Now Demosthenes Kivotides, Carlo Barenghi and David Samuels at the University of Newcastle in the UK have discovered another surprising feature of liquid helium. They have predicted that the coupled motion of the normal and superfluid components can give rise to an intriguing structure that contains three vortex rings (Science 2000 290 777).
In the February issue of Physics World, Charles Adams of the Department of Physics, University of Durham, UK, investigates this surprising phenomenon.
The so-called optical activity of materials has been studied extensively since then, so it is perhaps surprising that there is anything new to discover. However, José Goulon of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, and co-workers at the ESRF, the University of Paris and Purdue University in the US have recently observed a new type of optical activity using linearly polarized X-rays. The effect – known as non-reciprocal linear dichroism – was discovered in a chromium-doped vanadium-oxide crystal in which the magnetic moments of the atoms are aligned in opposite directions (J Goulon et al. 2000 Phys. Rev. Lett. 85 4385).
In the February issue of Physics World, Gerrit van der Laan of the Synchrotron Radiation Department, Daresbury Laboratory, Warrington, UK, describes the effect – which had been predicted theoretically – but has now been observed clearly for the first time.
Many icy bodies – ranging in diameter from under a metre to several kilometres – exist beyond the orbit of Neptune in a band known as the Kuiper belt. Current theories propose that the abundance of the objects grows exponentially as the size falls. But if very small objects are as common as this suggests, these Kuiper belt objects would violate Olbers’ principle – that is, they would make the night sky bright.
Scott Kenyon and Rogier Windhorst of the Smithsonian Astrophysical Observatory and Arizona State University realised that Olbers’ paradox imposed useful limits on the nature of Kuiper belt objects (KBOs). They analysed existing counts of larger KBOs and measurements of optical and infrared radiation from smaller objects to establish more accurately their size distribution. This in turn provides clues about evolution in the early solar system. “Our results suggest that – for the night sky to be dark – the size distribution of Kuiper belt objects must have two components”, Kenyon told PhysicsWeb. For large objects – that is, between 1 and 100 km across – Kenyon and Windhorst found that the abundance of objects towards the top of that range decreases sharply. But for objects less than a kilometre across, the distribution is much shallower – resulting in fewer very small fragments.
This discovery is consistent with classical theories of how particles collided and merged in the disk of gas and dust orbiting the Sun before the planets coalesced. “High speed collisions between smaller bodies produce smaller and smaller fragments. Low velocity collisions between the larger bodies produce larger and larger bodies,” said Kenyon. “The end result is two size distributions – a steep one for the larger bodies and a shallower one for the smaller bodies.”
The finding also lends weight to the idea that the KBOs formed around the same time as Neptune – if Neptune had reached its current size much earlier, it would have stirred up the small fragments before they had time to form the large KBOs we see today. Kenyon and Windhorst hope to use data from the Hubble Space Telescope to refine their results within the next year.
Liquid drops on a surface seek out areas that have a higher surface energy. The difference in surface energy produces a stress at the boundary between the liquid and the surface that pushes the droplet along. However, if the surface tension of the droplet varies across its surface, the droplet stretches out as it moves – this known as Marangoni flow. The surface tension can vary if the composition of the droplet is uneven – for example, if a volatile component of the liquid evaporates from the wall of the droplet – or if there is a temperature gradient across the droplet. But as the droplet stretches out into a thin half-cylinder shape, capillary forces emerge – and these oppose the flow.
Daniel and colleagues realised they could overcome the negative capillary flow by preparing a special surface for the droplets to travel across. They created a central water-repelling zone surrounded by a cooler, water-attracting surface. Steam was allowed to condense onto the water-repelling area, forming hot droplets – which quickly head towards the cool, water-attracting area. The high speed at which the droplet travels – up to 1.5 metres per second – reverses the direction of the capillary forces, and the droplet accelerates. The droplet coalesces with smaller droplets as it speeds across the surface, sweeping a path for fresh condensation to take place.
The phenomenon could improve many existing industrial processes. In particular, it could double the efficiency of traditional heat exchangers by rapidly dispersing the insulating layer of condensed steam that builds up on the ‘steam side’ of the heat-conducting surface, impeding heat exchange. Daniels and colleagues even suggest that their technique might help cool human body parts during cryogenic surgery.
When a certain type of star collapses into itself, the gravitational force is so strong that even light waves cannot escape – hence the expression ‘black hole’. In the 1980s, William Unruh of the University of British Columbia in Canada realised that sound waves in fluids can behave similarly to light waves in a gravitational field. Moreover, if a fluid travels faster than the light or sound waves within it, it should be possible to create an artificial black hole inside the fluid. The point at which the fluid speed overtakes the wave speed is equivalent to the event horizon of a true black hole – the ‘point of no return’ for energy and matter.
Ulf Leonhardt and Paul Piwnicki at the University of St Andrews in the UK plan to make artificial black holes using both light and sound waves. An ‘optical’ black hole may be created when laser light is ‘slowed down’ in a vapour of very cold atoms. Neil Turok of the University of Cambridge likens an optical black hole to a collection of mirrors: “The light will only be totally absorbed if you shine your torch in the right direction”, he told PhysicsWeb. ‘Sonic’ black holes, on the other hand, would exploit the unusual properties of a fluid known as a Bose-Einstein condensate. Peter Zoller’s team at the University of Innsbruck in Austria has devised a method to forcing the condensate through a nozzle to make it flow faster than the speed of sound. “Sonic black holes would be easier to create and will be more analogous to astrophysical black holes because the waves are completely trapped”, says Turok.
In the 1970s, Stephen Hawking of Cambridge University predicted that quantum effects at the event horizon of a black hole should release radiation into space. Astrophysicists have so far failed to observe this ‘Hawking radiation’ against the noise of the cosmic background radiation – and therefore to relate the large-scale structure of the Universe to the subatomic world. But it is likely that artificial black holes will exhibit a similar effect and this is a major goal of the new research.
But Turok cautions that we should not read too much into the analogy between artificial and real black holes because it is not perfect – for one thing artificial black holes do not involve gravity and distortions of space and time. “Nevertheless, this is a very exciting field”, he says, “and I’m sure something important will come out of it – even if it isn’t a true black hole”.
The eleven questions are:
* What is dark matter?
* What are the masses of the neutrinos, and how have they shaped the evolution of the universe?
* Are there additional spacetime dimensions?
* What is the nature of the dark energy?
* Are protons unstable?
* How did the Universe begin?
* Did Einstein have the last word on gravity?
* How do cosmic accelerators work and what are they accelerating?
* Are there new states of matter at exceedingly high density and temperature?
* Is a new theory of matter and light needed at the highest energies?
* How were the elements from iron to uranium made?
The committee is keen to bring physicists and astronomers together to tackle these questions, and is optimistic that the enormous leaps in technology – including the exponential growth of computing speed – and our understanding of the universe that have occurred over the last twenty years will bring fresh insights to the debate. A second report, due in late 2001, will prioritize the questions and make recommendations about funding.
Like all electromagnetic waves, radio waves contain vibrating electric and magnetic fields. In free space, these electric and magnetic fields are constrained to be perpendicular to each other, and to the direction of propagation. The waves can also be polarized. For instance, if the electric field vibrates only in the vertical direction, the wave is said to be vertically polarized. This polarization will not change as the wave travels through free space. In urban areas, however, radio signals are usually scattered by buildings and other large objects. This scattering effectively creates extra polarization states in all three spatial directions at a receiving antenna.
Now Michael Andrews and co-workers have exploited this fact using a group of three orthogonal antennae that can transmit or receive electric fields whatever their polarization. The new system – dubbed a tripole by the Bell Labs team – thus increases the rate at which data can be transmitted by three times compared with conventional ‘dual polarized’ radio signals.
Andrews and his colleagues demonstrated this improvement by transmitting an encoded image of a colour painting by Spanish artist Joan Miró. The red, green and blue components of the painting were transmitted at 880 MHz by each of the orthogonal antennae over a distance of 25 metres in the cafeteria at Bell Labs. The receiver was located around the corner from the transmitter so that it detected radio waves that had been scattered, and reconstructed them to form the full-colour image. The Bell Labs team has also demonstrated that it should be possible to further increase the capacity by a factor of six by using three magnetic-dipole antennae in addition to the tripole.
During the day, solar radiation excites atoms and molecules in the atmospheres of Earth and Venus to higher energy levels. As darkness falls, the atoms and molecules drop to lower energy states and emit radiation – this is ‘nightglow’. Slanger’s team used the Keck I telescope in Hawaii to study two distinct emission lines in the nightglow of Venus: green light emitted by atomic oxygen as it drops from a higher to a lower excited state, and red light subsequently emitted as it falls further to its ground state. In theory, every green emission could be followed by a red emission, but this only happens above an altitude of 200 km. Below this level the lower excited states are ‘quenched’ by collisions with other atoms and molecules.
Astronomers expect to see much greater levels of red emission because they believe that daytime solar energy breaks up carbon dioxide molecules high in the Venusian atmosphere to produce atomic oxygen that is mainly in its lower excited state. However, they found that the green emission is eight times stronger than the red emission. This could mean that the green light comes from a different process deeper in the Venusian atmosphere. Alternatively, scientists may need to modify their theories of the how excited states of oxygen are created and transported through the atmosphere.
The green line intensity of Venus is comparable with that from aurora on Earth – a phenomenon produced as the terrestrial magnetic field interacts with the solar wind. But the atmospheres of the two planets are enormously different in composition, temperature and pressure. “A green line does not mean that a planet has an oxygen atmosphere because Venus has extremely low levels of oxygen”, Slanger told PhysicsWeb. This result has implications for researchers studying the atmospheres of extra-solar planets.
The team also hopes its findings may shed light on the apparent variability of the emission lines: the Russian Venera orbiters visited Venus in 1975 and found no sign of the green signal. “We do not understand how the variability can be this large”, said Slanger, although the team speculates that the fluctuations could be connected with the solar cycle.
Ron Walsworth, Mikhail Lukin and colleagues at the Harvard-Smithsonian Center for Astrophysics trapped the pulses in a gas of rubidium atoms that had been cooled to between 70 and 90 kelvin (D F Phillips et al 2001 Phys. Rev. Lett. 86 783). Meanwhile, Lene Vestergaard Hau and co-workers at the Rowland Institute for Science and Harvard University used sodium atoms that had been cooled to 0.9 microkelvin in a magnetic trap (C Liu et al 2001 Nature 409 490).
In both experiments a coupling laser is used to drive transitions between two internal energy levels in the atoms. Next a weaker laser that is resonant with one of these levels and a third level is sent into the gas cell. Normally this probe laser would be absorbed by the gas. However, quantum interference effects caused by the first laser mean that the probe laser is slowed down rather than absorbed.
In 1999 Hau and co-workers showed that light could be slowed from 300 million metres per second to just 17 metres per second in a sodium gas. Now her group and that of Walsworth and Lukin have lowered the speed of light to absolute zero. The laser pulse in Hau’s experiment was 3.4 kilometres long in free space, yet it could be stored in a gas cell that was less than half a millimetre in size. In both experiments the laser pulse is trapped in the gas by turning off the coupling laser. The pulse can be regenerated by switching the coupling laser back on.
These experiments are the latest in a series of demonstrations of the novel properties of light in atomic gases. Last year, for instance, Lijun Wang and colleagues at NEC Research in Princeton used similar principles to send a laser pulse through a gas of caesium atoms at more than 300 times the speed of light.
Combustion often appears to occur spontaneously after a long period of stability. Charbel Tannous and Alain Fessant of the Université de Bretagne Occidentale in Brest recognised that this is similar to the behaviour of some stocks that tend to jump in value suddenly after long spells of stability – a quality known as ‘burstiness’. The pair decided to apply their knowledge of condensed matter physics to the fluctuations of the stock market. Their findings could help market analysts to predict when stock values will rise and fall, and by how much.
Simple models of combustion are based on changes in the concentration of fuel over time. Ignition takes place when the fuel concentration reaches a certain level. Tannous and Fessant modified the equations, replacing fuel concentration by the share prices of real companies. The researchers chose large and small companies from different industrial and economic backgrounds, and charted the variation in their share prices over five years.
For all six companies, the researchers found that the predictions of the combustion-inspired model very accurately matched the actual variation in the share price. “We would be delighted if financial analysts would consider deterministic approaches for predicting financial trends”, Tannous told PhysicsWeb. “Combustion theory offers a natural framework for the description of bursty financial time series.”
According to Tannous, this is the first time that combustion models have been used to predict the performance of companies. Tannous and Fessant do point out, however, that after the share value has jumped, market conditions become more important and the analogy with combustion models disappears.
