The Newman Rambler

Georges Lemaître: Priest and Scientist

4 May 2021 ‖ Jonathan Lunine

Jonathan I. Lunine is David C. Duncan Professor in the Physical Sciences and Chair of the Department of Astronomy at Cornell University. His research focuses in planet formation. Luline helped found the Society of Catholic Scientists. He earned a B.S. in Physics and Astronomy from the University of Rochester in 1980, followed by M.S. (1983) and Ph.D. (1985) degrees in Planetary Science from the California Institute of Technology. Lunine is the David Baltimore Distinguished Visiting Scientist at NASA's Jet Propulsion Laboratory. He is an interdisciplinary scientist on the Cassini mission to Saturn, and on the James Webb Space Telescope, as well as co-investigator on the Juno mission launched in 2011 to Jupiter. He is the Principal Investigator of a proposed astrobiology mission to Enceladus called Enceladus Life Finder. Lunine is a member of the U.S. National Academy of Sciences,[3] a fellow of the American Association for the Advancement of Science and the American Geophysical Union, and a member of the International Academy of Astronautics, which gave him its Basic Science Award in 2009. In 2015 he was awarded the Jean Dominique Cassini medal of the European Geosciences Union. Author of nearly 400 academic papers, he is also the author of Astrobiology: A Multidisciplinary Approach (2005) and Earth: Evolution of a Habitable World (2013).*

On October 29th of 2018, the International Astronomical Union (IAU) voted to recommend renaming Hubble’s Law the “Hubble-Lemaître Law.” That such a vote would take place today—during a time when science and faith are portrayed in the media as implacable foes—speaks to the remarkable character of Lemaître himself, the Belgian monsignor and astronomer who made a number of fundamental contributions to the science of cosmic structure and origins.

Georges Henri Joseph Édouard Lemaître was born July 17, 1894 in Charleroi, Belgium. At an early age he felt called to become a priest, but did not pursue ordination until after he completed his scientific education at the Catholic University of Louvain. Initially setting out to study civil engineering, he left the university to fight in World War I as an artillery officer for the Belgian army, for which he was awarded the Belgian War Cross. Returning to his education in 1918, he obtained a Docteur en Sciences from Louvain in 1920, with a thesis on pure mathematics. He then was ordained a priest in 1923, but having become aware of new developments in astronomy, he sought and obtained permission from his superiors to become a research associate at Cambridge University (U.K.) under the famous astronomer Sir Arthur Eddington. A year later he worked at Harvard College Observatory with Harlow Shapley and in 1927 was awarded a Ph.D. from MIT[1] with a thesis on the behavior of gravitational fields under general relativity.[2] In 1925 he returned to Louvain to take up a faculty position.

To appreciate Lemaître’s contribution requires that we recognize how different the astronomy of the early 20th century was from today. In 1917 Albert Einstein published his theory of “general relativity,” in which gravity is the geometry of the space and time we exist in. The size and structure of the universe was poorly known then. It was understood that the solar system is located in a large assemblage of billions of stars called the Milky Way Galaxy. However, the argument of the day was whether the Milky Way was in fact the entire universe. Up through the first decade of the 20th century, telescopes were not powerful enough to resolve the true nature of spiral-shaped “nebulae” as other galaxies like the Milky Way.[3] Thus when Einstein conceived his theory of general relativity a decade earlier, the simplest assumption was that the universe is static, unchanging over countless eons of time.[4] But this posed a serious problem for Einstein, because his theory of gravity required that matter distort space in such a way that a static universe—all matter, and space itself—would simply collapse upon itself. An alternative model of a static cosmos was developed in 1917 by the Dutch physicist Willem de Sitter. De Sitter solved the problem of a collapsing universe by postulating that space was empty—devoid of matter. As unrealistic as this may seem, the de Sitter universe was interesting in two ways. First, if two small bits of matter were introduced they would tend to move away from each other. Second, the space de Sitter considered was flat—a departure from Einstein’s model, in which matter imposed an overall positive curvature on space such that the latter resembled the surface of a ball. The actual universe seems to be very nearly flat and after an enormity of time will come to resemble a de Sitter universe.[5]

Lemaître wrestled with the problems of the de Sitter model while pursuing his Ph.D. By that time, 1924, astronomical observers using more powerful telescopes had succeeded in finding distance indicators that established the spiral nebulae as galaxies like the Milky Way. Hence the universe was not 100,000 light years across, but rather billions of light years in size. More significantly, observers found that the light of the more distant galaxies seemed shifted toward the red end of the color spectrum relative to nearby galaxies. Various explanation for this red-shift were offered.[6]

Lemaître’s time at Harvard enabled him to be engaged with the astronomical data, and by 1927 he understood how to interpret the galactic red shifts; galaxies were moving away from each other, but not by their own motion through fixed space. His was not the static universe of Einstein, or the empty cosmos of de Sitter, but rather a universe in which space itself was expanding, in which massive galaxies embedded in that space were carried into a future in which the cosmos became ever more dilute. Galaxies appear reddened not because of the classical Doppler Effect but because the light waves themselves are stretched out by the expansion of the space through which they travel. And unlike the original de Sitter model, no observer is in a special position, no galaxy occupies a “center.”[7]

In 1927 Lemaître published his seminal paper on the expanding universe.[8] His expanding cosmos filled with matter combined the best of both Einstein’s and deSitter’s cosmologies, directly confronted the astronomical data at hand, and did not require a cosmological constant. In his universe, the velocity of recession of a galaxy would be proportional to the distance to that galaxy. He used the available astronomical data on galactic distances and redshifts to compute the constant of proportionality.[9]

Lemaître’s paper was virtually unknown and unread. Two years later, in 1929, the American astronomer Edwin Hubble published in the prominent Proceedings of the (US) National Academy of Sciences,[10] in which he used the much larger body of data on galactic distances and velocities then available to show empirically that there was a linear relationship between the recessional velocity and distance of a galaxy. The velocity-distance relationship he derived by plotting data on a chart became known as Hubble’s law, and the constant of proportionality the Hubble constant.

Hubble interpreted the recessional velocities of galaxies by appealing to de Sitter’s cosmology, in which particles would fly apart in a fixed space. He also invoked what became known as “light fatigue”—light waves would lose energy and increase in wavelength as they traveled from source to observer. Neither is correct: de Sitter’s model did not apply to the universe in which we live, and light does not lose energy as it travels through the vacuum of space. It was Lemaître’s expansion of space itself that provided a natural mechanism for the ever-greater reddening of galaxies with distance. But Hubble was unaware of Lemaître’s 1927 paper, and in any event never accepted the idea of a universe in which space itself was expanding. As late as the 1940’s Hubble gave interviews in which he asserted the data to be consistent with a static cosmos[11]—an opinion now well established to be erroneous. Ironically, the man for whom the fundamental yardstick of cosmic expansion was named never accepted the idea that space was expanding.

The story would end here were it not for another consequence of Lemaître’s publishing in an obscure journal. In 1930 Arthur Eddington produced an expanding universe model virtually identical to Lemaître’s, and began to lecture on it. Upon learning from colleagues of his old Cambridge mentor’s reinvention, Lemaître reminded Eddington that he had sent him a copy of the 1927 paper. The gracious Eddington realized immediately that his former student’s choice of journal had doomed the work to obscurity and arranged for the editor of the Monthly Notices of the Royal Astronomical Society, a distinguished journal informally known to astronomers as MNRAS, to publish an English translation.[12]

The 1931 English translation of the 1927 seminal paper did nothing to establish Lemaître’s priority in deriving “Hubble’s Law,” because the key paragraph setting forth the relationship between the recession speed of galaxies and their distance, and the constant relating them, was missing. For decades intrigue swirled around this omission; theories ranged from anti-religious motives to Hubble himself intervening to save his own priority. In 2011 astronomer Mario Livio solved the mystery after combing the archives of the Royal Astronomical Society, where he discovered a cover letter enclosed with the translated manuscript to the editor of MNRAS.[13] The letter establishes that Lemaître had translated his own 1927 paper into English, and decided to omit the material on the galaxy velocity-distance relationship.

Why would Lemaître do such a thing? He knew well that by 1929, when Hubble wrote his paper, there were more data of higher accuracy that established the linear nature of the velocity-distance relationship than he had access to in 1927. When Lemaître wrote out the relationship in his original paper, he had derived it from his cosmological model, in effect predicting what better data would show two years later.[14]

By omitting the key paragraph, Lemaître lost the opportunity to have his name attributed to the famous and now fundamentally important cosmological relation. Although it was easy to go back to the original 1927 paper to see what Lemaître had done, few apparently did. Further, Hubble had a big personality and was in charge of what was the largest telescope at the time; as a public figure he easily overshadowed low-key Belgian priest-professor.

Lemaître would go on in 1931 to propose what later came to be known as the Big Bang model for how the cosmos began, and for this his priority is unquestioned. Why then is Lemaître’s name not as well known as Hubble, or even Einstein? By the end of World War II, the center of action in cosmology and the elaboration of the Big Bang model had moved from general relativity to nuclear physics, a field which simply did not interest Lemaître.[15] He remained a dedicated professor, pioneering high performance computing in Belgium, but in the end produced few students in cosmology as his legacy. By the 1970’s most of Lemaître’s peers had died, and his contributions in large part became undervalued in publications from then until about a decade ago when interest in his life was rekindled.[16]

The case for renaming Hubble’s law the Hubble-Lemaître law rests upon both the timing of the 1927 paper and Lemaître’s unique ability to provide the mathematically sound cosmologies while engaging directly with the astronomical data.

While few discoveries in science are correctly attributed to their discoverers[17], I would argue that this case is special, and that Lemaître really was undervalued despite awards earned in his lifetime. Lemaître’s religious identity is relevant here.  Appropriately recognizing Lemaître’s name in the history of astronomy, by accepting the recommendation of the IAU to use the term “Hubble-Lemaître law”, will benefit scientist-believers and scientist-atheists alike. For the former, it strengthens our case that science and faith are compatible. And for the latter, it might just help remove the suspicion that Lemaître has been treated differently from his peers, both in his lifetime and thereafter, because of the collar he wore.[18]

 * The biography is mostly based on the Wikipedia entry for the author. This text is based on a lecture delivered on 31 October 2018 at the University of Chicago, organized by the Lumen Christi Institute. The lecture was entitled: Georges Lemaître's Science, Faith, and Why "Hubble's Law" Ought to be Renamed. The text was first published in Church Life Journal of the University of Notre Dame on 4 April 2021 with the title Faith and the Expanding Universe of Georges  Lemaître. The editors of Church Life Journal have graciously accepted to allow The Newman Rambler to publish a shortened version of the original text.

Notes

[1] Graduate study in astronomy at Harvard did not officially begin until 1928 (https://astronomy.fas.harvard.edu/history). Thus, Lemaître, who arrived at Harvard in 1924, had to matriculate at nearby MIT in order to obtain his Ph.D.

[2] Georges H.J.E. Lemaître, (1) The gravitational field in a fluid sphere of uniform invariant density according to the theory of relativity; (2) Note on de Sitter’ Universe; (3) Note on the theory of pulsating stars. Ph.D. Dissertation, MIT, 1925, available from D-space@MIT at https://dspace.mit.edu/handle/1721.1/10753). The notes “on de Sitter’s universe” and “on pulsating stars” were not included in the thesis copy deposited in the library; the all-important first of these two was however published by Lemaître in the Journal of Mathematics and Physics, vol. IV, no. 3, May 1925.

[3] Ideas ranged from smaller systems of stars to individual solar systems in the process of formation; see Robert W. Smith “Cosmology 1900-1931” in Cosmology: Historical, Literary, Philosophical, Religious, and Scientific Perspectives, ed. Norriss S. Hertherington (New York, Garland Publishing, 1993), 329-345.

[4] By 1913 telescopic observations showed that the Andromeda nebula, soon to be revealed definitely as a spiral galaxy, was rushing toward us at a very high speed, while within a few years other galaxies would be shown to be receding. But the sparsity of data prevented inference of a general expansion of the cosmos until Lemaître and Edwin Hubble came on the scene a decade later. See Robert. W. Smith, op. cit.

[5] See, for example, Lawrence M. Krauss and Robert J. Scherrer, “The return of a static universe and the end of cosmology”, General Relativity and Gravitation, Vol. 39, No. 10, 2007, 1545-1550.

[6] The light of galaxies was spread out according to wavelength at the telescope through the use of “spectrometers”. Because galaxies are made up in large part of stars, whose atmospheres contain atoms that absorb light at definite wavelengths, astronomers could see the same pattern of dark lines in the spectrum from one galaxy to another, but in many cases shifted to the red relative to the pattern one would see in the laboratory. It is possible in this way to measure very precisely the amount of the so-called “red shift” for a given galaxy.

[7] It is difficult to imagine space without a center; after all, if the galaxies are receding, what are they receding from? The easiest way to visualize such a reality is to consider the surface of a balloon as a two-dimensional analog to three-dimensional space. Inflate the balloon, and draw dots all over the resulting surface. Note that no dot is at the center, every dot is at rest in its local spot on the balloon’s surface, and yet as you inflate the balloon, the perspective from every dot is that all other dots are moving away from it. (The dots themselves, drawn by pen, get bigger, but the real galaxies do not). By using a ruler, you can also show that the further one dot is from another, the faster it seems to recede—by just the proportionality law Lemaître inferred for his model. The difficulty with this analogy is that inevitably one fixates on the space outside and inside of the balloon—an extra spatial dimension that has no correspondence with anything in most models of the actual expanding universe.

[8] G. Lemaître, “Un univers homogene de masse constante et de rayon crossant, rendant compte de la vitesse radiale des nebuleuses extra-galactiques”, Annales de la Societe Scien- tifique de Bruxelles A, vol. 47, 1927, 49-59.

[9] A megaparsec is the conventional unit of distance used by extragalactic astronomers. One parsec is 3.26 light-years, and a megaparsec is a million parsecs, or roughly thirty million trillion kilometers.

[10] Edwin Hubble, “A relation between distance and radial velocity among extra-galactic nebulae”, Proc. National Academy of Sciences, Vol. 15, 1929, 168-173.

[11] Helge Kragh and Robert W. Smith “Who discovered the expanding universe” History of Science, vol. 41, no. 2, 2003, 141-162. In 1958 Lemaître stated that he was made aware of Friedmann’s papers in a meeting with Einstein in late October 1927, months after his own paper (note 12) appeared. In view of the subsequent events regarding Hubble’s work, described in this article, there is little reason to disbelieve Lemaître.

[12] Georges Lemaître, “A homogenous universe of constant mass and increasing radius accounting for the radial velocity of Extra-galactic nebulae”, Monthly Notices Royal Astronomical Society, vol. 91, 1931, 483-490.

[13] Mario Livio, “Mystery of the missing text solved”, Nature, Vol. 479, 171-173.

[14] In his 1927 paper, Lemaître averaged the data on galactic distances and velocities to obtain his constant, rather than fitting the data to a straight line. Given the limitations in the amount and precision of the data at the time, this was the right thing to do, since Lemaître knew that his model of the universe—the primary point of the paper—determined the form of the velocity-distance relationship.

[15] Ralph Alpher, Hans Bethe and George Gamow., “The origin of the chemical elements”, Physical Review 73, 1948, 803-804.

[16] I believe that John Farrell’s book [2] was influential in this regard, as was the collection of papers in Rodney Holder and Simon Mitton, eds. Georges Lemaître: Life, Science and Legacy (Heidelberg, Springer, 2012), along with other articles and books published in the last 15 years.

[17] Stigler’s law of eponymy states that “no scientific discovery is named after its discoverer.” Stephen M. Stigler, “Stigler’s law of eponymy”, Proceedings New York Academy of Sciences, vol. 39, no. 1, series II, 1980, 147-157.

[18] Lemaître was recognized in his lifetime with the Francqui Prize (he was nominated by Einstein).