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Home » Page 1807

Stars make an early entrance

Stars are born when dense clouds of molecular hydrogen gas implode under the influence of their own gravity. As more and more gas is pulled in, pressure builds up and eventually triggers nucleosynthesis, the process that forges all heavier elements. At the ends of their lives, many stars explode in supernovae that eject heavy elements into space.

With a redshift of 6.42, quasar J1148+5251 is the most distant – and therefore the oldest – object ever observed. Walter and co-workers used the Very Large Array telescope in New Mexico to observe the galaxy around quasar J1148+5251. It is difficult to detect molecular hydrogen directly, but previous studies have strongly linked carbon monoxide with regions of star formation. This is because the atomic hydrogen formed after the big bang must cool in order to form molecular hydrogen, and the presence of carbon monoxide allows this heat to radiate away.

According to the team’s calculations, stars in the ancient galaxy may have formed at a rate of 3000 solar masses a year. The researchers also spotted the spectral signatures of heavy metals in the infrared emissions, which suggests that at least one cycle of stellar evolution must have taken place by this early stage.

Astronomers hope that the next generation of ultra-high-resolution radio telescopes, such as the Atacama Large Millimeter Array, will allow them to peer even further back in time to establish exactly when star formation began.

VIRGO takes off

Gravitational waves are ripples in the fabric of space-time that are produced when massive bodies accelerate through space. However, the waves are very weak – even for the strongest astrophysical sources, such as supernova explosions or collisions between neutron stars and black holes – and therefore extremely difficult to detect.

When a gravitational wave reaches the Earth it should expand space in one direction and contract it in the perpendicular direction. VIRGO relies on laser interferometers to detect the effect of these changes on pairs of widely separated test masses. The arms of the interferometer are 3 kilometres long and they are at right angles to each other (see figure). Changes in the interference pattern produced by two laser beams reflected off mirrors on the test masses will reflect changes in the length of the arms.

However, the changes caused by any gravitational wave will be extremely small – only about 10-21m – so the detector must be very sensitive. In particular, it must be seismically isolated from its environment, and it must use an ultrahigh vacuum cavity and the best possible mirrors and other optical components.

Virgo has passed preliminary tests and will begin to record data in a few months. It will join a global network of gravitational-wave detectors that already includes the two LIGO detectors (which are both 4 km long) in the US, GEO600 in Germany (600 m) and TAMA in Japan (300 m).

Particles win prizes

David Gross of the University of California at Santa Barbara, David Politzer of the California Institute of Technology and Frank Wilczek of the Massachusetts Institute of Technology share the 2003 High Energy Particle Physics Prize. Gross, Politzer and Wilczek are best known for their work on QCD – the theory of the strong force. In particular they showed that the force between two particles in certain types of gauge theories is strong when they are far apart, and weak when they are close together. This ‘asymptotic freedom’ was a crucial step in the development of QCD.

Nima Arkani-Hamed of Harvard University receives the Gribov Medal. In 1998, working with Savas Dimopoulos and Gia Dvali, Arkani-Hamed suggested that the extreme weakness of gravity could be explained by the existence of “large” extra dimensions of space, perhaps as large as 100 microns in size, and that quantum gravitational effects would become important at the electroweak scale rather than the much higher Planck scale. This would allow the effects of quantum gravity to be probed at accelerators and even in table-top experiments.

Guillaume Unal from the Université Paris Sud in France received the Young Particle Physicist Award for his work on charge-parity (CP) violation in the kaon particle. CP violation explains why the universe is made of matter and not antimatter, even though equal amounts of both should have been created in the Big Bang. Unal was also part of the team that discovered the top quark.

The Outreach Prize was shared by Rolf Landua of CERN and Nicholas Tracas of the National Technical University in Athens for communicating the results of high-energy physics research to universities, school students, teachers and the public.

Nanoparticles reinforce steel

Creep is a type of deformation that weakens metals and alloys when they are exposed to stress and high temperatures. To strengthen steel against creep, engineers disperse small particles – mostly oxides – throughout the metal, but this technique is expensive and not suitable for large-scale manufacturing. Moreover, the particles ‘coarsen’ with time and this reduces their strengthening effect.

In many strengthened steels, the creep rate increases abruptly after a certain period and the sample fractures. This so-called time-to-rupture depends on the amount of carbon in the steel. By adding just 0.002% carbon to martensitic steel that already contains 9% chromium, Sawada and colleagues were able to increase the time-to-rupture at 923 Kelvin by a factor of 100 over the strongest creep-resistant steel currently available (which contains about 0.08% carbon).

Using transmission electron microscopy, the Tsukuba team observed that its sample contained a large number of fine particles, between 5 and 10 nanometres in size. In contrast, conventional strengthened steels contain much larger particles, typically 100 to 300 nanometres across. The small particles consist of metal alloyed with either carbon or nitrogen and they are found ‘linked up’ in vulnerable regions such as grain boundaries and other boundaries. These links strengthen the steel.

The researchers say that the tiny carbon-nitrogen nanoparticles form because the sample contains so little carbon. The nanoparticles also coarsen at a much lower rate than larger particles, which increase the time-to-rupture.

Zealots put the spin into voting

In an election most people weigh up the pros and cons of the candidates before they choose who to vote for, but a few zealots may have strong, unchangeable views. In his simulations, Mobilia likens this situation to an array of randomly oriented magnetic spins, each of which can point “up” or “down”. An impurity with a spin that is fixed in one of these directions represents the zealot.

The amount of influence this impurity has on its neighbouring spins depends on how many interactions it has with them, and this is determined by the number of dimensions in the system. Mobilia calculated the effect of the impurity in one, two and three dimensions, where it would interact with two, four and six nearest neighbours, respectively.

He found that in a one-dimensional string of spins, the impurity swiftly aligned all of the spins with itself – the equivalent of a unanimous vote. In two dimensions, a ‘unanimous vote’ was also reached, but it took much longer for the spins to line up. In three dimensions, Mobilia found that the influence of the impurity is limited to a small region, which means that the zealot’s influence is greatly restricted.

“In real life, each voter interacts with more than two others, so it seems reasonable to expect that the two- and three-dimensional models would be more realistic than the one-dimensional version,” Mobilia told PhysicsWeb. “In three dimensions, not all of the voters are doomed to follow the zealot, and this seems more or less in agreement with our everyday experience.”

Mobilia is one of several researchers now refining the simple ‘voter model’ to include randomly scattered zealots, zealots with a range of opinions, and zealots with opposite views – for example, Democrat or Republican. Such sophisticated models could also improve our understanding of many physical systems, including the kinetics of certain chemical reactions, magnetic arrays and diffusion processes.

Nanoscale sensor approaches the quantum regime

The uncertainty principle states that we cannot know both the position and velocity of a quantum particle at the same time, thus placing a fundamental limit of the possible accuracy of any measurement. However, extremely precise measurements would be needed to observe these limits in a macroscopic object. Nanoelectromechanical devices might be able to make such measurements. In these devices a mechanical element moves in response to an external force, and a highly sensitive detector is used to record the displacement of the mechanical element.

Robert Knobel and Andrew Cleland of the University of California at Santa Barbara have built a device in which the mechanical element is a beam made from gallium arsenide that is fixed at both ends. The beam is about 3 microns long, 250 nanometres wide and 200 nanometres thick. Knobel and Cleland positioned the beam about 250 nanometres from a single-electron transistor – the ‘detector’ – and coupled the two together via a capacitor. They then applied a voltage to make the beam vibrate.

As the beam moves towards the detector, and then away from it, the amount of current flowing through the transistor changes. “The single-electron transistor is the world’s best sensor of charge, with a sensitivity of about one millionth of an electron,” Cleland told PhysicsWeb. “By measuring the current through the transistor, we can measure the vibration of the beam,” added Knobel. “Our result shows a clear path by which quantum-limited measurements can be made on macroscopic devices,” said Cleland. “Such a path has not been demonstrated before.”

The pair now hopes to make measurements in the quantum regime. To do this they will have to increase the sensitivity by a factor of 100 and the vibration frequency by a factor of 10.

Rashid Sunyaev wins cosmology prize

Sunyaev is a pioneer in the fields of X-ray astronomy and the cosmic background radiation, and in 1972, working with Yakov Zel’dovich, he predicted that the cosmic background would be cooled as it passes through hot gas. This effect, now known as the Sunyaev-Zel’dovich effect, is widely used in astrophysics and cosmology to determine absolute distances.

Previous winners of the Gruber prize include Jim Peebles and Allan Sandage (2000), Martin Rees (2001) and Vera Rubin (2002).

Seeing our own world rotate: the magnificent splendour of Foucault’s pendulum

The first Foucault’s pendulum I ever saw was at the Franklin Institute in Philadelphia, the city where I was born. The pendulum hung in a stairwell. Its wire was attached to the ceiling four stories up, while its silver bob glided silently back and forth over a globe in the floor, with Philadelphia at dead centre. The plane of its oscillation slowly shifted clockwise at an unchanging rate throughout the day: 9.6 º an hour. I’d stare at it, trying to make myself see it was the floor and I that moved – as the sign informed me – and not the pendulum itself. It troubled me that I was never sure I succeeded.

The pendulum’s installation in 1934 was cause for an unusual parade. The wire, which weighed only nine pounds, could not be rolled up but had to be kept straight to prevent stresses that would interfere with its swing. It was therefore carried, stretched out, through the streets of Philadelphia from the manufacturer. The bizarre procession of 11 men carrying a long wire was accompanied by a police escort, and trailed by bemused onlookers and reporters.

The pendulum’s movement was unforgettable, as inexorable a performance as I knew – then or now. The only human influence on it was the museum staff member who started it swinging just before the museum opened at 10 a.m. Once I heard a museum supporter had arranged, as a birthday gift, for his son to start the pendulum one day. How I envied that child. Other kids may have dreamed of tossing out the first pitch at a baseball game. I dreamed of starting a Foucault’s pendulum.

Splendeur magnifique

The story is well known how Jean-Bernard-Léon Foucault discovered that an oscillating pendulum continued to move in the same plane when its mount was turned. He realized that this effect, if magnified enough, could demonstrate the rotation of the Earth. In January 1851 he succeeded, using a pendulum suspended from the vault of his basement. In March of that year, Foucault set up a demonstration at the Panthéon, where it had a “splendeur magnifique”.

Few scientific experiments met such instantaneous fame, and 1851 was the year of the pendulum. Foucault’s pendulums proliferated: Oxford, Dublin, New York, Rio de Janeiro, Ceylon, Rome. Today, hundreds of such pendulums exist all over the world. The pendulum illustrates that every location on this planet is equally in motion – all are to that extent on a par. The United Nations headquarters in New York has a Foucault pendulum in the grand ceremonial staircase of its lobby. The Smithsonian Institution – America’s national museum – once also had a Foucault’s pendulum on display, but it was removed to make way for the restoration project of the Star-spangled Banner, the national symbol. The pendulum now lies in a storage area of the museum.

Foucault’s pendulum fascinates not because it proves the world rotates. All educated Europeans in 1851 knew that the Earth moved, although the evidence was based on astronomical inferences. The pendulum fascinates because it raises puzzling questions about perception itself. Do we really see the pendulum move but know it is really the Earth that moves? Or do we really see the Earth moving? For Foucault, it was the latter, and the pendulum speaks “directly to the eyes”.

Seeing the Earth rotate

Foucault’s remark is philosophically disingenuous: nothing speaks directly to the eyes. Instinctively Cartesian, Foucault felt that what he knew to be true had to be what he saw. Mentally modelling the pendulum oscillating against the background of the solar system, he thought what he saw was the Earth turn.

But perception is more complicated than that. Perceiving what is in motion – and what is at rest – depends on what we take as foreground and what as background. Foucault’s pendulum seems to offer us the experience either of the pendulum turning on the Earth, or of the Earth turning. This either/or resembles French philosopher Maurice Merleau-Ponty’s description and explanation of the familiar experience of being on a stationary train beside another train on a nearby track. When this other train begins to move, we experience either that we are beginning to move or that the other train is beginning to move in the opposite direction. Which sensation we experience, according to Merleau-Ponty, depends on where our perception is invested – in this train or the other – and on what is its background.

So when we see the plane of the pendulum’s oscillation move, we are taking the pendulum as foreground and the surrounding room as background. To “see” the Earth moving, we would have to introduce a different and much bigger background in which the pendulum’s plane of oscillation would be perceived as stationary and the floor and all the surroundings as moving. This could only happen, I have come to think, if the pendulum were mounted not inside but outside, with the heavens as the background. Could one see the Earth turn on a starry night?

The critical point

Foucault’s pendulum is different from other scientific demonstrations because of its size, non-interactive character, and the counterintuitive experience it suggests. It seems to reveal the limitations of human perception in perceiving the workings of nature. Small wonder that Foucault’s pendulum has played a key role in at least two novels, including an eponymously titled best-seller by Umberto Eco, and is the subject of a forthcoming popular book by Amir Aczel, entitled Pendulum: Léon Foucault and the Triumph of Science (2003 Pocket Books/Atria Books).

The Franklin Institute pendulum performed the same way each time I saw it. But it never failed to enthrall me due to its unsettling simplicity. It moved, but stayed the same. It turned, but told me that I was the one who was turning. I looked at it, and what it reflected back was my mobility and that of everything else around me – providing me with a clear and dramatic sense, whose true meaning I sensed I would never entirely fathom, of the deceptions and limits of my own perception and experience.

A new way to flip bits

In data storage, the reversal of magnetization is used to ‘write’ bits of information – strings of 1s and 0s – onto magnetic materials. The magnetization is usually reversed by applying a local magnetic field. To increase the amount of information stored in a device, it is necessary to use materials with increased magnetic energy densities. However, this means that higher magnetic fields are required to switch the magnetization. Such high fields are difficult to generate and researchers are looking for other ways to reverse the magnetization.

The Tohoku team applied an electric field of 1.5 megavolts per centimetre to a ferromagnetic semiconductor sample, and found that they could reverse the magnetization at a coercive field that was 5 times lower than when no external voltage was applied. The coercive field is the magnetic field that needs to be applied to a material to reduce its magnetization to zero. The sample was made of indium arsenide – a semiconductor – that had been doped with manganese, which is ferromagnetic.

Ohno says that the technique is a ‘demonstration of principle’ because it has only been made to work at temperatures of 30 kelvin so far. The researchers now hope to demonstrate the technique at room temperature so that it can be used in applications.

Fast and slow light made easy

Light travels at a speed of 300 million metres per second in a vacuum, but in recent years physicists have managed to slow laser pulses down to speeds of metres per second – or to bring them to a complete halt – in ultracold gases. In similar experiments physicists have observed superluminal or faster-than-light pulse propagation. These effects have also been observed in crystals at cryogenic temperatures and in “hot” gases. Now Matthew Bigelow, Nick Lepeshkin and Robert Boyd have observed the same effects in a much simpler system – a crystal at room temperature.

All the experiments exploit changes in the refractive index of an optical medium caused by quantum interference effects. Whereas previous experiments relied on a process known as “electromagnetically induced transparency”, the Rochester team exploited “coherent population oscillations” in the crystal. This involves shining two lasers – a pump beam and a weaker probe beam – at the crystal. Under certain conditions the probe beam experiences reduced absorption over a narrow range of wavelengths. The refractive index also increases rapidly in this “spectral hole”, which leads to a much reduced group velocity – the velocity at which a laser pulse travels – for the probe beam.

Earlier this year, the Rochester team used this technique to reduce the group velocity of a laser pulse to 58 metres per second in a ruby crystal at room temperature. Bigelow and co-workers have now repeated this feat in a crystal of alexandrite. Moreover, by using different wavelengths they can make a spectral “antihole” in which the absorption is higher, and which leads to superluminal propagation. They observed light speeds of 91 metres per second for a laser with a wavelength of 488 nanometres, and minus 800 metres per second for wavelengths of 476 nanometres. Negative speeds indicate superluminal velocities because the pulses appear to leave the crystal before they enter it under these conditions.

“Our technique is applicable to many solid materials, not just alexandrite,” Lepeshkin told PhysicsWeb. “Another important feature of our approach is the ability to cover a fairly broad range of optical frequencies.” The researchers will now investigate solid state materials with higher bandwidth to use in their system that are suitable for communication applications.

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