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Home » Page 1533

Pulsar bursts move ‘faster than light’

Every physicist is taught that information cannot be transmitted faster than the speed of light. Yet laboratory experiments done over the last 30 years clearly show that some things appear to break this speed limit without upturning Einstein’s special theory of relativity. Now, astrophysicists in the US have seen such superluminal speeds in space – which could help us to gain a better understanding of the composition of the regions between stars.

Superluminal speeds are associated with a phenomenon known as anomalous dispersion, whereby the refractive index of a medium (such as an atomic gas) increases with the wavelength of transmitted light. When a light pulse – which is comprised of a group of light waves at a number of different wavelengths – passes through such a medium, its group velocity can be boosted to beyond the velocity of its constituent waves. However, the energy of the pulse still travels at the speed of light, which means that information is transferred in agreement with Einstein’s theory.

Now, astrophysicists claim to have witnessed this phenomenon in radio pulses that have travelled from a distant pulsar.

Modified pulses

The discovery has been made at the University of Texas at Brownsville, where Frederick Jenet and colleagues have been monitoring a pulsar – a rapidly spinning neutron star – more than 10,000 light years away. As pulsars spin, they emit a rotating beam of radiation that flashes past distant observers at regular intervals like a lighthouse. Because the pulses are modified as they travel through the interstellar medium, astrophysicists can use them to probe the nature of the cosmos.

Several factors are known to affect the pulses. Neutral hydrogen can absorb them, free electrons can scatter them and an additional magnetic field can rotate their polarization. Plasma in the interstellar medium also causes dispersion, which means pulses with longer wavelengths are affected by a smaller refractive index.

Timing is off

Jenet’s group thinks that anomalous dispersion should be added to this list. Using the Arecibo Observatory in Puerto Rico, they took radio data of the pulsar PSR B1937+21 at 1420.4 MHz with a 1.5 MHz bandwidth for three days. Oddly, those pulses close to the centre value arrived earlier than would be expected given the pulsar’s normal timing, and therefore appeared to have travelled faster than the speed of light.

The cause of the anomalous dispersion for these pulses, according to the Brownsville astrophysicists, is the resonance of neutral hydrogen, which lies at 1420.4 MHz. But like anomalous dispersion seen in the lab, the pulsar’s superluminal pulses do not violate causality or relativity because, technically, no information is carried in the pulse. Still, Jenet and colleagues believe that the phenomenon could be used to pick out the properties of clouds of neutral hydrogen in our galaxy.

‘Solid result’

“It seems to be very interesting indeed…a solid and rather nice result,” says Michael Kramer, an astrophysicist at the University of Manchester who was not involved with the study.

Andrew Lyne, a pulsar astrophysicist who is also based at Manchester, thinks it is an “interesting, if not unexpected result”. However, he has doubts that it could help in the understanding of neutral-hydrogen clouds because there are often several clouds in the same line of sight. “It is not clear from the paper quite what extra information will be obtained,” he adds.

The research will be published in the Astrophysical Journal. A preprint is available at arXiv:0909.2445v2.

Spotting fake paintings with stats

DACEY Bruegel.jpg
Winter Landscape with a Bird Trap Pieter Bruegel the Elder (1565)

By James Dacey

I wrote recently about a new technique for identifying the literary fingerprint of famous authors based on statistical patterns in their writing styles. Quantifying artistic style in painting, however, is a more difficult challenge as the constituent “words” are unknown, and whilst different artistic styles involve different brush strokes this variation is not enough to identify a well-produced fake. Now a trio of researchers in the US may have cracked the code by developing a model that can define the “spatial structure” of celebrated works of art.

Dan Rockmore of Dartmouth College, New Hampshire, and his colleagues have been inspired by vision science, which explores the way the human eye makes sense of the world. The human brain has evolved to recognize patterns in nature, but the researchers also realize that the world has been kind to us in possessing regular statistical patters – “neighbouring point tend to be correlated,” says Rockmore.

In their method, Rockmore’s team first take a painting, unaware if it is genuine or fake then apply random functions until they can successfully reconstruct a given section of that painting. Next, they apply the same set of functions or “filter” to both a genuine painting by the same artist and a known fake of this same picture. If the filter performs worse at reconstructing the genuine image, then they conclude that they have another fake on their hands.

The researchers apply their model to the works of Pieter Bruegel the Elder, the famous Dutch Renaissance painter, where they successfully distinguish authentic drawings from a set of well-known Bruegel imitations. “It turns out that such tests succeed the vast majority of the time, confirming that we have indeed chosen a ‘representation’ that picks out the visual elements that distinguish Bruegel from his imitators,” says Rockmore.

The Dartmouth researcher tells physicsworld.com that his team intend to develop their research by attempting to incorporate colour variations into their models. “We are planning to apply sparse coding to a number of other problems in art history,” he says. In particular, the team are keen to focus on the perennial challenge of differentiating works by Rembrant from those of his students.

This research is published in Proceedings of the National Academy of Sciences.

E8 symmetry spotted in ultracold magnet

An unusual form of symmetry known as E8 – which some physicists believe underlies a theory of everything – may have been spotted in a solid material for the first time. Physicists in Germany and the UK claim to have shown that the 8D symmetry group describes the spectrum of spin configurations that emerge when a 1D chain of spins is chilled to near absolute zero and subjected to a specific magnetic field. The experimental work also confirms a long-standing prediction of the spectrum at zero magnetic field.

Symmetry plays a fundamental role in our understanding of the physical world. Perhaps the most intriguing of all is E8 – essentially a fantastically beautiful 8D diamond-like lattice of spheres in which each sphere is surrounded by 240 others. E8 shot to fame in 2007 when the US freelance physicist Garrett Lisi, who has an unlikely day-job as a surfer in Hawaii, posted a paper on the arXiv preprint server suggesting that E8 could underpin a theory of everything by mapping out all known particles and how they interact with each other.

However, E8 had fascinated researchers well before Lisi’s paper appeared. In 1988 the Russian physicist Alexander Zamolodchikov showed that E8 symmetry could – under certain conditions – also describe the spectrum of spin excitations that occur in 1D Ising ferromagnets. These are chains of spins in which each spin can only interact with its two nearest neighbours. Below a critical temperature, neighbouring spins tend to align perpendicular to the direction of the chain in one of two directions (up or down).

Quantum fluctuations

If a magnetic field is applied perpendicular to the spins, it will encourage spins to flip spontaneously – or tunnel – between up and down. Such fluctuations can propagate though a material much like a particle and are therefore referred to as quasiparticles. These quantum fluctuations occur even at zero temperature and, if the field is strong enough, ferromagnetism is destroyed in a quantum phase transition.

Now, Radu Coldea of Oxford University and colleagues at the Helmholtz Zentrum Berlin (HZB), the University of Bristol and Rutherford Appleton Laboratory have measured the energies of several of these quasiparticles. They did this by cooling the sample of cobalt niobate to 40 mK and firing neutrons at it to create quasiparticles. When this occurs, the spin and energy of the scattered neutrons change relative to the incident beam by an amount that can be used to calculate the energies of the quasiparticles.

When the experiment was done at zero magnetic field, five quasiparticles were spotted. Their energies are described not by E8, but by a mathematical formula derived three decades ago by Barry McCoy and Tai Tsun Wu. McCoy, who is at Stony Brook University in New York, told physicsworld.com that he was “very impressed with the experiment and most gratified to see that our prediction of 1978 has been experimentally observed”.

‘Golden ratio’

Coldea and colleagues then repeated their measurements in a magnetic field. As the strength of the field was increased to the quantum critical value of 5.5 T, the ratio of energies of the first two quasiparticles approached 1.618. This number is the “golden ratio” and is precisely what should be measured if the quasiparticles are described by E8 – a prediction that was made more than 20 years ago by Zamolodchikov, now at Rutgers University.

Unfortunately the team was unable to study the system at the quantum critical field – where E8 should emerge – because they could only resolve the lowest energy quasiparticle above about 5 T. Although E8 predicts a total of eight quasiparticles, McCoy believes that it will be very difficult to use neutrons to see the higher energy quasiparticles. This is because their energies overlap a region dominated by continuum scattering involving two or more quasiparticles.

Supermassive black hole struggles to swallow Milky Way

Scientists were already aware that the huge black hole at the centre of our galaxy does not consume large amounts of matter, but it could be an even pickier eater than previously thought. That is according to new research done in the US that suggests that the black hole – called Sagittarius A* – has a tendency to blow away 99.99% of the matter available for its consumption.

Supermassive black holes are awesome phenomena that are believed to exist at the centre of most, if not all, galaxies. They are hundreds of thousands to billions of solar masses and expand by feeding on dust that is blown off massive young stars just outside the black hole’s event horizon – the zone beyond which not even light can escape.

In the Milky Way, these neighbouring stars are located a relatively large distance away from the black hole mass. For this reason, scientists had calculated that Sagittarius A* should consume only about 1% of the available dust. But now a team of astronomers, including Roman Shcherbakov of Harvard University, claims that it is consuming much less than that.

Gaseous lobes

The team studied an image constructed from a series of observations captured by NASA’s Chandra X-ray Observatory, over almost two weeks. This long exposure time, enabled the researchers to get a clear view of the gas surrounding the event horizon, which revealed a series of gaseous lobes stretched in various directions.

To explain this observation, Shcherbakov and his colleagues employ a model that considers the flow of energy between two regions around the black hole: an inner region that is close to the event horizon, and an outer region that includes the black hole’s fuel source – the young stars – extending up to a million times farther out than the event horizon. They conclude that collisions between particles in the hot inner region transfer energy to particles in the cooler outer region via conduction, which adds an additional outward pressure so that all but 0.01% of the incoming star dust is blown away.

These findings were presented at the 215th annual meeting of the American Astronomical Society (AAS), which is taking place this week in Washington, DC.

New pulsars could net gravitational waves

 

By combining observations at gamma-ray and radio wavelengths, astronomers have rapidly increased the known number of millisecond pulsars in the Milky Way. The newly discovered pulsars, found using NASA’s Fermi Gamma-ray Space Telescope and ground-based radio telescopes, could form part of a galactic-scale observatory for detecting hitherto elusive gravitational waves.

Gravitational waves are ripples in the fabric of space–time that Einstein’s theory of general relativity predicts will occur whenever massive bodies are accelerated. Gravity waves at relatively high frequencies of about 1 Hz could be produced by the merger of neutron stars, for example. Physicists hope to detect such waves using a number of huge laser interferometers. These devices rely on the interference of two laser beams arranged at right angles to one another, with any passing wave producing a miniscule shift in the interference pattern.

An alternative, and cheaper, way to detect gravitational waves is to utilize millisecond pulsars – the extremely dense remnants of burnt-out stars that emit well-defined beams of electromagnetic radiation that sweep round like a beam from a lighthouse. Certain kinds of older pulsar can spin extremely rapidly and send out hundreds of pulses per second. These frequencies vary by less than a microsecond over very long periods of time and such pulsars therefore make ideal timekeepers.

All in the timing

The time it takes for these pulses to arrive at a radio telescope would be changed very slightly by any gravitational wave passing between pulsar and telescope. Therefore, by measuring the changes in the relative timing of large numbers of such pulsars, each with a different line of sight to the Earth, it should be possible to reveal the presence of a gravitational wave as well as record its direction of propagation and polarization.

Separated from one another by thousands of light years, these pulsars would enable the detection of waves with frequencies of nanohertz, which could be produced from sources such as black-hole binaries, which form when galaxies merge, or the “cosmic strings” believed to have existed in the early universe.

Millisecond pulsars were first discovered almost 30 years ago but the huge amount of telescope and computing time needed to find them using sky surveys at radio wavelengths has meant they have remained a relative rarity. Until recently astronomers had uncovered only about 150 of them, some 90 of which were grouped tightly together in star clusters and therefore unsuited for detecting gravitational waves. Now, however, Paul Ray of the Naval Research Laboratory in Washington, DC and colleagues have accelerated this discovery process by using the Fermi satellite’s Large Area Telescope.

This instrument has been surveying the heavens at gamma-ray wavelengths since August 2008 and in that time has uncovered some 1000 new sources of gamma rays. Ray suspected that some of these sources were pulsars and so set up a team of astronomers to analyse them in finer detail using a number of radio telescopes around the world, including the Green Bank Telescope in West Virginia and the Parkes Observatory in Australia. After just three months of analysis on a fraction of the Fermi data, Ray and colleagues have found 17 new millisecond pulsars, and he reckons that they might double this number by studying the remaining unidentified sources.

With a little luck

Ray believes that “with continued good luck discovering millisecond pulsars and enough dedicated telescope time”, gravitational waves might be detected using a pulsar array within the next decade. He points out that astronomers in the US, Europe and Australia are currently putting together proposals to increase the fraction of time devoted to pulsar observations on facilities such as the Green Bank and Parkes telescopes and to develop advanced software to process the huge amounts of data involved. He adds there is a small chance that a pulsar array might scoop the interferometers in making the first direct detection of gravity waves.

One of the physicists working on interferometers, Jim Hough of the University of Glasgow, agrees that pulsar timing is a good way to search for gravity waves at extremely low frequencies. He believes that if astronomers observe 20 pulsars with a timing precision of better than 100 nanoseconds for five years then they would “have a very good possibility of observing gravitational-wave signals.”

The research was reported at a meeting of the American Astronomical Society in Washington, DC.

Physicists catch sight of trembling particle

European physicists have won the race to observe zitterbewegung, the violent trembling motion of an elementary particle that was predicted by Erwin Schrödinger in 1930. To observe this phenomenon, the team simulated the behaviour of a free electron with a single, laser-manipulated calcium ion trapped in an electrodynamic cage.

They took this approach because it is currently impossible to detect the quivering of a free electron, which has an amplitude of just 10–13 m and a frequency of 1021 Hz. Computational simulations are also ruled out, because today’s computers have insufficient power and memory capabilities.

The researchers claim that their triumph may also serve as an important step towards using trapped ions and atoms to simulate high-temperature superconductivity, magnetism and even black holes.

Relativistic realization

According to Christian Roos at the University of Innsbruck, Austria, one of the keys to success was to make their non-relativistic ion behave as if it was a relativistic particle. This is crucial because zitterbewegung is predicted by the Dirac equation, which describes relativistic quantum mechanics.

Roos did the work along with colleagues at Innsbruck and the University of the Basque Country. “When the right conditions are met, the Schrödinger equation that describes this ion as a quantum system looks identical to the Dirac equation of the free electron,” he explained. The trapped, laser-manipulated ion can then be studied as an analogue of a relativistic free electron.

Calcium ions were chosen because they can be excited with visible wavelength lasers. “In addition, calcium’s level structure is sufficiently simple to allow the experimentalist a near-perfect control over the internal states of the ion, but complex enough to carry out the quantum measurements needed for inferring the position of the particle.”

Simulations begin by putting the calcium ion into a particular quantum state. This is allowed to evolve for a certain time, before the researchers measure the position of the ion.

Tiny movements

“In these measurements the particle moves by much less than the wavelength of visible light, so we cannot directly use an imaging technique to determine the position of the ion,” explains Roos. “Instead, we use a suitably tailored laser-ion interaction that maps the information about the position of the particle onto the internal states of the ion.” The ion’s position is then determined from its internal state, and this uncovers the quivering motion.

The act of measuring the ion’s position collapses its wave function, so the researchers have to reconstruct the desired initial wave function for every single measurement. This process is relatively quick, however, and they are able to carry out 50 experiments per second.

Adjusting the output of the laser alters the simulated particle’s kinetic energy to rest-mass energy ratio, and opens the door to studies of relativistic and non-relativistic physics.

The researchers found that changes to the particle’s effective mass while its momentum was kept constant led to the disappearance of zitterbewegung in the non-relativistic and highly relativistic limits (large and small effective masses, respectively). However, the quivering motion was clearly present in the regime between these limits.

Inspirational work

Jay Vaishnav from Bucknell University, Pennsylvania, says that the work of Roos and his co-workers represents a major step forward for quantum mechanical simulations, and she believes that it will inspire other research groups to attempt similar things.

She says that the building of an atomic version of the Datta-Das transistor – a spin-based device that has never successfully been built with electrons – could lead on from Roos’ work. “The workings of this transistor are based on creating a relativistic set-up using cold atoms.”

The work is reported in Nature.

Ancient Mars wetter than we thought

 

Early Martian history may have involved more water on the planet’s surface than was previously thought. That is according to researchers in the UK who have identified a series of geological features on the planet’s surface, which they claim could only have been formed by running liquid. One tantalizing consequence is that primitive life could have had greater opportunity to evolve before the planet became a frozen wasteland.

Since the 1970s when NASA’s Viking mission returned detailed images from the Martian surface, scientists have been in broad agreement that water was present on Mars at the very beginning of its history. Most believe, however, that once the planet entered its “Hesparian Epoch” approximately 3.5 billion years ago, the temperatures at the Martian surface plummeted and any remaining water turned to ice. If life had begun to emerge on the early Mars then it would have become very difficult to sustain in these cold conditions.

In this latest research, a team led by Nicholas Warner of Imperial College London suggests that this is not necessarily the case. By studying images captured by a camera on NASA’s Mars Reconnaissance mission, the researchers focused on a series of depressions around parts of the planet’s equator. The formation of these features was dated at no more than 3 billion years ago by counting the number of crater impacts – a method originally developed by NASA scientists to determine the age of geological features on the Moon.

Previously, it had been assumed that these features were formed by ice converting directly to water vapour in the process of sublimation. However, the high resolution of the new images enabled Warner and his colleagues to spot a number of narrow channels connecting individual depressions, and this has led them to a different conclusion about the formation. They attribute these landscapes to the same processes responsible for the “thermokarst” landscapes common in Siberia and Alaska, in which areas of melting permafrost cause water to trickle to lower elevations under gravity.

“Scientists had largely overlooked the Hesperian epoch because it was thought that Mars was then a frozen wasteland,” says Warner. “Excitingly, our study now shows that this middle period in Mars’ history was much more dynamic than we previously thought.”

Richard Soare, a physical geographer at Dawson College in Canada agrees that the images do bear a strong resemblance to terrestrial thermokarst regions. He told physicsworld.com that very little has been written about permafrost degradation on Mars during this early period of Mars history. “This work provides a substantial first step in this direction,” he says.

Determining the extent and distribution of liquid water is an important aspect in probing the question of whether life could have emerged on the planet. Warner and his team intend to develop their research by analysing spectrographic data from the depressions and their surrounding regions in search of hydrated minerals. “Very little is known about these crater regions and it is still not clear whether they will be too dusty to probe the underlying chemistry,” Warner told physicsworld.com.

This research is published in the journal Geology.

Britain 'on the cold side of a meander'

bristolsnow.jpg
Redcliffe, Bristol

By James Dacey

Perhaps it’s the fear of appearing irresponsible, but why is it that the majority of the media always focuses on the negative aspects of snowfall? It’s always the walker stranded on a hill, the traffic standstills, the council’s failure to grit the roads; even the snow itself becomes personified with adjectives like “biting”, “terrible” or “vicious”. Fortunately, it seems that many of Bristol’s commuters were not put off by last night’s gloomy broadcasts, as there were many of us out there in the white stuff this morning snapping photos of our transformed city.

Britain is experiencing its most prolonged period of freezing conditions since 1981 – why is this? Well, the simple answer is that a zone of high pressure is sitting between Greenland and the UK, which is causing the warm, wet, westerly winds over the Atlantic to be deflected southward towards the Mediterranean. As a result, Britain is being blasted with the colder, dryer winds from Scandinavia, with up to 16 inches of snow falling last night alone over southern Britain. Many regions in northern England and Scotland have been coated in snow for the past three weeks.

But what has triggered and sustained this zone of high pressure?

Well, this is where it gets a bit more complicated but the answer, according to meteorologist Philip Eden, lies with the Atlantic jet stream – the well-defined core of strong wind that flows from northern Canada across northern Europe. These winds can exceed 400 kilometres per hour and they usually snake around to strike Britain from the south-west. At the moment, however, the jet stream has been disturbed and this is causing the winds to slow and change direction. Speaking on BBC Radio 4’s Today programme, Eden says that the jet stream now resembles a “sluggish river that meanders.”

Britain has unfortunately (or fortunately depending on situation or personality) found itself “on the cold side of a meander”, says Eden, meaning that a high pressure region from the polar region has been allowed to drift southward. You can see how this process is occurring at the website of the UK Met office.

So, I realize that by writing this post I am living up to the British stereotype of being way too fascinated by the weather, but I hope that these matters of meteorology are of some interest.

Kepler unveils its first five exoplanets

Scientists working on NASA’s Kepler mission have unveiled the first five exoplanets discovered by the space telescope since it was launched in March 2009. The exoplanets – planets orbiting stars other than the Sun – were found in Kepler’s first six weeks of operation. Although these bodies are nothing like the Earth, their rapid discovery suggests that Kepler is on track to discover a habitable exoplanet sometime in the next three years.

Kepler found the quintet by monitoring the brightness of over 156,000 stars: any orbiting exoplanet crossing the line of sight between a star and the Earth star makes the star appear dimmer. The orbital period of the planet can be determined from the period of fluctuation, while the size of the fluctuations gives the radius of the planet relative to its star.

The five planets are all much larger than the Earth, the smallest being about four times the diameter of Earth. They all have orbital periods of less than five days and are very near to their companion stars. As a result, the exoplanets are thought to have temperatures above 1500 K and are therefore not suitable for life – at least as we know it. The planets are similar to most of the 400 or so exoplanets discovered to date.

Towards longer orbits

The smallest of the new exoplanets, Kepler-4b, appears to be very similar to Neptune except that it receives much more radiation from its star. According to Kepler scientists, this implies that Kepler-4b has a higher rock-to-water ratio, or less hydrogen and helium than Neptune. Another exoplanet, Kepler-7b, has a density about 17% that of liquid water – making it the least dense exoplanet discovered to date.

“The quick discovery and confirmation of five new exoplanets shows that the Kepler mission is performing well,” says Kepler’s lead scientist William Borucki of NASA’s Ames Research Center. His team now plans to look for planets with longer orbital periods by searching through the additional eight months of data that they also have.

Finding planets with orbits of about one year is crucial for the mission to achieve its ultimate goal – the discovery of an Earth-like exoplanet in the “habitable zone” of a stellar system, where conditions are just right for life. “The emphasis in 2010 will be on the discovery of small planets,” says Borucki.

“Performing well”

Suzanne Aigrain, an astrophysicist at Oxford University in the UK, told physicsworld.com that the result “bodes very well for future discoveries based on longer datasets, which should enable the detection of smaller and longer-period planets”. She adds that both the satellite and the processes used to detect and confirm exoplanet discovery appear to be performing well.

In a bid to boost its efficiency, Kepler targets a catalogue of stars that have been pre-selected for their likelihood of harbouring detectable exoplanets. However, Aigrain observes that the preliminary results suggest that the catalogue could bias Kepler to discovering bodies orbiting larger than average stars. “This is something one will have to be careful about when attempting to extract general trends about the exoplanet population from the Kepler results,” she warns.

Kepler, which was named after the 17th century German astronomer Johannes Kepler, is expected to continue searching for exoplanets until at least November, 2012.

The results will be reported in Science.

Dirac book scoops biography prize

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Award winning biographer

By Hamish Johnston

It was our book of the year and now Graham Farmelo’s The Strangest Man: The Hidden Life of Paul Dirac, Quantum Genius has taken the 2009 Costa Biography Award.

The judges had this to say about the book: “The extraordinary mind and achievements of Britain’s Einstein are rendered here in the most compelling biography of the year.”

Last year Farmelo gave the inaugural Physics World online lecture about the life of Paul Dirac — you can watch it here.

This morning Farmelo was interviewed about his award on BBC Radio 4 — you can listen to the interview here — just scroll down to “0720”.

And if you’re interested in a physicist’s opinion of the book, you can read John Enderby’s review, which we published in April, 2009.

Farmelo — along with 2009 Costa winners in fiction, poetry and children’s literature — is now in the running for the Costa Book of the Year, which will be announced later this month.

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