威廉·德西特Willem de Sitter

威廉·德西特的父母是Lamoraal Ulbo 德西特和Catharine Theodore Wilhelmine Bertling。Lamoraal Ulbo 德西特是一名法官，坚定地遵循德西特家族成为律师的传统。他的儿子德西特被期望遵循家族传统，但他带着对数学和科学的热爱走上了不同的职业道路。德西特在阿纳姆上中学，就读于阿纳姆文理中学。从学校毕业后，他进入格罗宁根大学，打算攻读数学学位。除了对数学的热爱，他在本科期间对物理和进行物理实验的喜爱也与日俱增。他获得许可协助格罗宁根大学物理学教授Hermanus Haga正在进行的实验，因此开始在格罗宁根的天文实验室工作。在实验室里，天文学和理论力学教授Jacobus Kapteyn正在测量由天文学家弗洛伦斯·南丁格尔·大卫 Gill在开普敦天文台拍摄的南部天空摄影巡天的一部分照相底片。尽管Kapteyn是格罗宁根的天文学教授，但他自愿协助天文实验室，因为他没有天文台来进行自己的观测。

德西特一生的转折点出现在1896年，当时大卫吉尔访问卡普坦，讨论南天巡天调查的进展。进入实验室后，吉尔看到德西特在一台测量机旁，两人简短交谈[15]。在：-

……第二天早上，德西特正在房间里吃早餐时，收到一条消息，说Gill希望在实验室与他谈话。De Sitter当时还不能像后来那样流利地使用英语；Kapteyn当时正在讲课，Kapteyn夫人则在会面中担任口译。Gill邀请德西特到开普敦担任计算员，正如德西特后来在给Gill的信中所说，“从而完成我的天文学教育——或者更确切地说是开始它，因为在那之前我从未专攻天文学，而是打算成为一名数学家。”

德西特咨询了父母，决定听从他们的建议，先在格罗宁根完成考试，再接受Gill的邀请。他于1897年获得学士学位，并于同年夏天前往开普敦，8月抵达。他在南非开普敦天文台工作了两年，参与光度学和量日仪项目。正是在Gill的建议下，德西特还攻读博士学位，研究Gill于1891年对木星卫星所作的量日仪观测。德西特于1899年返回格罗宁根后，被任命为天文实验室的助理，并在Kapteyn的指导下继续攻读博士学位。他于1901年向格罗宁根大学提交了学位论文Discussion of Heliometer Observations of Jupiter's Satellites。Jan Hendrik Oort在[13]中写道：-

随着对德西特使用吉尔的日心仪观测木星卫星的讨论，他进入了一个似乎完全适合他多才多艺头脑的领域：一方面，具有清晰洞察最复杂数学问题并实际处理它们的才能；另一方面，对观测的价值和局限性的批判性洞察——这种洞察源于对观测和仪器的非常积极的兴趣。

1898年12月6日，德西特在南非开普敦与埃莱奥诺拉·苏尔蒙德结婚。埃莱奥诺拉出生于荷属东印度第二大城市泗水，她在开普敦当学校教师时遇到了德西特。他们的第一个孩子拉莫拉尔·乌尔博于1899年9月出生在开普敦，但不幸的是，在全家返回格罗宁根后，于1901年1月10日去世。他们的其他孩子有：西奥多拉，1900年9月28日出生于格罗宁根；拉莫拉尔·乌尔博，1902年3月6日出生于斯赫拉芬哈赫；阿尔诺特，1905年4月5日出生于格罗宁根；艾格尼丝，1908年5月14日出生于格罗宁根。在此记录其中两个孩子生活的一些细节。拉莫拉尔·乌尔博德西特作为构造地质学家而闻名，于1980年在尼斯特尔罗德去世。阿尔诺特德西特曾担任荷属东印度伦邦的博斯查天文台台长。在日本入侵荷属东印度后，他被逮捕并关入拘留营，于1944年9月5日在那里去世。

亨德里库斯·赫拉尔杜斯·范德桑德·巴库伊曾自1872年被任命以来一直担任莱顿大学天文学教授和大学天文台台长。他于1908年退休，其职责被分为两部分，天文学讲席与天文台台长职位分离。De Sitter被任命为天文学讲席，而H·G·范德桑德·巴库伊曾的兄弟欧内斯特-弗雷德里克·范德桑德·巴库伊曾被任命为天文台台长。就任讲席时，德西特作了关于The New Methods in Celestial Mechanics的就职演讲。E·F·范德桑德·巴库伊曾于1918年去世，次年德西特被任命为莱顿天文台台长，同时保留其教授职位。他对莱顿的天文学进行了彻底重组，将其分为三个部门：位置基础天文学或天体测量学；天体物理学；以及天体力学或理论天文学。在他的领导下，它成为世界领先的天文学中心之一。奥尔特写道[13]：-

在格罗宁根的这些年以及后来，德西特表现出几乎令人难以置信的活跃。翻阅他一些更重要的出版物，人们会惊讶于不同主题的讨论接踵而至的速度，以及每一个都体现出的深入批判性思考的量。无论是他多年来患有的严重疾病，还是他承担几乎完全重组的天文台台长职务，还是他与大学相关的许多事务中的持续活动，都无法减缓他科学研究的流动。

1913年，德西特基于对双星系统的观测提出了一个论点，证明光速与光源的速度无关。这平息了直到那时为止试图寻找依赖于光源速度但不与实验证据相冲突的光的发射理论的尝试。

德西特在1916年与保罗·埃伦费斯特通信，他提出四维时空将适合基于广义相对论的宇宙学模型。他发表了一系列论文（1916-17），关于阿尔伯特·爱因斯坦广义相对论的天文学后果。他在没有物质的情况下找到了阿尔伯特·爱因斯坦场方程的解。这很重要，因为马赫曾提出一个原理，即局部惯性参考系由宇宙中大规模的质量分布决定。德西特问道：-

如果除了试验物体之外不存在其他物质，它是否具有惯性。

德西特的工作直接导致了亚瑟·爱丁顿1919年的远征，以测量经过太阳附近的光线的引力偏折，当时只有在日食期间才能获得这样的结果。与阿尔伯特·爱因斯坦不同，德西特坚持认为相对论实际上意味着宇宙在膨胀，这些理论结果后来得到了观测验证，直到那时才被阿尔伯特·爱因斯坦接受。

事实上，阿尔伯特·爱因斯坦在1917年引入了宇宙学常数，以解决一个关于宇宙的重大问题，这个问题在他之前也困扰着艾萨克·牛顿，即为什么宇宙不会在引力吸引下坍缩。阿尔伯特·爱因斯坦引入的这个相当任意的积分常数，他承认它并没有得到我们对引力的实际知识的支持，后来被他称为“我一生中最大的错误”。然而，德西特在1919年写道，这一项：-

……损害了阿尔伯特·爱因斯坦原始理论的对称性和优雅性，该理论的主要吸引力之一在于它解释了很多现象，而没有引入任何新的假设或经验常数。

在20世纪90年代，观测证据表明宇宙的膨胀正在加速。将其纳入相对论模型的一种方法是重新引入宇宙学常数。这仍然是一个极其活跃的研究领域。

1932年，阿尔伯特·爱因斯坦和德西特发表了一篇联合论文，其中他们提出了阿尔伯特·爱因斯坦-德西特宇宙模型。这是广义相对论场方程对于膨胀宇宙的一个特别简单的解。他们在这篇论文中认为，可能存在大量不发光且尚未被探测到的物质。这种物质，现在被称为“暗物质”，此后通过观测其引力效应被证明存在。然而，阿尔伯特·爱因斯坦和德西特在1932年假设的暗物质仍然是一个谜，因为其性质仍然未知，但却是当今主要研究努力的主题。

德西特最著名的是他在相对论方面的工作，他还做出了许多其他具有重大意义的贡献。他的博士学位论文是关于木星的卫星的，他一生都对这个主题保持兴趣。他使用可追溯到1668年的卫星食数据，得出了四颗卫星的轨道要素和质量的确定性数据。他在1918年、1919年和1925年的三篇论文中发表了理论讨论。最后在1929年，他得出了确定性结果，但他在1934年去世时仍在研究卫星运动表。在他研究木星卫星食的时间时，他意识到他的数据受到地球自转速度变化的影响。德西特立刻有了另一个主题，他恰好拥有取得突破所需的技能。他的研究表明，存在变化的潮汐摩擦，影响地球和月球，此外，地球的转动惯量还会发生突然变化。

德西特进行的另一项研究是改进天文学基本常数的数据。西蒙·纽康在1895年发表了这些常数的值，并且在1896年巴黎的一次相当引人注目的国际协议中，决定世界上每个国家的历表都应使用西蒙·纽康的这些常数值。1915年，德西特发表了他的第一篇关于改进这些值的论文，这篇论文几乎完全关注地球的形状和组成。他在引言中写道：-

在各种场合，我发现自己面临着这样一个问题：对于某个基本天文常数，采用什么值最好。当然，我们有国家历书中正式采用的那套常数……有些被采用的常数彼此不一致。例如，采用的年长度和岁差常数是矛盾的，同样，地球质量和太阳视差也是如此。西蒙·纽康的伟大著作至今已超过三十年，但作为可靠批判性讨论的典范，它仍然无与伦比。……我只是出于这样的考虑：一方面，我的结果也许对处于类似情况的人有用；另一方面，像现在这样的批判性综述可能有助于引导天文学家的努力，指向那些最需要它们的地方。

他关于基本常数的第二篇论文发表于1927年，涉及与岁差、章动、太阳视差、月球视差和月球质量相关的常数。在他去世时，德西特几乎完成了这些常数的一次新更新。

由于他的杰出贡献，德西特获得了许多荣誉。1929年，他获得了国家科学院颁发的沃森金质奖章。该奖章由美国加利福尼亚大学天文学教授、沃森基金受托人委员会主席Armin Otto Leuschner在华盛顿授予他。Leuschner说：-

赋予我的这一特权涉及一项异常困难的任务，因为德西特的大部分工作是在复杂、思辨和数学领域中，涉及根据经典天体力学对太阳系天体运动的摄动，以及从纯思辨角度以及与观察到但未解释的摄动、以及观察尚未感知或尚无法感知的相对论理论摄动相关的相对论。因此，如果我未能清晰地呈现他对科学某些贡献的意义，我恳请你们的宽容。他的智力能力覆盖如此广泛的范围，如此深入和细致地渗透到实用天文学和解释所观察现象的数学理论中，只有对他辉煌工作的深入研究才能公正地评价这个人的伟大。

1931年，德西特获得了太平洋天文学会颁发的布鲁斯奖章。同样在1931年，他获得了皇家天文学会的金质奖章：-

……由于他对木星卫星轨道的理论研究，以及他对相对论的贡献。

他当选为国际天文学联合会主席，并于1925年至1928年间担任此职。

德西特患胸部疾病数年，但似乎已经克服。然而，他在年仅六十二岁时死于肺炎。以下是1934年11月25日发表在New York Times上对他的部分悼念文字；该文也收录于[8]中：-

在德西特的工作中，我们看到最具创造力的数学家处于最佳状态。他不是一个冷冰冰、不动感情地摆弄希腊字母的人，不是一个配平方程的人，而是一位艺术家，在他身上，想象的狂放飞翔受到符号语言的形式体系和观察证据的约束。只有音乐家才能完全领会，对德西特来说，看到宇宙在他的公式中以新的方式成形意味着什么。如同音符一样，那些代表被推测而非被知晓的力和质量的奇异符号，自行排列成连贯的信息。而当这信息被解读时，一个全新的宇宙展现出来。在这里，我们拥有某种对外部世界的直接个人体验，对自然奇观之意义的体验，这种体验只有贝多芬或弥尔顿才能获得。德西特的膨胀宇宙必须被视为不仅仅是从人类心智所熟悉的最严格的逻辑中得出的不可抗拒的结论。它是一种新式的诗——科学家撰写史诗的方式。

Willem de Sitter's parents were Lamoraal Ulbo de Sitter and Catharine Theodore Wilhelmine Bertling. Lamoraal Ulbo de Sitter was a judge and firmly in the de Sitter family tradition of becoming lawyers. His son Willem was expected to follow the family tradition, but he took a different career path with his love of mathematics and science. Willem attended secondary school in Arnhem, studying at the Arnhem Gymnasium. After graduating from the school, he entered the University of Groningen with the intention of taking a mathematics degree. In addition to his love for mathematics, his enjoyment of physics and carrying out physical experiments grew during his undergraduate years. He obtained permission to assist with experiments being carried out by Hermanus Haga, the professor of physics at the University of Groningen, and as a consequence began to work in the Astronomical Laboratory at Groningen. In the Laboratory Jacobus Kapteyn, the Professor of Astronomy and Theoretical Mechanics, was measuring photographic plates which had been taken by the astronomer David Gill as part of a photographic survey of the southern sky taken at the Cape Town Observatory. Although Kapteyn was the Groningen Professor of Astronomy, he had volunteered to assist in the Astronomical Laboratory since he had no observatory in which to conduct his own observations.

The turning point in de Sitter's life came in 1896 when David Gill visited Kapteyn to discuss progress on the southern sky survey. Entering the Laboratory, Gill saw de Sitter at a measuring machine and the two talked briefly [15]. On :-

... the following morning de Sitter, while having breakfast in his rooms, received a message that Gill wished to speak with him in the laboratory. De Sitter did not possess at that time the fluent command of the English language which he afterwards acquired; Kapteyn was lecturing at the time and Mrs Kapteyn acted as interpreter at the interview. Gill invited de Sitter to come to the Cape as a computer and, as de Sitter afterwards stated in a letter to Gill, "thereby complete my astronomical education - or rather begin it, for up to that time I had never made a speciality of astronomy and intended to become a mathematician."

De Sitter consulted his parents, and decided to take their advice and complete his examinations at Groningen before taking up Gill's offer. He received a Bachelor's Degree in 1897 and left for Cape Town in the summer of that year arriving in August. He worked at the Cape Observatory in South Africa for two years taking part in photometric and heliometer programmes. It was at Gill's suggestion that de Sitter also worked towards his doctorate studying heliometer observations of Jupiter's moons which had been made by Gill in 1891. After de Sitter returned to Groningen in 1899 he was appointed as an assistant in the Astronomical Laboratory and also continued to work towards his doctorate, advised by Kapteyn. He submitted his thesis Discussion of Heliometer Observations of Jupiter's Satellites to the University of Groningen in 1901. Jan Hendrik Oort writes in [13]:-

With the discussion of Gill's heliometer observations of Jupiter's satellites, de Sitter had entered upon the field which seemed so completely to suit his versatile mind: the talent, on one hand, for seeing clearly through the most intricate mathematical problems and dealing practically with them; on the other hand, the critical insight into the value and limitations of observations - an insight which sprang from a very active interest in observations as well as instruments.

On 6 December 1898 de Sitter had married Eleonora Suermondt in Cape Town, South Africa. Eleonora was born in Surabaya, the second largest city in the Dutch East Indies, and she had met de Sitter in the Cape where she was working as a school teacher. Their first child Lamoraal Ulbo was born in Cape Town in September 1899 but sadly died on 10 January 1901 after the family had returned to Groningen. Their other children were: Theodora born on 28 September 1900 in Groningen; Lamoraal Ulbo born on 6 March 1902 in 's-Gravenhage; Aernout born on 5 April 1905 in Groningen; and Agnes born 14 May 1908 in Groningen. Let us record a few details of the lives of two of these children at this point. Lamoraal Ulbo de Sitter became well-known as a structural geologist and died in Nistelrode in 1980. Aernout de Sitter served as director of the Bosscha Observatory in Lembang, Dutch East Indies. After the Japanese invasion of the Dutch East Indies he was arrested and put into a detention camp where he died on 5 September 1944.

Hendricus Gerardus van de Sande Bakhuyzen had been Professor of Astronomy and Director of the University Observatory at the University of Leiden from his appointment in 1872. He retired in 1908 and his duties were split into two, with the chair of astronomy being separated from the directorship of the Observatory. De Sitter was appointed to the chair of astronomy while Ernest-Frederich van de Sande Bakhuyzen, H G van de Sande Bakhuyzen's brother, was appointed as Director of the Observatory. On taking up the chair, de Sitter gave his inaugural lecture on The New Methods in Celestial Mechanics. E F van de Sande Bakhuyzen died in 1918 and in the following year de Sitter was appointed Director of the Leiden Observatory in addition to his professorship. He undertook a complete reorganisation of astronomy at Leiden dividing it into three divisions: Fundamental Astronomy of position or astrometry; Astrophysics; and Celestial Mechanics or theoretical astronomy. Under his leadership it was one of the leading astronomical centres in the world. Oort writes [13]:-

Throughout the years at Groningen, as well as later, de Sitter showed an almost unbelievable activity. In looking through a number of his more important publications, one is struck with the rapidity with which discussions on different topics follow each other and by the amount of thorough critical thinking of which each of these brings evidence. Neither a serious illness from which he suffered during some years, nor the directorship of an observatory which he undertook to reorganise almost entirely, nor his constant activities in many matters connected with the University could slow up the flow of his scientific investigations.

In 1913 de Sitter produced an argument based on observations of double star systems which proved that the velocity of light was independent of the velocity of the source. It put to rest attempts which had been made up until this time to find emission theories of light which depended on the velocity of the source but were not in conflict with experimental evidence.

De Sitter corresponded with Paul Ehrenfest in 1916, and he proposed that a four-dimensional space-time would fit in with cosmological models based on general relativity. He published a series of papers (1916-17) on the astronomical consequences of Einstein's general theory of relativity. He found solutions to Einstein's field equations in the absence of matter. This was significant since Mach had stated a principle that local inertial frames of reference were determined by the large-scale distribution of mass in the universe. De Sitter asked:-

If no matter exists other than the test body, does it have inertia.

De Sitter's work led directly to Arthur Eddington's 1919 expedition to measure the gravitational deflection of light rays passing near the Sun, results which, at that time, could only be obtained during an eclipse. De Sitter, unlike Einstein, maintained that relativity actually implied that the universe was expanding, theoretical results which were later verified observationally and only then accepted by Einstein.

In fact Einstein had introduced the cosmological constant in 1917 to solve a significant problem concerning the universe, which had also troubled Newton before him, namely why does the universe not collapse under gravitational attraction. This rather arbitrary constant of integration which Einstein introduced, admitting it was not justified by our actual knowledge of gravitation, was later said by him to be "the greatest blunder of my life." However de Sitter wrote in 1919 that the term:-

... detracts from the symmetry and elegance of Einstein's original theory, one of whose chief attractions was that it explained so much without introducing any new hypothesis or empirical constant.

In the 1990s observational evidence suggested that the expansion of the universe is accelerating. One way to incorporate this into the relativistic model is to reintroduce the cosmological constant. This remains an extremely active area of research.

In 1932 Einstein and de Sitter published a joint paper in which they proposed the Einstein-de Sitter model of the universe. This is a particularly simple solution of the field equations of general relativity for an expanding universe. They argued in this paper that there might be large amounts of matter which does not emit light and has not been detected. This matter, now called 'dark matter', has since been shown to exist by observing is gravitational effects. However the dark matter postulated by Einstein and de Sitter in 1932 still remains a mystery in that its nature is still unknown but is the subject of major research efforts today.

Although de Sitter is best known for this work on relativity, he made many other contributions of great significance. His doctorate had been on the satellites of Jupiter and he maintained an interest in this topic throughout his life. He used data on eclipses of the satellites dating back to 1668 to produce definitive data on the orbital elements and masses of the four satellites. He published theoretical discussion in three papers of 1918, 1919 and 1925. Finally in 1929 he produced his definitive results, but he was still working on tables of the motions of the satellites when he died in 1934. During his investigations of the timings of the eclipses of Jupiter's moons, he realised that his data was being affected by variations in the speed of rotation of the earth. Immediately de Sitter had another topic for which he had precisely the right skills to make a breakthrough. His study showed that there is varying tidal friction which affects both the earth and the moon and, in addition, sudden changes which occur in the moment of inertia of the earth.

Another study which de Sitter undertook was to refine the data for the fundamental constants of astronomy. Simon Newcomb had published values for these constants in 1895 and in a rather remarkable international agreement in Paris in 1896, it had been decided that the ephemerides of every country in the world should use Newcomb's values for these constants. In 1915 de Sitter published his first paper on improving the values, this one being concerned almost entirely with the figure and composition of the earth. He wrote in the introduction:-

On various occasions I have found myself confronted with the question which would be the best value to adopt for one of the fundamental astronomical constants. We have, of course, the set of constants officially adopted in the national ephemerides ... Some of the adopted constants are inconsistent with each other. This the adopted length of the year and the constant of precession are contradictory, and similarly the mass of the earth and the solar parallax. Newcomb's great work is now more than thirty years old, but it still stands unsurpassed as an example of sound critical discussion. ... I have only been led by the considerations that on the one hand my results may perhaps be useful to others in similar circumstances, and on the other hand that a critical survey like the present may be helpful to guide the efforts of astronomers to those points where they are most needed.

His second paper on the fundamental constants was published in 1927 and dealt with the constants associated with precession, nutation, solar parallax, lunar parallax and the mass of the moon. At the time of his death, de Sitter had almost completed a new updating of these constants.

Many honours were given to de Sitter for his outstanding contributions. He received the Watson Gold Medal from the National Academy of Sciences in 1929. The Medal was presented to him in Washington, USA, by Armin Otto Leuschner, Professor of Astronomy of the University of California and Chairman of the Trustees of the Watson Fund. Leuschner said:-

The privilege accorded me involves a task of unusual difficulty, for the major part of de Sitter's work is in intricate, speculative, and mathematical fields dealing with the perturbations of the motions of bodies of the solar system according to classical celestial mechanics, and with the theory of relativity from a purely speculative point of view as well as in relation to perturbations observed, but not explained, and relativistic theoretical perturbations not perceived or as yet perceivable by observation. I, therefore, beg your indulgence if I fail in a clear presentation of the significance of some of his contributions to science. His intellectual abilities cover so wide a range and penetrate so deeply and so minutely into practical astronomy and the mathematical theories to explain what is observed, that only an intensive study of his brilliant work could do justice to the greatness of the man.

In 1931 de Sitter received the Bruce Medal from the Astronomical Society of the Pacific. Also in 1931 he received the Gold Medal of the Royal Astronomical Society:-

... for his theoretical investigations on the orbits of the satellites of Jupiter, and for his contribution to the Theory of Relativity.

He was elected as President of the International Astronomical Union and served in this capacity from 1925 to 1928.

De Sitter suffered from chest complaints for several years but seemed to overcome them. However he died from pneumonia at the young age of sixty-two years. The following is part of a tribute to him which appeared in the New York Times on 25 November 1934; it is also reproduced in [8]:-

In [de Sitter's] work we see the creative mathematician at his best. He is not a cold, dispassionate juggler of Greek letters, a balancer of equations, but rather an artist in whom wild flights of the imagination are restrained by the formalism of a symbolic language and the evidence of observation. Only the musician can fully grasp what it must have meant to de Sitter to see the cosmos shaping itself in new ways in his formulas. Like musical notes, strange symbols, standing for forces and masses that were divined rather than known, arranged themselves into a coherent message. And when the message came to be read a totally new universe was revealed. Here we have something of the direct personal experience of the outer world, of the significance of nature's wonders, that comes only to a Beethoven or a Milton. The expanding universe of de Sitter must be regarded as something more than an inexorable conclusion drawn from the strictest kind of logic with which the human mind is familiar. It is poetry of a new sort - the scientist's way of writing an epic.