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Home » Page 1464

An inordinate fondness for bits

This is how it usually goes. I arrive at a party, get myself a beer and start chatting to people. Soon, it becomes apparent that I am a physicist. Various responses follow. Some people never talk to me again. Some politely say that they failed to appreciate physics in school because of the lack of a good teacher. And then there are some who simply exclaim “You must be brainy!”. But, eventually, I am almost always asked “Do you believe in God?”.

The God question is awkward for non-religious physicists like me. If I do not feel like continuing with a philosophical debate (or if I detect that I would be offending religious sensibilities), I usually just reply with a plain “no” and go and get myself another beer. But if I feel that the person inquiring is up for a challenging discussion, I ask them to define “God” in the first place. This request may sound pedantic, I explain, but the whole point of science is to make well-defined conjectures, so that conclusive tests could be performed that would (at least in principle) falsify these conjectures.

Despite attending many parties, I have still not heard a definition of God that is good enough to make me subscribe to it. Yes, we physicists frequently use the word God in our popular writings, but it is usually meant as a synonym for “the universe”, “reality” and “nature”. Science is all about understanding the world we live in, and sometimes this quest is metaphorically called “knowing the mind of God” – in Einstein’s rather poetic words.

A new book sets out to change all this by, among other things, providing an unusually physical definition of God. Information and the Nature of Reality: From Physics to Metaphysics is a collection of non-technical articles compiled by Paul Davies (a physicist) and Niels Henrik Gregersen (a theologian), and written by biologists, historians and philosophers, as well as physicists and theologians. Each article explores the hypothesis that information is at the root of everything. And I mean everything – from atoms to, perhaps, a deity.

The collection starts with historical essays by philosopher of science Ernan McMullin and philosopher-theologian Philip Clayton, who write about materialism (the worldview that states that the only thing that really exists is matter and that all other phenomena are just interactions between different pieces of matter) and its receding hold on philosophy. The stage being set, Davies and fellow physicist Seth Lloyd then present a physics perspective on information. Davies is without a doubt one of the best popular-science writers in the world, and his article demonstrates why. In it, he explains why, in light of modern physics discoveries, materialism is not the most viable philosophy. Lloyd then expands on this idea by introducing the notion that the universe is a giant information-processing device. This is a view that has emerged from my own field of research – quantum computation – and Lloyd is one of its most prominent advocates.

Next, we get a biological perspective, introduced by one of the finest UK evolutionary biologists, the late John Maynard Smith. Smith’s essay elaborates on the central dogma of biology, namely that information flows from genes to the phenotype but never backwards. In other words, the fact that your father was a bodybuilder when you were conceived does not mean you will have a nice six-pack when you grow up. Continuing through the book, however, we soon encounter a somewhat contrary view from anthropologist Terrence Deacon. He argues that Shannon’s theory of information – which states that the amount of information in an event is proportional to how surprising that event is – may not be sufficient to fully capture biological information. What Shannon’s definition lacks, Deacon says, is “semantics”. In other words, the information itself does not make much sense without the medium that interprets this information and gives it a meaning. The debate on meaning, expanded on by Bernd-Olaf Küppers and Jesper Hoffmeyer, naturally leads us into the realms of philosophy, metaphysics and, yes, theology.

The idea that information is the basic building block of reality very much resonates with me. I recently wrote a book, Decoding Reality (see review from August 2010), in which I explained why information is central to biology, economics and sociology, as well as quantum physics, computing and philosophy. At the end of the book, I muse on how information processing might give rise to reality itself. But the last section of Davies and Gregersen’s collection goes well beyond my own speculations. This is the part of the book that I enjoyed the most, even (or especially) when I disagreed with what was being argued.

This section starts with an article by the late Arthur Peacocke, to whom the book is dedicated. As both an ordained priest and a biochemist, Peacocke embodied two very different aspects of information explored in the present book. His essay is on the problem of evil, namely why an omnipotent and benevolent God would allow evil to exist in the world. Peacocke maintains that evil is simply necessary, since for any creation of good to occur, something first needs to be destroyed (he views the extinction of species in the process of natural selection in this way). So far, so good. Then the Roman Catholic theologian John Haught makes a curious suggestion: an event that has the maximum amount of information – in other words, the least likely one – ought to be taken to represent a religious revelation. The pinnacle of this “theological” section, however, is the proposal by Keith Ward that deity is a form of information-theoretic principle. Ward’s proposal is related to the observation that the universe walks a fine line between total disorder and compete order. Logically, it would seem that disorder is a more natural state of affairs if things were left to themselves, and so to get some order out we clearly need an extra guiding principle. And this extra principle, Ward suggests, is synonymous with God – albeit a very different sort of God than an all-powerful creator. Now, there is a definition of God with which I might almost agree!

The problem, of course, is that once we leave the scientific domain, it becomes rather easy to make unfounded speculations about the connections between information and various other religious concepts. For instance, in the last chapter of this book we encounter suggestions that the Christian Gospel of John could be interpreted in information-theoretic terms. Similar parallels are drawn in the essay by Michael Welker, who discusses the information content in the resurrection of the Christ. Carelessly extrapolated, this sort of exposition might lead to arguments similar to Frank Tipler’s nonsensical “proofs” of various Christian dogmas in his two books The Physics of Immortality and The Physics of Christianity. Amusing as such parallels might be, it is doubtful that they will ever lead to any greater enlightenment as to the nature of reality itself.

When the famous British geneticist J B S Haldane was asked if his research taught him anything about God, he replied “The Creator, if He exists, has an inordinate fondness for beetles”. The collection by Davies and Gregersen suggests, in line with my own views, that we could go deeper than Haldane: the ultimate answer might just turn out to be a Creator with an inordinate fondness for bits. Certainly, bits of information are present everywhere we look, and if you want to know more about this novel take on reality, then I highly recommend Davies and Gregersen’s erudite and entertaining collection.

Kepler bags first rocky exoplanet

The first rocky exoplanet has been discovered by NASA’s Kepler space telescope, according to the mission’s deputy science team leader Natalie Batalha. The planet is called Kepler-10b and has a density on par with that of iron – making it much denser than Earth. The exoplanet orbits a star about 560 light years from Earth.

Kepler-10b is about 20 times closer to its host star than Mercury is to the Sun – and as a result its surface temperature is expected to be as high as 1400 °C. It always shows the same side to its star and is likely to have oceans of molten rock on its day side and a solid night side, according to Batalha. It has an orbital period of about 0.84 days and its star is known to be about the same size as the Sun.

Since the discovery of the first extrasolar planet (exoplanet) in 1995, over 500 more have subsequently been unveiled. While most of these are gas giants like Jupiter, astronomers are getting better at finding smaller exoplanets that could be more similar to Earth. Before Kepler-10b was identified, the best candidate was Corot-7b. While Corot-7b could indeed be rocky, its star is very active, making it difficult to accurately determine its density.

Three key measurements

Kepler-10b, however, is a very different case because its star is very old – about 8 billion years – which means that it is very quiet and much easier to deal with. The team determined the planet’s density by making three different observations. First, they determined its radius relative to the star’s by measuring how much light it blocks when it transits between Earth and its star. Then they determined its mass (again relative to its star) by measuring the wobble of the star caused by the orbiting planet. The final, and crucial step was to determine the radius and mass of the star itself, which was done by measuring the vibrational frequency of “starquakes” on the star.

Putting it all together the team believes that the planet’s density is about 8.8 g/cm3, which is denser than iron.

“This is the first unquestionably rocky planet,” said Batalha today the American Astronomical Society meeting in Seattle. “Its discovery is an important milestone for humanity,” she added. Geoff Marcy of the University of California at Berkeley, who was not involved in the work, said that the discovery “will go into every textbook on astronomy”.

Night and day temperatures

Batalha said that the Kepler team is now studying a possible modulation in the amount of light that reaches the telescope during the planet’s transit. This could allow the researchers to determine the temperatures of the day and night surfaces of the planet. While Kepler-10b is similar to Earth in some ways, it is not within the habitable zone around its star, where life could emerge – the Kepler group already knows it is much to hot for life as we know it.

One mystery surrounding Kepler-10b is how it managed to get so close to its star. Edward Guinan of Villanova University believes that it could be the remains of a gas giant like Jupiter that got so close to its star that the gas was blown off and only the rocky core remained.

Cutting the cost of coffee and a space telescope

By Hamish Johnston at the AAS meeting in Seattle

Times are tough, and cutting costs was on the agenda for the two speakers who opened the 217th Meeting of the American Astronomical Society here in Seattle.

On the podium first was AAS president Debra Elmegreen, who had something to say about the cost of coffee at the Seattle Convention Center – which is astronomical. Indeed, catering is the single largest expense for the meeting, and free coffee adds over one hundred dollars per delegate. Yikes, that’s a lot of money considering that more than 10% of the 2700 folks here are undergraduates.

So no more free coffee breaks between sessions – with the exception of ticketed coffee in the exhibition – and not a groan in the audience. If only the bankers would take the same attitude towards their bonuses!

The next speaker was the Nobel laureate John Mather, who spoke about progress towards launching the James Webb Space Telescope in 2015. The big news is that construction of the telescope’s 18 primary mirrors is well under way and they should all be completed by this summer. “It’s huge,” said Mather, referring to the telescope, which is 6.5 m across, compared with Hubble at 2.4 m.

The next big step, according to Mather, is to place the entire optical system into a giant chamber at the Johnson Space Center to simulate the rigours of space.

Mather took a few questions, which is when the thorny issue of money came up. A recent article in the New York Times pointed out that cost overruns on the James Webb were going to sap funds from other NASA missions. Not surprisingly, Mather was adamant that the extra funds were needed, and expensive projects such as the Johnson testing must go ahead.

I’m afraid that at this point there was some muttering in the audience – and the person next to me said under his breath that the James Webb was consuming far too much of NASA’s astrophysics budget.

Let’s hope Mather and colleagues can keep costs under control for the sake of my neighbour’s blood pressure.

Dwarf galaxy solves supermassive mystery

Ever since supermassive black holes were found to lurk at the heart of most large galaxies, astronomers have wondered what came first: the galaxies or the black holes themselves? Now astronomers in the US have spotted the first known supermassive black hole at the heart of a very young “dwarf” galaxy, where stars are still breeding rapidly. The finding, obtained using data from the Very Large Array radio telescope in New Mexico and the Hubble Space Telescope, suggests that supermassive black holes form before their companion galaxies.

The mystery about which came first – galaxies or their supermassive black holes – initially arose when astronomers found that the mass of the black hole divided by that of the galaxy’s dense central core (or “bulge”) is the same for nearly all large galaxies, including our own Milky Way. It seemed that the black holes and bulges affect each other’s growth – and therefore develop at the same time. However, over the past few years observations seem to be suggesting that young galaxies harbour much more massive black holes than this ratio would allow – suggesting that their black holes formed first.

Super-fast jets

The latest evidence backing this black-holes-were-first theory comes from Amy Reines and colleagues at the University of Virginia and the National Radio Astronomy Observatory (NRAO) in the US. They examined the intense radio waves emanating from the centre of Henize 2-10 – a galaxy with a radius about 3% that of the Milky Way and 30 million light-years from Earth. The galaxy is forming stars very rapidly and some astronomers believe that it resembles the first galaxies to form in the early universe.

The radiation streaming from the centre of Henize 2-10 was found to resemble that expected from super-fast “jets” of material spewed outward from areas close to a black hole. The presence of a supermassive black hole was then confirmed by measurements from the Chandra X-ray Observatory. These revealed intense X-ray emission from the galactic core, which – together with the radio data – indicated what the researchers say is a supermassive black hole a million times more massive than our own Sun.

The case strengthens

While central black holes of about the same mass have been found in other galaxies, those galaxies are larger – and have much more regular shapes – than Henize 2-10. They also do not support the same high rate of star formation as Henize 2-10. “Now, we have found a dwarf galaxy with no bulge at all, yet it has a supermassive black hole,” says Reines. “This greatly strengthens the case for the black holes developing first, before the galaxy’s bulge is formed.”

That view is echoed by Reines’ colleague, Kelsey Johnson of the University of Virginia. “This galaxy probably resembles those in the very young universe, when galaxies were just starting to form and were colliding frequently,” she says. “All its properties, including the supermassive black hole, are giving us important new clues about how these black holes and galaxies formed at that time,” Johnson says.

The research is described in Nature doi:10.1038/nature09724 and is being presented by Reines today in Seattle at the 217th meeting of the American Astronomical Society.

Racetrack memory nears the finish line

IBM researchers have moved another step closer to commercializing “racetrack memory” – a new technology that uses magnetic nanowires as high-density data storage devices. Racetrack involves moving magnetic domain walls – the boundaries between regions of opposite magnetization – along a nanowire using small spin-polarized current pulses. It could make for a new type of magnetic memory that can store up to 100 times more data than existing random-access memories (RAMs).

A conventional computer hard drive uses a motor to rotate glass discs on which magnetic bits are stored in a thin film. Racetrack memory is radically different because it uses electric currents to move magnetic domain walls up and down a nanowire without displacing any atoms at all. Magnetic domain walls are narrow boundaries between regions in a material where the magnetic moments point “up” on one side of the wall and “down” on the other. Walls can be moved inside a material by applying an external magnetic field or by injecting a spin-polarized current pulse (a current of spin-polarized electrons that carries spin angular momentum).

In a racetrack memory, data are stored as a sequence of magnetic domains – separated by domain walls – along a nanowire and individual bits are stored and retrieved by moving the sequence along the nanowire and across magnetic read and write devices. A typical racetrack chip would contain arrays of nanowires a few microns long and about 30 nm wide and could store hundreds of gigabytes of data.

Do walls move instantly?

Stuart Parkin’s team at IBM Almaden Research Center in San Jose, California, has been working on the technology since 2004 and has already developed some basic racetrack prototypes that can read and write simple data sets. However, until now, the researchers did not know how magnetic domain walls move in a nanowire. Do the walls move instantly as soon as the current is applied and come to a stop straight away when the current is switched off, or do they take time to reach their peak velocity and come to a slow stop when there is no current?

Parkin and colleagues say that the second scenario holds true. The researchers came to their conclusion by measuring the time it takes for a domain wall to accelerate to its peak velocity and the distance travelled by the domain wall when it is excited by a current pulse. They did this by using an exciting current pulse and a second probe current pulse while also measuring the time it takes for the domain wall to decelerate from this peak velocity to zero when the current is switched off.

‘Surprisingly long’ time and distance

“We found that the time required for acceleration/deceleration is surprisingly long at around 10 ns and the distance travelled is long too at around 1 µm,” Parkin told physicsworld.com. “However, the distance lagged during acceleration is the same as the distance moved by the domain under its own inertia (or mass) when the current is switched off.” This latter finding is very important because it means that, although the domain wall has inertia, it still moves a distance that corresponds to the length of the applied current pulse. Knowing this, the researchers will thus now be able to precisely control the position of the domain wall along the racetrack by using carefully tailored current pulses or sequences of pulses and so accurately move and retrieve data on it.

“This is clearly an important breakthrough in our understanding of current-driven domain wall dynamics relevant to building racetrack memories,” stated Parkin.

The IBM team followed domain wall motion by measuring the resistance of racetracks made of permalloy – a soft magnetic alloy made of nickel and iron. The presence of a domain wall slightly lowers the resistance of the nanowire.

‘Ingenious technique’

“We use the resistance of the nanowire to determine not only whether a domain wall is present, but also the number of domain walls and even the detailed internal magnetic structure of the domain wall,” explained Parkin. “We used a rather ingenious technique that involves combining two current pulses – one to excite the domain wall and the second to probe the domain wall’s motion – somewhat akin to pump-probe techniques using photon pulses.”

The researchers now plan to build an integrated prototype of a racetrack memory with reading, writing and shifting elements built into the track itself.

The work is reported in Science 330 1810.

Talking astronomy in the shadow of the Space Needle

seattle.jpg


By Hamish Johnston

Greetings from Seattle, where I will be for the next few days reporting from the 217th Meeting of the American Astronomical Society.

Today was mostly one of leisure as my colleagues and I recovered from our 9.5 hour flight and prepared ourselves for the coming feast of astronomy and astrophysics.

Highlights of the day included a ferry ride across Puget Sound to Bainbridge Island. I took the above photo from the ferry and you can see that the skies were particularly leaden – but what you can’t see is the biting wind and chilly temperatures. That’s the futuristic Space Needle at the far left.

But now it’s down to business…or at least a bit of schmoozing at the pre-meeting reception.

Carbon nanotubes spin a yarn

Researchers in US are the first to produce electrically conducting yarns from webs of carbon nanotubes and various powders and nanofibres. The yarns, made by a technique called biscrolling, are very strong and can be woven, sewn, knitted and braided into a variety of structures. They could find applications in energy storage and harvesting, structural composites, photocatalysis and intelligent textiles.

Current methods to transform powders with useful properties into yarns involve using polymer binders to fix the powders in place. A major problem with this technique, however, is that not much powder can be incorporated into the yarn and the resulting composites are not very strong. What is more, the desirable properties of the powder usually degrade during processing.

Now, Ray Baughman and colleagues at the Nanotech Institute at University of Texas in Dallas have developed a new approach that exploits carbon nanotube (CNT) sheets, or webs, instead of polymers to transform nano- or micron-sized powders into electrically conducting, sturdy yarns.

Drawing and twisting

The researchers began by overlaying the “host” CNT sheets with “guest” powders using an electrostatic powder-coating gun and then twist-spinning the guest-host stack to form a biscrolled yarn. The technique is in fact adapted from traditional textile-spinning methods that have been around for millennia and involves drawing 10 nm diameter multiwalled carbon nanotubes from a “forest” of similar length tubes deposited on a substrate while applying a twist at the same time.

Depending on the final application desired, the twisting produces different-shaped structures – known as Archimedean, dual-Archimedean or Fermat scrolls. These structures are similar to the spirals commonly found in nature – for example, those seen in nautilus shells and the now-extinct ammonites.

The difference between the structures is as follows: Archimedean is where a sheet edge is buried deep in a scroll; dual-Archimedean means that the sheet edges are buried in different interconnected scrolls; in Fermat biscrolling, twisting starts from the centre of a symmetric spinning wedge, the edges of which then wrap in opposite directions around the scroll score.

CNT sheets are ideal for making such yarns; its sheets are nearly as light as air (they have a density of around just 1.5 mg/cm3) but are stronger than steel, with a specific strength that can reach 560 MPa cm3/g when densified. This is much higher than the values for Mylar and Kapton films used for ultralight air vehicles that have a strength of about 125 MPa cm3/g. The yarns can also happily be washed in an ordinary washing machine without suffering any measurable damage.

Making superconducting wires

The biscrolled yarns can be made from a variety of powders that can be chosen according to the final application. For example, Baughman and colleagues made superconducting yarn by biscrolling a mixture of magnesium and boron powders (up to 99 wt%) as the guest on CNT sheets, and then thermally annealing the biscrolled yarn. The technique has the added bonus in that it avoids the 30 or more drawing steps needed in conventional powder-in-tube methods to produce millimetre-sized, iron-clad, superconducting wires from a magnesium/boron/CNT precursor.

The team also made biscrolled yarns containing up to 98 wt% graphene oxide nanoribbons that were then converted to graphene nanoribbon yarn by reducing the graphene oxide. These yarns could be used to make weavable anodes for flexible lithium-ion batteries, says Baughman. The researchers showed that they could make electrodes for lithium-ion batteries using LiFePO4 (an environmentally friendly, inexpensive, high-performance lithium-ion battery cathode) as the guest powder.

My colleagues say that I’ll try to biscroll anything, as long as it has interesting properties for applications Ray Baughman, University of Texas at Dallas

The batteries perform well, with a high gravimetric electrical conductivity of 8 S cm2/g, are flexible and mechanically robust. These properties mean that they could be used in applications like energy storage and energy-generating clothing – so-called intelligent textiles. The high gravimetric electrical conductivity also makes for lighter batteries because biscrolled yarn cathodes no longer require the conventional aluminium current collector and conducting particle binders that can bump up electrode weight by over 30%.

And if that wasn’t enough, Baughman and co-workers also fabricated highly catalytic fuel cell cathodes by biscrolling nitrogen-doped carbon nanotube guests. These devices are promising for future applications because they do not contain any of the expensive platinum found in conventional fuel cells.

The biscrolling technique, reported in Science, is a generic one, Baughman says, and can easily be extended to electrically insulating hosts such as SiO2 and Si3N4 nanotubes, among other materials. “My colleagues say that I’ll try to biscroll anything, as long as it has interesting properties for applications!”

The work is described in Science 331 51.

Nobel Foundation stays firm on symposium cancellation

The Nobel Foundation is refusing to back down over its decision to cancel a Nobel Symposium on water in biology and medicine that was due to take place in August in Stockholm. The three-day-long conference was cancelled last September by the Nobel Symposium Committee because of the way in which the meeting was organized. However, physicists and other scientists who were set to speak at the conference are dismayed at the cancellation, saying that they have not been given any firm reasons as to why the meeting has been called off. The Nobel Foundation, which also awards the Nobel Prize for Physics, finances several official Nobel Symposia each year, with 2011 featuring meetings on international relations as well as on mind, machines and molecules.

The symposium on water in biology and medicine was first initiated in April 2009 by John Skår of the Karolinska Institute in Sweden. By May 2010 the proposal to hold the meeting was assessed by members of three Nobel-prize committees – physics, chemistry, and physiology and medicine – who each recommended that the meeting go ahead. The proposal was then sent to the board of the Nobel Foundation, which accepted the proposal in June and awarded it SEK 1.1m (about £100,000).

Following the board’s approval, the meeting was officially named as Nobel Symposium 151 (NS151) on water in biology and medicine, with Skår acting as co-ordinator of the meeting’s organizing committee. “Nobel Symposia are prestigious events,” says Peter Coveney, director of the Centre for Computational Science at University College London, who was also on the NS151 organizing committee. “They are taken as an indication of where future Nobel prizes may be heading.”

In September last year, after invitations were sent to speakers, the Nobel Symposium Committee (NSC), chaired by Michael Sohlman, who is also executive director of the Nobel Foundation, asked Bengt Nordén from Chalmers University of Technology in Gothenburg to assess the merits of the invited speakers. The review was initiated after two former members of the organizing committee wrote a letter to the Nobel Foundation complaining about the meeting’s organization and its speaker list.

Dissenting voices

However, one of the dissenting voices was apparently one of five people who gave evidence to Nordén for his report to the NSC. In an e-mail seen by Physics World, Nordén wrote in early October that the people he interviewed felt that the majority of the speakers “did not meet the criterion of being eligible for a Nobel Symposium”. Nordén concluded that there was “something” in the criticism of the former members.

It’s like scientific misconduct. You can’t just break a contract like the Nobel Foundation has done. Eugene Stanley, Boston University

On 30 September the NSC decided to cancel the meeting. It then sent a brief letter to the invited speakers on 5 November saying that “the planning of the Nobel Symposium 151 has not taken place in the way required for a Nobel Symposia”. The letter, sent by Sohlman, stated that the cancellation was made with “no judgements whatsoever about the scientific standing of any of the invited participants”.

Skår told Physics World that he received no explanation as to why the planning was not sufficient, given that the proposal had already been accepted, adding that the organization committee had also not been given any opportunity to refute the accusations. Indeed, many researchers are furious that the meeting has been cancelled at such a late stage. “This could become the major scientific scandal of the year,” says Eugene Stanley from Boston University, who was due to speak at the conference about anomalies in liquid water. “It’s like scientific misconduct. You just can’t just break a contract like the Nobel Foundation has done.”

Requirements not met

Sohlman told Physics World via e-mail that concerns regarding NS151 were raised in September when the NSC realized “that the work of the organizers did not meet the stipulated requirements”. When asked to elaborate what these requirements are, and whether Nordén’s report played any role in the cancellation, no response has yet been given.

Skår refutes Sohlman’s reasons. “There are no such stipulated requirements, and there are also no such criteria for selecting the eligibility of speakers,” says Skår. “One would expect the foundation to adhere to high standards of professional integrity and transparency.”

Stanley is now calling on the foundation to uphold its original decision to hold the meeting. “It is a matter of ethics,” says Stanley. “We have to stop such unethical behaviour before others cancel meetings months after they are announced.” Sohlman, however, told Physics World that the decision to cancel NS151 is “irrevocable”.

Thunder storm radiation amazes physicists

The radiation produced during a lightning storm is more energetic and potentially threatening to aircraft than previously thought, claim researchers in Italy. Studying this radiation in closer detail could help scientists to probe some of the big unanswered questions in the study of thunder storms, such as how lightning is triggered in clouds.

Scientists have known for a long time that the large electric fields and currents produced during thunder storms can also generate X-rays and gamma rays in the vicinity of clouds. But in the early 1990s a rare type of lightning event was discovered that can produce extremely bright, energetic gamma rays – known as terrestrial gamma ray flashes. So far, however, it has been difficult to determine specific details about this phenomenon such where the radiation originates from and its energy range.

Now researchers involved with the Italian Space Agency’s AGILE mission have been able to home in on terrestrial gamma ray flashes. Launched in 2007, this satellite is dedicated to observing gamma rays originating from terrestrial and cosmic sources. Its silicon-based gamma ray detector has recently been fitted with new software that enables the satellite to take snapshots of the radiation at sub-millisecond time intervals.

Flash strikes

The AGILE team led by Marco Tavani gathered data collected from 130 terrestrial gamma ray flash events occurring in the past two and a half years. Reporting its findings in Physical Review Letters, Tavini’s team noted radiation emerging in all directions from the upper atmosphere, covering a wide range of energies. In some cases gamma rays were up to 100 MeV – more than twice as energetic as previous measurements.

100 MeV is absolutely amazing considering that these things are made deep in our atmosphere by thunderstorms Joseph Dwyer, Florida Institute of Technology

“100 MeV is absolutely amazing considering that these things are made deep in our atmosphere by thunderstorms,” says Joseph Dwyer, a lightning physicist at the Florida Institute of Technology, who was not involved in this research. “For comparison, 100 MeV is even impressive coming from big explosions on the Sun.”

Tavani tells physicsworld.com that his team became interested in these gamma rays because they can reveal details about the processes occurring in thunderclouds. “People have sent probes directly into clouds but it is very difficult to catch this kind of strong lightning in action,” he says.

Runaway electrons

While the specific details of lightning initiation are not yet known, scientists do know that it requires large potential differences to be established within thunderclouds. This can transform the clouds into particle accelerators where electrons can be rapidly accelerated, producing an avalanche of particles as they liberate more electrons along the way. These “runaway electrons” are believed to be the source of gamma rays, and they may be involved in producing the initial spark for bolts of lightning.

Tavani’s team intends to continue observing gamma rays emitted during thunderstorms. The researchers are planning a programme for surveying the upper atmosphere in more detail, which will involve sending an aeroplane equipped with gamma ray detectors into the vicinity of a thunder cloud. “We will have to be careful and I think flying over the clouds will be best because you don’t get the dangerous discharge,” Tavani says.

In addition, Tavani is concerned with the potential threat the gamma rays may cause to aviation. In the next few months his team will be publishing a separate study that reviews the impacts of gamma rays on electrical equipment.

Physicists find new clue in coronal heating mystery

The latest research from a team of international astronomers could help to explain the long-standing mystery of why the Sun’s outer atmosphere – or corona – is so much hotter than its surroundings.

The corona, the vast gossamer atmosphere of plasma visible from Earth during a total solar eclipse, can notch up temperatures in excess of one million degrees Kelvin (MK). Several rival explanations have jostled to account for why the corona is unexpectedly over 200 times hotter than the visible surface, or photosphere, of the Sun. However, in recent years one theory has charged from the back of the pack to become a frontrunner in the race to solve the mystery: spicules – or fountain-like jets of plasma. These emanate from the chromosphere, a relatively thin layer separating the photosphere and corona.

Previously, the spicule theory was largely discredited due to an absence of correlating phenomena in the corona itself. Then, in 2007, researchers led by Bart De Pontieu at the Lockheed Martin Solar and Astrophysics Laboratory in California, US, found a new breed of spicule, which they dubbed “Type II”; Type II spicules are shorter-lived but faster moving than their Type I cousins. In his latest research, published in Science, De Pontieu and colleagues now believe they have found evidence implicating Type II spicules in the heating of the corona.

Tell-tale signature

“Spicules play a significant role in coronal heating, which doesn’t fit any of the current theories. This also suggests that there is significant heating going on in the first few thousands of kilometres [of the corona], which is very different from what people have assumed before,” De Pontieu told physicsworld.com. When the spicule jets occur on the solar disc they leave a tell-tale signature in the spectral lines observed in the chromosphere: fast-occurring blue-shifts, known as rapid blue-shift events (RBEs). De Pontieu used data from the Solar Optical Telescope (SOT), aboard the Sun-orbiting Hinode spacecraft, to build up a catalogue of RBEs, which he then compared to coronal data from NASA’s Solar Dynamics Observatory.

We haven’t completely solved the problem, but we’ve certainly added a significant new wrinkle to it Scott McIntosh, NCAR

“The high temporal and spatial resolution of this generation of solar observatories allowed us to discover that spicule events in the chromosphere are correlated to brightenings in the corona,” De Pontieu explained. The team found that the vast majority of the spicule plasma is only heated to between 0.02–0.1 MK and sinks back down into the chromosphere. However, the key finding is that a small but significant portion of the plasma is heated beyond 1 MK and uplifted into the corona. The researchers found this process to be ubiquitous across the Sun.

However, the search for a definitive answer to the coronal heating mystery isn’t over. “We haven’t completely solved the problem, but we’ve certainly added a significant new wrinkle to it,” second author Scott McIntosh, at the National Center for Atmospheric Research (NCAR) in Colorado, explained.

Combination of different mechanisms

Lucie Green of the Mullard Space Science Laboratory at University College London, who was not involved in the research, agrees: “My hunch is that the solution to the [coronal heating] problem is a mix of answers, a combination of different mechanisms. We shouldn’t be looking for just one ‘golden’ answer,” she said. “However, this research is something new and it will definitely sit alongside the other explanations,” she added.

Whatever mechanism, or mix of mechanisms, is responsible for the soaring temperatures of the corona, finding an answer is important. “Heating causes the corona to expand outwards, forming the solar wind. This is ultimately what drives lots of processes throughout the solar system, so it would be great to better understand how that heat is being put into the solar atmosphere,” said Green.

In order to pinpoint the exact role of spicules in coronal heating and to understand what drives and heats them in the first place, De Pontieu hopes to exploit an upcoming NASA mission. “Fundamentally we need new instrumentation. The Interface Region Imaging Spectrograph (IRIS), is due to launch in December 2012 and it is really focused on the physics of the region between the solar surface and corona,” De Pontieu explained. “That would really help us to follow up this research,” he added.

The research is described in Science.

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