Hubert Reeves Astronomy, Astrophysics and Space Science

World famous cosmologist and science communicator

"Modern physics allows us to shout 'Long live Freedom!'"

The Story

Hubert Reeves looked out the window. The Alps, magnificent and snowcapped, caught his eye as the train rounded a curve on its way from Geneva to Berne, Switzerland. It was October 1970. Beneath the mountains, an autumn patchwork of gold and vermilion framed a winding brook. Reeves briefly wondered if its source was melting snow, and then it came to him, the answer to the puzzle. The stream triggered a memory, a scene from a movie about mountains and cold water: La Bataille de l'eau lourde (The Battle for Heavy Water). The 1947 French film by Jean Dréville tells the true story of how, near the end of World War II, Allied commandos destroyed a top-secret heavy-water plant in the mountains of Norway. The Nazi invaders were going to use the heavy water to make an atom bomb.

Heavy water, heavy hydrogen ... Reeves’ mind flashed: You extract heavy water from ordinary water at super-cold temperatures and it takes a long time ... That was it! He now felt he could explain the huge discrepancy in recent experimental results about the nature of the solar wind, the huge flux of atomic particles blown into space by the burning sun. He was on his way to Berne to discuss this very problem with his colleague Johannes Geiss, the Swiss physicist who had conducted the experiment. Reeves grabbed a notepad and, as the scenic panorama unfolded outside, he tried to capture his thoughts on paper. His mind still resonated with ideas from the nuclear astrophysics lecture he had just presented at the Geneva observatory. At the same time, he felt he now had a theory that could explain why Geiss had found five times less heavy hydrogen in the solar wind than its natural occurrence on Earth. Theoretically, the sun and Earth originate from the same primordial matter, mostly hydrogen, so physicists were very bothered by this five-fold difference. What caused it?

Hydrogen is the simplest element in the universe. One proton in a nucleus orbited by a single electron: That’s it. However, another kind of natural hydrogen exists, called deuterium, or heavy hydrogen. It’s heavier because a deuterium nucleus contains a neutron as well as a proton. Deuterium is rare, but chemically it behaves like ordinary hydrogen, so on Earth, as with most of the planet’s hydrogen, it exists primarily as water. There are two hydrogen atoms in every water molecule (H2O). The water molecules in Earth’s oceans contain about one heavy hydrogen atom for every 2,000 plain hydrogen atoms.

This so-called heavy water is required in a certain type of nuclear reactor, such as Canada’s CANDU (from Canadian deuterium uranium) reactor, because it slows down fast neutrons, and if these neutrons are not kept in check a terrific nuclear explosion can occur. Heavy-water reactors also produce plutonium, which is a major component of an atom bomb. This is why the Nazis wanted heavy water in 1944.

In 1969, after long negotiations with the U.S. National Aeronautics and Space Administration (NASA), Johannes Geiss persuaded the Americans to conduct a simple experiment for him. On five of the 15 Apollo rocket trips to the moon, astronauts hoisted flags of aluminum foil and left them out for times varying from 77 minutes on Apollo 11 to 45 hours on Apollo 16. Each foil sheet was retrieved, brought back to Earth and examined by Geiss. With no atmosphere on the moon, Geiss expected it to be the ideal place to “feel” the solar wind. Particles in the solar wind would embed themselves into the aluminum foil flag.

When Geiss analyzed the foil, he found, among other things, that the solar wind consisted of one heavy hydrogen atom for every 10,000 regular hydrogens. Why should this be? If Earth and the sun originated from the same primordial stuff, why would there be five times as much heavy hydrogen on Earth as on the sun?

This was the puzzle Reeves had solved on the train. During the war, in much the same way as it is done today, the Nazis were separating heavy water from ordinary water by subjecting it to near vacuum at temperatures approaching absolute zero, or minus 273 degrees Centigrade — similar to conditions in outer space that favour the formation of heavy water. Current cosmological theory holds that solar systems originate when a nebula, a giant interstellar gas cloud, condenses under its own mass to form a hot, burning star at the centre of a rotating disc of particles and gas, from which planets eventually coalesce over millions of years. Reeves made the calculations and determined that the pressure and temperature of the solar disc near Earth’s orbit would favour the same chemical reaction used to produce deuterium for the nuclear industry. It’s a much slower process in space, but the time scale was right — about 10 million years, plenty of time to account for the five-times difference.

When Reeves got off the train he said to Geiss, who had come to meet him at the station, “You know your problem with the heavy-hydrogen abundance? I think I’ve got it figured out.”

“You’ve got a theory! Well, I’ve got one, too,” said Geiss. It turned out that both physicists had come to the same conclusion, but in slightly different ways. More than 30 years after that breakthrough they were awarded the Einstein Prize for their experiment and theory, first published in 1971, that estimates the density of ordinary matter in the universe. The predictions have since been confirmed in many ways and are still remarkably close to current observations.

Reeves likes to remind people that amazing scientific discoveries can come anywhere and at any time. “Going to the movies can help you do physics,” he says.

As A Young Scientist...

When Hubert Reeves was six he used to go with his family to visit Père Louis Marie, a friend of Reeves’ mother. Père Marie was a Trappist monk who lived at the monastery in Oka, Quebec. A naturalist and geneticist, Père Marie used to let Reeves turn the pages of his fabulous herbarium, a giant book of dried plant specimens. Reeves went for long walks in the woods with the naturalist, who would teach the boy how to identify plants and flowers.

When Reeves was in grade 10 his physics teacher took the class up to the roof to make a special telescope to view sunspots. They had a frame called an optical bench that accurately held lenses in precise alignment. With two lenses, a mathematical formula and a few measurements they were able to point the whole thing at the sun to make an image appear on the viewing plane, a sheet of white paper. After some fiddling, a bright, shining disc came into focus on the paper, with a series of black dots clearly visible across the centre of the glowing image.

“To be able to see sunspots was marvellous to me. It was like magic,” says Reeves. He was amazed that a few calculations and a bit of do-it-yourself could make the invisible visible. He imagined the emotion Galileo must have felt when, in 1610, he did something similar to see the moons of Jupiter for the first time. From then on, Reeves was hooked on astronomy and astrophysics.

He went on to study at the University of Montreal and at Cornell University in New York. He worked for several years as a professor at the University of Montreal, but in 1965 he became a top researcher for the National Centre of Scientific Research in Paris, France. He maintains an associate professorship at the University of Montreal and teaches courses there each year. Reeves holds dual Canadian and French citizenship. He is president of the Ligue ROC pour la Préservation de la Faune Sauvage, a French organization for the preservation of wild animals.

The Science

An astrophysicist is a nuclear physicist who studies thermonuclear reactions in the cores of stars, how stars are born, how all the chemical elements are created within them and how they die. Ultimately, Reeves is trying to discover the origin and fate of free energy in the universe.

He considers the idea of the big bang and an expanding universe to be the most important scientific discovery of the 20th century. Before this, scientists from Aristotle to Einstein considered the cosmos to be static and changeless, disconnected from the bustle of life on Earth. Questions about how stellar objects formed were considered meaningless or beyond the scope of science. But now we know the universe has a history, and Reeves considers himself to be a sort of cosmic historian. “I am fundamentally a nuclear physicist,” he says, “but the 100 or so chemical elements were formed as a result of nuclear reactions in stars. So my work is about trying to unravel how things went — the history of our origins.” He likes to point out that Earth and everything on it, including us, began as stardust.

Besides the famous paper with Geiss about the density of matter, Reeves has helped explain exactly how certain elements can originate from nuclear reactions in space. In particular he has elucidated the origins of the very light elements lithium, beryllium and boron. The formation of such light elements cannot be explained by fusion — the melding of two hydrogen atoms to form helium, for instance. Stars are constantly fusing hydrogen and helium into heavier and heavier elements, and generally this is how nearly everything originated. Lithium, beryllium and boron, however, cannot be made this way. What’s more, these three elements are fragile and easily split into other elements. Therefore, they must constantly be under production to account for their currently observed abundance in the universe. Where, then, do they come from and how are they made? The answer is a process called spallation.

How spallation works. Click to enlarge.

When a solar system forms, astrophysicists believe that it starts with a huge rotating nebular cloud of dust. Although nobody knows how such a protoplanetary disc starts or exactly how it works, scientists believe that gravitational forces, possibly caused by shock waves from the collapse of a nearby star or supernova, induce a central star to form from the cloud over tens of millions of years. When enough hydrogen is present, the collective gravity is enough to compress it to the point for nuclear fusion to occur and a star is born. Similarly, planets are thought to condense from the orbiting dust. The cloud probably consists mostly of hydrogen and helium, with carbon and oxygen being secondarily abundant.

Hubert Reeves has shown how numerous other elements can be created (over long periods of time) by chance collisions of high-energy protons, gamma rays or cosmic rays, with just these few primordial elements. The above illustration shows how lithium and helium can be created from oxygen via the process of spallation when the oxygen is hit by a very high-energy proton, a cosmic ray.

While he has published many scientific books and papers on the subject of spallation of the elements and other aspects of astrophysics, Reeves is best known, particularly in the French-speaking world, for his many popular books and tv shows on cosmology and astronomy. He is the French version of the popular American astrophysicist Carl Sagan, who wrote books and had a TV show before he died in 1996. Reeves is also an active environmentalist, and in that capacity he can be compared to the Canadian biologist David Suzuki.

As a scientist who focuses on origins, Reeves is sometimes challenged by those who feel the currently accepted big bang theory is a myth, equivalent to creation stories found in religious books like the Bible. Reeves never uses the word “creation” when he talks about the origin of the universe or the formation of galaxies and stars. He won’t even use “creation” to describe the big bang. “Creation in the philosophical sense means starting from nothing,” he says, whereas in science you can never create something from nothing; you always start with something you assume to be there. So where does the big bang start from? No one knows. Reeves says, “I like to think of the big bang as a horizon: the horizon of our knowledge. That’s as far as we go, and beyond this we don’t know.” It does not mean that nothing exists over the horizon. The big bang is where it’s at right now, but scientific knowledge progresses and the horizon moves with it as we find out more about our origins.

The stories from the Bible, Koran and other religious books are meant to teach lessons on how to live. For them it doesn’t really matter if the world was made in seven days or fifteen billion years. Religious books impart wisdom about living with each other. Reeves says, “These are stories related to the very important human desire that life must have a meaning. If life has no meaning, you die. You cannot live.” Their prime role, according to Reeves, is to teach morality, relations with our ancestors and how to live. Science offers something else.

Reeves grew up Roman Catholic. He doesn’t feel the Bible should be taken literally as a book of science. “Science is robust,” he says. “I have some basis to defend myself when I make statements in scientific papers. It’s not just something that I invent — a story that comes out of my mind and tomorrow I can make up another story.” The strength of science, he believes, is that if you ask why we believe in the big bang, scientists can point to a number of observations, physical measurements, that confirm the scenario of the big bang. But what’s even more important is an essential feature of science called prediction. A scientific “story” is not good enough if it just explains what we have seen. It must also go out on a limb and predict something new, something never before seen. Then a new experiment is devised to test that prediction and new observations are made. If they are in agreement with what was expected, that strengthens the theory.

As an example, Reeves points to a series of experiments designed to settle the problem of solar neutrinos. Neutrinos are particles with no electric charge and, scientists believed at the time, no mass. Hence they are very hard to detect, because they pass through all matter with little or no interaction. Three kinds of neutrinos are known to exist. The sun emits a type called electron neutrinos. Previous experiments had found a third fewer electron neutrinos coming from the sun than predicted based on how astrophysicists think the sun burns. This was the solar neutrino problem. Reeves says, “Either we don’t know the sun well enough, or we don’t know the neutrino.”

As it turned out, it was the neutrino. After almost 10 years of preparation, an experiment was conducted deep in an abandoned mine in Sudbury, Ontario, to detect solar neutrinos with much greater sensitivity than ever before. (The detector is located two kilometres below Earth’s surface to shield it from cosmic rays that would give false positive readings.) Researchers made an amazing discovery: solar neutrinos change type on their way from the sun to Earth. “Neutrinos can evolve into other forms, just like Pokemon characters,” says Reeves. The sun emits electron neutrinos, but by the time they reach Earth they transform into tau or muon neutrinos. To accomplish this they have to change the way they vibrate, and to do that they must have mass — not much, only about a millionth the mass of an electron. Reeves says, “So the experiment tells us our solar burning model is okay, but we learned something new about neutrinos and physicists must now incorporate these new ideas into their theories.”

Experiments are always being conducted to learn more about our origins. In September 2004, NASA’s Genesis space mission returned a relatively large sample of the solar wind to Earth, after two years of collecting while sitting 1.5 million kilometres away from our planet and facing the sun. It’s like having a piece of the sun here on Earth. Scientists believe these samples will tell us what the original primordial solar nebula disc consisted of five billion years ago. Unfortunately, the Genesis return module’s parachute failed to open and it crashed into the Utah desert at about 300 kilometres per hour. The sample-collection system was bent and cracked but still intact. Despite this mishap, scientists expect to learn more about the origin of our solar system from this experiment.


Only about 5 percent of the universe is made of known matter; 25 percent is dark matter; 70 percent is made of dark energy, a repulsive force that operates over very long intergalactic distances. We don’t know anything about dark matter or dark energy. In other words, we don’t know what 95 percent of the universe is made of.

Explore Further

Hubert Reeves, et al., La plus belle histoire du monde (published in English as Origins: Speculations on the Cosmos, Earth and Mankind), Editions du Seuil, 2004.

J. Geiss and H. Reeves, 1972, Astr. Ap. 18, 126.

H. Reeves, W. A. Fowler and F. Hoyle, Nature (London),1970, pp. 226, 727.

The 1971 Apollo 14 Solar Wind Composition Experiment on NASA's website.

Website for NASA's Genesis Mission.

How to make a telescope, by the Fun Science Gallery.

Cosmic rays and spallation at Macalester College.

Career Advice

So You Want to Be an Astrophysicist

At first it may appear that all of physics is based on mathematical equations, and indeed it was this theoretical basis that fascinated the young Hubert Reeves. Just by thinking, by calculating, he was amazed how accurately scientists could predict the positions of stars, planets, atoms and much more. But later he realized there was more going on. Something else must come before number and theory. That missing factor is observation. “You always need to start with an observation,” says Reeves. “Mathematics alone can do nothing about knowing the real world. It cannot even tell you what is the number of dimensions you feel.” First comes the observation, he says, then the theory. Ultimately, the theory allows us to do calculations that lead to more observations that confirm or improve the theory. It was this “dialogue” between what we see and what we think that fascinated Reeves, and he feels it should be the driving force behind any career in science.

Typical physics careers include specialties in electronics, communications, aerospace, remote sensing, biophysics, nuclear, optical, plasma or solid state physics, astrophysics and cosmology. Some physicists focus mostly on experiments, while others just do theory.

Career ideas:

  • research scientist, physics
  • research scientist, electronics
  • research scientist, communications
  • research scientist, aerospace
  • research scientist, remote sensing
  • nuclear physicist
  • optics physicist
  • plasma physicist
  • solid state physicist
  • astrophysicist
  • cosmologist
  • experimental physicist

The Person

July 13, 1932
Montreal, Quebec
Paris, France
Family Members
  • Mother: Manon Beaupré
  • Father: Joseph-Aimé
  • Spouse: Camille Scoffier
  • Children: Gilles, Nicolas, Benoit, Evelyne
  • Grandchildren: five
Congenial, private, charming
Favorite Music
Schubert's String Quintet in C, second movement
Other Interests
Associate Professor
U. of Montreal
  • B.Sc. U. of Montreal, 1953
  • M.Sc. McGill, Montreal 1955
  • PhD Cornell, New York, 1960
  • Chevalier de la Legion d’Honneur (France) 1986
  • Grand prix de la francophonie décerné 1989
  • Einstein Prize, Einstein Society, Bern 2001
Père Louis Marie, childhood family friend, geneticist and botanist who knew everything in the woods. Philip Morrison, Cornell physicist who combined culture, science and literature with an enthusiasm for the nature of reality.
Last Updated
May 15, 2020

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