Professor John Wheeler: Titan of 20th-century physics who coined the term 'black hole'

John Wheeler was one of the titans of 20th-century physics. He collaborated with the two greatest physicists of the past 100 years – Albert Einstein and Niels Bohr. He worked on the development of the American H-bomb. And he coined a term that not only galvanised a field of physics but so seeped into the general public's consciousness that it is used in everyday language. The term is "black hole".

Wheeler was born in Jacksonville, Florida. Switched on to science by a children's book called Ingenious Mechanisms and Mechanical Devices, he trained to be a physicist at Johns Hopkins University in Baltimore. It was while there, in 1934, that he won a National Research Council Fellowship to visit the great Niels Bohr in Copenhagen.

Bohr was the father of "quantum theory", the revolutionary picture of atoms and their constituents that provided a window into a scarcely believable, Alice-in-Wonderland world where things happened for absolutely no reason at all and a single atom could be in two places at once. The significance of Wheeler's visit to Bohr was that it led to a return match in the United States in 1939. And, on that trip, Bohr was carrying with him earth-shattering news.

In Berlin at the end of 1938, Otto Hahn and Fritz Strassman had fired subatomic particles called "neutrons" at atoms of uranium. An atom consists of a tight knot of matter called a nucleus orbited by tiny particles called electrons. Hahn and Strassman's neutrons had caused uranium nuclei to split in two, or "fission", liberating more neutrons and a large amount of "nuclear energy".

The news quickly reached Hahn and Strassman's ex-colleague Lise Meitner, exiled in Sweden because she was a Jew. Meitner and her nephew, Otto Frisch, quickly realised that a "chain reaction" was possible. All it would take would be two neutrons to be liberated during fission, which split two more uranium nuclei, which liberated four neutrons, which split four more uranium nuclei, and so on. The energy unleashed by such a runaway chain reaction would create a bomb of unimaginable violence.

This was the news Bohr, arriving at the docks in New York, imparted to Wheeler. And, with a fear of a German atomic bomb already infecting them, they began feverishly trying to understand the fission of uranium. Remarkably, the picture they had success with was of the nucleus like a liquid drop. Struck by a neutron, it wobbled violently, extending into a dumbbell, which thinned in the middle and finally split into two smaller droplets.

From their liquid drop model, Bohr and Wheeler discovered two key things. First, fission was possible only with a rare kind of uranium – uranium-235 – which accounted for just a small percentage of natural uranium. This would therefore have to be concentrated to make a bomb. Secondly, there was an element with an even heavier nucleus, which nobody had ever made, that would also undergo fission. It was called plutonium.

The rest, as they say, is history. In an industrial effort unmatched in human history, the Manhattan project led to the exploding of the first atomic bomb at Alamagordo in New Mexico on 16 July 1945 and the subsequent destruction of the Japanese cities of Hiroshima and Nagasaki. Wheeler was a key figure in the project, working at the Hanford site, in Washington state, on preventing the "poisoning" of the nuclear material needed to make plutonium.

And, later, with the Soviet Union substituting for Germany as America's nightmare personified, Wheeler steered the US towards a hydrogen bomb – thousands of times more destructive than the uranium and plutonium bomb that had erased Hiroshima and Nagasaki. It was first exploded in 1952.

Wheeler, however, was far more than just a bomb scientist. In the early 1950s, almost single-handedly in the US, he turned Einstein's theory of gravity – the general theory of relativity – which was very unfashionable, into a thriving field. His efforts at Princeton were mirrored in the Soviet Union by Yakov Zel'dovich at Moscow State University and in Britain by Dennis Sciama, at Cambridge, whose most famous student would be Stephen Hawking.

Wheeler had a talent for summarising complex ideas in simple phrases. The essence of Einstein's theory of gravity, he said, is that: "Matter tells space how to warp. And warped space tells matter how to move." These simple words, a mantra for all physics students, belie the difficulty of predicting anything with the theory. But Wheeler made progress.

One of the things that caught his attention was the claim in 1939 by Hartland Snyder and Robert Oppenheimer, the father of the atomic bomb, that, when a very massive star runs out of fuel to burn at the end of its life, no force in the universe can stop it being crushed down to an infinitely dense point, or "singularity". Since nothing can escape such an object, not even light, it is totally black.

Wheeler thought the idea of such an object totally ridiculous. However, when he applied Einstein's theory to burnt-out stars, he was surprised to reach the same conclusion as Oppenheimer. He discovered that the runaway shrinkage to a singularity was unavoidable for a star above a critical threshold mass of about twice the mass of the Sun. In 1967 – the exact date and circumstances are the subject of some debate – Wheeler came up with a name for such an object: a black hole.

Wheeler's former student, Richard Feynman, thought the term obscene. Possibly, he had a point. After all, you have to feel for the French for whom the term "trou noir" is slang for something entirely different. However, the name stuck.

If ever there was an example of a word being important in science, it is this. Before 1967, there was essentially no research on black holes, which the Russians called "frozen stars". After 1967, the field exploded. The term "black hole" crystallised the thoughts of physicists and caught the imagination of everyone, including lay people. Today, it is common for people to say things like "My e-mail must have disappeared down a black hole".

Wheeler, in 1957, also coined the term "wormhole", for a tunnel-like short cut through space-time, which is permitted to exist by Einstein's theory of gravity. In addition, he proclaimed that "black holes have no hair". This refers to the fact that black holes have no distinguishable features – no humps or bumps or "hair" sticking out of them. All information about the star from which they collapsed is erased, and they are characterised only by their mass and spin rate, a fact that led the Nobel Prize winner Subrahmanyan Chandrasekhar to remark: "The black holes of nature are the most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and time."

Black holes appear to be a reality with almost every galaxy, including our own Milky Way, harbouring a monstrous "supermassive" version in its heart.

But Wheeler's work was not only in gravity and nuclear physics. His interests ranged far and wide and were characterised by true fearlessness. Controversially, he favoured an extreme interpretation of quantum theory in which a conscious observer is required to actualise the world of atoms out of the haze of "probabilities". In Wheeler's "participatory universe", there was no universe until the rise of consciousness to perceive it. In other words, the universe, with all its unaccountable galaxies and stars, is brought into existence by the act of observing it with telescopes. Since light from distant celestial objects shows us what they looked like far back in time, the past is not only another country, it is a non-existent country without us.

Wheeler, like Bohr and many of the greatest physicists, was a magnet to talented and imaginative young physicists. Among them was Feynman, winner of the 1965 Nobel Prize for physics for "quantum electrodynamics" and arguably the greatest post-war American physicist. At one point, Feynman and Wheeler theorised that there was just one electron in the entire universe. By weaving backwards and forwards through time, it became every electron in existence. Other students included the Israeli physicist Jacob Bekenstein, who showed the unexpected connection between black holes and thermodynamics, the theory that had spawned steam engines, and Kip Thorne, who showed that Wheeler's wormholes were viable if kept open by "exotic matter" with repulsive gravity. Nobody yet knows whether such matter exists in the universe.

But history may show Wheeler's most important student to have been Hugh Everett III, whose idea that there exists an infinite number of realities, stacked like the pages of a never-ending book, in which all quantum possibilities are realised, is half a century old. For instance, there is a reality where you are not reading this obituary; where you were never born; where the dinosaurs were not wiped out by an asteroid 65 million years ago but went on to invent the motor car. The extraordinary idea, ignored by physicists for 30 years, is now increasingly being embraced as an explanation of the world of atoms.

Wheeler dressed like a businessman. He wore a tightly knotted tie and his cuffs were starched white. According to Bekenstein, he was a model of proper, gentlemanly behaviour. "From Wheeler I learnt of the importance of treating people, no matter how silly they may be, with respect. At one time he asked me to talk to a dentist who was pestering him with claims that relativity was wrong. I was instructed to hear the guy out and show him his error. It would have been so much simpler just to wave the guy away, but Wheeler insisted on the moderate approach."

Marcus Chown

John Archibald Wheeler, physicist: born Jacksonville, Florida 9 July 1911; Assistant Professor of Physics, University of North Carolina 1935-58; Assistant Professor of Physics, Princeton University 1938-42, Associate Professor 1945-47, Professor 1947-66, Joseph Henry Professor of Physics 1966-76 (Emeritus); physicist, Manhattan Project 1942-45; Director, Project Matterhorn, Princeton 1951-53; Professor of Physics, University of Texas at Austin 1976-86, Ashbel Smith Professor of Physics 1979-86, Blumberg Professor of Physics 1981-86 (Emeritus); married 1935 Janette Hegner (one son, two daughters); died Hightstown, New Jersey 13 April 2008.