[About describing atomic models in the 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.

Contraria sunt complementa.
Opposites are complementary.

~~[Doubtful attribution]~~ Today’s common sense is yesterday’s science.

~~[Misattributed?]~~ Prediction is difficult, especially the future.

A visitor to Niels Bohr's country cottage, noticing a horseshoe hanging on the wall, teasing the eminent scientist about this ancient superstition. “Can it be true that you, of all people, believe it will bring you luck?'
'Of course not,' replied Bohr, 'but I understand it brings you luck whether you believe it or not.'”

An expert is a man who has made all the mistakes, which can be made, in a very narrow field.

An expert is a person who has made all the mistakes that can be made in a very narrow field.

Anyone who is not shocked by the quantum theory has not understood it. [Attributed.]

As regards the co-ordination of all ordinary properties of matter, Rutherford’s model of the atom puts before us a task reminiscent of the old dream of philosophers: to reduce the interpretation of the laws of nature to the consideration of pure numbers.

But, but, but … if anybody says he can think about quantum theory without getting giddy it merely shows that he hasn’t understood the first thing about it!

Causality may be considered as a mode of perception by which we reduce our sense impressions to order.

Every sentence I utter must be understood not as an affirmation but as a question.
[A caution he gives his students, to be wary of dogmatism.]

How wonderful that we have met this paradox. Now we have some hope of making progress.

If we couldn’t laugh at ourselves, that would be the end of everything.

In physics we deal with states of affairs much simpler than those of psychology and yet we again and again learn that our task is not to investigate the essence of things—we do not at all know what this would mean&mash;but to develop those concepts that allow us to speak with each other about the events of nature in a fruitful manner.

One thought [spectra are] marvellous, but it is not possible to make progress there. Just as if you have the wing of a butterfly then certainly it is very regular with the colors and so on, but nobody thought one could get the basis of biology from the coloring of the wing of a butterfly.

Predictions can be very difficult—especially about the future.

Quantum provides us with a striking illustration of the fact that though we can fully understand a connection … we can only speak of it in images and parables. 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.

Science only advances by renouncing its past.

The existence of life must be considered as an elementary fact that can not be explained, but must be taken as a starting point in biology, in a similar way as the quantum of action, which appears as an irrational element from the point of view of classical mechanical physics, taken together with the existence of elementary particles, forms the foundation of atomic physics. The asserted impossibility of a physical or chemical explanation of the function peculiar to life would in this sense be analogous to the insufficiency of the mechanical analysis for the understanding of the stability of atoms.

The old saying of the two kinds of truth. To the one kind belongs statements so simple and clear that the opposite assertion obviously could not be defended. The other kind, the so-called “deep truths”, are statements in which the opposite also contains deep truth.

The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.

The present state of atomic theory is characterized by the fact that we not only believe the existence of atoms to be proved beyond a doubt, but also we even believe that we have an intimate knowledge of the constituents of the individual atoms.

The task of science is both to extend the range of our experience and to reduce it to order.

This interpretation of the atomic number [as the number of orbital electrons] may be said to signify an important step toward the solution of the boldest dreams of natural science, namely to build up an understanding of the regularities of nature upon the consideration of pure number.

Those who are not shocked when they first come across quantum mechanics cannot possibly have understood it.

Truth is something that we can attempt to doubt, and then perhaps, after much exertion, discover that part of the doubt is not justified.

We all agreed that your theory is crazy. The question which divides us is whether it is crazy enough to have a chance of being correct.

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.

What is that we human beings ultimately depend on? We depend on our words. We are suspended in language. Our task is to communicate experience and ideas to others.

When asked … [about] an underlying quantum world, Bohr would answer, “There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about Nature.”

When searching for harmony in life one must never forget that in the drama of existence we are ourselves both actors and spectators.

You must come to Copenhagen to work with us. We like people who can actually perform thought experiments!

Your theory is crazy...but it’s not crazy enough to be true.

When Bohr is about everything is somehow different. Even the dullest gets a fit of brilliancy.

But it is necessary to insist more strongly than usual that what I am putting before you is a model—the Bohr model atom—because later I shall take you to a profounder level of representation in which the electron instead of being confined to a particular locality is distributed in a sort of probability haze all over the atom.

At the beginning of this debate Stephen [Hawking] said that he thinks that he is a positivist, whereas I am a Platonist. I am happy with him being a positivist, but I think that the crucial point here is, rather, that I am a realist. Also, if one compares this debate with the famous debate of Bohr and Einstein, some seventy years ago, I should think that Stephen plays the role of Bohr, whereas I play Einstein's role! For Einstein argued that there should exist something like a real world, not necessarily represented by a wave function, whereas Bohr stressed that the wave function doesn't describe a 'real' microworld but only 'knowledge' that is useful for making predictions.

We [Frisch and Lise Meitner] walked up and down in the snow, I on skis and she on foot (she said and proved that she could get along just as fast that way), and gradually the idea took shape that this was no chipping or cracking of the nucleus but rather a process to be explained by Bohr’s idea that the nucleus was like a liquid drop; such a drop might elongate and divide itself.

You can talk about people like Buddha, Jesus, Moses, Confucius, but the thing that convinced me that such people existed were the conversations with Bohr.
About his time working with Niels Bohr in Copenhagen.

People were pretty well spellbound by what Bohr said… While I was very much impressed by [him], his arguments were mainly of a qualitative nature, and I was not able to really pinpoint the facts behind them. What I wanted was statements which could be expressed in terms of equations, and Bohr's work very seldom provided such statements. I am really not sure how much later my work was influenced by these lectures of Bohr's... He certainly did not have a direct influence because he did not stimulate one to think of new equations.
Recalling the occasion in May 1925 (a year before receiving his Ph.D.) when he met Niels Bohr who was in Cambridge to give a talk on the fundamental difficulties of the quantum theory.

I admired Bohr very much. We had long talks together, long talks in which Bohr did practically all the talking.
Recalling his Sep 1926-Feb 1927 stay in Copenhagen.

Our scientific work in physics consists in asking questions about nature in the language that we possess and trying to get an answer from experiment by the means at our disposal. In this way quantum theory reminds us, as Bohr has put it, of the old wisdom that when searching for harmony in life one must never forget that in the drama of existence we are ourselves both players and spectators. It is understandable that in our scientific relation to nature our own activity becomes very important when we have to deal with parts of nature into which we can penetrate only by using the most elaborate tools.

Physicists speak of the particle representation or the wave representation. Bohr's principle of complementarity asserts that there exist complementary properties of the same object of knowledge, one of which if known will exclude knowledge of the other. We may therefore describe an object like an electron in ways which are mutually exclusive—e.g., as wave or particle—without logical contradiction provided we also realize that the experimental arrangements that determine these descriptions are similarly mutually exclusive. Which experiment—and hence which description one chooses—is purely a matter of human choice.

Bohr’s standpoint, that a space-time description is impossible, I reject a limine. Physics does not consist only of atomic research, science does not consist only of physics, and life does not consist only of science. The aim of atomic research is to fit our empirical knowledge concerning it into our other thinking. All of this other thinking, so far as it concerns the outer world, is active in space and time. If it cannot be fitted into space and time, then it fails in its whole aim and one does not know what purpose it really serves.

After the discovery of spectral analysis no one trained in physics could doubt the problem of the atom would be solved when physicists had learned to understand the language of spectra. So manifold was the enormous amount of material that has been accumulated in sixty years of spectroscopic research that it seemed at first beyond the possibility of disentanglement. An almost greater enlightenment has resulted from the seven years of Röntgen spectroscopy, inasmuch as it has attacked the problem of the atom at its very root, and illuminates the interior. What we are nowadays hearing of the language of spectra is a true 'music of the spheres' in order and harmony that becomes ever more perfect in spite of the manifold variety. The theory of spectral lines will bear the name of Bohr for all time. But yet another name will be permanently associated with it, that of Planck. All integral laws of spectral lines and of atomic theory spring originally from the quantum theory. It is the mysterious organon on which Nature plays her music of the spectra, and according to the rhythm of which she regulates the structure of the atoms and nuclei.

Nobody knows how the stand of our knowledge about the atom would be without him. Personally, [Niels] Bohr is one of the amiable colleagues I have met. He utters his opinions like one perpetually groping and never like one who believes himself to be in possession of the truth.

[Niels Bohr] is a national pride to his fellow Danes. In Denmark, Bohr’s standing is only slightly less than that of the royal family and Hans Christian Anderson. When the wife of an American physicist casually told a gentleman seated next to her on a Copenhagen streetcar that her husband was studying under Professor Bohr, the old man jumped to his feet, swept off his hat with a flourish and bowed deeply.

[Student describing Niels Bohr's main gift, the ability to synthesize:] Like Socrates, he wages a fight to bring harmony out of chaos and diversity.

[About the great synthesis of atomic physics in the 1920s:] It was a heroic time. It was not the doing of any one man; it involved the collaboration of scores of scientists from many different lands. But from the first to last the deeply creative, subtle and critical spirit of Niels Bohr guided, restrained, deepened and finally transmuted the enterprise.

Relations between authors and referees are, of course, almost always strained. Authors are convinced that the malicious stupidity of the referee is alone preventing them from laying their discoveries before an admiring world. Referees are convinced that authors are too arrogant and obtuse to recognize blatant fallacies in their own reasoning, even when these have been called to their attention with crystalline lucidity. All physicists know this, because all physicists are both authors and referees, but it does no good. The ability of one person to hold both views is an example of what Bohr called complementarity.

It might interest you that when we made the experiments that we did not read the literature well enough—and you know how that happens. On the other hand, one would think that other people would have told us about it. For instance, we had a colloquium at the time in Berlin at which all the important papers were discussed. Nobody discussed Bohr’s paper. Why not? The reason is that fifty years ago one was so convinced that nobody would, with the state of knowledge we had at that time, understand spectral line emission, so that if somebody published a paper about it, one assumed “probably it is not right.” So we did not know it.

The Bohr atom was introduced to us by Bohr himself. I still have the notes I took during his lectures … His discourse was rendered almost incomprehensible by his accent; there were endless references to what I recorded as “soup groups”, only later emended to “sub-groups”.

Two years ago 1 tried to appeal to Rockefeller’s conscience about the absurd method of allocating grants, unfortunately without success. Bohr has now gone to see him, in an attempt to persuade him to take some action on behalf of the exiled German scientists.

I remember discussions with Bohr which went through many hours till very late at night and ended almost in despair; and when at the end of the discussion I went alone for a walk in the neighboring park I repeated to myself again and again the question: Can nature possibly be as absurd as it seemed to us in these atomic experiments?

The ingenious but nevertheless somewhat artificial assumptions of [Bohr’s model of the atom], … are replaced by a much more natural assumption in de Broglie’s wave phenomena. The wave phenomenon forms the real “body” of the atom. It replaces the individual punctiform electrons, which in Bohr’s model swarm around the nucleus.