Niels Bohr

Niels Henrik David Bohr (7 October 1885 – 18 November 1962) was a Danish physicist. He received the Nobel Prize for Physics in 1922 for his contributions which were essential to modern understandings of atomic structure and quantum mechanics.

Quotes



 * Those who are not shocked when they first come across quantum theory cannot possibly have understood it.
 * In a 1952 conversation with Heisenberg and Pauli in Copenhagen; quoted in Heisenberg, Werner, Physics and Beyond. (New York: Harper & Row, 1971) p. 206.


 * 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.
 * In his first meeting with Werner Heisenberg in early summer 1920, in response to questions on the nature of language, as reported in Discussions about Language (1933); quoted in Defense Implications of International Indeterminacy (1972) by Robert J. Pranger, p. 11, and Theorizing Modernism : Essays in Critical Theory (1993) by Steve Giles, p. 28


 * The grand discoveries which scientific experiment yielded at and about the turn of the century, in which investigators in many countries took an eminent part and which were destined all unexpectedly to give us a fresh insight into the structure of atoms, were due in the first instance, as all are aware, to the work of the great investigators of the English school, Sir Joseph Thomson and Sir Ernest Rutherford, who have inscribed their names on the tablets of the history of scientific research as distinguished witnesses to the truth that imagination and acumen are capable of penetrating the crowded mass of registered experience and of revealing Nature's simplicity to our gaze.
 * Niels Bohr's speech at the Nobel Banquet in Stockholm (December 10, 1922)


 * Even the mathematical framework helps nothing, I would first like to understand how Nature avoids the contradictions. (1927)
 * Quoted in Werner Heisenberg: Die Sprache der Atome (2010) by H. Rechenberg, p. 564.


 * The great extension of our experience in recent years has brought light to the insufficiency of our simple mechanical conceptions and, as a consequence, has shaken the foundation on which the customary interpretation of observation was based.
 * Niels Bohr, "Atomic Physics and the Description of Nature" (1934)


 * Isolated material particles are abstractions, their properties being definable and observable only through their interaction with other systems.
 * "Atomic Physics and the Description of Nature" (1934)


 * What is it that we humans depend on? We depend on our words... Our task is to communicate experience and ideas to others. We must strive continually to extend the scope of our description, but in such a way that our messages do not thereby lose their objective or unambiguous character ... We are suspended in language in such a way that we cannot say what is up and what is down. The word "reality" is also a word, a word which we must learn to use correctly.
 * Quoted in Philosophy of Science Vol. 37 (1934), p. 157, and in The Truth of Science : Physical Theories and Reality (1997) by Roger Gerhard Newton, p. 176


 * For a parallel to the lesson of atomic theory regarding the limited applicability of such customary idealizations, we must in fact turn to quite other branches of science, such as psychology, or even to that kind of epistemological problems with which already thinkers like Buddha and Lao Tzu have been confronted, when trying to harmonize our position as spectators and actors in the great drama of existence.
 * Speech on quantum theory at Celebrazione del Secondo Centenario della Nascita di Luigi Galvani, Bologna, Italy (October 1937)


 * Contraria Sunt Complementa
 * Opposites are complementary.
 * Motto he chose for his coat of arms, when granted the Danish Order of the Elephant in 1947.


 * However far the phenomena transcend the scope of classical physical explanation, the account of all evidence must be expressed in classical terms. The argument is that simply by the word "experiment" we refer to a situation where we can tell others what we have done and what we have learned and that, therefore, the account of the experimental arrangement and of the results of the observations must be expressed in unambiguous language with suitable application of the terminology of classical physics.
 * Niels Bohr, "Discussions with Einstein on Epistemological Problems in Atomic Physics," in, Albert Einstein: Philosopher Scientist (1949) pp. 199-241.


 * An expert is a person who has found out by his own painful experience all the mistakes that one can make in a very narrow field.
 * As quoted by Edward Teller, in Dr. Edward Teller's Magnificent Obsession by Robert Coughlan, in LIFE magazine (6 September 1954), p. 62
 * Variant: An expert is a man who has made all the mistakes which can be made in a very narrow field.
 * As quoted by Edward Teller (10 October 1972), and A Dictionary of Scientific Quotations (1991) by Alan L. Mackay, p. 35


 * We are all agreed that your theory is crazy. The question that divides us is whether it is crazy enough to have a chance of being correct.
 * Said to Wolfgang Pauli after his presentation of Heisenberg's and Pauli's nonlinear field theory of elementary particles, at Columbia University (1958), as reported by F. J. Dyson in his paper "Innovation in Physics" (Scientific American, 199, No. 3, September 1958, pp. 74-82; reprinted in "JingShin Theoretical Physics Symposium in Honor of Professor Ta-You Wu," edited by Jong-Ping Hsu & Leonardo Hsu, Singapore; River Edge, NJ: World Scientific, 1998, pp. 73-90, here: p. 84).
 * Your theory is crazy, but it's not crazy enough to be true.
 * As quoted in First Philosophy: The Theory of Everything (2007) by Spencer Scoular, p. 89
 * There are many slight variants on this remark:
 * We are all agreed that your theory is crazy. The question which divides us is whether it is crazy enough.
 * We are all agreed that your theory is crazy. The question is whether it is crazy enough to be have a chance of being correct.
 * We in the back are convinced your theory is crazy. But what divides us is whether it is crazy enough.
 * Your theory is crazy, the question is whether it's crazy enough to be true.
 * Yes, I think that your theory is crazy. Sadly, it's not crazy enough to be believed.


 * Physics is to be regarded not so much as the study of something a priori given, but rather as the development of methods of ordering and surveying human experience. In this respect our task must be to account for such experience in a manner independent of individual subjective judgement and therefore objective in the sense that it can be unambiguously communicated in ordinary human language.
 * "The Unity of Human Knowledge" (October 1960)


 * Every valuable human being must be a radical and a rebel, for what he must aim at is to make things better than they are.
 * As quoted in The World of the Atom (1966) by Henry Abraham Boorse and Lloyd Motz, p. 741


 * How wonderful that we have met with a paradox. Now we have some hope of making progress.
 * As quoted in Niels Bohr : The Man, His Science, & the World They Changed (1966) by Ruth Moore, p. 196


 * Two sorts of truth: profound truths recognized by the fact that the opposite is also a profound truth, in contrast to trivialities where opposites are obviously absurd.
 * As quoted by his son Hans Bohr in "My Father", published in Niels Bohr: His Life and Work (1967), p. 328
 * Unsourced variant: The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.
 * As quoted in Max Delbrück, Mind from Matter: An Essay on Evolutionary Epistemology, (1986) p. 167. It is the hallmark of any deep truth that its negation is also a deep truth


 * Every sentence I utter must be understood not as an affirmation, but as a question.
 * As quoted in A Dictionary of Scientific Quotations (1991) by Alan L. Mackay, p. 35


 * It is a great pity that human beings cannot find all of their satisfaction in scientific contemplativeness.
 * As quoted in Chandra: A Biography of S. Chandrasekhar‎ (1991) by Kameshwar C. Wali, p. 147


 * Anyone who is not shocked by quantum theory has not understood it.
 * As quoted in Meeting the Universe Halfway (2007) by Karen Michelle Barad, p. 254, with a footnote citing The Philosophical Writings of Niels Bohr (1998).
 * Variants: Those who are not shocked when they first come across quantum mechanics cannot possibly have understood it. Those who are not shocked when they first come across quantum theory cannot possibly have understood it. Anyone who is not shocked by quantum theory has not understood a single word. If you think you can talk about quantum theory without feeling dizzy, you haven't understood the first thing about it.


 * Some subjects are so serious that one can only joke about them.
 * As quoted in The Genius of Science: A Portrait Gallery (2000) by Abraham Pais, p. 24
 * Some things are so serious that one can only joke about them.
 * Variant without any citation as to author in Denial is not a river in Egypt (1998) by Sandi Bachom, p. 85.


 * Truth and clarity are complementary.
 * As quoted in Quantum Theory and the Flight from Realism : Philosophical Responses to Quantum Mechanics (2000) by Christopher Norris, p. 234


 * It is not enough to be wrong, one must also be polite.
 * As quoted in The Genius of Science: A Portrait Gallery (2000) by Abraham Pais, p. 24


 * Never express yourself more clearly than you are able to think.
 * As quoted in Values of the Wise : Humanity's Highest Aspirations (2004) by Jason Merchey, p. 63


 * Oh, what idiots we all have been. This is just as it must be.
 * In response to Frisch & Meitner's explanation of nuclear fission, as quoted in The Physicists - A generation that changed the world (1981) by C.P.Snow, p. 96


 * I go into the Upanishads to ask questions.
 * As quoted in God Is Not One : The Eight Rival Religions That Run the World and Why Their Differences Matter (2010), by Stephen Prothero, Ch, 4 : Hinduism : The Way of Devotion, p. 144


 * No, no, you are not thinking, you are just being logical.
 * In response to those who made purely formal or mathematical arguments, as quoted in What Little I Remember (1979) by Otto Robert Frisch, p. 95


 * I am absolutely prepared to talk about the spiritual life of an electronic computer: to state that it is reflecting or is in a bad mood... The question whether the machine really feels or ponders, or whether it merely looks as though it did, is of course absolutely meaningingless. 
 * As quoted in a letter written from J. Kalckar to John A. Wheeler dated June 10, 1977, which appears in Wheeler's "Law Without Law," pg 207.

Remarks after the Solvay Conference (1927)

 * Statements of Bohr after the Solvay Conference of 1927, as quoted in Physics and Beyond (1971) by Werner Heisenberg


 * I feel very much like Dirac: the idea of a personal God is foreign to me. But we ought to remember that religion uses language in quite a different way from science. The language of religion is more closely related to the language of poetry than to the language of science. True, we are inclined to think that science deals with information about objective facts, and poetry with subjective feelings. Hence we conclude that if religion does indeed deal with objective truths, it ought to adopt the same criteria of truth as science. But I myself find the division of the world into an objective and a subjective side much too arbitrary. The fact that religions through the ages have spoken in images, parables, and paradoxes means simply that there are no other ways of grasping the reality to which they refer. But that does not mean that it is not a genuine reality. And splitting this reality into an objective and a subjective side won't get us very far.

In quantum mechanics the departure from this ideal has been even more radical. We can still use the objectifying language of classical physics to make statements about observable facts. For instance, we can say that a photographic plate has been blackened, or that cloud droplets have formed. But we can say nothing about the atoms themselves. And what predictions we base on such findings depend on the way we pose our experimental question, and here the observer has freedom of choice. Naturally, it still makes no difference whether the observer is a man, an animal, or a piece of apparatus, but it is no longer possible to make predictions without reference to the observer or the means of observation. To that extent, every physical process may be said to have objective and subjective features. The objective world of nineteenth-century science was, as we know today, an ideal, limiting case, but not the whole reality. Admittedly, even in our future encounters with reality we shall have to distinguish between the objective and the subjective side, to make a division between the two. But the location of the separation may depend on the way things are looked at; to a certain extent it can be chosen at will. Hence '''I can quite understand why we cannot speak about the content of religion in an objectifying language. The fact that different religions try to express this content in quite distinct spiritual forms is no real objection. Perhaps we ought to look upon these different forms as complementary descriptions which, though they exclude one another, are needed to convey the rich possibilities flowing from man's relationship with the central order.'''
 * I consider those developments in physics during the last decades which have shown how problematical such concepts as "objective" and "subjective" are, a great liberation of thought. The whole thing started with the theory of relativity. In the past, the statement that two events are simultaneous was considered an objective assertion, one that could be communicated quite simply and that was open to verification by any observer. Today we know that 'simultaneity' contains a subjective element, inasmuch as two events that appear simultaneous to an observer at rest are not necessarily simultaneous to an observer in motion. However, the relativistic description is also objective inasmuch as every observer can deduce by calculation what the other observer will perceive or has perceived. For all that, we have come a long way from the classical ideal of objective descriptions.


 * In mathematics we can take our inner distance from the content of our statements. In the final analysis mathematics is a mental game that we can play or not play as we choose. Religion, on the other hand, deals with ourselves, with our life and death; its promises are meant to govern our actions and thus, at least indirectly, our very existence. We cannot just look at them impassively from the outside. Moreover, our attitude to religious questions cannot be separated from our attitude to society. Even if religion arose as the spiritual structure of a particular human society, it is arguable whether it has remained the strongest social molding force through history, or whether society, once formed, develops new spiritual structures and adapts them to its particular level of knowledge. Nowadays, the individual seems to be able to choose the spiritual framework of his thoughts and actions quite freely, and this freedom reflects the fact that the boundaries between the various cultures and societies are beginning to become more fluid. But even when an individual tries to attain the greatest possible degree of independence, he will still be swayed by the existing spiritual structures — consciously or unconsciously. For he, too, must be able to speak of life and death and the human condition to other members of the society in which he's chosen to live; he must educate his children according to the norms of that society, fit into its life. Epistemological sophistries cannot possibly help him attain these ends. Here, too, the relationship between critical thought about the spiritual content of a given religion and action based on the deliberate acceptance of that content is complementary. And such acceptance, if consciously arrived at, fills the individual with strength of purpose, helps him to overcome doubts and, if he has to suffer, provides him with the kind of solace that only a sense of being sheltered under an all-embracing roof can grant. In that sense, religion helps to make social life more harmonious; its most important task is to remind us, in the language of pictures and parables, of the wider framework within which our life is set.

Disputed



 * Anyone who is not shocked by quantum theory has not understood it.
 * Heisenberg recounts a personal conversation he had with Pauli and Bohr in 1952 in which Bohr says, "Those who are not shocked when they first come across quantum theory cannot possibly have understood it." Heisenberg, Werner, Physics and Beyond. (New York: Harper & Row, 1971) p. 206.
 * Bohr said this sentence in a conversation with Werner Heisenberg, as quoted in: "Der Teil und das Ganze. Gespräche im Umkreis der Atomphysik" . R. Piper & Co., München, 1969, S. 280. DIE ZEIT 22. Aug. 1969.
 * As quoted in Meeting the Universe Halfway (2007) by Karen Michelle Barad, p. 254, with the quote attributed to The Philosophical Writings of Niels Bohr, but with no page number or volume number given.
 * David Mermin, on pages 186–187 of his book Boojums All the Way Through: Communicating Science in a Prosaic Age (1990) noted that he specifically looked for pithy quotes about quantum mechanics along these lines when reviewing the three volumes of The Philosophical Writings of Niels Bohr, but couldn't find any: "Once I tried to teach some quantum mechanics to a class of law students, philosophers, and art historians. As an advertisement for the course I put together the most sensational quotations I could collect from the most authoritative practitioners of the subject. Heisenberg was a goldmine: 'The concept of the objective reality of the elementary particles has thus evaporated...'; 'the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them ... is impossible ...' Feynman did his part too: 'I think I can safely say that nobody understands quantum mechanics.' But I failed to turn up anything comparable in the writings of Bohr. Others attributed spectacular remarks to him, but he seemed to take pains to avoid any hint of the dramatic in his own writings. You don't pack them into your classroom with 'The indivisibility of quantum phenomena finds its consequent expression in the circumstance that every definable subdivision would require a change of the experimental arrangement with the appearance of new individual phenomena,' or 'the wider frame of complementarity directly expresses our position as regards the account of fundamental properties of matter presupposed in classical physical description but outside its scope.' I was therefore on the lookout for nuggets when I sat down to review these three volumes – a reissue of Bohr's collected essays on the revolutionary epistemological character of the quantum theory and on the implications of that revolution for other scientific and non-scientific areas of endeavor (the originals first appeared in 1934, 1958, and 1963.) But the most radical statement I could find in all three books was this: '...physics is to be regarded not so much as the study of something a priori given, but rather as the development of methods for ordering and surveying human experience.' No nuggets for the nonscientist."
 * Variants: Those who are not shocked when they first come across quantum mechanics cannot possibly have understood it. Those who are not shocked when they first come across quantum theory cannot possibly have understood it. Anyone who is not shocked by quantum theory has not understood a single word. If you think you can talk about quantum theory without feeling dizzy, you haven't understood the first thing about it.


 * Prediction is very difficult, especially about the future.
 * As quoted in Teaching and Learning Elementary Social Studies (1970) by Arthur K. Ellis, p. 431
 * The above quote is also attributed to various humourists and the Danish poet Piet Hein: "det er svært at spå – især om fremtiden"
 * It is also attributed to Danish cartoonist Storm P (Robert Storm Petersen).
 * Variant: It's hard to make predictions, especially about the future.


 * Stop telling God what to do with his dice.
 * A response to Einstein's assertion that "God doesn't play dice"; a similar statement is attributed to Enrico Fermi
 * Variant: Einstein, don't tell God what to do.
 * Variant: Don't tell God what to do with his dice.
 * Variant: You ought not to speak for what Providence can or can not do. – As described in The Physicists: A generation that changed the world (1981) by C. P. Snow, p. 84


 * Of course not ... but I am told it works even if you don't believe in it.
 * Reply to a visitor to his home in Tisvilde who asked him if he really believed a horseshoe above his door brought him luck, as quoted in Inward Bound : Of Matter and Forces in the Physical World (1986) by Abraham Pais, p. 210
 * In most published accounts of this anecdote such was Bohr's reply to his friend, but in one early account, in The Interaction Between Science and Philosophy (1974) by Samuel Sambursky, p. 357, Bohr was at a friend's house and asked "Do you really believe in this?" to which his friend replied "Oh, I don't believe in it. But I am told it works even if you don't believe in it."
 * Variant: No, but I'm told it works even if you don't believe in it.
 * Possibly originated by Samuel Goudsmit

Quotes about Bohr

 * Alphabetized by author


 * Bohr seemed to think that he had solved this question. I could not find his solution in his writings. But there was no doubt that he was convinced that he had solved the problem and, in so doing, had not only contributed to atomic physics, but to epistemology, to philosophy, to humanity in general. And there are astonishing passages in his writings in which he is sort of patronizing to the ancient Far Eastern philosophers, almost saying that he had solved the problems that had defeated them. It's an extraordinary thing for me—the character of Bohr—absolutely puzzling. I like to speak of two Bohrs: one is a very pragmatic fellow who insists that the apparatus is classical, and the other is a very arrogant, pontificating man who makes enormous claims for what he has done.
 * John S. Bell, quoted in Jeremy Bernstein, Quantum Profiles (1991), John Stewart Bell: Quantum Engineer


 * One of the favorite maxims of my father was the distinction between the two sorts of truths, profound truths recognized by the fact that the opposite is also a profound truth, in contrast to trivialities where opposites are obviously absurd.
 * Hans Henrik Bohr, writing about his father in "My father" in Niels Bohr - His Life and Work As Seen By His Friends and Colleagues (1967), S. Rozental, ed.


 * If quantum theory has any philosophical importance at all, it lies in the fact that it demonstrates for a single, sharply defined science the necessity of dual aspects and complementary considerations. Niels Bohr has discussed this question with respect to many applications in physiology, psychology, and philosophy in general.
 * Max Born in Natural Philosophy of Cause and Chance (1949) ch. 10, p. 127


 * Not often in life has a human being caused me such joy by his mere presence as you did.
 * Albert Einstein in a letter to Bohr (1920)


 * It is practically impossible to describe Niels Bohr to a person who has never worked with him. Probably his most characteristic property was the slowness of his thinking and comprehension. When, in the late twenties and early thirties, the author of this book was one of the "Bohr boys" working in his Institute in Copenhagen on a Carlsberg (the best beer in the world!) fellowship, he had many a chance to observe it. In the evening, when a handful of Bohr's students were "working" in the Paa Blegdamsvejen Institute, discussing the latest problems of the quantum theory, or playing Ping-pong on the library table with coffee cups placed on it to make the game more difficult, Bohr would appear, complaining that he was very tired, and would like to "do something." To "do something" inevitably meant to go to the movies, and the only movies Bohr liked were those called The Gun Fight at the Lazy Gee Ranch or The Lone Ranger and a Sioux Girl. But it was hard to go with Bohr to the movies. He could not follow the plot, and was constantly asking us, to the great annoyance of the rest of the audience, questions like this: "Is that the sister of that cowboy who shot the Indian who tried to steal a herd of cattle belonging to her brother-in-law?" The same slowness of reaction was apparent at scientific meetings. Many a time, a visiting young physicist (most physicists visiting Copenhagen were young) would deliver a brilliant talk about his recent calculations on some intricate problem of the quantum theory. Everybody in the audience would understand the argument quite clearly, but Bohr wouldn't. So everybody would start to explain to Bohr the simple point he had missed, and in the resulting turmoil everybody would stop understanding anything. Finally, after a considerable period of time, Bohr would begin to understand, and it would turn out that what he understood about the problem presented by the visitor was quite different from what the visitor meant, and was correct, while the visitor's interpretation was wrong.
 * George Gamow on Niels Bohr in "The Great Physicists from Galileo to Einstein" (1961) pg. 237


 * 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 neighbouring park I repeated to myself again and again the question: Can nature possibly be so absurd as it seemed to us in these atomic experiments?
 * Werner Heisenberg in Physics and Philosophy (1958)


 * The first thing Bohr said to me was that it would only then be profitable to work with him if I understood that he was a dilettante. The only way I knew to react to this unexpected statement was with a polite smile of disbelief. But evidently Bohr was serious. He explained how he had to approach every new question from a starting point of total ignorance. It is perhaps better to say that Bohr's strength lay in his formidable intuition and insight rather than erudition.
 * Abraham Pais, in testimony in Niels Bohr : His Life and Work as Seen by His Friends and Colleagues (1967) edited by Stefan Rozental, p. 218; later in his own work, Niels Bohr's Times : In Physics, Philosophy, and Polity (1991)


 * When asked whether the algorism of quantum mechanics could be considered as somehow mirroring 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." Bohr felt that every step in the development of physics has strengthened the view that the problem of establishing an unambiguous description of nature has only one solution. He regarded all attempts to replace our elementary concepts or to introduce a new logic to account for the peculiarities of quantum phenomena as not merely unnecessary but also incompatible with our most fundamental conditions, since we are suspended in a unique language.
 * Aage Petersen, "The philosophy of Niels Bohr" by in the Bulletin of the Atomic Scientists Vol. 19, No. 7 (September 1963); The Genius of Science: A Portrait Gallery (2000) by Abraham Pais, p. 24, and Niels Bohr: Reflections on Subject and Object (2001) by Paul. McEvoy, p. 291
 * Quotes about quote:
 * To my great pleasure, Victor Weisskopf was sitting in his usual place in the front row, smiling approvingly up at me. (It's surprising how much such encouragement from such a source can improve the quality of a talk.) His smiles continued right up to the moment when I read the Petersen quotation. No sooner had I finished reading it than Viki was on his feet. "That's outrageous," he proclaimed. "Bohr couldn't possibly have said anything like that!" Somewhat taken aback by this sudden flip from approbation to condemnation, I feebly protested that I wasn't attributing it to Bohr, merely to Aage Petersen's memory of Bohr. That did not extinguish the flames. "Shame on Aage Petersen," declared Viki, "for putting those ridiculous words into Bohr's mouth!"
 * N. David Mermin, "What's Wrong With This Quantum World?" Physics Today Vol. 52, No. 2 (February 2004), p. 10.


 * [Bohr was] a marvelous physicist, one of the greatest of all time, but he was a miserable philosopher, and one couldn't talk to him. He was talking all the time, allowing practically only one or two words to you and then at once cutting in.
 * Karl Popper, quoted in John Horgan, The End of Science (1996), Ch. 2 : The End of Philosophy


 * A big, athletic man, a terrible writer and famous mumbler, Bohr was, after Einstein, the greatest physicist of the era, remarkable not only for his intuitive brilliance but for a kindness that extended equally to his friends and to humanity.
 * Rebecca Solnit As Eve Said to the Serpent: On Landscape, Gender, and Art (2001)


 * "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," Dr. Wheeler said.
 * John A. Wheeler as quoted by Dennis Overbye in (14 April 2008)


 * Niels Bohr distinguished two kinds of truths. An ordinary truth is a statement whose opposite is a falsehood. A profound truth is a statement whose opposite is also a profound truth.
 * Frank Wilczek, The Lightness of Being (2008)