by Marcus Chown
But what compounded Fowler’s amazement was the manner of Hoyle’s prediction. He had predicted the 7.65 MeV energy state of the carbon-12 nucleus using an unprecedented argument: it had to exist because if it did not, the universe would contain no carbon or heavy elements. Nobody in the history of physics had ever used so preposterous an argument to make such a precise prediction about the world. Among the brotherhood of the magicians, Hoyle has a unique place.
*
When Hoyle had time to ponder the discovery of the 7.65 MeV energy state of the carbon-12 nucleus, he began to appreciate how the existence of the heavy elements out of which we are made appears to be dependent on not one but several pieces of remarkably good fortune. The first is the non-existence of a stable state of a nucleus of beryllium-8. The second is the existence of an excited state of a carbon-12 nucleus at precisely 7.65 MeV. But there is also a third piece of nuclear luck.
There is no energy state of an oxygen-16 nucleus at the combined energy of a carbon-12 and a helium-4 nucleus at the 100-million-degree temperature inside a red giant; if there was, the conversion of carbon-12 into oxygen-16 would be resonant. In other words, the instant carbon-12 was made in the triple-alpha process, it would immediately be converted into oxygen-16. The universe would end up with no carbon whatsoever, whereas in reality it contains roughly equal amounts of carbon and oxygen.
In 1973, the Australian physicist Brandon Carter popularised the idea that many of the ‘fundamental constants’ of nature, such as the strength of the electromagnetic force and the mass of the electron, have the values they have because, if they did not, it would be impossible for stars, planets and life to exist. In other words, the fact that we are here is a key observational fact. After all, if things were not the way they are, we would not be here to remark on the fact.
Not surprisingly, the topsy-turvy logic of this ‘anthropic principle’ has proved controversial. It does not help the idea’s credibility that its proponents have pointed out that the electromagnetic or gravitational forces have the strength they have only after observing the consequences of those forces in the universe. Hoyle’s prediction of the 7.65 MeV energy state of the carbon-12 nucleus is unique in that it was made in advance of any observation or experiment. And in the years since 1973, it has been hailed as the big success of the anthropic principle.12
The three pieces of nuclear good fortune may not actually be as necessary for our existence as they seem at first sight. Proponents of the anthropic principle point out that, if the strong nuclear force which binds the nucleons inside nuclei were a few per cent weaker, it would be impossible to make sufficient carbon-12, but it is not often pointed out that, if the strong nuclear force were a little stronger, it would make the nucleus of beryllium-8 stable. Crucially, this would open up an entirely new route to the building of carbon-12 and all heavier elements. So at best, the fact that beryllium-8 is unstable is a one-sided piece of luck.
In 1953, Hoyle in effect said, ‘Heavy elements exist, therefore there must exist a state of carbon-12 at an energy of 7.65 MeV to open the door to the building of heavy elements.’ However, he eventually saw things in anthropic terms and his statement metamorphosed into ‘I exist, therefore there must exist a state of carbon-12 at an energy of 7.65 MeV.’
The bypassing of the beryllium barrier opened the road for the building of heavy elements. As a massive star evolved and its core became ever denser and hotter, helium nuclei deep in its interior would stick to oxygen-16 to make neon-20; helium nuclei would stick to neon-20 to make magnesium-24; and so on. This ‘alpha process’ would culminate in the addition of helium to silicon, to make iron at a temperature of about three billion degrees. At this juncture, things would go awry for the star; ‘silicon burning’, unlike the preceding nuclear reactions, does not liberate energy but sucks it out of a star. And, since the heat from such nuclear energy is what provides the outward push that prevents gravity crushing a star, the core implodes. This is the process – which is still not well understood – that results in the ejection of the outer envelope of a star as a ‘supernova’, spraying into space many of the elements it has painstakingly built up over its lifetime.
The iron-group elements are created in the nuclear thermodynamic equilibrium that exists briefly in the supernova explosion, but there are many other processes such as the alpha process that are responsible for building up elements. In fact, in a monumental paper published in 1957 by Margaret and Geoffrey Burbidge, Fowler and Hoyle, and universally known as ‘B2FH’, eight distinct element-building processes were identified as producing the elements we see in the universe today.13 Two processes – the rapid and the slow-neutron processes – make nuclei by adding neutrons one at a time. Since neutrons have no electric charge, such processes overcome the problem of charged nuclei repelling each other before they can get close enough to stick. The rapid and slow-neutron processes make neutron-rich nuclei in supernovae explosions and red giant stars, respectively.
Willy Fowler won the 1983 Nobel Prize in Physics for figuring out why elements such as iron and nickel are common and elements such as lithium and beryllium are rare. Hoyle did not share the prize, although, as Fowler later remarked, he himself would probably have remained a run-of-the-mill nuclear physicist had it not been for the visit of Fred Hoyle to his office on that fateful day in the winter of 1953.
Despite all the successes of B2FH, the origin of a small number of elements such as gold and silver was mysterious, until recently. The puzzle was finally solved only on 17 August 2017, when gravitational waves were detected by the Laser Interferometric Gravitational-Wave Observatory, or LIGO. They came from the merger of two super-compact ‘neutron stars’. The gamma rays picked up on Earth carried the ‘fingerprint’ of gold and silver, and revealed the creation of a quantity of gold equivalent to twenty times the mass of the Earth.
The extraordinary story begun by Hoyle in 1944 has reached its final chapter. We are more connected to the stars than even the astrologers guessed. Would you like to see a piece of a star? Hold up your hand. The iron in your blood, the calcium in your bones, the oxygen you take in with every breath were all forged inside stars that lived and died before the Earth and Sun were born. You are stardust made flesh. You were literally made in heaven.
Notes
1 Home Is Where the Wind Blows: Chapters from a Cosmologist’s Life by Fred Hoyle (University Science Books, California, 1994, p. 264).
2 It did not go unnoticed that Hoyle had pursued his astronomical interests while in the US on official radar business for the Admiralty. Back in the UK, he was asked to explain his visit to the Mount Wilson Observatory, having been reported by someone at the British Embassy in Washington DC. Thinking on his feet, Hoyle replied that he was interested in the well-known temperature inversion in the Los Angeles Basin, which caused a jump in the density of the air, with possible consequences for the propagation of radar pulses. Anomalous propagation of such radar pulses had been the subject of the conference he had attended in Washington DC; by neatly tying together his astronomical excursions and his ‘tour of duty’, he escaped any official reprimand and punishment.
3 In fact, very heavy elements such as californium, plutonium, einsteinium and fermium were found to have been created in the fall-out from the world’s first large H-bomb test on Enewetak Atoll in the Pacific on 1 November 1952.
4 A key discovery that would be made by Paul Merrill in 1952 is the fingerprint of technetium in the light of stars. Since the element disintegrates, or decays, in only a few hundred thousand years, it can persist in stars only if it is made continually.
5 ‘The Chemical Composition of the Stars’ by Fred Hoyle (Monthly Notices of the Royal Astronomical Society, vol. 106, 1946, p. 255).
6 Hoyle proved to be right even when he was wrong, as dense hydrogen gas does turn out to exist in space. Such ‘Giant Molecular Clouds’ are the nurseries where stars are born. However, the idea of such clouds proved so controversial for decades after Hoyle and Lyttleton c
ame up with the idea that the only place Hoyle could present it was in a science-fiction novel, The Black Cloud (William Heinemann, London, 1957).
7 ‘Experimental and Theoretical Nuclear Astrophysics: The Quest for the Origin of the Elements’ by William Fowler (Nobel Lecture, 8 December 1983: https://www.nobelprize.org/uploads/2018/06/fowler-lecture.pdf).
8 Ward Whaling interviewed by Shelley Irwin (California Institute of Technology Archives, April–May 1999: http://oralhistories.library.caltech.edu/122/1/Whaling_OHO.pdf).
9 Home Is Where the Wind Blows: Chapters from a Cosmologist’s Life by Fred Hoyle (University Science Books, California, 1994, p. 265).
10 It actually took about three months to pin down the result precisely. Hoyle was back in Cambridge by the time Whaling and his team wrote their paper, to which they added Hoyle’s name. ‘A State in Carbon-12 Predicted from Astrophysical Evidence’ by F. Hoyle, D. N. F. Dunbar, W. A. Wenzel and W. Whaling (Physical Review, vol. 92, no. 4, 1953, p. 1095a).
11 William Fowler interviewed by Charles Weiner (American Institute of Physics, 6 February 1973: https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4608-4).
12 ‘When Is a Prediction Anthropic? Fred Hoyle and the 7.65MeV Carbon Resonance’ by Helge Kragh (University of Aarhus, May 2010: http://philsci-archive.pitt.edu/5332/1/3alphaphil.pdf).
13 ‘Synthesis of the Elements in Stars’ by E. M. Burbidge, G. R. Burbidge, W. A. Fowler and F. Hoyle (Reviews of Modern Physics, vol. 29, 1957, p. 547). Similar ideas were also published almost simultaneously by Alastair Cameron in ‘Nuclear Reactions in Stars and Nucleogenesis’ (Proceedings of the Astronomical Society of the Pacific, vol. 69, 1957, p. 201).
* An electronvolt (eV) is a convenient unit of energy used by physicists. It is the energy acquired by an electron when it is accelerated by a voltage difference of one volt. A megaelectronvolt (MeV) is the energy acquired by an electron accelerated through a voltage difference of a million volts.
† Atomic nuclei contain positively charged particles known as protons and chargeless particles known as neutrons. The two particles, which have essentially the same mass, are together known as ‘nucleons’. Hydrogen-1 has one nucleon in its nucleus; helium-4 has four; lithium-6 has six; and so on. Since the protons are balanced by an identical number of electrons orbiting the nucleus, and the electrons determine how an atom connects with other atoms – in short, its fundamental character – the number of protons in a nucleus determines the particular type of atom. Hydrogen atoms contain one proton in their nucleus; helium atoms two; lithium atoms three; and so on. All nuclei except those of hydrogen also contain neutrons, which do not affect the behaviour of an atom but contribute to its mass.
‡ Atoms of a particular element absorb and emit light at only certain wavelengths, which act as a fingerprint for the element. It is by means of such fingerprints that different elements reveal themselves to astronomers in the light of stars. The wavelengths correspond to the energy absorbed or shed by electrons moving between different orbits inside an atom.
§ In fact, Gamow had provided this vital ingredient. The first person to apply quantum theory to the atomic nucleus, he had discovered in 1928 that, in the ‘alpha decay’ of a heavy element such as radium, an alpha particle, or helium nucleus, can escape the nucleus, even though it appears to have insufficient energy to do so. This phenomenon of ‘quantum tunnelling’ is possible because the quantum wave associated with the alpha particle extends outside the nucleus, giving the alpha particle a small probability of being found there at any time. In 1929, Robert Atkinson and Fritz Houtermans turned Gamow’s idea on its head, showing how inside the Sun one nucleus could tunnel into another, despite their ferocious mutual repulsion appearing to make it impossible. As a by-product, such a nuclear reaction created sunlight.
¶ As would later be discovered, lower-mass stars like the Sun are powered by another sequence of nuclear reactions, known as the ‘proton–proton chain’.
5
Ghost busters
Neutrino physics is largely an art of learning a great deal by observing nothing.
HAIM HARARI1
I have done a terrible thing: I have postulated a particle that cannot be detected.
WOLFGANG PAULI
Savannah River, South Carolina, 14 June 1956
Frederick Reines was singing as he drove to the bomb plant. He loved to sing almost as much as he loved to do physics. Back in college in New Jersey, he had even taken lessons from a voice coach at the Metropolitan Opera and sung solos in Handel’s Messiah.2 When he was working on a particularly tough theoretical problem, he had been known to sing for hours on end while locked away in his office. But on this June morning there was a very specific reason why his fine baritone boomed out through the wound-down car window, turning the heads of pedestrians walking by on the sidewalk. After almost a year of exhausting work – five years, if you counted the total effort that had led up to this day – he was in celebration mode. He and his team were about to achieve the impossible.
It was eight miles from Aiken, the community where they had been domiciled since November, to the Savannah River Plant. As he drove out of town, the sweet smell of camellias and magnolias came in through the window on the hot damp air, reminding him of how exotic South Carolina had seemed when they had arrived from the high desert of Los Alamos. On their first drive out from Aiken through the swampy Savannah River Valley, their car lurched over something in the road and they were tossed about like rag dolls. Looking back, they saw that what they had assumed was a speed bump was actually a giant rattlesnake.3
At the gates to the Savannah River Plant, Reines pulled up behind a long double-line of cars. The site, with its five nuclear reactors, separation facilities and waste dumps, covered an area larger than New York City and employed almost forty thousand people. When the US government had announced its plan to build the facility, it had said it was not for the ‘manufacture of atomic weapons’, but that was splitting hairs. Everyone knew the truth: it made the fuel for nuclear weapons – which is why, even in the shops and bars of Aiken, Savannah River was referred to as the ‘bomb plant’.4
In early September 1949, a US Air Force B-29 bomber had sniffed the air high above the Pacific coast of the Soviet Union and caught the unmistakable aroma of an atomic bomb blast. Like most of his colleagues at Los Alamos, Reines had worked on the Manhattan Project to build the first atomic bomb, and he still remembered his shock at the announcement, a mere four years after Hiroshima, that the Russians had caught up and the US no longer had an atomic monopoly.
To counter the Soviet threat, President Harry Truman embarked on a drive to build a ‘superbomb’ whose destructive power would dwarf an atomic bomb. It involved the construction of vast facilities across the country, not only to make the fuel for such ‘hydrogen bombs’ but to assemble them. As part of the programme, on 28 November 1950 the US government announced the seizure of almost 500 square kilometres of land by the Savannah River to make two key components of nuclear bombs: tritium and plutonium. Four towns were bulldozed and six thousand people moved, and, by early 1952, the plant was in full production mode.5
On 1 November 1952, the US exploded a hydrogen bomb on Elugelab, part of Enewetak Atoll, a Pacific island liberated from the Japanese in the Second World War. With 700 times the destructive power of the bomb dropped on Hiroshima, it vaporised the island, creating a radioactive mushroom cloud 150 kilometres across and gouging a hole in the ocean floor more than two kilometres wide and as deep as a sixteen-storey building. But a mere nine months later, in August 1953, came the scarcely believable news that the Russians had detonated their own hydrogen bomb. Their design could not be scaled up to make bigger explosions, but everyone knew it was only a matter of time. Sure enough, on 22 November 1955, at the Soviet test site at Semipalatinsk in Kazakhstan, the Russians exploded their first true hydrogen bomb.
Reines reached the head of the line of cars, flashed his ID card through the open window and
accelerated towards the hulking shape of P Reactor. The Savannah River facility boasted five reactors – R, P, K, L and C – whose letter designations had been chosen entirely at random. They were built at two-and-a-half mile intervals, so they could not be wiped out by a single Soviet nuclear strike, and were spread along a horseshoe-shaped curve to make them immune to a straightline bombing run. Each reactor rose sixty metres into the air and was sunk twelve metres into the ground, for even more protection. It was this last feature that was of key importance to Reines and his team. It was what had brought them from New Mexico in search of their impossible quarry: a ghostly subatomic particle that had been predicted a quarter of a century earlier and whose existence would almost certainly be confirmed that day.
Zurich, December 1930
The elusive particle had been predicted by the Austrian physicist Wolfgang Pauli. A one-time infant prodigy, Pauli had at twenty-one written such a masterful survey of the theory of relativity that it had astonished even the theory’s creator, Albert Einstein. In fact, Pauli – his confidence bordering on arrogance – had famously stood up at the end of a lecture given by Einstein, turned to the audience and reassured them that ‘What Professor Einstein said is not entirely stupid.’6
In the mid-1920s, Pauli was one of the principal architects of ‘quantum theory’, the revolutionary description of the submicroscopic world of the atom and its constituents. His name is immortalised in the ‘Pauli exclusion principle’, which, by preventing electrons from piling on top of each other, makes atoms and the everyday world possible.
