Georges Lemaître: A Life in Five Chapters

Chapter 1 · Charleroi and the Yser

An engineering student in the trenches

1894 – 1919 · Charleroi · Louvain · The Yser front

Georges Lemaître was born on 17 July 1894 in Charleroi, an industrial city in southern Belgium, where his father ran a glass factory. He was educated by the Jesuits and decided young that he wanted to be both a priest and a scientist. His teachers advised him to finish a secular education first.

He began studying engineering at the Catholic University of Louvain. In August 1914 Germany invaded Belgium, and Lemaître, aged twenty, volunteered for the army with his brother. He fought at the Yser, where the Belgian army held a small strip of its own country behind flooded fields for four years, and later served in the artillery. He was decorated with the Croix de Guerre.

The war left him with a lasting dislike of grand claims made by people who had not checked the facts. It also gave him time to read. He studied physics in the gaps between bombardments and, according to one story, irritated an officer by pointing out an error in an artillery manual.

When the war ended he did not return to engineering. He switched to mathematics and physics, completed a doctorate in 1920, and then entered the seminary for what were called late vocations. He was ordained a priest of the archdiocese of Mechelen in 1923 by Cardinal Mercier, the archbishop who had defied the German occupation.

From the start, then, he lived two lives at once: the life of a priest and the life of a physicist. He never thought that was a contradiction, and he spent his career explaining why.

Why this matters

Lemaître chose both the priesthood and physics from youth, and his life is the clearest counter-example to the idea that science and faith must be enemies.

A volunteer at the Yser, a Croix de Guerre, and a double vocation. What would you ask him?

Ask Lemaître

• “Why did you never see being a priest and a physicist as a contradiction?”
• “How did four years at the Yser change the way you think about evidence?”
• “Why did your teachers tell you to finish a secular education before the seminary?”
• “Why did you switch from engineering to mathematics after the war?”
• “What did Cardinal Mercier's resistance to the occupation mean to you?”

Chapter 2 · Eddington and Einstein

Learning the new physics in Cambridge and Massachusetts

1923 – 1927 · Cambridge · Harvard · Brussels

After his ordination Lemaître won a scholarship to study abroad. In 1923–24 he worked at Cambridge with Arthur Eddington, the astronomer who had led the 1919 eclipse expedition that tested Einstein's general theory of relativity. Eddington, a Quaker, thought Lemaître a brilliant mathematician with a clear mind, and the two became lifelong friends.

He then spent a year in the United States, at the Harvard College Observatory and the Massachusetts Institute of Technology. He met the astronomers who were measuring the distances and speeds of the spiral nebulae — the objects we now know to be other galaxies. Vesto Slipher had found that most of them were moving away from us. Edwin Hubble was establishing how far away they were.

Einstein's equations of general relativity describe how matter shapes space and time. Einstein himself had assumed the universe was static — unchanging in size — and had added a term to his equations to keep it so. The Russian mathematician Alexander Friedmann had shown in 1922 that the equations also allowed a universe that expands or contracts, but Friedmann treated it as a mathematical possibility and died in 1925.

Lemaître put the two things together: the mathematics of general relativity and the astronomers' measurements. If the universe was expanding, distant galaxies would all be moving away from us, and the further away they were, the faster they would recede. That was a prediction that could be checked against observation, and it was what he set out to do.

Why this matters

Lemaître's achievement was to join Einstein's equations to real astronomical measurements, turning an abstract possibility into a testable claim about the actual universe.

Eddington in Cambridge, the nebulae in America, and Einstein's static universe. What would you ask him?

Ask Lemaître

• “What did you learn from Eddington that you could not have learned in Louvain?”
• “Why did Einstein believe the universe must be static?”
• “How do the speeds of distant galaxies show that the universe is expanding?”
• “What was the difference between your work and Friedmann's?”
• “Did your Quaker friend Eddington and you ever discuss faith?”

Chapter 3 · The Expanding Universe

A paper nobody read, and physics called abominable

1927 – 1931 · Louvain · Brussels · London

In 1927, now a professor at Louvain, Lemaître published a paper in French in the Annales de la Société scientifique de Bruxelles. Its title promised a homogeneous universe of constant mass and increasing radius. In it he showed that general relativity allowed an expanding universe, and — using published figures for galaxy speeds and distances — he calculated the rate of expansion. That relationship between distance and recession speed was published by Edwin Hubble in 1929, and for most of the twentieth century it was called Hubble's law.

Almost nobody noticed Lemaître's paper. The journal was obscure, and it was in French. At the Solvay conference in Brussels that year Lemaître spoke to Einstein, who told him — as Lemaître later recalled — that his calculations were correct but his physics was abominable. Einstein did not yet accept an expanding universe.

In 1930 Eddington realised what Lemaître had done and arranged for an English translation to appear in the Monthly Notices of the Royal Astronomical Society in 1931. The translation left out Lemaître's numerical estimate of the expansion rate. For decades some people suspected that this had been done to protect Hubble's priority. In 2011 the astronomer Mario Livio found letters showing that Lemaître had made the translation himself and dropped the estimate because Hubble's newer figures were better.

Lemaître never complained about credit. What interested him was whether the theory was true. In 2018 the International Astronomical Union recommended that the law be called the Hubble–Lemaître law.

Why this matters

The 1927 paper shows that the expansion of the universe was predicted from theory and measurement by a priest, two years before the observations that made it famous.

An obscure Belgian journal, a verdict from Einstein, and a missing number. What would you ask him?

Ask Lemaître

• “Why do you think almost nobody read your 1927 paper at first?”
• “How did you feel when Einstein called your physics abominable?”
• “Why did you leave your own estimate of the expansion rate out of the English translation?”
• “Does it matter who gets the credit for a discovery?”
• “What would have convinced Einstein that the universe really was expanding?”

Chapter 4 · The Primeval Atom

A universe with a beginning — and a warning about what that means

1931 – 1949 · Louvain · Pasadena · London

If the universe is expanding, then in the past it was smaller. Run the expansion backwards far enough, and everything would once have been packed together. In 1931 Lemaître published a short letter in the journal Nature proposing exactly that: the universe began as a single 'primeval atom', a super-dense state of all matter, which broke apart like a radioactive atom decaying, and from it came space, time and everything else. He described it vividly as a fireworks display whose ashes and smoke we now see.

Many physicists disliked the idea. A beginning sounded too much like religious creation, and some suspected the priest of smuggling Genesis into physics. Lemaître insisted that his hypothesis was physical and stood or fell by observation.

In 1933 he lectured in California with Einstein in the audience. By then Einstein had accepted the expanding universe and dropped the term he had added to keep it static; he is reported to have praised Lemaître's account, though the exact words come from later newspaper reports.

The main rival was the steady-state theory of Fred Hoyle, Hermann Bondi and Thomas Gold, which held that the universe had no beginning and new matter was continually created. On BBC radio in 1949 Hoyle called Lemaître's idea a 'big bang' theory. The phrase was meant dismissively. It stuck.

Lemaître also wrote that his beginning was not the same as creation. Cosmology could describe the earliest state of the universe, he argued, but it could not say whether that state was created or why there is anything at all. Those questions belonged somewhere else.

Why this matters

The primeval atom is the origin of the Big Bang theory, and Lemaître's own insistence that it was not proof of creation is central to debates on science and religion.

A primeval atom, a fireworks display, and a name meant as a joke. What would you ask him?

Ask Lemaître

• “Why did some physicists suspect you of smuggling Genesis into science?”
• “Is the beginning of the universe in your theory the same thing as creation?”
• “What would count as evidence against your primeval atom?”
• “What did you make of Fred Hoyle calling your idea a 'big bang'?”
• “Can science ever answer why there is something rather than nothing?”

Chapter 5 · A Pope and Two Paths

Saying no to a proof of God

1951 – 1966 · Rome · Louvain

On 22 November 1951 Pope Pius XII addressed the Pontifical Academy of Sciences. He spoke enthusiastically about modern cosmology and suggested that it seemed to confirm that the universe had a beginning in time, and so supported belief in creation by God.

Lemaître was dismayed. He thought this was the mistake called concordism — trying to make scientific results agree with scripture, or using science to prove religious doctrine. His reasons were both scientific and religious. Scientifically, his theory was a hypothesis that might be revised or overturned; if faith rested on it, faith would fall with it. Religiously, he thought a God reached by calculation would be one more object inside the universe, not its creator. The Bible, he believed, did not teach physics, and physics could not reach God.

He worked quietly with Daniel O'Connell, the Jesuit director of the Vatican Observatory, and the Pope did not repeat the claim in his address to the International Astronomical Union in 1952.

Lemaître continued teaching and became fascinated by electronic computers, installing one at Louvain. In 1960 he was appointed President of the Pontifical Academy of Sciences. In June 1966, in hospital after a heart attack, he was told by a colleague of the discovery of a faint microwave radiation filling the sky — evidence that strongly supported a hot early universe. He died on 20 June 1966.

He had said that there were two paths to truth and that he followed both. He meant that they were different paths, not one disguised as the other.

Why this matters

Lemaître's opposition to Pius XII shows a religious scientist refusing to use science as proof of faith — a key example for questions on whether science and religion conflict.

A Pope's address, a quiet objection, and a radiation from the beginning. What would you ask him?

Ask Lemaître

• “Why did you object when Pius XII said cosmology supported creation?”
• “What would happen to faith if it rested on a scientific theory that was later overturned?”
• “Why do you say the Bible does not teach physics?”
• “How did you persuade the Vatican not to repeat the claim?”
• “What did it mean to you to hear of the microwave radiation just before you died?”

What Lemaître changed

Lemaître discovered the expansion of the universe from theory and data, proposed the origin of what became the Big Bang theory, and gave a lasting model of how a religious believer can do science without making either serve the other. The Hubble–Lemaître law now carries his name.

A debate that continues

Does the Big Bang theory support the cosmological argument for God, as some believers say, or was Lemaître right that a physical beginning says nothing either way about creation?

Keep exploring — ask Lemaître

• “Does the Big Bang leave room for a creator, or remove the need for one?”
• “Is it possible to accept both scientific cosmology and religious belief?”
• “Should religious texts ever be read as scientific accounts?”

Related lives

• Edwin Hubble — Who Made the Universe Much Larger
• Albert Einstein — Nobel Laureate · Author of Relativity
• Arthur Eddington — Who Told Them To Find A Hotter Place
• Galileo Galilei — Father of Modern Science

Related themes

The Big Bang theory · Science and religion · Creation and Genesis · The cosmological argument