Page 1921 – Physics World

Home » Page 1921

Heat can produce ultrasound

Currently most ultrasound devices convert an electrical signal directly into mechanical vibrations through the piezoelectric effect. But these devices only operate over a narrow frequency range and have limited output. The idea of using heat to generate sound has been around for over 80 years, but until recently the thermal conduction of porous silicon was considered too low for this approach to work.

This new device does not produce mechanical vibrations and is therefore more reliable than standard piezoelectric transducers. Moreover, its wide frequency range makes it suitable for recording images through many different types of material. And as the base of the device is made from crystalline silicon, it should be straightforward to integrate it with conventional electronic circuitry.

New angle for optical memories

The medium is made of nanocrystals of zinc sulphide doped with samarium. Although the nanocrystals are only 3 nanometres long, they clump together into larger particles to form a powder that can be used as the storage medium. In their experiments Kurita and co-workers used a laser to measure the energy excitation spectrum of the crystal. Then they used a much stronger laser to burn a narrow “hole” in this spectrum, probably by ionizing the samarium in the crystal. This hole can be measured by using the first laser again.

The depth of the hole decreased with time, but as much as half of the initial hole was found to remain after 24 hours. Moreover, the position of the hole can change – or even disappear – as the crystal is rotated with respect to the laser beam. This behaviour has not been observed before and could, say the researchers, “bring a significant advantage over conventional two-dimensional optical memories.”

Plasma forms new forms of matter

Colloidal plasmas contain three different components – electrons, ions and charged microspheres. Although these unusual plasmas can be studied in the laboratory, researchers have only been able to study two-dimensional effects because gravity can overwhelm the electrostatic interactions between the particles. These interactions are responsible for the unusual properties of colloidal plasmas.

By reducing the effects of gravity for up to six minutes at a time, Morfill and colleagues have been able to observe three-dimensional structures in the plasma, including voids that did not contain any microspheres. The structures they observed included “crystalline plasmas” and “liquid plasmas”. Properties exhibited by the latter included ordered flows and vortices.

Slow period upsets pulsar theories

Inside a pulsar the neutrons are thought to decay into protons and electrons, which are then accelerated by the pulsar’s powerful magnetic field. Since protons and electrons are charged particles, they release intense synchrotron radiation at radio wavelengths from the magnetic poles of the pulsar. Previously astronomers believed that once a pulsar slowed beyond a certain point, the radio emissions would stop. However PSR J2144-3933 – which is now believed to be 280 million years old – defies this result. “By rights it should be a corpse,” says Young.

The team discovered the pulsar while using the Parkes radio telescope in Australia to look for something else. “I was looking for missing pulses, points at which the pulsar skips a beat,” says Young. “At first glance it seemed that this pulsar was missing two beats out of three. Then it became clear that it was really only beating a third as fast as we’d thought.”

“This is a bit of a problem for the theories,” says Manchester. “Perhaps the matter and anti-matter process can happen at lower spin rates than we thought. Or perhaps the pulses are powered by something else. Whatever the case, the theory needs a rethink.”

The discovery also highlights the dangers of relying on fully automated software to locate interesting astrophysical objects. When PSR J2144-3933 was discovered in 1993, a software package determined its period to be 2.84 seconds. Almost half of the pulsars discovered in the past few years were detected by automated software at the Parkes telescope.

A double-slit experiment in a single molecule

Photons from the laser can excite electrons in the water molecules into a so-called anti-bonding state. Molecules in this state can adopt one of two distorted shapes, both of which can dissociate into a hydroxide radical and a hydrogen atom. However the two different shapes can result in the hydroxide radical being created in different rotational energy states. According to Dixon and colleagues, the water molecule is the equivalent of a photon, and the two geometries can be thought of as the slits in Young’s experiment. The quantum wave functions arising from the two dissociation routes interfere with one another, creating an interference pattern that can be observed in the distribution of the hydroxide radicals among their different rotational states.

Foreign scientists lead US science

An earlier report from the National Research Council has already expressed concern that the US may not retain its leadership in science and technology. The report speculated that the supply of foreign researchers moving to the US could dry up as the world’s economy improves. The report concluded that more US students had to be attracted to work in the physical sciences. Between 1985 and 1997, however, the number of students studying physics in the US dropped by 24%.

In this latest report Sharon Levin from the University of Missouri-St Louis and Paula Stephan of Georgia State University looked at the number of foreign-born researchers who had achieved any of six key scientific milestones. The milestones included election to the National Academy of Sciences or the National Academy of Engineering, various publication or patent impacts (such as being an author of a ‘citation classic’ or ‘hot paper’ according to the Institute for Scientific Information), and playing a major role in the launch of a biotechnology firm.

Levin and Stephan discovered that in the physical sciences a large number of US-based researchers who had been born in German or the UK had produced the most important papers. Over 64% of the most-cited authors and 56% of the citation classics were from foreign-born researchers. Levin and Stephan now plan to study whether “native-born talent is disadvantaged by this inflow and, if so, whether the benefits outweigh the costs.”

The mystery of Neptune’s rings

Neptune’s ring arcs were first observed by the Voyager spacecraft in 1984. In the absence of some external source of stability the rings would have been expected to decay within a matter of months. But the three rings – called Liberté, Egalité and Fraternité – are still visible, and it has been assumed that their stability was due to the influence of a small moon called Galatea. However, when Sicardy and colleagues observed the satellite with the Canada-France-Hawaii Telescope on Mauna Kea, they discovered that the arcs fall outside the gravitational resonance’s associated with Galatea.

In a second paper, based on observations with the Hubble Space Telescope, Christopher Dumas of the Jet Propulsion Laboratory in California, and colleagues from Hawaii, the University of Arizona and the University of California at Los Angeles, reached the same conclusions (Nature 400 733). They suggest that instead of one moon, Galatea, there must be two moons keeping the arcs in position. However, no such moon has yet been discovered, and Sicardy and others have dismissed this explanation as ‘ad hoc’.

Evidence for supersymmetry found

Supersymmetry is an attractive theory to physicists as it provides a connection between all known elementary particles and can simplify many complex interactions. High energy supersymmetry suggests that instead of two types of basic elementary particles – bosons and fermions – there is just one. It also implies that high-energy physics experiments should detect twice as many particles than researchers have currently seen. For example quarks and electrons, which are fermions, should have bosonic equivalents called squarks and selectrons, while photons and gluons, which are bosons, should have fermionic equivalents called photinos and gluinos. Current research suggests that these particles are superheavy and hence only new accelerators, such as the powerful Large Hadron Collider at CERN – which is due to come on-line in 2005 – are likely to see them. Supersymmetry is also used in nuclear and solid state physics.

In 1990 Francesco Iachello of Yale University in the US proposed that it would be possible to see the effects of supersymmetry in the nucleus. He suggested that it should be possible to create an algebraic transformation that would allow a nucleus with an even number of protons and neutrons (even-even), two nuclei consisting of either an odd number of protons or neutrons (even-odd or odd-even), and a nucleus with an odd number of both neutrons and protons (odd-odd) to transform into each other via supersymmetry. These four types of nuclei can form what is known as a supersymmetric quartet. It was later realised that platinum-194 (even-even), gold-195 (odd-even), platinum-195 (even-odd) and gold-196 (odd-odd) formed the ideal quartet in which to search for nuclear symmetry .

Now Jan Jolie of the University of Fribourg in Switzerland and co-workers at the Ludwig-Maximilians University in Munich, the University of Bonn, and the University of Kentucky in the US have discovered experimental proof of nuclei supersymmetry. They used the Tandem accelerator in Munich to study the transfer reactions of very thin films of gold-196 nuclei.

In their experiments, they bombarded gold-197 with polarised deuterons. This causes a nucleon to be transferred from the gold to the deuteron to create triton – a tritium nucleus – leaving gold-196 behind. As the triton leaves the target, it is analysed by a magnetic spectrograph to determine its energy and spin. This, in turn, provides information on the excited energy states in the gold-196 nucleus.

Once they obtained the energy spectrum of gold-196, they applied their supersymmetry transformations to the results and found that they could produce the energy-level spectra of the other three nuclei in the quartet. “It’s a fascinating paper,” says Georgis Kraniotis from Royal Holloway and Bedford College in the UK, “and encouraging that they have used super-algebra in a non-accelerator experiment.”

“The fact that they claim to have found a realization of supersymmetry in nature is exciting,” adds Herbi Dreiner of the Rutherford Appleton Laboratory. “It would definitely further encourage us to look for supersymmetry in particle physics.”

“This finding is extremely important because firmly establishes the experimental occurrence of supersymmetry in physics,” says Iachello. “It represents, in my opinion, a major achievement in the study of symmetries in physical systems.”

Parisi wins Dirac medal

Parisi won the award for his work in a wide number of fields. The official citation highlights “his study of scaling violations in deep inelastic processes (Altarelli-Parisi equations), a model that links supersymmetry and superconductors together, the introduction of multifractals equations into turbulence studies, the stochastic differential equation for growth models for random aggregation (the Kardar-Parisi-Zhang model) and his groundbreaking analysis of the replica method that has permitted an important breakthrough in our understanding of glassy systems and has proved to be instrumental in the whole subject of Disordered Systems.”

Parisi graduated from Rome University in 1970 and worked as a researcher at the National Laboratories in Frascati, Italy, until 1981. Since then he has been a professor of physics at Rome University and at the universities of Rome I and II. He has also taught in the US and France. In 1986 he received the Feltrinelli prize for physics and was awarded the Boltzman medal in 1992 and the Italgas prize in 1993. Parisi is a fellow of both the Accademia dei Lincei and the French Academy.

J D Bernal: genius yes, sage no

At the age of two, John Desmond Bernal was taken by his American mother from their farm in Ireland to see his grandmother in California. He amazed other passengers on the steamship by talking in both English and French. Later on, when he was at Cambridge, his fellow undergraduates would nickname him “Sage” because of his seemingly limitless knowledge.

J D Bernal: A Life in Science and Politics is a collection of essays about the crowded life of this famous crystallographer, Marxist and political activist. Most of the essays are by authors who knew Bernal, who lived from 1901 to 1971, but few of them can approach the drama and descriptive power of Bernal’s own essay entitled “D-day diaries”, which is one of the book’s highlights. Indeed, some chapters contain turgid sociological analyses; in “Irish roots”, for example, Bernal’s free-spirited parents are burdened with complexes about class and religion that they were unlikely to have experienced in reality.

There are also some remarkably sympathetic accounts of Bernal’s totalitarian beliefs. This partiality even extends to the “Notes on contributors”, where we are informed that Ivor Montagu, who worked with Bernal for the World Peace Council, was for many years “president of the English Table Tennis Association” – but not that he was president of the British Communist Party.

Bernal’s seminal contributions to X-ray crystallography are lucidly recounted and set in context by Peter Trent. As an undergraduate at Cambridge, Bernal studied mineralogy and mathematical concepts of symmetry, which culminated in a thesis entitled “On the Analytical Theory of Point Systems”. This gained him a research post in 1923 at the Royal Institution in London with William Henry Bragg, who set him to work on the structure of graphite. Bernal dissected out a single crystal, mounted it at the centre of an alarm-clock, and took X-ray photographs as the crystal rotated. According to reciprocal-lattice theory, each plane in a crystal is represented by a point, and can therefore be directly correlated with one X-ray diffraction spot. Bernal used these ideas to develop a chart from which two co-ordinates could be read for each diffraction spot, thereby allowing a stereographic projection of the plane to be made.

In 1927 he returned to Cambridge as the first lecturer in structural crystallography, and clearly saw the enormous potential of his chosen field, namely that X-ray diffraction patterns could be used “to fix the nature and orientation of the minute crystals which go to build up common substances”. This, Bernal realized, would provide a way of analysing “all the textures of natural and artificial products. This is the type of information which links crystallography to biology on the one hand and technology on the other.”

Bernal’s achievement was to realize this ambitious agenda by his own brilliant work and also by inspiring the next generation of crystallographers at the Cavendish Laboratory and at Birkbeck College in London, where he was appointed professor of physics in 1937. Between them, these two groups of researchers did much to establish the three-dimensional structures of nucleic acids, proteins and viruses. Indeed, the field of molecular biology was sterile until Bernal’s simple observation that protein crystals could be studied only in the wet state. Prominent students of his were Rosalind Franklin, Dorothy Hodgkin, Aaron Klug and Max Perutz.

Bernal’s odyssey from uncommitted Irish nationalist, through undergraduate socialist to unswerving Marxist is minutely catalogued. He became a passionate proponent of state planning, particularly in the integration of scientific research and industrial technology, with a subjugation of individual liberty that he especially would have found intolerable. Indeed, his work was inextricably linked to his revolutionary ideas. One of the attractions of biology was that it was the academic discipline of J B S Haldane, Joseph Needham and Lancelot Hogben, who together with Bernal constituted the intellectual hardcore of British Marxism.

The purges in the Soviet Union of the 1930s no doubt caused these British Marxists moments of private anguish, but the rise of Trofim Lysenko as Stalin’s favourite biologist should have presented them with an unavoidable challenge. Lysenko promised a quick fix for the woeful state of Soviet agriculture and began a crusade against the conventional theory of genetics. However, the ambivalent reaction of Haldane and Bernal to Lysenko’s actions was craven. It was left to Sergei Vavilov – the leading Soviet geneticist whom Lysenko denounced to Stalin – to defend the science of genetics, saying: “We shall go to the pyre, we shall burn, but we shall not retreat from our convictions.”

Vavilov was not exaggerating – he was arrested in 1941 and refused to admit to false accusations, despite being tortured. For Hilary Rose and Steven Rose to therefore write, as they do here, that Vavilov died “in transit to a labour camp” is equivalent to stating that Joan of Arc died in a fire. Even the Roses feel compelled to issue a mild rebuke for Bernal’s obituary of Stalin as “a great scientist [who combined] a deeply scientific approach to all problems with his capacity for feeling and expressing himself in simple and direct terms”.

Like so many Marxists, Bernal had an outstanding war against the Nazis, but he did not have to wait for the Hitler-Stalin pact to be revoked. Despite his presidency of the Cambridge Scientists’ Anti-War Group and his known communism, Bernal was recruited in 1939 as a free-ranging consultant by Sir John Anderson, then minister in charge of civilian defence. (Bernal had been invited to a lunch with Anderson as his most outspoken critic, and Anderson had immediately recognized the quality of Bernal’s intellect.) When questioned about the suitability of his choice, Anderson replied: “Even if he is as red as the flames of hell, I want him.”

Bernal’s initial aim was to establish objective data about explosions, a subject that was until then largely based on assumptions. He worked closely with the physiologist Solly Zuckerman, and together they tested their theories about blast injury by exposing themselves to explosions in slit trenches. Indeed, a similar experiment later in the war nearly ended in disaster for John Kendrew (who shared the 1962 Nobel Prize for Chemistry for his elucidation of the structure of myoglobin) after Bernal misplaced a decimal point on his slide rule.

Bernal and Zuckerman were also responsible for the first detailed survey of the effects of bombing on a civilian population. Their findings that people did not become demoralized and that industrial production did not suffer from such bombings were, however, misrepresented by Churchill’s friend and scientific adviser, Frederick Lindemann, as justification for the bombing of German cities. One memorable vignette supplied in the book is from Lord Ritchie Calder. He recounts that Bernal’s own successful pitch to Churchill that the Allied forces should land during D-day at Mulberry harbours on the Normandy beaches was accomplished in front of the prime minister by floating paper boats in a bathtub on the Queen Mary.

All in all, this book makes a persuasive case for Bernal, the original and inspirational scientist; his sagacity as a philosopher of science and political figure remains questionable.

Browse articles by content type