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6 May 1872 
Sneek, Netherlands 
20 Nov 1934 
Leiden, Netherlands 
Willem De Sitter studied mathematics at Groningen and then joined the Groningen astronomical laboratory. He worked at the Cape Observatory in South Africa (189799) then, in 1908, de Sitter was appointed to the chair of astronomy at Leiden. From 1919 he was director of the Leiden Observatory. 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 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 (191617) 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 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 accepted by Einstein . In fact Einstein had introduced the cosmological constant in 1917 to solve the problem of the universe which had 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 1932 Einstein and de Sitter published a joint paper with Einstein 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.
Source:School of Mathematics and Statistics University of St Andrews, Scotland
