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When less is better

Many physicists, including the staff at Physics World, gain a certain wry satisfaction from thinking about how easy today’s school science is. Priding ourselves on how difficult our exams were, we note that in the UK 29.4% of A-level physics students gained A grades this year, compared with 13.9% in 1990.

This disdain could be reinforced by new science GCSEs being introduced for 14–16 year olds this month. These courses, designed to appeal to the broad mass of students rather than just those who go on to study science at higher levels, emphasize the process of science, in other words how scientific knowledge is amassed and used (see pp12–13; print version only). As a result, they contain less actual scientific content.

For example, many students taking just the compulsory core GCSE in science will not learn Ohm’s law. Those who take either two or three science GCSEs will learn the famous equation, but only the latter will have to manipulate it. As the OCR’s Twenty First Century Science curriculum spells out, “rearrangement of the equation [R = V/I] is expected only on the higher tier”.

It is hard not to be shocked by how little maths today’s students of GCSE science actually need. But the new courses do have much to commend them. As the developers of the courses point out, the vast majority of students taking GCSE science will never become scientists themselves. They do not need a specialist’s training, but rather a broad understanding of, and appreciation for, modern science. Learning about the major themes in science and considering how science works should allow pupils to think more critically about the scientific issues that affect their lives, be it the supposed health risks of mobile phones or the claimed effects of global warming.

A broader approach with less rote learning of scientific content can also benefit pupils who are more able. Again, thinking of electric circuits, it is better that students have a clear physical understanding of why components connected in series and parallel exhibit different behaviour, rather than becoming expert in manipulating the associated equations. These details can be filled in later.

But will the courses encourage students to stick with science? Since 1982, the number of students sitting A-level physics has more than halved – a situation that the Confederation of British Industry last month warned will have serious implications for national and individual prosperity. Unfortunately, pilot studies of the new-style GCSEs indicate that while students appear to enjoy the courses, they are not that much more likely to take A-level physics as a result.

What is really needed to transform physics education in the UK is an injection of talented, enthusiastic physics teachers – currently only one in 10 science teachers in the UK has a physics degree. The government has taken some steps to address this problem but it remains a huge challenge. However, in the meantime, the new GCSE curricula should allow existing science teachers to provide lessons that are more inspiring. Doing so will be difficult for some teachers, who are more used to transmitting a fixed body of knowledge rather than discussing the often quite sophisticated ideas associated with the scientific process, such as uncertainty, causality and risk. But mastering this change of teaching style will be well worth it. The guiding principle should be to teach less and teach it better.

Nanotube resonators break gigahertz barrier

When taken together, these features are described as a “Holy Grail” of NEMS resonator performance (Phys.Rev.Lett 97 087203). Previous CNT resonators were limited to 200 MHz and could not operate at atmospheric pressure.

Created by Alex Zettl and colleagues at the University of California, Berkeley and the Lawrence Berkeley National Laboratory, the NEMS also proved to be very sensitive mass detectors with 10^-18g (attogram) resolution. The measurement of mass at the attogram scale was first demonstrated in 2004 by Harold Craighead and colleagues at Cornell University. Craighead used laser techniques to observe the changes in the resonant frequency of a NEMS cantilever.

While the Berkeley researchers have not achieved better mass resolution, their system employs electrical self detection and does not require the use of a laser. This could make the design more appropriate for the development of highly sensitive chemical and biological sensors.

According to Nickolay Lavrik, a nanotechnology and sensor specialist at Oak Ridge National Laboratory in the US, the Berkeley resonators represent an important step towards the development of these sensors. Lavrik told PhysicsWeb that CNTs show great promise for the development of ultra sensitive mass detectors because they can be self-assembled into very small structures down to the molecular scale. Indeed, Lavrik believes that CNTs have a good chance at achieving another Holy Grail of mass detection, zeptogram (10^-21g) sensitivity.

The Berkeley resonators comprised a CNT clamped between source and drain electrodes on a silicon substate. The midsection of the nanotube is free to vibrate above a trench containing a gate electrode. The gap between the source and drain was 300-1000 nm and the gap between the CNT and the gate was 200-500 nm.

The resonator is actuated by applying an RF signal at the gate. A carrier signal (which is a slightly different frequency than the actuation signal) is applied at the drain and the source is monitored using a lock-in amplifier. The CNTs were operated both bare and after being coated with metals such as indium. The researchers we also able to determine the quantity and mobility of excess charge that can build up on the CNT by employing two different methods for measuring the resonant properties of the NEMS.

Hidden hydrogen could force galaxy rethink

Deuterium — an isotope of hydrogen containing a proton plus a neutron — is cosmologically significant because it is continually destroyed in star cores by the same nuclear reactions that create helium and other heavier elements. The amount of deuterium is therefore far lower now than it was in the early universe. Indeed, the shortfall in the amount of deuterium allows scientists to track our galaxy’s chemical evolution over billions of years.

But what has mystified astronomers over the years is the fact that while primordial concentrations are about 27 parts deuterium per million hydrogen atoms (ppm), values in the Milky Way today range from 5-22 ppm. Three years ago, Bruce Draine from Princeton University developed a model that could explain this anomaly. He said that deuterium, compared to hydrogen, might preferentially bind to interstellar dust grains, changing from an easily detectable gas to an unobservable solid form.

That theory is now strongly supported by the new data from NASA’s Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, which can monitor the ultraviolet fingerprints of gaseous deuterium. The data, collected over the past six years, show that areas with large amounts of interstellar dust have low abundances of gaseous deuterium, while areas with little interstellar dust have high concentrations.

But surprisingly, the FUSE team has found present-day deuterium abundances are less than 15% below primordial concentrations — much larger than current assumptions based on theory that at least one-third was destroyed over time. This implies two possibilities: either significantly less material has been converted to helium and heavier elements in stars; or much more primordial gas rained down onto the galaxy over its lifetime than previously thought.

“Our results indicate some essential physical processes have been left out of the models,” says team member Jeffrey Linsky from the JILA lab in Boulder, Colorado. “New models of the galaxy’s chemical evolution need to be computed that can explain the new deuterium abundance.”

Neutrino Nobel laureate dies

Born in New York City in 1932, Schwartz studied physics at Columbia University, where he graduated in 1953. He stayed on at Columbia to do a PhD, carrying out experiments on Brookhaven’s first accelerator, the Cosmotron. In 1958 he was appointed assistant professor at Columbia, before becoming associate and then full professor in 1963.

It was while at Columbia, that Schwartz, Lederman and Steinberger performed their Nobel-prize-winning work. They wanted to find a way of producing beams of neutrinos so that they could study how the weak force behaves at high energies. At the time, the only way to study the weak force was through radioactive decay, which occurs at relatively low energies.

Schwartz, Lederman and Steinberger therefore designed an experiment using Brookhaven’s Alternating Gradient Synchrotron, in which a beam of high-energy protons was slammed into a metal beryllium target. The resulting cascade of particles included some that always came out in pairs consisting of a muon, which is a heavy version of an electron, and a neutrino.

A steel wall made from material from scrap warships was used to remove all particles except the neutrinos, which passed unhindered through the wall in the same direction as the protons. Schwartz and his co-workers were then able to detect the neutrinos by watching the sparks created when the particles interacted very occasionally with the atoms in a ten-ton aluminium detector.

Because the neutrino reactions only ever produced muons, rather than electrons, they concluded that neutrinos must come in at least two types — muon-neutrinos and electron neutrinos. In fact, we now know a third flavour of neutrino — tau neutrinos — also exists.

In 1965 Schwartz moved to Stanford University in California. But annoyed by the increasing size of physics collaborations, he took the unusual step of founding a computer security company called Digital Pathways in Silicon Valley in 1970. He initially remained at Stanford, but joined the firm full time in 1983 as chief executive.

In 1991 Schwartz moved back to physics, becoming associate director of high-energy and nuclear physics at Brookhaven, where he oversaw the construction of the four detectors at the lab’s Relativistic Heavy Ion Collider. He stayed there until 1994 before returning to Columbia until 1997.

Pluto relegated to dwarf status

Dissenting astronomers have called the decision arbitrary, pointing out that some other planets (including Earth) share their orbits with other bodies.

In addition to orbiting the sun and being rounded by its own gravitational field, the IAU definition of a “classical planet” requires an object to be the sole occupant of its orbit. A dwarf planet must only meet the first two criteria and cannot be a satellite. All other bodies in the solar system are referred to as “small solar system bodies”. IUA members voted yesterday to adopt this classification scheme and to classify Pluto as a dwarf planet.

There are three other dwarf planets – Charon, Ceres and UB313, which is informally known as Xena. Charon was once considered a moon of Pluto, but now the two bodies are called a “double-dwarf planet system”. Pluto was discovered in 1930 and Charon in 1978. Pluto also has two moons, Nix and Hydra, which were discovered in 2005.

The controversy was sparked by the discovery in 2005 of UB313. With a diameter of about 3000 km, UB313 is larger than Pluto (2300 km diameter) and occupies an orbit well beyond that of Pluto. More objects like UB313 are expected to be discovered in the future and many in the astronomical community do not wish to call these bodies planets. Others, however, are happy to see the number of planets increase with every discovery.

UPDATE 25 September 2006: NASA and others in the astronomical community are now calling Charon a moon of Pluto, not a dwarf planet.

Gravity lens reveals dark matter

NASA’s Chandra X-ray Observatory, the Hubble Space Telescope, the European Southern Observatory’s Very Large Telescope and the Magellan optical telescopes were used to observe the violent collision between two large galaxy clusters 3 billion light years away. The force of the collision separated the dark and luminous matter, allowing a clear identification. Although scientists are yet to determine what form this mysterious dark matter may take, the observations are strong evidence that most of the matter in the universe is dark (Astrophysical J. and Astrophysical J. Letters to be published).

Dark matter was originally hypothesized to explain the abnormally high rotation speeds of galaxies, which would otherwise be torn apart if they did not contain hidden mass. It is fundamentally different from normal “luminous” matter such as stars as it is invisible to modern telescopes, giving off no light or heat, and seems to interact only through gravity.

However, some scientists do not believe that dark matter exists and have proposed alternative theories — where gravity is stronger on intergalactic scales – to explain galactic dynamics. The new results are a blow to such theories. “Regardless of how one modifies gravity, it should still generally point to where most of the mass is,” says Maxim Markevitch at the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, who was involved in the research. “If the only matter in this cluster was visible matter, the mass map would approximately follow the interstellar gas map. Instead, we found most of the mass elsewhere, exactly where if it were dominated by collisionless dark matter.”

Behind these observations lies a remarkable bullet-shaped cloud of hot gas produced by the collision of two clusters. As they cross at 10 million miles per hour, the luminous matter in each interacts with the other and slows down. But the dark matter does not interact at all, passing right through without disruption. This causes the dark matter to sail ahead, separating each cluster into two components: dark matter in the lead and luminous matter lagging behind.

To detect this separation, researchers compared x-ray images of the luminous matter with measurements of the cluster’s total mass through gravitational lensing. This involves the observation of the distortion of light from background galaxies by the cluster’s gravity — the greater the distortion, the more massive the cluster. The team discovered four separate clumps of matter: two large clumps of dark matter speeding away from the collision, and two smaller clumps of luminous matter trailing behind, proving two types of matter exist.

The results have captured the imagination of the cosmology community. “This is an exciting discovery — dark matter is not merely a trick of the light,” says Robert Caldwell, who is a cosmologist at Dartmouth College, in New Hampshire. “This result helps confirm we’re on the right track in trying to solve the mystery of dark matter.”

Satellite protection plan could backfire

The calculations were carried out by a team led by space physicist Craig Rodger from the University of Otago in New Zealand. The work also suggest that the system would have a short-lived impact on the ozone layer, similar to that caused by solar storms or volcanic eruptions (Annales Geophysicae 24 2025-2041).

There are currently about 250 satellites in low-Earth orbit at altitudes of 160–480 km. These devices provide a diverse range of essential services such as long-distance radio communications, international broadcasting and GPS navigation services. However, satellites are vulnerable to failure caused by high-altitude nuclear airbursts or intense solar storms. Even a “small” nuclear detonation equivalent to 10 kilotons of TNT at heights of 125-300 km could cause 90% of satellites to fail within a month and delay manned spaceflights for a year or more.

A nuclear detonation releases vast numbers of electrons that would “pump up” the Van Allen belts, which are two bands of charged particles trapped high in the atmosphere by the Earth’s magnetic field. As these intense streams of belt electrons hit the satellites, they produce penetrating X-rays causing significant damage to onboard electronics and corrupting data sensors.

To mitigate this effect, the US Air Force and the US Defense Advanced Research Projects Agency have proposed a remediation system that uses radio waves at very low frequencies of about 20 kHz to flush the excess energetic particles from the belts and dump them into the upper atmosphere, well away from the satellites.

Rodgers and colleagues from the British Antarctic Survey, the Finnish Meteorological Institute and the Sodankylä Geophysical Observatory in Finland have performed calculations that suggest this process would significantly alter the upper atmosphere, albeit only temporarily. In particular, it would modify the ionosphere, which is vital for radio communications because it reflects signals making long-distance radio transmissions possible. The modified ionosphere would absorb radio waves, effectively shutting down communications – a disruption that worsens the longer the system is operated.

Unlike severe solar flares, where the effect is limited to the sunlit sector of the Earth, the disturbances from the remediation system could affect both the sunlit and night sectors worldwide and isolate some locations for up to a week. Airplane pilots and ships could lose contact and GPS systems are also unlikely to be immune as the deluge of particles will distort signals between satellites and receivers, the researchers say.

Although Rodger thinks the remediation system may be warranted in the extreme case of high-altitude nuclear explosions, he says it may be unsuitable for mitigating intense natural events such as solar flares and that policymakers should carefully consider its implications.

Top hydrogen-storing polymer revealed

A low-cost, high-capacity hydrogen-storage medium is essential for the commercialization of hydrogen fuel-cell technologies. Researchers had previously looked at carbon nanotubes, hydrogen-clathrate-hydrates and other nanostructured materials as ways of storing hydrogen, but they only work in fuel cells at low temperatures or high pressures. Now, Ihm and co-workers have shown that polymers covered with metal atoms can store a significant amount of hydrogen under more practical working conditions.

The large storage capacity is predicted because numerous hydrogen molecules are attracted to the metal atoms that lie along the polymer chain. Using a series of first-principles electronic-structure calculations, the physicists worked out how much energy the hydrogen molecules need to bind to the metal atoms. They looked at a wide combination of metal atoms (including titanium, scandium and vanadium), polymers (including polyacetylene, polypyrrole and polyaniline) and bonding sites for the hydrogen on the metal atoms.

The researchers found that a form of polyacetylene “decorated” with titanium atoms was the best. This molecule consists of a series of carbon atoms linked together in a chain by alternating single and double bonds. Each carbon atom has one hydrogen atom that can be replaced by a particular atom like titanium.

They found that up to five hydrogen molecules can be attached to each titanium atom in this particular form of polyacetylene, allowing the material to reversibly store 7.6 wt% of hydrogen, or 63 kilograms per cubic metre under practical working conditions. This value is higher than a target of 45 kilograms per cubic metre that the US Department of Energy said should be reached by 2010 (Phys. Rev. Lett. 97 056104).

“Our results will have considerable importance for experimentalists and engineers to synthesize metal-decorated polymers for hydrogen storage,” Ihm told PhysicsWeb. “Indeed, we have already begun to make some titanium-decorated polymers in collaboration with other researchers and are measuring their hydrogen-storage capacity now”.

See-through silver

It is well known that light travelling through a solid block will undergo total internal reflection if it strikes the surface at a very shallow angle. Some of the electromagnetic field, however, strays into the air — a so-called evanescent wave. This can form a beam of lower intensity if it passes into another solid block placed nearby. Hooper and colleagues then wondered what happens if each block is coated with another material. According to their calculations, light should be transmitted with perfect efficiency.

To find out what happens in practice, the researchers coated the surface of a silica prism with a film of zinc sulphide just 200 nm thick. They then clamped two such prisms together, leaving a very thin air gap sandwiched between them. When light of the correct wavelength was used, it was found to pass through the sandwich with about 85% efficiency. This was not quite perfect transmission because the zinc sulphide absorbs some light.

The physicists then replaced the air gap with a 40-nm thick layer of silver. On its own, silver of this thickness is almost opaque to light. But when sandwiched between the two coated prisms in this way, it was found to transmit light with an efficiency as high as 35% at certain wavelengths.

According to the team, the light is transmitted because light reflected from the silica/zinc selenide and the zinc selenide/silver interfaces are 180 degrees out of phase with each other and of equal amplitude. They therefore cancel out to give no net reflection – that is, all the light is transmitted. “The reflection from the front interface combined with the multiple reflections from the subsequent interfaces also interfere such that they cancel,” says Hooper. With no net reflection, and with a non-absorbing system, all the light must be transmitted.”

The researchers say their technique could be used to improve the efficiency of a new generation of “top emitting” OLEDs, whose performance is limited by the light passing through a metal cathode. It could also be used to improve semiconductor devices, where an analogous quantum effect should be seen. Here, semiconductors could be layered together to create a barrier through which a current could tunnel with less loss.

World’s most creative physicist revealed

Soler’s method involves calculating the number of references, n, that a particular paper makes to previous papers as well as the number of citations, m, that it receives from papers written at a later date. According to his definition of creativity, a paper that has lots of references but only a few citations will have a low level of “creativity”, while a paper with just a few references and lots of citations, in contrast, will have a very high creativity. The creativity index (C_a) of a particular scientist can then be calculated by summing the total creativity for every paper that author has written, normalized for the number of co-authors in each case.

Soler calculated a creativity index for the 10 physicists who have been cited most often according to data held in the Thomson-ISI Web of Knowledge. He found that Anderson — a condensed-matter theorist from Princeton University — has the highest C_a of 36.9. Witten, a string theorist from the Institute for Advanced Study in Princeton, has a C_a of 35.9, while Weinberg — the University of Texas theorist who shared the 1979 Nobel prize for electro-weak unification — is third with a C_a of 29.3. The average C_a of these ten most-cited physicists is 18.5.

The advantage of the new technique is that “review” articles, which are often highly cited even though they do not necessarily contain much new information, will have a relatively low creativity index because they contain so many references to previous work. The technique also means that citing your own paper will not boost your creativity index because the reference and citation counts will cancel out. Conventional citation analyses, in contrast, would not take this effect into account.

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