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Grand challenges for physics in the US

The challenges and recommendations are listed in the final report of the Physics in a New Era series, which has been produced for the board on physics and astronomy of the US National Research Council (NRC) by a committee chaired by Thomas Appelquist of Yale University. Previous reports in the series have covered all the major subfields of physics. The NRC traditionally conducts a survey of physics every decade.

The six grand challenges identified by the panel are: developing quantum technologies; understanding complex systems; applying physics to biology; creating new materials; exploring the universe; and unifying the forces of nature. The committee identified the six areas “based on their intrinsic scientific importance, their potential for broad impact and application, and their promise for major progress during the next decade.”

The committee also recommends that federal investment in basic physics research in the US relative to GDP should be restored to the levels of the early 1980s, thereby reversing a fall of 20% in real terms over this period. The panel also advises physics departments to revise their curricula to ensure that they appeal to a wide range of students and make connections to other areas of science and technology. Other recommendations cover the need to support small groups and single investigators at universities, to re-establish long-term basic research related to national security, and to encourage partnerships between government, universities and industry.

The panel also calls on the US government to “develop effective mechanisms for US participation and leadership in international scientific projects, including clear criteria for entrance and exit.”

Linear collider race gets serious

An international group of particle physicists based at the DESY laboratory in Hamburg, Germany, has staked its claim to host the world’s next major accelerator by unveiling plans for a Euro 3.136bn (about $2.8bn) linear collider. Last month the TESLA collaboration revealed its design for a 33 km long collider that would smash electrons and positrons together at energies of 500 billion electron volts (0.5 TeV) to probe physics beyond the Standard Model. The machine would also incorporate a powerful X-ray laser to carry out research in condensed-matter physics, chemistry, biology and materials science.

TESLA is one of three proposals for a next-generation linear collider, but is the first to be fully costed and made public. The Stanford Linear Accelerator Centre in the US and the KEK laboratory in Japan are collaborating on R&D for two similar designs – the Next Linear Collider (NLC) and the Japanese Linear Collider (JLC), while CERN is working on a design called CLIC (Physics World September 1999 pp8-9).

Different approaches

Most particle physicists believe that a linear collider is the obvious machine to complement the 14 TeV Large Hadron Collider (LHC), which is due to come on line at CERN in 2006. By colliding point-like particles, a linear collider would be able to make precise measurements on new particles discovered in the higher-energy collisions between protons, which are composite particles, at the LHC. These new particles could include the Higgs boson and more exotic “supersymmetric” particles not predicted by the Standard Model.

But at several billion dollars each, the world will almost certainly pay for only one linear collider. CLIC would operate at the highest energies – at about 3 TeV – but it would not be ready to be built until well into the next decade. That leaves TESLA and the NLC or the JLC as the most likely candidates, since all could be constructed by about 2010.

The German Science Council, Germany’s most senior scientific advisory body, will now review the TESLA technical-design report and is expected to present its conclusions to the federal government by 2002. Albrecht Wagner, director of the DESY lab and chairman of the board of the TESLA collaboration, says the government has “expressed an interest in hosting the collider, provided that the international community is behind the project and the technology is in place”.

Unlike the other designs, which would use conventional accelerator technology, the TESLA machine would be made from superconducting cavities. At 33 km long this limits the collider to a maximum energy of 0.8 TeV – less than the 1 TeV that the NLC or the JLC could reach in theory – but the collision at TESLA rate would be higher.

The TESLA design report states that the “advantage of superconducting technology, combined with the high efficiency to convert electrical energy to beam energy, has been acknowledged from the very beginning of the R&D on linear colliders”. Superconducting cavities, however, have always been considered prohibitively expensive. But the TESLA collaboration has succeeded in reducing the cost significantly over the past decade, and the cost of the collider is now comparable to the other designs. The NLC would cost about $4bn, but it would not include a free-electron laser. TESLA’s detector would cost Euro 210m, with the X-ray laser laboratory adding an extra Euro 531m.

Researchers at DESY have operated a 300 m prototype of TESLA for more than 8600 hours. “TESLA has worked in a test environment, so we can put forward a proposal where we can be confident that we can build a machine tomorrow if we were given the money,” says Wagner.

Dave Burke, the NLC project leader, believes that both the TESLA and the NLC/JLC designs are “very strong”, and says that the American particle-physics community “must now put its cards on the table” by endorsing the concept of a linear collider. This backing will probably come in the Autumn when a high-level panel of particle physicists presents its conclusions on the future of the discipline (Physics World March p9, print version only).

Burke says that until now US particle physicists have maintained that any linear collider should operate at 1-1.5 TeV in order to fully complement the LHC, and they have not given their support to a 0.5 TeV machine. “But the panel may not take this view in the light of the low Higgs mass tentatively discovered at CERN,” he says. He adds that there are still technical problems to be overcome if the NLC is to reach energies of 1 TeV or more.

The International Committee for Future Accelerators is carrying out a comparative study of the different designs and is due to publish a report by the end of the year. Both camps are confident that the ICFA will endorse their designs, but the decision about which proposal to fund and where to build any linear collider may simply come down to politics. In the end, says Burke, it could be the government that puts up the most money that hosts the machine.

A global venture

Wagner believes that some of the political heat about where to build the machine, if such a machine is to be built, could be removed by modelling it on the so-called “Global Accelerator Network”. Devised by the ICFA, this is a blueprint for any future large scientific facility. “Projects of the size and complexity of TESLA should be truly international,” says Wagner. “The idea behind the network is to ensure that all the key players have a stake in any future accelerator. Each country or lab would be responsible for building and running a part of the machine and so the site selection should become less of an issue.” He adds that there could be three control rooms for a linear collider: one in the US, one in Europe and one in Japan. “This would mean that each region is involved in the day-to-day running of the machine – and they would have no more night shifts to cope with.”

Burke underlines the importance of building a linear collider, of whatever variety, rather than choosing a particular design. But he admits that American particle physics would suffer if the US could not host a major facility in the future: “Scientists thrive on excitement. Personally speaking, in order to attract high-energy physicists we need to have a frontier machine on our soil.”

First sighting of dark matter

Oppenheimer and co-workers suspected that some very faint objects in existing photographs could be white dwarfs – Earth-sized remnants of ancient stars. The team collected spectra of the mysterious bodies at the Cerro Tololo Interamerican Observatory in Chile and found that over half of them were a new breed of ultracool white dwarf. “We’ve found a previously undetected population of stars in the galactic halo that represents a fraction of the dark matter in the galaxy”, says Oppenheimer.

It is thought that ‘cold’ white dwarfs – so-called because their surface temperatures are below 4500 kelvin – are very faint because of the way hydrogen molecules behave in their atmospheres. Astronomers believe the molecules collide and take on temporary molecular moments, which makes them absorb light at most visible wavelengths.

An earlier investigation hinted at the existence of a population of dim stars when it found that light from distant bright stars was bent by invisible objects closer to the Milky Way – a phenomenon known as microlensing (see Controversy reigns over ‘dark matter’ claim). “These indirect observations indicated that white dwarfs may make up a substantial fraction of dark matter”, explains Oppenheimer, “but the results were interpreted differently by some astronomers”.

Theories of star formation and the history of the galaxy will also benefit from the new discovery. “These cool white dwarfs are fossils of the early population of halo stars”, says team member Didier Saumon. “This discovery will branch out into many different areas of astrophysics”.

Neutrino messages from across the Universe

Neutrinos are fundamental particles with very low mass and no electric charge. They can travel enormous distances across the Universe – carrying information about their sources – because they barely interact with matter. Low-energy neutrinos from the Sun have already been observed in small detectors. But high-energy neutrinos from cosmic sources are extremely difficult to spot, and detectors must be very large to pick up them up.

The ‘Antarctic muon and neutrino detector array’ – AMANDA – spans a cylindrical volume of ice 500 metres deep and 120 metres in diameter, and is buried 1.5 kilometres beneath the surface. As neutrinos travel through the ice they collide with nucleons – protons and neutrons – within the ice and surrounding rock. These collisions create muons, heavier cousins of the electron, which emit blue and ultraviolet light as they travel through the ice. This light is ‘Cerenkov radiation’, which is emitted by all charged particles travelling faster through a medium than the speed of light in that medium.

AMANDA’s 302 photomultipliers detect the Cerenkov radiation to reveal the ‘decay path’ of the muon, and in turn, information about the neutrino and its source. The experiment collected more accurate data in 138 days than existing experiments had gathered over much longer periods. “We are already working hard to scrutinize the data for signs of any astrophysical phenomena – from gamma-ray bursts to dark matter or magnetic monopoles”, Halzen told PhysicsWeb.

The experiment successfully captured muons created by high-energy neutrinos travelling ‘upwards’ through the ice, which must have passed through the Earth. It filtered out ‘downward’ muons created by cosmic ray interactions in the atmosphere rather than by neutrinos. Now that the technique has been proved, Halzen’s team plans to build a much larger detector to pick up more neutrinos from deep in the Universe. The ‘IceCube’ experiment will spread 4800 photomultipliers over a square kilometre.

“Whenever a new instrument for studying the sky is launched, unexpected discoveries are made – and they are invariably more interesting than what had been anticipated”, Halzen says. “We very much hope to continue this trend”.

Deepest ever picture of the universe reveals new quasar

The Chandra telescope scrutinised X-ray signals from an exceptionally clear patch of the southern sky known as the Chandra Deep Field South. The high-resolution telescope identified over 300 separate X-ray sources. Astronomers at the Very Large Telescope then obtained the infrared and visible spectra of over 100 of the sources.

The X-ray sources are ‘active galactic nuclei’ (AGNs) and astronomers believe that they are the main contributors to the X-ray background. At about 8000 million light years away, we see the AGNs as they were when the universe was about half its present age. “In essence, it is like seeing galaxies similar to our own Milky Way at much earlier times in their lives”, says Ann Horschemeier of Pennsylvania State University. The AGNs are enveloped in clouds of gas and dust, and are probably powered by enormous black holes. “The Chandra data show us that giant black holes were much more active in the past than at present”, says Riccardo Giaconni of Johns Hopkins University.

The deep field study also identified for the first time a so-called type-II quasar, which has a black hole at its core. The quasar is extremely remote and shrouded in gas and dust. “The discovery of this object is key to understanding how dense clouds of gas form galaxies with massive black holes at their centres”, says Colin Norman of Johns Hopkins University.

Astronomers tune in to brown dwarf

LP944-20 came to the attention of Berger’s team after the space-based Chandra X-ray Observatory detected a burst of X-rays from the brown dwarf. Investigating the object further with the Very Large Array telescope in New Mexico, Berger and colleagues found that it also emits an unexpectedly intense radio signal – around 20 000 times stronger than theory predicts. This posed questions about the power sources responsible for the radiation.

Brown dwarfs are not true stars because they are not massive enough to kickstart the burning of hydrogen in their cores. But at over 12 times the mass of Jupiter, they can fuse hydrogen into deuterium, although this energy source peters out after about 10 million years. LP944-20 is thought to be much older than this, suggesting the radio waves have a different energy source.

Synchrotron radiation is a likely candidate for the brown dwarf’s battery. As electrons spiral around a star’s atmosphere under the influence of its magnetic field, they release synchrotron radiation at radio wavelengths. When the electrons reach the outer layer of the star’s atmosphere – the corona – their energy is converted into heat and emitted as X-rays. This physical process leads to a remarkably close and predictable relationship between X-ray and radio emissions from individual stars, including the Sun. But LP944-20 does not fit into this model because of the imbalance in its radio and X-ray emissions.

Berger and co-workers believe that the strong radio signal from LP944-20 could best be explained by it having a very weak magnetic field. Electrons circulating in a weak field would travel more slowly and spend more time emitting synchrotron radiation – resulting in a disproportionately intense signal. When the electrons finally reach the corona, they would have less energy to contribute to the X-ray emission process, and this would explain the relatively dim X-ray emission.

Success for Irish physicist

Hegarty, who is 52, is a graduate of St Patrick’s College in Maynooth and obtained a PhD in physics from University College Galway. After spells at the University of Wisconsin-Madison and Bell Laboratories in the US, he moved to Trinity as professor of laser physics in 1986. Since then he has also served as head of the physics department and dean of research.

X marks the atom

Korecki and Materlik’s method is novel because, unlike existing methods, it uses ‘white X-rays’ that cover a wide spectrum rather than X-rays of a single wavelength. It also images atomic structure in ‘real space’ – other techniques collect complex data about the phases of scattered waves, which must then be mathematically ‘inverted’ to obtain a real image.

X-rays are ideal probes of atomic structure because they have wavelengths comparable with the separation of atoms in crystals. But existing techniques based on X-rays are limited in resolution or application. Devices that focus X-rays through a lens suffer because the lens needs to be small – but this limits their resolution to about 10 nm, which is too coarse to resolve individual atoms. X-ray diffraction, on the other hand, are hindered by the so-called phase problem: the intensity of a diffraction pattern is independent of the exact position of an atom and this makes it impossible to convert the phase information into a real-space picture of the atomic structure.

The sample used by Korecki and Materlik was a silicon wafer 300µm thick. But the wafer also acts as a photodiode and uses the current produced to deduce the degree of X-ray interference at each point in the crystal. Combining this ‘photocurrent data’ with the direction of the incoming X-rays produces a two-dimensional image of the crystal structure. Korecki and Materlik slowly turned the crystal and collected eight such images, which they reconstructed to create the three-dimensional picture.

Korecki and Materlik also found that as the X-ray spectrum becomes broader – that is, as it includes a greater spread of wavelengths – the image becomes clearer. This is because the fainter fringes towards the edges of the diffraction pattern disappear as the range of wavelengths widens. Ultimately, only the central strong peak in the diffraction pattern – the ‘zero-order’ fringe – remains.

The new technique may allow physicists to establish exactly where dopant atoms are sited within crystals, which is not possible with existing methods. And because it makes direct imaging possible, single planes of the crystal can easily be scrutinized: existing devices require data from all angles of the crystal to form a real space image.

Physicists create first superconducting polymer

It has long been known that certain polymers – containing alternating single and double bonds along the polymer ‘backbone’ – conduct electricity. Electrons move by hopping from one bond to another. Introducing ‘foreign’ atoms to donate extra electrons can make some polymers conduct as well as metals. Superconductivity, however, has never been observed before.

Batlogg and co-workers allowed a solution of poly(3-hexylthiophene) – or P3HTP – to solidify into thin films. The films were found to consist of tiny crystals of polymer interspersed with amorphous regions. Resistance-free current flows when the films are cooled below 2.35 K, and the relationship between conductivity and temperature is typical of that in polycrystalline superconductors. This is thought to result from the co-existence of the superconducting nanocrystals and the insulating amorphous areas.

The Bell team believes that the ‘self-organization’ is the key to superconductivity in P3HTP. The polymer chains within the crystals spontaneously align themselves as the films are cooled. Indeed, interfering with this self-imposed order was found to suppress superconductivity. For this reason, Batlogg and co-workers injected charge carriers into the polymer – one for every five polymer chains – using a field-effect transistor, which disrupts the structure less than the traditional chemical doping. “Using our method, many organic materials could potentially become superconductors”, said team member Zhenan Bao.

The polymer has a clear metal-to-insulator transition, and its superconductivity disappears when a strong magnetic field is applied – the acid test of a true superconductor. But the mechanism for superconductivity in P3HTP is unclear. The Bardeen-Cooper-Schrieffer theory describes, for conventional superconductors, how thermal vibrations of the crystal lattice – or phonons – help electrons to pair up and flow without resistance. There are strong hints that phonons play a role in the P3HTP effect, but other types of electron-electron interaction have not been ruled out.

Within the last year, the prolific Bell Labs team has chalked up the first electrically powered organic laser, and achieved superconductivity in organic crystals and the fullerene, C60. Elsewhere, the discovery of surprisingly high-temperature superconductivity in the metallic compound magnesium diboride is under intense investigation.

Cosmological model gets a boost

The cosmic microwave background (CMB) has a perfect black-body spectrum with a temperature of 2.73 kelvin. But this temperature varies very slightly at different points in the sky, and this fluctuation – related to the microwave power spectrum – is thought to reflect the aggregation of matter in the early universe into galaxies and galaxy clusters. The recent Boomerang experiment, which measured the CMB by converting microwave signals into heat, detected a peak in the power spectrum predicted by the standard cosmological theory. But it failed to find a dip that was also thought to exist.

Unlike Boomerang, the Cosmic Background Imager (CBI) is a radio-interferometer – a type of instrument that has only recently become sufficiently refined to measure the CMB directly. The new telescope has imaged the CMB around three times more accurately than Boomerang, at an angular resolution of up to a twelfth of a degree – and it has found the trough in the data that Boomerang did not observe. This is the first time that a single experiment has measured this feature, which is a fundamental prediction of the standard model of cosmology.

According to Joseph Silk, an astrophysicist at the University of Oxford, this is good news for the ‘inflation’ theory, which links the echoes of the big bang to the power spectrum of the CMB. “These results support current theories, but most importantly they prove that the new technique is very promising”, Silk told PhysicsWeb. Forthcoming radio-interferometer experiments include the Very Small Array (VSA) and the Degree Angular Scale Interferometer (DASI).

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