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Magnetic fields put the heat on neutron stars

Formed by the collapse of massive stars, neutron stars are extremely dense objects containing mostly neutrons. They are typically only about 10 kilometres in diameter, but are at least 40% heavier than the Sun, which means that their core density is several times that of the density of an atomic nucleus. As they age, neutron stars are thought to cool first by emitting neutrinos, and then by emitting photons. By measuring the rate at which neutron stars cool, physicists can gain important insights into the subatomic physics that govern the innards of these objects.

José Pons and colleagues at Alacant University in Spain and a collaborator at Montana State University in the US used data from satellite X-ray telescopes and ground-based radio telescopes to show that magnetic heating appears to be happening in neutron stars with magnetic field strengths between about 10¹²-10¹⁵ Gauss. Astrophsyicists had previously thought magnetic heating would only be signficant in magnetars with magnetic fields above 10¹⁴ Gauss.

The next step for the researchers is to further test the relationship between temperature and magnetic field by analysing data from more neutron stars. However, this may have wait until the next-generation of X-ray telescopes such as NASA’s Constellation X satellite array or the ESA’s XEUS satellite become available sometime in the next decade. Pons also believes that improved computer simulations of the interaction between neutron stars and their magnetic fields could shed further light on how neutron stars cool.

Magnet misbehaves near absolute zero

In a conventional continuous phase transition, a change in temperature causes matter to transform from one state to another (magnetic to non-magnetic, for example). As the material approaches the critical temperature, thermal fluctuations cause “bubbles” of the new state to appear and grow within the old state, eventually taking over the material. During the transition, the difference in energy between the two states approaches zero in a manner that defines the “universality class” of the transition. All know continuous phase transitions can be described using one of a small number of universality classes.

At extremely low temperatures near absolute zero, there is little energy available for thermal fluctuations and quantum “zero-point” fluctuations are expected to play a role in phase transitions. These fluctuations keep matter in constant motion — even at zero temperature. However, it isn’t clear whether “quantum phase transitions” driven by these fluctuations will belong to the same universality classes as phase transitions driven by thermal fluctuations.

Now, Philip Gegenwart and co-workers at the Max Planck Institute for the Chemistry of Solids in Germany along with colleagues in the US have reported that quantum fluctuations appear to be driving two very distinct phenomena in an antiferromagnetic metal. This unexpected behaviour was observed at temperatures below 0.8 K in YRS, which is a compound of ytterbium, rhodium and silicon.

The researchers believe that the unexpected change they observed at higher fields could be related to the “entanglement” of magnetic spins and conduction electrons. Magnetic spins are electrons that are fixed to individual atoms and normally have little to do with the conduction electrons. But at higher magnetic fields and very low temperatures they appear to become entangled with conduction electrons to create quasi-particles that behave like very heavy electrons. Gegenwart and collegues may have observed a transition to this “heavy electron liquid” state of matter.

According to Andrew Schofield of the UK’s Birmingham University, if quantum fluctuations were simply taking over the role of thermal fluctuations, only the magnetic transition should have been observed. He told Physics Web that the appearance of a second distinct feature in the phase diagram associated with the quantum critical point defies our current understanding of universality classes and a new physical theory is required to describe what is a purely quantum phenomenon.

Bacterium battles against the current

Bacteria that have trailing, screw-like tails (known as flagella) propel themselves forward in short bursts when the tails rotate. Normally, these bursts are hindered by drag forces, which counteract a bacterium’s propulsion until it eventually tumbles to a halt. At the surface of a calm fluid, however, a lack of drag can allow bacteria to adopt some surprising trajectories.

Now, Jane Hill and colleagues from Yale University in the US and Bogazici University in Turkey have spotted that, given the right conditions, the bacterium E. coli – notorious in certain strains as a deadly pathogen – can even swim upstream. The physicists designed flat microfluidic channels between 50 and 450 µm deep to study how cells of the bacterium behave near the surfaces of flowing liquids, and took snapshots of the channels to track their progress.

Hill’s team found that, at low flow rates, the drag is greater around the nose of the cell, causing it act as a pivot and thus orient the rest of the body to face roughly upstream – similar to the way a weather vane points to the direction of oncoming wind. The torques from swimming and drag then balance to direct the cell left, leading the cell to one edge of the channel. Finally, for certain orientations, the cell can continue its leftward path along the channel wall, moving in a narrow trench upstream.

This peculiar surface interaction could have put E. coli and similar flagellated bacteria at an evolutionary advantage, allowing them to seek efficient routes to swim upstream in a wide range of flow conditions. Hill told Physics Web that she is now using this knowledge as a basis for understanding how E. coli moves, for example, in urinary tract infections. “Paediatrics patients are particularly susceptible to infection, and I think this phenomenon in these patients, as well as others, will be quite relevant,” she said.

Photons denied a glimpse of their observer

The famous double slit experiment applied to photons neatly highlights the mysterious influence of an observer in quantum mechanics. In the experiment, single photons are fired at a distant screen, partially obstructed midway by a wall containing two slits. If one neglects to check which slit a photon passes through, the photon appears to interfere with itself, suggesting that it behaves as a wave by travelling through both slits at once. But if one monitors the slits carefully (i.e. observes), the interference disappears, and each photon travels through one of the slits as a particle would.

In 1978, however, John Wheeler pointed out that a photon could somehow know in advance whether an observation was going to be made, and change its behaviour to that of a wave or particle accordingly. To test for this possibility he thought of an experiment in which the decision to observe the photons is made only after they have been emitted.

Now, Jean-François Roch and colleagues from the École Normale Supérieure de Cachan have for the first time faithfully realized Wheeler’s thought experiment. The team substituted the two slits in Young’s apparatus (which would be unfeasible) for two paths in an interferometer (see figure: “Choose the right path”). These paths led directly to two different detectors, allowing one to clearly observe the path each photon took. However, the physicists also devised an automated system that randomly inserted a beam splitter at the last moment. When the beam splitter was in place, it was impossible for an observer to know which path a photon had taken.

Without the beam splitter, the photon took one path or the other, behaving as a particle. But with the beam splitter, the detectors registered interference – as though the photon was behaving as wave and going down both paths simultaneously. However, unlike all previous “two-slit” experiments, the system made the decision to observe after the photon had to commit itself to one path, the other, or both. Therefore if some conceivable source was secretly informing the photon, it would have to be sending a message faster then light – something that relativity forbids.

“Due to this constraint, we can be sure the photon does not know what will be at the end of the interferometer when it enters,” Roch said. “This really emphasizes the tension between quantum mechanics and relativity.”

Firm claims first “commercial” quantum computer

In a classical computer, bits of information are stored as either “0” or “1”. Any working quantum computer would need to exploit the ability of quantum particles to also be in superpositions of both states. Such “qubits” can, in principle, allow a quantum computer to outperform a classical computer for certain tasks. D-Wave, which was founded in 1999 by the physicist Geordie Rose, says that its device has 16 qubits that are based on circuits made from two superconducting materials – aluminium and niobium.

Electrical circuits made from such materials are promising candidates for quantum computers because they can be made from thin films using conventional microchip fabrication technology. The difficulty, however, is connecting and controlling the qubits in such a way that they can be used to perform calculations. In particular, qubits rapidly lose the information they are holding by interacting with their surroundings.

D-Wave claims to have minimized this problem of “decoherence” using a novel version of a technique called adiabatic quantum computation, or AQC, which controls the computer in a slow and continuous manner. However, AQC itself has yet to be demonstrated experimentally, leaving many in the quantum computation community wondering exactly how Orion works.

“If they have achieved control over 16 bits they have done something exciting,” says Andrew Steane, a physicist at Oxford University in the UK. However, Steane says that D-Wave’s announcement came completely out of the blue and he remains highly sceptical until the company releases more information.

Plasma wakefield “turbocharges” particle accelerator

By the time CERN’s Large Electron-Positron (LEP) collider was dismantled to make way for the forthcoming Large Hadron Collider (LHC) in 2000, it had pushed the record for accelerated electron energies over 100 GeV. But such energies are not easy to come by, and the LHC will come with a final price tag of about $8 billion.

Now, Mark Hogan and his team from the SLAC and two Californian universities have shown that devices based on “plasma wakefields” – a much smaller and potentially cheaper technology – can supplement existing, conventional accelerators by “turbocharging” the particles as they leave. They have developed a device just 85 cm long that took the 42 GeV electron beam at SLAC up to 85 GeV.

Recently, physicists have accelerated electrons to GeV energies in plasma wakefields created by firing a laser into a jet of gas. Hogan’s team, on the other hand, used the electron beam from SLAC as an input for their device, which was filled with lithium vapour. Electrons from the gas were dragged away from the lithium nuclei by the beam, only to snap back and overshoot their initial position. This oscillating movement, which occurred in the wake of the original electron pulse, occasionally captured some of the pulse’s electrons and accelerated them to much higher energies.

Because SLAC’s beam contained compressed bunches of electrons, Hogan’s device could maintain a stable accelerating wakefield for almost a metre. Previous devices have been limited to a few centimetres by a beam instability known as “hosing”, named after a similar effect in cartoons when untamed hosepipes thrash about as water is pumped through them.

However, several challenges remain before the technique can be used in practical particle accelerators. The team now need to reduce the energy spread of the electrons, and prove that positrons can also be accelerated in the same manner. “The problem with plasmas is that they’re nearly always unstable,” Robert Bingham, a physicist at Rutherford Appleton Laboratory, told Physics Web. “This [result] leads the way to build longer plasma columns and hence produce higher energy particles.”

Ocean waves keep Earth humming

Over the past decade geophysicists have become increasingly aware that the Earth is vibrating at a series of well-defined “infrasonic” frequencies between about 1-10 mHz. The origins of this hum have been the subject of heated debate. Earthquakes were an obvious candidate, but they were ruled out along with interactions between turbulence in the atmosphere and the Earth’s surface.

Then in 2004, researchers found that the strongest hums appeared to be coming from the oceans. Now, Webb has shown that two or more infragravity waves interact to create pressure waves that set the Earth humming at its natural mHz vibrational modes. Webb believes that these interactions occur over the continental shelves – the shallow water surrounding land masses where infragravity waves are most common.

Infragravity waves are not the familiar wind-driven waves that form on the surface of the ocean. Rather, they are produced when higher frequency wind-driven waves (above 40 Hz) interact with the shore. These infragravity waves have frequencies in the 1-40 mHz range and much smaller amplitudes than typical ocean waves.

Extra dimensions could leave trace in Big Bang’s aftermath

The Standard Model of particle physics does a good job of describing the three strongest fundamental forces: electromagnetism, the strong interaction and the weak interaction. However, it has so far failed to incorporate the fourth: gravity.

One of the ways in which physicists have tried to unify gravity with the other forces is to create theories that allow for many more spatial dimensions. In such “string” theories, the point-like particles long thought to be the building blocks of all matter are replaced by tiny strings that wrap themselves up in the extra dimensions. But although some claim string theories are the best contenders yet for a “theory of everything”, the extra dimensions are far too small to be found using conventional methods.

Now, however, Gary Shiu and Bret Underwood from the University of Wisconsin have done calculations to show that the shape of extra dimensions could have effects that show up in the cosmic microwave background (CMB) – an indelible map of the universe as it was some 380,000 years after the Big Bang. Although these effects alone would not present a way to prove or disprove string theory, it would mean that the details of extra dimensions have an observable impact on cosmological measurements. “Like anthropologists looking at fossils to reconstruct our history, detailed snapshots of the early universe may allow us to ‘see’ additional dimensions,” said Shiu.

These snapshots could be taken with CMB experiments such as NASA’s existing WMAP or the ESA’s Planck satellite, due to be launched in July 2008. But the precise way in which physicists interpret them will depend on how they model inflation – a period of exponential expansion in the universe preceding the creation of the CMB. Shiu and Underwood based their calculations on Dirac-Born-Infeld (DBI) inflation, a model frequently used in conjunction with string theory. However, regardless of which model is eventually established, the shape of the extra dimensions will always have an impact.

“String theory is by far the most promising framework to address questions about the early universe and the high energy frontier,” Shiu continued. “We are eagerly waiting for more and more data from precision cosmology.”

New particle resolves erratic sightings of axions

Axions were first proposed 30 years ago to resolve a discrepancy between experimental findings and quantum chromodynamics (QCD), a theory that describes how the strong force binds quarks together within protons, neutrons and other hadrons. However, physicists have since realized that axions could have just the right properties – a small mass and a coupling to light – to be the elusive particles that make up dark matter, a substance thought to constitute up to 95% of matter within the universe.

In March last year, researchers working at the PVLAS experiment in Italy shone a laser beam through a strong magnetic field in a vacuum and noted the beam’s polarization rotated slightly. At the time many physicists thought that this was due to an ultralight particle coupling with photons in the beam, and so heralded it as the first glimpse of the axion. If it were an axion, however, the implied mass would contradict findings of the CAST experiment based at CERN, where researchers have been looking to capture axions travelling from the Sun by converting them to x-rays.

Now, Rabi Mohapatra from the University of Maryland and Salah Nasri from the University of Florida say that the two results can be reconciled if one considers the source of the axions. Whereas in the PVLAS experiment axions would have been produced locally in the laser beam at room temperature or colder, the CAST experiment was monitoring axions produced in the Sun’s core at about 10 million degrees Celsius.

CAST

Mohapatra’s theory is that a phase transition could occur at Sun’s high temperatures that destroys the coupling altogether, rather like a magnet losing its magnetism when heated. This would account for the null findings of CAST. However, this theory would also require a new particle – a force-carrying boson with a mass of about 100 MeV – to account for the coupling observed in PVLAS. Although such a particle has not been seen, Mohapatra says that it could be sought in future experiments involving the decay of upsilon particles.

Mohapatra and Nasri are not the first physicists to try to reconcile PVLAS and CAST’s findings. Last October, for example, Eduard Massó and Javier Redondo from the Universitat Autònoma de Barcelona in Spain claimed to have constructed a plausible theory using multi-charged particles (Phys. Rev. Lett. 97 151802). But Mohapatra dismisses their suggestion: “Our proposal uses conventional ideas, whereas Massó’s proposal invokes unusual effects inside the Sun.”

“We are confident that if PVLAS experimental results are confirmed, our model will be one of the best ways to understand the effect, providing a new and interesting direction for new physics,” he added.

A moon that puts a shine on its neighbours

Albedo is a measure of how well a surface reflects light and is used by astronomers to study the surfaces of planets, moons and asteroids. Newly created surfaces on geologically active bodies tend to reflect light very well and have relatively high albedos — whereas geologically inactive bodies with ancient surfaces shaped by meteorite impacts tend to have lower albedos.

Enceladus has an extremely high albedo because it is very geologically active – indeed a plume of material from eruptions at its south pole gives rise to a vast ring that envelops at least 11 of Saturn’s other satellites. These other moons, which are very old and have geologically inactive surfaces, were expected to have low albedos.

But new direct albedo measurements made by Hubble at the precise moment when the Sun, Earth and Saturn were in alignment tell a much different story. Anne Verbiscer of the University of Virginia and colleagues at Wellesley College, the SETI Institute and Cornell University discovered that four moons within this ring have albedos that are about 50% greater than previously thought.

The astronomers believe that these moons are being constantly bombarded by high-velocity particles from the plume. This sandblasting is though to produce clean, icy, and very reflective surfaces even though the moons are geological inactive. Verbiscer and colleagues believe that their theory is supported by the fact that enhanced albedos were not observed in two other moons that were inside the ring and therefore not subjected to the sandblasting.

Verbiscer told Physics Web that this is the first time that such a sandblasting effect has been observed directly. However, the sandblasting effects of small meteorites have already been put forth as an explanation for the differences in the albedos of the leading and trailing hemispheres of Saturn’s moons.

Despite the dramatic effects of sandblasting, Verbiscer says that rather than being a “driving rain” of particles, the flux would be so low that an observer on one of the moons would not be aware of the sandblasting process.

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