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Home » Page 1806

Magnetic angle for gamma rays

Gamma-ray bursts are the most energetic events in the universe, typically corresponding to the conversion of 1% of the mass of the Sun into energy. Every day an average of about one of these brief flashes of gamma rays – which come from any direction in the sky and last a few tens of seconds – are detected. Observations of the optical afterglow of the bursts reveal that they originate in galaxies that are several billions of light-years away. But what causes the bursts, and how the gamma rays are actually produced, have, until recently, remained a mystery.

It has been suggested that gamma-ray bursts result from supernovae – the explosions of massive stars that have used up all their fuel – and that the bursts might be the “birth cries” of black holes. Some of this evidence is circumstantial. It is known, for example, that the host galaxies of the bursts are regions where massive stars are copiously forming. However, observations of the afterglow of GRB980425 in 1998 revealed a spectral signature that would be expected for a supernova. This provided the first compelling, but inconclusive, direct evidence for the link between gamma-ray bursts and supernovae.

In March this year, however, this link was dramatically confirmed. The HETE-2 satellite detected a spectacular gamma-ray burst that was among the closest and brightest ever observed. GRB030329 was just two billion light-years away, which in terms of gamma-ray bursts is right next door.

In the August issue of Physics World Donald Lamb from the University of Chicago explains how optical observations of the burst’s afterglow revealed that it had the characteristic spectrum of a supernova explosion. This spectrum was remarkably similar to GRB984025 just before it reached maximum brightness.

IBM gears up to gene challenge

In 1999 IBM embarked on an ambitious $100m project to build, by 2005, a supercomputer to simulate the folding of proteins. The computer is known as Blue Gene – after IBM’s nickname Big Blue, which originated in the 1960s thanks to the blue covers on its mainframe machines and other products.

The initial press release called the project a “grand challenge”, which the company defines as “a problem that by virtue of its degree of difficulty and the importance of its solution, both from a technical and societal point of view, becomes a focus of interest to a specific scientific community”. At first glance, the goal might seem arbitrary – a public-relations gimmick to tether a computer-engineering project to a quest of the life sciences. In fact, it was shrewdly chosen, and reflects dramatically changing forces in large scientific-industrial projects.

Grand challenges…

The discovery of the structure of DNA in 1953 – perhaps the single most important scientific event in the second half of the 20th century – made many scientists confident that they at last understood the “secret of life”. It also led to high hopes of imminent breakthroughs across medicine. DNA, after all, provides the structural blueprint for how to make the tens of thousands of proteins that are the body’s building blocks and carry out its basic cellular processes.

But in the intervening years, the breakthroughs failed to keep pace with expectations. A major hurdle was the failure to understand the process by which the strings of amino acids that comprise proteins fold into their final, complex, functional shapes. Protein folding remains the mysterious bridge between structure and function. Many deadly diseases, including cystic fibrosis, sickle-cell anaemia and “mad-cow” disease, are thought to arise from misfolding.

The ability to simulate protein folding could transform medical science and technology. Just as physics gained from Monte Carlo simulations, so biological experimentation could become more reliant on computing. IBM was also betting that the ability to simulate folding would transform computer technology. While computers are already improving at a torrid rate, building a version 500 times more powerful than existing models – as Blue Gene will need to be – requires extensive improvements in hardware, software and computer architecture. The project will also generate new kinds of large-scale, interdisciplinary collaborations among biologists and other kinds of scientists, especially computer engineers.

Finally, as IBM was keenly aware when it debated the project, success with Blue Gene would transform the computing industry. The ability to simulate protein folding would make supercomputers of Blue Gene’s capabilities indispensable for cutting-edge research in the life sciences, meteorology and other fields besides.

…and grand risks
A true challenge also involves risk. The history of computing – and of science – is full of ambitious, multi-year projects with worthy goals that were later abandoned, hopelessly compromised, or – worst of all – turned into white elephants that soak up money and resources, thanks to interim changes in science and technology. Examples include the Argonne National Laboratory’s zero-gradient synchrotron, which was completed – years late – in 1963, as well as the Brookhaven National Laboratory’s colliding-beam accelerator ISABELLE (cancelled in 1983) and the US Superconducting Super Collider (cancelled in 1993).

A country or even a national lab can absorb such failures. But to a private corporation, a grand challenge is something else again. IBM, however, has become adept at such challenges. Its last such venture was Deep Blue, the computer that defeated world chess champion Gary Kasparov in 1997. IBM planned this project with more in mind than grandmaster-level chess. Such a computer would be invaluable to scientists, researchers and engineers in artificial-intelligence research, and in the growing numbers of fields where it is critical to evaluate huge amounts of multiple variables.

Still, even among grand challenges, Blue Gene seems different. What IBM is doing is like throwing yourself into a dangerously hostile environment to ensure that you learn survival skills. To make matters easier, the project is based as much as possible on existing technologies. IBM is also seeking other possible uses for the computer – such as climate research, financial data analysis, weapons research and earthquake prediction – and recruiting potential users in those fields and incorporating them in the project.

The first of Blue Gene’s specially designed chips have just started to roll in from its factories. A key landmark will take place this autumn when IBM tests a “half-rack” – a big enough assembly of the chips to allow researchers to see if the overall design is going to work.

The critical point
In their recent book Analog Days: The Invention and Impact of the Moog Synthesizer, sociologists of science Trevor Pinch and Frank Trocco contrast the approach of Robert Moog with that of Don Buchla in developing electronic instrumentation in the 1960s. The authors want to understand why Moog’s synthesizer became an indispensable musical instrument while Buchla’s more innovative and ambitious device did not.

The reason, Pinch and Trocco conclude, had to do with the way Moog interacted with musical culture. Moog retained the traditional keyboard, for instance, meaning that musicians did not have to invent techniques from scratch. It was easy therefore to apply existing musical expertise and develop it in innovative ways. Moog “let the culture help shape his instrument” conclude the authors. “He listened to his users.” Allowing himself to be guided by culture, Moog strongly influenced it in turn.

The story of Blue Gene may well come to illustrate a similar lesson for contemporary science. It attempts to select a goal that is sufficiently ambitious to reward investing precious resources over the time needed to reach it, but sufficiently practical to incorporate, extend and co-ordinate existing techniques. Meanwhile, project leaders are continuing to cultivate users and incorporate their needs into the project as it evolves. How the Blue Gene project unfolds is likely to show much about how ambitious science will have to be practised in the 21st century.

From strength to strength


Medical physics can sometimes get overlooked in our enthusiasm for all things genetic, but the links between physics and medicine have a long history, and the opportunities for future collaboration are just as exciting as those in biology. Moreover, while there is much talk about the need to break down the barriers between physical and biological scientists – different “languages”, funding bodies and so on – medical physics has the advantage of being a well established discipline in its own right with all the accompanying “infrastructure”. In addition to medical-physics departments in hospitals and universities, this infrastructure includes learned societies, professional bodies, journals and big international conferences – like the World Congress on Medical Physics and Biomedical Engineering taking place in Sydney this month. In this issue we celebrate the state of the art in medical physics.

In “The rise and rise of medical imaging”, Paula Gould describes how medical physicists are improving the performance of all the major imaging “modalities” – X-ray computed tomography, positron emission tomography, magnetic resonance imaging, ultrasound and others. A major trend in medical imaging is the move towards combining data and images from complementary techniques: for instance, positron emission tomography can provide information on metabolic activity, while computed-tomography scans reveal the related anatomical detail.

Many physics-based techniques are also being used to treat cancer, and each has its own distinct advantages and disadvantages. The basic approach is to use photons or particles to deposit enough energy in the cancerous cells to kill them. With X-rays the challenge is to make sure that the dose is as high as possible at the tumour and as low as possible everywhere else. This is because the X-ray photons start losing energy as soon as they enter body tissue.

Particles are different and tend to deposit most of their energy at a specific depth that depends on their energy. This allows more targeted treatment, but the downside is that it is much more difficult and expensive to accelerate particle beams than it is to produce X-rays. However, medical physicists are working hard to improve the performance of both approaches (see “How particle physics can be therapeutic” and “X-rays pinpoint tumour targets”). Within particle therapy, a range of different approaches is being explored – these include proton therapy, ion therapy and boron neutron-capture therapy, as well as the relatively mature electron-beam therapy and the futuristic antiproton approach.

Returning to photons, lasers are also being used in medical treatments and procedures as diverse as cancer diagnosis, dentistry and the monitoring of newborn babies in intensive-care units (see “Lighting up medicine with lasers”). Of course, all breakthroughs in medical-physics research must pass through rigorous trials before they can be used routinely on patients, and in “From the laboratory to the patient” we describe the experiences of two US-based companies in commercializing ultrasound technology and a new approach to making radioactive “seeds” for brachytherapy.

The bottom line is that medical physics is a thriving subject that offers many challenges for young physicists with the right skills and outlook. And as Natalka Suchowerska explains (p47, print version only) the good news for physicists excited by these opportunities – but not so good news for hospitals and patients – is that there is a shortage of medical physicists across the globe.

Molecules build a bridge to spintronics

Conventional electronic devices manipulate the flow of electronic charge, but spintronic devices would also exploit the intrinsic angular momentum or spin of electrons. Several proposals to build a so-called solid-state quantum computer rely on using electron spins as “quantum bits”.

To make such devices work it is necessary to trap electrons and protect their spins from outside influences. The obvious way to do this is to store the electrons on quantum dots – tiny islands of a semiconductor material embedded in another semiconductor with a different band gap. But until now, physicists have failed to transfer spins between quantum dots, a key feature of any quantum computer.

The Santa Barbara team has overcome this hurdle by building structures made of alternate layers of 7-nm and 3.4-nm cadmium selenide quantum dots. The dots are linked by chain-like organic molecules, which both bind the array together and act as channels for the transfer of spin. Ouyang and Awschalom start by using a ultrashort circularly-polarized laser pulse to get the electron spins pointing in the right direction, followed by linearly-polarized pulse to measure the degree of electron polarization at a later time.

Red pulses are used to polarize the electrons in the larger quantum dots, and green pulses are used for the smaller dots. However, when Ouyang and Awschalom fire red pulses at their assembly, followed by green pulses, they find that the small quantum dots absorb far less green light than they do in experiments in which only green pulses are used. According to the pair, this shows that the spins in the large quantum dots had migrated across the molecular bridges to the small quantum dots.

Moreover, the efficiency of the process jumps from 12% at very low temperatures to 20% at room temperature. Together with the simplicity of their assembly process, Ouyang and Awschalom believe that these advantages could make their technique an important step towards a practical spintronic device.

Ytterbium joins the condensates

Bose-Einstein condensation occurs when a gas of atoms is cooled until the de Broglie wavelength of the atoms is comparable with the average distance between them. When this happens the atoms all collapse into the same quantum ground state and the entire ensemble can be described by a single wavefunction. This means that the quantum properties of the gas become evident on the macroscopic scale.

Physicists have already made Bose condensates from five alkali metals – rubidium, sodium, lithium, potassium and cesium – as well as hydrogen and helium. All of these atoms, apart from helium, have one valence electron and are therefore paramagnetic. Helium atoms have two electrons, but the helium atoms that have been condensed so far have been in a triplet state, which is also magnetic. Researchers have therefore been keen to condense other atoms with two electrons such as ytterbium and the alkali-earth elements such as calcium and strontium.

The two valence electrons in ytterbium can pair up in two different ways. If their spins point in opposite directions the result is a ‘singlet’ state with zero spin that is not influenced by an applied magnetic field – unlike all the other atoms that have been condensed so far. If the spins point in the same direction, a magnetic ‘triplet’ state is formed.

Many of the techniques employed to make Bose condensates rely on magnetic fields, so Takahashi and colleagues had to use an ‘all-optical’ method. First they first trapped about a million ytterbium atoms between two laser beams at about 180 microkelvin, and then modified the intensity of the beams. This forced the most energetic atoms to leave the trap, which in turn reduced the temperature of the remaining atoms. The condensate contained about 5000 atoms and lasted for around 500 milliseconds.

“The insensitivity of the spinless ground state to a magnetic field is a great advantage for many atom optics and atom laser experiments,” Takahashi told PhysicsWeb. The transition between the singlet and triplet states is also very narrow, he adds, and this could be used to make a very precise atomic clock or optical frequency standard. The Kyoto team also hopes to exploit the fact that there are seven different stable isotopes of ytterbium – five bosons and two fermions – in further experiments on the fundamental properties of Bose condensates and the equivalent state for Fermi gases.

Llewellyn Smith to lead UK fusion programme

Llewellyn Smith is a theoretical particle physicist who has spent much of his career at Oxford University, where he was responsible for merging five separate departments into a single physics department. He was director general of the CERN particle physics laboratory in Geneva from 1994 until 1998, when he moved to University College London (UCL) as provost and president. He resigned from UCL in 2002 following opposition to his plans for modernization. At Culham he replaces Derek Robinson, who died last December.

“The UK has a great track record, and a crucial role to play in developing fusion as a source of clean, safe and renewable energy,” says Llewellyn Smith. “Culham also has an important role to play in technology transfer and in fostering science and engineering talent, and has links with universities that range from plasma physics to materials science.”

Llewellyn Smith received the Glazebrook Medal and Prize of the Institute of Physics in 1999 and was awarded a knighthood in 2001. His move from particle physics to fusion follows the announcement at the end of last year that Robert Aymar, currently leader of the ITER fusion project, would be moving in the opposite direction to become the next director general of CERN.

The story behind Infinities

The work that I provided under the title Infinities was enthusiastically taken up by Ronconi and our collaboration began. I chose this subject because, despite being abstract, it is nonetheless familiar to everyone, or at least so they think. Ronconi told me early on in our discussions of an extraordinary space in Milan that he believed would provide a natural home for the production. It allows five adjacent theatrical sets to be created with radically different shapes and sizes that naturally evoked the subject matter.

Needless to say I had no idea whether the production would succeed. But in the end it was an almost overwhelming success. Every seat was sold for the first season in 2002 and Infinities won the Premi Ubu for the best play in the Italian theatre that year. The second season in Milan, which has just finished, ran 10 performance cycles each night to meet demand but all seats for the run were sold more than two months before the opening night.

There have been performances in Spanish to inaugurate the Ciutat de les Arts Escéniques in Valencia and Infinities has been the subject of symposia at the Theatre Populaire in Lyons and the Spoleto Festival. It is hoped that there will be a production in Arabic at the restored Library of Alexandria in the future.

Superconductors could help Europe meet Kyoto target

Under the Kyoto agreement, the EU needs to reduce its emissions of greenhouse gases by 8% from their 1990 levels by the end of the decade. The burning of fossil fuels to produce electricity is one of the major sources of greenhouse gases such as carbon dioxide, so any increase in the efficiency of electric generators, transformers and motors will lead to a reduction in the emission of greenhouse gases. Superconducting devices are more efficient than existing devices because their reduced resistance to electric current means that they waste less energy as heat.

Now Risto Mikkonen and colleagues from Tampere University of Technology have analyzed data from the Finnish power grid and calculated the potential reduction that would result from using high-temperature superconductors (T Hartikainen et al. 2003 Supercond. Sci. Technol. 16 963).

The team found that Finland could reduce its carbon dioxide emissions by between 0.8 and 1.53 million tonnes per year if it replaced all its existing power plant generators, transformers and synchronous motors with high-temperature superconductor (HTS) devices. When applied to the entire EU, this corresponds to a reduction of between 27 and 53 million tonnes, which is 33-65% of the reduction stipulated in the Kyoto treaty.

However, market penetration models suggest that it could take 20 years to achieve half of this potential reduction. At present, Mikkonen and colleagues argue that it is more important to improve the efficiency of HTS devices than it is to manufacture more devices.

Nanoactuator gets in a spin

“Without question the potential of NEMS actuators/motors is huge – only the timescale for this to be realized is an unknown,” Alex Zettl of the University of California at Berkeley told Nanotechweb.org “Many of my MEMS – microelectromechanical systems- colleagues are chomping at the bit to incorporate such nanoscale bearings, motors and actuators into higher order NEMS/MEMS systems. Apparently they just don’t have good low-friction, high-Q materials that operate effectively at this size scale.”

The Berkeley device’s components are integrated on a silicon chip. The nanotube support shaft for the rotor plate spans the gap between two electrically conducting anchors. The rotor plate assembly is also surrounded by three fixed stator electrodes – one to each side and one buried beneath the substrate surface. By applying a voltage of up to 5 volts to the rotor plate and stators, the scientists were able to control the position, speed and direction of rotation of the rotor plate.

The nanotube both acts as the support shaft and provides an electrical connection to the rotor plate. To bring low-friction properties to the nanotube shaft and to enable the rotor plate to rotate through a full 360°, the team sheared away the outer nanotube shells by applying very large stator voltages – around 80 volts.

“Our synthetic devices have many advantages over the molecular motors already known in biology,” said Zettl. “Nature has been exceedingly clever in designing various kinds of molecular (i.e. nanoscale) linear and rotational motors – such as kinesin, ATP synthase, flagellar motors of bacteria – but these are pretty darn specific. They cannot survive over a very wide temperature range, need aqueous environments with tightly controlled chemical balance, and are actually quite slow – they cannot exceed kHz.”

In contrast, Zettl says there is no reason why the synthetic actuators/motors should not operate from cryogenic temperatures up to at least hundreds of degrees C, in environments from vacuum to fluids, and from dc to even microwave frequencies. “While our synthetic motors will not compete with biomotors in certain applications, they most certainly break the applications field wide open, to arenas where nature’s existing motors simply were not meant to, and cannot, survive,” he said.

The rotor plate could act as a moveable mirror for optical switches, for example, or as a paddle for fluid manipulations. “While many of these applications will revolve around communications and mechanical sensor systems, there is an equally large potential in the chemical/biological arena, including microfluidics, chemical sensing and nanotube-bio-motor hybrids,” continued Zettl. “Just like projections made in the early days of lasers, integrated circuits and MEMS, I think no matter how broad and visionary we try to be, we will greatly underestimate the impact – and no doubt also misjudge where the most successful applications will be.”

Now the team is pursuing “chemically and biologically inspired extensions of this work” as well as developing aligned arrays of nanoactuators/motors, and incorporating them into higher order structures with sophisticated functionality.

Why is the tropopause getting higher?

The height of the tropopause varies from about 15 kilometres above the Earth at the equator and to about 8 kilometres above the poles. It marks the limit of most clouds and storms, and plays an important role in the development of large weather systems. Its height is sensitive to changes in atmospheric temperature caused by pollution and global warming.

Santer and colleagues modelled climate changes caused both by man-made factors such as greenhouse gases, atmospheric ozone and sulphate aerosols, and by natural phenomena such as changes in solar activity and volcanic eruptions. “We examine the output from the models and compare this with observations,” Santer told PhysicsWeb. “In the model world we can change one factor at a time, while holding the others constant. This allows us to isolate and quantify the effect of individual factors.”

Santer and colleagues say that increased amounts of greenhouse gases warm the troposphere, while reduced levels of ozone cool the stratosphere. Both these effects increase the height of the tropopause. The results provide independent support for troposphere warming and overturn controversial claims based on satellite measurements that showed little or no tropospheric warming over the last 20 years.

The team found that its results agreed well with other ‘fingerprint’ studies. These include increases in Earth and sea -surface temperatures, sea-level pressure patterns and the melting of the polar ice caps.

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