Independent analyses of data from the Fermi Gamma-ray Space Telescope have found no trace of low-mass dark matter – the mysterious substance thought to make up much of the universe. The results appear to go against recent direct evidence for low-mass dark matter, although some physicists believe there is no conflict.
Dark matter is an invisible substance thought to make up nearly a quarter of the mass/energy of the universe. While its gravitational pull is needed to explain the properties of massive structures such as galaxies, it does not interact strongly with light and has therefore yet to be observed directly. The most popular candidates for dark matter are so-called weakly interacting massive particles (WIMPs). To spot these WIMPs directly, researchers have built detectors in underground labs where the low background noise ought to allow any signals to stand out. These detector experiments include DAMA and CRESST, both based underground at the Gran Sasso laboratory in central Italy, and CoGeNT, based in the Soudan mine in the US.
For just over a decade, the team behind the DAMA experiment has claimed to see an annual modulation in its data that would be consistent with the Earth’s orbit passing through a prevailing “wind” of dark-matter WIMPs in our galaxy. Those signals were joined last year by hundreds of WIMP-like blips in the detectors at CoGeNT and, in September, a few dozen WIMP-like blips in the detectors at CRESST. Although the signals of these three experiments do not match perfectly, they seem to be pointing to a relatively light WIMP with a mass of the order of 10 GeV/c2.
Colliding WIMPs
If such a light WIMP does exist, it should leave additional, indirect evidence in data obtained by the orbiting Fermi telescope. This telescope, which is a joint mission between NASA and other international space agencies, ought to be able to record any gamma rays produced when WIMPs collide and annihilate one another. But now two independent analyses of the Fermi data have found – for at least two major types of annihilation – no gamma rays for light WIMPs.
The first analysis was done by Johann Cohen-Tanugi and others in the Fermi-LAT (Fermi Large Area Telescope) collaboration. The team used a computer model that calculates the output of known sources of gamma rays coming from the vicinity of our galaxy’s companion “dwarf spheroidal galaxies”. By comparing the model with the actual Fermi data, the researchers found no significant extra contribution from WIMPs with a mass of less than 30 GeV/c2 annihilating into either bottom quarks or tau leptons.
The second analysis was carried out by Alex Geringer-Sameth and Savvas Koushiappas of Brown University in the US. It also examines gamma rays coming from the Milky Way’s dwarf spheroidal galaxies, but it relies on a different method. The researchers use gamma rays recorded from regions surrounding the dwarf galaxies as the background, and then compare these with the gamma rays emitted from the direction of the dwarf galaxies themselves to look for a WIMP component. The analysis revealed no WIMPs with a mass less than 40 GeV/c2 annihilating into bottom quarks, and no WIMPs with a mass of less than 19 GeV/c2 annihilating into tau leptons.
Two analyses are better than one
“The two studies are complementary to each other, and they represent two different ways that one can approach the problem,” says Koushiappas. “In my opinion, the complementarity of these two papers makes the derived constraints much stronger than any single, one analysis could do.”
These constraints might appear to go against the evidence reported by DAMA, CoGeNT and CRESST, but those collaborations do not necessarily see any conflict. “One would be naively tempted to claim that there’s some tension there,” says Juan Collar, spokesperson for CoGeNT. But the type of annihilation considered by the Brown and Fermi-LAT researchers is not the one primarily being considered by those working on other dark-matter experiments. “When the exact meaning of those bounds and hints is examined, things look a lot less confusing,” he adds.
Rita Bernabei, spokesperson for the DAMA collaboration, says the DAMA signal is also compatible with WIMP masses above the new Fermi limits. “The results in the papers…are model-dependent and strong speculations should have been applied to derive such model-dependent limits,” she says. A spokesperson for the CRESST collaboration could not be reached for comment.
Underlying physics not yet understood
Michael Kuhlen, a theoretical astrophysicist at the University of California, Berkeley who was not involved with the research, also believes there is not necessarily any conflict between the Fermi data and those of the direct dark-matter searches. He says it is difficult to relate the annihilation probabilities used in the latest studies with the probabilities of WIMP interactions inferred by direct dark-matter searches without properly understanding the underling particle physics.
But are we now further from knowing what dark matter actually is? “Certainly we are not any closer,” adds Kuhlen.
The results of both the Brown group and the Fermi-LAT collaboration are due to be published in Physical Review Letters and are available as preprints at arXiv.org:1108.2914 and arXiv.org:1108.3546, respectively.
The International Union of Pure and Applied Chemistry (IUPAC) has unveiled the proposed names for elements 114 and 116. Named after Georgi Flerov, founder of the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, element 114 will, if approved, be called flerovium and have the symbol Fl. Element 116, meanwhile, will be named livermorium after the Lawrence Livermore National Laboratory (LLNL) and given the atomic symbol Lv.
The elements were created by researchers at the JINR back in 2004 and were both confirmed by scientists at the LLNL in California and the Centre for Heavy Ion Research (GSI) in Darmstadt, Germany.
Commenting on the suggested names has now opened to anyone for a five-month period, which will end in April. So what do you think? In this week’s _Facebook_ poll, we want you to answer the following question.
Do you like the element names livermorium and flerovium?
I like both of them
I like livermorium but not flerovium
I like flerovium but not livermorium
They’re both boring and unimaginative
To cast your vote, please visit our Facebook page; you are also free to suggest your own names by posting a comment.
In last week’s Facebook poll we asked you when you thought we will see the first working nuclear-fusion reactor supplying electricity to a grid? Nearly half of you (49%) chose the most optimistic option, saying that we could be running our toasters on fusion within 30 years. Some 21% foresee fusion reactors in 30–60 years and 7% think they will be a reality within 60–90 years. However, 23% of you believe that it’s unlikely ever to happen.
Commenting on a related Facebook posting about an article on the Canadian company General Fusion, Michael Simmons wrote “In high-school physics in 1968, I was told practical fusion power was 20 years away. In 1971, in college-modern physics, it was 20 years away. As a high-school physics teacher, I attended a conference on the energy future in 1985 and fusion was 20 years away. In 2005, at a local conference on future energy sources, fusion was mentioned as being 20 years from becoming economically feasible. I don’t believe it is never, but I have come to believe it won’t be in my lifetime.”
There’s nothing better in physics than a bit of a ding-dong, and you can, of course, rely on string theory to supply the ammunition for it.
String theory, after all, polarizes opinion seemingly like nothing else: its proponents deem it a rigorous framework that could unify the fundamental forces, while its critics dub it preposterous guff that makes no testable predictions of the world.
One of string theory’s masterminds – Michael Duff of Imperial College London – has now hit back at his critics with a paper in a special issue of the journal Foundations of Physics published to mark 40 years of the theory. You can read Duff’s 19-page paper either in Foundations of Physics, which is open to all until 31 December 2011, or as a preprint on arXiv.
Duff reckons that “much of the criticism has been misguided or misinformed” and goes on to outline why string theory is valid, before taking a pop at various critics – not only other researchers, notably Lee Smolin and Peter Woit (who he calls “a single-issue protest group”), but also the media, including Physics World.
Duff’s complaints about the media are a little confused in my eyes, stemming in part from the fact that journalists paid too much attention, in Duff’s eyes, to the work of Garret Lisi, who in 2007 published a (non-peer-reviewed) paper entitled “An exceptionally simple theory of everything” that controversially claimed to unify “all fields of the standard model and gravity”.
Although Duff says Lisi is “by no means a crackpot”, he complains that “journalists love [crackpots]” and seems to suggest it was for that reason that so much coverage was given to Lisi’s work, even though the latter does not have much to do with string theory. All I can say is that we at Physics World are no fan of crackpots either.
Duff’s paper has, not surprisingly, drawn a vigorous response from Woit himself, whose blog post can be read here. Woit thinks that attempts by Duff and other string theorists to respond to their critics has “damaged not just the credibility of string theory, but of mathematically sophisticated work on particle theory in general”.
Traditionally, physics and biology have been viewed as separate disciplines, but the interface between the two sciences is an exciting place to be working right now. A new website called biologicalphysics.iop.org has been created to assist in the teaching of biological physics to undergraduate students.
In this video report, Physics World meets the project’s director Athene Donald in the Cavendish Laboratory at the University of Cambridge, where she is based. Donald explains why she believes physicists should look beyond narrow academic boundaries because they have the tools to tackle certain problems in biology. “The laws of physics apply to biological systems, so one could argue that the divisions are slightly artificial and certainly not necessarily very helpful now,” she says.
Donald, who worked in polymer physics before moving into the field of biological physics, explains that the idea for the new website emerged following the Engineering and Physical Sciences Research Council’s 2005 International Review of Physics. This review highlighted the fact that many undergraduate physics students get no exposure to biological physics and that this is partly because many universities do not have researchers with the relevant expertise. The new website is designed to help remedy this situation by providing resources for lecturers to use alongside their standard material; it is hosted by the Institute of Physics, which also publishes Physics World.
The Cavendish, of course, holds an important place in the history of biological physics, being the lab where Watson and Crick helped to uncover the double-helix structure of DNA. The lab was relocated in the 1970s from the centre of Cambridge to its outskirts, but despite this, Donald says that she still finds the lab’s history to be a source of great personal inspiration. “Everyday, when I walk to my office from where I park my bike, I walk past the museum, so it’s a constant reminder of all the great things that have happened here,” she says.
In a separate video report, Donald discusses her own group’s ongoing research into how proteins aggregate and how this process is involved in Alzheimer’s disease.
Then, in a final video, Physics World meets Pietro Cicuta, one of Donald’s colleagues at the Cavendish’s biological-physics lab. Cicuta’s group is interested in the mechanical properties of red blood cells and how they are affected by malaria parasites. Cicuta also explains how biological physics differs from biophysics, and how he developed an interest in the field after starting out as a physicist.
Public outreach has long been a central component of the scientific enterprise. In the 19th century, travelling lecturers gave public scientific talks and demonstrations to general audiences – Michael Faraday’s 1860 “Christmas lecture” on the chemistry of candles being a notable early example. This tradition continued in the early days of radio (when audiences familiar with on-air interviews with real scientists found Orson Welles’ fictional War of the Worlds broadcast all too convincing) and moved on to television in the 1950s with programmes such as Mr Wizard, as scientists sought new ways of using mass-communication technology to reach a broader audience. Today, outreach-minded scientists have another option for communicating science and the scientific method: they can become consultants on big-budget Hollywood films.
A scientist who consults for a science-fiction or superhero film can easily reach an audience of millions – far more than will ever read the research papers he or she publishes in refereed journals. Hollywood’s demand for such consultants has increased in the past few years and the scientific community has responded. In the US alone, there are now two separate groups that aim to match academics with film and television creators: the National Academy of Sciences’ Science and Entertainment Exchange and the National Science Foundation’s Creative Science Studio. But are such arrangements beneficial for science? What is driving Hollywood’s interest in the opinions of scientists, and what do the scientists get out of it? And what are the advantages and pitfalls of this deal?
These questions are addressed in Lab Coats in Hollywood: Science, Scientists and Cinema by David Kirby. A senior lecturer in science communication studies at the University of Manchester, UK, Kirby has written that rare book: a scholarly work at the intersection of popular culture and serious science that is accessible and highly readable. In it, he provides a thorough history of the ways scientists have assisted filmmakers, beginning with Fritz Lang’s 1929 Woman in the Moon, which employed rocket-science pioneers Hermann Oberth and Wernher von Braun as consultants.
To uncover this history, he has interviewed an admirably broad cross-section of participants on both sides of the academia/Hollywood divide, including Jack Horner, paleontology consultant for the Jurassic Park films; the makers of the 1998 comet thriller Deep Impact and the scientists they relied upon; and Alex McDowell, production designer for Minority Report and Watchmen. (Disclosure: he also interviewed me, since I was one of the first participants in the Science and Entertainment Exchange programme and I have worked pro bono as a consultant for the Warner Bros films Watchmen, Green Lantern and next year’s The Amazing Spider-Man.)
Hollywood’s interest in telling engaging stories has brought filmmakers to the halls of academia because some of them realize that if the audience is noticing an egregious scientific blooper, or thinking that the scene they are watching is not an accurate representation of a real laboratory, they are not paying attention to the story. This can be fatal for science-fiction and superhero films, where it is crucial that once the audience has suspended disbelief and bought into the film’s fantastic premise, they are not subsequently jolted by obvious bad science.
Kirby documents various small ways that scientists can help filmmakers avoid such jolts, including making sure the set of a biological science lab contains a box of KimWipes. One of my favourite examples of little things “ringing true” is a scene in Iron Man (2008) where the actor Robert Downey Jr – playing the industrialist genius Tony Stark – is constructing a hi-tech suit of armour in his basement laboratory. We see Downey doing soldering on the suit – and he is doing it right, using the same soldering iron I have in my own condensed-matter physics laboratory. Although I am probably the only person who saw Iron Man and applauded the soldering, such attention to detail is necessary when creating a believable fake reality. Leave it out, and the audience may find it harder to accept that they are watching Stark create his superhero suit, and not Downey playing a role.
Scientists can also get involved in bigger issues, such as helping with script development. With Deep Impact, the filmmakers brought together a team of scientists to provide as much realistic information as possible, and their extensive and substantial suggestions were, for the most part, incorporated into the film. In contrast, the director Michael Bay mostly ignored the advice of his science consultant on Armageddon, a similar meteor-impact adventure that came out the same year. As a result, Bay’s film is riddled with bad science, though sadly this does not appear to have hurt it at the box office – demonstrating that, ultimately, engaging visuals and dramatic storytelling can trump scientific accuracy.
So what do the scientists get out of being consultants? Kirby’s answer is that interactions with Hollywood provide not only personal exposure and enhanced outreach opportunities, but sometimes also a chance to shift the terms of a scientific debate. Jack Horner’s proposals about the common ancestry of dinosaurs and birds, for instance, were controversial among paleontologists back in 1993, but you would not know it from watching the first Jurassic Park film, which treated his theory (more or less) as established fact. Similarly, the experts who consulted on Deep Impact disagreed about the appearance of a comet’s surface, the intensity of its outgassing and the size of debris in its coma; inevitably, the filmmakers went with the answers that made for more dramatic visuals. In some cases, Hollywood exposure can even help scientists bring about new technologies, such as space travel or virtual reality, using films as a vehicle to get public opinion behind increased research funding or better preparation for a deadly contagion. Such silver-screen arguments can reach more people and be much more persuasive than those that come from, say, NASA or the Center for Disease Control.
In the best-case scenario, as Kirby states, “scientists…can help filmmakers craft images and narratives that convey the excitement of scientific research or communicate a sense of awe about the natural world”. Crucially, it is in scientists’ interest to help bring about this best-case scenario, since less-credible “authorities” wait in the wings. It may be frustrating, though understandable, when sound scientific advice is ignored in favour of exciting visual story-telling, but as Kirby points out near the end of Lab Coats, the alternative is to leave the job to “individuals involved in fringe pseudo-sciences such as cryptozoology, UFOlogy and parapsychology [who] have jumped at the chance to consult on films”. Kirby makes a strong case that this predominant form of mass communication gives scientists the power to effect real, positive change in the public’s opinion of scientific research. And from the Spider-Man films, we all know what comes with great power.
In his declining years, Albert Einstein cut a saintly figure, moved to anger only when clamorous journalists, photographers and quote-hunters tried to disrupt his working routine. When they knocked on the door of his home in Princeton, he almost always refused to meet them, though his colleague Nandor Balazs once told me that the world’s most famous scientist could not resist talking with a young journalist who had travelled 10 000 miles from India to ask him a few questions about religion. As he got up from the kitchen table and prepared to meet her, he sighed to Balazs, “Oh well, I suppose I’d better go and play God again.”
Einstein was, however, a soft touch as a correspondent, and we now know that he spent many hours patiently replying to letters from people who tried their luck at getting him to put pen to paper. In 1953, two years before his death, one of his correspondents was John Moffat, then a 21-year-old physicist at the Niels Bohr Institute in Copenhagen. Moffat was working on unified field theory and his colleagues were giving him more derision than encouragement, so he wrote to Einstein for advice.
Moffat was surprised three weeks later when Einstein replied sympathetically, noting that “…every individual and every study circle has to retain its own way of thinking, if he does not want to get lost in the maze of possibilities”. This was to be the first of several courteous and helpful letters the Sage of Princeton wrote to Moffat, and now, some five decades later, the latter has understandably made it the centrepiece of his memoir Einstein Wrote Back.
Elsewhere in the book, Moffat – now an affiliate member of the Perimeter Institute in Canada – introduces us to the gallery of theoretical physicists he has met during his long career working in relativity and particle physics. These include Abdus Salam, Murray Gell-Mann, Robert Oppenheimer, Kurt Symanzik, Steven Weinberg and even an infant Edward Witten, whose father, Louis Witten, was leading the Research Institute for Advanced Study in Baltimore.
While exercising the memoirist’s prerogative of name-dropping on almost every page, Moffat simultaneously gives the impression of being an outsider. Although he has made the acquaintance of many stellar physicists, he has apparently never worked closely with them or got to know any of them intimately. He also has his share of prejudices. At one point he comments – I suspect, revealingly – on the modern, “draconian” system of peer review, without actually saying that he has often been on the wrong end of it. He wonders how the young Einstein “could ever have succeeded in publishing many of his iconoclastic papers” under such a system. The truth is that those papers had to pass muster with chief editor Max Planck, a theorist of generous spirit but conservative inclination.
Among the character portraits painted by Moffat, that of the brilliant if egoistic cosmologist Fred Hoyle is especially amusing. When Moffat visited him at St John’s College, Cambridge, he found Hoyle sitting at his desk, with a large oil painting of himself on the wall behind. Reflecting on the juxtaposition, Moffat wonders whether all of Hoyle’s students had to sit through his tutorials and “contemplate Hoyle’s blunt Yorkshire features in duplicate”. Later, Moffat had a similar experience in Hoyle’s cottage, where the great cosmologist intellectually disembowelled a divinity student over tea and cakes. The poor student was sitting beneath another oil painting of Hoyle hung over the fireplace.
Moffat’s stories are always entertaining, though some of them seem to have been a tad embellished in repeated re-tellings. For example, Moffat repeats an anecdote from Nathan Rosen, who recalled that when Einstein received a rejection letter from Physical Review, he “leapt out of his chair and threw the envelope with the letter and the manuscript into his trash can, which he kicked loudly around his office”. This story does lend support to Moffat’s view that Einstein’s work on unified theory led him to be “ostracized by his physicist peers”, but I suspect Rosen was exaggerating somewhat. Like all the other anecdotes in the book, this one is not referenced.
The picture Moffat paints of Paul Dirac mostly rings true, though, especially when he describes an excruciating scene in which Dirac sat at home in complete silence while Moffat waited in vain for conversation to begin. It is entirely believable, too, that Dirac’s wife told her husband that he was “so stupid” that he couldn’t “even put on [his] own trousers”. However, I find it hard to picture Dirac uttering the chirpy greeting to Pauli quoted here: “I have been reading your recent work on quantum field theory, Wolfgang.” Nor do I find it easy to imagine him spending afternoons in the garage “repairing his old Rolls-Royce”. Dirac had no taste for car maintenance or any other mechanical chore and never, according to his family, owned such a fancy car.
Pauli, a brilliant if charmless critic of everyone’s new ideas, including his own, makes several appearances here, always as a charismatic figure, though devoid of grace. At one point, Moffat gives us a vivid, detailed account of the Austrian theorist’s visit to Abdus Salam’s new group at Imperial College, London. This was apparently a red-letter day for the dapper Salam, who put on “what appeared to be a new, dark, three-piece suit, white shirt and St John’s College tie, with his black hair and moustache gleaming”. Moffat tells us that the visit took place in 1959, so it appears that Salam and his students were welcoming a ghost – Pauli having died the previous year.
In Einstein Wrote Back, Moffat has given physicists a wealth of new stories about several of its greatest characters. It is a pleasant read, but it would have been even better if he had submitted himself to the discipline of a tougher editor.
A climate model based on the “global energy balance” has provided new evidence for human-induced climate change, according to its creators. Using this simple model, researchers in Switzerland conclude that it is extremely likely (>95% probability) that at least 74% of the observed warming since 1950 has been caused by human activity.
Previously, climate scientists have used a technique called “optimal fingerprinting” to pinpoint the causes of global warming. This involves using complex models to simulate the climate response to different “forcings”. These include greenhouse gases, aerosols and ozone, as well as natural factors such as solar and volcanic variability. The relative contribution of each forcing is then assessed by a statistical comparison of the model outputs to the real-life warming pattern.
However, this method relies on the ability of climate models to accurately simulate the response patterns to each forcing, and also assumes that the responses can be scaled and added. Furthermore, changes in the energy balance of the climate system are not explicitly considered.
A conservative model
Now, Reto Knutti and Markus Huber at the Institute for Atmospheric and Climate Science in Zurich, Switzerland, have developed a model based on the simple fact that Earth’s energy must be conserved. When the Earth is in equilibrium, the thermal energy it emits is equal to the amount of energy received from the Sun. However, evidence shows that this energy balance has become disrupted, with less energy being emitted back into space. The trapped energy in the climate system thus acts to heat up our planet, causing a rise in global temperature.
The researchers used their energy-balance model to investigate the cause and magnitude of this warming. The model, driven by observational records of climate forcings, surface temperature and ocean heat uptake, was run many thousands of times with different parameter combinations. The combinations that best matched the observations were then fed through the model a second time in order to simulate the climate response to each individual forcing.
The model predicts a global temperature increase of 0.51 °C since the 1950s, similar to the observed estimate of 0.55 °C. Greenhouse gases provide the largest contribution to this warming, responsible for a temperature increase of 0.85 °C, with approximately half of this greenhouse warming offset by the negative forcing of aerosols. On the other hand, the contribution of solar and volcanic forcing was close to zero.
Different but similar
The model was also used to simulate the future evolution of the climate system. A temperature increase of 1.29 °C was found for 2050–2059 compared with the 2000s, almost entirely due to greenhouse gases, with carbon dioxide being the dominant contributor.
These findings are consistent with the latest report by the Intergovernmental Panel on Climate Change (IPCC), as well as other studies that use the optimal fingerprinting approach. The researchers believe that their energy-balance model can be used in tandem with alternative climate-attribution methods.
“We don’t criticize optimal fingerprinting – it is a very powerful technique – but to almost all people it’s a black box,” says Reto Knutti. “It’s statistically complex, makes a number of assumptions and is not physically intuitive. At least to a physicist, conservation of energy is fundamental. The fact that our results are entirely consistent with optimal fingerprinting is an argument for even higher confidence in human-induced climate change.”
“Independent evidence”
Paul Williams, a Royal Society University Research Fellow in climate modelling at the University of Reading, UK, agrees that the model is a useful tool. “Even the most hardened climate sceptic with a basic knowledge of physics could not possibly object to the application of energy conservation to the climate problem,” he says. “The energy-balance method provides further independent evidence for the anthropogenic origin of the majority of 20th-century global climate change.”
This study comes less than two months after the Berkeley Earth Surface Temperature project announced its preliminary findings. Motivated by criticisms of the current temperature datasets, this independent study is building a historical temperature record from scratch, using measurements from more than 39,000 stations. Its first results, for land-surface temperatures only, reveal a global warming of 0.91 °C over the past 50 years, in close agreement with previous observational records. And now, the study by Knutti and Huber provides new evidence that this warming is due to human activity.
Researchers in Japan have developed what may be the first string-theory model with a natural mechanism for explaining why our universe would seem to exist in three spatial dimensions if it actually has six more. According to their model, only three of the nine dimensions started to grow at the beginning of the universe, accounting both for the universe’s continuing expansion and for its apparently three-dimensional nature.
String theory is a potential “theory of everything”, uniting all matter and forces in a single theoretical framework, which describes the fundamental level of the universe in terms of vibrating strings rather than particles. Although the framework can naturally incorporate gravity even on the subatomic level, it implies that the universe has some strange properties, such as nine or ten spatial dimensions. String theorists have approached this problem by finding ways to “compactify” six or seven of these dimensions, or shrink them down so that we wouldn’t notice them. Unfortunately, Jun Nishimura of the High Energy Accelerator Research Organization (KEK) in Tsukuba says “There are many ways to get four-dimensional space–time, and the different ways lead to different physics.” The solution is not unique enough to produce useful predictions.
These compactification schemes are studied through perturbation theory, in which all the possible ways that strings could interact are added up to describe the interaction. However, this only works if the interaction is relatively weak, with a distinct hierarchy in the likelihood of each possible interaction. If the interactions between the strings are stronger, with multiple outcomes equally likely, perturbation theory no longer works.
Matrix allows stronger interactions
Weakly interacting strings cannot describe the early universe with its high energies, densities and temperatures, so researchers have sought a way to study strings that strongly affect one another. To this end, some string theorists have tried to reformulate the theory using matrices. “The string picture emerges from matrices in the limit of infinite matrix size,” says Nishimura. Five forms of string theory can be described with perturbation theory, but only one has a complete matrix form – Type IIB. Some even speculate that the matrix Type IIB actually describes M-theory, thought to be the fundamental version of string theory that unites all five known types.
The model developed by Sang-Woo Kim of Osaka University, Nishimura, and Asato Tsuchiya of Shizuoka University describes the behaviour of strongly interacting strings in nine spatial dimensions plus time, or 10 dimensions. Unlike perturbation theory, matrix models can be numerically simulated on computers, getting around some of the notorious difficulty of string-theory calculations. Although the matrices would have to be infinitely large for a perfect model, they were restricted to sizes from 8 × 8 to 32 × 32 in the simulation. The calculations using the largest matrices took more than two months on a supercomputer, says Kim.
Physical properties of the universe appear in averages taken over hundreds or thousands of matrices. The trends that emerged from increasing the matrix size allowed the team to extrapolate how the model universe would behave if the matrices were infinite. “In our work, we focus on the size of the space as a function of time,” says Nishimura.
‘Birth of the universe’
The limited sizes of the matrices mean that the team cannot see much beyond the beginning of the universe in their model. From what they can tell, it starts out as a symmetric, nine-dimensional space, with each dimension measuring about 10–33 cm. This is a fundamental unit of length known as the Planck length. After some passage of time, the string interactions cause the symmetry of the universe to spontaneously break, causing three of the nine dimensions to expand. The other six are left stunted at the Planck length. “The time when the symmetry is broken is the birth of the universe,” says Nishimura.
“The paper is remarkable because it suggests that there really is a mechanism for dynamically obtaining four dimensions out of a 10-dimensional matrix model,” says Harold Steinacker of the University of Vienna in Austria.
Hikaru Kawai of Kyoto University, Japan, who worked with Tsuchiya and others to propose the IIB matrix model in 1997, is also very interested in the “clear signal of four dimensional space–time”. “It would be a big step towards understanding the origin of our universe,” he says. Although he finds that the evolution of the model universe in time is too simple and different from the general theory of relativity, he says the new direction opened by the work is “worth investigating intensively”.
Will the Standard Model emerge?
The team has yet to prove that the Standard Model of particle physics will show up in its model, at much lower energies than this initial study of the very early universe. If it leaps that hurdle, the team can use it to explore cosmology. Compared with perturbative models, Steinacker says, “this model should be much more predictive”.
Nishimura hopes that by improving both the model and the simulation software, the team may soon be able to investigate the inflation of the early universe or the density distribution of matter, results which could be evaluated against the density distribution of the real universe.
The research will be described in an upcoming paper in Physical Review Letters and a preprint is available at arXiv:1108.1540.
With all those rumours flying around of possible sightings of the Higgs boson in among the proton–proton collisions at CERN’s Large Hadron Collider, you might find this video of a very different type of collision interesting.
It involves not protons but pool balls, as performed by Philadelphia-based “professional pool trick-shot artist” Steve Markle.
“The trick shots I do are an excellent showing of defining the laws of physics,” Markle claims.
And if you think pool balls behave in a pretty predictable way according to the rules of classical physics, well yes they do, but it’s still surprising to see what some good old-fashioned spin can do. Take a look, for example, at 3.46 min, when Markle manages to bend a pool ball in a curve through an entire 90° angle.
And if you want to see Markle in action for real, he’s due to be performing at the Artistic Pool World Championship (yes, there is such a thing) in Oaks, Oaklahoma next March.
As for whether the Higgs is going to show up at CERN, you’ve now got just a week to wait. In the meantime, these pool-ball collisions are sure to keep you amused.
We are in the midst of a culinary revolution, as high-end chefs around the world exploit scientific knowledge and technological advances to create spectacular dishes. Ferran Adrià, known for his world-acclaimed restaurant El Bulli in Catalonia, has pioneered the use of hydrocolloids to create yogurt spheres, carrot foam and other novel foods. Other chefs, such as Heston Blumenthal at the Fat Duck in Bray, UK, Grant Achatz at Alinea, Chicago, and Wylie Dufresne at wd~50, New York, are exploring science-based techniques, including the use of liquid nitrogen, enzymes and controlled temperature baths, to create remarkable juxtapositions of new flavours and unexpected textures.
The same trend is happening, in parallel, with cocktails. For years bartenders have relied on trial and error to refine recipes, but now the same techniques that fuelled the culinary revolution are allowing a more systematic approach to developing new drinks. Tools and techniques borrowed from research laboratories in physics and chemistry, such as rotary evaporators, thermocouples and centrifuges, are helping bartenders to put their innovative drinks ideas into practice. Concepts from thermodynamics as well as the physics of colloids, gels and other forms of “soft matter” can help explain the flavour, appearance and “mouthfeel” of these beverages. So get your cocktail shakers and bar spoons ready as we take you through what you need to know to create cocktails that look, taste and feel fantastic.
Full of flavour
Be it in beer, wine or spirits, the physical properties of ethanol, especially its solubility and volatility, help to deliver flavours that are impossible to achieve using water alone. What we think of as “flavour” actually has two main components: taste and aroma. As food-science author Harold McGee puts it, “Tastes provide the foundation of flavour, and aromas provide the tremendous variety.” Although we can perceive just five basic tastes on the tongue (sweet, sour, salty, bitter and savoury), there are thousands of aromas that we can sense through olfactory receptors in the nose – be they the caramel notes of rum or the oaky smell of bourbon.
Alcohol is far more effective than water at delivering these aromatic components, since typically they are not especially water-soluble. Water molecules are polar and so prefer to be near other polar molecules to minimize their interaction energy. This encourages nonpolar molecules, such as the aromatics, to leave the liquid phase and vaporize into the surrounding air, where they contribute to the aroma of the drink. The presence of ethanol mediates this polar/non-polar interaction and allows high concentrations of aromatics to remain in an aqueous solution. For this reason, ethanol is used to extract and deliver flavours from a range of sources, including flowers, spices, nuts, fruits and herbs.
Distilled alcoholic liquids, called spirits, are the essential component of any cocktail. Naturally fermented alcoholic beverages, such as beer and wine, rarely exceed about 20% ethanol by volume, since higher levels are toxic to most of the strains of yeast that produce them. Higher concentrations must therefore be reached through distillation, in which the fermented beverage is heated to preferentially extract the ethanol, which has a lower boiling point than water. The plant material used during fermentation, such as molasses for rum or agave for tequila, gives an intense flavour to the final distilled beverage. Additional plant materials supplied during or after distillation, such as the juniper berries used for making gin, also contribute to the flavour. Because of the high concentration of aromatic molecules extracted from the plants during the production process, spirits are some of the most intensely flavoured foods. Indeed, only a few drops of Chartreuse, a French liqueur made using nearly 130 herbal extracts, can entirely change a cocktail’s flavour.
1 From lab to kitchen A rotary evaporator, typically found in a science lab, can be used to extract delicate aroma molecules that would be destroyed by the higher temperatures of traditional distillation. The liquid in the rotating chamber (red) is gently heated while under a vacuum, causing it to vaporize at a lower temperature. The condenser coils (blue) liquefy the vapour, which is collected in the flask on the left (green). (Courtesy: Dave Arnold, Cooking Issues)
Distillation has been used for thousands of years to create spirits, going as far back as Mesopotamia and ancient China, but continues to be improved through applications of scientific knowledge. For example, some bartenders, such as the award-winning Tony Conigliaro of London bar 69 Colebrooke Row, are experimenting with a device commonly found in the science lab – the rotary evaporator (figure 1). This device extracts aroma molecules that would otherwise be destroyed by the higher temperatures in traditional distillation techniques. The rotary evaporator lowers the pressure inside a rotating container holding the liquid to be distilled, causing the more volatile components to evaporate and leaving behind the undesirable water, sugar, pigments and other large molecules. A condensing coil uses a coolant to turn the vapours back into a liquid – the final intensely flavoured product – which is collected in a separate flask. A habañero liqueur is one illustrative example: the capsaicin that makes the chilli taste so hot is non-volatile, so only the fruity and floral compounds end up in the distillate, yielding a liqueur that retains all the flavour of the chillies but without any nasty burn.
Another way to intensely flavour spirits is to soak ingredients in high concentrations of ethanol, thereby infusing the aromatics into the alcohol. This process traditionally requires many days for the ethanol to fully penetrate the ingredients and extract the desired compounds. Now, however, flavour infusion can be achieved in just a few minutes, using a technique pioneered by Dave Arnold, author of the blog Cooking Issues and director of culinary technology at the French Culinary Institute in the US. Coffee-flavoured vodka, for example, can be made by combining ground coffee beans and vodka in a whipped-cream dispenser – a pressurized device typically used to create foams such as whipped cream at the touch of a button, now well known by the commercial name “iSi Whip”. What happens is that nitrous oxide, which is also in the canister and under high pressure, dissolves in the vodka. The high pressure of the liquid displaces any air bubbles in the coffee grounds. When the pressure is released, the nitrous oxide rapidly bubbles out of the solution, just as when a can of carbonated drink is opened. Releasing these bubbles draws flavour molecules from the coffee grounds into the vodka, flavouring the alcohol and turning it brown. This versatile technique works for a range of porous substances, such as cocoa nibs and a variety of herbs.
By combining spirits with other ingredients, a full spectrum of flavours can be achieved. Tastes can be added through the sweetness of syrups, the sourness of citrus juice, the salt around the rim of a glass or numerous other methods. Aromas can be enhanced with a variety of highly concentrated alcohol-based solutions called tinctures and bitters. Compared with mixed drinks, there is less flexibility in what can be produced with beer or wine because their flavours can only be manipulated through the fermentation and ageing process.
Hot or cold
Whether by dare or by choice, many of you will have experienced the hot, burning sensation you get in your throat and chest if you drink neat vodka or tequila. In fact, too much spirits in a cocktail can overwhelm the desired mix of flavours. The alcohol burn can, however, be reduced by lowering the temperature of the beverage, which is why aquavit, vodka and other straight spirits are often served cold, at temperatures of around –18 °C. Unfortunately, such low temperatures can also diminish the perception of the other tastes and aromas in the drink, so most mixed drinks are served at somewhat warmer temperatures.
The flavour of a drink also depends on how dilute it is, which in practical terms means how much ice has been mixed into it. Vigorously shaking the mixture rapidly cools the drink within seconds, whereas cooling can take upwards of a few minutes if it is only gently stirred. In both cases, the final temperature of the diluted mixture can be several degrees below the initial temperature of the ice, for essentially the same reason that roads are de-iced by spreading salt on them. Because the entropy of the diluted mixture is far larger than the entropy of the crystalline ice, the ice continues to melt and absorb heat from the mixture even as the mixture cools below 0 °C.
The precise temperature of the drink also strongly affects the complex balance between these flavours. A chilled martini, for example – consisting of gin and vermouth – is crisp and balanced, whereas the gin can overwhelm the flavour near room temperature. As McGee explains, “the bartender’s challenge is to make drinks that have a balanced taste foundation and aromas that suit that foundation, and retain that overall structure reasonably well over the drink’s lifetime, as it becomes diluted or warms up”.
Appearance is everything
2 Shaken or stirred? These manhattan cocktails have identical ingredients, but the shaken version (left) has a froth on the surface and a cloudy appearance, caused by vigorous shaking that incorporates air bubbles, while the stirred version (right) is clear. (Courtesy: Mike Betancourt and Leo Stein)
The flavour of a cocktail is of course important but its appearance and texture also contribute to the overall experience of the drink – be it the layers of the graphically named squashed frog, the creaminess of an eggnog or the showiness of a blue blazer, which is poured between two cups after being set on fire. Flames and decorations aside, a cocktail’s appearance results from a combination of its colour and opacity, both of which can be controlled by the bartender. For a coloured drink, the mixologist selects ingredients that absorb specific wavelengths of light. For example, a rich brown can be obtained using a spirit that has been aged in oak barrels, as this imparts pigment molecules that produce this colour. If you want the finished drink to be clear, all the pigments and particulates must be removed, to prevent light absorption or scattering.
But even with clarified components, the mixing technique can have a dramatic impact on the light-scattering properties of the finished drink. For example, a manhattan, which contains whisky, vermouth and bitters, can become cloudy when shaken. This results from small air bubbles introduced into the beverage while shaking, which are then stabilized by the bitters. A stirred manhattan, in contrast, is clear (figure 2), which is why it is typically served stirred, not shaken, unlike James Bond’s martinis. As for drinks that are cloudy, their appearance is often caused by the presence of small particulates, although these can be removed by a variety of clarification techniques. Surprisingly, the most common method of clarification – filtration – is rarely used. Instead, some technology-minded bartenders are using other techniques such as centrifugation, which rapidly produces a clear liquid by accelerating the settling of particulates. Indeed, this technique is a particularly good way of clarifying lime juice, which can then be used for transparent gin and tonics or clear, stirred margaritas. Another technique, also developed by Arnold, uses gels made from agar – a naturally occurring polysaccharide – to trap particulates from citrus juices and other non-transparent liquids. Water is boiled with agar to hydrate it, the juice is then added and the solution is allowed to cool to form a gel. The longer pectin fibres and other plant materials become trapped in the agar gel, and a clear liquid weeps out, which contains the much smaller flavour molecules.
3 Settling out nicely This half sinner, half saint cocktail is clear before the absinthe is added (a). But after the absinthe is poured in (b), the top of the drink becomes cloudy as a spontaneous emulsion of the oils in the absinthe is formed. After several minutes (c), the emulsion drops spread halfway down the glass, producing an opaque white layer. (Courtesy: Mike Betancourt and Leo Stein)
Half sinner, half saint: This is a traditional-style cocktail made by John Gertsen, bar manager at Drink, Boston. To make it yourself you will need 2 oz sweet vermouth, 2 oz dry vermouth and 1/4 oz absinthe. Add crushed ice to a rocks glass, pour in both vermouths and mix. Float the absinthe on top, as shown above.
There is also plenty of interesting physics going on in cocktails that include anise-flavoured spirits such as pastis, ouzo and absinthe, which contain water-insoluble anethole compounds. Although the anethole dissolves in ethanol because of the alcohol’s unique structure, when these compounds are diluted with water they are no longer soluble, so they form spontaneous emulsions. What happens here is that a highly concentrated suspension of microscopic droplets has been created in the drink that strongly scatters light. Because the droplets are small, these emulsions are stable for months without having to add any stabilizing “surfactant” molecules. This effect is exemplified in a drink called half sinner, half saint, in which a layer of absinthe is floated on top of a mixture of sweet and dry vermouth. The absinthe spreads downwards, leading to a white layer, caused by the droplets of anethole that travel from the top to the bottom of the glass over the course of several minutes (figure 3).
Tactile textures
In addition to flavour and appearance, the “mouthfeel” of a drink is another parameter manipulated by bartenders. Incorporating air via shaking results in a more viscous texture. Egg whites are used in fizzes and sours to stabilize these air bubbles. An extreme example is a Ramos gin fizz, which calls for an exhausting 12 minutes of shaking in the original recipe. The effort is worth it, however, as it results in an extremely creamy, frothy texture. A layer of foam protrudes several centimetres above the rim of the glass and is stiff enough to hold a metal straw vertically at its centre. The long mixing time is needed to divide the air into progressively smaller bubbles, resulting in a stiffer foam. Another class of drinks, called flips, uses whole egg to form an emulsion, leading to a more creamy texture.
Several of the chefs leading the innovations in haute cuisine are also pushing the frontiers of texture in cocktails. Adrià serves several novel types of cocktail in his establishments, including a hot and cold gin fizz (see below). Instead of the lengthy shaking of the Ramos gin fizz, an iSi Whip introduces nitrous-oxide bubbles into the top foam layer, which sits on top of a frozen juice layer. At Grant Achatz’ bar, Aviary, Chicago, the cocktail chefs use techniques developed in the Alinea kitchen to create novel forms for the drinks. For instance, they use a modified starch called tapioca maltodextrin to produce a powdered gin and tonic and ultralow temperatures to make a chewy Pisco sour. Other mixologists, such as Eben Freeman of the Altamarea group, use similar techniques to create a variety of solid cocktails.
These elements of flavour, appearance and texture all contribute to the final perception of the drink. Classic cocktail recipes have survived and evolved as we have learned to improve the balance of these components. But today’s bartenders are seeking inspiration from science to improve these recipes and to invent new concoctions. So let’s all raise a glass to science!
At a glance: cocktail physics
• Having perfected yoghurt spheres, carrot foam and other novel foods using new technological tools, some chefs are now turning their attention to cocktails
• Cocktails have traditionally been developed by trial and error but can now be understood in terms of thermodynamics and soft-matter physics
• The physical properties of ethanol, the basis of every spirit, enable the delivery of flavours impossible to achieve using water alone
• Equipment such as rotary evaporators and whipped-cream dispensers are now used to extract flavours, along with traditional distillation and soaking methods
• The appearance and texture of drinks can be controlled by clarification, the decision to stir or shake, or the production of foam, for example using egg whites to stabilize air bubbles
Get mixing!
Here is a cocktail recipe you can try for yourself, from leader in the field Ferran Adrià.
Hot and cold gin fizz
Ingredients For the base syrup:
150 g sugar
150 g water For the frozen lemon mix:
250 g lemon juice
150 g base syrup (see above)
150 g gin For the hot lemon foam:
150 g egg whites
130 g lemon juice
70 g gin
145 g base syrup (see above)
Equipment
0.5 litre iSi Whip
1 cartridge of N2O
Method For the base syrup:
Mix ingredients and bring to a boil.
Remove from heat, cool, then refrigerate. For the frozen lemon mix:
Mix all ingredients cold, then freeze.
Once frozen, blend in a blender until fluid. Keep in the freezer. For the hot lemon foam:
Break egg whites with a whisk. Add the remaining ingredients. Strain and pour into the iSi Whip using a funnel. Load the iSi Whip and keep in a water bath at 80 °C, shaking occasionally.
To serve, 3/4 fill a cocktail glass with frozen lemon mix. Top up with hot foam.
More about: cocktail physics
D Arnold Cooking Issues www.cookingissues.com
T Conigliaro Drink Factory http://drinkfactory.blogspot.com
H McGee 2004 On Food and Cooking (Scribner, New York)
N L Sitnikova, R Sprik and G Wegdam 2005 Spontaneously formed trans-anethol/water/alcohol emulsions: mechanism of formation and stability Langmuir 21 7083
