Astronomy, Astrophysics and Space Science
World famous cosmologist and science communicator
"Modern physics allows us to shout 'Long live Freedom!'"
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.
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.