as divided into finite portions, proportional to frequency by a constant he called h. He presented this on 14 December 1900. He describes it as a purely formal assumption made because nothing else worked.

The honest part follows. Planck spent the next several years trying to make h go away, to recover the result within classical physics, and he says plainly that the effort failed and cost him a great deal. Einstein took the quantum seriously as a physical fact in 1905, for light itself, and Planck did not accept that step for years. He was the discoverer and the last holdout at once.

Relativity and the constants. He welcomed Einstein’s 1905 paper immediately, and was one of the first established physicists to work on relativity. The reason fits the pattern: relativity removes a privileged human frame of reference and replaces it with an invariant, the speed of light, and Planck read that as another absolute coming into view. He also notes with satisfaction that h, c and G together fix units of length, mass and time that owe nothing to any human convention.

The reality of the world. A section, expanded in the accompanying lectures, arguing against the view that physics is only an economical summary of sense impressions. Planck had admired Mach and turned against him. His argument is that the constants physics has found were not chosen, that they came out of the measurements whether anyone liked them or not, and that this is what having a real world outside us amounts to.

Causality and freedom. The last topic. Planck holds that physical law is causal and that the will is free, and reconciles them by pointing out that no one can predict his own decision in advance, because the act of predicting would itself enter the situation. Causality applies to the past, where an observer can trace it; freedom belongs to the future as the agent faces it. Whether this is a solution or a restatement is the question to take away.

How to read it

It is sixty pages. Read the whole thing in an evening.

Slow down at the black-body section. That is the moment classical physics ends, described by the man who ended it and did not want to, and the detail about the formula preceding the derivation is the important one.

Read the Wood letter about the act of desperation beside it if the edition includes it, and note the difference in register between a private letter and a public memoir.

Read the accompanying lectures. The free will piece and the one on exact science carry the philosophical argument the memoir only sketches.

Keep in mind what the book leaves out. Planck stayed in Germany, kept his positions, made the accommodations that kept the Kaiser Wilhelm Society functioning, lost his son to the resistance, and wrote a memoir about entropy. That silence is part of the document.

Pair it with Heisenberg’s Physics and Philosophy or Bohr’s essays for the contrast. The next generation took the quantum as the nature of things and let go of determinism. Planck never did, and the memoir is the argument he could not win.

Questions it raises

• Planck says opponents of a new theory are not converted but outlived. If that is true, what does it do to the idea of scientific rationality?
• Is there anything wrong with guessing a formula and deriving it afterwards, or is that how discovery usually goes?
• Why did the discoverer of the quantum resist it longer than anyone else?
• Planck argues that universal constants prove a world independent of us. Does it follow?
• His reconciliation of causality and free will turns on what an agent can predict about himself. Is that a real distinction or a trick of perspective?
• What does it mean that a man who lived through both wars wrote his life story and mentioned neither?