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Inner-ear mystery solved

The cochlea is a small seashell-shaped organ in the inner part of the ear where sound vibrations are converted into nerve impulses. These are then sent to the brain as electrical signals. The human cochlea occupies a volume of about 1 cubic centimetre and operates at frequencies between 20 hertz and 20 kilohertz. It can detect sounds over a range of 120 decibels.

Scientists have long wondered whether the cochlea’s coiled-up shape plays an important role in how it processes sound. Although intuition said it should, theoretical models did not support the idea. The new work by Manoussaki, who is an applied mathematician, and colleagues has now shown that the spiral shape probably increases hearing sensitivity to low frequency sounds by as much as 20 decibels.

When we hear something, the incoming sounds are transmitted via the tympanic membrane (or eardrum) to the basilar membrane, which runs along the length of the cochlea. The sound waves then cause the basilar membrane to vibrate. Manoussaki and co-workers looked at the equations that describe the mechanical interaction between this membrane and its surrounding fluid. In this way, they were able to describe how the amplitude of the membrane’s movement changes across the width in each section of the spiral tube.

The scientists found that the spiral shape makes the membrane deflect more towards the outside of the cochlea wall and less towards the inside. This difference in the membrane’s motion, which is effectively a tilt about a line running along its centre, increases with increasing radius of curvature and so enhances hearing sensitivity. “Since low frequency sounds are processed where the spiral curvature is greatest – at the apex of the cochlea – the effect will be more important for low-frequency sound waves,” explains Manoussaki.

The spiral structure may have developed because animals use low-frequency sounds, which travel the furthest of all sound waves, for communication and survival. The US team will now try to confirm its results by comparing cochlea in different mammals.

Mars mission passes key test

Past missions to the Red Planet have shown that water once flowed across its surface. For example, in 1999 the Mars Global Surveyor took pictures of gullies that indicated sources of liquid water at or near the planet surface, while in 2002 the Mars Odyssey Orbiter detected large amounts of hydrogen, implying vast water ice deposits in the upper three metres of Mars’ soil.

Despite these successes, it remains a mystery whether water existed for long enough on Mars to support life. The MRO is designed to find out when water existed on the Martian surface and where that water is now. To do so, it will fly in a low-altitude orbit over the next three years and collect data using a powerful suite of instruments.

A telescopic camera – the most powerful ever flown on a planetary exploration mission and capable of resolving objects as small as a dinner table – will take extreme close-up shots, while a spectrometer will map and analyze minerals. A radar instrument will look for subsurface water and rock, and others will trace the distribution of dust and water in the atmosphere, as well as monitor the Martian weather.

The mission should be able to establish whether the underground ice discovered by the Mars Odyssey Orbiter is the top layer of a deep ice deposit or merely a shallow layer linked to the seasonal cycle of water vapour in the Martian atmosphere. As an added bonus, the MRO will also look for safe and scientifically important landing sites for future exploration.

Over the next six months, the MRO will transform its current elongated elliptical orbit into a smaller circular orbit suitable for making its scientific measurements. It will do this by slowing itself down through a series of carefully calculated dips into the Martian atmosphere. Meanwhile, astronomers around the world will be waiting impatiently.

Tiny motor turns giant rods

Researchers in the Netherlands have made a light-driven nano-scale motor that can rotate microscale objects that are 10,000 times bigger than itself. The motor consists of a molecule embedded in a liquid-crystalline film with a glass rod placed on top. As the molecule changes shape, it alters the structure of the film, which in turn makes the rod move (Nature 440 163).

The motor, which was developed by Ben Feringa of the University of Groningen and colleagues at Eindhoven University of Technology and Philips Research Laboratories, consists of a dumb-bell shaped molecule with a central carbon-carbon double bond that functions as an axle. The upper part of the molecule acts as a rotor while the lower part is the stator. The molecules are incorporated into a liquid-crystalline film.

When the molecule is illuminated with light that has a wavelength of 365 nm, the “helicity” of the molecule changes from right- to left-handed. In fluid mechanics, helicity is the extent to which corkscrew-like motion occurs and a moving fluid has helicity if it rotates about an axis parallel to the direction of motion. If the rotation is clockwise when viewed from the “front” of the fluid, the helicity is right-handed and if anticlockwise, it is left-handed.

Nanomotor team

The changes to the molecule’s helicity in turn alter the direction of the molecules of the rod-shaped molecules in the liquid-crystal film itself. Removing the light source makes the molecule’s helicity switch back again, as can heating the molecule. The team found that the molecule could be rotated through 360° by carrying out two photochemical steps, each followed by a thermal step.

The team used the motors to move glass rods with dimensions of 5 microns by 28 microns. The rod rotated at an average speed of 0.67 rpm during the photochemical steps and at 0.22 rpm during the thermal steps. Feringa points out that the rotation is, however, caused by the collective action of several motor molecules. “The work is a stepping stone to functional motors that can act as tiny machines or perhaps power a nanocar”, says Feringa.

How to calculate hardness

Hardness is a measure of a material’s resistance to being scratched or dented and is measured using various experimental techniques, including the Vicker’s and Knoop tests. However, the values obtained often vary depending on the testing method — the Knoop diamond, for example, is sharper than the Vicker’s and gives a lower hardness. Indeed, experimental values of hardness can vary by more than 10% for the same material. Scientists have therefore been keen to devise a theoretical technique for predicting the hardness of a material with more certainty.

Three years ago, a team led by Faming Gao of Yanshan University in China took an important step towards this goal by developing a semi-empricial formula for the hardness of a material based on the length of the bonds between its components atoms, the number of electrons available for bonding, and the “ionicity”, which is the degree to which each pair of atoms shares the electrons between bonds. (In “covalent” materials like silicon the electrons are shared equally, whereas in “ionic” materials one atoms takes over its neighbour’s electrons entirely; “polar covalent” materials lie in-between.)

Simunek and Vackar have now taken this work a step further and devised a method for calculating the hardness of single crystals — both covalent and ionic — from first principles. Their trick is to introduce a new expression for hardness that describes the strength of a bond based on quantities inherently linked to the atomic structure of the material. Until now, scientists were not able to easily define hardness at the atomic scale.

Their new theory says that the hardness of an ideal single crystal is proportional to the bond strengths and to the number of bonds in a unit cell volume of the crystal. Using simple mathematics, Simunek and Vackar can then calculate the material’s hardness. Using their equation, the researchers have also found an unexpected result that contradicts conventional wisdom: atoms surrounded by relatively few other atoms — that is, those having a lower “co-ordination” number — are harder than those surrounded by lots of other atoms.

“Our work will lead to a deeper understanding of hardness,” Simunek told PhysicsWeb. “The approach will also tell materials scientists how to arrange atoms to make a hard structure for the first time.” The team now hopes to develop a complete theory for metallic bonds too.

The oldest explosion in the universe

Gamma-ray bursts are violent explosions that give off intense flashes of gamma rays that can last from a few milliseconds to about a hundred seconds. The initial burst of gamma rays is followed by an “afterglow” of longer wavelength radiation that can last for weeks or even years. Many astronomers believe that gamma-ray bursts happen when a massive star undergoes a supernova explosion at the end of its life and collapses to form a black hole.

GRB 050904, which lasted just 80 seconds, was detected on 4 September 2005 by NASA’s Swift satellite, which contains a gamma-ray monitor that was able to determine the position of the burst. Giancarlo Cusumano of the National Institute for Astrophysics in Palermo in Italy and colleagues describe how, within seconds, the satellite turned around to direct its built-in X-ray telescope at the region of sky where the burst had occurred (Nature 440164). This then allowed astronomers at other optical and infrared telescopes around the world to search and measure the burst’s afterglow.

A second team led by Nobuyuki Kawai at the Tokyo Institute of Technology in Japan reports on how it measured the afterglow of GRB 050904 at optical wavelengths (Nature 440 184). In the final paper, a group led by Daniel Reichart of the University of North Carolina in the US describes how it measured the afterglow at longer, near-infrared wavelengths (Nature 440 181).

In astronomy, distance, time and the wavelength at which observations are made are all linked. Light travels at a finite speed and so takes a certain time to reach us. Distant objects are therefore seen as they were in the far-off past. Cosmologists generally use the “redshift”, z, to define distance, or how far away in the past an object actually is.

With a redshift of about 6.3, GRB 050904 is the most distant stellar explosion ever observed and occurred when the universe was only about 900 million years old. The current age of the universe is thought to be 13.7 billion years. The researchers believe that other distant gamma-ray bursts could now be detected. Such bursts could be used to study how the first generation of stars in the early universe formed.

Antiproton co-discoverer dies

Born in San Francisco on 10 July 1920, Chamberlain obtained a bachelor’s degree from Dartmouth College in New Hampshire in 1941, before entering graduate school at the University of California, Berkeley. However, his studies were interrupted by America’s entry into the Second World War and in early 1942 Chamberlain joined the Manhattan atomic-bomb project, first at Berkeley and then at Los Alamos. There he worked under the supervision of Segrè, investigating the properties of neutrons and the spontaneous fission of heavy elements. Chamberlain was present at the first atomic-bomb test in New Mexico, losing a $5 bet that it would not go off.

After the war, Chamberlain resumed his graduate work at the University of Chicago under the supervision of Enrico Fermi. He completed his PhD on the diffraction of “slow” neutrons in liquids in 1948 and the same year took up a teaching position back at Berkeley. Chamberlain also began a series of experiments with Segrè and Clyde Wiegand on the “Bevatron” accelerator at Berkeley’s Radiation Laboratory, including several that involved smashing ordinary protons into copper. Chamberlain found that a few of the particles that streamed out from the collisions had the same mass as protons but curved the opposite way in a magnetic field: proof of the existence of the antiproton. Chamberlain announced the existence of the new particle in October 1955.

The existence of the antiproton was soon confirmed by his colleague Gerson Goldhaber, who saw tracks of antiprotons meeting protons in photographic emulsions and then annihilating in a burst of energy. Ironically, Goldhaber’s brother, the particle theorist Maurice Goldhaber, had laid down a $500 bet that the antiproton did not exist.

In 1958 Chamberlain was appointed a full professor at Berkeley and he soon adopted the laid-back mannerisms of the university, growing a large beard and long hair. He also spoke out on various issues such as human rights, free speech, and peace, and became involved in attempts to initiate a nuclear test-ban treaty. He remained at Berkeley until retiring in 1989, although he continued to attend weekly colloquia — including one the day before he died.

Writing in Physics World last year, his one-time PhD student Herbert Steiner said that although graduate students at Berkeley learned a lot from Segrè, they usually prefered to go to Chamberlain when they wanted to understand a tricky concept in physics. “Chamblerlain was very approachable and [was someone who] could invariably find his own unique explanation to [a] problem,” wrote Steiner.

Towards entangled-photon LEDs

Entanglement allows particles to have a much closer relationship than is possible in classical physics, and means that we can know the state of one photon by measuring the state of the other. For example, if one photon is horizontally polarized, then its entangled counterpart must have a vertically polarized spin, even if it is many kilometres away. In the source made by Shields and colleagues, the correlated photons that the quantum dot produces have the same polarisation and correlation is seen for not only horizontal and vertical polarizations but also for all possible directions of polarization.

Quantum dots

The team produced entangled photons from a crystal just 12 nm in diameter made from indium arsenide embedded within a gallium arsenide and aluminium arsenide cavity. When excited by a laser pulse, the quantum dot captures two electrons and two holes to form a “biexciton” state in the dot. One of the electrons recombines with a hole to create a photon, leaving behind an intermediate “exciton” state in the dot of one electron and one hole. The other electron-hole pair then combines to create a second photon.

In the lab

According to the team, the polarizations of the two emitted photons are governed by the spins of the electron and hole in the intermediate exciton state, which has two possible spin configurations. Recombination via one of these two states leads to the emission of a random mixture of two vertically polarized or two horizontally polarized photons. The researchers found that entangled photons were only produced by certain dots that have a symmetric shape.

Earlier work by the UK team was only able to produce entangled photons with an efficiency of 49%. The researchers have now improved on this result and have increased the efficiency to 70%, which approaches that required for useful applications. They did this by suppressing the amount of background light emitted by layers other than the quantum dot itself.

A unique feature of the new source is that it generates a pair of entangled photons “on demand”, that is, in response to an external trigger. “Such a source is essential for many applications, like quantum communications or computing, where gate operations are triggered by an external clock,” says Shields. Most entangled photons are currently produced through the technique of “parametric down-conversion” by shining a laser onto certain crystals.

Keeping cool with oxide films

Traditionally, most household fridges used environmentally damaging chemicals, such as chlorofluorocarbons, which damage the ozone layer. Although these chemicals have been phased out from domestic appliances, the search is still on for alternative cooling methods that do not use hazardous substances or are more efficient.

Such alternatives include magnetic refrigeration systems and the use of electrocaloric materials, which change temperature when an applied electric field is removed from them. These materials generated much interest in the 1960s and 1970s but were not exploited commercially because the electrocaloric effects were too small – just 2.5° for the best material at applied fields of 750 volts.

Mischenko and co-workers now report on a “giant electrocaloric effect” in perovskite PZT, which is an insulating oxide containing lead, zirconium and titanium. By measuring the polarization of thin films of the PZT at different temperatures coupled with some basic thermodynamic equations, the scientists calculated that the material cools down by as much as 12° in a field of just 25 V. This means that it is about a 100 times better at cooling surrounding matter than other known substances. Unfortunately, the effect is strongest at 222°C, which is well above room temperature.

Although practical applications are still some years away, electrocaloric thin films could be used to cool down electronic components like computer chips, and in biotechnology systems such as reactors and sensors and microelectromechanical and infrared imaging systems. The technology might also find use in automotive and aerospace applications, air conditioning and even domestic refrigeration.

The team now plans to introduce some dopants into the PZT to bring down the working temperature to near room temperature. It will also look for new lead-free electrocaloric materials.

Milky Way X-ray mystery solved

The conventional view of the galactic X-ray background is that it is produced by clouds of gas that have been heated to extremely high temperatures by supernova explosions. But there are problems with this picture: calculations suggest that there are not enough supernovae in our galaxy to replace the gas as it speeds out of the Milky Way.

Now, however, Revnivstev and colleagues think that the X-ray glow is produced by individual stars that are not very bright and so were missed in previous searches, like those made by the Chandra X-ray Observatory. The team came to their conclusion by analyzing a decade’s worth of data produced by NASA’s Rossi X-Ray Timing Explorer between 1995 and 2005. In doing so, they have also produced the most detailed ever map of the Milky Way at X-ray wavelengths.

According to the astronomers, the glow comes from two main sources: “cataclysmic variables” and “active stellar coronas”. Cataclysmic variables consist of a normal star and a white dwarf — an Earth-sized remnant of a star like our Sun that has run out if its nuclear fuel. Normally, a white dwarf is very dim and so cannot be detected, but in a binary it can pull matter from its companion and heat this gas up, which releases X-rays. Active stellar coronas are binary systems in which a normal star “stirs up” another star’s outer atmosphere (or corona) causing flares like those produced by our Sun. These also release X-rays.

The study suggests that there are about one million cataclysmic variables and around one billion active coronas, which suggests the number of stars in our galaxy has been greatly underestimated until now. The team now plans to confirm its results by making more sensitive measurements of the central part of the galaxy with Chandra.

According to the researchers, the X-ray background is like seeing the glow of a city by night when looked at from an aeroplane. “Only when you get closer do you see individual sources that make up the glow,” explains Revnivtsev.

Retrieving data from a black hole

Ever since Stephen Hawking showed that black holes radiate, physicists have wondered if the radiation contains information about the matter that formed the black hole. “It is a question that has fascinated researchers and excited controversy,” says Lloyd. This problem has been known as the black-hole “information paradox” since the 1970s, when Hawking applied quantum theory to black holes.

Classical physics says that black holes are regions of space where gravity is so strong that nothing, not even light, can escape from the “event horizon” surrounding the hole. However, Hawking showed that black holes actually have a temperature, which means they give off thermal radiation (now known as “Hawking radiation”) and should eventually evaporate altogether.

Hawking initially believed that this radiation did not contain any information. This meant that any information carried by light or matter falling into the black hole should disappear forever — even though this would violate quantum mechanics. In 2004, however, Hawking famously admitted to being wrong and said that information can escape from a black hole after all. In announcing his change of mind, he also lost a long-standing bet that he and Caltech theorist Kip Thorne had made with John Preskill (also of Caltech) about black holes.

Lloyd’s new calculations show that Hawking radiation contains a processed version of the information that went in and that this information escapes from the black hole via a model called “final-state projection”. This hypothesis says that quantum information can only take on a certain final state at “singularities”, such as at the end of the universe or at the centre of a black hole.

The idea that information might escape from a black hole via this model was first put forward in 2004 by US physicists Gary Horowitz and Juan Maldacena, but was criticized because interactions between the information inside the black hole and the Hawking radiation could interfere with the escape of information. Moreover, the final-state projection hypothesis is controversial in itself because it apparently allows faster-than-light travel of information escaping a black hole.

By using techniques from the theory of quantum information — in particular, quantum teleportation — Lloyd has now overcome these problems. He has shown that all, or almost all, of the information that goes into a black hole is “entangled” with the Hawking radiation and is preserved. As the black hole evaporates, the escaping information can be recovered with a “fidelity” of 0.85. This means that only about a half a bit of information is lost, regardless of how many bits were in the hole to start with.

According to Lloyd, the result means that black holes could one day function as information processors. However, knowing how to program them will depend on a full knowledge of quantum gravity, which we do not have yet, and an experimental confirmation of final-state projection.

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