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Single molecules demonstrate resolving power

However, the resolution possible with scanning near-field optical microscopy (NSOM), as this approach is called, is essentially limited by the size of the aperture. To improve the resolution it is necessary to use ever smaller apertures, or to replace the aperture with an extremely small light source. A team of physicists at the University of Konstanz in Germany has now taken this latter approach to the extreme and used the light from a single molecule instead of an aperture (J Michaelis et al. 2000 Nature 405 325). The technique was used to obtain images of a hexagonal lattice of 25 nanometre high triangles on an aluminium surface.

Vahid Sandoghdar and colleagues started by growing micron-sized crystals of p-terphenyl that contained about one molecule of terrylyne for every ten million molecules of the host. A suitable microcrystal was then glued to the end of an optical fibre. The low doping concentration of the crystal and operation at 1.4 kelvin meant that a single terrylene molecule could be selectively excited by laser radiation. The photons emitted from the molecule then passed through the sample and various optics before being detected by an avalanche photodiode.

By moving the sample relative to the fibre, it was possible to build up an image. Moreover, the quality of the image improved as the sample-probe separation was reduced from 350 nm to 20 nm. Although the images are not yet as good as those obtained with the best optical near-field microscopes, the Konstanz team hope to improve the resolution of their technique by using smaller microcrystals and different ways to excite the molecules near the end of the crystals.

How good is physics in the UK?

The 11-strong panel spent a week in the UK in April and drew on the comments of more than 150 physicists from around the world. The panel’s conclusions were presented at a meeting of UK physics professors in London last Friday by panel chairman, Alex Bradshaw of the Max Planck Institute for Plasma Physics in Germany and president of the German Physical Society. The executive summary of the panel’s report is available on the Web, and the full report will be published soon.

The panel conclude that condensed-matter physics in the UK “still maintains a substantial research base with some examples of world-leading work.” Nuclear physics, the panel states, “has secured international strength by a judicious focussing of scarce resources.” Although UK leadership is apparent in some sub-fields of atomic and molecular physics, the panel also points to “missed opportunities”. Particle physics in the UK is world class but it would be even stronger if its funding matched that of Germany, Italy or France, Bradshaw told the meeting in London. Astronomy and astrophysics are also strong, but there are worries about long-term investments in these areas.

The report stresses the need to attract and retain young people in physics research and education, and school teaching in particular. It also emphasizes the need to attract more women into physics and “to facilitate their retention at all levels in the profession.” The panel also criticizes salaries in the UK as “not internationally competitive”, and says that the regulations governing some postgraduate schemes are “excessively rigid and formulaic.”

The survey was sponsored by the Institute of Physics, the Particle Physics and Astronomy Research Council, the Engineering and Physical Sciences Research Council, and the Royal Astronomical Society.

Fusion: the final frontier for plasmas

One of the standing jokes that those of us who work on fusion have to suffer every now and then from other physicists is that the best-conserved time invariant in physics is the time to achieve a controlled and sustainable fusion reaction. And certainly that prospect is still at least 25 years away, which only goes to prove the optimism of researchers in the field, who have continually been able to convince politicians that fusion is worth funding from the public purse. More importantly, the fact that controlled fusion is still so distant highlights the difficulty of the problem to be solved.

Despite the sarcasm, nations all over the world continue to dedicate important resources to harness on Earth the source of energy that is more common than any other in the universe – nuclear fusion. Such stubbornness is only justified given the promised land that fusion offers: namely a virtually inexhaustible source of energy that is safe and environmentally friendly. And the compelling evidence is that we are very close to the goal.

Some 675 million Joules of energy have already been obtained via the Tokamak experiment at the Joint European Torus lab in the UK from fusion reactions between deuterium and tritium nuclei. And the International Thermonuclear Experimental Reactor (ITER) – an engineering design of a device that could deliver 400 Megawatts of fusion power over a period of 10 minutes – is almost ready, waiting only for the appropriate authorities to give its construction the green light. ITER will prove beyond any doubt the scientific and technical feasibility of harnessing fusion reactions as a source of energy.

These and numerous other experiments around the world have now proved that all of the parameters of a plasma that are needed to achieve fusion – including density, temperature, confinement time and pressure – are practically possible, even if those parameters have yet to be obtained together at the same time. That task is impossible to fulfil with current devices, and will require a next-generation fusion device, such as ITER.

With fusion as the focus to this book, Hans Wilhelmsson from the Chalmers University of Technology in Sweden invites us to go on a fascinating tour of the universe to see for ourselves just how valid is his claim that “99% of known matter [in the universe] is in a plasma state”. Starting in our own atmosphere, we pass through the magnetosphere and on to the Sun, from where we are taken to distant supernovae, pulsars and quasars. Along the way we learn in an entertaining fashion the strong links between phenomena that naturally occur in plasmas in the universe and those produced in fusion reactors.

Having heard many fusion scientists say how much researchers in these two fields could learn from each other – without either side actually doing much about the problem – Wilhelmsson’s book provides a good opportunity for both communities to strengthen their links. The book even suggests, using several good examples, the most straightforward issues of common interest where the benefit of closer ties would be immediate.

Wilhelmsson is a theorist, and the mind of a theorist clearly surfaces in the section that is entitled “fusion on Earth”, which covers plasma modelling, waves and instabilities, nonlinear effects and turbulence. However, at this particular stop on the journey through the universe, where the author discusses “inertial-confinement fusion” – in which lasers or magnetic fields are used to confine the fusion material – I feel he should have mentioned the defence applications of the Mégajoule laser-fusion project. After all, in 1995 after the end of French nuclear explosions at Mururoa in the Pacific Ocean, it was President Jacques Chirac himself who said that the construction of the huge Mégajoule laser laboratory in Bordeaux would make such tests a thing of the past.

Another slight criticism is that the author passes perhaps too quickly and superficially through the technical applications of plasmas by scientists on Earth. Having said that, it may be that the author’s approach makes sense because his journey takes us to some of the furthest reaches of the universe, which does not leave him with much time to dwell on such simple Earthling problems.

Although “plasma physics is a comparatively young science”, as the author points out, it already has a history that people like Wilhelmsson have a duty to tell younger generations about. I therefore greatly enjoyed the fact that he includes various personal reminiscences and descriptions of his interactions with many well known physicists over the years, including several Nobel-prize winners. For example, he tells how the Russian physicist Piotr Kapitza, who won the Nobel prize for his work on low-temperature physics, spoke enthusiastically in his Nobel lecture on fusion. These intelligent anecdotes make the book a pleasure to read. I also enjoyed the classic papers by Hannes Alfvén, Stig Lungquist, Subrahmanyan Chandrasekhar and Enrico Fermi that appear in the appendices.

“When the game comes to an end,” concludes the author about the future of fusion, “the benefits will be enormous and Nature will help to provide a peaceful source of energy as it has done for eternity in the rest of the universe.” Let us hope that one day that vision is indeed reached.

How Britain was saved by radar

I was recently 60 feet underground in the museum attached to the old operations room of 11 group at Uxbridge, one of the nerve centres of the Battle of Britain, staring at pictures of senior Royal Air Force officers like Sir Hugh Dowding and Sir Arthur Harris. Why, I wondered, were there no pictures of scientists like Sir Henry Tizard or Sir Robert Watson-Watt? After all, it was Watson-Watt who in 1935 first suggested to the Tizard committee, which had been set up by the British government to evaluate scientific air-defence techniques, that radio waves could be used to detect enemy aircraft. No-one who reads this book on the history of radar during the Second World War could possibly fail to agree to the addition of those pictures of Tizard or Watson-Watt; it makes it quite clear that without radar the allies would have lost the Battle of Britain.

Radar was not simply a new gadget; it was a radical extension of human vision. In the Second World War it empowered us to see our enemies coming from afar in the dark and fog, so that we could defend ourselves more easily. It also helped us to seek them out at night for attack. In peacetime, radar empowers our aircraft and ships to find their way about the world in much greater safety. Indeed, its importance to navigation was as vital as that of the chronometer.

In the first two chapters of this book, Louis Brown sets the scene into which radar was born and gives us an account of its conception in Britain, Germany, Japan and the Netherlands. Most of this material can be found elsewhere, but it has not been collected and presented in such an admirably wide perspective as this one.

In the section on the origins of radar, we are left in no doubt that any discussion of the question “who invented radar?” is as futile as asking which of the many votes won an election. The more interesting question is “who made radar into a weapon that really mattered?”, and on that topic the book sheds some light by comparing what happened in different countries. It shows that the early British experiments at Orfordness on the detection of aircraft by radar were turned with remarkable speed into a highly effective defence system (Chain Home – CH) in time for the outbreak of the war by the combined efforts of Watson-Watt, Tizard and Dowding.

An essential factor in this achievement was the development of an extensive system of communications, special display equipment and, importantly, new military procedures. It is particularly instructive to compare the operational results of this work with what happened in the Japanese attack on the US naval base at Pearl Harbour, where there was adequate radar but no effective plan for using it. The comparison shows perfectly the difference between developing radar and making it into an effective weapon.

The basic problem in developing a new weapon is to put the person who knows what can be done technically in touch with the person who knows what needs to be done militarily. One major difficulty in doing this is to overcome the barrier of military caste, which, so often, has frustrated innovation. Another difficulty is, of course, the scientists’ lack of information about military operations.

In Britain both these difficulties were overcome by Watson-Watt and Tizard, and later by A P Rowe’s “Sunday soviets” – weekend meetings at which scientists and air-force officers got together at the Telecommunications Research Establishment (TRE) at Swanage and Malvern – and by establishing “operational research”. Reading in this book about what happened in other countries, including the US, it is clear that this close co-operation between the scientists and the military was a distinctive feature of the development of radar in Britain and that it paid off.

Even so, I was left wondering how it was that Fighter Command and the TRE failed to foresee that the development of radar coverage inland (Ground Control Interception – GCI) would be as essential to the air defence of Britain at night as the development of radar coverage out to sea (CH) was to air defence by day. The introduction of radar (GCI) to control fighters was delayed until late in 1940, and the use of radar in night defence (Air Interception – AI + GCI) did not therefore become an effective weapon until early in 1941 – sadly too late for the night raids by the German air force on British cities in 1940.

I was also surprised to read that the idea of putting a powerful lamp in the nose of anti-submarine aircraft (Leigh Light) arose rather late in the war from a casual conversation between two junior Royal Air Force officers. Had the idea been put into practice sooner, it would have increased the efficiency of airborne radar (Air-to-Surface Vessel – ASV). One can’t help wondering why those concerned with anti-submarine warfare failed to see that in the last stages of attacking a submarine in the dark you needed to see it!

In the next seven chapters, Brown surveys the application of radar in all the major theatres of war, starting with the Battle of Britain and ending with the destruction of Hiroshima and Nagasaki in 1945. In each case he tries to present the use of radar in its operational context and to include all types and nationalities of radar – marine, airborne and anti-aircraft. Given the large number of military operations and the astonishing variety of radar equipment, such a survey is, necessarily, rather confusing, and some of the more interesting operations could do with more space. Nevertheless, if you want to know more about a particular operation there are copious references.

Given the breadth of his canvas the author does a pretty good job, and I came out of the book with a much better understanding of the use of radar in the last war, particularly in the Pacific and Mediterranean, than I had gathered from the dozen or so books already on my shelf. The final chapter, entitled “The Measure of Radar”, includes a brief account of the application of radar techniques to navigation, air-traffic control and radio astronomy; a discussion of the effects of secrecy; and an attempt to evaluate the use of radar in the last war. There is a short appendix on radar techniques, which had me wondering whom it was written for! Finally, there is a comprehensive list of sources, an index, and plenty of interesting pictures of different types of radar.

I am glad to welcome this edition to my family of books on radar.

Impossible things usually don’t happen

Sam Treiman was a distinguished particle theorist. The famous Goldberger-Treiman relation was, at the time of its discovery in 1958, an amazing connection between the strong and weak interactions. Colleagues used to credit him with “Treiman’s theorem” – impossible things usually don’t happen.

Shortly before his untimely death late last year, Treiman wrote a book that, he said, was “aimed at a wide audience of the curious, scientists as well as non-scientists”. The publishers advertise it as “a concise account of quantum mechanics written for general readers”.

Writing a popular book on quantum mechanics is a perilous endeavour. When I was asked to review the book, my predatory instincts led me to expect numerous errors and misconceptions that I would happily criticize. That hope has not been fulfilled: I found only one error, and it is not about quantum mechanics but is instead in the preliminary discussion on classical physics. (The error concerns an incorrect statement that the solution of the twin paradox requires general relativity.)

The price paid for this dearth of errors is that the book is not at all of the popular kind. Its unpopular features include Maxwell’s equations, on which Treiman writes: “The Maxwell equations are too irresistible not to display, if only for aesthetics… .” The reader is not, however, asked to understand or solve the Maxwell equations, only to appreciate their elegance (cf Matthew, 7:6). Readers will also encounter complex numbers, multiple integrals, bra-ket notations, spherical harmonics and casual statements of familiar truths such as “according to the principles of statistical mechanics, the atoms at low temperature are mainly in the ground state”.

It is therefore necessary to redefine the set of readers: they should have a general education in physics and be eager to understand the curious features of the quantum. For this type of reader, Treiman’s book is outstanding. It is an informal textbook on quantum mechanics, without detailed proofs of the theorems, only hints on how to prove them. This is not at all a qualitative pictorial description of the quantum world, but a rigorous formulation of its axioms in a casual language. For example, Treiman carefully avoids saying that a wavefunction describes properties of a physical system. Rather, he says that the wavefunction “tells us all we can know about the system” – that is, all we can predict on the results of potential measurements that we may perform on that system.

The uncertainty principle is correctly explained in terms of dispersions in statistical distributions, not of mutual disturbances, as many textbooks present it. Quotation marks are scrupulously used for the “uncertainty relation”, DE/Dt~ hw. Treiman stresses that this is not a true uncertainty relation, in the above sense. Quantum mechanics does imply limitations involving time and energy differences, but their physical meaning is not that of uncertainties.

It is inevitable that some points are oversimplified. In discussing the hydrogen atom, for example, Treiman replaces the radial wavefunction u(r) by the product rR(r), and asserts that “this product must vanish at the origin, since r vanishes there”. Quantum mechanics does not demand this, but only that wavefunctions be square-integrable and belong to the domain of definition of the Hamiltonian. Going beyond the hydrogen atom, the book discusses various applications of quantum mechanics to atomic structure, nuclei, the solid state and astrophysics.

Chapter 7, entitled “What’s Going On?”, is the highlight of the book. It begins with the following statement: “Quantum mechanics deals with probabilities. Observers deal with facts [but] nothing within quantum mechanics tells us how to convert probabilities into facts.” This is the infamous quantum measurement problem, which lies at the interface of classical and quantum physics. It is not part of the standard physics curriculum, and suffers from a vast and mostly misleading literature.

Indeed, some leading contemporary scientists have promoted wild speculations about the quantum measurement problem – for example, that human consciousness plays a role in it. Treiman, however, is faithful to his no-nonsense approach and is exceedingly careful. As he writes: “The quantum assertion is [that] the state of the system ‘collapses’ into the eigenstate that corresponds to the eigenvalue obtained in the measurement”, and quickly adds, “for the present, let’s stick with the naive proposition enunciated above.”

There is the inevitable Schrödinger’s cat parable, followed by a mundane explanation: “We are actually all of us, daily, in the position of Schrödinger’s cat…To the outside observer, we are superpositions until the observation is made.” One brief paragraph is devoted to Hugh Everett’s many-worlds interpretation. “It is undoubtedly amusing to contemplate,” says Treiman, “[but] it can neither be falsified nor built on.”

The last two chapters present elementary particles and quantum fields. These were the author’s research areas and the reader gets an up-to-date education on these subjects. Cross sections are defined, and then time, charge and parity symmetries. The text also discusses in a qualitative way neutrino oscillations, Feynman diagrams, propagators and renormalization.

But for Treiman being qualitative does not mean being sloppy! With admirable care, he asserts that “the virtual particle concept is actually only a proxy for certain mathematical ingredients…Virtual particles are not real objects. [We may] describe them as corresponding to virtual reality.” Treiman stops short of explaining current algebra, on which he co-authored two other books.

There are also many funny anecdotes on the early history of quantum theory and on its founding fathers. In summary, this book is a wonderful guided tour through quantum mechanics, and I recommend it without hesitation to every physicist.

Data, data everywhere

Examples of lost data include the results of heavy-ion experiments at the Bevelac accelerator at Berkeley. The accelerator stopped running in 1993 but much of the data – which are relevant to research into solar neutrinos, nucleosynthesis and cosmic rays – was never published in any form. “Scientists will have to wait decades before these data are remeasured,” say Firestone and colleagues. They also criticize plans for the long-term storage and dissemination of data from the $600m relativistic heavy-ion collider (RHIC) at the Brookhaven National Laboratory and the CEBAF accelerator at the Thomas Jefferson National Accelerator Facility. However, plans to build ‘virtual’ observatories (see Astronomers look to ‘virtual’ observatories) to analyze data from telescopes and satellites are praised by Firestone and colleagues.

But storing the data is not enough, writes Declan Butler in a related article, you must also be able to search or index it. Only 50% of the billion pages on the Web today have been catalogued, and it is expected that over 100 billion pages will be added over the next two years. The only solution is to develop a new generation of search engines for scientists, according to Butler. New tools, such as XML (eXtensible Markup Language) – the successor to HTML – should make it possible to restrict search terms to scientific papers, while the rise of specialised science portals will also help, he writes. And new search algorithms – which take account of how many times different pages have been accessed – should help direct users to the most relevant material. The first two prototypes for such advanced search engines – Google and ResearchIndex – can already be accessed on the Web.

Mini Earth created in the lab

A conducting fluid needs to have a small ‘seed’ magnetic field before it can generate a self- sustaining field. The seed field induces electric currents in the fluid that in turn create a more powerful, and stable, magnetic field. This creation of the field relies on a positive feedback mechanism. However, the process only works if the metallic fluid is moving very quickly or if it exists in very large quantities.

The Latvian-German team applied a small magnetic field to a container filled with two cubic metres of molten sodium that was rotating at 15 metres per second. The high conductivity and velocity of liquid sodium was fast enough to generate an oscillating magnetic field in the fluid, and this field was maintained when the external magnetic field was turned off.

Quantum dots detect single photons

The quantum dot device consists of a transistor made of different layers of gallium arsenide and aluminium gallium arsenide. One of the layers consists entirely of quantum dots just nanometers across. The quantum dots are extremely sensitive to photons. A photon hitting the detector liberates an electron trapped in the one of the dots. A nearby conducting channel, just a few nanometres above the quantum dot layer, picks up the electron – which causes a detectable change in the resistance of the channel. Currently the device only operates at 4 Kelvin but the team hopes to develop detectors that will work at 77 Kelvin and, eventually, at room temperature.

PhysicsWeb editor moves on

“When I helped create PhysicsWeb nearly three years ago, I was sure that it would be a successful site. However, sites such as PhysicsWeb do not rely solely on one individual, and I would like to thank Lloyd Fletcher, James Counihan, Martin Kelly, Chris Brown and all the Physics World editorial team for their support and guidance and support. This has been one of the best jobs that anyone could ask for. Keep reading!”

Peter Rodgers
Editor, Physics World

Heat flow surprise in space

On Earth, convection or thermal diffusion dominates heat transfer. But these effects are extremely weak in zero gravity, and adiabatic compression can dominate. To observe local overheating and adiabatic compression, Régis Wunenburger from the University of Bordeaux and colleagues placed a small cylinder containing sulfur hexafluoride (SF₆) in both its liquid and gas forms on Mir last year. The compound was heated to just below 318.5 Kelvin – its liquid-vapour transition point – to reduce the effect of heat diffuison on the liquid and to induce adiabatic compression.

When the SF₆ was slowly heated above this point, Wunenburger and co-workers discovered that the expanding fluid outer layer squeezes the inner gas bubble within seconds. This process heats the inner core to temperatures some 23% higher than the cylinder walls. However, after 130 seconds or so, the system strives to reach equilibrium, and diffusion cools the gas to the same temperature as the liquid and walls.

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