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The endurance of the Heisenberg mystery is nothing short of astonishing. There is a rich paper trail documenting almost everything he did in Germany; there are hundreds of pages of secret recordings made without his knowledge; there are countless interviews with people who knew him well; there are memoirs written by his wife, volumes from his own hand, commentary from his associates. And yet the mystery remains unsolved, a black hole of intrigue that continues to pull historians tightly into its orbit. What exactly was Heisenberg doing in Germany during World War II?

Some questions remain long after their owners have died. Lingering like ghosts. Looking for the answers they never found in life.

Margrethe Bohr, in Copenhagen, Act 1

The story of Werner Heisenberg starts out as the story of the development of quantum mechanics, the most complex intellectual revolution in the history of physics and arguably the most amazing theory in the history of science. No other theory has been as difficult to develop, as complex to interpret, and as metaphysically suggestive as quantum mechanics.

Quantum theory emerged as an attempt to understand electromagnetic radiation, of which light is the most familiar example. At the turn of this century, radiation was understood to be a wave, and we still use terms such as "radio waves" for one particular portion of the electromagnetic spectrum. But Max Planck discovered in 1900 that energy was also, in some strange sense, a particle. A hot object, as it cooled, emitted what seemed to be particles of energy; we call them photons now. The question naturally arose, Was light then a particle or a wave?

Strangely, the answer depended on the context in which one was observing or measuring it. One could do "wave physics"—pass waves around posts or through openings that would give rise to interference patterns— and the radiation would always behave like a wave. One could do "particle physics"—have particles hit detectors or go through various openings—and the radiation would always behave like a particle. All this happened without apparent contradiction, for one could design no experimental setup that would simultaneously look at waves and particles. This dual character of electromagnetic radiation became known as the "wave-particle duality."

A French historian-turned-physicist, Prince Louis de Broglie argued, in a seven-page doctoral dissertation so short, so speculative, and so odd that his advisor sent it to Einstein for review, that maybe this "wave-particle duality" of light could clarify the nature of electrons. De Broglie's suggestion, later borne out by experiment and rewarded with a Nobel Prize, was that the electron, which had always been thought of as a particle, might also be, in some way that could not be pictured, a wave. Just as wave-like radiation had suddenly taken on a new particle-like aspect, so the particle-like electron was adding a new wave-like aspect. Long live symmetry and the ghost of Pythagoras.

De Broglie's wave-particle duality became a cornerstone of the new quantum physics and Janus-like progress occurred as physicists alternately uncovered waveness in nature and then particleness, but never both together. Along the way it became clear that these small bits of reality—the photons and the electrons—really could not be modeled in any meaningful sense of that term, and that reality—itself now a complex concept—simply could not be pictured at the level of the very small.

The atom, for example, was not appropriately modeled as a miniature solar system, as had been recently popular (and as introductory texts continued to picture the atom even decades after the quantum revolution). Familiar "orbits" in which little electronic "planets" circled heavy nuclear "suns," were replaced by "orbitals"—ephemeral, wave-like clouds of purely mathematical probability, wrapped around and even through the nucleus of the atom, like fog on a lake.

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