I made a number of changes to the text of the play, as I have explained (in the Postscript to the Anchor Book Original, August 2000 printing, ISBN 0-385-72079-3) in response to suggestions and criticisms I received during the run of the play in London, and to new material I came across. The production in New York, however, opened up a much broader and more fundamental debate. A number of commentators expressed misgivings about the whole enterprise. Paul Lawrence Rose, the most outspoken of the play's critics, even managed to detect in it a 'subtle revisionism... more destructive than Irving's self-evidently ridiculous assertions - more destructive of the integrity of art, of science, and of history.'

One of the most frequent complaints about the play in America was that I should have laid more stress on the evils of the Nazi regime, and in particular upon the Holocaust; it was pointed out that Heisenberg's visit to Copenhagen in 1941 coincided with the Wannsee Conference. It was argued that I should have put the visit in the context of a number of subsequent trips he made during the course of the war to other occupied countries. It was also felt that I should have laid more stress than I did on Heisenberg's stated view that Germany's conquests, at any rate in Eastern Europe, were justified, and that her victory over Russia was to be welcomed.

With hindsight I think I accept some of these criticisms. I should perhaps have had Heisenberg justify Germany's war aims on the Eastern front direct, instead of having Bohr refer to his arguments in one angry but passing aside. I should perhaps have found some way to make the parallel with all the other trips that were found offensive, and about whose purpose there was none of the mystery which had seemed to attach to the one to Copenhagen.

About a greater stress on the evil of the Nazi regime I'm not so sure. I thought that this was too well understood to need pointing out. It is after all the given of the play; this was precisely why there was (or should have been) a problem facing Heisenberg, and us in understanding him. In any case the play returns to the persecution of Jews in Nazi Germany again and again, from the suppression of so-called 'Jewish physics' (relativity) to the enforced flight of all the Jewish physicists, the death of Goudsmit's parents in Auschwitz, and the attempt by the SS to deport the Jewish population of Denmark to the death camps, which Margrethe Bohr describes as 'that great darkness inside the human soul... flooding out to engulf us all.'

Some of the criticisms were even more radical. The play turns on the difficulty of determining why Heisenberg made his trip. For a number of commentators there was no problem at all - they knew the correct explanation for certain; though what that explanation was varied from one to another. For some it was Heisenberg's desire to persuade Bohr of the rightness of Germany's war aims and of its inevitable victory; for Rose and others he was on a spying mission, to find out through Bohr if the Allies were also working on an atomic bomb.

I agree that Heisenberg may have wished to present the German case to Bohr; but he surely didn't go all the way to Copenhagen just to do that. I also agree about the spying. But then so does my Heisenberg. He tells Bohr that he wanted 'some hint, some clue' about whether there was an Allied nuclear programme. This seems to me to be commonsense; he would have had to be insanely incurious not to seize any chance he could to find out whether the Allies might drop atomic bombs on his country. There is surely no contradiction at all with what he himself claimed his purpose was - to discuss whether the German team were justified in working on a German weapon. Any information he could get about the other side's intentions would have been a prerequisite for deciding what to do.

Some criticisms I reject, and I should like to put the record straight. Professor Rose suggested that I had 'fantasized' Heisenberg's fear that he was in danger of his life from the Gestapo for talking to Bohr. Not so - I was simply expanding upon what the real Heisenberg said. Jonothan Logan, a physicist writing in American Scientist, dismissed as misleading the fictitious Bohr's assertion that in June 1942 Heisenberg had been slightly ahead of Fermi in Chicago. The context makes plain that this was in terms of neutron multiplication, and the claim was based on what David Cassidy says in his biography of Heisenberg. The correctness of Cassidy's assessment was verified for me, after much inquiry on my part, by Al Wattenberg, one of the editors of Fermi's Collected Papers.

All these are at any rate debatable points. Other criticisms I found extremely difficult to make sense of - some even to credit. Professor Rose, who detected the subtle revisionism of the play, found a particularly sinister significance in one particular detail - the fictitious Heisenberg's remarking upon the neatness of the historical irony whereby the crucial calculation (of the critical mass), which persuaded the Allies of the possibility of building a nuclear weapon, was made by a German and an Austrian, driven into exile in Britain because they were Jewish. This Professor Rose saw this as an attempt to blame 'the Jews' for the bomb's invention.

A little more extraordinary still was the view of the play taken by Gerald Holton, Professor of Physics and Professor of the History of Science Emeritus at Harvard. He saw it as being 'structured in good part' to reflect the thesis advanced by Powers, that Heisenberg had correctly calculated the critical mass, but concealed it by 'cooking up' a false result. By the time the play was produced in New York, he believed, I had been forced (by Bernstein) to lay this idea aside, so that I now had an 'unsolvable problem' with the motivation of the play.

I can only suppose that Professor Holton was misled because in the Postscript I speak warmly and gratefully about Powers's book. It has been much attacked, but I continue to admire the generosity of its tone, and to admire the range of Powers' research. I also agree with the first part of his thesis (lack of zeal). But then so does Holton himself, and so, he says, does everyone else who has studied the matter. In the Postscript, however, I make abundantly clear that I don't accept Powers' view about the 'cooking up' and never did.

But you don't even need to read the Postscript to discover this, because it's all over the play itself. The central argument turns on Heisenberg's confession to Otto Hahn that he had not attempted the calculation. By my count there are something like 35 speeches devoted to establishing this, to asking why he hadn't attempted it, and to suggesting what might have happened if he had. How anyone could give the play even the most cursory glance and fail to notice this is difficult to understand.

Even harder to credit was the reaction in some quarters to the 'strange new quantum ethics' proposed by the fictitious Heisenberg. I suppose I should have erected a flashing 'IRONY' sign in front of it. The allusion is to his insight, in his original introduction of quantum mechanics, that physics should be limited to the measurement of what we could actually observe - the external effects of events inside the atom. We should need a similar kind of ethics, he suggests in my play, if we judged people purely on the external effects of their actions, without regard to their intentions. According to Professor Holton, Heisenberg 'exults' that under the new dispensation there would be a place in heaven even for him. Professor Holton fails to mention that Heisenberg also 'exults' that, under the new quantum ethical rules, there would also be a place in heaven for the SS man who seemed ready to murder him in 1945, simply because in the end he settled for a pack of American cigarettes instead. Jonothan Logan manages to believe that I am seriously proposing even the SS man's assumption into heaven.

Let me make it absolutely unambiguous: my Heisenberg is saying that we do have to make assessments of intention in judging people's actions. (The epistemology of intention is what the play is about!) He is saying that Bohr will continue to inspire respect and love, in spite of his involvement in the building of the Hiroshima and Nagasaki bombs; and that he himself will continue to be regarded with distrust in spite of his failure to kill anyone. The reaction of Holton, Rose, and others to the play is perhaps an oblique testimony to the truth of this judgement.

*

One of the most striking comments on the play was made by Jochen Heisenberg, Werner Heisenberg's son, when I met him, to my considerable alarm, after the premiere of the play in New York. 'Of course, your Heisenberg is nothing like my father,' he told me. 'I never saw my father express emotion about anything except music. But I understand that the characters in a play have to be rather more forthcoming than that.'

This seems to me a chastening reminder of the difficulties of representing a real person in fiction, but a profoundly sensible indication of the purpose in attempting it, which is surely to make explicit the ideas and feelings that never quite get expressed in the confusing onrush of life, and to bring out the underlying structure of events. I take it that the nineteenth-century German playwright Friedrich Hebbel was making a similar point when he uttered his great dictum (one that every playwright ought to have in pokerwork over his desk): 'In a good play everyone is right.' I assume he means by this not that the audience is invited to approve of everyone's actions, but that everyone should be allowed the freedom and eloquence to make the most convincing case that he can for himself. Whether or not this is a universal rule of playwriting it must surely apply to this particular play, where a central argument is about our inability, in our observation of both the physical world and the mental, ever to escape from particular viewpoints.

I suppose that this is what sticks in some people's throat - that my Heisenberg is allowed to make a case for himself - even to criticise others. His claims about his intentions are strongly contested by another character in the play, Margrethe Bohr. Neither Heisenberg nor Margrethe Bohr, so far as I can see, is presented as winning the argument. I don't see why my Margrethe shouldn't be allowed to express her suspicions of Heisenberg much more sharply and woundingly than the real Margrethe's habitual courtesy would ever have permitted, and I don't see why my Heisenberg shouldn't be free to express the deeper feelings that the real Heisenberg remained silent about. Why shouldn't he have the same conflicting loyalties and the same mixed motives and emotions that we all have? Why shouldn't he try to juggle principle and expediency, as we all do? Why shouldn't he fear his country's defeat, and its destruction by nuclear weapons? Why shouldn't he lament its ruin and the slaughter of its citizens?

I can imagine it being asked how far I think this principle should be carried. Do I believe that a fictitious Hitler should be accorded the same privileges? I can see all the problems of exhibiting Hitler on the stage, but I can't see any point in attempting it at all if he is to be simply an effigy for ritual humiliation. Why should we be asked to to endure a representation of his presence if he doesn't offer us some understanding of what was going on inside his head from his own point of view? The audience can surely be trusted to draw its own moral conclusions

*

The most surprising result of the debate set off by the production of the play, though, has been the release of the Bohr documents.

I was told privately about the existence of at any rate one of the documents at a symposium on the play organised in Copenhagen by the Niels Bohr Archive in the autumn of 1999. Heisenberg had made public his own version of the 1941 meeting with Bohr, chiefly in two places: a memorandum written in 1957 to Robert Jungk, who was preparing the material for Brighter Than a Thousand Sons, and his memoirs, published in 1969. Bohr, however, had never publicly given his side of the story, and historians had been obliged to rely upon what other people (chiefly his son Aage - also a physicist, and later a Nobel prizewinner himself - and his colleague Stefan Rozental) recalled him as saying about it.

In 1957, however, Bohr had apparently been so angered by Heisenberg's version, when he read it in Jungk's book, that he had written to Heisenberg dissenting, and giving his own account. He had never sent the letter, though, and at his death in 1962 it had been placed in the Archive by his family, not to be released for another fifty years. This was all my informant was prepared to tell me.

I said nothing about this because I believed that I had been told in confidence. The existence of the letter was first publicly mentioned, so far as I know, by Professor Holton, at a further symposium on the play organised in New York in March 2000 on the occasion of its production there. He said that he had actually seen the letter - he had been shown it by the Bohr family. He felt bound not to divulge its contents, but I recall him as promising that when it was finally made public, in 2012, it would entirely change our view of the meeting.

Now the cat was out of the bag, and at yet another symposium on the play, at the Niels Bohr Archive in September 2001, it was announced that the Bohr family had decided to release the letter early. It also turned out that there was not just the one letter but various alternative drafts and notes relating to it. When they were finally published on the web in February 2002 the whole question of the visit was accorded even wider attention in the press than ever before.

The documents seem to me to bear out remarkably well the very detailed reconstruction made of Bohr's attitude by Powers from other sources. The most surprising thing to me in Bohr's first attempt at the letter is its remarkably sharp tone - particularly coming from a man so celebrated for his conciliatoriness:

It is a revelation to have all this in Bohr's own voice, and I wish it had been available when I wrote the play. I recognise that the real Bohr remained much angrier for much longer than my character, that he claimed to have paid much closer attention to what Heisenberg said, and that he claimed to recall it much more clearly.

Does it really modify our view of what Heisenberg said, though, and of what his intentions were?

Slightly, I think, but not fundamentally. There has never been any disagreement, for a start, that Heisenberg publicly told various people at the Institute that Germany was going to win the war, and that her aims, at any rate in the East, were justified. Then again, Aage and Rozental were both already on record as recalling Bohr's saying that Heisenberg had talked about the military applications of atomic energy. According to Aage: 'My father was very reticent and expressed his scepticism because of the great technical difficulties that had to be overcome, but he had the impression that Heisenberg thought that the new possibilities could decide the outcome of the war if the war dragged on.' According to Rozental: 'I can only remember how excited Bohr was after that conversation and that he quoted Heisenberg for having said something like, "You must understand that if l am taking part in the project then it is in the firm belief that it can be done."'

The letter, however, is the first direct confirmation that Bohr believed he was being urged to accept German 'offers of cooperation,' which is what Weizsäcker suspected he may have understood Heisenberg to be suggesting. It's not clear from the letter what Bohr thought this 'co-operation' would entail, and the recollection may not be entirely at odds with what Weizsäcker recalls Heisenberg as telling Bohr - that he ought to establish contact with the staff of the German Embassy for his own safety.

Some of the differences between Bohr's account of the meeting and Heisenberg's are less clear-cut than Bohr's indignation makes them appear. According to Heisenberg, in his memorandum to Jungk, he told Bohr he knew that the use of uranium fission for making weapons was 'in principle possible, but it would require a terrific technical effort, which one can only hope cannot be realized in this war.' Bohr, he said, was shocked, 'obviously assuming that I had intended to convey to him that Germany had made great progress in the direction of manufacturing atomic weapons.' This is not all that different in substance, it seems to me, from what Bohr recalls.

The same is true when Bohr goes on to dispute Heisenberg's interpretation of his reaction:

The difference between the 'shock' that Heisenberg diagnosed and the more dignified 'silence and gravity' that Bohr himself recalled dissolves a little in a later draft of the letter, where Bohr refers to his reaction as 'alarm'. His assertion that he already understood about the possibility of producing a weapon based on fission is moreover a simplification which is not quite supported by his subsequent behaviour. He had in fact up to that moment believed that it was a practical impossibility, because of the difficulty of separating the fissile U235, and Heisenberg could not tell him why the balance of probability had now changed - because of the German team's realisation that a reactor, if they could get one going, would produce plutonium as an alternative. After Heisenberg's visit, according to Rozental, he was sufficiently shaken by Heisenberg's confidence to go back to the blackboard and rework all his calculations. Even so he seems to have remained unconvinced when he got his guarded report on the meeting through to Chadwick, his contact with British intelligence, and said: 'Above all I have to the best of my judgment convinced myself that in spite of all future prospects any immediate use of the latest marvelous discoveries of atomic physics is impracticable.'

The real kernel of the apparent disagreement about the meeting emerges only in later drafts of the letter, where Bohr says that 'there was no hint on your part that efforts were being made by German physicists to prevent such an application of atomic science.' This appears to be a rebuttal of some claim made by Heisenberg. The belief that Heisenberg made some such claim seems to be widespread. Professor Holton suggests that my the play is 'based in large part on Heisenberg's published claim that for him an impeding moral compunction may have existed about working on atomic energy.'

But nowhere, so far as I know, did Heisenberg ever make the claim that Bohr seems to have attributed to him. There is no mention of it in the memorandum to Jungk. Even in the expanded account of the meeting that he gave in his memoirs he remained extremely cautious: 'I hinted that... physicists ought perhaps to ask themselves whether they should work in this field at all... An enormous technical effort was needed. Now this, to me, was so important precisely because it gave physicists the possibility of deciding whether or not the construction of atom bombs should be attempted. They could either advise their governments that atom bombs would come too late for use in the present war, and that work on them therefore detracted from the war effort, or else contend that, with the umost exertions, it might just be possible to bring them into the conflict. Both views could be put forward with equal conviction...'

One might think this sounds a quite implausibly judicious rendering of anything he might have said. The fact remains, however, that he is not claiming to have made any efforts to prevent work on weapons. He is not even claiming that up to this point the German team had exercised the option of offering discouraging advice, only that they might at some point if they so chose. In any case, Heisenberg says that Bohr 'was so horrified by the very possibility of producing atomic weapons that he did not follow the rest of my remarks.'

Some reports on the release of the documents have suggested that they refute a claim made by Heisenberg to have offered Bohr a 'deal', whereby the German physicists would discourage their government from proceeding with nuclear weapons if Allied physicists would do likewise. I suppose the implication of Heisenberg's indeterminate phrase 'the physicists' is that this applied to the physicists on both sides, but the only evidence I can find for Heisenberg having made any more definite suggestion than this is in a part of the memorandum to Jungk which is quoted by Powers: 'I then asked Bohr once again if, because of the obvious moral concerns, it would be possible for all physicists to agree among themselves that one should not even attempt work on atomic bombs...' This might perhaps be interpreted as a tentative hint at some possible arrangment, though in the interview he gave to David Irving for The Virus House in 1965 he seems to be retreating even from this, and says merely that Bohr 'perhaps sensed that I should prefer it if physicists in the whole world would say: We will not make atom bombs.' The remark to Jungk was not quoted by him in his book, and so presumably not seen by Bohr in 1957. In his letter, in any case, Bohr makes no reference to any such claim, or to having understood any such offer at the time.

There are discrepancies in every other aspect of the evidence relating to this meeting, and it is scarcely surprising that there are some to be found between the two participants' own accounts. In both cases they are attempting to recollect something that happened sixteen years earlier, and their perceptions are inevitably coloured by strong feelings and conflicting loyalties. On the whole, I think, what's surprising is how slight the differences of substance are, and how readily most of them can be understood in the circumstances.

*

The only really clear-cut disagreement between the two accounts is about a circumstantial detail - where the meeting took place. Bohr talks about 'our conversation in my room at the Institute' (a version supported by Abraham Pais, Bohr's biographer, after making inquiries among Bohr's surviving colleagues just before his own death in 2000). Heisenberg, on the other hand, recalls in his memoirs visiting the Bohrs' home in Carlsberg, and finally broaching 'the dangerous subject' on their evening walk. This version is reinforced by what he recalls of his attempt to reconstruct with Bohr the 1941 meeting when he returned to Copenhagen in 1947. He was convinced, he said, that the conversation had taken place during 'a nocturnal walk on Pile Allé,' which is very close to Carlsberg, and four kilometres away from the Institute. (Bohr at the time, according to Heisenberg, thought it had been in his study - but in his study at home in Carlsberg.)

Even Heisenberg's own testimony, though, is not entirely consistent. According to his biographer, David Cassidy, he made an earlier statement in which he 'remembered that his most important talk with Bohr occurred one evening as they strolled along a tree-lined path in the large and secluded Faelledpark, just behind Bohr's institute.'

On the other hand, Heisenberg's wife, although not a first-hand witness, seems to be supporting the Carlsberg version in her own memoirs, presumably on the basis of what she recalled her husband as saying to her at the time: 'In spite of their difficult political position, the Bohrs received Heisenberg with great warmth and hospitality...' There is also a secondhand account of the meeting given to Thomas Powers by Ruth Nanda Anshen, Heisenberg's American editor, who said that she was told it by Bohr, and that his assistant Aage Petersen confirmed it. According to Powers, in Heisenberg's War, Bohr told Anshen that 'the invitation had cost him much agony - he wanted to sit down to dinner with Heisenberg, but his wife, Margrethe, objected, and Bohr couldn't make up his mind what to do. Finally his assistant Aage Petersen suggested that Bohr should write down his objections to Heisenberg's visit, then read them carefully a day or two later, and decide. This Bohr did; the old friendship seemed to him stronger than the objections, and he told his New York friend that he finally obtained Margarethe's agreement with a solemn promise to discuss only physics with Heisenberg - not politics.'

Jochen Heisenberg recalls his father showing him the street where he said he had walked with Niels Bohr in 1941, though he can't now remember the name of it, only that it was tree-lined (which Pile Allé is). Heisenberg's version is perhaps also given colour by Bohr's own remark in the letter that 'every word of our conversations... made a strong impression both on Margrethe and me'. I don't think that anyone who took part in any of the various meetings at the Institute mentioned Margrethe's presence, which suggests that Heisenberg did at some point go to the Bohrs' home. In any case Margrethe's strongly expressed opinion later that the visit had been 'hostile' sounds as if it was based on a first-hand impression.

I can't help being moved, though, by the picture that the new documents give of Bohr drafting and redrafting the text of the letter over the last five years of his life - and still never sending it. He was famous for his endless redrafting of everything he wrote, and here he was trying not only to satisfy his characteristic concern for the precise nuance, but also to reconcile that with his equally characteristic consideration for Heisenberg's feelings. There is a sad parallel with the account which Professor Hans-Peter Dürr gave, at the Heisenberg Centenary symposium in Bamberg last year, of Heisenberg's rather similar efforts to understand what had happened.

Professor Dürr, who worked for many years with Heisenberg in Göttingen after the war, said that Heisenberg had contined to love Bohr to the end of his life, and he recalled his going over the fatal meeting again and again, trying to work out what had happened. Professor Dürr offered what seems to me the most plausible common-sense estimate of Heisenberg's intentions that has yet been advanced. He thought that Heisenberg had simply wanted to have a talk. Heisenberg and Bohr had been so close that they could finish each other's sentences, and he assumed that he would have only to hint at what was on his mind for Bohr to grasp the significance of it. What he had entirely failed to grasp was that the situation had changed, and that Bohr's anger about the German occupation would make the old easy communication entirely impossible.

Whatever was said at the meeting, and whatever Heisenberg's intentions were, there is something profoundly characteristic of the difficulties in human relationships, and profoundly painful, in that picture of the two ageing men, one in Copenhagen and one in Göttingen, puzzling for all those long years over the few brief moments that had clouded if not ended their friendship. It's what their shades do in my play, of course. At least in the play they get together to work it out.

[end]