Page 1895 – Physics World

Home » Page 1895

Old galaxies, new problems

The expansion of the universe means that the light from distant galaxies is red-shifted when observed on Earth. The larger the red shift, the greater the distance to the galaxy. Moreover, when we view a galaxy with a high redshift, we are seeing it as it was billions of years ago. By measuring the red shifts of galaxies, therefore, astronomers can tell how far away the galaxy is and how old it was when it emitted the light that they are observing.

The Durham team, working with the 4.2 metre William Herschel Telescope in the Canary islands, the Calar Alto Telescope in Spain and the Hubble Space Telescope, counted the number of galaxies at different redshifts in a small region of the sky. They found that the number density of galaxies with very high redshifts – about 5 or 6 – was similar to the number of galaxies with small redshifts. A redshift of 6 corresponds to light emitted about 10 billion years ago, so the results imply that most galaxies were formed before this time – which is early in the history of the universe.

Most theories predict that galaxies formed fairly recently, at redshifts of about 1. Observing galaxies so much older than this means that a radical rethink is clearly required. Furthermore, the results open up the question of whether galaxies exist at even higher redshifts. The Durham team plan to search for these galaxies with the VISTA telescope, which is due to open in Chile in 2004, and the Infrared Telescope on Hawaii, which will be upgraded soon.

Sandage and Peebles win cosmology prize

Sandage is best known for his efforts to pin down the values of the Hubble constant, the age of the Universe and its deceleration parameter through observations. Peebles is a theoretical cosmologist who has worked on problems ranging from light-element synthesis to the nature of dark matter. The awards will be presented at the Pontifical Academy of Science at the Vatican on November 9.

Organic superconductivity

Hendrik Schon, Christian Kloc and Batlogg investigated three different acenes: anthracene (which contains three linked rings), tetracene (four) and pentacene (five). The crystals are normally insulating, but when a thin layer of the material was included in a field-effect transistor, it became superconducting. The superconducting transition temperature ranged from 2 Kelvin for pentacene to 4 Kelvin for anthracene. This variation is in line with what is expected if the superconductivity is based on electron-phonon coupling – the mechanism that explains conventional low-temperature superconductivity.

The electrons in the acenes are confined to move in two dimensions, and Batlogg and co-workers recently observed the fractional quantum Hall effect – one of the most widely studied phenomena in two-dimensional electron gases – in pentacene. The Bell Labs team also demonstrated laser action in tetracene earlier this year – the first time that electrically-driven laser action has been achieved in an organic material.

Writing in Nature about the experiment, Philip Phillips of the University of Illinois states that “the achievements of Batlogg and colleagues offer a new avenue for tuning the magnitude of the electron interactions to study almost any aspect of two-dimensional solid-state physics.”

DNA ‘tweezers’ take shape

Bernard Yurke from Lucent Technologies in the US, Andrew Turberfield from Oxford University in the UK and Lucent, and co-workers constructed the tweezers from three separate strands of DNA. DNA molecules are chains of four different bases – adenine, cytosine, guanine and thymine. Adenine will only bind to thymine, and cytosine will only bind to guanine. The first strand contained 40 bases, with the middle four acting as a hinge. Strand B bonded with the 18 bases on one side of the hinge, and strand C with the bases on the other side. Since two strands bonded together are much stiffer than a single strand, the tweezers, which are open to begin with, consist of two rigid arms with a flexible hinge in between, and the loose ends of strands B and C dangling freely.

The tweezers can be closed by adding a ‘fuel’ strand which bonds with the loose ends of both B and C, and so pulls the two rigid arms together. The tweezers can be opened again by adding an ‘anti-fuel’ strand, to which the fuel strand will bond in preference to B and C. The fuel and anti-fuel strands bond together to form a waste product that floats away, allowing the tweezers to open. To observe the opening and closing of such a tiny pair of tweezers, Yurke and co-workers added fluorescent dye molecules or “tags” to the ends of strand A.

The tweezers could be used to investigate chemical interactions, by attaching chemical components to its arms, or to control nanoscale machines. The Lucent team is also trying to attach electrically conducting molecules to the DNA strands to build molecular scale electronic circuits. “This technology has the potential to replace existing manufacturing methods for integrated circuits”, says Yurke.

Antimatter factory opens at CERN

Unlike most experiments at CERN, the £3.2 million Antimatter Decelerator (AD) is designed to slow down particles rather than accelerate them. The antiprotons are created when high-energy protons from the lab’s Proton Synchrotron strike an iridium target. The antiprotons are siphoned off and directed towards the AD, a 188m circumference ring, where they are focussed into a beam and slowed down.

The deceleration process causes the spread of energies in the beam to increase, and so the beam must be stabilized with a variety of stochastic and electron cooling techniques. The beam momentum is reduced from 3.57 GeV/c to about 100 MeV/c, which is low enough for detailed antimatter experiments, in several stages. The deceleration process takes about one minute and has an efficiency of about 25%.

Two of the experiments at the decelerator, ATRAP and ATHENA, will study antihydrogen, while the ASACUSA experiment will look at the ‘atomcules’ produced when an electron in a helium atom is replaced with an antiproton.

Antihydrogen was first produced with high-energy antiprotons at CERN in 1995, but the antiatoms did not survive long enough for them to be studied in experiments. A source of low energy antihydrogen would open up a number of exciting research possibilities. “For the first time we will be able to isolate and trap antihydrogen in a way which will enable precise analysis,” said CERN spokesman Neil Calder.

The ATRAP experiment is designed to produce ‘cold’ antihydrogen atoms by combining the antiprotons with positrons in a single Penning trap. The ATHENA project will try a slightly different approach, firing the antiprotons through a cloud of positrons in the hope that some will stick together. A major aim of both experiments is to carry out a detailed comparison of the atomic structures of hydrogen and antihydrogen. Any differences could help explain why our universe is dominated by matter.

Cluster takes shape

The Rumba and Tango satellites were sent into space on board a French-Russia launcher. Over the next week they will rendezvous with the Salsa and Samba satellites that were launched on July 16. The four satellites will undergo three months of tests before beginning their two-year scientific mission. Cluster is a joint mission between ESA and NASA, the European and American space agencies, and it will join two other joint missions – SOHO and Ulysses – that are studying the solar wind. Today’s launch comes four years after the rocket carrying the original Cluster mission exploded shortly after take off in 1996.

Dirac medal goes to particle theorists

Georgi, Pati and Quinn were honoured for their “pioneering contributions to the quest for a unified theory of quarks and leptons and the strong, weak and electromagnetic interactions.” Working with Salam, Pati developed the first gauge theory version of the standard model. Working with Sheldon Glashow – who shared the 1979 Nobel Prize for Physics with Salam and Steven Weinberg – Georgi discovered many of the most significant models for the grand unification of the strong and electroweak forces. Quinn is best known for her work on charge-parity symmetry. Georgi, Quinn and Weinberg also did important work together on unification.

New planets and star birth in Manchester

A team from the University of Colorado at Boulder will present evidence for what it believes is the youngest massive star cluster ever detected in the Milky Way. It contains about 100 type-O stars, the hottest and most massive stars in our galaxy. Astronomers believe they are less than a million years old. ‘These massive stars are in the birthing process’, explains Peter Conti of Boulder, ‘and they would make a very luminous ensemble if it were not for the absorption of optical light by dust in the Milky Way’. Clouds of gas and dust masking the cluster are a tell-tale sign of star formation.

Other talks at the conference cover the crop of newly found planets outside our solar system. Astronomers at the University of Texas have discovered a Jupiter-mass planet orbiting the nearby sun-like star Epsilon Eridani. A team from the University of California at Berkeley has observed three new extra-solar planets, and has tentative evidence that multi-planet systems may be more common than originally thought.

The IAU 24th General Assembly opens at the University of Manchester today and runs until 18 August.

Abraham Pais dies

His best-known work was a biography of Einstein, Subtle is the Lord: The Science and the Life of Albert Einstein, which was published in 1982 and won the 1983 American Book Award. Pais went on to publish, among other books, Niels Bohr’s Times in 1991 and Einstein Lived Here in 1994, and completed his autobiography, A Tale of Two Continents: a physicist’s life in a turbulent world in 1997. His most recent book, The Genius of Science: A Portrait Gallery, was published earlier this year.

Pais was born in Amsterdam in 1919, and became a talented mathematician and theoretical physicist. After World War II, during which many of his relatives died in Nazi concentration camps, Pais briefly worked with Niels Bohr in Copenhagen and then with Robert Oppenheimer at the Institute of Advanced Study at Princeton in the US. Pais made several important contributions to the foundations of the modern theory of particle physics during his time at Princeton. In 1954 he became a US citizen and moved to Rockefeller University, in New York City.

Promising new materials for better nuclear waste storage

The so-called ‘complex oxides’ are a group of ceramics with a common chemical formula consisting of two pairs of metallic cations and seven oxygen atoms. Sickafus and co-workers at Los Alamos National Laboratory, Imperial College and the University of Osaka used computer simulations to predict that the relative size of the cation pairs would determine a material’s resistance to radiation damage. Radioactive emissions from nuclear waste knock atoms out of place in container walls. The simulation showed that different-sized pairs of cations tend to produce highly regular atomic structures, similar to the mineral pyrochlore, in which the cation pairs can’t change places easily. The displaced atoms therefore tend to build up, eventually causing the material to swell and fracture.

‘If a material wants to be highly ordered, and radiation defects are putting atoms where the material doesn’t want them, that raises energy in the structure. Ultimately, the material may have so much energy that it will suffer unwanted structural damage’, Sickafus explains.

In contrast, ceramics with similar-sized cation pairs have less regular structures, like the mineral fluorite. But the pairs can easily swap places and the material is therefore likely to be more resistant to radiation damage because the displaced atoms can dissipate, rather than building up to critical concentrations. Sickafus thinks this could be a basic rule that applies to materials beyond those in this study, but says there is more work to be done.

The team has also performed some preliminary experiments, irradiating crystals with pyrochlore- and fluorite-type structures. These initial results strongly support their theory, and they plan to extend their investigations to other ceramics. ‘We are continuing irradiation studies on fluorite/pyrochlore compounds’, Sickafus told PhysicsWeb, ‘and we have already synthesized some cerium-containing fluorite compounds which we have shown are highly tolerant to radiation. We also think that similarly structured oxides containing actinides like depleted uranium will be robust under irradiation’.

According to the group, these new findings will be invaluable for the development of chemically durable and radiation-tolerant containers for safe and reliable storage of radioactive waste.

Browse articles by content type