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Georges Lemaitre was born on 1894 July 17 in Charleroi, Belgium, and was educated at the Jesuit school in that town. He entered the engineering school of the University of Louvain in 1911 where he remained until the outbreak of the first world war in 1914. Lemaitre joined the Belgian Army as a volunteer, served throughout the war and was decorated with the Belgian 'Croix de Guerre'. On his return to the University of Louvain in 1918 he turned from engineering to mathematical and physical sciences and obtained his doctorate with highest honours in 1920. The subject of his thesis was approximations to functions of many variables and his supervisor was de la Vallée Poussin. Thereafter Lemaitre entered the seminary of Malines and was ordained a priest of the Roman Catholic Church in 1923. Simultaneously with his religious studies, he prepared a thesis on relativity and gravitation which won him, in 1923 July, a travelling scholarship from the Belgian Government. This, together with a fellowship from the C.R.B. Educational Foundation, enabled him to spend one year at Cambridge University and the year 1924/25 at the Harvard College Observatory and the Massachusetts Institute of Technology. In 1925 October he returned to the University of Louvain where he spent the rest of his life. By 1927 he was a full professor, teaching relativity, the history of physical and mathematical sciences and mathematical methods. Lemaitre visited the United States again in 1932 and 1935 and taught at the Catholic University of America during one year. Many honours came his way, among which may be mentioned the Prix Franqui (1934) and memberships in the Pontifical Academy of Sciences (1940), the Belgian Academy (1941), the American Philosophical Society (1945), the Academi Nazionale Dei XL (1961) and the Academia Internationale 'Neocastrum' (1966). In 1960 March, Lemaitre became the President of the Pontifical Academy of Sciences and 'Domestic Prelate' of the Roman Catholic Church. His death occurred at the University Hospital of Louvain on 1966 June 20.
By 1920, Lemaitre had become deeply interested in Einstein's theory of general relativity and had mastered it by his own efforts. The tensor calculus, for example, was not taught in Belgian universities in those days and he had to learn it without the benefit of an instructor. At Cambridge he came under the influence of Eddington, who turned his attention to stellar dynamics, and he also studied numerical methods. During the year 1924/25, while in the United States, he benefited from conversations with Harlow Shapley and he was present at the meeting of the National Academy of Sciences at which Hubble announced his discovery of what is now called the redshift in the spectra of extra-galactic nebulae. 1927 April saw the publication of Lemaitre's famous paper 'Un univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extragalactiques' (Annls Soc. Sci. Brux., 47A, 49 (1927)). Curiously enough this work at first attracted little attention. Nearly three years later, I was a research student of Eddington's and he had suggested that I work on the redshift problem. I well remember the day when Eddington, rather shamefacedly, showed me a letter from Lemaitre which reminded Eddington of the solution to the problem which Lemaitre had already given. Eddington confessed that, though he had seen Lemaitre's paper in 1927, he had completely forgotten about it until that moment. The oversight was quickly remedied by Eddington's letter to Nature of 1930 June 7, in which he drew attention to Lemaitre's brilliant work of three years before.
With the benefit of hindsight, Lemaitre's work of 1927 is easily described: it was a far harder matter to carry it out at a time when investigators were groping their way forward to a theory of the expanding universe. If the material content of the universe is regarded as a hydrodynamical fluid whose pressure and density vary with the time alone, then Einstein's field equations predict that the problem of the expansion is solved by the determination of a single function of the time, the scale-factor . The differential equation satisfied by R shows that it is, in general, an elliptic function of the time, even under the simplifying assumption of zero pressure. Two parameters enter into the equation, the cosmical constant and the space-curvature constant. For certain values of the parameters, the scale-factor is expressible in terms of elementary functions and Lemaitre discovered one of these cases, by no means the simplest one. It was characterized by a relation between the two parameters which meant that the expansion of the universe began from an unstable state of finite volume, the Einstein Universe. The cosmical constant and the space-curvature both had positive values. It is true that, unknown to Lemaitre, the Russian mathematician A.Friedmann had, in 1922 and 1924, published papers in which the general differential equation for was considered. But Lemaitre went beyond the determination of a specific form for : he showed that his model could be used to interpret the observed redshift in the spectra of extragalactic nebulae, regarded as a Doppler effect. With this step, cosmology, as it is known today, was launched.
Throughout his life, Lemaitre felt certain that the cosmical constant had a positive value and defended this position in his characteristic jovial and good-humoured fashion. In Newtonian terms, a positive cosmical constant may be said to represent a universal force that exists between aggregates of matter and repels these aggregates from one another. Perhaps he saw in this force a 'cause' for the expansion of the universe. At any rate, his later papers on cosmology appear to take it for granted that the cosmical constant is positive. In one of these (Annls. Soc. Sci. Brux., 53A, 51 (1933)) he arrives incidentally at an interesting result which is not primarily cosmological. He shows that the Schwarzschild metric for the gravitational field external to a spherically symmetric body can be expressed in non-static coordinates in terms of which the central singular region disappears. He argues that the singularity is therefore coordinate-dependent and so is presumably physically unreal.
From 1945 onwards, Lemaitre put forward the notion of the 'Primeval Atom'. He was clearly guided in this by his work of 1927 in which the expansion of the universe begins from a highly condensed initial state. The primeval atom was said to be 'nearly an isotope of a neutron'; it was unstable and, in shattering, not only started the expansion, but produced electrons, protons, alpha particles, etc. Cosmic rays were to be regarded as remnants of the original cataclysm. It must be confessed, however, that Lemaitre never gave a detailed theory of the physical processes by which the primeval atom eventually produced the multitudinous astronomical objects that are observed today.
Another of Lemaitre's interests was cosmic rays. After Compton had established their corpuscular nature, Lemaitre, in co-operation with M.S.Vallarta and L.Bouckaert, reduced the problem of their motion near the Earth to that of charged particles moving in the dipole magnetic field of the Earth. The latitude effect, the density of rays arriving at a point on the Earth's surface and their intensity could thus be calculated. These researches were carried out during the period 1933-1939.
Finally, there are Lemaitre's investigations in celestial mechanics, in which he considered such questions as Stormer's problem, symmetrical coordinates in the three-body problem and the method of dealing with a collision between two of the three bodies. He was also much interested in numerical methods and wrote some ten papers on this subject.
Lemaitre was made an Associate of the Royal Astronomical Society in 1939 and was the first recipient of the Society's Eddington Medal, in 1953.
I am particularly indebted to Dr Ch.Manneback of the Academie Royale de Belgique and to Professor O.Godart of the University of Louvain for their kindness in supplying me with much of the information on which this notice is based.
G.C.MCVITTIE
By 1920, Lemaitre had become deeply interested in Einstein's theory of general relativity and had mastered it by his own efforts. The tensor calculus, for example, was not taught in Belgian universities in those days and he had to learn it without the benefit of an instructor. At Cambridge he came under the influence of Eddington, who turned his attention to stellar dynamics, and he also studied numerical methods. During the year 1924/25, while in the United States, he benefited from conversations with Harlow Shapley and he was present at the meeting of the National Academy of Sciences at which Hubble announced his discovery of what is now called the redshift in the spectra of extra-galactic nebulae. 1927 April saw the publication of Lemaitre's famous paper 'Un univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extragalactiques' (Annls Soc. Sci. Brux., 47A, 49 (1927)). Curiously enough this work at first attracted little attention. Nearly three years later, I was a research student of Eddington's and he had suggested that I work on the redshift problem. I well remember the day when Eddington, rather shamefacedly, showed me a letter from Lemaitre which reminded Eddington of the solution to the problem which Lemaitre had already given. Eddington confessed that, though he had seen Lemaitre's paper in 1927, he had completely forgotten about it until that moment. The oversight was quickly remedied by Eddington's letter to Nature of 1930 June 7, in which he drew attention to Lemaitre's brilliant work of three years before.
With the benefit of hindsight, Lemaitre's work of 1927 is easily described: it was a far harder matter to carry it out at a time when investigators were groping their way forward to a theory of the expanding universe. If the material content of the universe is regarded as a hydrodynamical fluid whose pressure and density vary with the time alone, then Einstein's field equations predict that the problem of the expansion is solved by the determination of a single function of the time, the scale-factor . The differential equation satisfied by R shows that it is, in general, an elliptic function of the time, even under the simplifying assumption of zero pressure. Two parameters enter into the equation, the cosmical constant and the space-curvature constant. For certain values of the parameters, the scale-factor is expressible in terms of elementary functions and Lemaitre discovered one of these cases, by no means the simplest one. It was characterized by a relation between the two parameters which meant that the expansion of the universe began from an unstable state of finite volume, the Einstein Universe. The cosmical constant and the space-curvature both had positive values. It is true that, unknown to Lemaitre, the Russian mathematician A.Friedmann had, in 1922 and 1924, published papers in which the general differential equation for was considered. But Lemaitre went beyond the determination of a specific form for : he showed that his model could be used to interpret the observed redshift in the spectra of extragalactic nebulae, regarded as a Doppler effect. With this step, cosmology, as it is known today, was launched.
Throughout his life, Lemaitre felt certain that the cosmical constant had a positive value and defended this position in his characteristic jovial and good-humoured fashion. In Newtonian terms, a positive cosmical constant may be said to represent a universal force that exists between aggregates of matter and repels these aggregates from one another. Perhaps he saw in this force a 'cause' for the expansion of the universe. At any rate, his later papers on cosmology appear to take it for granted that the cosmical constant is positive. In one of these (Annls. Soc. Sci. Brux., 53A, 51 (1933)) he arrives incidentally at an interesting result which is not primarily cosmological. He shows that the Schwarzschild metric for the gravitational field external to a spherically symmetric body can be expressed in non-static coordinates in terms of which the central singular region disappears. He argues that the singularity is therefore coordinate-dependent and so is presumably physically unreal.
From 1945 onwards, Lemaitre put forward the notion of the 'Primeval Atom'. He was clearly guided in this by his work of 1927 in which the expansion of the universe begins from a highly condensed initial state. The primeval atom was said to be 'nearly an isotope of a neutron'; it was unstable and, in shattering, not only started the expansion, but produced electrons, protons, alpha particles, etc. Cosmic rays were to be regarded as remnants of the original cataclysm. It must be confessed, however, that Lemaitre never gave a detailed theory of the physical processes by which the primeval atom eventually produced the multitudinous astronomical objects that are observed today.
Another of Lemaitre's interests was cosmic rays. After Compton had established their corpuscular nature, Lemaitre, in co-operation with M.S.Vallarta and L.Bouckaert, reduced the problem of their motion near the Earth to that of charged particles moving in the dipole magnetic field of the Earth. The latitude effect, the density of rays arriving at a point on the Earth's surface and their intensity could thus be calculated. These researches were carried out during the period 1933-1939.
Finally, there are Lemaitre's investigations in celestial mechanics, in which he considered such questions as Stormer's problem, symmetrical coordinates in the three-body problem and the method of dealing with a collision between two of the three bodies. He was also much interested in numerical methods and wrote some ten papers on this subject.
Lemaitre was made an Associate of the Royal Astronomical Society in 1939 and was the first recipient of the Society's Eddington Medal, in 1953.
I am particularly indebted to Dr Ch.Manneback of the Academie Royale de Belgique and to Professor O.Godart of the University of Louvain for their kindness in supplying me with much of the information on which this notice is based.
G.C.MCVITTIE
Georges Lemaître's obituary appeared in Journal of the Royal Astronomical Society 8:3 (1967), 294-297.