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Liquid crystals single out microwaves

The refractive index of a liquid crystal depends on the polarization of the light travelling through it. This means that horizontally polarized light is transmitted when vertically polarized light is absorbed, and vice versa. Since the orientation of liquid crystals can be controlled by an applied voltage, the light travelling through a crystal can be switched on and off. This is the basis of liquid crystal displays.

Scientists have long known that this effect also works at microwave frequencies, but Yang and Sambles are among the first to exploit it. The device is based on a grating made from 55 one-mm-thick aluminium sheets interleaved with layers of liquid crystal 75 micrometres thick. This number of slits per millimetre is chosen to make the structure act as a ‘zero-order’ grating – which does not diffract the microwaves – for wavelengths greater than two millimetres. Yang and Sambles used microwaves with wavelengths between seven and 12 millimetres.

When the microwaves reach the grating, they create electromagnetic surface waves in the aluminium. These produce standing waves in the liquid-crystal-filled cavities of the grating, and microwaves with the same frequencies as these standing waves are transmitted. “Were it not for the resonant modes in the small gaps there would be no transmission”, Sambles told PhysicsWeb.

By varying the voltage applied across the liquid crystals between the aluminium slats, the researchers could adjust their orientation. This alters the frequency of the standing waves that arise in the slits, and the frequency of the microwaves that are transmitted.

Scientists realised only recently that these standing waves allow microwaves to pass through such narrow slits. “It had not been appreciated before that one could easily transmit so much energy through such a tiny area”, says Sambles. Yan and Sambles are optimistic that their device will prove useful in the communications industry, which is heavily dependent on high-frequency microwaves.

Magnetic polymer makes its debut

The building blocks of the polymer made by the Nebraska team contain 14 benzene molecules – rings of six carbon atoms and six hydrogen atoms. The electrons in the benzene ring become ‘dissociated’ and move freely around the rings – that is, the system is ‘conjugated’.

In each building block, the benzene rings are arranged into two ‘modules’. One module is a ring, or ‘macrocycle’, of eight benzene molecules. The other module contains the remaining six benzene molecules, and forms two branches that lead to the next building block. Magnetism arises in the polymer because the electrons behave differently in these two modules.

Dissociated electrons move freely through the macrocycle module of the unit, and their spins are strongly related, which gives the module a large magnetic moment. In contrast, the motion of the electrons in the two branches is limited, and this gives the second module a small magnetic moment.

When the building blocks are joined together to form the polymer, these strong and weak magnetic moments alternate along its length. The strong magnetic moments all point ‘upwards’ and the weak magnetic moments point either ‘upwards’ or ‘downwards’. As a result, the polymer has both ferromagnetic and antiferromagnetic properties.

Because the relationship between the alternating ‘modules’ in the polymer is weak, the magnetic properties only exist below 10 kelvin. At higher temperatures, the weak coupling is drowned out by thermal motion.

According to Rajca, the new polymer has a ‘saturation magnetization’ around twenty times weaker than iron at room temperature, but a hundred times stronger than recently discovered ‘magnetic carbon’. Saturation magnetization is the magnetic moment per unit volume when the spins in the material are fully aligned by an external magnetic field – this is a key measure of how ‘magnetic’ a material is.

Rajca is open-minded about possible uses for his team’s polymer, which is rather soft. Polymeric conductors are now used as light-emitting diodes in large-area displays, he points out, but researchers could not have predicted this application when the materials were invented 20 years ago. “The importance of our polymer is that it illustrates that conjugated organic polymers can be made magnetic” he told PhysicsWeb.

Star signs first light in cosmos

For 30 years astronomers have argued that clusters of densely packed stars, super-massive black holes and even planet-sized objects were the first objects in the universe. The diversity of these predictions is due to the thermal instability of the primordial gas and its chemically reactive nature – both of which are difficult to account for in analytical calculations. In contrast, the simulation by Abel and colleagues follows the chemical and radiative processes that occur in atoms and ions of hydrogen and helium.

Starting with a flat universe filled mostly with invisible ‘dark’ matter and 6% of ordinary matter, the simulation charts events that occurred some 13 million years after the big bang. Small density fluctuations prompted the formation of pre-galactic objects, which merge to form more massive structures. As these objects assemble, the primordial gas collects, cools and sinks to form a cold, dense cloud. Some 140 million years later, the core of this cloud had grown to be 100 times more massive than the Sun and it is sufficiently dense for hydrogen molecules to form.

According to the simulation, this ‘three-body’ formation of molecular hydrogen prevents the core from breaking up. Instead, a single proto-star, similar in mass to the Sun, forms at the centre of the core and rapidly accretes more mass to become a fully-fledged metal-free star. Abel and co-workers predict that the final mass of the star is between 30 and 300 solar masses, although the exact value depends on the detailed physics of the accretion. The team believes that such massive primordial stars offer a natural explanation for the absence of small metal-free stars in the Milky Way.

Accident grounds neutrino lab

The Super-Kamiokande experiment – located a kilometre underground in central Japan -consists of a tank filled with 50 000 tons of water surrounded by photomultiplier tubes. These tubes detect the faint flashes of light known as Cerenkov radiation given off by electron- and muon-neutrinos when they interact with electrons in the water molecules.

One of these tubes – each of which contains a vacuum – is thought to have imploded as the detector was being refilled with water following maintenance work. It is believed that the energy released from this implosion caused a shock wave in the water, leading to a chain reaction of implosions in the remaining tubes. According to the New York Times, about 7000 of the tubes imploded, which puts the cost of the accident at about $20 – $30m since each tube costs about $3000.

The director of the Kamioka Observatory, the organization that owns Super-Kamiokande, is putting on a brave face. “We will rebuild the detector,” says Yoji Totsuka. “There is no question”. Totsuka proposes to restore the number of photomultiplier tubes to about half of the original number in order to resume the K2K experiment as soon as possible, perhaps within a year. This experiment – in which a beam of muon neutrinos travels from the KEK particle physics lab on the east coast of Japan to Super-Kamiokande – needs to run for about another two years in order to confirm its initial results on neutrino mass.

Once this experiment is complete, Totsuka says that Super-Kamiokande will be upgraded in time to receive neutrinos from the Japanese Hadron Facility (JHF), which is due to be commissioned in 2007. The JHF will provide a beam of neutrinos much more intense than that from K2K, allowing more precise measurements of neutrino oscillations and other properties of neutrinos. The upgraded facility should also be able to detect the thousands of neutrinos produced by rare supernovae explosions in the Milky Way.

“It would be a great loss to the field not to get these results,” says Dave Wark, a particle physicist at the Rutherford Appleton laboratory in the UK. “In light of that I hope that Super-Kamiokande gets the support it needs to recover completely from this disaster. One should always remember that building state-of-the-art experiments like Super-Kamiokande always involves pushing the edge of what is technologically possible, so setbacks are inevitable”.

Old star caught in the act

When a star like the Sun approaches the end of its life, it ejects a shell of gas before it collapses into a dense white dwarf. The star loses gravitational energy as it collapses, and this energy is converted into ultraviolet light that streams out towards the ejected shell of gas, or nebula. This ultraviolet radiation excites atoms and molecules in the nebula and makes it emit radiation at a range of wavelengths, which can be studied by astronomers.

Gomez and colleagues used the Very Large Array telescope in New Mexico to analyse radio waves emitted by nebula K3-35, which is 16 000 light years from Earth. They found a tell-tale peak in the spectrum at 22 gigahertz, which is characteristic of water vapour ‘masers’ – short for microwave amplification by spontaneous emission of radiation. These masers amplify radio waves and show that water vapour is present in the nebula.

White dwarfs heat up as they collapse, and they reach temperatures of 30 000 kelvin within a hundred years. But astronomers believe that this extreme heat destroys the water vapour in the surrounding nebula. This means that the nebula observed by Gomez and team is less than a century old. “We are seeing this star during an extremely brief transition period in its life”, she says.

In order to establish when the K3-35 system began to collapse, the researchers compared their results with previous studies from the 1980s and 1960s. These indicated that the process began in 1984. “This is extremely exciting because we now have a laboratory for watching this process take place,” says Gomez. “We don’t fully understand everything we see in this object, but we can learn by watching it develop.”

Quickening pulses probe plasma

The force between two charged particles in a vacuum – whether attractive or repulsive – is governed by the Coulomb potential, and it gets stronger as the particles move closer together. But when many particles are present, this simple relationship gets complicated: a positively charged particle attracts a layer of negatively charged particles – and vice versa – and this shields it from interactions with more distant particles. This effect is known as screening.

Previous experiments appeared to show that this screening effect arose instantaneously, but this is because they could not detect changes on very short timescales. Now Leitenstorfer’s team have used a technique known as ultrafast laser spectroscopy, which is based on pulses of light just femtoseconds – 10^-15 seconds – long.

The team fired a pulse of red laser light at a slice of gallium arsenide to create a plasma of electrons and positive holes. Femtoseconds later, an infrared pulse probes the “complex dielectric function” of the plasma, which is a measure of how easily the electrons can be separated by an applied electric field. Leitenstorfer and colleagues repeated this process several times, with different intervals between the first and second pulses. This allowed them to build up a picture of how the dielectric function changes on very short timescales, and showed that the screening effect took around 70 femtoseconds to emerge.

“Our example is the build-up of Coulomb screening in an electron-hole plasma, but the phenomenon is very general”, Leitenstorfer told PhysicsWeb. “It could be important for future semiconductor devices, ultrafast photochemistry, nuclear collisions, superconductors and biological complexes”.

The discovery also shows that quantum effects play an important – but usually neglected – role in the dynamics of many-body systems. Conventional – or “semi-classical” – theories of condensed matter treat particles as perfect spheres that interact with no loss of energy, and these models work well in many cases. “But wave features like interference are ignored by semi-classical models,” explains Leitenstorfer, “and we have shown that these are important on short timescales”.

Canaries sing simple harmonics

Similar to the human larynx, a bird’s vocal organ – or syrinx – consists of folds of tissue in the passage that connects the lungs to the throat. As the bird expels air, these folds open and close to produce notes with frequencies between 1 and 2 kilohertz. Individual ‘syllables’ in the song last between 10 and 300 milliseconds.

Two factors control how sounds are produced – the pressure of the air entering the syrinx from the lungs, and the elasticity of the folds that make up the walls of the organ. When the air pressure exceeds a certain level, the folds oscillate. Gardner and colleagues recognised that this behaviour is analogous to the motion of a simple harmonic oscillator such as a mass on a spring.

The team developed a formula based on the equations of simple harmonic motion to relate the air pressure and elasticity to the pitch of the note produced. Using the formula, they accurately modelled the spectral characteristics of three typical notes in a canary’s song – a short falling note, a long rising note and a medium-length note that rises then falls.

Biologists are interested in the link between brain activity and song because fledglings learn to sing by listening to adult birdsong. This research requires an understanding of the underlying physical processes, which Gardener and colleagues have now helped to provide. Such studies may even shed light on how humans learn to speak – a neural process that is thought to be similar to the mimicking behaviour of young birds.

Shrinking electronics charges ahead

Schön and colleagues based their experiment on an insulating compound that contained just a few conducting molecules (J Schön et al 2001 Science 294 2138). They deposited a one-molecule-thick layer of this compound onto a silicon substrate, which acts as a gate electrode. The layer then ‘self-assembled’ on the surface – that is, the conducting molecules distributed themselves evenly among the more numerous insulating molecules. Gold source and drain electrodes were then added to each end of the organic layer.

Schön’s group measured the conductance of the layer at a variety of gate voltages, and showed that the layer behaved as a field-effect transistor. This means that the researchers could control its conductance by changing the gate voltage. But most importantly, the team found that, at low temperatures, the conductance was quantized in units related to electric charge. This suggests that individual conducting molecules are producing the conductance – which means that each molecule must behave as a field-effect transistor.

Schön and colleagues admit that further studies are needed to establish exactly how their device works, but believe that it is an important step towards molecular electronics.

Meanwhile, Leiber and colleagues used another so-called bottom-up technique to create logic circuits, including OR, AND and NOT gates (Yu Huang et al 2001 Science 294 1313). ‘Top down’ methods of miniaturization – such as etching and lithography – depend on carving ever-smaller structures from bulk material. These techniques will soon reach their limit, but Leiber’s group instead fabricated transistors from nanowires grown in solution.

The germanium and silicon wires – which are just tens of nanometres in diameter – are crossed and heated to create an insulating layer between them. These structures behave as transistors when electrodes are added, and Leiber and co-workers combined them to make logic circuits. Leiber’s team believes that this bottom-up approach could one day lead to chips that contain a million million devices per square centimetre.

Fresh light shed on black holes

Black holes are difficult to study because their gravity is so powerful that even light cannot escape it. But the gas and dust attracted to a black hole swirl around it in a flat ‘accretion disk’ before they are sucked in – and the particles in this disk glow because they are heated by friction. This radiation allows us to study the structure and formation of the disk, and can reveal clues about the black hole itself.

Spruit and colleagues observed X-rays and visible light from the disk around the black hole XTE J118+480. They found that the intensity of both signals varied periodically, but every jump in the optical output lagged behind the corresponding increase in the X-ray output by half a second.

Spruit and colleagues initially thought that the well-known ‘X-ray reprocessing’ effect could explain their observations. In this process, X-rays from the inner portion of the accretion disk heat up gas and dust at the outer edge of the disk, making it emit visible light. But the team ruled this out when they found that the optical signal changed in intensity every few tens of milliseconds, whereas the X-ray intensity changed only every few seconds.

The researchers studied the fluctuations in the intensity of the visible light to determine the size of the region that emitted it. Since the intensity changed significantly on timescales of around hundred milliseconds, the region that produced the light could not be larger than the distance light can travel in that time – that is, about 30 000 kilometres.

To propose an alternative to X-ray reprocessing, Spruit and colleagues combined the estimate of the size of the emitting region with luminosity measurements of the visible light. Their calculations led them to suggest that the light is cyclosynchrotron radiation – radiation generated by charged particles accelerated in a circular path by a strong magnetic field. According to the team, the strength of the magnetic field at around 20 000 kilometres from the black hole would generate light at the luminosity that they observed.

Rave review for Hawking’s new book

The Universe in a Nutshell
Stephen Hawking
2001 Bantam Press 224pp £20.00/$35.00hb

It is said that one of the world’s most eminent quantum-gravity theorists was once asked to explain in his institution’s annual report what he did. He declined, claiming that his work was far too complicated for the general public to understand. All the more credit to Stephen Hawking for having seized the challenge.

Hawking’s previous attempt at popularization – A Brief History of Time – succeeded in sales beyond his wildest dreams. However, I never managed to steer my way through the realm of imaginary time and neither, I would guess, did 99.9999% of the other 25 million or so readers. Indeed, one can be virtually certain that very few of them went beyond chapter 3.

In fact, Hawking has got the message. The Universe in a Nutshell is different. It is far more accessible, and it is full of Hawking’s wry humour. One can select chapters to read without needing to have mastered earlier chapters. One can even cherry pick the juicy bits with little loss of content. And the highlights are well worth reading, in large part because of Hawking’s caustic asides and his infallible optimism.

The ebb and flow of time is a recurring theme that appears and reappears in the book. I must confess that I have still not completely got to grips with imaginary time, but the nutshell metaphor conjures up a much warmer image. The nutshell represents a “hypersphere” in which the roles of space and time have become reversed. It provides a somewhat individualistic approach to quantum gravity, but one that is near to the forefront of current research. In a nutshell, this mixing up of space and time is what happens just inside a black hole as one approaches the black-hole horizon and crosses the point of no return. This is standard black-hole physics, as laid down by Einstein’s theory of gravitation, the theory of general relativity.

The classical view of a black hole is marred by one ugly feature: at the core of the black hole lies a singularity. This is a forbidding concept, since literally all hell may break loose should one get too near to the singularity. Hawking is convinced that such a singularity is never accessible, or “naked”: it is always shrouded by the black-hole horizon. In other words, we can live our lives without undue fear of the horrors of confronting a singularity, with the inevitable breakdown of the physical laws that govern our existence and even our sanity.

Were we to find a naked singularity, it would immediately allow us to extract unlimited resources from other universes. Miracles could be performed. Time travel would become feasible, since space and time reverse their roles. One could travel in time, either far into the future to escape any of the unfortunate calamities of our current era, or into the past, to pursue our dreams of long-lost Elysian fields.

It is this prospect that horrifies Hawking, for one could, if sufficiently perverse, go back in time, seek out one’s grandfather in his infancy, and murder him in order to challenge future generations of physicists. For now our notions of causality would be overturned: the impossible is possible, and there is a fundamental contradiction in the laws of physics.

Kip Thorne proposed a solution to this paradox. Quantum gravity, he argued, tells us that such ventures are subject to the laws of uncertainty. The probability of actually finding a particular individual at a particular place and time would be vanishingly small. Time travel is a good exercise for the statistically inclined. Hawking’s take on time travel is different: he argues that one could never succeed in constructing a workable time machine.

In one of the great tours de force of modern cosmology, Hawking – working with Roger Penrose – predicted that if the universe were causal, and time machines could never exist, then the universe must have begun from a physical singularity. This would be a catastrophe for cosmologists with fundamentalist inclinations.

As with religious movements, cosmologists who deal with the very early universe fall into two schools: the phenomenologists and the fundamentalists. The former deal with data, adopting empirical theory with its lack of rigour and all of its inevitable flaws to match. The latter start from pure mathematics, appeal to beauty and simplicity to guide the physics, and say to hell with any data that happen to clash with the theory. Not that there is usually much in the way of data, apart from rare exceptions.

Here is where the nutshell cosmology enters. The flaw in the deduction about the past singularity is that the theory of general relativity made no allowance for quantum gravity. According to Hawking, quantum gravity mathematically (thanks to imaginary time) provides a dual and singularity-free description of the universe, in which the roles of space and time are reversed. Time has no boundaries, nor does space.

Can we believe a word of this? The answer seems to be that as long as two alternative theories make identical predictions, it is meaningless to debate which one is true. This is positivist thinking. Theory never advances by proving anything is actually true, but rather by leading to predictions that conflict with observable data. All we can ever hope to do is to falsify a theory; then we move on to the next one.

The nutshell theory makes absolutely no predictions that are verifiable, at least not yet. But then neither does its immediate rival, superstring theory, now incorporated into the theory of M-branes, which consist of higher-dimensional space-time manifolds collectively called “p-branes”, where p stands for any integer that represents the dimensionality of space. Physicists would very much like to be able to predict p from first principles.

We know that p cannot be 1 or 2, for we would be, respectively, sausage-like or pancake-like, with the accompanying adverse effects on our digestive systems, among other problems. Quantum gravity can be resolved, at least in principle, in spaces of higher dimensions. This has been one of the great messages from superstring theory. Troublesome infinities (and infinity really is troublesome to a physicist) can be removed if we settle for p in some higher-dimensional space. The preferred number is 10, although some hold out for 4. Certainly 3 is insufficient, for one needs the extra freedom of higher dimensionality.

However, there is a price to pay. The nutshell – and indeed any quantum-gravitational theory – gives rise to too many descriptions of our past. Most can have no bearing on reality. The universe would not resemble our observed universe. It might be immensely more chaotic or quiescent. Either would be a disaster for the predictive power of a fundamental theory. In the absence of any predictions, this line of reasoning about possible outcomes only takes us so far. Hawking, in good company, supplements it with the “anthropic principle”.

Scientists, especially cosmologists, love principles. After all, the cosmological principle carried Einstein far, if at first in the wrong direction. The anthropic principle asserts that the universe is just so because we are here. If it were any different, there would be no cosmologists to observe it (see “Life, the cosmos and everything” Physics World October pp23-25).

One can now supplement the cosmic nutshell with this cosmic principle, and our very own big bang emerges. One can understand, so we are told, why the universe is so vast and relatively uniform, and why it is just beginning to undergo a phase of acceleration away from the big bang.

So we seem to be in good shape. Never mind that the most powerful minds in physics, so they tell us, have been working on superstring theory for two decades and have yet to show us a single unambiguous and experimentally verifiable prediction. Certainly, it is a phenomenally difficult theory. Is the anthropic principle part of the solution? I do not wish to overly dampen the party spirit, but I am underwhelmed by it.

The anthropic principle is one of the more remarkable swindles in physics. Indeed it is metaphysics, and that is the essence of the problem for most physicists in accepting it. The anthropic logic is either immensely subtle, by arguing that we, via our mere existence, control the cosmos, or unabashedly naive, by setting aside any physics explanations that any ultimate theory of physics might reasonably be expected to deliver. Metaphysics lacks predictive power, the very core of physics. The anthropic principle is an extreme expression of our ignorance.

It may well be that the ultimate theory of cosmology will have anthropic ramifications. We are some way yet from this promised land. In the meantime, Hawking’s book is a delight to read. It discusses questions that are at the forefront of current thinking about quantum gravity, yet for the most part is highly readable. It conveys the author’s sense of wonder and awe at the cosmos, and – like a child stepping into the darkness – illustrates his tentative gropings towards the ultimate theory of everything.

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