马克斯·普朗克Max Planck

马克斯·普朗克出身于一个学术家庭，他的父亲尤利乌斯Wilhelm Planck在他出生时是基尔大学的宪法学教授，他的祖父和曾祖父都曾是哥廷根的神学教授。他的母亲艾玛·帕齐希是他父亲的第二任妻子。普朗克的父母在他出生时都相对年长，父亲41岁，母亲37岁。他出生在一个大家庭中，是他父亲的第六个孩子（其中两个孩子来自他与玛蒂尔德·福格特的第一次婚姻），他在一个极其尊重学术、诚实、公正和慷慨的传统中长大。他幼年时被赋予的价值观很快成为他一生珍视的价值观，对国家机构和教会表现出最大的尊重。

普朗克在基尔开始上小学。1867年春天，他父亲被任命为教授后，全家搬到了慕尼黑。这座城市为这个年幼的男孩提供了激励人心的环境，他喜爱这里的文化，尤其是音乐，并且当全家去上巴伐利亚远足时，他喜欢在山中散步和攀登。他在那里上了中学，于1867年5月进入著名的马克西米利安文理中学。他在学校表现不错，但并非出类拔萃，通常在班上排第三到第八名之间。音乐也许是他最好的科目，他几乎每年都获得学校教义问答和品行优良奖。人们可能会期望他在数学和科学方面出类拔萃，但在他早期的学校岁月里，尽管他表现良好，这些科目却没有任何杰出才华的迹象。然而，在他学校生涯接近尾声时，他的老师雅各布·赫尔曼 埃米尔·穆勒提高了他对物理和数学的兴趣，他深深折服于能量守恒定律的绝对性。一份1872年的学校报告写道：-

理所当然地受到老师和同学的喜爱……尽管有些孩子气，但他头脑非常清晰、有逻辑。显示出巨大前途。

1874年7月，16岁的他以优异成绩通过了中学毕业考试，但由于他在广泛科目上都有才能，尤其是音乐（他钢琴和管风琴都弹得极好），他仍然不清楚自己应该在大学学习什么。在慕尼黑大学开始学习之前，他与一位音乐家讨论了从事音乐职业的可能性，那位音乐家告诉他，如果他不得不问这个问题，那最好还是学点别的。

他于1874年10月21日进入慕尼黑大学，由Philipp von Jolly和Wilhelm Beetz教授物理学，由路德维希·赛德尔和Gustav Bauer教授数学。在课程开始时主要选修数学课程后，他向慕尼黑大学物理学教授Philipp von Jolly询问了物理学研究的前景，被告知物理学本质上是一门完整的科学，几乎没有进一步发展的前景。幸运的是，尽管向他展示了研究前景黯淡，普朗克还是决定学习物理学。

在[7]中，普朗克描述了他为什么选择物理：

外部世界是某种独立于人的东西，某种绝对的东西，而对适用于这个绝对的定律的探求，在我看来是生活中最崇高的科学追求。

他物理学教授的那些令人反感的评论，显然为他在慕尼黑大学的时光定下了基调，普朗克后来写道：-

我没有幸有一位著名科学家或教师指导我具体的教育进程。

1875年夏季学期他生病了，这使他一度放弃了学业。当时德国学生在大学之间转学是惯例，事实上普朗克于1877年10月转学到柏林大学，他的老师包括卡尔·魏尔斯特拉斯、赫尔曼·冯·亥姆霍兹和古斯塔夫·基尔霍夫。他后来写道，他非常钦佩古斯塔夫·基尔霍夫，但觉得他作为教师枯燥单调。然而，很可能正是他在慕尼黑的老师与柏林老师的研究态度之间的对比，促成了我们上面引用的那句话（多年后所说）。不过，他在柏林受教育的一个重要部分来自独立学习，因为在这个阶段他阅读了Rudolf 鲁道夫·克劳修斯关于热力学的文章。热力学第二定律的绝对性质再次给他留下了深刻印象。

普朗克于1879年7月回到慕尼黑，时年21岁，凭借一篇关于热力学第二定律的学位论文On the Second Law of Mechanical Theory of Heat获得博士学位。1879年7月28日，该博士学位以“最优等”成绩授予。此后，普朗克继续为取得他的教授资格论文（Habilitation）而努力，在提交了关于熵和热的机械理论的论文后，于1880年6月14日获得该资格，并成为慕尼黑大学的Privatdozent。这样的教学职位是无薪的，因此普朗克没有收入来维持生计。在担任此职的五年间，他与父母同住，但对于继续依靠他们的花费生活感到相当内疚。在此期间，他与Carl 卡尔·龙格成为朋友，这段友谊发展为长久且学术上富有成果的关系。

1885年5月2日，普朗克被任命为基尔大学理论物理学编外教授，并担任此讲席四年。这使他经济上有了保障，因此得以与相识多年的玛丽·默克结婚。她是慕尼黑一位银行家的女儿，两人于1887年3月31日成婚。他此时研究热力学，发表了关于物理化学和热电学应用的三篇优秀论文。

1887年10月古斯塔夫·基尔霍夫去世后，柏林大学寻找一位世界顶尖物理学家来替代他，并成为赫尔曼·冯·亥姆霍兹的同事。他们接触了Ludwig 路德维希·玻尔兹曼，但他不感兴趣，海因里希·鲁道夫·赫兹也同样如此。1888年，柏林大学哲学系提议任命普朗克，并得到赫尔曼·冯·亥姆霍兹的大力推荐：-

普朗克的论文与他大多数同事的论文相比，有着非常有利的显著区别，因为他试图建设性地贯彻热力学的严格推论，而不添加额外假设，并仔细区分可靠与可疑之处……他的论文……清楚地表明他是一个有独创思想、走自己道路的人[并且]他对科学的各个领域有全面的了解。

普朗克于1888年11月29日被任命为柏林大学理论物理学特聘教授，同时成为理论物理研究所所长。他于1892年5月23日晋升为正教授，并担任该讲席直到1927年10月1日退休。他的同事和朋友包括Émile du Bois-Reymond（著名生理学家，Paul du Bois-Reymond的兄弟）、赫尔曼·冯·亥姆霍兹、Pringsheim、威廉·维恩，以及神学家、历史学家和语文学家。他继续沉溺于对音乐的热情，让人制造了一台每个八度有104个音的风琴，并在自己家中举办音乐会。

在柏林期间，普朗克 做出了他最辉煌的工作，并发表了杰出的演讲。他研究热力学，特别是考察能量按波长的分布。通过结合 威廉·维恩 和 瑞利 的公式，普朗克 于1900年10月宣布了一个现在被称为 普朗克 辐射公式的公式。在两个月内，普朗克 放弃经典物理学并引入能量量子，对他的公式做出了完整的理论推导。1900年12月14日，他在柏林物理学会的一次会议上提出了涉及能量量子的理论解释。这样做时，他不得不放弃他关于热力学第二定律是自然界绝对定律的信念，并接受 路德维希·玻尔兹曼 的解释，即它是一个统计定律。在一年后写的一封信中，普朗克 描述了他提出辐射公式的理论解释，说：-

……整个过程是一种绝望之举，因为无论如何都必须找到一个理论解释，无论代价可能有多高。

普朗克 因这一成就于1918年获得诺贝尔物理学奖。他在1920年6月2日所作的诺贝尔演讲中，比我们上面所给出的要详细得多地描述了他如何做出他的发现。我们现在给出该演讲的一些摘录：-

多年来，[我的目标]是解决辐射热正常光谱中的能量分布问题。在 古斯塔夫·基尔霍夫 表明，在任何处于均匀温度的发射和吸收材料所包围的空腔中发生的热辐射状态完全独立于材料的性质之后，一个普遍函数被证明只依赖于温度和波长，而完全不依赖于材料的性质。这个非凡函数的发现承诺更深入地洞察能量与温度之间的联系，这实际上是热力学中以及因此所有分子物理学中的主要问题。……

当时我抱有今天会被认为天真迷人而令人愉快的期望，即经典电动力学的定律，如果以足够一般的方式处理，避免特殊假设，将使我们能够理解我们所预期的过程中最重要的部分，从而实现所期望的目标。……

[若干不同的进路]越来越清楚地表明，必定缺少了一个重要的连接要素或项，而它对于完全把握问题的基础是必不可少的。……

从[我建立一个新的辐射公式]那天起，我就忙于……寻找该公式的真正物理解释这一任务，而这个问题自动引导我去考虑熵与probability之间的联系，也就是路德维希·玻尔兹曼的思路；最终，在我一生中最艰苦的工作过了几周之后，光明进入了黑暗，一个不可思议的新前景在我面前展开。……

因为[辐射定律中的一个常数]代表能量与时间的乘积……我把它描述为作用量子。……只要它被视为无限小……一切都没问题；但在一般情况下，某处会打开一个宽阔的缺口，所考虑的振动越弱越快，这个缺口就越显著。所有弥合这一鸿沟的努力都失败了，这很快就几乎没有疑问了。要么作用量子是一个虚构的量，那么辐射定律的整个推导本质上是一种幻觉，只代表毫无意义的公式空玩，要么辐射定律的推导基于健全的物理概念。在后一种情况下，作用量子必须在物理学中起根本作用，而这里有些全新的、前所未闻的东西，似乎要求我们从根本上修正我们所有的物理思维，这种思维自 哥特弗里德·威廉·莱布尼茨 和 艾萨克·牛顿 建立微积分以来，就建立在接受所有因果联系的连续性之上。实验决定是第二种选择。

起初该理论遇到阻力，但由于 尼尔斯·玻尔 在1913年使用该理论计算谱线位置的出色工作，它被普遍接受。普朗克 本人在[7]中解释了尽管他发明了 quantum theory，他起初自己并不理解它：-

我立即尝试以某种方式将基本作用量子纳入经典理论的框架。但面对所有这些尝试，这个常数都表现得顽固不化……我将基本作用量子纳入经典理论的徒劳尝试持续了多年，耗费了我大量精力。

普朗克在做出历史性的量子宣告时42岁，但在量子理论的后续发展中只起了很小的作用。这留给了阿尔伯特·爱因斯坦的光量子理论、儒勒·昂利·庞加莱从数学上证明量子是普朗克辐射定律的必然结果、尼尔斯·玻尔的原子理论、Paul保罗·狄拉克等人。遗憾的是，在他卓越地开创量子力学研究之后的岁月里，他的生活充满了悲剧。他的妻子Marie于1909年10月17日去世。他们有四个孩子：两个儿子Erwin和Karl，以及双胞胎女儿Margarete和Emma。在第一任妻子去世两年后，普朗克于1911年3月14日再次结婚，娶了Margarete von Hösslin，即他第一任妻子Marie的侄女。他们有一个孩子，一个儿子赫尔曼。Karl是普朗克第一次婚姻所生儿子中较小的一个，于1916年第一次世界大战期间阵亡。他的两个女儿都死于分娩，Margarete于1917年，Emma于1919年。他的儿子Erwin成了他最好的朋友和顾问，但正如我们下面所述，Erwin死于更加可怕的境况。

普朗克除了研究工作之外，还一直承担行政职务，例如柏林科学院数学与自然科学部的秘书，他从1912年到1943年担任该职。他于1894年当选为科学院院士。普朗克深度参与德国物理学会，担任司库和委员会成员。他从1905年到1908年担任学会主席，然后从1915年到1916年再次担任。普朗克还于1927年当选为荣誉会员，这是一种荣誉。两年后设立了一个奖项，即普朗克奖章，普朗克本人成为首位获奖者。他从1916年起担任Kaiser Wilhelm Gesellschaft（德国主要研究组织）的委员会成员，并从1930年到1937年担任该学会主席（该学会更名为Max Planck Society）。这正是纳粹上台的时期，他尽力阻止政治问题取代科学问题。他无法阻止纳粹对学会的改组，并拒绝接受改组后学会的主席职位。

第二次世界大战期间他留在德国，经历了想必是最艰难的时期。1942年，他解释了自己为什么仍在柏林：-

自1889年以来，我一直在柏林这所大学……所以我算是个老资格了。但实际上并没有真正的老柏林人，即出生在这里的人；在学术界，每个人都频繁地搬来搬去。人们从一所大学转到下一所大学，但在这个意义上，我其实非常安土重迁。然而，一旦我来到柏林，要搬走就不容易了；因为归根结底，这里是整个德国所有知识活动的中心。

然而，在第二次世界大战末期身处柏林非常危险。他于1943年搬到马格德堡附近的Rogätz。他在柏林Grunewald郊区的家于1944年2月一次空袭后被大火烧毁。失去家和财产很糟糕，但失去他无可替代的科学笔记本对他和科学来说都是一场悲剧。更糟的还在后面。他的儿子Erwin被怀疑参与了1944年7月20日刺杀希特勒的阴谋，并于1945年初被盖世太保处决。在[4]中，汉斯·海尔布龙描述了战争对普朗克及其家人的影响：-

即使到了晚年，他仍会记得六岁时普鲁士和奥地利军队开进他家乡的情景。在他一生中，战争给他带来了深切的个人悲痛。他在第一次世界大战中失去了长子。第二次世界大战中，他在柏林的家在一次空袭中被烧毁。1945年，他的另一个儿子因被判定参与刺杀希特勒的阴谋而被处决。

普朗克在第二次世界大战结束时已经87岁，他被盟军带到了哥廷根。考虑到他的年龄，他能够致力于重建德国科学，这非常了不起，并且在1945-1946年他再次成为Kaiser Wilhelm Gesellschaft的主席。他第二次在一个异常困难的时期捍卫了德国科学。

Max Planck came from an academic family, his father Julius Wilhelm Planck being Professor of Constitutional Law in the University of Kiel at the time of his birth, and both his grandfather and great-grandfather had been professors of theology at Göttingen. His mother, Emma Patzig, was his father's second wife. Both Max's parents were relatively old when he was born, his father being 41 and his mother being 37. He was born into a large family, being his father's sixth child (two of the children were from his first marriage to Mathilde Voigt), and he was brought up in a tradition which greatly respected scholarship, honesty, fairness, and generosity. The values he was given as a young child quickly became the values that he would cherish throughout his life, showing the utmost respect for the institutions of state and church.

Max began his elementary schooling in Kiel. In the spring of 1867 his family moved to Munich when his father was appointed Professor there. This city provided a stimulating environment for the young boy who enjoyed its culture, particularly the music, and loved walking and climbing in the mountains when the family took excursions to Upper Bavaria. He attended secondary school there, entering the famous Maximilian Gymnasium in May 1867. He did well at school, but not brilliantly, usually coming somewhere between third and eighth in his class. Music was perhaps his best subject and he was awarded the school prize in catechism and good conduct almost every year. One might have expected him to excel in mathematics and science, but certainly in his early school years, although he did well, there was no sign of outstanding talent in these subjects. However, towards the end of his school career, his teacher Hermann Müller raised his level of interest in physics and mathematics, and he became deeply impressed by the absolute nature of the law of conservation of energy. A school report for 1872 reads:-

Justifiably favoured by both teachers and classmates ... and despite having childish ways, he has a very clear, logical mind. Shows great promise.

In July 1874, at the age of 16, he passed his school leaving examination with distinction but, having talents for a wide variety of subjects particularly music (he played piano and organ extremely well), he still did not have a clear idea of what he should to study at university. Before he began his studies at the University of Munich he discussed the possibility of a musical career with a musician who told him that if he had to ask the question he had better study something else.

He entered the University of Munich on 21 October 1874 and was taught physics by Philipp von Jolly and Wilhelm Beetz, and mathematics by Ludwig Seidel and Gustav Bauer. After taking mostly mathematics classes at the start of his course, he enquired about the prospects of research in physics from Philipp von Jolly, the professor of physics at Munich, and was told that physics was essentially a complete science with little prospect of further developments. Fortunately Planck decided to study physics despite the bleak future for research that was presented to him.

In [7] Planck describes why he chose physics:-

The outside world is something independent from man, something absolute, and the quest for the laws which apply to this absolute appeared to me as the most sublime scientific pursuit in life.

The off-putting comments from his physics professor clearly set the tone for his time at the University of Munich for Planck wrote later:-

I did not have the good fortune of a prominent scientist or teacher directing the specific course of my education.

He was ill during the summer term of 1875 which caused him to give up studying for a while. It was customary for German students to move between universities at this time and indeed Planck moved to study at the University of Berlin from October 1877 where his teachers included Weierstrass, Helmholtz and Kirchhoff. He later wrote that he admired Kirchhoff greatly but found him dry and monotonous as a teacher. However it is likely to be the contrast between the research attitude of his teachers at Munich and those at Berlin which prompted the quote we gave above (made many years later). One important part of his education at Berlin came, however, through independent study for at this stage he read Rudolf Clausius's articles on thermodynamics. Again the absolute nature of the second law of thermodynamics impressed him.

Planck returned to Munich and received his doctorate in July 1879 at the age of 21 with a thesis on the second law of thermodynamics entitled On the Second Law of Mechanical Theory of Heat. The award of the doctorate was made "summa cum laude'' on 28 July 1879. Following this Planck continued to work for his habilitation which was awarded on 14 June 1880, after he had submitted his thesis on entropy and the mechanical theory of heat, and he became a Privatdozent at Munich University. Such a teaching post was unpaid so Planck received no income to support himself. He lived with his parents during the five years that he held this post, but felt rather guilty that he was continuing to live at their expense. During this time he became friends with Carl Runge and it turned into a long lasting and academically fruitful friendship.

On 2 May 1885 Planck was appointed extraordinary professor of theoretical physics in Kiel and held this chair for four years. This now made him financially secure so he could now marry Marie Merck whom he had known for many years. She was the daughter of a Munich banker, and the pair were married on 31 March 1887. He now worked on thermodynamics publishing three excellent papers on applications to physical chemistry and thermoelectricity.

After the death of Kirchhoff in October 1887, the University of Berlin looked for a world leading physicist to replace him and to become a colleague of Helmholtz. They approached Ludwig Boltzmann but he was not interested, and the same proved true for Heinrich Hertz. In 1888 the appointment of Planck was proposed by the Faculty of Philosophy at the University of Berlin, strongly recommended by Helmholtz:-

Planck's papers are very favourably distinguished from those of the majority of his colleagues in that he tries to carry through the strict consequences of thermomechanics constructively, without adding additional hypotheses, and carefully separates the secure from the doubtful ... His papers ... clearly show him to be a man of original ideas who is making his own paths [and] that he has a comprehensive overview of the various areas of science.

Planck was appointed as an extraordinary professor of theoretical physics at the University of Berlin on 29 November 1888, at the same time became director of the Institute for Theoretical Physics. He was promoted to ordinary professor on 23 May 1892 and held the chair until he retired on 1 October 1927. His colleagues and friends included Émile du Bois-Reymond (the famous physiologist and brother of Paul du Bois-Reymond), Helmholtz, Pringsheim, Wien, as well as theologians, historians, and philologists. He continued to indulge his passion for music having a harmonium built with 104 tones in each octave, and holding concerts in his own home.

While in Berlin Planck did his most brilliant work and delivered outstanding lectures. He studied thermodynamics, in particular examining the distribution of energy according to wavelength. By combining the formulae of Wien and Rayleigh, Planck announced in October 1900 a formula now known as Planck's radiation formula. Within two months Planck made a complete theoretical deduction of his formula renouncing classical physics and introducing the quanta of energy. On 14 December 1900 he presented his theoretical explanation involving quanta of energy at a meeting of the Physikalische Gesellschaft in Berlin. In doing so he had to reject his belief that the second law of thermodynamics was an absolute law of nature, and accept Boltzmann's interpretation that it was a statistical law. In a letter written a year later Planck described proposing the theoretical interpretation of the radiation formula saying:-

... the whole procedure was an act of despair because a theoretical interpretation had to be found at any price, no matter how high that might be.

Planck received the Nobel Prize for Physics in 1918 for his achievement. He described in his Nobel Lecture given on 2 June 1920, in much more detail than we have given above, how he made his discoveries. We now give some extracts from the lecture:-

For many years, [my aim] was to solve the problem of energy distribution in the normal spectrum of radiating heat. After Gustav Kirchhoff has shown that the state of the heat radiation which takes place in a cavity bounded by any emitting and absorbing material at uniform temperature is totally independent of the nature of the material, a universal function was demonstrated which was dependent only on temperature and wavelength, but not in any way on the properties of the material. The discovery of this remarkable function promised deeper insight into the connection between energy and temperature which is, in fact, the major problem in thermodynamics and so in all of molecular physics. ...

At that time I held what would be considered today naively charming and agreeable expectations, that the laws of classical electrodynamics would, if approached in a sufficiently general manner avoiding special hypotheses, allow us to understand the most significant part of the process we would expect, and so to achieve the desired aim. ...

[A number of different approaches] showed more and more clearly that an important connecting element or term, essential to completely grasp the basis of the problem, had to be missing. ...

I was busy... from the day I [established a new radiation formula], with the task of finding a real physical interpretation of the formula, and this problem led me automatically to consider the connection between entropy and probability, that is, Boltzmann's train of ideas; eventually after some weeks of the hardest work of my life, light entered the darkness, and a new inconceivable perspective opened up before me. ...

Because [a constant in the radiation law] represents the product of energy and time ... I described it as the elementary quantum of action. ... As long as it was looked on as infinitely small ... everything was fine; but in the general case, however, a gap opened wide somewhere or other, which became more striking the weaker and faster the vibrations considered. That all efforts to bridge the chasm foundered soon left little doubt. Either the quantum of action was a fictional quantity, then the whole deduction of the radiation law was essentially an illusion representing only an empty play on formulas of no significance, or the derivation of the radiation law was based on a sound physical conception. In this case the quantum of action must play a fundamental role in physics, and here was something completely new, never heard of before, which seemed to require us to basically revise all our physical thinking, built as this was, from the time of the establishment of the infinitesimal calculus by Leibniz and Newton, on accepting the continuity of all causative connections. Experiment decided it was the second alternative.

At first the theory met resistance but, due to the successful work of Niels Bohr in 1913 calculating positions of spectral lines using the theory, it became generally accepted. Planck himself in [7] explains how despite having invented quantum theory he did not understand it himself at first:-

I tried immediately to weld the elementary quantum of action somehow in the framework of classical theory. But in the face of all such attempts this constant showed itself to be obdurate ... My futile attempts to put the elementary quantum of action into the classical theory continued for a number of years and they cost me a great deal of effort.

Planck who was 42 years old when he made his historic quantum announcement, took only a minor part in the further development of quantum theory. This was left to Einstein with theories of light quanta, Poincaré who proved mathematically that the quanta was a necessary consequence of Planck's radiation law, Niels Bohr with his theory of the atom, Paul Dirac and others. Sadly his life was filled with tragedy in the years following his remarkable initiation of the study of quantum mechanics. His wife Marie died on 17 October 1909. They had four children; two sons Erwin and Karl, and twin daughters Margarete and Emma. Two years after the death of his first wife, Planck married again, to Marga von Hösslin the niece of Marie his first wife, on 14 March 1911. They had one child, a son Hermann. Karl, the younger of Planck's sons from his first marriage, was killed in 1916 during World War I. Both his daughters died in childbirth, Margarete in 1917 and Emma in 1919. His son Erwin became his best friend and advisor, but as we relate below Erwin died in even more terrible circumstances.

Planck always took on administrative duties, in addition to his research activities, such as Secretary of the Mathematics and Natural Science Section of the Berlin Academy of Sciences, a post he held from 1912 until 1943. He had been elected to the Academy in 1894. Planck was much involved with the German Physical Society, being treasurer and a committee member. He was chairman of the Society from 1905 to 1908 and then again from 1915 to 1916. Planck was also honoured by being elected an honorary member in 1927. Two years later an award, the Max Planck Medal, was established and Planck himself became the first recipient. He was on the committee of the Kaiser Wilhelm Gesellschaft, the main German research organisation, from 1916 and was president of the Society from 1930 until 1937 (it was renamed the Max Planck Society). This was the time that the Nazis rose to power, and he tried his best to prevent political issues to take over from scientific ones. He could not prevent the reorganisation of the Society by the Nazis and refused to accept the presidency of the reorganised Society.

He remained in Germany during World War II through what must have been times of the deepest difficulty. In 1942 he explained why he was still in Berlin:-

I've been here in Berlin at the university since 1889 ... so I'm quite an old-timer. But there really aren't any genuine old Berliners, people who were born here; in the academic word everybody moves around frequently. People go from one university to the next one, but in that sense I'm actually very sedentary. But once I arrived in Berlin, it wasn't easy to move away; for ultimately, this is the centre of all intellectual activity in the whole of Germany.

However, being in Berlin towards the end of World War II, was very dangerous. He moved to Rogätz, near Magdeburg, in 1943. His home in the suburb of Grunewald in Berlin was destroyed by fire after an air raid in February 1944. Loosing his home and possessions was bad, but losing his irreplaceable scientific notebooks was a tragedy for him and for science. Worse was to follow. His son Erwin was suspected of being involved in the plot to assassinate Hitler on 20 July 1944 and was executed by the Gestapo early in 1945. In [4] Heilbronn describes the impact of wars on Planck and his family:-

He would remember, even in his old age, the sight of Prussian and Austrian troops marching into his native town when he was six years old. Throughout his life, war would cause him deep personal sorrow. He lost his eldest son during World War I. In World War II, his house in Berlin was burned down in an air raid. In 1945 his other son was executed when declared guilty of complicity in a plot to kill Hitler.

Planck was 87 years old at the end of World War II and he was taken to Göttingen by the allies. Remarkably, given his age, he was able to put effort into reconstructing German science and he again became president of the Kaiser Wilhelm Gesellschaft in 1945-1946. For the second time he defended German science through a period of exceptional difficulty.