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Niels Bohr [via Werner Heisenberg] on breaking with Newton and the poetry of atoms

Heisenberg prefaces Physics and Beyond with the note that “needless to say, conversations cannot be reconstructed literally after several decades….”

[Niels Bohr, as recalled Heisenberg]: “I realize full well how cautious one has to be with assertions about the structure of atoms. I had best begin by telling you a little about the history of this theory.

“My starting point was not at all the idea that an atom is a small-scale planetary system and as such governed by the laws of astronomy. I never took things as literally as that. My starting point was rather the stability of matter, a pure miracle when considered from the standpoint of classical physics.

“By ‘stability’ I mean that the same substances always have the same properties, that the same crystals recur, the same chemical compounds, etc. In other words, even after a host of changes due to external influences, an iron atom will always remain an iron atom, with exactly the same properties as before. This cannot be explained by the principles of classical mechanics, certainly not if the atom resembles a planetary system.

“The existence of uniform substances, of solid bodies, depends on the stability of atoms; that is precisely why an electron tube filled with a certain gas will always emit light of the same color, a spectrum with exactly the same lines. All this, far from being self-evident, is quite inexplicable in terms of the basic principle of Newtonian physics, according to which all effects have precisely determined causes, and according to which the present state of a phenomenon or process is fully determined by the one that immediately preceded it. This fact used to disturb me a great deal when I first began to look into atomic physics.

“Now, this was really a hopeless task, quite different from those physicists normally tackle. For in all previous physics, or in any other branch of science, you could always try to explain a new phenomenon by reducing it to known phenomena or laws. In atomic physics, however, all previous concepts have proved inadequate. We know from the stability of matter that Newtonian physics does not apply to the interior of the atom; at best it can occasionally offer us a guideline.

It follows that there can be no descriptive account of the structure of the atom; all such accounts must necessarily be based on classical concepts which, as we saw, no longer apply. You see that anyone trying to develop such a theory is really trying the impossible. For we intend to say something about the structure of the atom but lack a language in which we can make ourselves understood. We are in much the same position as a sailor, marooned on a remote island where conditions differ radically from anything he has ever known and where, to make things worse, the natives speak a completely alien tongue. He simply must make himself understood, but has no means of doing so. In that sort of situation a theory cannot ‘explain’ anything in the usual strict scientific sense of the word. All it can hope to do is to reveal connections and, for the rest, leave us to grope as best we can… to do more than that is quite beyond our present means.”

I [Heisenberg] asked: “If that is all we can do, what is the point of all those atomic models you produced and justified during the past few lectures? What exactly did you try to prove with them?”

“These models,” Bohr replied, “have been deduced, or if you prefer guessed, from experiments, not from theoretical calculations. I hope that they describe the structure of the atoms as well, but only as well, as is possible in the descriptive language of classical physics.”

We must be clear that, when it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images and establishing mental connections.”

Werner Heisenberg – Physics and Beyond: Encounters and Conversations

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