Hubert Reeves

Astronomy, Astrophysics and Space Science

World famous cosmologist and science communicator

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

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.

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