In October 1926, Schrödinger arrived by train in Copenhagen. Niels Bohr was at the station to welcome him and the two began talking immediately as they made their way to the tram. Bohr had invited Schrödinger up north to give a lecture on the “Foundation of Undulatory Mechanics,” although Bohr did not believe a word of the theory. Proving the mathematical equivalence of all existing forms of quantum mechanics had merely intensified the ferocity of the dispute between the partisans on either side. Quantum physicists had become like philosophers who agree on every point about, say, archaeology, excepting only whether it should be called a science or not. With so much authority resting on so slender a hook the disputants showed no mercy.
Heisenberg had become Bohr’s chief assistant and he witnessed the quarrel. It went on for days while Schrödinger was a guest in Bohr’s house. “Although Bohr was otherwise most considerate and amiable in his dealings with people,” Heisenberg recalled, “he now appeared to me almost as an unrelenting fanatic.” Schrödinger had begun to face the full Bohr treatment, the sort that had eventually turned Kramers into Bohr’s lapdog.
Bohr had, from the beginning of his days as Rutherford’s student, come at quanta from the material side of things. He had been the one
to link quanta and atoms. Schrödinger came at quanta from another point, arriving via de Broglie, Bose, Einstein, and ultimately Planck and his little chunk of radiation, hυ. Einstein’s E = mc2 meant that ultimately atoms and radiation were like matrix algebra and wave mechanics—different forms of the same thing. But they did not feel like the same thing and the people who came at quanta from these separate directions had different tastes.
Bohr liked the jumps of quantum mechanics and their defiance of geometry.
“If all this damned quantum jumping were really here to stay,” Schrödinger gave voice to his exasperation, “then I should be sorry I ever got involved with quantum theory.”
Bohr, Heisenberg said, “was not prepared to make a single concession to his discussion partner.”
Schrödinger held his ground. Wave theory had formidable strengths. In his mechanics, matter and energy passed through space in the usual way of moving continuously from point to point and without ever disappearing down a rabbit hole.
Heisenberg appears to have prudently stood aside during this dispute, although wave theory’s lack of physical surprises was the very thing he disliked most. The world of quanta seemed too weird to be accounted for in the ordinary way of space, time, and motion.
Bohr and Schrödinger continued disputing relentlessly, like nineteenth-century boxers before there were limits set to the number of rounds. Neither had a knockout punch left in him, but neither was willing to throw in a towel.
Bohr rather liked Max Born’s suggestion of a probabilistic interpretation of the psi wave.
Schrödinger would have none of this rejection of physical explanation.
Bohr thought particles could not be described as wave packets because wave packets become unstable over time.
Schrödinger thought that matter jumping from point to point—“liberated” from the constraints of space and time—was absurd.
“It will hardly be possible,” Heisenberg recalled, “to convey the
intensity of the passion with which the discussions were conducted on both sides.”
Bohr would not, according to Heisenberg, “tolerate the slightest obscurity,” on Schrödinger’s part, but, of course, as usual, Bohr himself was deeply obscure.
Schrödinger said Bohr talked “often for minutes almost in a dreamlike, visionary and really quite unclear manner, partly because he is so full of consideration and constantly hesitates—fearing that the other might take a statement of his point of view as an insufficient appreciation of the other’s.”
Bohr argued like a thoughtful dentist who is persistent in his drilling but regularly interrupts his labor to ask, “Does it hurt? Let me know if it hurts.”
After days of this treatment, perhaps by coincidence, perhaps not, Schrödinger became ill and had to stay in bed. Mrs. Bohr proved as faithful a nurse as Walt Whitman, attending the guest regularly, and spoon-feeding him broth. All the while, Bohr sat nearby speaking of the folly of taking wave mechanics literally as a picture of what really goes on in the world.
And when it was time for Schrödinger to leave Copenhagen, neither side had conceded any ground. Neither side could answer all the objections of the other, and neither could give a complete account of how his favored system was supposed to work. That was the way of it during quantum’s heroic era: the toolkit grew considerably while the meaning of it all was as sharply disputed as the meaning of a close election.