尼尔斯·玻尔Niels Bohr

尼尔斯·玻尔的父亲是Christian Bohr，母亲是艾伦·阿德勒。Christian Bohr于1880年在哥本哈根大学获得生理学博士学位，并于1881年成为该大学的Privatdozent。同年晚些时候，他与艾伦结婚，艾伦是弗洛伦斯·南丁格尔·大卫阿德勒的女儿，阿德勒是一位在丹麦政治和商业生活中地位很高的犹太政治家。克里斯蒂安和艾伦有三个孩子。最大的孩子是珍妮，1883年出生在大卫阿德勒拥有的豪宅中，该豪宅位于丹麦议会所在的克里斯蒂安堡宫对面。艾伦的母亲在丈夫大卫阿德勒于1878年去世后继续住在这所房子里，艾伦回到母亲家生下孩子。两年后，尼尔斯·玻尔在同一个庄严的宅邸中，于母亲25岁生日那天出生，艾伦再次回到母亲家生孩子。这个家庭的第三个孩子，后来成为著名数学家，是哈拉尔·玻尔，他比尼尔斯·玻尔小两岁。

当尼尔斯·玻尔只有几个月大时，他的父亲克里斯蒂安被任命为讲师，填补哥本哈根大学生理学教授彼得·帕努姆去世后留下的空缺，不久之后全家搬进了哥本哈根的帕努姆教授住宅。

Kennedy在[5]中写道：-

尼尔斯·玻尔、哈拉尔·玻尔和他们的姐姐Jenny在一个有教养且富有激励性的家庭中长大。从最早的时候起，他们就接触到一个充满思想和讨论的世界，各种相互冲突的观点得到理性而温和的审视，他们逐渐尊重所有寻求更深知识和理解的人。

1891年10月，尼尔斯·玻尔进入Grammelholms学校。他在这所学校完成了全部中学教育，他的兄弟哈拉尔·玻尔也是如此，并于1903年参加Studenterexamen考试。他在学校表现良好，但从未出类拔萃，通常在大约20名学生的班级中排名第三或第四。如果说他真正擅长某一科目，那也许是体育，这或许令人惊讶。他是一名出色的足球运动员，但不如他的兄弟哈拉尔·玻尔，后者曾代表丹麦参加足球比赛并获得银牌。尼尔斯·玻尔在学校时结交了一些好朋友，但他一生中最好的朋友是他的兄弟哈拉尔·玻尔。

在学校的最后两年里，尼尔斯·玻尔专攻数学和物理学。确实有一些证据表明，他很快就意识到数学老师对这门学科的掌握并不如应有的那样好，并且这位老师对他这位非凡的学生尼尔斯·玻尔有些畏惧。在物理学方面，尼尔斯·玻尔也提前研读课本，并从中发现错误。与其说是学校的老师，不如说是他的父亲激发了他在数学和物理学方面的研究热情。他在1922年写道：-

我对物理学研究的兴趣是在我还上学时被唤醒的，这在很大程度上要归功于父亲的影响。

尼尔斯·玻尔于1903年进入哥本哈根大学学习。他以物理学为主修，同时辅修数学、天文学和化学。他的物理学老师是Christian Christiansen，哲学老师是Harald Hoffding。他认识这两人已有多年，因为他们是他父亲的密友，并且是一个定期讨论小组的成员，兄弟俩尼尔斯·玻尔和哈拉尔·玻尔一到能有所贡献的年龄就参与其中。尼尔斯·玻尔在大学里的数学老师是托瓦尔·尼古拉·蒂勒。

在大学里，尼尔斯·玻尔无法进行物理实验，因为没有物理实验室。然而，他的父亲有一个生理学实验室，他的第一篇论文描述了他在那个实验室里进行的物理学实验工作。他向他的兄弟哈拉尔·玻尔口述了这篇论文。一位同学这样描述尼尔斯·玻尔和哈拉尔·玻尔：-

这两人形影不离。我从未见过有人像他们这样亲密。

这篇论文是尼尔斯·玻尔唯一一篇描述他所做实验的论文。凭借这篇论文，他因分析水注振动作为测定表面张力的一种方法而获得了丹麦皇家科学院1906年的金质奖章。他于1909年获得哥本哈根大学的硕士学位，并于1911年5月获得博士学位，学位论文题为Studies on the electron theory of metals。这是一篇基于经典物理学的论文，因此必然无法解释某些效应。尼尔斯·玻尔在这部著作中写道：-

在电子理论发展的现阶段，似乎不可能从这一理论出发来解释物体的磁性。

尼尔斯·玻尔将他的学位论文献给了几个月前，即1911年2月，因心脏病发作去世的父亲。此时尼尔斯·玻尔已与Margrethe Norlund订婚。两人于1912年8月1日结婚，理查·科朗特在尼尔斯·玻尔去世后谈到他们的婚姻时这样说：-

有些人猜测是幸运的境遇共同造就了尼尔斯·玻尔如此成功。我认为他生命的要素绝非偶然，而是深深植根于他的人格结构之中……不是运气，而是深刻的洞察力，使他在年轻时就找到了他的妻子，众所周知，她在使他整个科学和个人活动成为可能并和谐融洽方面起到了决定性的作用。

尼尔斯·玻尔于1911年5月向嘉士伯基金会申请了一笔旅行资助，获得批准后，于1911年9月前往英国，在剑桥跟随J J Thomson爵士学习。他本打算整个学习期间都留在剑桥，但他与Thomson相处得不好，因此，在1911年12月在剑桥与Ernest Rutherford会面后，尼尔斯·玻尔于1912年3月转到了曼彻斯特的维多利亚大学（现曼彻斯特大学）。时机非常偶然，因为在尼尔斯·玻尔和卢瑟福相遇前不久，卢瑟福发表了一项重要工作，表明原子的大部分质量集中在原子核中。

尼尔斯·玻尔与卢瑟福的小组合作研究原子结构。卢瑟福因其个人和科学品质成为尼尔斯·玻尔的榜样。利用马克斯·普朗克和阿尔伯特·爱因斯坦提出的quantum思想，尼尔斯·玻尔猜想原子只能存在于一组离散的稳定能量状态中。今天存在着尼尔斯·玻尔科学进展的显著证据，因为他经常与他的兄弟哈拉尔·玻尔通信。他在1912年6月12日写信给哈拉尔·玻尔：-

你可以想象，在这里很好，有这么多人可交谈……而且是和那些对这些事情知道最多的人；卢瑟福教授对所有他认为有价值的事情都抱有如此浓厚的兴趣。在过去几年里，他研究出了一种原子结构理论，这似乎比迄今为止存在的任何理论都更加稳固。

写完这封信一周后，即6月19日，尼尔斯·玻尔向哈拉尔·玻尔报告了进展：-

也许我已经对原子的结构有所发现。不要跟任何人谈起这件事，否则我就不能这么快写信告诉你了。……你明白，我可能还是错的；因为这件事还没有完全弄清楚（但我认为它没错）。……相信我，我急于赶快完成它，为此我从实验室请了几天假（这也是个秘密）。

到7月13日他写道：-

事情进展得相当顺利，因为我相信我已经发现了一些东西；但可以肯定的是，我并没有像我愚蠢地以为的那样快地把它弄出来。我希望准备好一篇小论文，在我离开之前给卢瑟福看，因此我太忙了，太忙了。

尽管卢瑟福和尼尔斯·玻尔性格完全不同，但他们对物理学有着巨大的热情，而且彼此个人之间也很喜欢。然而，他们的关系从来都不是那种亲密朋友的关系，因为尼尔斯·玻尔总是把卢瑟福看作自己的老师。从1911年他们相识起，直到1937年，也就是卢瑟福去世的那一年，他们一直通信。

1912年7月24日，论文尚未完成，尼尔斯·玻尔离开了卢瑟福在曼彻斯特的小组，回到哥本哈根继续发展他的新原子理论，并于1913年完成了这项工作。同年，他发表了关于原子结构理论的三篇具有根本重要性的论文。第一篇关于氢原子，后两篇关于比氢更重的原子的结构。在这些论文中尼尔斯·玻尔[5]：-

……阐述了他那令人惊叹的尝试，将经典物理学的某些方面与马克斯·普朗克的作用量子概念结合起来。……这三篇著名论文……构成了尼尔斯·玻尔早期声誉的基础。他的工作虽然并未立即被所有人接受，却引起了同时代人的兴趣，使他们意识到需要一种描述原子层面事件的新方式。尼尔斯·玻尔原子虽然在科学上已被取代，但即使在今天，仍作为原子样貌的生动形象和物理学的象征，留存在许多人的心中。

1913年7月，尼尔斯·玻尔被任命为哥本哈根的一个讲师（Docent）。然而，这并非令他满意的处境，因为他无法继续发展自己正在形成的数学物理风格。1914年3月10日，他致信教育事务部：——

署名者冒昧请求该部促成在大学设立理论物理学教授职位，并另外可能将该职位委托于我。

这是一项大胆的举动，但尼尔斯·玻尔已有的崇高声誉意味着他会受到认真对待。大学评议会推荐他担任理论物理学讲席，但教育事务部决定推迟确认该职位。当然，1914年时局不定，尼尔斯·玻尔意识到不太可能迅速作出决定。因此，他欣然接受了卢瑟福的邀请，以Schuster Reader的身份加入其曼彻斯特团队。他预计在曼彻斯特待一年，期待届时他在哥本哈根的理论物理学讲席能得到确认。他在前往曼彻斯特之前于蒂罗尔度假时，第一次世界大战爆发，使他的旅程极为艰难，但他和妻子在途中绕过苏格兰北部、历经猛烈风暴后，于1914年10月抵达曼彻斯特。

尼尔斯·玻尔在曼彻斯特待的时间比他预期的更长，因为他的讲席直到1916年4月才得到确认。然而，那是一段非常富有成效且愉快的时期。Pais在[13]中写道：——

1916年初夏，玻尔一家返回丹麦。四年前，尼尔斯·玻尔离开曼彻斯特时，满脑子关于原子的激动人心却未经消化的想法。如今他离开时已是该领域的大师，作为哥本哈根的教授，身边是正怀着他们第一个孩子的妻子。

1917年，尼尔斯·玻尔当选为丹麦皇家科学院会士，并开始筹划在哥本哈根建立理论物理研究所。该研究所是为他而创建的，自1921年成立起，他便担任所长，直至终生[3]：-

该研究所很快成为世界各地理论物理学家的圣地，1933年后又成为许多逃离希特勒德国的科学家的避难所。他们的社交中心是“老嘉士伯”宅邸，由著名啤酒厂的创始人捐给国家，并于1932年交由尼尔斯·玻尔使用。在这里，在尼尔斯·玻尔美丽的妻子玛格丽特的慈母般照料下，来自各国的学生和学者聚集在一起吃饭、交谈、听音乐，还常常真真切切地坐在尼尔斯·玻尔的脚下，努力捕捉他以柔和的丹麦口音说出的挑战性言论、微妙的评论和温和的玩笑。

尼尔斯·玻尔最为人所知的是上文提到的原子结构研究，以及在辐射方面的工作，这使他获得了1922年诺贝尔物理学奖。1922年12月11日，他在斯德哥尔摩就获奖工作发表了演讲。他谈到了原子稳定性和电动力学理论，叙述了量子理论的起源、氢光谱，解释了元素之间的关系。他的解释涵盖了光谱线的吸收和激发，以及他在1918年至1922年间在三篇论文On the quantum theory of spectra中提出的对应原理。

1923年，尼尔斯·玻尔总结了这些想法：-

尽管这些假设从根本上背离了经典力学和电动力学理论的思想，但仍有可能在原子发射的辐射与粒子运动之间找到一种联系，这种联系与经典辐射起源思想所主张的有着深远的相似性。

量子力学可以说在1925年诞生，两年后维尔纳·海森堡提出了他的不确定性原理。1927年9月，在意大利北部科莫举行的一次会议上，尼尔斯·玻尔提出了他的互补性原理，为维尔纳·海森堡的不确定性关系提供了物理解释。他提出了感知与图景、粒子与波、共轭变量、量子演化-经典测量等的互补性，作为量子理论基础的一种全新解释。尼尔斯·玻尔关于互补性的思想在[30]中得到了充分探讨。

尼尔斯·玻尔认为他的互补性思想可以在量子物理以外的领域发挥重要作用，他余生都在研究这些思想。他考虑了在生物学、心理学和认识论中的应用。有人提出互补性思想来自物理学之外，一些人认为这一思想的根源来自他还在上学时与父亲、Christiansen以及哲学家Hoffding的讨论。其他人，如[13]中的Pais，给出了令人信服的论据表明尼尔斯·玻尔并未有意受Hoffding哲学的影响。

最终被接受的是尼尔斯·玻尔的量子理论观点。阿尔伯特·爱因斯坦对尼尔斯·玻尔的解释表示严重怀疑，尼尔斯·玻尔、阿尔伯特·爱因斯坦和保罗·埃伦费斯特花了很长时间深入讨论，但尼尔斯·玻尔的观点占了上风。尼尔斯·玻尔表达了这一观点，他说：-

在不同实验条件下获得的证据无法在单一图景中被理解，而必须被视为互补的，其意义在于只有现象的整体才能穷尽关于客体的可能信息。

H B G Casimir写道，描述了在他的研究所与尼尔斯·玻尔共事是什么样子：-

尼尔斯·玻尔比我们任何人都更专注、更有耐力，他在填字游戏、体育运动和诙谐的讨论中寻找放松。

除量子理论外，尼尔斯·玻尔的其他主要贡献还包括他在1920年前后对元素周期表的理论描述、1936年关于原子核是复合结构的理论，以及1939年他对铀裂变以同位素235为基础的理解。

1937年，尼尔斯·玻尔、他的妻子和他们的儿子汉斯进行了一次环球旅行。他们前往美国、日本、中国和苏联。同年，他参加了在伦敦威斯敏斯特教堂举行的卢瑟福的葬礼，并发表了感人的讲话：-

当我第一次有幸在他的亲自启发下工作时，他已经是声名最显赫的物理学家，但尽管如此，他当时以及始终都愿意倾听一个年轻人心中所想。……在我们心中，对他的思念将永远是鼓励与坚毅的宝贵源泉。

尼尔斯·玻尔虽然曾在基督教会受洗，但母系有犹太血统，因此当纳粹于1940年占领丹麦时，他的生活变得极其艰难。1943年，他不得不乘渔船逃往瑞典。从那里他被飞机送往英国，开始从事制造核裂变炸弹的项目。几个月后，他随英国研究团队前往美国的洛斯阿拉莫斯，他们在那里继续该项目的工作。

尼尔斯·玻尔深切关注核武器的控制，并从1944年起试图说服丘吉尔和罗斯福有必要进行国际合作。他于1950年写了一封致联合国的公开信，主张理性、和平的原子政策：-

人类将面临前所未有的危险，除非能及时采取措施，防止在如此可怕的军备上进行灾难性的竞争，并建立对强大材料制造和使用的国际控制。

尼尔斯·玻尔的儿子奥格也成为物理学家，并分享了1975年诺贝尔物理学奖。（这只是同一家族中著名科学家的一个例子。其他还有Van Vlecks以及布拉格父子和居里夫人及其女儿伊雷娜·约里奥。）

尼尔斯·玻尔于1957年获得首届美国原子能和平奖。1962年，他在家中因心脏病发作去世，此后世界各地的科学家和知名人士纷纷向他致敬。肯尼迪总统写道（例如见[53]）：-

美国科学家，实际上所有知道尼尔斯·玻尔博士的名字及其伟大贡献的美国公民，在超过两代人的时间里都尊敬和崇敬他……

Niels Bohr's father was Christian Bohr and his mother was Ellen Adler. Christian Bohr was awarded a doctorate in physiology from the University of Copenhagen in 1880 and in 1881 he became a Privatdozent at the university. Late in the same year he married Ellen, who was the daughter of David Adler, a Jewish politician with a high standing in Danish political and commercial life. Christian and Ellen had three children. The eldest was Jenny born in 1883 in the mansion which David Adler had owned opposite Christiansborg Castle where the Danish Parliament sat. Ellen's mother had continued to live in this house after her husband David Adler died in 1878 and Ellen had gone back to her mother's home to have her child. Two years later Niels was born on his mother's 25th birthday in the same stately home, Ellen again having returned to her mother's house for the birth of her child. The third child of the family, who went on to become a famous mathematician, was Harald Bohr who was two years younger than Niels.

When Niels was only a few months old his father Christian had been appointed as a lecturer to fill a post left vacant by the death of Peter Panum, the professor of physiology at the University of Copenhagen, and a short while later the family moved into the Panum's professorial house in Copenhagen.

Kennedy writes in [5]:-

Niels, Harald, and their older sister, Jenny, grew up in a cultured and stimulating home. From their earliest days they were exposed to a world of ideas and discussion, of conflicting views rationally and good-temperedly examined, and they developed a respect for all who seek deeper knowledge and understanding.

In October 1891 Niels entered the Grammelholms school. He attended this school, as did his brother Harald, for his complete secondary education taking his Studenterexamen in 1903. He did well at school without ever being brilliant, usually coming third or fourth in a class of about 20 students. If he really excelled at a subject it was, perhaps surprisingly, physical education. He was an excellent soccer player, yet not as good as his brother Harald who won a silver medal playing soccer for Denmark. Niels made some good friends while at school but his best friend throughout his life was his brother Harald.

During his last two years at school Niels specialised in mathematics and physics. There is certainly some evidence that he soon realised that the mathematics teacher did not have as good a grasp of the topic as he should have had, and that he became somewhat frightened of his exceptional pupil Bohr. In physics too Bohr studied texts ahead of the class finding errors in them. It was his father, more than his school teachers, who inspired him in his studies of mathematics and physics. He wrote in 1922:-

My interest in the study of physics was awakened while I was still in school, largely owing to the influence of my father.

Bohr studied at the University of Copenhagen which he entered in 1903. He studied physics as his main subject but took mathematics, astronomy and chemistry as minor subjects. He was taught physics by Christian Christiansen and philosophy by Harald Hoffding. He had known both of them for many years since they were close friends with his father and had met as part of a regular discussion group, with both brothers Niels and Harald Bohr taking part as soon as they were old enough to contribute. Bohr was taught mathematics at university by Thorvald Thiele.

At university Bohr could not carry out physics experiments since there was no physics laboratory. However his father had a physiology laboratory and his first paper describes experimental work in physics which he carried out in that laboratory. He dictated the paper to his brother Harald. A fellow student wrote of Niels and Harald:-

The two are inseparable. I have never known people to be as close as they are.

This paper is the only one that Bohr wrote describing experiments which he had carried out. With it he won the Gold Medal for 1906 from the Royal Danish Academy of Sciences for his analysis of vibrations of water jets as a means of determining surface tension. He received his Master's degree from the University of Copenhagen in 1909 and his doctorate in May 1911 for a thesis entitled Studies on the electron theory of metals. It was a thesis based on classical physics and as such necessarily failed to explain certain effects. Bohr wrote in this work:-

It does not seem possible at the present stage of the development of the electron theory to explain the magnetic properties of bodies from this theory.

Bohr dedicated his thesis to the memory of his father who had died from a heart attack a few months earlier in February 1911. By this time Bohr was engaged to Margrethe Norlund. The pair married on 1 August 1912 and Richard Courant, speaking after Bohr's death, had this to say of their marriage:-

Some people have speculated about the lucky circumstances which combined to make Niels so successful. I think the ingredients of his life were by no means matters of chance but deeply ingrained in the structure of his personality ... It was not luck, rather deep insight, which led him to find in young years his wife, who, as we all know, had such a decisive role in making his whole scientific and personal activity possible and harmonious.

Bohr applied to the Carlsberg Foundation for a travel grant in May 1911 and, after the award was made, went to England in September 1911 to study with Sir J J Thomson at Cambridge. He had intended to spend his entire study period in Cambridge but he did not get on well with Thomson so, after a meeting with Ernest Rutherford in Cambridge in December 1911, Bohr moved to the Victoria University, Manchester (now the University of Manchester) in March 1912. The timing was very fortuitous since shortly before Bohr and Rutherford met, Rutherford had published a major work showing that the bulk of the mass of an atom resided in the nucleus.

In Manchester Bohr worked with Rutherford's group on the structure of the atom. Rutherford became Bohr's role model both for his personal and scientific qualities. Using quantum ideas due to Planck and Einstein, Bohr conjectured that an atom could exist only in a discrete set of stable energy states. Remarkable evidence exists today of Bohr's scientific progress since he corresponded frequently with his brother Harald. He wrote to Harald on 12 June 1912:-

You can imagine it is fine to be here, where there are so many people to talk with ... and this with those who know most about these things; and Professor Rutherford takes such a lively interest in all that he believes there is something in. In the last years he has worked out a theory of the structure of atoms, which seems to be quite a bit more firmly founded than anything which has existed up to now.

A week after writing this letter, on 19 June, Bohr was reporting progress to Harald:-

Perhaps I have found out a little about the structure of atoms. Don't talk about it to anyone, for otherwise I couldn't write to you about it so soon. ... You understand that I may yet be wrong; for it hasn't been worked out fully yet (but I don't think its wrong). ... Believe me, I am eager to finish it in a hurry, and to do so I have taken a couple of days off from the laboratory (this is also a secret).

By the 13 July he wrote:-

Things are going rather well, for I believe I have found out a few things; but, to be sure, I have not been so quick to work them out as I was stupid to think. I hope to have a little paper ready and to show it to Rutherford before I leave, and I therefore am so busy, so busy.

Although Rutherford and Bohr had completely different personalities, they shared an enormous enthusiasm for physics and they also liked each other personally. However the relationship was never quite that of close friends since Bohr always saw Rutherford as his teacher. They corresponded from the time they met in 1911 until 1937, the year of Rutherford's death.

On 24 July 1912, with his paper still unfinished, Bohr left Rutherford's group in Manchester and returned to Copenhagen to continue to develop his new theory of the atom, completing the work in 1913. The same year he published three papers of fundamental importance on the theory of atomic structure. The first paper was on the hydrogen atom, the next two on the structure of atoms heavier than hydrogen. In these papers Bohr [5]:-

... set out his startling attempt to combine aspects of classical physics with the concept of Planck's quantum of action. ... The three famous papers ... formed the foundation of Bohr's early reputation. His work, although not immediately accepted by everyone, intrigued his contemporaries and made them aware of the need for a new way of describing events at atomic level. The Bohr atom, although it has been superseded scientifically, persists even today in the minds of many people as a vivid image of what atoms look like and a symbol of physics.

In July 1913 Bohr was appointed as a docent in Copenhagen. However it was not a situation which pleased him since he could not pursue the style of mathematical physics which he was developing. On 10 March 1914 he wrote to the Department of Educational Affairs:-

The undersigned takes the liberty of petitioning the department to bring about the founding of a professorship in theoretical physics at the university and in addition to possibly entrust me with that position.

It was a bold move but Bohr's already high reputation meant that he would be taken seriously. The Faculty of the University recommended him for a chair of theoretical physics but the Department of Educational Affairs decided to delay confirming the post. Of course in 1914 times were uncertain and Bohr realised that no quick decision was likely. He therefore was delighted to accept an offer by Rutherford to join his Manchester group as Schuster Reader. He expected to be in Manchester for a year, anticipating that his chair of theoretical physics in Copenhagen would be confirmed by then. The outbreak of World War I while he was on holiday in the Tyrol before travelling to Manchester made his journey extremely difficult, but he and his wife arrived in Manchester in October 1914 having sailed round the north of Scotland through severe storms on their way.

Bohr was in Manchester longer than he expected since his chair was not confirmed until April 1916. However, it was a very productive and happy period. Pais writes in [13]:-

In the early summer of 1916 the Bohrs returned to Denmark. Four years earlier Bohr had left Manchester full of exciting but undigested ideas about the atom. Now he departed as the master of that field, as a professor in Copenhagen, with his wife who was expecting their first child at his side.

In 1917 Bohr was elected to the Royal Danish Academy of Sciences and he began to plan for an Institute of Theoretical Physics in Copenhagen. This was created for him and, from its opening in 1921, he became its director, a position he held for the rest of his life [3]:-

That Institute soon became a Mecca for theoretical physicists from all over the world, and after 1933 a refuge for a good many scientists who had fled from Hitler's Germany. Their social centre was the mansion "Gamle Carlsberg", given to the nation by the founder of the well-known brewery and placed at Niels Bohr's disposal in 1932. Here, under the motherly care of Bohr's beautiful wife, Margrethe ... students and scholars of all nations gathered to eat and talk and listen to music, and often to sit quite literally at the feet of Bohr, trying to catch his challenging remarks, subtle comments and gentle jokes, spoken in his soft Danish voice.

Bohr is best known for the investigations of atomic structure referred to above and also for work on radiation, which won him the 1922 Nobel Prize for physics. He gave a lecture on the work for which he was awarded the Prize on 11 December 1922 in Stockholm. He talked of atomic stability and electrodynamic theory giving an account of the origins of quantum theory, the hydrogen spectrum, explaining the relationships between the elements. His explanation covered the absorption and excitation of spectral lines and the correspondence principle which he had set out in three papers On the quantum theory of spectra between 1918 and 1922.

In 1923 Bohr summed up the ideas:-

Notwithstanding the fundamental departure from the ideas of the classical theories of mechanics and electrodynamics involved in these postulates, it has been possible to trace a connection between the radiation emitted by the atom and the motion of the particles which exhibits a far-reaching analogy to that claimed by the classical ideas of the origin of radiation.

Quantum mechanics may be said to have arrived in 1925 and two years later Heisenberg stated his uncertainty principle. In a meeting at Como in north Italy in September 1927 Bohr put forward his principle of complementarity which gave a physical interpretation of Heisenberg's uncertainty relations. He proposed complementarity of perceptions and pictures, particle-wave, conjugate variables, quantum evolution - classical measurements etc. as a fundamentally new interpretation of the foundations of quantum theory. Bohr's ideas on complementarity are fully explored in [30].

Bohr thought that his idea of complementarity could play an important role in fields other than quantum physics and he worked on these ideas throughout the rest of his life. He considered applications to biology, psychology and epistemology. It has been suggested that the idea of complementarity came from outside physics, some arguing that the roots of the idea came from the discussions with his father, Christiansen and the philosopher Hoffding when he was still at school. Others, such as Pais in [13], give convincing arguments to show that Bohr was not knowingly influenced by Hoffding's philosophy.

It was Bohr's view of quantum theory which was eventually to become accepted. Einstein expressed grave doubts about Bohr's interpretation and Bohr, Einstein and Ehrenfest spent many hours in deep discussion, but Bohr's view prevailed. Bohr expressed this view saying:-

Evidence obtained under different experimental conditions cannot be comprehended within a single picture, but must be regarded as complementary in the sense that only the totality of the phenomena exhausts the possible information about the objects.

H B G Casimir wrote describing what it was like working with Bohr in his Institute:-

Even Bohr who concentrated more intensely and had more staying power than any of us, looked for relaxation in crossword puzzles, in sports, and in facetious discussions.

Bohr's other major contributions, in addition to quantum theory, include his theoretical description of the periodic table of elements around 1920, his theory of the atomic nucleus being a compound structure in 1936, and his understanding of uranium fission in terms of the isotope 235 in 1939.

In 1937 Bohr, his wife and their son Hans, made a world tour. They travelled to the United States, Japan, China, and the USSR. In the same year he attended Rutherford's funeral in Westminster Abbey in London, giving a moving speech:-

When I first had the privilege of working under his personal inspiration he was already a physicist of the greatest renown, but nevertheless he was then, and always remained, open to listen to what a young man had on his mind. ... The thought of him will always be to us an invaluable source of encouragement and fortitude.

Bohr, although he had been christened in the Christian Church, had Jewish origins on his mother's side and so, when the Nazis occupied Denmark in 1940, his life became exceeding difficult. He had to escape in 1943 by being taken to Sweden by fishing boat. From there he was flown to England where he began to work on the project to make a nuclear fission bomb. After a few months he went with the British research team to Los Alamos in the United States where they continued work on the project.

However Bohr was deeply concerned about the control of nuclear weapons and from 1944 he tried to persuade Churchill and Roosevelt for the need to have international cooperation. He wrote a public letter to the United Nations in 1950 arguing for rational, peaceful atomic policies:-

Humanity will be confronted with dangers of unprecedented character unless, in due time, measures can be taken to forestall a disastrous competition in such formidable armaments and to establish an international control of the manufacture and use of powerful materials.

Bohr's son Aage also became a physicist and shared the Nobel prize for Physics in 1975. (This is just one instance of famous scientists in the same family. Others are the Van Vlecks as well as the Braggs and Madame Curie and her daughter Irene Joliot.)

Bohr received the first U.S. Atoms for Peace Award in 1957. He died from a heart attack in his home in 1962 and following this scientists and leading figures world-wide joined in paying tributes to him. President Kennedy wrote (see for example [53]):-

American scientists, indeed all American citizens who knew Doctor Bohr's name and his great contributions, have respected and venerated him for more than two generations ...