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Home » Page 1847

Antique dealers turn to physics

With works of art now fetching significant amounts of money at auction, it is not surprising that counterfeiters are becoming ever more skilful at producing copies of antiques. While art dealers can easily spot fake porcelain figures that are the wrong shape or have the wrong glaze, the more sophisticated copies only come to light following scientific analysis. Indeed, counterfeit pottery is so prevalent that dealers are unable to sell a piece without a certificate to show that it has passed a physics-based test known as thermoluminescence dating.

Physicist Doreen Stoneham has been testing ceramics this way for over 30 years. She joined the research laboratory for archaeology at Oxford University in 1970, shortly after one of the students there had discovered how to estimate the age of pottery and porcelain using a tiny sample drilled from the object. This form of “luminescence testing” is used in archaeology to date burnt flint and materials that have been “fired” in a kiln, such as pottery.

Previous luminescence dating involved crushing the material to find large grains of quartz, and also required knowledge of the prevalent local radiation levels for accurate measurements. Both of these requirements made it impossible to date valuable antiques using traditional techniques. However, the new “fine grain” method only needed a sample weighing about 100 mg and was sufficiently accurate (to within 20%) to tell if an antique came from a particular period or if it was a recent fake.

Stoneham recognized the commercial potential of thermoluminescence dating and began testing ceramic works of art for Sotheby’s, Christie’s and other art dealers. Looking for a new challenge, in 1997 she decided to set up her own company with her own savings. “One of the best pieces of advice I have ever been given”, recalls Stoneham, “is never to start off with a loan. You’ll be in debt for the rest of your life.” Oxford Authentication now employs two other people and tests about 3000 samples every year in a lab near Oxford. Moving away from the academic environment was a shrewd move for Stoneham, who has trebled the amount of business she used to do at Oxford University.

Almost 90% of the pottery, porcelain and stoneware that Oxford Authentication tests is Chinese. The rest is mostly a mixture of South American ceramics that are at least 500 years old, Italian Renaissance pieces from the 14th and 15th centuries, and bronzes from south-east Asia. Although thermoluminescence testing is unsuitable for metals, many bronze sculptures are made by moulding the metal around a pottery figure. Often this “casting core” remains jammed inside the bronze and can be used to authenticate the figure.

Stoneham admits that the work is fairly routine. When the company is asked to test a sample, she or one of her colleagues – including 40 representatives around the world – visit the dealer and take several samples from the antique. The powders are then brought back to the lab where they are sorted into grains 4-12 microns across and then reheated.

Thermoluminescence dating is essentially a measure of the radiation to which the ceramic has been exposed since it was fired. Radioactive elements in the sample or in the surrounding environment ionize the atoms in crystalline quartz, and the electrons become trapped by crystal defects. When the grains are heated, the electrons are released from the traps and combine with luminescence centres to produce light. Firing the pottery in a kiln resets the thermoluminescence clock to zero. Put simply, the more light that is produced when the sample is re-heated, then the older the ceramic is.

In practice, thermoluminescence testing is much more complex and the technique requires careful calibration with a known radioactive source. Young European pottery from the Renaissance period, in particular, usually requires more sophisticated tests, including measurements of the potassium content, because so many copies were made in the 19th century. If the measurements are consistent with the reported age of the antique, the dealer receives a one-page certificate of authentication.

But canny forgers have cottoned on to thermoluminescence dating too. Stoneham finds that counterfeiters incorporate pieces of genuine porcelain into areas where they think the test holes will be drilled (see figure). To get round this problem – and to identify pastiches made from an assortment of genuine and fake fragments – Stoneham and co-workers will sometimes take several samples from different parts of a piece.

Counterfeiters in China are even trying to pass the thermoluminescence test by making copies from ground-up bits of ancient clay found near the factory in Ching-te-chen where Ming pottery was produced in the 14-17th centuries. Other forgers are irradiating porcelain in an attempt to artificially age it, while others just try to fake the certificates of authenticity although “they are not doing it very successfully,” says Stoneham.

So do Stoneham and co-workers check to see how much the pieces they have tested fetch at auction? “We do look at the catalogues,” she admits. “And sometimes I see an object in the newspapers and wish I’d charged the dealer more. But at the end of the day, we just want to get at the truth.”

Bubble fusion: fact or fiction?

When Rusi Taleyarkhan of the Oak Ridge National Laboratory and colleagues reported last month that they had observed evidence for nuclear fusion in a beaker of deuterated acetone, controversy was sure to follow. The researchers claimed to have detected neutrons and tritium when bubbles in the acetone collapsed to generate temperatures of millions of degrees in sonoluminescence or “sound-into-light” experiments (R Taleyarkhan et al. 2002 Science 295 1868).

However, other physicists at Oak Ridge have failed to reproduce the results, and many researchers in the field have voiced doubts about the experiment as well. Some are worried that the incident will prove to be a repeat of the discredited “cold fusion” claims in the late-1980s, but they all agree that “bubble fusion” would be a major discovery if it proves to be real.

Let there be light – and fusion?

In sonoluminescence, flashes of light are emitted by bubbles that are forced to expand and then collapse by sound waves, although this process is still not fully understood (see Physics World May 1998 pp38-42, print version). Physicists have long speculated that the considerable compression forces inside the collapsing bubbles could be large enough to spark nuclear reactions. If this was the case, it could lead to a new source of clean energy.

In their experiment, Taleyarkhan and co-workers at Oak Ridge, the Rensselaer Polytechnic Institute in the US, and the Russian Academy of Sciences used 14 MeV neutrons to create bubbles about 10 nanometres across in acetone in which the ordinary hydrogen atoms had been replaced by deuterium atoms (C3D6O). An acoustic signal then forced these bubbles – and the acetone vapour inside them – to first expand to about a millimetre in size, and then collapse.

Two deuterium nuclei can fuse to produce either a tritium nucleus and a proton, or a helium-3 nucleus and a neutron with an energy of 2.45 MeV. Taleyarkhan and co-workers claim to have seen evidence for both tritium and 2.45 MeV neutrons in their experiments. They have also calculated that temperatures of a million or even 10 million degrees – the temperature at the Sun’s core – must have existed inside the bubbles for these reactions to proceed.

Given the highly surprising nature of the results, the Oak Ridge management asked two other physicists – Dan Shapira and Michael Saltmarsh – to repeat the experiments. However, Shapira and Saltmarsh failed to obtain convincing evidence for tritium or 2.45 MeV neutrons. Indeed, they detected at least three orders of magnitude fewer neutrons than the fusion of deuterium into helium-3 should generate, even though their neutron detector was much more efficient than the device used by Taleyarkhan’s group. They also noted that traces of tritium exist naturally in deuterium.

Shapira and Saltmarsh are now preparing two reports for Science that outline the technical shortcomings of Taleyarkhan’s original work, and give a fuller account of their own attempts to reproduce it. They welcome the fact that both reports will have to be peer-reviewed, but point out that the lab’s management also sent their first report to researchers outside Oak Ridge for peer review before it was published on the lab’s Web site (www.ornl.gov/slsite/SLan5av2.pdf).

However, Taleyarkhan and colleagues counter that Shapira and Saltmarsh misinterpreted their results because they failed to calibrate their detector properly (www.rpi.edu/~laheyr/SciencePaper.pdf). “We have reproduced our experiment tens of times, starting from scratch each time,” Taleyarkhan told Physics World.

Other reactions

Mathias Fink of the Université Denis Diderot in Paris thinks that the work by Taleyarkhan’s group is very exciting. “Many of us working in the field of cavitation have speculated that ‘sonofusion’ could occur, but few of us expected to see it so soon,” says Fink. He believes this is a first step towards a fusion source but emphasizes that a lot of work remains to be done.

However, Lawrence Crum of the University of Washington in Seattle is sceptical about the claims. He dismisses the idea that the technique could become a viable energy source, but concedes that the discovery would have far-reaching consequences for science if it proves to be real and is planning to repeat the experiment in his laboratory.

Detlef Lohse of the University of Twente in the Netherlands is also unconvinced. “I am very surprised this paper was published,” he says. “The temperature they are claiming is grossly overestimated – the gas inside the bubble would heat up, but most of the energy would be eaten up by vibration, rotation and chemical reactions.”

Seth Putterman of the University of California at Los Angeles (UCLA) has been investigating “sonofusion” for some years, but is unconvinced by the Oak Ridge results and does not intend to try and repeat them himself. “We will not morph our project in order to reproduce an experiment that provides no new evidence for sonofusion,” he told Physics World.

Meanwhile, Taleyarkhan and colleagues are designing new experiments that, they hope, will give higher yields of tritium and neutrons. Taleyarkhan adds that at least six groups in the US – at the universities of Illinois and Michigan, Purdue University, Rensselaer Polytechnic, NASA and UCLA – are planning to repeat the work, as are groups in Argentina, Canada, France and the UK. Oak Ridge is also assembling another team to investigate bubble fusion.

Despite the controversy surrounding the results, the editor-in-chief of Science, Donald Kennedy, believes that his journal’s decision to publish the paper was the right one. Writing in an editorial that appeared in the same issue as the original paper, Kennedy states that Science‘s mission is “to put interesting, potentially important science into public view”. He emphasizes that the quality of the paper was established through the conventional process of peer-review.

But Putterman does not know who approved the paper. “I haven’t been able to locate a single academic scientist who is willing to say something positive about the experimental data,” he told Physics World.

Mike Moran, a nuclear physicist at the Lawrence Livermore National Laboratory, is unhappy that the paper was ever published. “Every time an incorrect scientific result is announced with premature hoopla, proponents of things like perpetual-motion machines demand their fair share of media exposure and government funding,” he says. “These trends undercut the credibility of valid scientific results and do not bode well for the success of science in the future.”

Superconductors tell left from right

Campuzano and co-workers studied thin films of bismuth strontium calcium copper oxide (Bi2Sr2CaCu2O8+x), known as Bi-2212 for short. The cuprates are normally insulators, but when dopants are added – extra oxygen atoms in the case of Bi-2212 – they lose all resistance to electrical current. This happens at the superconducting transition temperature, Tc, which varies with the amount of doping. The maximum value of Tc occurs for optimal doping, and samples with more or less than optimal doping are said to be overdoped or underdoped.

One of the essential characteristics of a superconductor is the energy gap – this is the binding energy of the pairs of electrons or “holes” that are responsible for the superconductivity. However, in the mid-1990s physicists discovered evidence for a “pseudogap” that revealed itself in underdoped materials at temperatures well above Tc.

A key question is whether a phase transition occurs at the pseudogap temperature, T*, as this would imply the existence of a new form of quantum order, just as superconductivity itself is a form of order. Campuzano and co-workers measured the photocurrents that were produced when samples of Bi-2212 were irradiated with left- and right-handed circularly polarized radiation. No difference in the photocurrents was observed for overdoped samples. However, the photocurrents produced by left- and right-handed radiation were different below T* for underdoped samples.

This behaviour, which persists below Tc, suggests that time-reversal symmetry is broken in the pseudogap state, as had previously been predicted by one of Campuzano’s co-authors, Chandra Varma of Bell Labs. Knowledge of the symmetry and order properties of the pseudogap state could lead to a better understanding of high-temperature superconductivity.

Photons get the quantum cloning treatment

Conventional computers store information as ‘bits’, which can have a value of either 1 or 0. As electronic components become smaller, physicists have suggested that information could be stored in certain two-level quantum systems. These include the horizontal and vertical polarization states of photons, or the ‘spin up’ and ‘spin down’ states of electrons. But the crucial difference is that these quantum bits – or ‘qubits’ – can exist in both possible states at the same time, a phenomenon known as superposition.

Many conventional computing tasks rely on ‘parallel processing’, in which bits are duplicated and operated upon simultaneously to solve a problem more quickly. But the state of a quantum system can never be fully known, so perfect duplication of qubits is forbidden. This is the ‘no-cloning’ theorem, which is the basis of quantum cryptography.

Lamas-Linares and co-workers based their experiment on two photons created by the ‘down-conversion’ – or splitting – of a single higher-frequency photon. These photons are linked – or ‘entangled’ – so that a measurement of the polarization of one reveals the polarization of the other.

The Oxford team sent a photon from one such pair into an optically active crystal where it stimulated the emission of a further photon. There is an increased chance that the new photon will have the same polarization as the ‘input’ photon. In contrast, photons that are emitted spontaneously are equally likely to be in either polarization state.

Since the input photon was one of an entangled pair, Lamas-Linares and colleagues were able to compare the polarizations of the new photon and the second photon created in the down-conversion step – and therefore with the polarization of the original photon. They believe that the wave qualities of the new photon overlapped with those of the original photon by the maximum possible amount calculated by theorists – that is, five sixths.

“This is the first time that cloning has been demonstrated for individual quantum systems,“ Lamas-Linares told PhysicsWeb. “The use of stimulated emission is a particularly natural choice.”

In earlier experiments, physicists at Oxford cloned large assemblies of quantum systems (arXiv.org/abs/quant-ph/0111098), and a group at the University of Science and Technology of China imprinted the polarization and the motion of a single photon onto two clones (Phys. Rev. A 64 012315). But Mark Hillery of Hunter College of the City University of New York believes that the work by Lamas-Linares and colleagues is the best demonstration yet, because it produced copies of the highest theoretical quality. “It is nice to see cloning moving off paper and into the laboratory,” he told PhysicsWeb.

Earth’s magnetic field linked to changing orbit

Current theories of geomagnetism state that the ‘dynamo’ that powers the Earth’s magnetic field is maintained by heat and gravitational energy. But previously observed long-term patterns in the intensity and inclination of Earth’s magnetic field cannot be explained by these effects, which change on relatively short time-scales.

In order to study long-term variations in the magnetic field, Yamazaki and Oda extracted a column of sediment 42 metres long from the sea floor near the equator, and measured the magnetization of over 1700 samples from this ‘core’. The magnetization of the samples is determined by the orientation of magnetic grains in the sediment. This study revealed that the intensity and orientation of the magnetic field changes over a cycle that lasts 100 000 years.

After ruling out a number of possible candidates – such as climate effects – Yamazaki and Oda proposed that the 100 000 year cycle could arise from changes in the eccentricity of the Earth’s orbit. Eccentricity is a measure of how much the orbit of a planet deviates from a circle, and ranges from zero for a circular orbit to one for a highly elliptical orbit.

Astronomers know that the eccentricity of the Earth’s orbit varies between 0 and 0.06 every 100 000 years. This causes the Earth to pass slightly closer to the Sun during certain epochs. Yamazaki and Oda believe that this could induce slight changes in the Earth’s iron core that affect the generation of the magnetic field, and therefore the way that sediment is deposited in the ocean.

The researchers are optimistic that their theory will prove easy to test because the variations in magnetic field are expected to be very pronounced in certain regions of the Earth’s surface.

CERN delays collider start-up

In September last year CERN shocked its member states – which fund the lab – with the news that the LHC would cost some SFr850m more than originally budgeted. In response the Council of the member states told the management to revise its spending so that CERN could pay for the extra costs without increasing its overall budget.

The saving of SFr500m identified by the management would not lead to a reduction in the number of CERN’s own staff but would involve cuts in support staff provided by industrial contractors, including engineers. It would also mean a drop in the number of student researchers and scientific fellows that the lab could fund in coming years. In addition, the plan would mean limiting the operating time of certain experiments, including shutting down the Super Proton Synchrotron in 2005; scaling back travel and other general overheads; and reducing the amount of R&D on potential new facilities, such as a neutrino factory or an advanced linear collider.

Some savings would also be made by postponing the start-up of the LHC, although a spokesman for the lab said that the collider would have been delayed anyway because of a shortage of superconducting cable needed for the collider’s magnets.

All of these measures, however, would still leave the lab facing a shortfall of SFr350m. This could be paid for by holding back final payment for the LHC until 2010 or by increasing CERN’s overall budget. The latter would allow financing of the machine to be complete by 2009 and would enable limited R&D to continue but would not be popular among member states. Switzerland, however, has agreed to advance CERN SFr90m to pay for the LHC, money that will be deducted from Swiss contributions in later years.

CERN’s Council will decide on the management’s plans at a meeting in June, allowing it to take into account the findings of an external review committee set up in October to identify budget cuts.

The physics of classical poetry

Mansilla and Bush examined poems that used a form of verse known as the hexameter. Each verse in this form of poetry contains six meters or “feet”, which in turn are made up of two or three syllables that can be long or short, and stress or unstressed. The long syllable is always at the start of the foot and is always stressed, while the short syllables are unstressed. This means that the rhythm of the hexameter is “TA-ta-ta” or “TA-ta” depending on the combinations in each foot. This allowed Mansilla and Bush to translate the poems they studied into a three-letter “alphabet” in which zero represented a long syllable, one was a short syllable and two was a pause or a “caesurae”. The pair then analysed the poems using techniques from information theory.

Their analysis involved calculating quantities such as the mutual and partial information functions of a string of characters, and then computing their Fourier transforms to look at the temporal and rhythmical patterns of the strings. The same techniques have recently been used by physicists to analyse the information content in DNA.

Mansilla and Bush studied the first hundred verses of four Greek poems (the Iliad and the Odyssey by Homer, Works and Days by Hesiod, and Idylls by Theocritus) and four Latin poems (the Aeneid and the Georgic by Vergil, Metamorphoses by Ovid, and On the Nature of Things by Lucretius). They found that Latin poetry was more complex than Greek poetry, mainly because Latin poets tended to “break the rules” more than their Greek counterparts. The authors speculate that since written verse did not exist at the time of the Greeks, the more rigid rhythmical structure found in the Greeks poems enabled them to be memorized and recited more easily.

Mansilla and Bush now plan to turn their attention to Homer. Classical scholars have debated for some time whether Homer actually existed and when and where the Iliad and the Odyssey were composed. “There is considerable debate on whether the Iliad and the Odyssey were composed by one author or many,” says Mansilla. “We feel that the present study can throw light on the question regarding singular or multiple authors of the two poems. Given our results, it is probable that the Iliad was composed by one rhapsode, while the Odyssey was composed by many.” However, Mansilla stresses that the results in their paper are preliminary and that they are going to study other ancient poems and poets.

Anne Mahoney, a classicist at Tufts University in the US, says that she welcomes the application of mathematical techniques to the study of metrics, so long as it is done well. “The word ‘complex’ is perhaps unfortunate, since it is clear to a classicist that Latin hexameters are less complicated than Greek, not more complicated,” she says. “The metrical rules for classical Latin poetry were deliberately borrowed from Greek, starting somewhere in the third century BC. Over the next few hundred years, Latin poets gradually came to apply the rules more and more strictly, with the result that there are many types of hexameter lines that a Greek poet could use more freely than a Latin one.”

Mahoney also points out that Latin has more long syllables than Greek, which could explain some of the differences noted by Mansilla and Bush.

Superconductivity clue comes out of the blue

Superconductors are materials that lose their electrical resistance below a certain temperature. This phenomenon arises when electrons overcome their mutual repulsion to form pairs, which do not experience electrical resistance. BCS theory successfully describes the behaviour of many superconductors, in which electrons gain the energy they need to pair up through interactions with vibrations of the crystal lattice. But physicists are unsure how electrons pair up in high-temperature – or cuprate – superconductors, which were only discovered in 1986.

van der Marel and colleagues studied a high-temperature superconductor made of bismuth, strontium, calcium, copper and oxygen – known as Bi-2212 – which has a superconducting transition temperature of 88 kelvin. By analysing the light reflected from the material above and below 88 kelvin – that is, in its normal and superconducting states – the researchers could tell which frequencies of light it absorbed. The energies of these photons are related to the binding energy of the electron pairs in the superconductor.

As the sample of Bi-2212 was cooled below its transition temperature, the team found that the light reflected from it shifted towards higher frequencies and away from the infrared region of the spectrum. This means that the material absorbs more infrared light – that is, photons with lower energies – when it is in its superconducting state.

According to van der Marel, this suggests that the electrons in Bi-2212 pair up because they have the lowest kinetic energy in this arrangement. He believes that this could lead to a stable superconducting state even if the electrostatic repulsion between two electrons tends to push them apart.

If this shift of ‘spectral weight’ occurs in all high-temperature superconductors, it would prove that the BCS mechanism is not the only way to achieve superconductivity. But van der Marel emphasizes that this is not yet clear. “We are just at the beginning of this type of experiment,” he told PhysicsWeb. “At the moment we can’t say whether this is a generic property of all cuprate superconductors or a peculiarity of the Bi-2212 system.”

Binary star breaks speed record

In 1994, astronomers using the German ROSAT space observatory discovered a strong X-ray source in the constellation of Cancer, and noticed that its intensity fell sharply every five minutes. Binary star system are known to emit X-rays, and this prompted Israel and co-workers to study the system – known as RX J0806.3+1527 – more closely, using the Very Large Telescope in Chile and the Italian Telescopio Nazionale Galileo in the Canary Islands.

The larger star in a binary system has the stronger gravitational field, which means that it draws gas and matter from its companion. Astronomers believe that the energy of this infalling material can make the ‘receiving’ star so hot that it emits X-rays. These X-rays are blocked from view each time the ‘donor’ star passes in front of its companion, leading to a sharp dip in the signal.

High-resolution studies of the binary system by Israel’s group showed that the stars are only 80 000 kilometres apart – around a fifth the distance from Earth to the Moon – which means that they must be travelling at around 1000 kilometres per second. The researchers believe the system may be in a brief transition phase of its evolution, because the orbital periods of binary stars are thought to lengthen over time. The stars are probably different-mass white dwarf stars, each about the size of the Earth.

Israel and colleagues think that the stars in binary system J0806.3+1527 could be moving fast enough to generate gravitational waves – ripples in the fabric of space-time that are predicted to arise when a massive object moves through space. The researchers hope these waves will be detected by the planned Laser Interferometer Space Antenna – or LISA – which is due for launch in ten years’ time. So far only indirect evidence for gravitational waves has been found, but astronomers hope that detectors such as VIRGO and LIGO will find direct evidence for them within a decade.

The research is to appear in the journal Astronomy and Astrophysics Letters.

A recipe for female success

Few women currently choose careers in physics, and of those that do, very few make it to the top of their chosen professions. The International Union of Pure and Applied Physics organised the Paris conference to uncover the reasons for this shortfall, and to develop strategies to tap this under-used pool of female talent. Eight resolutions emerged from the conference, addressing the particular problems that women face in education, employment and research.

Many of the general recommendations apply to a wide range of organizations, including universities, governments, industrial labs and research institutes. These measures include the provision of child-care facilities and flexible working hours. But crucially, these institutions must give equal consideration to men and women applying for jobs and promotion. Selection criteria should be freely available, and unfair practices must be abolished. The well-known ‘glass ceiling’ effect – in which few women reach senior positions – leads to a lack of female role models that further discourages women. Promoting more women to decision-making roles would help to enshrine in working practices an awareness of the problems faced by women.

Among the more specific recommendations were those to encourage girls and young women to study physics. One resolution calls upon schools and universities to highlight the positive effects that physics has on people’s lives, because research has shown that girls are strongly motivated to improve the lives of other people. Schools should also use teaching methods that interest both girls and boys.

Funding bodies must be more flexible when they allocate research grants. Lifting age restrictions on certain grants would help women who find that critical stages in their scientific careers – such as gaining experience through stints of post-doctoral research – coincide with their child-bearing years. Flexible periods of ‘family leave’ – without penalties – would help many women to integrate the birth of children with their research careers.

The crucial role of physics in the health of national economies was reflected in a recommendation that governments should make good physics education equally available to everybody. Governments should also appoint more women on to national planning committees, and allocate public funds only to organizations with active equal opportunities policies.

Scientific societies were called upon to identify women working in physics and publicize them as role models. Societies could also help by publishing data on the number of women working at all levels in physics. Working groups could be set up in each society to monitor women’s issues and make recommendations to the organization as a whole.

According to the results of a survey presented at the conference, marriage, family life and the attitudes of a minority of male physicists are the biggest barriers for women attempting to forge a career in physics. Statisticians at the American Institute of Physics questioned over 800 female physicists world-wide, and found that the employment prospects of many women are damaged by the difficulty of finding a job near their partner – the so-called two-body problem. Many women, however, cited the support of partners and other family members as crucial to their success as physicists.

Conference organizer Marcia Barbosa, a Brazilian physicist, has already pronounced the event a success. She believes that the networks established at the conference – consisting of 65 teams from all over the world – will enable female physicists to coordinate their efforts in future. “The Brazilian team hopes to organize another event in 2004,” she told PhysicsWeb. “We hope by then that we will see significant changes.”

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