THE 20th century has seen the human scale of things squeezed into a tiny slice in a vast spectrum of distances. From the submicroscopic building blocks of matter to a cosmos so huge that even light takes aeons to reach us, the range of scales in the Universe spans more than 40 powers of 10. Hardly less extraordinary are the tools that enable us to look at the micro and macrocosmos. Telescopes and spectrometers record the telltale patterns of radiations from dim and distant worlds, millions of light years away. To take us inwards, we have particle accelerators and detectors that make real the debris from particle collisions unseen since matter first formed in the early Universe.

In The Quark Machines, Gordon Fraser tells, from the European perspective, the story of this inward exploration with particle accelerators. It is a task for which he is well qualified. For the past 18 years he has been editor of The CERN Courier, the magazine of CERN, the world’s biggest particle physics laboratory.

During Fraser’s time there, the laboratory near Geneva has built and run the world’s largest scientific instrument: the Large Electron Positron Collider. It has also changed its name, while retaining the acronym, from the enigmatic Conseil Européen pour la Recherche Nucleaire to the far more comprehensible European Laboratory for Particle Physics. And, although constitutionally still European, CERN serves physicists from across the globe and is clearly world-class.

It was not always so, as Fraser explains in his careful and fascinating account of particle physics in the 20th century. The subtitle “How Europe Fought the Particle Physics War” suggests progress through battles for scientific supremacy, with particle physicists on opposing sides bringing bigger and bigger guns to bear on disputed territory deep within the atom. The story bears far stronger analogies to the space race, only here Europeans and Americans were competing to be first to raise their flags at the heart of matter. This is precisely the tone that Fraser takes up in his introduction, entitled “Subnuclear liftoff”. But the main theme has more to do with the changing nature of Europe in the 20th century, leading Fraser to the striking conclusion that a united Europe can work.

Discoveries in particle physics—the quarks and gluons, the theories such as quantum electrodynamics and chromodynamics—belong mainly to the fifty years following the Second World War. The background to these ideas and phenomena, in turn, lies in the remarkable last five years of the 19th century, which saw the discovery of X-rays, radioactivity and the first subatomic particle, the electron. But as Fraser says, the Second World War, as well as the events that led up to it and brought it to a close, shaped the future of research in particle physics as dramatically as they changed the political maps of the world.

As some of the brightest stars in particle physics fled the terrors of Hitler’s Europe, the nascent discipline’s centre of gravity shifted westward towards the US. Through the war effort, in particular the Manhattan Project, physicists learnt, in Fraser’s words, “the value of close collaboration” and found that “if enough minds and sufficient resources were brought to bear on a central problem, it would eventually crack”.

After the war, the seed that would eventually flourish as the modern CERN was sown at a European Cultural Conference in Lausanne in 1949. The idea was to create a European centre for atomic research. By the end of 1950 this had become a concrete proposal to build a European laboratory dedicated to particle physics. The ambitious aim was to build a particle accelerator that would reach higher energies than any under construction elsewhere. Nine years later, the new CERN laboratory had succeeded, as its proton synchrotron reached more than double the highest energy of the machine at Dubna, near Moscow.

But, as Fraser reveals, the jubilation was short-lived. Having put all their effort into making a world-beating machine, the physicists at CERN had neglected to build the instruments to measure the newly opened territory. It was if they had landed first on the Moon, but not brought a ladder to reach its surface. The Americans were well on their way and, although late, they had the equipment to get on with the exploration. Within six months a similar accelerator was working at the Brookhaven National Laboratory on Long Island, New York. It was to lead to important discoveries, keeping the balance of power in particle physics tilted towards the US.

It is in this postwar period that Fraser’s tale comes to life, as he climbs out of the well-trodden history of modern physics in the early 20th century to less well-known but more recent events. Untimely deaths have changed the leadership of CERN in far-reaching ways; the British have repeatedly gone out of step with the rest of Europe; and CERN itself, while proving masterly at building machines, has sadly lagged behind the Americans in the all-important discoveries. Throughout the 1960s and into the 1970s, the breakthroughs came mainly from the US.

The balance began to change—a small but significant wobble to the scales—in 1972, with studies at CERN of the ghostly particles known as neutrinos in a giant bubble chamber called Gargamelle. Results from Gargamelle confirmed the picture of a proton as a cluster of three particles, or quarks, with charges that are fractions of the proton’s charge. This was a vital step in demonstrating the reality of quarks, which had been proposed in 1964 by the American theorist Murray Gell-Mann. It was, as Fraser says, “the most important new physics result to have emerged from CERN—the laboratory’s first major physics discovery”.

A year later, Gargamelle supplied evidence that neutrinos could take part in “neutral current” interactions, predicted by a new theory that intimately entwined the familiar electromagnetic force with the enigmatic weak force of radioactivity. Sadly, the excitement of this discovery was marred by an acrimonious battle with a team in the US who had contradictory results. It was, says Fraser “an exhausting struggle”, but in the end fate seems to have deprived CERN of a Nobel prize. André Lagarrigue, “father” of the remarkable Gargamelle chamber, died of a heart attack in 1975, only two years after the discovery; his successor, Paul Musset, died while mountaineering in 1985.

The Nobel Prize for Physics eventually came famously to CERN in 1984, to Carlo Rubbia and Simon van der Meer, whose efforts made possible the discovery of the W and Z bosons that convey the weak force between other particles. This followed the discovery in 1979 of the gluons that carry the strong force between quarks, at the DESY laboratory in Hamburg.

Now, with the end of the 20th century in sight, CERN can boast the largest particle accelerator in the world. And it has embarked on building what will become the highest-energy accelerator, the Large Hadron Collider (LHC). With four major experiments being planned, involving expertise from across the world, it seems unlikely that CERN will be caught unprepared, as it was with its first record-breaking machine in 1959. The LHC has not had an easy passage

CERN continues to be buffeted by world events, such as the reunification of Germany and the signing of the Maastricht treaty, both with their implications for European economies. The laboratory has proved, as Fraser concludes: “managed correctly, Europe works”.

The value of colliders is that their head-on collisions make more energy available than machines which fire a single particle beam at a stationary target. But how to make these machines was initially far from obvious and required many ingenious inventions. Original papers describing these ideas are reproduced in The Development of Colliders, the latest in the series “Key Papers in Physics”, published by the American Institute of Physics. The physicists behind the ideas are in general the unsung heroes of particle physics. The papers have been selected and set in their historical context by Claudio Pellegrini and Andrew Sessler, who themselves played no small part in the development of colliders as their papers testify. The result is a valuable reference volume – for anyone with some technical knowledge.

I suspect that the main reason for such misunderstandings is that textbooks (and lecturers) have enough ground to cover without getting tangled up in historical detail. So the history is summarised as briefly as possible, and as references become increasingly remote from the original source, an inaccurate picture begins to take shape. It’s rather like Chinese whispers, where the message is gradually altered from its original form to something quite unrelated by the time it reaches the end of the chain.

Another example of rewritten history came to my attention when I was preparing this week’s Inside Science, “Heart of the atom”. For some reason I decided to check when it was that the famous Danish physicist Niels Bohr began to think of the atomic nucleus as a liquid drop. To my surprise, I discovered that the idea did not originate with him, but with George Gamow, a Russian who is probably best known as one of the originators of the big bang theory of the early Universe and the author of the splendid Mr Tompkins stories. Yet most of us who have studied physics, link Bohr with the liquid drop model, just as we link Rutherford with the gold-foil experiment.

The book that set the record straight for me is Niels Bohr: A Centenary Volume, edited by A. P. French and P. J. Kennedy (Harvard University Press, 1985). In the chapter on “Niels Bohr and nuclear physics”, Roger Stuewer, nuclear physicist and historian at the University of Minnesota, describes how the young Gamow left Leningrad in June 1928, and having impressed Bohr, was awarded a year-long fellowship at Bohr’s Institute of Theoretical Physics in Copenhagen. It was near the end of 1928 that the liquid drop model first took shape in Gamow’s fertile mind, and in February 1929, during a visit to Cambridge, Rutherford invited him to present his ideas at the Royal Society in London, during a “Discussion on the structure of atomic nuclei”, opened by Rutherford himself. The following year Gamow – who by now was a visiting fellow at the University of Cambridge – published a more detailed quantitative analysis in the Proceedings of the Royal Society.

Gamow was ahead of his time. In 1930, the neutron – a basic constituent of nuclei – had yet to be discovered. Gamow’s original model of the nucleus contained mainly alpha particles, plus some protons and electrons, which he assumed were bound together by forces rather like those at work in a drop of liquid. The basic idea was not unreasonable, since both alpha particles and electrons emerge from the nucleus in different forms of radioactivity. However, the discovery of the neutron in 1932 was to lead eventually to our present picture of the nucleus as being built from protons and neutrons, although for a year or so various hypotheses appeared based on different combinations of protons, neutrons, electrons and alpha particles.

In October 1933, at the Solvay Conference in Brussels – then a leading international scientific meeting – the German physicist Werner Heisenberg discussed the various hypotheses for the nucleus, beginning with Gamow’s liquid drop model. A major difficulty with this model concerned the presence of electrons. Moreover, Bohr, who took part in the discussion following Heisenberg’s talk, worried that even a heavy nucleus would not contain many alpha particles. Indeed, he did not seem very impressed. Three years later, however, the German physicist Carl Friedrich von Weizsäcker developed a formula for nuclear masses based on Gamow’s work, which he acknowledged in his paper in the journal Zeitschrift für Physik.

So how is it that we now usually associate the liquid drop model with Bohr rather than Gamow? Stuewer points out that when the Strasbourg-born Hans Bethe published the second part of his authoritative review of nuclear physics in Reviews of Modern Physics in 1937, he based his discussion of the liquid drop model on a paper by Bohr and his colleague Fritz Kalckar, which made no reference to Gamow. Bethe’s review became a “bible” of basic nuclear physics, and since then, asserts Stuewer, “virtually everyone has assumed, incorrectly, that Bohr conceived the liquid drop model”.

Indeed, says Stuewer, it seems that in 1937 Bohr probably inclined more towards the idea of the nucleus as an elastic solid (although at the time of the 1933 Solvay Conference he didn’t seem to think much of this either). It was the discovery of nuclear fission late in 1938 that was subsequently to focus his ideas on the nucleus as a liquid drop. In a classic paper submitted to Physical Review in June 1939, Bohr and the American theorist John Wheeler developed a detailed analysis of the fission process, based on an analogy with the break-up of a large drop of liquid into two smaller droplets. This completed the association of the liquid drop model with Bohr.

Interestingly, although Bohr and Wheeler refer to “the liquid drop model of atomic nuclei” without crediting it to anyone, their discussion of the energy released by nuclear fission begins with an acknowledgment to Gamow for a related idea – the relationship between the mass and binding energy of a nucleus, which can be calculated from the liquid drop model.

Today, particle physicists – in a sense, the modern descendants of those philosophers – have been privileged to gain a real view into a rich and exotic world, unimagined by their predecessors. Over the past 30 years or so, they have discovered a new underlying layer to matter, composed of the particles called quarks and leptons – the tiny constituents of the more familiar atoms. Before these discoveries, the subatomic world was truly a secret garden.

In The Particle Garden, Gordon Kane, professor of physics at the University of Michigan, opens up the secrets of the garden to anyone who has heard something of its wonders and wishes to know more. The difficulty in writing a “guide book” like this lies partly in deciding how much you need to tell visitors for them to appreciate the wonders without overburdening them with so much detail that they become lost and never find their way to the end.

Several physicists have tried their hand at such guides, with varying degrees of success. Kane is experienced in teaching non-science students and in giving talks to the general public. His guide is, therefore, a blend of what he believes is the minimum people need to know to appreciate the particle garden, together with answers to the kind of questions they are most likely to ask. Kane is like a tour guide who pauses here and there to explain some of the ways in which particle physicists work and think. The result is an idiosyncratic tour that is sometimes didactic, sometimes challenging, but not so long as to be truly arduous. It also contains delightful moments.

In one instance, Kane considers the important question of what we mean by “understanding” – a question that often arises in teaching particle physics and in explaining the basic ideas to nonspecialists. It often comes as a shock, especially to undergraduates, to learn that as physicists we understand electric charge no better than the much less familiar but equally important property of colour charge, responsible for binding quarks together to make the world about us.

Kane identifies three levels of understanding: “descriptive” understanding, “input and mechanism” understanding, and “why” understanding. He identifies particle physics as having completed the first stage, where we know the basic features of the garden: the quarks, the leptons, and the forces that bind them together to form the observable Universe. We have an excellent theory, called the standard model, which correctly describes the results from experiments in particle physics, but which as yet contains no explanation at all for the properties of the particles. To use the theory, we have to measure the masses and charges of the particles and insert those values into the equations.

In the next stage, which, as Kane explains, we are about to embark upon, we hope to gain an “input and mechanism” understanding, where we discover the mechanisms that give rise to those values we currently put into the theory. This will happen in the coming years as we explore particle collisions at energies high enough to reveal whatever mechanism gives particles their masses – which may involve the proposed Higgs particle, or which may even be due to something as yet unthought of.

Only when we have firm experimental evidence for a mechanism that correctly predicts masses, will we be able to advance towards Kane’s third stage of understanding, the “why” understanding, in which we answer those very basic questions, such as explaining colour charge and electric charge. And even then we will not understand why particles ever came together to form the plants in a garden of flowers.

In this way, Kane presents a more humble attitude to the physicist’s quest to understand the Universe than is found in a number of other books. By and large, he succeeds in his task of leading the reader through the particle garden. Although his guide is brief, it encompasses not only the present state of the garden but also the scope for future developments. For anyone who has already been peeping into the garden, yet lacked a broad view, it provides an excellent guide. But those for whom it is completely new should be careful to take it steadily or risk being overwhelmed. However, a glossary provides valuable help for those unfamiliar with the exotic names needed to describe the special features of this garden.

If I have one major criticism, it is of the lack of a bibliography or suggested material for further reading. In particular, in the second half of the book, which looks more to the future, Kane introduces a variety of topics on which he spends little time. While this approach prevents his tour from stalling, it must no doubt also raise many unanswered questions for the reader. With this in mind, I would readily recommend The Particle Garden not only for non-specialists but also for students learning about particle physics, for it portrays well the grander view of an exciting subject.

Now a team of physicists has suggested a way around this difficulty. They
have proposed a mechanism in which the vast numbers of neutrinos emitted in
the supernova could play a crucial role in keeping the shock wave going.

Stars derive their energy from nuclear reactions in which the nuclei of
light elements fuse to make heavier elements. The light nuclei that fuse are
slightly heavier than the nucleus that forms, and the difference emerges as
energy.

The fusion process begins with hydrogen, which is converted to helium.
Helium is then converted to carbon, and so on up to iron. This is the heaviest
element that can be made in this way, because the balance of forces within
nuclei is such that forming heavier nuclei, far from releasing energy,
requires additional energy from outside.

Energy flowing out from the fusion reactions at the core prevents the star
from collapsing in on itself due to its own gravity. But when the core has
turned to iron, no further fusion reactions are possible and gravity is
unchallenged, so the star begins to collapse. The centre of the core is
crushed to the density of an atomic nucleus, hundreds of millions times as
dense as normal matter.

As the collapsing matter from the outer core slams into this very dense
inner core, it rebounds. This causes a shock wave which blasts its way out
through the infalling material, colliding with outer layers of the star as it
does so. It is the energy released in this collision that causes the rapid
brightening we see as a supernova.

One problem with this theory is that before the shock wave even leaves the
core it loses so much energy by blasting apart iron nuclei that it should come
to a halt. Instead of producing a supernova the star should simply collapse to
form a black hole.

However, in 1982, James Wilson at the Lawrence Livermore Laboratory in
California stumbled upon a way in which the shock wave could get going again.
Wilson had accidentally left a computer simulation of a supernova running
overnight. In the morning he found that the shock wave in the simulation,
after nearly stalling, had revived, apparently because it had gained energy
from neutrinos. Astrophysicists already knew that by far the greater part of
the energy released by the core’s collapse escapes not as the light we see as
a supernova, but as neutrinos. These very weakly interacting particles are all
that can easily escape from the extremely dense matter-in the core.

Wilson discussed his findings with Hans Bethe of Cornell University in New
York, an acknowledged expert on stellar physics and on neutrinos. Together,
they calculated that the neutrinos needed to deposit only 1 to 2 per cent of
their energy in the outer core in order to revive the all-important shock
wave. However, the actual mechanism for the energy transfer was far from
clear, and the idea that neutrinos gave the shock wave a second wind remained
controversial.

A few years later, after SN1987A had exploded onto the scene, Bethe
happened to discuss the problem of reviving the shock wave with John Dawson of
the University of California in Los Angeles. Dawson is an expert on the
interactions of lasers with plasma, the high-energy state of matter in which
electrons are no longer bound in atoms, but move around independently of the
positive ions they leave behind. He had found that electrons in a plasma can
absorb energy from photons in an unusual way. The problem for the supernova
was very similar, because the collapsing matter would be in the form of a
plasma. But this time the electrons would have to pick up energy from
neutrinos rather than photons.

Now Dawson and Bethe, together with Bob Bingham at the Rutherford Appleton
Laboratory near Oxford, and Jang-Jau Su at the National Central University in
Taiwan believe they have found a means by which the electrons can pick up
sufficient energy from the neutrinos (Physics Letters A, vol 193, p 279). The
key lies not in individual reactions between neutrinos and electrons, but in
the way the vast numbers of neutrinos affect wave-like fluctuations in the
density of electrons in the plasma, known as “plasma waves”.

If the neutrinos encounter a plasma wave, interactions between the
neutrinos and the electrons can cause the wave to grow rapidly. In this way,
the plasma wave, and ultimately the electrons, pick up extra energy from the
neutrinos.

The mechanism is basically the same as the one Dawson had discovered
between laser photons and electrons. Bingham and his colleagues applied the
same calculations, but with the electromagnetic coupling between photons and
electrons replaced by the far weaker coupling that exists between neutrinos
and electrons.

The results confirm that with the large numbers of neutrinos emitted in a
stellar collapse, the transfer of energy to the plasma wave can occur. With
fewer neutrinos it would not happen, and astronomers would be deprived a
phenomenon that has fascinated them for centuries.