Niels Bohr

Atomic Physics and Human Knowledge

Category: Lectures

Date: 25 June 1962

Duration: 53 min

Quality: HD

Subtitles: EN

Niels  Bohr (1962) - Atomic Physics and Human Knowledge

In 1962, Niels Bohr participated in his first Lindau Meeting. Since he was taken ill during the meeting and passed away in the autumn of the same year, the lecture he gave seems to have been his last public lecture

Ladies and gentlemen, It is a wonderful opportunity to a scientist who have had the good fortune to be occupied with studies and have moderately succeeded in giving some contributions to the common human inheritance of knowledge,to be able to come together with, and especially to meet, young people who helped to bring new blood into the work. Now I will say that everything in science, every contribution, is a large one. We live on the contributions of previous generations, and the work is to be compared with the bringing together of stones. But the interest, the pleasure, is to see how the whole edifice rises by the common contributions. Now, it seems, for the talk I have to give, that there is nothing new to most of you, but we will only assume there's some line in what we have learned in atomic physics and what helped the field to look into other things. I shall like to just say a few words about what of course everybody knows, that the idea of the limited divisibility of matter goes back to Antiquity. It was introduced by the atomists in order to try to the order that exists in nature, in spite of the very varied, immense variability of the physical phenomena. To take the very simple example of conservation, we think of a substance like water and the wonderful experience that the state of this system can be changed. By heating it up, the water may evaporate ? form steam or gas. And if it is cooled, it may form a rigid property, may form ice, which even, as we know, will form beautiful snow crystals on a beautiful summer day at the lake here. But the point is, that without any superstition, like witchcraft, it would be still more difficult to understand that the steam can be condensed, and like the ice that made it, form exactly the same water with the old properties. But, of course, if we assume that the water molecules just remain unaltered during the process, then we can explain these very properties by the ever-present particles being removed from each other, and with ice there is a possibility due to the smaller motions of the more regular order and that is then again changed by the melting of the ice. Now these are only too simple matters to speak about. The interesting thing is that for a long time people thought that the coarseness of our senses was too great to allow the actual observations of the individual atoms. Now we know that that was underrating our possibilities, that by the marvelous development of the technique or the art of experimentation, especially of wonderful quantification devices, it is possible to see effects of individual atoms. But this very fact we must admire: the caution which the old wisdom held and applied with regard to ideas and pictures used for the communication of all the experience in the new field, based on the experience. We also know that in this new field there are other laws ? it is not possible to keep to the older physical laws, which we call classical physics, and we have actually to go very much further. Now this is getting too long, but the point is that in the development of classical physics, there it was possible. The development of mechanics after the work of Galilei was completed with this wonderful genius of Newton, to describe the working even of the planets around our sun. A continuous chain of events described the state of the system by its position and power and the motion of its parts, and it was assumed that these different chains were uniquely connected in a so-called ? not a precise description ? relation method through the laws of conservation of energy and momentum. Now it was found that the same kind of description was possible also in the field of the electric and magnetic phenomena. We know how Maxwell was able, on the basis of the discovery of Faraday, to develop a description of such a kind. It was necessary, though, not only to describe an electromagnetic system ? the position of the electrified and magnetized bodies ? but also to keep account of the duration and tendency of the magnetic and electric fields at any point of space. Now I have come to something which has also become known to everybody. I might just say a few words about the development in the theory of relativity, which developed by the observation that it was impossible to find any change in the velocity of light dependent on the motion of the earth around the sun. That led, as we all know, Einstein to recognise not only that different observers moving relative to each other use different velocities, but in the arrangements of the phenomena it was even so that one had to use a different time, that two events which for one observer seemed to happen simultaneously might for another happen before and after each other. Or, to speak about relativity, or to say a few remarks. As we all know, then it was possible for Einstein not to introduce this as a complication, but as a way to find general laws of physics, common to all observers, and one is, of course, the relationship between energy and mass. To do this one has to introduce – one way to do it ? is to introduce mathematical abstractions as fundamental non-Euclidean geometry. But now it is essential, and that also applies to all the other things I'm speaking about, that mathematics is not something similar to science to be developed by getting new evidence by experimentation. Mathematics is a development of language and it is even so that these mathematical abstractions ? the mathematical definitions and their operations ? have to be described in the common language. Otherwise nobody could learn it, for it is a very practical tendency of language which makes it possible simply to describe relations in ordinary language. Now it is very drastic, that as regard the description in relativity theory, every observer distinguishes sharply between space and time. If that was not connected with the formalism, then, of course, it was not possible to speak about and say anything. Another point in relativity theory is that it assumes that the number of atoms in the world is very large, so large that we can make instruments that allow us to measure. Unless it was for those two reasons, of course, there was not the slightest physical content in the theory of relativity. Further it is the simplicity that, just like in the classical mechanics, we can order the phenomena in a prescription in which we reckon with cause and effect in the ordinary way. In all these ways, then, the theory of relativity is a wonderful generalization of classical physics. But now, the great change that we have been leading up to came with the discovery of the universal quantum of action in the first year of this century by Planck. This discovery revealed a feature of wholeness, of a duality in the atomic processes, which went far beyond the old doctrine of the limited divisibility of matter. It meant that the ordinary theories of classical physics were generalisations, which could only be applied in cases of phenomena on such a scale that the action involved in all points was so large compared with the quantum that it can be neglected. This was outside the ordinary experience. But when it comes to atomic forces we have new regulations, of a kind for which we cannot give simple pictures, but which, nevertheless, are responsible for the stability of atomic systems, on which the properties of all matter ? and of which our tools and all the bodies are made ? depend. Now just let me say that the difficulties of picturing such phenomena were brought, in a very serious way, to the forefront by Einstein, who really, at the time of the discovery, tried to explain the individual photoeffect by assuming that actually we had to do with what we call a transfer of a light quantum. But that was a very odd situation because there could be no question after returning to the corpuscular description of light, that the great picture of electromagnetic theory accounted for all the interference phenomena. And thereby we had two features which had to be used in what we will now call a complementary manner. That was the first time that such things came to the knowledge of physics. Now I shall not go on in that way. It was found to be possible ? by leaning on the ideas of Einstein and making use of the knowledge we got from the discovery of the atomic nucleus ? to account for their bringing order in certain fields of experience. But that was of a very unsatisfactory nature, because one could not get logical consistency before one had a harmonius generalisation of classical physics in such a way that all our definitions of the world we take from classical physics, and unless we have a harmonious generalisation, we cannot apply that in a consistent manner. I shall not try to speak about how this came about. You see here many of those people who contributed most decisively to this thinking ? Heisenberg, Born, Dirac and the others. I think I will only say that, had they gathered like this, we would have missed Schrödinger, whose death a few years ago was such a loss to science. Now, but the point is that the development has taught us something about what we may call objective description of atomic physics. Now what do we mean by objective description? That is all so philosophical. We simply mean to describe in a way which do not depend on subjective judgement. And now if you speak quite simply about it, it means that the answers to the questions we encounter in our experience must be given in ordinary human English, just refined by the terminology of classical physics. Now that is actually what we do in all experimentation, in all human experimentation, in all the fields. Then how do we go about it? We use in measurements bodies that are large enough and heavy enough so that we actually can tell their relative positions, relative motions, without taking into account any common feature which is involved in their constitution and their stability. Now, the observations, they are just recordings, permanent recordings, of such measurements. Measurements are of many kinds ? diagrams, sensors and photographic plates. The marks put on these ? like just this spot which can be developed on a photographic plate and set by an electron. Now there is a point that these recordings involve complicated irreversible processes. But that is no difficulty whatsoever in interpreting the research, because it just reminds us and stresses the irreversibility which is in the concept of observation itself. That's a point which is only spoken about in the classical theory, but actually it is what one learns. We say that the moon goes around the earth, and we see it sometimes and so on. But can we purely for psychological reasons – philosophical reasons ? say it has been there between that times that we have observed it? But the point is that surely classical mechanics is so to be understood. So that actually, the very steps in a phenomenon are really carried out and completed, but now in quantum physics, there we have just this limited divisibility of the phenomena. Therefore, now in the classical physics we just assume that generally in observations we can account for the interaction of the instruments and the objects in the observations or at any rate compensate for them. But in quantum phenomena this interaction forms an integral part of the phenomena for which no separate account is possible ? it is logically impossible ? if the instrument shall serve the purpose of defining the experimental arrangement. Now let me remind you of a few main points. When we specify conditions, then we may observe in quantum physics under one and the same fine experimental arrangement, as far as we can define it at all ? we have to define it ? different attributes corresponding to different individual quantum processes. Therefore, a statistical account is simply imperative, is indispensible. Another point is, that if you have a phenomenon observed under different experimental arrangements, it cannot be brought together in one and the same classical picture. Then we say that however much at first sight such phenomena contrast with each other, they have to be adapted as complementary in the sense that they together exhaust all the information which can be obtained and defined about the objects. Now the great thing in the development of the mathematical methods of quantum physics is that such simple considerations can never involve any logical contradiction, just due to the mathematical consistency of the quantum mechanical formalism. This formalism serves to predict or to develop the statistical laws holding for the observations at given experimental conditions. Actually, over a very large part of all our experience, nothing is finished; we will have to learn of the problem which Heisenberg spoke of the last day, about the kind of further departures from pictorial description, if it comes to phenomena where we may say that they take place within very small extensions of space. But that doesn’t mean that the description we have in the other fields can ever be changed. But new further things have a possibility to come in just because of the way of defining the things, by experimental arrangements just to use if we can make something very much smaller than in atomic physics. Now I do not feel how well I've done it ? I have tried to be as free as possible. But I would like to say that this complementary description to which we are referred in quantum physics means first of all that in order to really define what we understand, it will help to be reminded of the essential part of the experimental arrangement ? that it has to do with conditions of experimenting and the definition of research. Now such a situation is new in physics, but not in other fields of human interest, where in a very familiar way, we use language which implicitly or explicitly refer to the situations under which experience is obtained. Now I will just add something about these problems, as regards our attitude to various fields. First of all I would say a few words about the psychical experience, psychological experience. That is certainly very, very far from physics, and it is a field where it is not so simple to speak about objective description. That is, in the description of such experience ? communication of such experience ? we use words like thought and sentiment which refer to mutually exclusive situations, like the different experimental arrangement in atomic physics. For we cannot at the same time have a feeling of volition and ponder about the motives for our actions. Such words are used in a typical complementary manner since the very origin of human language. Now we say that that is how it is with objective description. In the objective description in physics, or atomic physics, we never refer to the observing subject. Sometimes one speaks as we do, but that is absolutely not necessary and disturb the actual meaning, because we need not say who has put up the instruments, and the actual observations are absolutely independent of how many, or how few, look at them. The actual effective content is never to be referred to the subject. When we speak about the scientific sphere, we say ‘I’ ? ‘I think’ or ‘I mean’ or so, and now, how does that actually correspond? It corresponds in the way that we in atomic physics must tell how and under what condition experience is obtained. Now this seems all so philosophical, maybe so unclear. I want just to refer to a story in Danish literature, very briefly. Poul Møller a hundred years ago wrote a book which is very much beloved among Danish students today, called ‘Adventures of a Danish Student’. The writer speaks about connections between young people, discussions among them and so on, and of different situations. And what I want to refer to is especially a talk between two cousins, one a very practical one, very efficient, one of those who students in Denmark used to call philistines. I don’t know what the word is in Germany ? ‘Spitzburgher’? And the other was a man who was very apt to philosophical meditation, but to a great disturbance for his form of life. Now the situation is that the former ? this is very funny ? tries to help the latter get his things better arranged so he can get some sensible occupation, to have a job as one especially say in America. And he arranges all things for him, but he does not respond at all. Then he says how can that be, and then this poor Licenciate ? he is called Licenciate ? says, It is very easy for you to speak of me, but I must think of that ‘I’, which controls what you call ‘me’. But as soon as I start thinking, it’s getting much worse. Because then it is clear that I am really thinking of an ‘I’, which think of that ‘I’ which control what you call ‘me’, and then I get a whole series of ‘I’s, in between one another who shall determine, and then they say that they try very hard to get order in my ‘I’s, but I must give it up and get a terrible headache.” Now this is told with very fine humour, which I have not been able to produce with these short words. But the point is, that’s a very true description of the position of every one of us. It is a position which we are in, where children learn from their parents, or from playing with each other, how to use the language in a way which is not too impractical. It also helps us learn that one has to get a good deal of humour into it, as there also is in children’s play. The point is that it is the possibility of so to say changing the separation between subject and object that gives the possibility for describing the rich conscious life. Similarly, the new widened framework of complementarity allows us in physics to account for regularities of very much finer character in a very much more varied way than anything that could at all be defined within the framework of classical physics. It also forms the view when one speaks about mysticism and pragmatism and so on ? they are all parts of what normally one can be interested in, and gives room for the art, poetry, pictorial art, or music, to play on all strings of our sentiments. But, I should like to say a few more words, maybe about the social relations. There we come to the so-called ethical problems. And now it is a new point I want to make, how we speak about it nowadays; we then ask, what are the words we use? The words in such fields are essentially “justice” and “charity”. And all societies try to combine these two things. To be clearer, you can not have some orderly society without some kind of fair play, regulated by judiciary rules. But it’s also clear that life would be very poor if we cannot use words like friendship or love. But the point is, in a situation where we have unambiguous applicability of judiciary rules, then what do to? Then there is no room for applied world charity. But conversely ? the great poets in Greece wrote on the subject ? compassion and goodwill can bring any one of us in absolute conflict with any judiciary rule. So there we have two words again which are used in a complementary manner. It’s very interesting that just such simple matters can be described in analogy with space and time description, conservation of energy?momentum, yet that these things have to be treated differently if you go to the fine details. So it is also that the finer issues of human life strongly present in the old Chinese philosophy, a long time ago, thousand years ago, looking for harmony in life, involved that we are ourselves both the actors as well as the spectators. When we compare different cultures, compare the behaviour in different societies, everything is often very different. One might say this is analogous to the theory of relativity because there we know how differently different observers may describe one and the same experience. But the situation is very different from that, because in relativity theory it is possible by means of a common principle, common terminology, for each observer to know how any other in a different frame of reference will arrange descriptions; that is what gives the value to the theory's general laws. But the traditions on which nations are based are of a very historical origin, and it is not such a simple thing to appreciate traditions of that kind on the basis of tradition alone. You may even say that it is hardly possible to appreciate the prejudices of one nation in terms of the prejudices of another. So therefore we might sometimes say that these cultures are complementary to each other, but that is very, very rough, because the complementary description in atomic physics and in psychology is simply unavoidable, but nations can by contact exchange, to a certain degree, traditions, and there can arise new cultures which contain valuable elements of the cultures of others. Now, this is all so very simple, and I only want to say that it is most essential actually to be aware of the common human basis in life. And this is especially so now, due to the development of atomic physics, which is meant for no other thing than augmenting knowledge of that nature of which we are part. Then, of course, at the same time, the human position is bound up with the means of destruction which have come into human hands. And unless we get some kind of cooperation between the nations, it may threaten human survival. Now this is a new situation. It has no precedence in history, and it is therefore something which must occupy everyone. And it’s a very, very difficult situation. But one must try to see how can one increase confidence between nations. And I would just end by saying that one of the greatest elements of having taken part in scientific cooperation is the possibility, in the common service for truth, to establish friendship and understanding between scientists of different nations, quite independently of traditions of other kinds. And I think that that is one ? we hope it works quickly ? of the ways by which some basis for confidence can be created, I think especially when we are together with the young people who will have to see that such a development is promoted and realised in the future. There is therefore hope that such a kind of cooperation, which I told of today ? which we have learnt just from pure science? will evolve in many kinds of human relationships, and I hope that it may contribute something to the great task now before us. Applause


In 1962, Niels Bohr participated in his first Lindau Meeting. Since he was taken ill during the meeting and passed away in the autumn of the same year, the lecture he gave seems to have been his last public lecture. But he was a Nobel Laureate already 31 years before the first Lindau Meeting on physics was held in 1953, so he had been invited a number of times. The Niels Bohr Archive in Copenhagen holds all the letters of invitation and also copies of Bohr’s replies. These express his wish to attend, but also give reasons why he cannot do so because of his many other commitments, for example a meeting about CERN in 1953 and another with the Atomic Energy Commission in 1959. In 1955 - a chemistry year in Lindau but also the year when Werner Heisenberg had suggested that all Nobel Laureates connected with atomic science should be present - Bohr was invited, but could not attend, giving his best regards to Hahn and the others. So when Dr. Mecke in Freiburg, one of the organizers, sent out the letter of invitation in the spring of 1962, he cannot really have been expecting a positive answer. But on March 12, Bohr accepted the invitation and gave his title as “Atomphysik und menschlische Erkenntnis”, i.e., “Atomic Physics and Human Knowledge”. This was a title that Bohr had used before and which signalled that he wanted to speak on questions of the theory of knowledge that became necessary to address with the advent of the quantum theory. Looking back, this was a fitting title for Bohr’s last lecture, since his deep analysis of these questions probably represents his most lasting contribution to science in general. In their old age, Niels Bohr and his wife Margrethe always brought one of the grandchildren to scientific meetings like the Lindau one. In 1962, the companion was the grandson Christian Bohr, who was going to celebrate his sixteenth birthday on 30 June. Some time after the meeting, young Christian wrote down his memories of the eventful journey and meeting. I have been allowed to quote from his quite long, detailed and interesting Danish text. In my own translation to English, Christian wrote:

“Grandpa and Grandma had invited me to accompany them to a Nobel Laureate Meeting in Lindau in southern Germany, with a stop-over in Cologne, where Grandpa was to give a lecture at Delbrück’s. I was to sit in the front with Jørgensen in the big Lincoln and perhaps be of some help with practical matters. But now I realize that what was most important was that I, with such a journey, should develop in a direction that Grandpa and Grandma would like.”

Max Delbrück received the Nobel Prize in Physiology or Medicine in 1969. Jørgensen was the chauffeur.

“We arrived in Cologne in the afternoon and drove towards Delbrück’s home. I remember the area as very elegant with large lawns that were mown by tractor-like mowers. Grandpa asked a passer-by about the address and could with pride tell us that he had managed to get hold of someone who knew Delbrück very well."

Bohr had been invited on the occasion of the inauguration of the Institute for Genetics in Cologne.

“What happened the next day is not completely clear to me. I know that Grandpa held an interesting lecture on ‘Light and Life’. As far as I remember, it was with regard to this lecture that Grandma told me that Grandpa impressed the audience by translating on the spot a lecture written in English to German. This makes me wonder, though, since I don’t think that Grandpa spoke that good German.”

The title “Light and Life” was used by Niels Bohr already in 1932. The unfinished manuscript of the lecture held by Bohr in Cologne on 21 June 1962 has been published as “Light and Life Revisited”. It has been regarded as his last public address.

“We passed through the city of Konstanz and arrived at Lake Constance. We drove through a smiling landscape with fruit orchards. Grandpa seemed to be busy with his lecture, but rested now and then. It was hot, so I, who sat in the front beside Jørgensen, had the window down and my arm outside, something that I was to regret. We arrived at the hotel. It was not in Lindau (‘im Bodensee’, as its name really is) itself, but on the mainland close to the lake. It looked like a very fashionable hotel.”

Christian’s arm got sunburnt. The hotel, Hotel Bad Schachen, is still used today.

“As I remember it, this or the next was the evening that Grandpa worked very hard on his lecture. Not to disturb Grandma, he put one of the lamps close to the window so that it would shine on a table outside of the window, where he sat to finish the lecture. I don’t know how long Grandpa worked that night, but I am afraid that it influenced what happened the next day. The best rooms at Hotel Bad Schachen have balconies towards the lake.Grandpa’s lecture was scheduled for one of the first days [Monday 25 June], and I remember quite clearly some things from it. I guess that to some extent it only existed in Danish, which had to be translated on the spot. It was about complementarity. But how the argument was presented, I don’t remember. But I do remember that Grandpa at the end said that there was an illustrative example in the classical Danish literature, namely in ‘Licenciaten’ from ‘En dansk students eventyr’. Here Grandpa was seeking a word in English and couldn’t find it. As I remember it, there followed a rather embarrased silence, after which Grandpa asked to be forgiven by the audience. The lecture was over and Grandpa left the podium under applause. As I remember it, there was something depressing about the situation. This also can have contributed (as I felt already at that time) to the heart failure.During the afternoon, I must have done something or the other away from the hotel. Thus it was only sometime after it had happened that I, on my return, learnt that Grandpa had suffered a stroke. Grandma was worried, and even though I didn’t really understand how dangerous the situation was -and, in particular, had been - I couldn’t help noticing that everybody else took it very seriously. It wasn’t very long before I was allowed to visit the sickbed. Only for a short while."

"Also some of Grandpa’s old pupils appeared there. And at least one of them, Dirac, stayed much too long to Grandma’s taste. Dirac was really very fond of Grandpa, and similar sentiments could be felt from many of the other participating physicists. I remember in particular how Hevesy put himself totally at Grandma’s disposition, also to lend her some money."

"By and large I got an interesting impression of many of the personalities behind the names I had heard so much about at home and at Carlsberg: the really kind and warm Heisenberg, the very abstract Dirac, the much respected Lise Meitner (participating outside the program), whom I remember as a tall, thin and rather reserved lady. About almost all of the participants, I heard Grandma say: ‘Yes, they were in Copenhagen in this and that year’.”

According to the standard biography of Bohr, it was a minor cerebral haemorrhage that occurred. In early July the car brought everybody to Frankfurt, from where Bohr and his wife were flown to Copenhagen. Christian and Jørgensen returned by car. Bohr recuperated during the summer, but on November 18 he passed away at the age of 77.

The lecture in Lindau was delivered in English. Although the quality of the tape recording is quite good, it is very difficult for someone not used to listening to Bohr to understand what he is saying. Therefore a transcription has been made by Gro Næs, assisted by Finn Aaserud and Christian Bohr.

Anders Bárány