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Home » Page 1470

Au revoir, kilogram

 

Once a year, a century-old ritual unfolds at the International Bureau of Weights and Measures (BIPM) outside Paris. It takes place in the basement of the BIPM’s main lab building, which houses a vault secured with three locks. Three antique keys, held by citizens of different countries, open these locks. At the annual meeting of the BIPM’s governing board – the International Committee for Weights and Measures (CIPM) – the three key holders produce the keys, open the vault, unlock a safe inside the vault and inspect its principal contents: the platinum–iridium cylinder that defines the kilogram.

This year’s ritual, on 13 October, was unusual. It was not just that BIPM director Andrew Wallard (UK) and CIPM president Ernst Göbel (Germany) are retiring, making this their last year as key holders. Nor was it that the third key holder, Claire Béchu of the French Archives, was an hour late, delayed by a strike that snarled French traffic for most of the week. The atmosphere was different because a major change is afoot: the cylinder may soon be removed and dethroned.

End of an era?

The BIPM was created by the Treaty of the Metre in 1875, a landmark in the history of measurement, globalization and international co-operation. The ritual began a few years later when the BIPM’s two international prototypes – of the metre and kilogram – were moved to the safe. In 1960 the metre was redefined in terms of the wavelength of light, thus dethroning the metre bar as the standard. (The metre was redefined again in 1983 in terms of the speed of light.) For most of the rest of the 20th century, metrologists could not even foresee the possibility of similarly replacing the kilogram with a natural standard.

However, advancing technologies now make this not only possible but inevitable. One agenda item at this year’s CIPM meeting, the 99th, was to draft a resolution to be submitted next year to the organization to which the CIPM reports – the General Conference on Weights and Measures (CGPM) – that sets out a plan to redefine the kilogram and three other SI base units. If adopted, all SI units will eventually be defined in terms of natural constants.

In short, the platinum–iridium cylinder may not keep its status in the vault much longer, as it will cease to define the kilogram. Within the next few years, Wallard expects to move it from the safe to a laboratory for measurements in the transition to a new definition.

The 18 CIPM members engaged in last-minute debate on how to phrase the redefinition. Some wanted the language to be aimed at the general public, others preferred technical language for professional metrologists, while still others thought the definition ought to be technical but with accompanying explanations.

In the end, the group agreed on a “statement of intention” to redefine using fundamental constants, but only to proceed when the mise en pratique – in other words, the way in which the definition is to be technologically implemented – is available, agreed and producing consistent results. Furthermore, the group agreed to spread awareness of the potential redefinition and its consequences. Part of this effort will take place at an upcoming meeting of the Royal Society in London next month.

The looming redefinition meant that more than the usual number of spectators lined up to see the kilogram standard; not only CIPM members but BIPM staff and a few invited outsiders, including me. One could not help but be awestruck. For 121 years this cylinder has ruled over an international network of masses and scales that stretches from laboratories in national and local metrology offices into everyday life in the form of grocery stores, post offices and home scales. The cylinder is both a thing and an institution. And its days are numbered.

“It’s so small!” exclaimed many of those who squeezed down the narrow staircase into the tiny room containing the safe. The cylinder is, in fact, a mere 39 mm high and as much in diameter. Another surprise for many of the spectators – including me – was that although the cylinder is protected by three bell-jars, it is not in a vacuum. The prototype turns out to be most stable in air, and might outgas in a vacuum. Spectators excitedly held up cameras and mobile phones to take pictures, the way they do at the Mona Lisa, housed at the Louvre across the Seine. Unlike the Mona Lisa, the international kilogram prototype is unharmed by flash photography – though by my extrapolation, Leonardo da Vinci’s iconic work suffers more photography in 15 min than the prototype does in a century.

The critical point

The CIPM’s draft resolution must still be debated by the CGPM at its 24th meeting in October next year. But two technologies appear on the verge of satisfying criteria for implementation, and the CGPM is expected to take steps towards a revision. “It’s a significant step,” says Wallard. “It would provide, for the first time, an anchoring of all the base units of SI to fundamental constants, from which we can build up the whole system.”

The visitors began to walk back upstairs and headed to the traditional champagne reception in the BIPM’s fabulously beautiful gardens overlooking Paris, with the Eiffel Tower in the distance. Wallard, Göbel and Béchu recorded the temperature and relative humidity, and signed a document attesting that they had verified the presence of the international kilogram prototype and its official copies. They secured the safe, and shut and locked the door of the vault with their three keys, securing the cylinder for what may be its last sovereignty as the ruler of the international network of weights, although it will still have a key role to play in implementing a future redefinition.

The kilogram seemed unperturbed.

• A full feature article on the redefinition of the kilogram will appear in the March 2011 issue of Physics World

How the other half lives

This past summer I spent the long US Independence Day weekend at a reunion with three of my undergraduate classmates. All of us went on to earn PhDs in physics, but I am the only practising physicist. The others work in finance – one at a hedge fund, the other two at major banks. They all seem to be enjoying their work, and they have obviously been very successful: our reunion took place near a famous ski resort, where one of the financiers has built a 1200 m2 retreat complete with gym, indoor pool and wine cellar.

Even the brightest graduate students in maths and physics know that their chances of assuming a position like that of their PhD supervisors are slim. For the last 20 years the cream of the crop of physicists leaving the field has gone on to positions in finance, typically in places such as New York or London. Once there, these individuals become hedge-fund managers, derivatives traders and risk managers, to take just a few examples from my own cohort.

So what do these people do? I have always been surprised that scientists in academia are not more curious about the lives of their former peers working in the “real world”. For those who are interested, Scott Patterson’s The Quants does an admirable job of exploring the increasingly mathematical and technological world of high finance, and the activities of the many physicists, mathematicians and engineers who inhabit it.

Patterson writes for the Wall Street Journal, and regular readers of the newspaper will recognize him as an insightful reporter who has covered a number of important topics over the years, most recently the rise of high-frequency trading. In researching the book, Patterson had access to a Who’s Who of prominent “quants”, a colourful group of characters with backgrounds and personalities that will be strangely familiar to anyone who has spent time among physicists. The term “quant” is short for “quantitative” and refers to those who apply mathematical or computational methods to finance.

Patterson begins by introducing the reader to ideas ranging from the basics of modern finance theory, such as efficient markets and random walks, to more esoteric topics such as the late Benoît Mandelbrot’s use of Lévy distributions, or Gaussian copula and their role in the mortgage meltdown. It is worth noting that Patterson focuses primarily on proprietary trading and hedge funds, while spending much less time on the derivatives business or structured finance, two other areas where quants tend to be employed.

All this information makes an effective backdrop to the book’s main story: the August 2007 quant-fund collapse that presaged the mortgage meltdown and subsequent global financial crisis. Patterson chronicles in detail how losses in the mortgage portfolios of banks and hedge funds forced them to sell off liquid assets such as the “vanilla” stocks (those of large Standard & Poors 500 companies) typically traded by large quant funds. This unanticipated deviation from the behaviour of financial models – in particular the wave of fear and paranoia that quickly spread among traders – led to huge losses for the quant funds. One prominent fund, Process-Driven Trading, lost $500m in a single month. This episode reminds us of the central role that human psychology – what the economist John Maynard Keynes called “animal spirits” – plays in markets, rendering them ultimately resistant to mathematical prediction.

To highlight the human element in this tale of financial collapse, Patterson includes many sketches of prominent quants. Among these, Ed Thorpe deserves Patterson’s description as the godfather of quants. An academic mathematician who became famous in 1962 for his statistical analysis of the card game blackjack (recounted in his classic book on card counting Beat the Dealer), Thorpe later became one of the earliest and most successful hedge-fund managers. The similarities between games such as blackjack or poker and modern finance are significant: strategies in each can be refined through mathematical analysis, but success is ultimately still dependent on luck and the ability to manage risk.

Perhaps the most successful of all quants is Jim Simons. In the early 1970s Simons was a graduate student of the mathematician Shiing-Shen Chern at the University of California, Berkeley. I doubt he was a physics student, as Patterson states, although Chern–Simons theory does play an important role in theoretical physics. In 1978 Simons left academia in favour of finance. Today, the Renaissance Technologies hedge funds he set up employ mainly former scientists, including mathematicians, physicists and statisticians, and its Medallion Fund boasts consistently positive returns that are unequalled by any other fund. Simons attributes this to his employees’ training in science, having once told the Wall Street Journal that “the advantage scientists bring into the game is less their mathematical or computational skills than their ability to think scientifically. They are less likely to accept an apparent winning strategy that might be a mere statistical fluke.”

Among the younger quants profiled, Peter Muller seems the most interesting. An idealist who was frequently conflicted about his career in finance, Muller built the Process Driven Trading (PDT) group at Morgan Stanley, which at one point in the late 1990s accounted for a quarter of all profits for the entire firm. During this period, he would sometimes jet off to the Hawaiian island of Kauai for some hiking, leaving his lieutenants and algorithms to manage PDT’s multi-billion-dollar financial positions. Like many of his fellow quants, Muller is an avid poker player, but he is also a musician who occasionally performs in the New York City subway and has released two CDs.

The broader financial crisis that followed the quant-fund collapse is not the major focus of this book. Readers seeking a deep discussion of its causes and consequences should look elsewhere. From my own investigations, I would say that while quants played an important role in the crisis, their influence was secondary to other causes, such as a housing bubble, misaligned incentives within banks and imprudence on the part of government-sponsored entities such as Fannie Mae and Freddie Mac, which provided funding for countless high-risk mortgages.

But that does not absolve quants entirely. It is a pity that although Patterson gives us a broad survey of quant finance, he devotes little space to the bigger question: are developments such as the mathematization of markets and the flow of top brains to financial activities good for society? On this matter, perhaps it is best to leave the last word to Charlie Munger, the long-time investment partner of financial guru-in-chief Warren Buffet. Writing in 2006, Munger had the following to say about the “brain drain” of top talent into finance.

“I regard the amount of brainpower going into money management as a national scandal. We have armies of people with advanced degrees in physics and math in various hedge funds and private-equity funds trying to outsmart the market. A lot of…older people…can remember when none of these people existed…At Samsung, their engineers meet at 11 p.m. Our meetings of engineers [meaning our smartest citizens] are also at 11 p.m., but they’re working on pricing derivatives. I think it’s crazy to have incentives that drive your most intelligent people into a very sophisticated gaming system.”

Between the lines: Christmas special

A picture of a dog with a dunce's cap on
It’s a dog’s life. (Courtesy: iStockphoto.com)

Sit! Roll over! Quantum tunnel!

Chad Orzel talks to his dog about quantum physics. It is not clear what the dog gets out of this arrangement, but the rest of us ought to be grateful for it, because Orzel’s book about their “conversations” is sure to become a classic. Unlike many accounts of quantum mechanics aimed at the general public, How To Teach Quantum Mechanics to Your Dog avoids giving the impression that all the really interesting stuff happened before 1960. Even in the first few chapters, which discuss familiar topics such as wave–particle duality and the thorny question of wavefunction collapse, Orzel makes it clear that quantum physics is still an active area of research. This is certainly true on the experimental side, where lasers have made it possible to perform some Einstein-era “thought experiments” for real. As an atomic physicist at New York’s Union College and author of the Uncertain Principles blog, Orzel is well qualified to explain both old and new findings to a diverse audience. However, he is not the only reason for the book’s success. The other is Emmy, his lively and inquisitive German Shepherd mix. For the purposes of the book, Emmy functions as a doggy Greek chorus: popping up to ask naive-but-illuminating questions. Orzel often plays these interruptions for laughs, and his weary patience with Emmy’s persistent squirrel/food/ tummy-rubbing obsessions will set many dog owners chuckling. However, her crazy suggestions (like the one about getting through the neighbour’s fence using quantum tunnelling) also allow Orzel to counter some common misconceptions about quantum mechanics, and to expand on his explanations without appearing to talk down to the reader. As a quantum physicist, Emmy still has a lot to learn – but as a plot device, she’s fantastic.

  • 2010 Oneworld Publications £7.99 pb 224pp

d(y)/d(braaains)

A paper was published last year with the intriguing title “When zombies attack!: mathematical modelling of an outbreak of zombie infection”. Its authors modelled the spread of flesh-eating zombies using coupled differential equations, and concluded that “the most effective way to contain the rise of the undead is to hit hard and hit often”. Such is the, erm, cerebral flavour of Jennifer Ouellette’s The Calculus Diaries: How Math Can Help You Lose Weight, Win in Vegas, and Survive a Zombie Apocalypse. An English graduate, science writer Ouellette has only recently embraced calculus, and can therefore reveal its simplicity without making any assumptions about what the reader should know. Her examples are blended with witty narrative, including Ouellette’s adventures with her husband, the California Institute of Technology physicist Sean Carroll. Reading the book is a bit like watching a stand-up comedian: you are never quite sure if the reminiscences are real, but you are willing to suspend disbelief as they are actually quite funny. One example features what is possibly the most underrated theme park ride: the tea cups. Using simple force vectors, Ouellette explains why riding them means hanging on for dear life and trying not to throw up one minute, but feeling perfectly fine as you frantically spin the wheel to reach the same giddy gyration the next. As for the log-flume ride, Ouellette offers useful tips on how to stay as dry as possible. Unfortunately, she and Carroll fail to follow their own advice, and are left sitting in wet clothes, lamenting the exponential decay of water evaporation and asking “Will we forever be slightly damp?”.

  • 2010 Penguin £9.46/$15.00 pb 336pp

Who’d be an astronaut?

The hours are long. The work environment is cramped and smelly. You don’t see your family much, and your bosses at Mission Control schedule your daily chores and even your bathroom breaks almost down to the minute. So why do so many people want to become astronauts? Mary Roach’s Packing for Mars does not really explain this, but it does explore almost every other aspect of astronaut life – particularly the icky ones. No question is too embarrassing or scatological for the intrepid Roach, who asks a Russian cosmonaut about blow-up dolls (vetoed by ground control, he says, because “we would need to put it in your schedule for the day”) and cheerfully quizzes astronaut Jim Lovell on what his Gemini VII capsule smelt like when it splashed down after a two week, bath-free mission (“Different than the fresh ocean breezes outside,” Lovell replies politely). That Roach gets away with such queries is probably down to the astronauts themselves. With missions on the International Space Station lasting months rather than the days or weeks of the Apollo era, an even temperament and a sense of humour have long since become integral parts of an astronaut’s “right stuff”. A Mars mission will test those qualities to breaking point, Roach notes, since astronauts bound for the red planet will face all the challenges that their predecessors did (loneliness, space sickness, toilet malfunctions) and more. The underlying seriousness of putting people into space is usually well hidden in this book. However, on the one occasion when it breaks through, the result is deeply moving: as Roach listens to NASA flight surgeon Jon Clark describe what happened to the astronauts on the Space Shuttle Columbia when it disintegrated during re-entry in 2003, she suddenly realizes that Clark’s wife Laurel was on board. And for once, this witty and insightful writer is lost for words.

  • 2010 Oneworld Publications/W W Norton £12.99 hb/$25.95 312pp

Top 20 cosmic conundrums

Why has this book got an elasticated cord on the back cover? Sadly, that is not one of the 20 “biggest cosmic mysteries” tackled in The Big Questions: The Universe by UK author and astronomer Stuart Clark. What we get instead is a trawl through some well-trodden territory in cosmology, astronomy and astrophysics, including the age and size of the universe, the origin of the Earth, and the nature of dark matter, dark energy and black holes. Each chapter wisely gives plenty of historical background and the explanations are clear and concise, even if the writing itself is a tad functional. This is a book to be dipped in and out of, which explains that elasticated cord – it is a stretchable book mark to stop you losing your place.

  • 2010 Quercus £12.99 pb 208pp

A tale of three neutrinos

Obituaries are all about endings, but the one that the Oxford physicist (and prolific popular-science author) Frank Close penned for Nobel laureate Ray Davis in 2006 became a beginning as well. In the 1960s Davis began looking for neutrinos produced in solar fusion, but however hard he tried, the number he detected was never more than half the value that theory predicted. It took decades for physicists to determine that the so-called solar-neutrino problem stemmed from an inadequate understanding of neutrino physics; it took even longer for the Nobel committee to reward Davis’ efforts. After sketching this story in the obituary, Close decided that Davis’ life was worth a whole book. While researching it, however, he found himself increasingly interested in Davis’ longtime collaborator John Bahcall, and the Italian–Soviet physicist Bruno Pontecorvo, who suggested that neutrinos could oscillate between three different flavours. The resulting three-part story in Neutrino is fascinating, although occasionally less clear than it could be. The distinction between neutrinos and antineutrinos, for example, is almost as muddled here as it was for the scientists who stumbled upon it in the 1950s. It is perhaps just as well that Close avoids discussing the even more confusing possibility that the neutrino could be its own antiparticle – although it seems a pity that the book leaves out Ettore Majorana, who suggested it. Then again, since Majorana disappeared without trace in 1938, there is no convenient obituary upon which to base his story.

  • 2010 Oxford University Press £9.99/$18.95 hb 176pp

Space-telescope extravaganza

If Physics World’s September feature “Hubble’s greatest hits” left you hungry for more images obtained by the 20-year-old space telescope, then you are in luck. Two recent books – Hubble: A Journey Through Space and Time and Hubble: Window on the Universe – offer a veritable feast of Hubble photographs, plus a bit of text to explain what you are looking at. In addition to photographs of cosmic wonders, both books contain a fair amount of information on the shuttle missions that serviced the telescope over its lifetime. This is a welcome inclusion, since the first repair mission, in particular, transformed the scope from an orbiting flop to a NASA success story. Journey Through Space and Time is written by NASA astrophysicist Edward Weiler and is slightly more technical than UK science writer Giles Sparrow’s Window on the Universe. Both are aimed at the coffee-table-book market, but Window is practically a table in its own right, measuring a whopping 30 × 36.5 cm, compared with the relatively puny 25 × 31 cm of Journey. Either one, though, would make a beautiful addition to the pile of presents under the tree.

  • 2010 Abrams £19.99/$29.95 hb 144pp; 2010 Quercus £20.00 hb 224pp

Fear rises among Iranian physicists

Academics in Iran have been left in a state of critical fear following the murder in Tehran yesterday of nuclear physicist Majid Shahriari and the attempted assassination of another nuclear researcher, Fereydoon Abbasi.

The separate attacks occurred yesterday morning and both were carried out by unidentified assailants on motorbikes who attached explosives to the victims’ vehicles as they travelled through the capital. Both scientists were based in the faculty of nuclear engineering at the Shahid Beheshti University in Tehran and they are both said to be key figures in Iran’s controversial nuclear programme.

Iranian president, Mahmoud Ahmadinejad, immediately blamed the attacks on foreign enemies saying that “undoubtedly the hand of the Zionist regime and Western governments is involved”. As yet, however, no nation or group has claimed responsibility.

“Everyone is shocked. Programmed assassination of scientists is the last thing we could imagine,” says Reza Mansouri, a cosmologist at Sharif University in Tehran and a former Iranian deputy science minister. He confirmed that the widely held view in Iran is that this attack was carried out by foreign agents.

Medical physicist?

Shahriari was a nuclear researcher and his publication record suggests that he specialized in medical applications of nuclear physics, with a particular interest in modelling neutron transport processes. His role in Iran’s nuclear programme was confirmed by Akbar Salehi, head of Iran’s atomic energy agency, who says that Shahriari was involved in “one of the great projects” of the agency. “Majid Shahriari was one of my students for years and had a good cooperation with the organization,” Salehi told the Islamic Republic News Agency.

Fereydoon Abbasi, the scientist who survived, appears to have a larger involvement with Iran’s nuclear programme. In 2007 his name was added to the United Nations Security Council’s sanctions list for his involvement in “nuclear or ballistic missile activities”.

Monday’s attacks follow the assassination in January of another Iranian physicist, Masoud Alimohammadi, a quantum mechanics and field theory specialist at Tehran University, who was killed by a remote-controlled bomb attached to the side of a motorcycle. The similarities between the attacks have left Iranian academics in an acute state of fear.

Looking for a motive

Despite the government’s conviction that the murders have been carried out by Israeli or US special services, there is also speculation that the attackers could have belonged to one of a number of organizations within the Middle East. The speculation is fuelled by the continuing secrecy that surrounds Iran’s nuclear programme and the increasingly complicated political situation in the Middle East.

Similarly, the motive for assassinating these particular men is not clear, particularly given their seemingly disparate research interests. One thing they do share is that all three men held roles in SESAME – Synchrotron-light for Experimental Science and Applications in the Middle East – a project that aims to create the region’s first major international research centre by building a synchrotron light source in Jordon. Shahriari is listed as having been an adviser to the project, while Abbasi is a member of the Iranian advisory committee to SESAME, and Alimohammadi was a member of the council.

SESAME president Chris Llewellyn Smith, who is also a former director-general of CERN, says that he does not remember Shahriari though the official records state that he did attend one council meeting. Llewellyn Smith does, however, recall meeting Alimohammadi but he, likewise, was only able to attend one meeting before he was killed.

Llewellyn Smith is keen to point out that synchrotrons such as SESAME are not in any sense nuclear facilities. “They are accelerators that are designed to produce intense light with wavelengths ranging from the infrared to X-rays, which are used to study matter on scales ranging from biological cells to atoms,” he says.

Big job, long title

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By Matin Durrani

The BMO Financial Group Isaac Newton Chair in Theoretical Physics at Perimeter Institute.

It’s a bit of a mouthful, but sooner or later someone – in fact “a scientist of the very highest international calibre” – will have to squeeze that title onto their business card.

That shouldn’t be a problem though as the BMO Financial Group Isaac Newton Chair in Theoretical Physics at Perimeter Institute comes with $4m of funding from the BMO Financial Group, or what used to be the Bank of Montreal. So there ought to be plenty of loose change hanging around for a nice set of cards.

The whopping $4m investment – said to be the biggest in the Perimeter Institute’s 10-year history – will be matched by $4m from the institute’s existing endowment.

The BMO Financial Group Isaac Newton Chair in Theoretical Physics at Perimeter Institute will in fact be just the first of five new top chairs, with the others named in honour of James Clerk Maxwell, Niels Bohr, Albert Einstein and Paul Dirac.

In case you missed it, the Perimeter Institute is based in Waterloo, Ontario, and was founded in 1999 by local-boy-made-good Mike Lazaridis, who made his fortune as the founder of Research in Motion – the company that makes Blackberry handheld devices.

The institute, which is keen to attract the brightest and best theorists from around the world, is already undergoing a huge expansion that will see it doubling in size by autumn 2011 with the opening of the Stephen Hawking Centre. Hawking’s already been over as one of the Perimeter’s 20 “distinguished research chairs” after retiring from Cambridge University.

The centre will see Perimeter expanding from its bases in quantum theory, quantum fundamentals, quantum gravity and string theory out into condensed-matter physics, particle physics, cosmology and complex systems.

As for what the BMO Financial Group Isaac Newton Chair in Theoretical Physics at Perimeter Institute will do during their 10-year stint, well, they’ll be “free to engage in investigator-driven research, without limits or mandates”.

But if you’re thinking of applying, don’t. The chair will be “identified through a highly competitive international search, and only scientists of the highest international calibre will be considered”.

Still, it would be nice to think the future chair is reading this right now.

New particle links dark matter with missing antimatter

Physicists in the US and Canada have proposed a new particle that could solve two important mysteries of modern physics: what is dark matter and why is there much more matter than antimatter in the universe?

The yet-to-be-discovered “X” particle is expected to decay mostly to normal matter, whereas its antiparticle is expected decay mostly to “hidden” antimatter. The team claims that its existence in the early universe could explain why there is more matter than antimatter in the universe – and that dark matter is in fact hidden antimatter.

Dark matter is a mysterious substance that appears to make up about 80% of the material universe. Although its existence can be inferred from its gravitational pull on normal matter, physicists have yet to detect it directly and therefore don’t know what it is made of. Antimatter, on the other hand, is easy to create and study in the lab. However, the Standard Model of particle physics cannot explain why antimatter is so rare in a universe that is dominated by matter – a mystery called baryon asymmetry.

Hypothetical and hidden

Now, Hooman Davoudiasl of Brookhaven National Laboratory and colleagues at TRIUMF and the University of British Columbia have proposed a new particle dubbed X that could solve both of these mysteries. X has a mass of about 1000 GeV – making it about a thousand times heavier than a proton. This particle can decay to a neutron or to two hypothetical hidden particles called Y and Φ. Both hidden particles would have masses of about 2–3 GeV. Its antiparticle, anti-X, decays to an antineutron or to the pair anti-Y and anti-Φ.

Physicists have tried to try to explain the baryon asymmetry by invoking a violation of the charge–parity (CP) symmetry – the result being that decaying particles are more likely to generate matter than antimatter. CP violation has been observed in laboratories, but the preference for matter is far too small to account for the proportion of matter in the universe.

X also commits CP violation in a way that author Kris Sigurdson of the University of British Columbia calls a “yin yang” decay pattern. While X decays to neutrons more often than anti-X decays to antineutrons, it is balanced by anti-X, which decays to anti-Y and anti-Φ more often than Y and Φ. When almost all particles with an available antiparticle annihilated one another in the early universe, these discrepancies left a chunk of visible matter and a heavier chunk of dark antimatter to form the cosmos.

Look for proton decays

The team has also thought of how the anti-Y and anti-Φ particles could be detected. Unlike weakly interacting massive particles (WIMPs) – which dominate many theories of dark matter – anti-Y and anti-Φ do not annihilate each other. However, the antiparticles would cause protons to decay, which is forbidden by the Standard Model. If an anti-Y particle collides with a proton, for instance, a virtual interaction with particle X can break the proton apart, transforming it into a positively charged kaon, and turning the anti-Y particle into a Φ particle.

A detector looking for proton decays, such as SuperKamiokande in Japan’s Kamioka mine, could catch the kaon. Kaons produced this way would have much higher energies than those generated by proton decays allowed by other theories that go beyond the Standard Model. Although protons are expected to be fairly resilient to this decay process, Sigurdson says, “This scenario could be on the boundary of detectability.”

“It looks like a very interesting model,” says Dan Hooper of Fermilab. Although at least three more models linking the production of dark matter to the baryon asymmetry are in development, he says that the proton-decay signature sets this scenario apart.

Driven by experiments

Matthew Buckley of Fermilab says that there is a sudden interest in linking dark matter with the baryon asymmetry because of recent experiments that have tried (unsuccessfully) to detect dark matter. Although WIMP models prefer dark-matter particles with masses around 100 GeV, the experiments suggest that dark-matter particles have masses nearer 7 or 8 GeV.

Having such a small mass “definitely isn’t what a WIMP is ‘supposed to look like’,” says Buckley. However, dark matter that also explains the baryon asymmetry seems to be more in line with recent experimental results – which is why Buckley believes it deserves further exploration.

The work is reported in Phys. Rev. Lett. 105 211304.

Guest molecule builds 3D nanostructures

The self-assembly of artificial 3D nanostructures on a surface has been achieved for the first time, claims a team of researchers in the UK. Until now, such assembly was only possible in 2D and the scientists say that the new technique could be useful for creating molecular computers and information storage devices of the future.

Self-assembly is an attractive bottom-up method for fabricating nanostructures. It is simple and quick, and does not require expensive equipment or extreme conditions. Until now, however, the technique has only been used to make simple periodic structures and making more complex architectures remains a major goal in nanotechnology today.

Introducing a guest molecule

Neil Champness and Peter Beton’s team at the University of Nottingham has now used self-assembly to build molecules upwards and outwards from a surface by introducing a “guest” molecule onto the surface. When additional “host” molecules are then added, these spontaneously arrange themselves around the guest, forming stable extra layers around the molecule.

“It is the molecular equivalent of throwing a pile of bricks up into the air and then as they come down again they spontaneously build a house,” said Champness in a University of Nottingham press release. Before now, the molecular “bricks” could only form flat structures – like patios or paths in the house building analogy.

The Nottingham team employed carbon-60, or buckyballs, as the guest molecules. The researchers introduced the C60 onto a surface patterned by an array of host tetracarboxylic acid molecules. These molecules provide an array of nanopores that are stabilized by hydrogen bonding and they can selectively trap other simple molecules, like C60.

Reversible process

Because the C60 is a spherical-shaped structure, the acid molecules assemble around it, forming a 3D network. The technique is a completely new way to build up additional layers around a molecule, says Champness. The self-assembly process is also reversible – for example, by adding planar molecules such as coronene that displace the C60.

“Our work opens up the possibility of preparing increasingly complex molecular arrangements, whose organization can be controlled,” he told physicsworld.com. “This means that the interactions between the molecules, be they magnetic, optical or electronic, can be controlled – something that will be essential for applications in which intermolecular interactions are important.”

Magnetic and electronic functionality

The team would now like to extend its work to create structures that assemble into larger arrays away from the surface, and introduce functionality – such a magnetic and electronic properties – into the arrays.

Ultimately, the technique could help to build new responsive materials with potential for sensing and possibly data storage, added Champness. “Such applications of our work are still a long way off though!”

The results were published in Nature Chemistry.

Graphene supercapacitor breaks storage record

Researchers in the US have made a graphene-based supercapacitor that can store as much energy per unit mass as nickel metal hydride batteries – but unlike batteries, it can be charged or discharged in just minutes or even seconds. The new device has a specific energy density of 85.6 Wh/kg at room temperature and 136 Wh/kg at 80 °C. These are the highest ever values for “electric double layer” supercapacitors based on carbon nanomaterials.

Supercapacitors, more accurately known as electric double-layer capacitors or electrochemical capacitors, can store much more charge than conventional capacitors. An important feature of supercapacitors is that there is an extremely narrow gap between the electrodes – which are ultrathin layers. This means that a large amount of electrical charge can be stored in a tiny volume.

The new device was made by Bor Jang of US-based Nanotek Instruments and colleagues. It has electrodes made of graphene mixed with 5wt% Super P (an acetylene black that acts as a conductive additive) and 10wt% PTFE binder. A sheet of carbon just one atom thick, graphene is a very good electrical conductor as well as being extremely strong and flexible.

The researchers coat the resulting slurry onto the surface of a current collector and assemble coin-sized capacitors in a glove box. The electrolyte-electrode interface is made of “Celguard-3501” and the electrolyte is a chemical called EMIMBF4.

Fast charging

We believe that this is truly a breakthrough in energy technology Bor Jang, Nanotek Instruments

The energy density values of the supercapacitor are comparable to that of nickel metal hydride batteries. “This new technology makes for an energy storage device that stores nearly as much energy as in a battery but which can be recharged in seconds or minutes,” Jang explained. “We believe that this is truly a breakthrough in energy technology.” The device might be used to recharge mobile phones, digital cameras and micro-EVs, he adds.

The team, which includes scientists from Angstron Materials in the US and Dalian University of Technology in China, are now working hard to further improve the energy density of the device. “Our goal is to make a supercapacitor that stores as much energy as the best lithium-ion batteries (for the same weight) but which can still be recharged in less than two minutes,” said Jang.

His team first discovered that graphene could be used as a supercapacitor electrode material in 2006. Since then, scientists around the world have made great strides in improving the specific capacitance of these electrodes but the devices still fall short of the theoretical capacitance values of 550 F/g.

“Despite the theoretically high specific surface area of single-layer graphene (which can reach up to 2.675 m2/g), a supercapacitance of 550 F/g has not been reached in a real device because the graphene sheets tend to re-stack together,” explained Jang. “We are trying to overcome this problem by developing a strategy that prevents the graphene sheets from sticking to each other face-to-face. This can be achieved if curved graphene sheets are used instead of flat ones.”

The work was reported in Nano Letters.

Galaxies pin down dark energy

A new way of measuring the geometry of the universe confirms that dark energy dominates the cosmos and bolsters the idea that this unusual form of energy is described by Einstein’s cosmological constant. The technique, developed by physicists in France, involves a relatively easy measurement of the orientation of distant pairs of galaxies.

Over the past decade or so, several kinds of observation, such as measurements of the distances of remote supernovae, have provided strong evidence that the expansion of the universe is accelerating. Cosmologists believe that this expansion is being driven by what is known as dark energy – a substance with negative pressure that opposes the pull of gravity. Unfortunately, however, they have little idea of what dark energy actually is, having been unable to measure its properties well enough to distinguish between rival hypotheses.

The new approach, devised by Christian Marinoni and Adeline Buzzi of the University of Provence in Marseille, should help narrow down the options as well as provide another means of working out the geometry of space. It involves comparing the known shape of very distant objects with the shape of those objects as revealed by astronomers’ observations. Astronomers don’t measure distances, and hence shapes, directly, but instead measure the extent to which the wavelength of radiation from a distant object has increased – or redshifted. This tells them the speed at which the object and Earth are moving apart.

Unusual geometry

Hubble’s law states that the speed at which objects within the universe move apart from one another is proportional to the distance between them, so knowing the speed of a distant object reveals how far away it is (although this is only approximately true at very great distances). But if the space between that object and the measurer has an unusual geometry or if the expansion of the universe is actually accelerating then the distance measured will not be accurate. So the idea is to vary the quantities that represent the geometry and the strength of dark energy until the distances of interest match up with expectations.

This principle was first proposed by the astronomers George Alcock and Bohdan Paczyński in 1979 but has been difficult to carry out in practice because the redshift due to the local motions of the objects themselves tends to mask that caused by the expansion of the universe. What Marinoni and Buzzi have done is to study a system for which the local motions can be filtered out in quite a straightforward way. They don’t measure a shape as such but instead the orientation of pairs of galaxies several billion light years from Earth that are in orbit around one another in binary systems. They reason that such galaxy pairs should be randomly oriented and so a large set of these binary systems should have an even distribution of orientations. Any deviation from that even distribution would reveal the influence of spatial geometry and dark energy, once the local effects have been removed.

To compare their technique against real observations they measured the orientations of galaxy pairs using data from the DEEP2 galaxy redshift survey and then used more local data from the Sloan Digital Sky Survey to calibrate the motion of the galaxies themselves. Their analysis agreed with the standard cosmological model regarding both the geometry of the universe and the abundance of dark energy – confirming that the universe is flat, in other words that it follows the ordinary laws of Euclidean geometry, and that dark energy makes up around 70% of the energy-matter content of the universe.

Cosmological constant is best bet

They also calculated a value for the strength of dark energy that suggests this substance comes in the form of the cosmological constant – a term that Einstein added to (and then removed from) his equations of general relativity. If correct, this means that the repulsive force is constant throughout the evolution of the universe and that it is mathematically is equivalent to the quantum-mechanical energy of the vacuum.

If you keep the technique simple you can avoid biases. Cosmology is a science where systematic errors are just behind the door Christian Marinoni, University of Provence

Marinoni argues that their technique represents a valuable additional approach to understanding dark energy, since, he says, it is “simple, transparent and faithful”. In particular, he says, it does not rest on any questionable physical assumptions. “If you keep the technique simple you can avoid biases,” he says. “Cosmology is a science where systematic errors are just behind the door.”

Alan Heavens of the University of Edinburgh, who wrote a commentary piece to accompany the paper, agrees that the new method is “nice and direct”. But he warns that it does contain an assumption that must be tested – that the orbital properties of local galaxy pairs are equal to those of galaxies from 7 billion years ago, when the light left the objects catalogued in the DEEP2 survey.

The research is described in Nature 468 539.

A fusion of fusion

By Matin Durrani

We’ve had fusion on our minds quite a lot here at Physics World in recent months.

First we published “Hot fusion” – a great feature by Steve Cowley, chief executive of the UK Atomic Energy Authority at the Culham Centre for Fusion Energy, in which he expresses his optimism that ITER – the huge international fusion experiment being built in France – “will achieve its goal of a burning plasma in the mid-2020s”.

While Cowley examined some of the technical challenges in building ITER, we then hooked up with Sir Christopher Llewellyn Smith who outlined ITER’s many political and financial challenges in a special video interview “ITER – a fusion facility worth building”.

Getting a huge multinational project like ITER off the ground is never easy and Sir Chris – who was chair of ITER council until last year – does a good job of making the case that ITER is a project worth pursuing, despite its price tag of €13bn (and rising). “We cannot afford not to develop fusion,” Sir Chris insists.

In a second video interview “Fusion: from here to reality”, I spoke to David Ward from the Culham Centre for Fusion Energy (CCFE) in the UK, who’s been involved in the fusion game for 25 years. He talked about some of the challenges in going from ITER to a working fusion plant, dubbed DEMO. What’s interesting is that he predicts not just one version of DEMO, but lots, with China and India potentially leading the way.

But fusion research, like all scientific research, would be nowhere without public funding and public support. In our third fusion video “A passion for fusion: nuclear research and science communication”, former CCFE researcher Melanie Windridge describes some of the challenges in communicating the excitement of fusion research to the public – and to school children in particular.

The Institute of Physics, which publishes physicsworld.com, selected Windridge as this year’s Schools Lecturer, a role that has seen her travelling the UK delivering an interactive lecture show about fusion energy to more than 13,000 school students between the ages of 14 and 16.

She talks passionately about her role as a science communicator and offers plenty of practical tips for researchers who want to communicate their work to a more general audience.

Summing up her 2010 lecture tour, Windridge believes that she is lucky with her area of expertise. “Fusion is inherently very interesting and energy is a very emotive subject, so it’s relevant to people’s lives,” she says.

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