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Reality check at the LHC

If January blues are getting the better of you, cast your mind back to the summer of 2008 when speculation about potential discoveries at CERN’s Large Hadron Collider (LHC), which was then about to switch on, went into overdrive. The world’s media descended into a frenzy about what the LHC might cook up: new dimensions of space, “sparticles”, dark matter and – who could forget? – planet-eating black holes. Two and a half years later and – no surprise, really – planet Earth prevails. Yet so does our well-established picture of the fundamental workings of nature.

It is still early days at the LHC, but the 27 km-circumference machine’s first year of smashing protons into each other at record energies is beginning to tame theorists’ imaginations. Researchers on the Compact Muon Solenoid (CMS) experiment, for example, have reported that, at the energies probed so far, quarks do not exhibit substructure (arXiv:1010.4439), exotic particles such as colorons and E6 diquarks have not shown up (arXiv:1010.0203) – and nor have leptoquarks (arXiv:1012.4031) or new heavy gauge bosons (arXiv:1012.5945) either. Although these entities cannot be ruled out completely, LHC data have allowed them less room to hide – principally by allowing researchers to place stringent limits on the particles’ masses.

CMS scientists have also found no evidence for micro black holes in their 12,500 tonne detector (arXiv:1012.3375). This result, reported just before Christmas, will not have come as a shock to anyone who thinks such black holes will destroy the planet. (For them, it’s only a matter of time…) But it has not surprised many physicists either, given that miniature black holes could only appear at the LHC if space has more than three dimensions. So what does CMS’s black-hole blank mean for such outlandish pictures of space–time? Can we now simply start ruling them out?

“The fascinating science of black-hole production and evaporation still stands,” insists Steve Giddings of the University of California, Santa Barbara, who a decade ago co-proposed the possibility that the LHC might create black holes. “The CMS results begin to rule out the most extreme configurations of extra dimensions, although it is true that such configurations are believed unlikely by many. It’s still a possibility that black holes will be made at the LHC, but it’s not a prediction unless you know the configuration of the extra dimensions!”

Bridging the gap

The models of extra dimensions that underpin the black-hole prediction were originally proposed in 1998 by Nima Arkani-Hamed of the Institute for Advanced Study in Princeton and others to address what is known as the “hierarchy problem”: why gravity is more than 30 orders of magnitude weaker than the forces that govern the quantum world. They posit that everything bar gravity is confined to a 3D brane that exists in a higher-dimensional space from which the true strength of gravity leaks, reducing the Planck scale (at which gravity and the other forces have similar strengths, as is thought to have been the case in the first instants of the universe) from its conventional value of 10¹⁶ TeV to just a few TeV – exactly the energy the LHC is exploring.

The fascinating science of black-hole production and evaporation still stands Steve Giddings, University of California, Santa Barbara

In such a higher-dimensional universe, argue Giddings and others, micro black holes could be squeezed into existence at the rate of one per second at the LHC by particles encountering the true strength of gravity at short distances – before decaying almost instantaneously into a flash of regular particles. However, Arkani-Hamed says that he never thought the black-hole signal was plausible. “Even if extra dimensions exist, black holes would be the last thing you’d discover because you’d see other, larger effects at lower energies first, such as gravitational radiation into the extra dimensions,” he says. “Whatever the reduced Planck scale is, you would have to go to energies several times that to start making what you would recognize as a black hole.”

But Georgi Dvali of Ludwig-Maximilians Universität in Munich, Germany, who worked with Arkani-Hamed on large-extra-dimension models, insists that micro black holes do exist. “It follows from the existence of big black holes,” he told physicsworld.com. “A micro black hole is just what happens to a big one at the very last stage of evolution once it has all but evaporated via Hawking radiation. We know this should happen – we just don’t know at what length [i.e. energy] scale.”

The problem, Dvali points out, is that our current theoretical understanding of micro black holes is not sufficient to accurately predict their properties. To search for these (and other exotic entities) at the LHC, researchers have to model the background events that could mimic them, mostly involving copious jets formed by quarks and gluons – a process that itself is not precisely known theoretically. So far the CMS team has found no signal against this background, allowing researchers to exclude black holes with minimum masses of 3.5–4.5 TeV/c².

A blow for string theory?

But is the lack of black holes seen by the CMS a setback for string theory? After all, this vast theoretical framework also invokes extra dimensions to connect gravity with the other three forces, describing elementary particles as facets of fundamental strings vibrating in a compact 6D or 7D space. However, Arkani-Hamed, for one, rejects the notion that string theory has been given a bloody nose, describing any such claim as “ridiculous”.

Lisa Randall of Harvard University, who in 1999 co-developed a similar extra-dimensional approach called “warped geometries” to address the hierarchy problem, explains that both hers and Arkani-Hamed’s large-extra-dimension (LED) models use string-theory ingredients and might even be derived from string theory, but that neither implies the other. “Ours are really effective theories that are defined at low energies,” she says. “These models don’t necessarily originate in string theory, and string theory doesn’t necessarily imply this low-energy realization.”

Colour graphic of a multi-jet proton-proton event seen by CERN's CMS detector

Closing in

In fact, string theory illustrates how hard it can be to test mathematics linking gravity to the quantum world. String theory describes an inordinate number of possible compactifications of the extra dimensions, each corresponding to a different possible universe, many of which contain features of both LED and warped-geometry models. “In this much broader class of ‘more realistic’ compactifications it appears possible to have particular ones where gravity gets strong at the LHC,” says Giddings. “But there may be many more where it doesn’t.”

Before theoretical physicists get a reputation for being disconnected from the realm of measurement, Arkani-Hamed points out that even before experiment renders an ultimate verdict, consistency conditions – mathematical ones as well as consistency with existing experiments – are so tough to satisfy that the vast majority of new ideas die immediately. Models of large extra dimensions, which were the first new attempt to crack the hierarchy problem in nearly 20 years, are plausible in part because experiments have only tested the sanctity of 3D space at the relatively gargantuan scale of about 0.1 mm.

Calling the super-world

One hotly anticipated find at the LHC would, however, boost string theory and solve the hierarchy problem at one fell swoop: supersymmetry (SUSY), which posits new quantum dimensions to space–time that give rise to a whole spectrum of heavy partners to the known Standard Model particles, dubbed “sparticles”. “Supersymmetry is a much deeper idea than extra dimensions, and there are also strong circumstantial hints that low-energy SUSY is correct,” says Arkani-Hamed. “So if I had to bet, I would bet (by a lot) that some variation of SUSY will show up at the LHC.”

Even if extra dimensions exist, black holes would be the last thing you’d discover Nima Arkani-Hamed, Institute for Advanced Study, Princeton

Searches for SUSY at previous CERN colliders and at the soon-to-be-shut-down Tevatron collider at Fermilab in the US have turned up nothing, allowing physicists to place lower limits on the sparticle masses. The LHC’s higher-energy collisions allow the machine to produce heavier sparticles, should they exist. But earlier this month CMS reported that it had found nothing new so far, ruling out supersymmetric particles with masses of less than about 0.5 TeV/c² (arXiv:1101.1628). And in November the experiment reported no signs of long-lived supersymmetric gluons (gluinos), which should arise if a more recent take on SUSY called split-supersymmetry is correct (arXiv:1011.5861).

CMS’s sister experiment, ATLAS, is expected to report on its own searches for such exotic new particles in the next few weeks, although it has already ruled out quark sub-structure (arXiv:1009.5069) and exotic particles lighter than 1.26TeV/c² (arXiv:1008.2461). And with the LHC due to restart next month after its winter shutdown and accumulate data at an even higher rate – perhaps also at a higher energy – decades of theoretical research into physics beyond the Standard Model (along with, of course, the mechanism that gives elementary particles their mass) will soon be put squarely on the line.

“Given that the LHC is just starting to dig into the territory relevant for the hierarchy problem, it’s hardly surprising that the machine isn’t turning up evidence for anything new in the very first analyses of the data collected at half its ultimate energy,” says Arkani-Hamed. “But even if no extra dimensions are found, that would be perfectly fine by me – after all, we’re in the truth business!”

Heat engine may be world’s smallest

Physicists in the Netherlands have built a heat engine that might be the tiniest ever created. Based on “piezoresistive” silicon, and smaller than a typical biological cell, the engine could find applications in watch mechanisms or as a mechanical sensor.

Engines come in a variety of sizes. The smallest include biological engines such as the flagella that bacteria use for locomotion, which are driven by chemical reactions, or manmade electrostatic engines, which drive ions with electric fields.

But heat engines, which usually rely on the expansion and contraction of liquids or gas, are trickier to downscale. As the devices get smaller, engineers find it harder to design structures that can handle the high pressures and fluid velocities required for a reasonable power output. The efficiency also tends to decrease, because it requires large temperature differences as given by the famous Carnot heat-engine equations. For these reasons, liquid- or gas-driven heat engines rarely get smaller than around 10⁷ µm³.

Driven by stress

In a paper published today in Nature Physics, however, Peter Steeneken and colleagues at NXP Semiconductors in Eindhoven easily overcome this threshold with a heat engine driven by the movement of a solid – in particular, a piezoresistive mass of crystalline silicon. Piezoresistive materials are unique in that their electrical resistance changes with applied stress: when a piezoresistive material is compressed, its resistance increases, and when it expands, its resistance decreases.

The tiny engine consists of a flat resonator of crystalline silicon, 1125 µm³ in size, with two small parallel beams, 0.34 µm³ in size, at one end – rather like a tuning fork with a heavy base. Both beams are anchored such that the compression or extension of one beam, the “engine” beam, heated by a DC current of just over one milliamp, bends the entire device up or down.

Schematic of the small-scale heat engine

Schematic of the device and electrical connections

The key to the device’s operation is an interplay between the engine beam’s temperature, compression and resistance. When it is compressed (device bent up) its resistance is greatest, and this resistance, owing to the DC current, increases the temperature. But the increased temperature makes the engine beam expand (device bent down), which lowers the resistance and hence lowers the temperature. The low temperature again makes the engine beam compressed, and the process thereon repeats in an oscillatory motion of over 1.2 MHz.

“If the volume of the engine beam is taken, this is the smallest heat engine I know of,” says Steeneken. “[But] if the resonator is considered part of the heat engine, it is probably not the smallest in the world.”

A timely breakthrough

Steeneken thinks the device could replace the quartz oscillators and electronic amplifiers used in clocks and wristwatches, because, he says, mechanical devices are more stable than purely electrical oscillators. He also believes it could act as a sensor, because the oscillation frequency changes dramatically with mass, so you could tell if an object comes to rest on the device.

What is more, the heat engine can also double as a refrigerator. In this case, the engine beam must be connected to a voltage rather than a current source, so that any oscillation imposed on the device via Brownian motion – the thermal jiggling of surrounding molecules – is lessened, thereby cooling the surroundings. A tiny fridge could be used to cool other tiny mechanical sensors or mirrors, reducing thermal noise to improve their precision.

Richard Peterson, an engineer who specializes in heat engines at Oregon State University in Corvallis, US, believes Steeneken’s group has “hit upon a new micro-scale thermal engine having phenomenological interest”. But Peterson points out that the engine’s heat is supplied by electricity, which could just as easily perform the mechanical work directly. “When the authors demonstrate driving their oscillator with a thermal source, then I believe they will really have something,” he adds.

Life’s asymmetry may come from space

Processes taking place in outer space, and not on Earth, are likely to have led to the biological molecules found exclusively in either a left-handed or right-handed form. That is the conclusion drawn from recent experiments carried out at the SOLEIL synchrotron facility near Paris in which a number of simple molecules found in star-forming regions exposed to polarized radiation created amino acids with an imbalance of left- and right-handed molecules.

So-called chiral molecules can exist in two forms, with one being the non-superimposable mirror-image of the other, even though both have the same chemical make-up. Although laboratory experiments will tend to produce equal quantities of the left- and right-handed versions of a given chiral molecule, many of the chiral molecules found in living organisms come in only one variety. For example, the amino acids that make up proteins only exist in the left-handed form, while the sugars found in DNA are exclusively right-handed.

Result of evolution

Scientists have long debated the reasons for this asymmetry in living matter. Some have argued that equal numbers of both versions of each chiral molecule were present at the onset of life and that it was only during biological evolution that the imbalance occurred. That view has become increasingly unpopular, however, with the realization that the fundamentally important process of protein folding seems to require chiral imbalances, while for nature to have selected the left- or right-handedness of each molecule during evolution would involve extraordinarily complex processes.

The latest work, published in Astrophysical Journal Letters, provides further backing for the alternative view, that the asymmetry existed before life got going. A group of astrophysicists, physicists and chemists in France, led by Louis le Sergeant d’Hendecourt of the University of Paris South, irradiated molecules of water, ammonia and methanol at low temperatures using circularly polarized ultraviolet light at SOLEIL. The idea was to recreate the conditions found in star-forming regions, where partially circularly polarized light has been observed, and to test the hypothesis that this polarization could induce an imbalance in the creation of left- and right-handed versions of certain amino acids. Other researchers have previously shown experimentally that chiral organic molecules can be created in space-like conditions, and that organic matter might therefore have its origins in space, but could not induce any asymmetry because they lacked a suitable source of radiation.

D’Hendecourt and colleagues found what they were looking for. Irradiation of the interstellar-like matter created an organic residue that contained a noticeable asymmetry in the chiral amino acid alanine. Specifically, they found a 1.3% reduction or a 0.7% increase in the amount of left-handed alanine, depending on the orientation of the light polarization (with the lower magnitude in the second case, they say, being explained by a lower concentration of photons arriving at the sample). Using linearly polarized light, in contrast, they found no noticeable asymmetry.

Conjuring up chirality

The researchers therefore conclude that it is possible to create “asymmetrical molecules of life” in space-like conditions from a mixture that does not initially contain any chiral substances. Team member Laurent Nahon, who works at SOLEIL, points out that the figure of 1.3% is of the same order of magnitude as the asymmetric fraction of amino acids discovered in primitive meteorites and so lends further weight to the idea that chiral asymmetry originated in space.

Nahon says that previous laboratory experiments have shown how a slight imbalance such as this can then lead to 100% asymmetry in a chiral substance, but says it is too early to pin down exactly the mechanism that creates the initial imbalance. It is not clear, he points out, whether the polarized radiation creates more of one kind of handedness than the other or whether it creates equal quantities of both and then destroys one of them more readily, but adds that his group is carrying out additional experiments to try and resolve this.

Laurence Barron, a chemist at Glasgow University, believes that D’Hendecourt and co-workers have carried out a “most interesting” experiment but points out that circularly polarized light is not the only mechanism that has been put forward to explain living matter’s asymmetry. Indeed, he notes, there are a number of other candidates, including the combined effect of unpolarized light and a static magnetic field, spin-polarized electrons from beta decay, and even charge–parity violation. “Whether this latest work has anything to do with the origin of biological chirality is not clear,” he says. “But it certainly merits inclusion in future discussions of the problem.”

Electron beams do the twist

A new twist on transmission electron microscopy (TEM) could enable the technique to unlock even more secrets on the nanoscale. Researchers in the US have produced a helical-shaped beam of electrons that could produce significantly higher-resolution images than is possible with conventional TEM, and it could be used to capture images of hard-to-spot bacteria and proteins.

TEMs work by firing a beam of electrons through a material and measuring how it absorbs and deflects the particles to build up an image of the sample. A microscope equipped with twisted electron beams should be able to produce images with even greater resolution thanks to the fact that the beams exchange large amounts of orbital angular momentum with the materials they interact with.

Twisted beams are already in used in optical microscopy, but it is much more difficult to twist beams of electrons. That is because electrons, like all other particles, have an associated wave whose wavelength is much shorter than that of light, so electron waves need to pass through much tinier structures to become twisted.

A special hologram

This has now been achieved by a group of researchers, including Ben McMorran of the National Institute of Standards and Technology (NIST), who fire electron beams through a specially designed hologram, which causes the beams to diffract. The diffraction created an ordinary plane wave beam, along with several helical-shaped beams, and the researchers were able to confirm the shape of beams and analyse how they evolve in time.

Although there are other ways to produce helical electron beams, the researchers say they used diffraction holograms because they more easily generate controllable beams with precise quantized large orbital momentum. The holograms were fabricated using a very finely focused ion beam to cut a pattern of extremely small slits just 20 nm across though a thin silicon membrane 30 nm thick. The free-standing silicon nitride structures are also quite mechanically robust and can withstand irradiation by the 300 keV electron beam in a TEM. And, they are small enough to be placed in the microscope without having to modify the instrument.

In addition to biological applications, the twisted electron beams could also be ideal for imaging magnetic materials because they can induce torques on charges in a sample by transferring angular momentum to them. “At its most fundamental, magnetism in a material is entirely due to the angular momentum of constituent charges, so being able to probe that using these beams will provide a new way to look at magnetic samples with unprecedented resolution,” said McMorran. “Quite recently another group confirmed this effect, which is very encouraging to us.”

Building on recent work

Indeed, a separate team based in Japan recently described an electron vortex beam produced by a different method and provided data on a single set of fringes showing that, while the electrons had spiral wavefronts, they were not single quantized orbital states. And a third group, based in Europe, described a similar technique to NIST’s but the holograms made were on the micro-scale as opposed to the nano.

“We made more complex, tinier holograms that enable us to achieve 10 times the separation angle between beams – important for applications – and 100 times the orbital angular momentum on electrons,” explains McMorran. “This is possible because each grating in our hologram produces multiple beams with higher diffraction orders containing proportionally larger amounts of angular momentum.”

The team is now working on ways to make the holograms even smaller. “We are taking a more detailed look at the fundamental properties of these helically shaped electron beams too, which is interesting stuff in itself. And to top it all, we are developing theory to understand all of this,” says McMorran.

The Dougal effect and the Partridge

By Hamish Johnston at the AAS meeting in Seattle

The jetlag and non-stop astronomy must be getting to me because I can’t stop thinking about various aspects of astrophysics in terms of my favourite sitcoms.

For example, Father Ted brings us the “Dougal effect”, whereby the actual size of an astronomical object cannot be inferred from its observed size alone. Distance must also be considered and Ted explains this to Dougal using nearby toy cows and a distant herd of real cows. “These are small… but the ones out there are far away,” is the best way to define the effect.

Then there’s that astronomical unit of temperature defined in I’m Alan Partridge. Alan uses a microwaved apple pastry as a weapon, discovering “It’s hotter than the Sun”. To calibrate your thermometer to one “Partridge” put a petrol-station pastry in the microwave for eight minutes and presto.

Well, that’s all from me in Seattle. I’m about to fly back to Blighty and I’ll be looking for astronomy references in The Inbetweeners, which is featured on the in-flight entertainment system.

Tevatron still churning out exciting physics

Fermilab’s ageing Tevatron may be due to cease operations at the end of September but for the time being it continues to produce new physics results. Researchers have found that pairs of top quarks and anti-top quarks are produced at the Tevatron with a greater spatial asymmetry than is expected from theory. The result suggests the existence of particles outside of the Standard Model, but this will need to be backed up with more data before physicists overhaul their current theories.

The Tevatron, located at the Fermilab near Chicago, collides protons with antiprotons. Among the many different kinds of particle produced in these collisions are pairs of top quarks and anti-top quarks, generated via the strong force. Detailed calculations reveal that charge should introduce a slight asymmetry when these particles are produced. The reason for this is that the positive charge of a quark contained within an incoming proton tends to repel a top quark very slightly while attracting an anti-top quark, and vice-versa for an incoming antiproton, thereby introducing a small asymmetry into the distribution of outgoing top quarks and anti-top quarks.

Analyses published in 2008 by the CDF and D0 collaborations at Fermilab did indeed provide evidence for this asymmetry. In fact, by carefully measuring the momenta of the particles into which the top and anti-top quark pairs decay, the researchers found that this asymmetry was larger than predicted by the Standard Model. But the discrepancy between theory and experiment was not that significant – the measured value of the asymmetry lying within two standard deviations, σ, of the predicted value.

Now, however, having accumulated a lot more data in the last two years, CDF has found the discrepancy to be more substantial. In particular, the collaboration studied how the asymmetry varies according to the total energy of the top/anti-top pair. At energies of less than 450 GeV (gigaelectronvolts), they found the asymmetry to actually be slightly negative, at –12±15%, but still therefore in line with the Standard Model prediction of 4%. Above 450 GeV, in contrast, they measured an asymmetry of 48±11%, compared with 8% predicted by theory. This anomaly, say the researchers, has a statistical significance of 3.4σ, or less than a 1 in 100,000 chance that it is simply a statistical fluctuation.

Wouldn’t bet their house on it

Despite the apparently very slim chance of the result being a fluke, CDF co-spokesperson Robert Roser says that no-one in the collaboration is “prepared to bet their house on it”. He points out that “sometimes three sigma results turn into five or six sigma whereas others turn into zero sigma”, adding that theorists are publishing new papers every day regarding the possible new particles that might be able to explain this result, but refusing to be drawn into theoretical speculation himself.

Indeed, Tommaso Dorigo, a member of the CMS collaboration at CERN in Geneva, which is also concerned with the analysis of symmetry in particle decays, cautions that the anomaly might not be so large if the uncertainty in the predicted asymmetry has in fact been underestimated. One potential cause of such an underestimation could be the modelling of how quarks and gluons are distributed within the colliding protons and antiprotons. So-called parton distribution functions, he points out, are measured in other experiments and then extrapolated to the energies at which the Tevatron operates and it is possible, he says, that this extrapolation produces a larger uncertainty than estimated. “The predicted asymmetry is just like a soup,” he adds, likening the soup’s recipe to the Standard Model. “There are many ingredients in the soup, and if the soup tastes bad, this may mean that one ingredient was not fresh; it does not necessarily mean that the recipe is wrong.”

Establishing whether or not the latest result is watertight will require collecting more data at the Tevatron, says Roser. As he points out, the Tevatron is ideally suited to studying this asymmetry because the proton/anti-proton collisions automatically lead to pairs of top quarks and anti-top quarks. Producing anti-top quarks at the LHC, in contrast, requires collecting huge amounts of data because the accelerator’s collision energy must be just right to produce an anti-top quark out of the vacuum. Roser adds that he is “disappointed but not surprised” that the Tevatron has not been granted a hoped-for extension beyond September, but expects the US collider to “still dominate the physics landscape for the next year to 18 months”.

The results are presented in a paper submitted to the arXiv preprint server.

Astronomers say goodbye to the ‘millicrab’

X-ray astronomers have for decades calibrated their detectors using the Crab Nebula – a supernova remnant that appeared to have an extremely steady brightness. But now an international team of astronomers has discovered that the X-ray output of the Crab has dropped by 7% in the last two years. Although astronomers may now have to look elsewhere for X-ray calibration, the discovery could help astrophysicists to gain a better understanding of how the Crab and similar structures generate vast amounts of high-energy radiation.

One of the most studied objects in the sky, the Crab Nebula is the remnant of an exploded star 6500 light-years away from Earth. At its core is a neutron star that spins 30 times per second, driving processes that are responsible for it X-ray and gamma-ray emissions. Until recently the X-ray intensity of the Crab was considered to be so stable that it is used as a “standard candle” to judge the relative brightness of other objects in the sky. Indeed, X-ray brightness is often expressed in units of “millicrab”.

Steady decline

But now Colleen Wilson-Hodge at NASA’s Marshall Space Flight Center and colleagues have published a painstaking study using data from five different instruments that clearly shows variability in the Crab’s output. Team members first became suspicious when they analysed recent data from the Fermi space telescope’s Gamma-ray Burst Monitor (GMB). It revealed a clear and steady decline in the intensity at X-ray energies between 15 and 50 keV.

Not convinced that the decline was real, the team then looked at data from four other instruments – some of which stretched back as far as 1999. The measurements suggest that the intensity increases before dropping by 7% in just two years (see figure). “We’re clearly seeing how much our candle flickers,” said Wilson-Hodge who was at the 217th Meeting of the American Astronomical Society in Seattle.

Giant slinky

Also at the meeting was Roger Blandford of Stanford University, who believes that the variability could be related to the rapid motion of magnetic field lines surrounding the spinning neutron star. Electrons spiral around these field lines, creating synchrotron X-rays. The high energy of this radiation suggests that these lines are moving close to the speed of light, Blandford reckons.

“The magnetic field lines resemble a giant slinky,” he says, “and anyone who has played with a slinky knows how unstable they are”. Blandford believes that such instabilities could lead to variations in the X-ray intensity.

Until recently, astronomers had also believed that the Crab was a very steady source of gamma rays. But last week Marco Tavani of the University of Rome and colleagues published a paper in Science showing that the nebula has produced two powerful gamma ray flares since 2007. Both flares contained radiation in the 100 MeV – 10 GeV range and were about three times the normal intensity of the Crab in that energy range.

The flares both lasted for several days and were spotted by the Italian Space Agency’s AGILE probe. The gamma rays from the Crab are produced by electrons accelerated to extremely high velocities and Tavani said that the flares show that the Crab is a “very efficient” particle accelerator. “We have lost a standard candle for astrophysics but have gained insight into the acceleration process,” he says.

The X-ray study is described in Astrophysical Journal Letters (727 L40).

Gulf Stream edging northwards along Canadian coast

The Gulf Stream off eastern Canada appears to have advanced northward of its historical position in recent decades, possibly in response to anthropogenic climate change. That is according to researchers in North America and Switzerland who say that the changes could have some profound implications for marine life off the coast of Canada.

The new study focuses specifically on a region just off the coast of Nova Scotia. This section of the Atlantic is fed from the north by the cold waters of the Labrador Current, and from the south by the warmer waters of the Gulf Stream. The mixing of these two water flows creates a nutrient-rich ecosystem for species such as cod, which has attracted a large fishing industry.

By analysing micro-organisms preserved in deep-sea sediment, scientists have suspected for several years that the balance between the Labrador Current and the Gulf Stream waters has been changing. Water from the Gulf Stream tends to be more stratified and richer in nutrients so it leaves a different signature in the sediment record than the waters coming from the north.

A smeared record

The trouble with these studies, however, is that their temporal resolution has been poor so researchers have only been able to attribute changes to some point in the past 150 years. “The limitations are partly because there are a lot of organisms down there that stir the sediments, smearing the historical records,” explains Owen Sherwood of the Memorial University of Newfoundland in Canada, the lead author of the new study.

Sherwood’s team has now managed to improve this resolution by turning its attention to corals, which exist in abundance in this part of the north-west Atlantic. Because these organisms – which can live for thousands of years – extract nitrogen from the ocean, they document the changes in the balance between the waters of the Gulf Stream and the Labrador Current, which contain differing nitrogen signatures.

The analysis involved determining the ¹⁵N/¹⁴N ratio within individual amino acids taken from gorgonian corals. As these deep-sea corals grow new rings in their endoskeleton every year, Sherwood’s team was able to determine annual variations in water composition stretching back 1800 years. According to Sherwood, one of the big challenges his team faced was collecting corals for analysis, but these were collected by remotely operated vehicles and others were supplied by the fishing industry, which accidentally scoops up corals in its nets.

Recent climate changes

Reporting their findings in Proceedings of the National Academy of Sciences, the researchers say that the dominance of the warm Gulf waters since the early 1970s appears to be largely unique within this bimillennial period. Although Sherwood’s team links these changes with recent changes in global climate, it says that further analysis is need to investigate the effects on wider ocean circulation. “These water masses do appear to have changed significantly in recent years, though I must emphasize that we have only looked at a very specific region off the coast of Nova Scotia,” says Sherwood.

This is a very exciting result, and one of the nicest demonstrations of the importance of cold-water corals of archives of environmental change Murray Roberts, Heriot-Watt University

Murray Roberts, a bioscientist at Heriot-Watt University in the UK, is impressed by the experimental work. “This is a very exciting result, and one of the nicest demonstrations of the importance of cold-water corals of archives of environmental change,” he says. “[The researchers] have uncovered new evidence that the recent warm and nutrient-rich conditions in the north-west Atlantic off Nova Scotia may be rather different to earlier times – and this change seems to be related to anthropogenic climate change.”

Roberts warns that ecologists cannot make specific predictions about how these changes will impact on local marine life. “Any broad changes in seawater temperature and nutrient supply are very likely to affect sedentary species like cold-water corals and sponges,” he says. “In simple terms they can’t get up and crawl or swim away if conditions become less suitable for them.”

The research group intends to develop its study and has recently obtained a grant from the US National Science Foundation to study the coral record in this part of the Atlantic in more detail.

How do supermassive black holes form?

A supermassive black hole could look like this: but how did it form? (Courtesy: NASA)

By Hamish Johnston at the AAS meeting in Seattle

The universe is full of supermassive black holes (SBHs). Indeed, they make up the core of just about every galaxy. These monstrosities can be a billion times more massive than the Sun. But despite their size and ubiquity, astrophysicists don’t really understand how they are formed.

That was the topic of a fascinating talk by Mitch Begelman of the University of Colorado, who is an expert on SBH formation.

According to Begelman there are two competing theories – the small seed that takes a long time to grow, and the large seed that grows quickly.

The small seed refers to the collapse of a massive star of about 100–1000 solar masses to form a black hole that grows slowly by sucking in surrounding gas and merging with other structures until it is an SBH.

The large seed refers to the direct collapse of a huge cloud of gas to create a supermassive star that could be heavy as a billion Suns. According to Begelman, such stars would be very fragile and would only last a few million years until their cores collapsed to create a black hole.

But instead of exploding in a supernova like much smaller stars, the remaining matter would puff out to become a “quasistar” – resembling a red giant. This surrounding matter is rapidly sucked in and what remains is a black hole that Begelman believes could be as large as one million solar masses. This is around the lower limit of an SBH, and it could keep growing.

Sounds great, but is there any chance of seeing a supermassive star or quasistar?

Unlikely for supermassive stars, says Begelman, because they would be very hard to distinguish from clusters of hot stars. He is a bit more hopeful about quasistars, because they could stand out in the optical and infrared wavelengths. However, he concedes that this would be a tough job, even with the upcoming launch of the James Webb Space Telescope.

To paraphrase Begelman’s conclusion, SBH formation models are getting more sophisticated but the problem has not yet been solved.

Quantum communications boosted by solid memory devices

Two independent groups have demonstrated how a pair of entangled photons can transfer their entanglement to and from a solid – the process that should one day form the backbone of so-called quantum memories or repeaters. These devices would enable quantum communication systems to transmit information over larger distances, with significantly reduced degradation.

“While I was sceptical a few years ago that a useful quantum repeater or quantum network could be built, I am now very confident…that this goal can be achieved in the next five to ten years,” says Wolfgang Tittel of the University of Calgary, Canada, an author of one of the papers that appear in Nature today.

Quantum communication is a means of sending information that is fundamentally secure from eavesdroppers. Two photons must be entangled – that is, have their quantum states inextricably linked – at either end of a channel, over which a “key” for decoding encrypted information can be established. Thanks to the uncertainty principle in quantum mechanics, it is impossible to intercept this key without corrupting it, so the official communicators can always tell if a third party has tried to eavesdrop.

One of the limitations to quantum communication, however, is signal degradation. In conventional information networks, engineers get around this problem by installing repeaters, which record a decaying signal and then re-emit it at its optimum strength. Yet, because it is impossible to record a quantum signal without corrupting it, a quantum repeater must be able to absorb and re-emit the entangled photons without disturbing the entangled state. Quantum memories, a more primitive form of repeaters, have been demonstrated before in single atoms or atomic vapours but not, until now, in the solid state, as is required in a robust communications system.

Choose your crystal

This is the advance made by Tittel’s group, which includes members at the University of Paderborn, Germany; and, similarly, by Nicolas Gisin and colleagues at the University of Geneva in Switzerland. Both groups have shown how one photon in an entangled pair can be absorbed by a crystal doped with a rare-earth ion, so that its quantum state becomes stored as an atomic excitation. A fraction of a second later, a new photon is emitted with that entangled state intact.

There are differences between the group’s demonstrations. For crystals, Tittel’s group used thulium-doped lithium niobate, whereas Gisin’s group opted for neodymium-doped yttrium silicate. In addition, a different type of laser set-up has favoured Gisin’s group, which reports a maximum storage time of some 200 ns at an efficiency of more than 20%; Tittel’s group reports a storage time of 7 ns at an efficiency of 2%. On the other hand, the quantum memory of Tittel’s group functions at a bandwidth of 5 GHz – some 40 times greater than Gisin’s group – which means, potentially, far more information could be sent in the same time.

Val Zwiller, a quantum physicist at the Delft University of Technology in the Netherlands, says the two groups have made “clearly important steps” towards quantum repeaters, but notes several limitations that suggest engineering challenges still lie ahead. One of these is the low efficiency, and the fact that the storage times are not variable, as would be required in a practical device. Another is that the wavelength of the stored photons is not the international standard used in telecommunications, around 1300 nm. “The work presented in these two articles still lacks on several fronts,” Zwiller concludes.

Members from both groups admit there is some way to go before quantum memories or repeaters can be implemented in practical systems, but believe there are no insurmountable hurdles. “Several solutions are already actively pursued in the world, and the rapid progress that has been achieved recently leads us to believe that these will be significantly improved in the coming years,” says Mikael Afzelius, a member of Gisin’s group.

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