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Unmanned plane tries transatlantic record

Each plane has a 3m wingspan and weighs 13.1 kg. During normal operations the craft will measure temperature, humidity and pressure over the ocean. At a cost of just $25000 each, the planes could dramatically cut the costs of gathering such information.

Meanwhile, during a test flight in Hawaii, NASA’s Pathfinder-Plus – a remote controlled propeller driven plane – reached a record altitude of more than 23km (80000 ft). Unlike Aerosonde, Pathfinder-Plus is solar powered, giving it the potential to stay aloft for several months at a time. The craft is a prototype for a larger plane called Centurion which has a wing span of 62m and intended to fly at an altitude of 30.5 km. Scientists on the project believe this will allow similar aircraft to act as stratospheric satellites at a fraction of the cost of space-based observation platforms.

Field theorists win Dirac prize

The citation highlights Adler’s derivation of a sum rule for pion-nucleon scattering that marked a breakthrough in our understanding of the currents and broken symmetries of the strong interaction. One of Jackiw’s major contributions was his discovery (with Rebbi) of fractional charge and spin when field theories are applied to condensed matter physics.

The paths of Adler and Jackiw crossed in the late 1960s when the “Bell-Jackiw-Adler anomaly” was able to explain how a neutral pion could decay into two photons. The decay was forbidden by the so-called PCAC hypothesis but had been observed in experiments. A pair of papers – one by the late John Bell and Jackiw, the other by Adler – showed that radiative corrections caused the PCAC hypothesis to break down. The citation calls this “one of the most profound examples of the relevance of quantum field theory to the real world”.

In brief: Yuri Gagarin and Gregory Benford

Starman: Truth Behind the Legend of Yuri Gagarin
by Piers Bizony and Jamie Doran
Bloomsbury p256 £12.59

On April 12, 1961 Yuri Gagarin became the first human in history to leave the Earth’s atmosphere and venture into space. This biography is based on material from sensitive KGB files and restricted documents from the Russian space authorities. It also includes a number of interviews with people who knew him. A fascinating and readable account of the early soviet space race. Recommended.

COSM
by Gregory Benford
Orbit books pp384 £10/$16.10

In this scientific thriller by Gregory Benford, a physicist at University of California at Irvine, something goes wrong with a young physicist’s ambitious experiment. Fortunately, it will soon be seen as a significant historical breakthrough, for the explosion has left something behind – a sphere made of nothing known to science.

The novel is set in the not too distant future at two locations: the Relativistic Heavy Ion Collider on Long Island (currently being constructed) and Benford’s own university campus. Many physicists reading the book will recognise the hardwork needed to get an experiment scheduled on an accelerator – and the infighting and politics involved. COSM explores how scientists work in a much more realistic manner than many current mainstream novels or movies. Surprisingly Benford says that the structure of the book was determined by the media’s reaction to Dolly, the cloned sheep. He wondered how people would react to a similar newsworthy event in physics. Recommended (especially to particle physicists).

Making light work of gravity gradients

The interferometer was built by Mark Kasevich and colleagues at Yale University in the US and used individual atoms, rather than macroscopic man-made objects, to measure the gradient. By observing the phase and frequency shifts of laser light passed through two cooled ensembles of atoms, they believe that the interferometer has the potential to be at least thirty times more accurate than any instrument currently available (M J Snadden et al. 1998 Phys. Rev. Lett. 81 971).

The apparatus consists of two magneto-optical traps at different heights – the first trap about one metre above the second – in which clouds of caesium atoms are cooled to 3 microKelvin. Various laser beams are used to excite the caesium atoms, and the probability of finding the atoms in an excited state after a sequence of laser pulses is related to the value of gravity at that point. Therefore differences in gravity at the different heights will be reflected in the different levels of excitation of the atoms, which can also be measured with lasers.

Unlike other devices which measure gravity gradients, this new instrument automatically cancels out background noise caused by vibrations or sudden knocks on the equipment, making it more suitable for field work.

Physicist named as Japan’s minister for education and science

Arima has long been a keen advocate for reforming Japanese science. Previously he helped shape the 1996 Basic Law for Science and Technology, which aimed to increase Japan’s spending on science. He also proposed educational and administrative reforms during the last government.

Monbusho has an annual budget of 5819 billion yen (£24.6 billion) and accounts for some 7.5% of total government spending. In 2001 it will be merged with the Science and Technology Agency as part of an attempt to cut costs and reduce duplication of effort. Arima’s appointment has increased confidence that the merger between the two departments will be successful.

Arima has announced that education will be his top priority, beginning with the reforms of primary and secondary schools that were outlined earlier this year by a committee which he chaired. There are also plans to reform the university admission system to make it easier for students to attend the university of their choice, and to introduce life-long learning programmes.

Talking physics with the Dalai Lama

Zeilinger had invited the Dalai Lama to his laboratory following a meeting at Dharamsala in Northern India last October at which he and four other physicists had, over the course of five days, discussed physics and cosmology with the Buddhist leader. In Dharamsala, Zeilinger had demonstrated some basic quantum phenomena – such as wave-particle duality – using a laser-based double-slit experiment with a photomultiplier tube connected to a loud-speaker. The Dalai Lama’s visit to Innsbruck allowed other quantum effects to be demonstrated for him.

Zeilinger says that the Dalai Lama did not have a problem with photons having both particle and wave-like properties, but was reluctant to accept that individual quantum events are random. For example, he refused to accept that we cannot know which path a photon takes in a two-path quantum interference experiment. Zeilinger notes that continuity of existence is very important to Buddhists because it leads to reincarnation.

However, observation plays a key part in what we can know in both quantum theory and Buddhism, and Zeilinger was surprised to learn that the Dalai Lama agreed that there are not only limits on what we can measure, but also limits on what we can know, even in principle.

So what is Tibetan Buddhism? And what possible connection can it have with physics? According to Alan Wallace, an interpreter at the meeting, Buddhism is a spiritual tradition with strong empirical, philosophical and religious components, including a belief in the after-life and reincarnation and healthy doses of meditation. Buddhism, he explains, is based on the four noble truths: the reality of suffering and conflict; the inner origins of suffering and conflict; the possibility of the cessation of suffering and its sources; and that Buddhism presents a path to this cessation through spiritual practices. The bottom line is that the root of suffering and conflict is ignorance and delusion, and that the path to spiritual freedom is the path of knowledge and insight.

Wallace is well placed to discuss the links between science and Buddhism. After spending 14 years as a Buddhist monk in India, Switzerland and the US, he graduated in physics from Amherst College in the 1980s. It was at Amherst that he met Arthur Zajonc, the physicist who was the scientific co-ordinator for the Dharamsala meeting. Wallace is now professor of Tibetan studies at the University of California at Santa Barbara.

“It is natural for Buddhists to be interested in science, ” he says, “because science is the most complete and successful theory of the physical universe we have. The Buddhist pursuit of truth includes not only the nature of consciousness, about which modern science knows very little, but also the entire world of which we are conscious.”

Wallace agrees that quantum mechanics and Buddhism have many similarities: neither is fully objective (i.e. certain quantum properties only have meaning in the context of a measurement) nor fully subjective. Consciousness and various mind-body problems are also similar in this respect, he adds. According to Wallace, the Dalai Lama had not realized before that these sorts of philosophical questions could be so clearly demonstrated in the laboratory, while physicists were surprised that the introspective approach of Buddhism led to similar questions.

Zajonc says that he found many elements of Buddhism potentially quite helpful to the philosophical treatment of quantum mechanics. “It quickly became clear, ” he adds, “that Tibetan Buddhism offers a vast and subtle set of philosophical approaches that we in the West would benefit by knowing, even in the sciences.”

The conference in Dharamsala was the sixth in the “Mind and Life” series in which the Dalai Lama meets with scientists, but the first on the physical sciences. The previous five had been concerned with the brain, consciousness and related topics. According to Wallace, the meetings are a response to the Dalai Lama’s own fascination with science and his belief that Tibetan Buddhism must not turn its back on modern knowledge. There are two main reasons for conferences: to provide a high-level tutorial for the Dalai Lama; and to encourage scientists to explore new ideas, inspired by Tibetan Buddhist philosophy, psychology and meditation.

George Greenstein, a theoretical astrophysicist at Amherst College, spoke about cosmology in Dharamsala. The other physicists present were Piet Hut of the Institute of Advanced Studies in Princeton, David Finkelstein of the Georgia Institute of Technology, Zajonc and Zeilinger.

“The dialogues, ” says Greenstein, “included a good deal of factual information in which the Dalai Lama was extremely interested.” Philosophical questions were also discussed. For example, is the big bang a moment of creation, as opposed to a transformation from one state to another? How can we understand creation? And did time and the laws of physics exist before the creation?

Greenstein says that philosophical questions are never answered, just discussed in new and interesting ways. His hope was that the Dalai Lama would ask the questions in new ways. “This happened in general, ” he says, “but I cannot quote chapter and verse about specific topics. It was more amorphous and subterranean.”

So why did Zeilinger agree to visit the Dalai Lama in the first place? “Science is an endeavour on which we have just started, ” he says. “We are just fledglings and it is important to pull together all the intellectual traditions in the world.”

Would Greenstein like to continue the discussions? “I would love to go back to Dharamsala, ” he says. “Considering we were discussing all of physics and astronomy, we only scratched the surface. We flew over a new continent. I’d love to go back and land and walk around a bit.”

Tiling theory could make DNA chips a reality

Previous attempts to use DNA as a molecular building block failed because its double helix structure is so flexible. However, Seeman and colleagues realized that if two synthetic DNA chains were placed side-by-side and linked at two cross over junctions, they would form a very rigid structure – called a DX molecular unit – that would be suitable for building patterns. In their experiments they created an A-B repeating structure of DX molecular units, which was visible as evenly-spaced stripes under an atomic force microscope.

Now that two-dimensional structures have been demonstrated, further development work could result in molecular algorithms suitable for high speed computation, novel photonic materials, biochips and components for molecular electronics.

Measuring up to advanced materials

Physics, chemistry and physical chemistry have undergone tremendous changes over the course of this century. In physics, the focus has shifted from atoms to subatomic particles, namely nuclear physics and particle physics. Meanwhile, the physics of collections of atoms in the liquid and solid states have slowly emerged as separate, independent fields.

After the Second World War, another interdisciplinary field emerged in the form of materials science, which combines metallurgy, physics, chemistry and physical chemistry. The goal of the subject is to synthesize materials such as metals, ceramics and polymers based on thermodynamic phase equilibria, reaction kinetics and our ability to characterize materials from the atomic level upwards. Particular attention is paid to the relationship between a material’s microstructure and its bulk properties. Materials science also includes theoretical studies that deepen our understanding of the properties of materials, helping us to create new materials on a rational basis – rather than through trial and error alone.

Materials science can therefore be said to encompass all of the classical parts of science. Early on, materials scientists and metallurgists tried to maximize a particular property, such as hardness, toughness, magnetization or conductivity. Great demands were made on developing materials with exceptional properties, such as ultimate strength. However, it soon became obvious that when it came to specific applications, advanced materials with various special properties were of little use unless they could be processed simply and straightforwardly. Thousands of materials have therefore been developed over the past 20 years – many with well defined and reproducible properties – but they have not been widely taken up by industry because they cost too much to make and are not particularly durable.

Having all of these materials at our disposal therefore begs the question: do we really need more research? Or is our task complete, now that we have developed the techniques that allow us to produce the materials that cutting-edge technologies require? Philip Ball, an associate editor of the journal Nature, does not think so. In this new book, he describes ten groups of materials that he thinks will be at the forefront of technology in the coming century. Each chapter covers a different material.

Photonic materials allow light to be transmitted through solids and will therefore be able transmit information at very high speeds. Offering immense data-storage capabilities, they are certain to be important tools over the next hundred years, which has been hailed as the century of information technology.
New types of magnetic materials will allow increased amounts of information to be stored. This is already one of the main areas of research in materials science.
Smart materials, which can react to outside stimuli, are already part of many of the important components found in the appliances used in daily life.
Biomaterials have inspired materials scientists to study and copy nature’s machinery. Together with biologists, they have developed biomaterials using well defined proteins under specific environmental conditions. Materials scientists are also looking at ways to use DNA to find new ways of storing information.
Biomedical materials have many important practical applications, serving, for example, as implants in the human body. They need to be compatible with human tissue, and one day could even function as spare parts for the human body.
Materials that can generate clean energy and store energy without polluting the environment could solve one of the biggest challenges of the next century – the provision of plentiful amounts of clean energy for the Earth’s ever-increasing population.
Porous materials with pores from a few atoms in size to thousands of atomic diameters act like sieves that can be used to select different compounds. They could be important for synthesizing other materials.
Diamond and other hard materials can be used as very thin surface layers to toughen materials, such as industrial tools.
Polymers that can be made by chain reaction allow scientists to develop materials with specific molecular architectures.
Surface and interface science will be critical in the development of new materials. Techniques need to be developed that allow atoms to be imaged either directly on the surfaces of materials or with advanced electron microscopy.

Ball writes about the enormous progress that has been made over the past 20 years in these ten areas in an interesting and entertaining way. However, after reading the book, one might conclude that materials science has now done all that it can, and that engineers can simply pick whatever material they need from the shelf. This is, of course, not true. The 21st century still holds many challenges for materials science. Information technology and energy technology – two key areas over the next hundred years – will demand materials with ever more specific properties at ever smaller scales.

The nanoscale and the sub-nanoscale will probably be the most important dimensions in the future, although this may well lead to a revitalization of more traditional fields, such as friction, wear and corrosion. After all, advanced wear-free materials will be of great interest in these applications. Materials scientists may also start to become interested in waste disposal and recycling.

The materials that Ball describes are all advanced – some, indeed, are quite exotic. They will provide challenges that are at the cutting-edge of research. But a by-product of this research will be the discovery of processes that are also relevant to more conventional materials. It is clear, after reading this book, that materials science is by no means coming to an end.

Global challenges

Fears over climate change and the increasing problem of pollution have put public concerns about the environment at an all-time high. Governments have responded to public pressure by introducing legislation to reduce pollution, and at the Kyoto summit last year they agreed to reduce emissions of greenhouse gases by an average of 5.2%. Europe has set the most ambitious targets: last month it was announced that Germany, for example, will cut emissions by 22.5% by 2012.

Most of these measures have a direct impact on industry, which must tread a fine line between short-term economic gains and the long-term effect on the environment. This tension has led to the idea of “sustainable development”, first promoted by the United Nations in 1987. Put simply, this policy encourages economic growth but in a way that does not harm the environment.

A good example of how this can work in practice is developed by a university-industry collaboration in the UK. They have worked with industry to develop an optics-based sensor for monitoring emissions of toxic gases. The instrument was invented by the university team in the early 1990s, and the industrial partners have helped to develop it into a practical and commercial instrument. Full-scale production is expected to start later this year.

But some environmental problems are outside human control. Last year global weather patterns were severely disrupted by El Niño, which causes the tropical Pacific Ocean to become warmer than usual. Researchers are now familiar with the effects of El Niño, and are beginning to understand the mechanisms that cause it. As Ping Chang and David Battisti explain, interactions between the ocean and atmosphere play a crucial role in both initiating and ending El Niño events. Computer models of the coupled ocean-atmosphere system have allowed researchers to predict El Niño events several seasons in advance.

Although El Niño is a natural phenomenon, research by the National Oceanic and Atmospheric Agency in the US indicates that its effects could be exacerbated by global warming. The link between the two is still unclear, but underlines the fact that emissions of greenhouse gases are having a major impact on both the weather today and the climate tomorrow.

So what can be done to reduce emissions? The burning of fossil fuels is the largest source of greenhouse gases, with over 85% of the world’s energy coming from oil, coal and gas. An obvious way forward is to develop renewable energy sources. Nuclear energy and hydroelectric power are already well established, while solar power and wind energy are slowly becoming more popular.

Another possibility is thermophotovoltaics (TPV), suggested in 1956 by Pierre Aigrain of the Massachusetts Institute of Technology in the US. These devices work in a similar way to solar cells, but generate electricity from thermal radiation rather than sunlight. As Timothy Coutts and Mark Fitzgerald explain, TPV systems provide higher efficiencies than solar cells, and could provide power in all parts of the globe – not just those with lots of sunshine. The Swiss and Swedish governments are investigating if TPV generators could be used to heat residential homes, and commercial systems are now becoming available.

But the biggest challenge for renewable energy will be the transport sector. Although conventional engines are now more efficient than ever, traffic pollution will continue to get worse as more cars fill the roads. One of the most promising alternatives is fuel cells, which are more efficient and only produce water as a waste product. As Gregor Hoogers points out, several companies have revealed prototype vehicles powered by fuel cells, and millions of dollars are being invested in improving their performance and reducing manufacturing costs. However, fuel cells will still be more expensive than conventional engines, and government incentives will be needed for them to gain widespread acceptance.

These are just a few examples of how physicists are helping to reduce our impact on our planet. But there is the potential to take an even more active role in making new technologies sustainable. Progress made today will help to safeguard the natural environment for many generations to come.

See the features in the August issue of Physics World magazine

Atoms join in the race for lithography in the next century

The problem is that the features made by conventional optical lithography cannot be shrunk indefinitely. The laws of diffraction dictate that the only way to make feature sizes smaller is to shorten the wavelength of the light. Yet the materials needed for lenses and other optical components are not effective at shorter wavelengths, and so optical lithography cannot pattern feature sizes less than about 100 nm.

Now, Mara Prentiss and colleagues at Harvard University in the US have demonstrated an intriguing new type of lithography that could be used for patterning devices below the 100 nm barrier (Science 1998 280 1583). This approach uses a beam of metastable rare-gas atoms, each one carrying a tiny payload of energy, which are guided onto a surface by laser light. The researchers have used the technique to pattern a silicon wafer with an array of lines 65 nm wide and separated by 401 nm.

Lithography in any form is the art of transferring a pattern onto a substrate. Optical lithography uses special lenses to project the reduced image of a mask onto a wafer covered with a photosensitive resist. Photons striking the resist alter it chemically, making it harder or softer (depending on the type of resist). Resist in the softer regions is removed with chemicals and an etching step transfers the pattern to the silicon wafer.

But the use of lithography to produce feature sizes smaller than 100 nm is a critical problem for the semiconductor industry. Research teams around the world are now investigating several alternative techniques for achieving the required resolution in a cost-effective way, including electron-beam, ion-beam and x-ray lithographies. Each technique has its advantages and disadvantages, and no-one yet knows which will be the method of choice in the semiconductor factories of the 21st century.

The new process, often dubbed “atom lithography”, reverses the conventional roles played by light and matter. Instead of using a solid mask to pattern a light beam, atom lithography patterns a beam of atoms with a “mask” made of light. Strange as this might seem, it works surprisingly well. While it is still too early to say whether atom lithography will ever be practical for chip manufacture, it boasts several features that make it worthy of further study.

Consider the diffraction problem. Atoms hardly diffract at all because their de Broglie wavelength is generally only a few picometres. Atoms are also electrically neutral, which means that a large flux can be squeezed into a small area. But how can neutral atoms expose a resist, and how can an atom beam be patterned? Research at Harvard and the National Institute of Standards and Technology in the past few years has provided answers to both of these questions. The trick lies in using a metastable rare gas such as argon.

Rare gases are unique in that they are chemically inert in the ground state, but have long-lived metastable states that can release significant energies when they strike a surface. Metastable helium atoms carry 20 eV of energy, while other rare gases provide a range of energies down to 10 eV for xenon. Once in the metastable state, quantum-mechanical selection rules forbid the atoms from releasing their energy provided that they are moving through a vacuum and are not interacting with anything else. If they strike a surface, the rules no longer hold and the energy is released. This is the basis of atom lithography. Instead of depositing photon energy in the resist, as with optical lithography, the energy of the metastable atoms is deposited in the resist.

The metastable nature of these atoms also provides the means for patterning a beam: the atoms alter the surface in certain areas, while leaving it unaffected in others. We therefore need some way to “defuse the bomb” in selected areas, releasing the energy stored in the metastable atoms to return them to the harmless ground state.

The Harvard team achieved this by using laser light tuned to an optical absorption line in the metastable atoms. When the atoms absorb light at this specific frequency, they are promoted to an even higher energy level – but only temporarily. From this level the quantum mechanics allow the atoms to relax into the ground state by emitting an ultraviolet photon. Light can therefore “quench” the metastable atoms, and a light beam containing a pattern of different intensities can impose the same pattern on a beam of metastable atoms. High-intensity regions of the light beam quench the metastable atoms into the ground state, while low-intensity regions allow the metastable atoms to pass through unaffected.

The Harvard team used a standing wave with an undulating intensity that falls to zero at integer multiples of half the laser wavelength (see right). The wavelength was 801.5 nm, which matches a transition in metastable argon. Metastable atoms only penetrated the light field in regions very close to the zeros in intensity, since even low-intensity light was enough to eliminate all of the metastable atoms. Metastable atoms striking the surface deposit their energy in the resist, which in this case consisted of residual hydrocarbons that adhere to the substrate more strongly when struck by the energetic atoms. This formed an array of lines with a linewidth of 65 nm and periodicity of 400.7 nm.

Interesting as this new lithography technique seems, it has several obstacles to overcome before it becomes a serious contender in tackling the problem of lithography below 100 nm. For instance, it is not an easy task to pattern a beam of light in exactly the right way for printing a complex electronic circuit. Also, today’s state-of-the-art metastable atom sources are pretty weak and would have to be improved to make the technique economically viable. But none of the existing contenders seems to be a clear winner for the demands of the future, so it makes sense to look for more radical approaches.

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