The antimony wars

The August issue of La Recherche has the theme of ‘controversies in science’. I wrote several pieces for it – this is the first, on the battle between the Galenists and Paracelsians in the French court in the early 17th century.

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“I am different”, the sixteenth-century Swiss alchemist and physician Paracelsus once wrote, adding “let this not upset you”. But he upset almost everyone who came into contact with him and his ideas, and his vision of science and medicine continued to spark dispute for at least a hundred years after his death in 1541. For Paracelsus wanted to pull up by its roots the entire system of medicine and natural philosophy that originated with the ancient Greeks – particularly Aristotle – and replace it with a system that seemed to many to have more in common with the practices of mountebanks and peasant healers.

Paracelsus – whose splendid full name was Philip Theophrastus Aureolus Bombastus von Hohenheim – had a haphazard career as a doctor, mostly in the German territories but also in Italy, France and, if his own accounts can be believed, as far afield as Sweden, Russia and Egypt. Born in the Swiss village of Einsiedeln, near Zurich, into a noble Swabian family fallen on hard times, he trained in medicine in the German universities and Ferrara in Italy before wandering throughout Europe offering his services. He attended kings and treated peasants, sometimes with a well-filled purse but more often penniless. Time and again his argumentative nature ruined his chances of a stable position: at one time town physician of Basle, he made himself so unpopular with the university faculty and the authorities that he had to flee under cover of darkness to avoid imprisonment.

Paracelsus could be said to have conceived of a Theory of Everything: a system that explained medicine and the human body, alchemy, astrology, religion and the fundamental structure of the cosmos. He provided one of the first versions of what science historians now call the ‘chemical philosophy’: a theory that makes chemical transformation the analogy for all processes. For Paracelsus, every natural phenomenon was essentially an alchemical process. The rising of moisture from the earth and its falling back as rain was the equivalent of distillation and condensation in the alchemist’s flask. Growth of plants and animals from seeds was a kind of alchemy too, and in fact even the Biblical creation of the world was basically an alchemical process: a separation of earth from water. This philosophy seems highly fanciful now, but it was nonetheless rational and mechanistic: it could ascribe natural and comprehensible causes to events.

Although Paracelsus was one of the most influential advocates of these ideas in the early Renaissance, they weren’t entirely his invention (although he characteristically exaggerated his originality). The chemical philosophy was rooted in the tradition known as Neoplatonism, derived from the teachings of Plato but shaped into a kind of mystical philosophy by the third-century Greek philosopher Plotinus. One of the central ideas of Neoplatonism is the correspondence between the macrocosm and the microcosm, so that events that occurred in the heavens and in the natural world have direct analogies within the human body – or with the processes conducted in an alchemist’s flasks and retorts. This correspondence provided the theoretical basis for a belief in astrology, although Paracelsus denied that our destiny is absolutely fixed by our horoscope. He proposed that the macro-micro correspondence led to ‘signatures’ in nature which revealed, for example, the medical uses of plants: those shaped like a kidney could treat renal complaints. These signatures were signs left by God to guide the physician towards the proper use of herbal medicines. They exemplify the symbolic character of the chemical philosophy, which was based on such analogies of form and appearance.

What the chemical philosophy implied for medicine conflicted with the tradition taught to physicians at the universities, which drew on ideas from antiquity, particularly those attributed to the Greek philosopher Hippocrates and the Roman doctor Galen. This classical tradition asserted that our health is governed by four bodily fluids called humours: blood, phlegm, and black and yellow bile. Illness results from an imbalance of the humours, and the doctor’s task was to restore this balance – by drugs, diet or, commonly, by blood-letting.

Academic doctors in the Middle Ages adopted the humoral system as the theoretical basis of their work, but its connection to their working practices was generally rather tenuous. Often they prescribed drugs, made from herbs or minerals and sold by medieval pharmacists called apothecaries. Doctors charged high fees for their services, which only merchants and nobles could afford. They were eminent in society, and often dressed lavishly.

Paracelsus despised all of this. He did not share the doctors’ disdain of manual work, and he hated how they paraded their wealth. Worse still, he considered that the whole foundation of classical medicine, with its doctrine of humours, was mistaken. When he discovered at university that becoming a doctor of medicine was a matter of simply learning and memorizing the books of Galen and Avicenna, he was outraged. He insisted that it was only through experience, not through book-learning, that one could become a true healer.

By bringing an alchemical perspective to the study of life and medicine, Paracelsus helped to unify the sciences. Previously, alchemy had been about the transmutation of metals. But for Paracelsus, its principle purpose was to make medicines. Just as alchemists could mimic the natural transmutation of metals, so could they use alchemical medicines to bring about the natural process of healing. This was possible, in fact, because human biology was itself a kind of alchemy. In one of his most fertile ideas, Paracelsus asserted that there is an alchemist inside each one of us, a kind of principle that he called the archeus, which separates the good from the bad in the food and drink that we ingest. The archeus uses the good matter to make flesh and blood, and the bad is expelled as waste. Paracelsus devised a kind of bio-alchemy, the precursor to modern biochemistry, which indeed now regards nature as a superb chemist that takes molecules apart and puts them back together as the constituents of our cells.

Most of all, Paracelsus argued that medicine should involve the use of specific chemical drugs to treat specific ailments: it was a system of chemotherapy, which had little space for the general-purpose blood-letting treatments prescribed by the humoral theory. This Paracelsian, chemical approach to healing became known in the late sixteenth century as ‘iatrochemistry’, meaning the chemistry of medicine.

Paracelsus was able to publish relatively little of his writings while he was alive, but from around 1560 several publishers scoured Europe for his manuscripts and published compendia of Paracelsian medicine. Once in print, his ideas attracted adherents, and by the last decades of the century Paracelsian medicine was exciting furious debate between traditionalists and progressives. Iatrochemistry found a fairly receptive audience in England, but the disputes they provoked in France were bitter, especially among the conservative medical faculty of the University of Paris.

That differing reception was partly motivated by religion. Paracelsus belonged to no creed, but he was widely identified with the Reformation – he even compared himself to Martin Luther – and so his views found more sympathy from Protestants than Catholics. The religious tensions were especially acute in France when the Huguenot prince of Navarre was crowned Henri IV in 1589. Fears that Henri would create a Huguenot court seemed confirmed when the new king appointed the Swiss doctor Jean Ribit as his premier médicin, and summoned also two other Huguenot doctors with Paracelsian ideas, the Gascon Joseph Duchesne and another Genevan, Theodore Turquet de Mayerne.

In 1603 Jean Riolan, the head of the Paris medical faculty, published an attack on Mayerne and Duchesne, asserting the supremacy of the medicine of Hippocrates and Galen. Although these two Paracelsians sought to defend themselves, they only secured a retraction of this damning charge by agreeing to practice medicine according to the rules of the classical authorities.

But the Paracelsians struck back. Around 1604, Ribit and Mayerne helped a fellow Huguenot and iatrochemist named Jean Béguin set up a pharmaceutical laboratory in Paris to promote chemical medicine. In 1610 Béguin published a textbook laying out the principles of iatrochemistry in a clear, straightforward manner free from the convoluted style and fanciful jargon used by Paracelsus. When this Latin text was translated into French five years later as Les elemens de chymie, it served much the same propagandizing role as Antoine Lavoisier’s Traité élémentaire de chemie did for Lavoisier’s own system of chemistry at the end of the eighteenth century.

But the war between the Galenists and the Paracelsians raged well into the seventeenth century. Things looked bad for the radicals when Henri IV, who had been prevented in 1609 from making Mayerne his new premier médicin, was assassinated the following year. Lacking royal protection, Mayerne took up an earlier offer from James I of England and fled there, where he flourished.

Yet when Riolan’s equally conservative son (also Jean) drew up plans for a royal herb garden in 1618, he did not anticipate that this institution would finally be established 20 years later as the Jardin du Roi by the iatrochemist Gui de la Brosse. In 1647 the Jardin appointed the first French professor of chemistry, a Scotsman named William Davidson, who was an ardent Paracelsian.

Most offensive of all to the Paris medical faculty was Davidson’s support for the medical use of antimony. Ever since the start of the century, Paracelsians and Galenists had been split over whether antimony was a cure or poison (it is in fact quite toxic). Davidson’s claim that “there is no more lofty medicine under heaven” so enraged the faculty that they hounded him from his post in 1651, when the younger Riolan republished his father’s condemnation of Duchense and Mayerne.

Yet it was all too late for the Galenists, for the Jardin du Roi, which became one of the most influential institutions in French chemistry and medicine, continued to support iatrochemistry. The professors there produced a string of successful chemical textbooks, most famously that of Nicolas Lemery, called Cours de chimie, in 1675. These men were sober, practical individuals who helped to strip iatrochemistry of its Paracelsian fantasies and outlandish jargon. They placed chemical medicine, and chemistry itself, on a sound footing, paving the way to Lavoisier’s triumphs.

What was this long and bitter dispute really about? Partly, of course, it was a power struggle: over who had the king’s ear, but also who should dictate the practice (and thus reap the financial rewards) of medicine. But it would be too easy to cast Riolan and his colleagues as outdated reactionaries. After all, they were right about antimony (if for the wrong reasons) – and they were right too to criticize some of the wild excesses of Paracelsus’s ideas. Their opposition forced the iatrochemists to prune those ideas, sorting the good from the bad. Besides, since no kind of medicine was terribly effective in those days, there wasn’t much empirical justification for throwing out the old ways. The dispute is a reminder that introducing new scientific ideas may depend as much on the power of good rhetoric as on the evidence itself. And it shows that in the end a good argument can leave science healthier.

Before it gets too previous, here is an earlier piece for BBC Future.

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It’s time for one of those imagined futures which always miss the mark by a mile – you know, “Imagine setting off for work with your jet-pack…” But here we go anyway: imagine that photographs, newspapers and books speak, that you can play music out of your curtains, that food wrapping calls out “I’m nearly past my sell-by date!” OK, so perhaps it’s all a bit nightmarish rather than utopian, but the point is that some weird and wonderful things would be possible if a loudspeaker could be made as thin, light and flexible as a sheet of paper.

That’s what is envisaged in a study by Andrew Barnard and colleagues at the Pennsylvania State University. They have revisited an idea nearly a hundred years old, and sounding decidedly steampunk: the thermophone or thermoacoustic loudspeaker, in which sound is generated by the effect of a material rapidly oscillating between hot and cold. In 1917 Harold Arnold and I. B. Crandall of the American Telephone and Telegraph Company and Western Electric Company showed that they could create sound by simultaneously passing alternating and direct currents through a very thin platinum foil. This heats up the foil, and the heat is conducted into the air surrounding it, in pulses that are paced by the frequency of the a.c. current.

A sound wave in air corresponds to an oscillation of the air pressure. An ordinary loudspeaker generates those pressure waves via a mechanical vibration of a membrane. But air pressure is also altered when the air gets hotter or cooler. So the thermal oscillations of Arnold and Crandall’s platinum film also generated a sound wave – without any of the cumbersome, heavy electromagnets used to excite vibrations in conventional speakers, or indeed without moving parts at all.

The problem was that the sound wasn’t very loud, however, and the frequency response wasn’t up to reproducing speech. So the idea was shelved for almost a century.

It was revitalized in 2008, when a team in China found that they could extract thermoacoustic sound from a new material: a thin, transparent film made from microscopic tubes called carbon nanotubes (CNTs), aligned parallel to the plane of the film. These tiny tubes, whose walls are one atom thick and made from pure carbon, are highly robust, need very little heat input to warm them up, and are extremely good heat conductors – just what is needed, in other words, to finally put the idea of Arnold and Crandall into practice and create gossamer-thin loudspeakers.

The Chinese team, led by Lin Xiao at Tsinghau University, showed that they could get their CNT films to emit sound. But that’s not the same as making a loudspeaker that will produce good-quality sound over the whole frequency range of human hearing, from a few tens of hertz (oscillations per second) to several thousand. So while the CNT speakers might have valuable applications such as sonar – they work perfectly well underwater – it isn’t yet clear if they can produce hifi-quality sound in your living room.

That’s what Barnard and colleagues have sought to assess. One of the factors determining the loudness of the devices is how efficiently heat can be transferred into the surrounding gas to induce pressure waves. This depends on how much the gas heats up for a given input of heat energy: a property called the heat capacity. A low heat capacity means that only a small energy input can create a big change in temperature, and thus in pressure. So the sound output can be improved by surrounding the CNT film with a gas that has a lower heat capacity than air, such as the inert gases helium, argon or xenon. Xiao’s team has already demonstrated this effect, but Barnard and colleagues now show that it offers perhaps the best avenue for improving the performance of these devices. To transmit the acoustic vibrations of the inert gas to the air beyond, so that we can hear the results, one would separate the gas and air with a flexible membrane.

Another way to improve the sound output is to make the surface area of the film bigger. That can be done without ending up with a carpet-sized device by stacking several sheets in layers. The Pennsylvania group has shown that this works: a four-layer speaker, for example, is significantly louder for the same power input.

All things considered, Barnard and colleagues conclude that “a high power CNT loudspeaker appears to be feasible.” But it won’t be simple: the CNT films will probably need to be enclosed and immersed in xenon, for example, which would pose serious challenges for making robust ‘wearable’ speakers.

And there is already competition. For example, a small start-up British company called Novalia has created an interactive, touch-sensitive printed poster that can generate drum-kit sounds through vibrations of the paper itself. Curiously, that technology uses electrically conducting inks made from a pure-carbon material called graphene, which is basically the same stuff as the walls of carbon nanotubes but flattened into sheets. So one way or another, these forms of ‘nanocarbon’ look destined to make our isles full of noises.

Reference: A. R. Barnard et al., Journal of the Acoustical Society of America 134, EL280 (2013).

Seven Ages of Science

I hope people have been listening to Lisa Jardine’s Seven Ages of Science on BBC Radio 4. It is very nice – a refreshingly personal and idiosyncratic take on the history of science, rather than the usual plod through the usual suspects. I made a few modest contributions to some episodes, plucked from some long but fun conversations.

Why you should appear in your papers

Here’s my latest Crucible column for Chemistry World.

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The strange thing about Einstein’s classic 1905 papers on relativity, quantum theory and Brownian motion is that he is largely absent from them. That’s to say, he hardly ever uses the first person singular to put himself in the reference frame. “We have now derived…”, “We now imagine space to be…” – we and Einstein do it all together. He pops up a little thrillingly at the start of the extraordinarily brief “E=mc2” paper, but quickly vanishes beneath the passive voice and the impersonal “one concludes”.

It wasn’t his intention but this all makes Einstein sound magisterial. Lavoisier was already vacillating 130 years earlier, when he is sometimes “I” and sometimes “we” – calculatedly so, for he’s very much present in person when distinguishing his own discoveries from Priestley and Scheele, but tells us bossily that “we shall presently see what we ought to think” when it comes to choosing amongst them.

I’m left thinking about these questions of voice after reading a paper by ‘science studies’ researchers Daniele Fanelli of the University of Edinburgh and Wolfgang Glänzel of the Catholic University of Leuven (PLOS ONE 8, e66938; 2013). They report that bibliometric analysis of around 29,000 papers ranging across all the sciences from maths and physics to social sciences, as well as some in the humanities, show significant differences in style and content which point to a genuine hierarchy of sciences, along the lines first postulated by the French philosopher Auguste Comte in the 1830s. As we would put it today, physics and maths are the ‘hardest’ sciences, and they become progressively ‘softer’ as we move through chemistry, the life sciences, and the social sciences. The key criterion the authors use for this classification is the degree of consensus in the field, as revealed for example by the number, age and overlap of references.

There’s a lot to discuss in these interesting findings; but one aspect that caught my attention was the authors’ comparison of whether or not papers use personal pronouns. “Scientists aim at making universal claims, and their style of writing tends to be as impersonal as possible”, say Fanelli and Glänzel. “In the humanities, on the other hand, the emphasis tends to be on originality, individuality and argumentation, which makes the use of first person more common.” They found that indeed the ‘harder’ sciences tend to use personal pronouns less often.

The assumption here is that an impersonal, passive voice suggests a universal truth. It really does suggest that – and that’s the whole point. Fanelli and Glänzel’s implication that the passive voice reflects science’s ability to deliver absolute knowledge is a case of science falling for its own tricks. Scientists actively cultivated the impersonal tone as a rhetorical device to persuade and convey authority. This process began with the institutionalization of science in the seventeenth century, and it was a feature of what historian Steven Shapin has called the “literary technology” of that age: a style of writing calculated to sound convincing.

There were good reasons for this, to be sure. Experimental scientists like Robert Boyle wanted to free themselves from the claims of the Renaissance magi to have received deep insights through personal revelation; on the contrary, they’d found stuff out using procedures that anyone (with sufficient care and education) could conduct. So it didn’t matter any more who you were, an attitude encapsulated in Claude Bernard’s remark in 1865 that “Art is I; Science is We.” Or better still, science is “It is shown that…”

Yet the pendulum is swinging. Many books advising how to write scientific papers tend now to recommend the active voice. For example, in Successful Scientific Writing (Cambridge University Press, 1996), Janice Matthews and Robert Matthews say “Many scientists overuse the passive voice. They seem to feel that every sentence must be written in passive terms, and they undergo elaborate contortions to do so.” But the passive voice, the authors say, “often obscures your true meaning and compounds your chances of producing pompous prose.” The American Institute of Physics, American Chemical Society and American Medical Association all recommend the active voice and use of pronouns, although they accept the passive voice for methods sections.

I would go further. If scientists care about precise reporting, they should insist on planting themselves in their papers. Their fallibility, preconceptions and opinions are a part of the picture, and it’s misleading to imply otherwise. For many of the scientists who, during my years as an editor at Nature, balked at writing “I” rather than “We” in their single-author papers, the worry was not that they’d seem less authoritative but rather, too arrogant. But I suspect “I” also seemed disturbingly exposing. Either way, if you did the work, you’ve got to admit to it.

How plastics got under control

Several things to catch up with after the holidays, and here’s the spoddiest first: a leader for Nature Materials celebrating the 50th anniversary of the chemistry Nobel for Ziegler and Natta.

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One could tell the history of the twentieth century through the medium of polymers. In a weird and ramshackle way that is almost what American author Thomas Pynchon attempted in his novel Gravity’s Rainbow, which shows the German cartel IG Farben clandestinely orchestrating the Second World War and making rockets with unnerving, sensory polymer skins. But the truth is scarcely less strange and no less dominated by the agencies of conflict, commerce and politics.

Karl Ziegler, who 50 years ago won the Nobel Prize in Chemistry alongside Italian chemist Giulio Natta for their work on the stereoselective catalysis of alkene polymerization, began his work on polymerization during the Second World War to make synthetic rubber for the German war effort as supplies from the Asian rubber plantations were cut off. When the war ended Ziegler was in Halle, soon to become Russian-occupied territory, and the American authorities encouraged him to take a post at Mülheim to preserve his expertise for the West. It was there in 1953 that he discovered the organometallic compounds, such as triethylaluminium, that would not only catalyse ethylene polymerization at lower temperatures and pressures than the standard industrial process then prevailing but would produce orderly straight-chain molecules without random branching, creating a high-density product with new potential uses.

Natta, working in Milan, was also drawn into synthetic-rubber work during the war, and once he heard about Ziegler’s discovery he realised that it could be used to make ordered polymers from other alkenes. He and his coworkers quickly discovered that ethylaluminium chloride and vanadium tetrachloride would catalyse the formation of polypropylene with a stereoregular isotactic chain structure: all the methyl side-chains on the ‘same’ side, enabling orderly crystalline packing into a solid, high-density form. The Italian chemicals company Montecatini, which funded Natta’s research, immediately developed this process on an industrial scale, and were marketing isotactic polypropylene at Ferrara by 1957 as a bulk plastic, a fibre and a packing film. Natta went on to conduct pioneering work on the synthesis of rubbers by controlled polymerization of butadiene.

Yet the stereoselective polymerization of propylene into a high-density plastic was in fact discovered independently before Ziegler and Natta, by American chemists J. Paul Hogan and Robert Banks working at the Phillips Petroleum Company in Oklahoma. They too were stimulated by the war – but in this case by its termination, which reduced the demand for oil and prompted Phillips to diversify its products. Hogan and Banks began in the early 1950s to look for ways to convert the small alkenes from oil refining into petrol. When they used a catalyst of nickel oxide and chromium oxide to process propylene, they found a solid white crystalline product.

This new, stiff plastic, marketed by Phillips from 1954 as Marlex, owed its commercial success to a craze that swept the United States in the late 1950s: the hula hoop. Demand for this toy consumed the Phillips plant’s entire output, and boosted production to a level that paved the way for more practical uses: industrial tubing, baby bottles and other household products. But the patent application filed by Hogan and Banks was contested by Ziegler’s rival claim, leading to a court battle that lasted three decades. Because of this, and since the American chemists were slow to publish, their discovery was eclipsed by the Ziegler-Natta Nobel – even though chromium catalysts are still widely used.

Even this is not the full extent of the priority dispute, for Alexander Zeltz and Ron Carmody of Standard Oil in Indiana also made a partially crystalline isotactic form of polypropylene in 1950 using a molybdenum catalyst. But there’s more to a discovery than being first: it’s not clear that they knew quite what they had made, and in any case there were complex questions to be addressed about the degree of stereoselectivity created by the different catalysts.

Basic science is here more the beneficiary than the begetter, for the work of Ziegler and Natta pointed the way to approaches to stereoselective formation of carbon-carbon bonds that remain a rich field of science today. Its value has occasionally surfaced in unexpected ways – it was an inadvertent excess of Ziegler-Natta catalyst, for example, that led Hideki Shirakawa to discover the first electrically conducting polymer, a form of polyacetylene, in Tokyo in 1967. The scale of the polyolefin industry, meanwhile, scarcely needs emphasizing: close to 50 million tons of polypropylene alone is produced each year.

One moral of these stories is that true discovery requires that you know what you’ve done, and show it. But they also reveal how the conventional narrative of technological advance, whereby ‘pure’ fundamental science leads to applications, is seldom of much relevance in fields such as materials chemistry. Social and cultural drivers often determine what gets explored – if not necessarily what comes out. And success may be determined by the fickle whims of the market rather than the merit of the product. One might add the lesson that, if you want recognition, publish quickly and get a good lawyer – not perhaps the most edifying moral, but that’s the way of the world.

Appearances matter most in musical performance

This is a long version of the news story I’ve just published with Nature – there is just so much to talk about here.

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Our judgements of quality depend more on how a musician moves than what they sound like.

He’d whip his long hair around as he played, beads of sweat flying into the audience, and women would swoon or throw their clothes onto the stage. No, this isn’t the young Mick Jagger or Jimmy Page but Franz Liszt, sometimes dubbed the first rock star, of whose famously theatrical piano recitals Robert Schumann once said that, if he played behind a screen, “a great deal of poetry would be lost”.

But who cares about the histrionics – it’s the music that matters, right? Not according to a new study which shows that people’s judgements about the quality of a musical performance are influenced more by what they see than by what they hear [1].

These findings by social psychologist Chia-Jung Tsay of University College London, who is also an acclaimed classical pianist, may be embarrassing and even shocking to music lovers. The vast majority of participants in Tsay’s experiments – around 83 percent of both untrained participants and professional musicians – insisted at the outset that sound was their key criterion in assessing video and audio recordings of performances.

Yet it wasn’t. The participants were presented with recordings for the three finalists in each of 10 prestigious international competitions, and were asked to guess the winner. With just sound, or sound plus video, novices and experts both guessed right at about the same level as chance (33 percent of the time), or a little less. But with video alone, the success rate for both rose to about 46-53 percent. The experts did no better than the novices.

In experiments where participants were randomly assigned to receive the silent videos, Tsay says “they expressed much frustration and lack of confidence in their choices, not realizing that they were the ones to best approximate the original decisions.” She would receive comments like “It’s impossible to take seriously without sound” and “This is meaningless since I can't hear [the performer]”

This is a brilliant paper”, says philosopher of music Vincent Bergeron of the University of Ottawa in Canada. “The fact that the judgments of both novice and expert participants are affected in the very same way suggests that the visual channel constitute a powerful and robust factor in the evaluation of musical performances.”

Rethinking performance

The results raise provocative questions about what musical performance really is. Classical audiences in particular might like to claim that they are there to enjoy the exquisite sounds the performers are making – but it seems their assessments are based primarily on what they are seeing.

“As an academic I was delighted to find these counterintuitive results”, says Tsay. “As a classical musician, I was initially somewhat disturbed. It was surprising to find that there is such a wide gap between what we believe matters in the evaluation of music performance and what is actually being used to judge performances.”

But Bergeron isn’t perturbed. He has previously argued that visuals do play a part in how we experience music when we see it performed [2]. “One could plausibly argue that music made for performance, such as classical music, is a visual as well as a sonic art, and that it should also be evaluated on the basis of how it looks”, he says.

Bergeron’s earlier case built partly on the work of Jane Davidson on the University of Western Australia in Crawley, Australia, who also found that judgements of quality depend on sight as well as sound [3]. Music neuropsychologist Daniel Levitin of McGill University in Canada agrees that Tsay’s results might have been anticipated, both because of earlier work on the subject [4,5] and because of what we know about cognition in general.

“In a sense, the visual channel is more primordial than the auditory”, he says. Besides, “there are lots of ways in which our intuitions about our own cognition are wrong”, he says. “The whole field of perception and cognition is full of these, such as visual and auditory illusions.”

But Davidson says that there seem to be nuances in such judgements. For example, in her studies “musicians were still able to differentiate between poorer and better quality musical sound”, she says.

In Davison’s experience, the assessments of non-musicians may rely more on visuals than that of professional musicians. “In my own studies, musicians were able to use sound and vision independently”, she says. “It was only non-musicians who relied mainly on the visual information.”

She adds that some studies, including her own, that lay sound from one performance on top of visuals from another find that, although the visuals dominate perceptions, such tricks “don’t fool experts”.

What kind of messages do we take away from visual information? Tsay was able to rule out the possibility that a performer’s gender, race or attractiveness influence judgements, at least in her experiments, by tests in which she reduced the video data to black-and-white outlines of the performers. Participants still guessed the competition winners correctly with much the same better-than-chance success rate (48 percent) as before.

Tsay thinks that, at least for this kind of music, visual cues carry implications about the degree of passion and motivation that the performer displays. These are qualities that many participants cited as important in their evaluations, and even musical novices can identify them visually. Perhaps they do it even more accurately than the ‘experts’, Tsay says, because they are unencumbered by the sound “that professional musicians unintentionally and non-consciously discard.”

Looking good

One has also to wonder if musicians already unconsciously know that it matters what they look like in performance. Tsay suspects they do. “Many performers do have the intuition that the role of visual information is an important one”, she says. Moreover, having studied at top musical institutions such as the Juillard School in New York, she says “Some teachers at conservatories seem to be quite attuned to the important role that visual information plays in the judgment of music, and they make their students aware of its impact for effective performance.”

Davidson agrees that performers sense the significance of how they look. “Look at the artistry of Judy Garland”, she says. “Every move is integrated into a smooth action plan, as if it were created in the moment, yet it is totally rehearsed and polished as an integrated essential element of her vocal performance.”

“Really good musicians do this too,” she adds. “There are data from Glenn Gould’s career which shows how he moved very differently when only concerned with creating a sound recording rather than when in the recital room.”

The topic is probably an under-researched aspect of musical performance, however. (Musicologist Susan Fast of McMaster University in Hamilton, Canada, has provided a rare analysis of the visual body language cultivated by the rock group Led Zeppelin [6].) “I think the claims of the current paper need some really good social psychological contextualizations and clarifications”, says Davidson.

Whether or not musicians do learn to enhance their “visual merit”, the question is sure to arise: “should they?” Isn’t this a bit like cheating? Is the celebrated Chinese pianist Lang Lang (see above), for example, fooling audiences as he makes great sweeping gestures with his arms, eyes closed and head thrown back in ecstasy?

Perhaps the key question is whether the visual information is reliable – helping us to pick out the most deserving winner, say – or misleading, making us prefer performers who rely on visual flair rather than musical depth. “It is possible that some performances can be ‘trained’ or ‘choreographed’ in a way that may not be authentic or true to the meaning of the musical composition, but may still remain effective as a performance as judged by audiences”, Tsay admits.

“The video is a "bad" signal if it leads to bad outcomes, that is, if we reward musicians in competitions conducted this way and then find that those musicians fail to sustain creative careers”, says Levitin. “I don't know of any study that looks at these outcome measures.”

But one might also argue that a competition is seeking only to identify who is “best” on the day. In which case, what should “best” mean?

“I would say that it depends on one's ontology”, says Bergeron. “Someone who thinks that musical performances are essentially sonic events should recognize that our aesthetic evaluations of musical performances might be systematically mistaken. However, someone who is not prepared to accept that our aesthetic evaluations of musical performances might be systematically mistaken should recognize that musical performances might be visual as well as sonic events.”

In any event, says Bergeron, the fact is that ‘experts’ seem to be swayed by visuals whether they like it or not. “This might be a practical reason to embrace the idea that music made for performance is a visual as well as a sonic art, since it might be psychologically impossible to distinguish, in our experience of performances, those aesthetic qualities that belong to the sound from those that belong to the visual aspects.”

In the end, says Tsay, this comes down to a matter of priorities. “It may be less a question whether the visual channel gives us ‘good’ or ‘bad’ data, and more a question of what we as musicians and audience members believe truly reflects quality”, she says. “This likely changes with time and with changes in technologies and the consumption of music.” After all, in Liszt’s day live performance was the only way audiences would ever hear music. And with the cult of the “artist as expressive genius” firmly established since Beethoven’s day, it made sense for him to perform with flair. Evidently, it still does.

References
1. Tsay, C.-J. Proc. Natl Acad. Sci. USA doi: 10.1073/1221454110.
2. Bergeron, V. & Lopes, D. M. Philos. Phenomenol. Res. 78, 1 (2009).
3. Davidson, J. W. Psychol. Music 21, 103 (1993).
4. Vines, B. et al., Ann. N. Y. Acad. Sci. 1060, 462 (2005)
5. Vines, B. et al., Cognition 101, 80 (2006).
6. Fast, S. In the Houses of the Holy (Oxford University Press, Oxford, 2001).

Bohr's beginnings

Here’s a book review, of sorts (I was asked to write something more like an essay review) just published in New Scientist.

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Love, Literature and the Quantum Atom Finn Aaserud & J. L. Heilbron Oxford University Press, 2013 ISBN 978-0-19-968028-3

Niels Bohr was one of the most profound thinkers among the early pioneers of quantum theory. He was the first truly to recognize and confront the philosophical problems that the theory posed, and the solutions he offered, such as the idea of complementarity and the Copenhagen interpretation, are still debated today. One hundred years ago he devised the first quantum picture of the atom, and he also anticipated quantum effects in biology.

What impresses most about Bohr’s scientific thought is that he could leave consistency to littler minds. Like James Clerk Maxwell, another genuinely deep physicist, he was happy to leave some matters unresolved and to accept contradiction. So what if the Bohr atom violates classical electrodynamics, which says it should decay? So what if wave can be particle? That’s just how things are (or how they seem, which for Bohr was much the same).

Love, Literature and the Quantum Atom is valuable for reminding us of this. But it’s a peculiar beast all the same, bearing signs of having been cobbled together for the Bohr atom centenary. In the first section, Finn Aaserud, director of the Niels Bohr Archive, offers a fresh perspective on Bohr’s early family life through newly released correspondence, especially with his wife Margrethe. Then the science historian John Heilbron, who collaborated with Thomas Kuhn in 1969 on a study of the Bohr atom, supplies a new account of the development of that seminal work in which he considers Bohr’s interests in literature, particularly that of Goethe and Ibsen. Finally the book reprints Bohr’s three-part paper (the so-called “Trilogy”) from 1913, “On the Constitution of Atoms and Molecules”.

To link Bohr’s extra-curricular reading with his science, Heilbron has been set a more or less impossible task. At times he can pursue it only by finding apt quotes from Ibsen’s Peer Gynt or Goethe’s Faust with which to punctuate Bohr’s professional life, irrespective of whether Bohr himself had the words in mind. Heilbron’s account of Bohr’s scientific journey is as insightful and informative as we’d expect from him. But once we get to the equipartition principle and the Balmer series, Goethe doesn’t have much to add.

This experiment fails not because a scientist’s interest in arts and literature can tell us nothing about his or her science, but because it seems Bohr’s cannot. He read widely and thought deeply, but on this showing was addicted to the strain of Germanic-Nordic romanticism that today looks like sentimentality, even chauvinism: great men striving to be great, while their pure-hearted, maidenly lovers pledge placid and dewy-eyed support. Margrethe was in fact Bohr’s staunch and sometimes steely ally, as he knew and appreciated – which is why all his talk of “my little one” who he would (using Ibsen’s words) “lock away as heart’s treasure” makes you realise why modernism and Virginia Woolf were so badly needed.

In a soul as noble as Bohr, this kind of sentiment has its touching aspect. But it’s not hard to see why, for less principled men, these visions of struggle and destiny, of heroes and Vikings, led down darker paths. It’s not too much to suggest that this was a Germanic thing (including Dutch and Danish – as physicist Hendrik Casimir attested, in the Netherlands too the intellectual elite was saturated in Germanic Kultur). There’s no inevitable path from Goethe to Goebbels, but the notion of Bildung – the particularly German character development all professors had to undergo – did breed the sort of patriarchal and patriotic conservatism that, as Heilbron showed in his splendid biography of Max Planck (The Dilemmas of an Upright Man, 1986), made it all but impossible for the traditional academics to muster any resistance to the Nazis.

This is why I’m left with mixed feelings about this glimpse at Bohr’s hinterland. On the one hand it is refreshing to see a great scientist being passionate about a difficult philosopher like Kierkegaard instead of coming up with empty soundbites about philosophy being dead. On the other hand, such an education evidently did little to build a moral framework; those few who, like Bohr and Max von Laue, behaved with something approaching heroism in the face of Hitler did so from some inner reserve of integrity that drew little on their broad education. Their generation was in this respect neither better nor worse than the culturally unsophisticated Feynman or the later generations brought up on Star Trek, Star Wars or Tomb Raider. Whatever it is that makes truly noble and responsible (let alone successful) scientists, it isn’t great art.

Colour for free

I have written up my “history of colour chemistry” talk for publication in the little-known journal Interfaces, produced by the Université Paris Diderot and others. This stems from a conference on colour held at the university in early 2012, which, as the other contributions to this volume indicate, was a very diverse affair. The kind folks in Paris have made this and some of the other articles available online for free as pdfs - you can find it here.

Fuelling physics envy?

Here’s an opinion piece I have just published in Physics World, before it was edited.

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Physics envy might get a fresh stimulus from a new paper which claims to present “bibliometric evidence for a hierarchy of the sciences”. By analysing features such as authorship, mode of expression and range of citations in about 29,000 papers from maths to the social sciences and humanities, bibliometrics experts Daniele Fanelli of the University of Edinburgh and Wolfgang Glänzel of the Catholic University of Leuven say that there are good objective reasons to support the hierarchy that proclaims maths and physics the ‘hardest’ and most solidly grounded of the sciences [1].

The work provides a fascinating glimpse at the stylistic and methodological differences that exist between disciplines. It’s telling us something worth knowing: that fundamental differences in style and content across the sciences are real, so that it might be a mistake to evaluate and manage all the sciences in the same way. What it is not telling us is that physics is the most exemplary or exalted of the sciences.

The authors are mostly scrupulous in avoiding that implication. They suggest that this hierarchy is only to be expected because, progressing from physics to sociology, the complexities of the subject matter – the degrees of freedom, if you like – are increasing. So it is scarcely surprising that the phenomena become harder to interpret and consensus becomes harder to achieve: as Fanelli and Glänzel put it, the data “become less able to speak for themselves.” In this much, they are endorsing the view first espoused in the 1830s by the French philosopher Auguste Comte, who also posited a hierarchy from mathematics to physics, chemistry, biology, psychology and sociology based on the level of complexity involved. Comte was the father of positivism, which asserts that all authoritative knowledge derives from an objective, data-driven, scientific study of the world.

Comte’s hierarchy is typically expressed in terms of the ‘hard’ and ‘soft’ sciences. Fanelli and Glänzel embrace these terms, saying that they “seem to capture an essential feature of science”, and that pretending they do not exist could be a “costly mistake”. The authors don’t deny that all disciplines have cultural and “non-cognitive” components, but say that they seem nevertheless shaped “by objective constraints imposed by the subject matter”.

Before grappling with those assertions, let’s look at what the duo did. They figured that a defining characteristic of a ‘hard’ science is the ability to reach a shared interpretation of phenomena. Consensus might be expected to be reflected in several general features of papers. For example, they will be shorter, since there is less need to justify and explain a study; the references will tend to be more recent (key questions are resolved faster), fewer, less diverse and dominated by tightly focused papers rather than general monographs. But titles might be longer, since the issues addressed will be more precisely defined, and the number of coauthors might be greater, since more researchers share commonly agreed goals and because increased specialization makes collaboration essential. Fanelli and Glänzel analysed these parameters in thousands of papers on the Thomson Reuters’ Web of Science, categorized into disciplines such as physics, chemistry, plant and animal sciences, and psychiatry/psychology, and find that the expected trends are borne out by the data.

So what’s the problem? Let’s start with semantics: ‘hard’ and ‘soft’ are prejudicial terms. It is very difficult to avoid reading them both as “hard-headed/soft-headed”, suggesting that the social sciences are pervaded by woolly thinking, and as “hard/easy”, suggesting that the physical sciences are more intellectually challenging and reinforcing the snooty conviction that the most brilliant scientists choose physics. But arguably (most) questions in physics are in fact the easiest to answer securely because they tend to be the easiest to isolate and interrogate experimentally. Economics is failing to answer our real-world questions not because economists are less able, but because economics is so complex, with few if any universal laws and very patchy data. (There’s another reason too, which I’ll come to shortly.)

Yet more invidious than the ‘hard/soft’ terminology is the whole notion of a hierarchy. By definition, this implies a judgement of status: there’s a top and a bottom. At best it invokes condescension towards those disciplines unlucky enough not to be physics; at worst, we’re invited to feel impatient that these ‘softer’ sciences haven’t yet got themselves physics-ified. Comte certainly felt that all sciences aspire to the condition of physics, and he looked forward to the time when the social sciences reached this stage of higher evolution. It was in Comte’s time that historians of science began to construct the narrative in which the mathematization of nature, as displayed in Newton’s Principia, was the defining achievement of the Scientific Revolution, ignoring the fact that this approach was of no value in, say, zoology, botany, chemistry, geology and medicine. When Immanuel Kant declared that the chemistry of his day was “not science” because it was insufficiently mathematical, he was exposing his limited understanding of what chemistry was about, both then and now.

Not only is mathematization, with its consequent opportunities for reductive subdivision of problems, of limited value in some sciences, but they – the life and social sciences particularly – have a dependence on context and history that offers scant purchase for physics-style universal rules, and means different data sets may tell different stories. When those dependencies are neglected for the sake of simplification, as in mainstream neoclassical economic theory, the result is a model so abstracted and simplistic that no amount of empirical input – not even the near-collapse of the global economy – can make much impression on the ramparts of its ivory towers.

I happen to believe that many sciences, from biology to sociology, can in fact benefit from physics-based ideas. But placing physics at the top of the tree doesn’t help, because it blurs the view of where “physics thinking” is and isn’t appropriate. And presenting science in terms of “consensus deficit” is not just misguided but potentially dangerous. A quest for consensus tacitly accepts Comte’s assumption that all questions can be given a single, scientifically based answer. But many cannot, not just in the humanities but also in history, politics, ethics, the social sciences, economics and beyond. Even in the so-called ‘hard’ sciences, the value of having complementary but not entirely compatible models is under-rated. For some questions about humanity, we may be better served by a diversity of views – including old ones – than by a doomed dream of consensus.

1. D. Fanelli & W. Glänzel, PLoS ONE 8, e66938 (2013).

The transformation of Paris

Here is my previous Under the Radar story for BBC Future – the next should be along very shortly. There’s a bigger story to this stuff – I hope I might get to tell it some time.

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They are what make Paris so distinctive: the grand, wide boulevards that march in straight lines through the city, lined with bustling cafés and tempting patisseries. But this isn’t how Paris looked at the time of the Revolution in the late eighteenth century. The city is one of the most striking examples of rational urban planning, conducted in the middle of the nineteenth century during the ‘Second Empire’ of Napoleon III to ease congestion in the dense network of medieval streets.

It’s not hard to see how the redesign, conducted by Baron Georges-Eugène Haussmann at the emperor’s command, transformed Parisian life. You only have to compare the cityscape today with the narrow, convoluted passageways of the Marais district, one of the few parts of Paris largely untouched by Haussmann’s plans. But what exactly did these so-called ‘Second Empire reforms’ really do to the properties of the road network? How did they alter the way residents navigated the city? Can we be sure that other changes, whether contemporaneous or subsequent, didn’t have equally profound impact?

Until recently, these are questions that will have relied largely on the subjective impressions of urban theorists. After all, we can’t make measurements to compare today’s traffic flow with that from the days of Robespierre. But a new study by a collaboration of mathematical physicists and social historians in France shows that, simply by analysing old and new maps of the city, it’s possible to quantify what effect Haussmann’s plans had on the shape and life of Paris. The results offer a case history of how cities may evolve through a combination of spontaneous self-organization and top-down central planning.

Marc Barthelemy of the CEA Institute of Theoretical Physics in the Parisian suburb of Gif-sur-Yvette and his colleagues have analysed maps of the city road network at six moments in time since the Revolution: 1789, 1826, 1836, 1888, 1999 and 2010. They looked at some basic properties of the networks, such as the numbers of nodes (intersections) and edges (roads between nodes), as well as using more sophisticated concepts from the modern theory of complex networks, such as the quantity called ‘betweenness centrality’ (BC) that measures the importance of individual nodes to the navigability of the network.

The results are revealing. Whether or not Haussmann made a difference depends on what you look at. For example, between 1836 (before the changes) and 1888 (when they were essentially complete), the total number of nodes and their total length both increase very sharply – more or less doubling – while changing rather little thereafter. You might say that Haussmann added a lot of ways of getting from place to place. But this growth is mirrored by a steep rise in the city’s population, suggesting that this factor, rather than planning in itself, drove the increases: they might have happened anyway, albeit not necessarily in the same way.

What’s more, changes in the average BC values of the network also suggest that there was nothing unusual about the Haussmann developments, compared to what came before and after. Rather, the web of streets just got steadily denser, as has been found for some other cities.

A quite different picture emerged, however, when the researchers looked at the spatial patterns of change. When they plotted maps of the nodes with the largest BC values – the intersections that are most important for finding a shortcut between any two other nodes – the results look quite different up to 1836 and after 1888. In the earlier period, most of the high-BC nodes are clustered around the city centre, although between 1826 and 1836 an important traffic channel opened up in the Saint Martin region in the east of Paris, where several large properties owned by the church or aristocrats were sold and divided up to create new houses and roads.

But after Haussmann, the high-BC nodes form a more open, widely spaced system of key channels, somewhat like the vein network of a leaf. In other words, Haussmann’s avenues and boulevards helped to prevent routes becoming funnelled through the congested city centre, and gave Paris space to breathe.

The new roads also altered the typical shape of blocks. It’s been found previously that many urban road networks tend to intersect at right angles, dividing up the space ever more finely into square or rectangular blocks a bit like the crack networks of ceramic glazes. That’s what Paris looked like before the 1850s. But the new boulevards sliced boldly through this grid, creating a wider variety of block shapes, especially triangles and elongated rectangles.

So whether the Second Empire reforms transformed the face of Paris is a subtle question. Some of the changes over the nineteenth century, such as higher street density and increase in intersections, might have happened anyway thanks to the growth in population. In other ways, Haussmann stamped a ‘non-natural’ geometry on the city’s evolving network. Although Haussmann’s plans were criticized both at the time and by later architects, it looks as though they did a pretty good job, making the city centre less congested in a way that Parisians still benefit from today. London, in contrast, missed its chance: the grand new streets proposed by Christopher Wren after the Great Fire in 1666 weren’t built in time to prevent the city’s natural, spontaneous evolution from reasserting itself. All the same, using the tools that Barthelemy and colleagues have developed, it might now be possible to probe Haussmann’s scheme more closely – to ask, for example, how close it came to finding the very best solution to the problems it tackled.

Reference: M. Barthelemy, P. Bordin, H. Berestycki & M. Gribaudi, Nature Scientific Reports 3, 2153 (2013).

Plastic fantastic

Here’s the initial version of a leader I wrote for last week’s Nature.

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The transition from basic science to practical technology is rarely linear. The common view – that promising discoveries need only patience, hard work and money to shape them into commercial products – obtains only rarely. Often there are more factors at play: all kinds of technical, economic and social drivers must coincide for the time to be right. So dazzling forecasts fail and fade, but might then re-emerge when the climate is more clement.

That seems to be happening for organic electronics: the use of polymers and other organic molecules as the active materials in information processing. That traditionally insulating plastics could be made to conduct electricity was discovered serendipitously in the late 1960s by Hideki Shirakawa in Tokyo, in the form of silvery films of polyacetylene. Chemists Alan Heeger and Alan MacDiarmid collaborated with Shirakawa in 1976 to boost the conductivity of this material by doping with iodine, and went on to make a ‘polymer battery’. Other conducting polymers, especially polyaniline, were mooted for all manner of uses, such as antistatic coatings and loudspeaker membranes.

This early work was greeted enthusiastically by some industrial companies, but soon seemed to be leading nowhere fast – the polymers were too unstable and difficult to process, and their properties hard to control and reproduce reliably. That changed in the late 1980s when Richard Friend and coworkers in Cambridge found that poly(para-phenylene vinylene) not only would conduct without doping but could be electrically stimulated to emit light, enabling the fabrication of polymer light-emitting diodes. The attraction was partly that a polymer’s properties, such as emission colour and solubility, can be fine-tuned by altering its chemistry. Using such substances for making lightweight, flexible devices and circuits, via simple printing and coating techniques rather than the high-tech methods needed for inorganic semiconductor electronics, began to seem possible. The genuine potential of the field was acknowledged when the 2000 Nobel prize for chemistry went to Shirakawa, Heeger and MacDiarmid.

The synthesis of gossamer-thin organic electronic circuits reported by Martin Kaltenbrunner in Tokyo and colleagues (Nature 499, 458-463; 2013) is the latest example of the ingenuity driving this field. Their devices elegantly blend new and old materials and techniques. The substrate is a one-micron-thick plastic foil, while organic small molecules provide the semiconductor for the transistors, other organic molecules and alumina constitute the insulating layers, and the electrodes are ultrathin aluminium. The featherweight plastic films, 27 times lighter than office paper, can be crumpled like paper, and on an elastomeric substrate the circuits can be stretched more than twofold, all without impairing the device performance. Adding a pressure-sensitive rubber layer produces a touch-sensing foil which could serve as an electronic skin for robotics, medical protheses and sports applications.

Wearable and flexible electronics and optoelectronics have recently taken great strides, propelled in particular by the work of John Rogers’ group at Illinois (D.-H. Kim et al., Ann. Rev. Biomed. Eng. 14, 113-128 (2012)). Such devices can now be printed on or attached directly to human skin, and can be made from materials that biodegrade safely. Especially when coupled to wireless capability, both for powering the devices and for reporting their sensor activity, the possibilities for in situ monitoring of wound care and tissue repair, brain and heart function, and drug delivery are phenomenal; the challenge will be for medical procedures to keep pace with what the technology can offer. At any event, such applications reinforce the fact that organic electronics should not be seen as a competitor to silicon logic but as complementary, taking information processing into areas that silicon will never reach.

At the risk of inflating another premature bubble, these technologies look potentially transformative – more so, on current showing, than the much heralded graphene. The remark by Kaltenbrunner et al. that their circuits are “both virtually unbreakable and imperceptible” says more than perhaps they might have intended. In this regard the new work continues the trend towards the emergence of a smart environment in which all kinds of functionality are invisibly embedded. What happens when packing film (one possible use of the new foldable circuitry), clothing, money, even flesh and blood, is imbued with the ability to receive, process and send information – when more or less any fabric of daily life can be turned, unseen, into a computing and sensing device? Most narratives currently dwell on fears of surveillance or benefits of round-the-clock medical checks and diagnoses. Both might turn out to be warranted, but past experience (with information technology in particular) should teach us that technologies don’t simply get superimposed on the quotidian, but both shape and are shaped by human behaviour. Whether or not we’ll get what’s good for us, it probably won’t be what we expect.

Radio DNA

Another cat among the pigeons, perhaps… here is my latest Crucible column for Chemistry World.

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It has to rate as one of the most astonishing discoveries of this century, and it came from a Nobel laureate. Yet it was almost entirely ignored. In 2011 Luc Montagnier, who three years earlier was awarded the Nobel Prize in medicine for his co-discovery of the AIDS virus HIV, reported that he and his coworkers could use the polymerase chain reaction (PCR, the conventional method of amplifying strands of DNA) to synthesize DNA sequences of more than 100 base pairs, without any of the target strands present to template the process [1]. All they needed was water. Water, that is, first subjected to very-low-frequency electromagnetic waves emitted and recorded from solutions of DNA encoding the target sequence. In other words, the information in a DNA strand could be transmitted by its electromagnetic emissions and imprinted on water itself.

Maybe you’re now thinking this work was ignored for good reason, namely that it’s utterly implausible. I agree: it doesn’t even begin to make sense given what we know about the molecular ingredients. But the claims were unambiguous. The authors say they took a 104-base-pair fragment of DNA from HIV (and who knows about that better than Montagnier?) and copied it, reproducibly and with at least 98% fidelity, by adding the PCR ingredients to the irradiated water. If you choose to ignore this, are you saying Montagnier is lying?

What you’re actually saying is that science doesn’t always work as it is ‘supposed’ to, by claims being tested and then accepted or rejected depending on the result. Of course, many trivial claims never get replicated (that’s another story), but really big ones – and they don’t come much bigger than this – are immediately interrogated by other labs, right? That’s what happened with cold fusion, however implausible it seemed. True, some results can’t be replicated without highly specialized kit and expertise – no one has rushed to verify the Higgs boson sighting. But Montagnier and colleagues used nothing more than you’d find in most molecular biology labs worldwide.

So what’s going on? What we’re really seeing tested here are the unwritten social codes of science. Montagnier has long been seen as something of a maverick, but in recent years some have accused him of descending into quackery. Since claiming in 2009 that some DNA emits EM signals [2], he has suggested that such signals can be detected in the blood of children with autism and that this justifies treating autism with antibiotics. He has seemed to suggest that HIV can be defeated with diet and supplements, and commended the notorious ‘memory of water’ proposed by French immunologist Jacques Benveniste [3]. Although he is currently the head of the World Foundation for AIDS Research and Prevention in Paris, his unorthodox views have prompted some leading researchers to question his suitability to lead such projects.

But science judges the results, not the person, right? So let’s look at the paper. At face value making a simple claim, it is in fact so peppered with oddness that other researchers probably imagine any attempt at replication will be deeply unrewarding. There are hints that the EM emissions come from a baffling and bloody-minded universe: their strength doesn’t correlate with concentration, they seem to appear in some ranges of dilution and then vanish in others, and there is no rhyme or reason to which organisms or sequences produce them and which don’t. That the authors show the signals not as ordinary graphs but as a screenshot adds to the misgivings.

Then there’s the ‘explanation’. Montagnier has teamed up with Italian physicist Emilio Del Giudice and his colleagues, who in 1988 published a “theory of liquid water based on quantum field theory” [4] which proposed that water molecules can form “coherent domains” about 100 nm in size containing “almost free electrons” that can absorb electromagnetic energy and use it to create self-organized dissipative structures. These coherent domains are, however, a quantum putty to be shaped to order, not a theory to be tested. They haven’t yet been clearly detected, nor have they convincingly explained a single problem in chemical physics, but they have been invoked to account for Benveniste’s results and cold fusion, and now they can explain Montagnier’s findings on the basis that the EM signals from DNA can somehow shape the domains to stand in for the DNA itself in the PCR process.

Make of this what you will; the real issue here is that it all looks puzzling, even prejudiced, to outsiders, who understandably cannot fathom why a startling claim by a distinguished scientist is apparently just being brushed aside. Perhaps it might help to stop pretending that science works as the books say it does. Perhaps also, given that Montagnier says his findings are motivating clinical trials to “test new therapeutics” for HIV in sub-Saharan Africa, it might be wise to subject them to more scrutiny after all.

References
1. L. Montagnier et al., J. Phys. Conf. Ser. 306, 012007 (2011).
2. L. Montagnier et al., Interdiscip. Sci. Comput. Life Sci. 1, 81 (2009).
3. E. Davenas et al., Nature 338, 816 (1988).
4. E. Del Giudice, G. Preparata & G. Vitiello, Phys. Rev. Lett. 61, 1085 (1988).

Maxwell's fridge

I haven’t generally been putting up here the pieces I’ve been writing for Physical Review Focus, as they can tend to be a bit technical. But as I’ve been writing this and that about Maxwell’s demon elsewhere, I thought I’d post this one. The final version is here.

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In 1867 the physicist James Clerk Maxwell described a thought experiment in which the random thermal fluctuations of molecules might be rectified by intelligent manipulation, building up heat that might be used to do useful work. Now in Physical Review Letters a team at the University of Maryland outline a theoretical scheme by which Maxwell’s nimble-fingered ‘demon’ might be constructed in an autonomous device that in effect uses computation to transfer heat from a cold substance to a hotter one, thereby acting as a refrigerator.

Maxwell believed that his demon might oppose the second law of thermodynamics, which stipulates that the entropy of a closed system must always increase in any process of change. Because this law seems to be statistical – an entropy increase, or increase in disorder, is simply the far more likely outcome – the demon might undermine it, for example by physically reversing the usual scrambling of hot and cold molecules and thereby preventing the diffusion of heat.

Most physicists now agree that such a demon wouldn’t defeat the second law, because of an argument developed in the 1960s by Rolf Landauer [1]. He showed that the cogitation needed to perform the selection would have a compensating entropic cost – specifically, the act of resetting the demon’s memory dissipates a certain minimal amount of heat per bit erased.

Despite this understanding, there have been few attempts to postulate an actual physical device that might act as a Maxwell demon. Last year, Christopher Jarzynski and Dibyendu Mandal at Maryland proposed such a ‘minimal model’ of an autonomous device [2]. It consisted of a three-state device (the ‘demon’) that can extract energy from a reservoir of heat and use it to do useful work. The transitions in the demon are linked the writing of bits into a memory register – a tape recording binary information – which moves past the it, according to particular coupling rules.

In collaboration with their colleague Haitao Quan, now at Peking University, Mandal and Jarzynski have now refined their model so that the demon is a two-state device coupled to heat exchange between a hot and a cold reservoir. Again, the operation of the demon is ensured by the coupling rules imposed between its transitions, the reservoirs and the memory, resulting in a mathematically solvable model whose performance depends on the model’s parameters.

The demon can absorb heat from the hot reservoir to reach its excited state, and reverse that process, without altering the memory. But the rules say that energy may only be exchanged with the cold reservoir by coupling to the memory. The demon can absorb heat from the cold reservoir if the incoming bit is a 0, or release it if the bit is a 1. And whenever energy is exchanged with the cold reservoir, the demon reverses the bit, which affects the entropy of the outgoing bit stream. So each 0 allows the chance for energy to move from the cold reservoir into the demon – and potentially then out to the hot reservoir.

The researchers find that the behaviour of the system depends on the temperature gradient and the relative proportions of 1s and 0s in the incoming bit stream. In one range of parameters the device acts as a refrigerator, drawing heat from the cold reservoir colder while imprinting a memory of this operation as 1s in the outgoing bit stream. In another range it acts as an information eraser: lowering the excess of 0s in the bit stream and thus randomizing this ‘information’, while allowing heat transfer from hot to cold.

Jarzynski says that, while the model couples heat flow and information, it doesn’t have Landauer’s condition explicitly built in. Rather, this condition emerges from the dynamics, and so the results provide some support for Landauer’s interpretation.

How might one actually build such a system? “We don’t have a specific physical implementation in mind”, Jarzynski admits, but adds that “we are exploring a fully mechanistic Rube Goldberg-like contraption where the demon and memory are represented by wheels and paddles that rotate about the same axis and interact by bumping into one another.”

Trying to figure out how a physical device might act like Maxwell’s demon is “an important task”, according to Franco Nori of the University of Michigan. “To build such a system in the future would be another story, but this is a very important step in the right direction,” he says.

Although he sees this as “an interesting theoretical model of Maxwell's demon”, Charles Bennett of IBM’s research laboratory in Yorktown Heights, New York, thinks it could be made even simpler. “It’s somewhat unrealistic and unnecessarily complicated to have the tape move at a constant velocity”, he says – the parameter describing the tape speed could be eliminated “by coupling each 0→1 tape transition to a forward step of the tape and each 1→0 transition to a backward step.”

References
1. R. Landauer, IBM J. Res. Dev. 5, 183 (1961).
2. D. Mandal & C. Jarzynski, Proc. Natl Acad. Sci. USA 109, 11641-11645 (2012).

What the bees know

I’ve written a news story for Nature on a new paper claiming that the bees’ honeycomb is made hexagonal by surface tension, rather than the engineering skills of the bees. They just make cylindrical cells, the researchers say, and physics does the rest. This isn’t a new idea, as I point out in the story: D’Arcy Thompson suggested as much, and Darwin suspected it. However, it seems to be to be potentially underestimating the role of the bees. The weird thing about the work is that it essentially freezes the honeycomb in an unfinished state, by smoking out the worker bees, and they find that the incomplete cells are circular in cross-section – but there’s apparently no reason to believe that the bees had done all they were going to do, leaving the rest to surface tension. Who’s to say they wouldn’t have kept shaping the cells if left undisturbed? It may be that the authors are right, but this current work seems to me to be some way from a proof of that. Well, here first is the story…

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Physical forces rather than bees’ ingenuity might create the hexagonal cells.

The perfect hexagonal array of the bees’ honeycomb, admired for millennia as an example of natural pattern formation, owes more to simple physical forces than to the skill of the bees, according to a paper published in the Journal of the Royal Society Interface [1].

Engineer Bhushan Karihaloo of the University of Cardiff in Wales and his coworkers say that the bees simply make cells of circular cross-section, packed together like a layer of bubbles, and that the wax, softened by the heat of the bees’ bodies, then gets pulled into hexagonal cells by surface tension.

The finding feeds into a long-standing debate about whether the honeycomb is an example of exquisite biological engineering or blind physics.

To make a regular geometric array of identical cells with simple polygonal cross-sections, they can only have one of three forms: triangular, square or hexagonal. Of these, hexagons divide up the space using the least amount of wall area, and thus the least amount of wax.

This economy was noted in the fourth century by the mathematician Pappus of Alexandria, who claimed that the bees had “a certain geometrical forethought”. But in the seventeenth century the Danish mathematician Erasmus Bartholin suggested that they don’t need any such foresight, since the hexagons would result automatically from the pressure of each bee trying to make its cell as large as possible, much as the pressure of bubbles packed in a single layer creates a hexagonal foam.

In 1917 the Scottish zoologist D’Arcy Thompson argued that, again by analogy with bubbles, surface tension in the soft wax will pull the cell walls into hexagonal, threefold junctions [2]. A team led by Christian Pirk of the University of Würzburg in Germany showed in 2004 that molten wax poured into the space between a regular hexagonal array of cylindrical rubber bungs will indeed retract into hexagons as it cools and hardens [3].

Karihaloo and colleagues now seem to clinch this argument by showing that bees do initially make cells with a circular cross-section – as Charles Darwin suspected – and that these develop into hexagons by the flow of wax at the junctions where three walls meet.

They interrupted honeybees in the act of making a comb by smoking them out of the hive, and found that the most recently built cells have a circular shape while those just a little older have developed into hexagons. They say the worker bees that make the comb knead and heat the wax with their bodies until it reaches about 45 oC – warm enough to flow like a viscous liquid.

Karihaloo thinks that no one thought previously to look at cells before they are completed “because no one imagined that the internal profile of the cell begins as a circle” – it was just assumed that the final cell shape is the one the bees make. He says they got the idea from experiments on a bunch of circular plastic straws which changed to the hexagonal form when heated [4].

The question is whether there is anything much left for the bees to do, given that they do seem to be expert builders. They can, for example, use their head as a plumb-line to measure the vertical, tilt the cells very slightly up from horizontal to prevent the honey from flowing out, and measure cell wall thicknesses extremely precisely. Might they not continue to play an active role in shaping the circular cells into hexagons, rather than letting surface tension do the job?

Physicist and bubble expert Denis Weaire of Trinity College Dublin in Ireland suspects they might, even though he acknowledges that “surface tension must play a role”.

“I have seen descriptions of bees steadily refining their work by stripping away wax”, he says. “So surely those junctions of cell walls must be crudely assembled then progressively refined, just as a sculptor would do?”

While Karihaloo says “I don't think the bees know how to measure angles”, he admits that further experiments are needed to rule out that possibility.

Weaire adds that “if the bee’s internal temperature is enough to melt wax, the temperature of the hive will always be close to the melting point, so the wax will be close to being fluid. This may be more of a nuisance than an advantage.”

But Karihaloo explains that not all the bees act as 'heaters'. "The ambient temperature inside the comb is just 25o C", he says. Besides, he adds, the insects strengthen the walls over time by adding recycled cocoon silk to it, creating a kind of composite.

References
1. Karihaloo, B. L., Zhang, K. & Wang, J. J. R. Soc. Interface advance online publication doi:10.1098/rsif.2013.0299 (2013).
2. Thompson, D. W. On Growth and Form (Cambridge University Press, 1917).
3. Pirk, C. W. W., Hepburn, H. R., Radloff, S. E. & Tautz, J. Naturwissenschaften 91, 350–353 (2004).
4. Zhang, K., Zhao, X. W., Duan, H. L., Karihaloo, B. L. & Wang, J. J. Appl. Phys. 109, 084907 (2011).

Now I want to add a few further comments. It seems the authors didn’t know that Darwin had looked extensively at this issue. He felt some pressure to show how the hexagonal hive could have arisen by natural selection. He conducted experiments himself at Down House, and corresponded with bee experts, noting that bees first excavate hemispherical pits in the wax which they gradually work into the cell shapes. There is some fascinating correspondence on this in the link given above, though Darwin never found the evidence he was looking for.

One of the problems with leaving it all to surface tension, however, is what happens when you get an irregular cell, either because the bees make a mistake (as they do) or because edge effects create defects. As Denis Weaire pointed out,

“Bees do make topological mistakes, or are led into them by boundary conditions. Surface tension would entirely destroy their work, because of this, if unchecked! (five-sided cells shrink etc...):there is no equilibrium configuration!”

Another worry that Denis voiced is what happens to the excess wax if the cell walls are thinned and straightened by flow. This does seem to have an explanation: Karihaloo says that wax is not actually removed, it just begins in a somewhat loose, porous state, which gets consolidated.

I also wondered about the cell end caps. The cells in the honeycomb are made in two back-to-back layers, married by a puckered surface made from end caps that consist of three rhombi in a fragment of a rhombic dodecahedron. This turns out – as Denis showed in 1994 (Nature 367, 123) – to be the minimal surface for this configuration. So one might imagine it too could result from surface tension, if the authors’ argument is right. But when I asked about it, Karihaloo said “Pirk et al. have shown that the end caps are not rhombic at all; it is just an optical illusion.” I was surprised by this, and asked Weaire about it – he said this was the first time he’d heard that suggestion, and that he has pictures of natural combs which show that these polygonal end faces are certainly not illusory. Indeed, Darwin and his correspondents mention the rhombi, and those old gents were mighty careful natural historians. So this suggestion seems to be wrong.

Preparing for a new second

A bit techie, this one, but I liked the story. It’s a news piece for Nature.

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A new type of atomic clock could transform the way we measure time.

The international definition of a second of time could be heading for a change, thanks to the demonstration by researchers in France that a new type of ‘atomic clock’ has the required precision and stability.

Jérôme Lodewyck of the Observatoire de Paris and his colleagues have shown that two so-called optical lattice clocks (OLCs) can remain as perfectly in step as the experimental precision can establish [1]. They say that this test of consistency is essential if OLCs are to be used to redefine the second, currently defined according to a different sort of atomic clock.

This is “very beautiful and careful work, which gives grounds for confidence in the optical lattice clock and in optical clocks generally”, says Christopher Oates, a specialist in atomic-clock time standards at the National Institute of Standards and Technology (NIST) in Boulder, Colorado.

Defining the unit of time according to the frequency of electromagnetic radiation emitted from atoms has the attraction that this frequency is fixed by the laws of quantum physics, which dictate the energy states of the atom and thus the energy and frequency of photons of light emitted when the atom switches from one state to the other.

Since 1967, one second has been defined as the duration of 9,192,631,770 oscillations of the microwave radiation absorbed or emitted when a caesium atom jumps between two particular energy states.

The most accurate way to measure this frequency at present is in an atomic fountain, in which a laser beam is used to propel caesium atoms in a gas upwards. Emission from the atoms is probed as they pass twice through a microwave beam – once on the way up, once as they fall back down under gravity.

The time standard for the United States is defined using a caesium atomic-fountain clock called NIST-F1 at NIST. Similar clocks are used for time standards elsewhere in the world, including the Observatoire de Paris.

The caesium fountain clock has an accuracy of about 3x10**-16, meaning that it will keep time to within one second over 100 million years. But some newer atomic clocks can do even better. Monitoring emission from individual ionized atoms trapped by an electromagnetic field can supply an accuracy of about 10**-17.

The clocks studied by Lodewyck and colleagues are newer still – first demonstrated under a decade ago [2]. And although they can’t yet beat the accuracy of trapped-ion clocks, they have already been shown to be comparable to caesium fountain clocks, and some researchers suspect that they’ll ultimately be the best of the lot.

That’s for two reasons. First, like trapped-ion clocks, they measure the frequency of visible light, with a frequency tens of thousands of times higher than microwaves. “Roughly speaking, this means that optical clocks divide a second into many more time intervals than microwave caesium clocks, and so can measure time with a higher precision,” Lodewyck explains.

Secondly, they measure the average emission frequency from several thousand trapped atoms rather than just one, and so the counting statistics are better. The atoms are trapped in a so-called optical lattice, rather like an electromagnetic eggbox for holding atoms.

If OLCs are to succeed, however, it’s essential to show that they are reliable: that one such clock ticks at exactly the same rate as another prepared in an identical way. This is what Lodewyck and colleagues have now shown for the first time. They prepared optical lattices each holding about 10,000 atoms of the strontium isotope strontium-87, and have shown that the two clocks stay in synchrony to within a precision of at least 1.5x10**-16, which is the detection limit of the experiment.

But if the definition of a second is to be switched from the caesium standard to the OLC standard, it’s also necessary to check that two types of clock are in synchrony. The French team have done that too. They found that their strontium OLCs will keep pace with all three of the caesium clocks in the Observatoire, to an accuracy limited only by the fundamental limit on the caesium clocks themselves.

“These sorts of comparisons have historically been critical in laying the groundwork for redefinitions of fundamental units”, says Oates.

Accurate timing is crucial to satellite positioning systems such as GPS, which is why GPS satellites have onboard atomic clocks. But their accuracy is currently limited more by other factors, such as air turbulence, than by the performance of their clocks. There are, however, other good reasons for going beyond the already astonishing accuracy of caesium clocks.

For example, in astronomy, if the arrival times of light from space could be compared extremely accurately for different places on the Earth’s surface, this could allow the position of the light’s source to be pinpointed very precisely – with a resolution that, as with current interferometric radio telescope networks, is “equivalent to a continent-sized telescope”, says Lodewyck.

Better time measurement would also enable high-precision experiments in fundamental physics: for example, to see if some of nature’s fundamental constants change over time, as some speculative theories beyond the Standard Model of physics predict.

Before switching to a new standard second, says Lodewyck, there are more hurdles to be jumped. Optical clocks are needed that can run constantly, and there must be better ways to compare the clocks operating in different institutes.

“This measurement is a significant advance towards a new definition of the second”, says Uwe Sterr of the Physikalisch-Technische Bundesanstalt in Braunschweig, Germany, which also operates an atomic-clock standard. “But to agree on a new standard for time the pros and cons of the different candidates that are in the play needs to be evaluated in more detail”, he adds.

“It’s not yet decided which atomic species nor which kind of optical clocks will be chosen as the next definition of the SI second”, Lodewyck concurs. “But we believe that strontium OLCs are a strong contender.”

References
1. Le Targat, R. et al., Nature Communications 4, 2109 (2013).
2. Takamoto, M., Hong, F. -L., Higashi, R. & Katori, H. Nature 435, 321–324 (2005).

Gangs of New York

Here’s my latest piece for BBC Future, pre-editing.

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One of the big challenges in fighting organized crime is precisely that it is organized. It is run like a business, sometimes literally, with chains of command and responsibility, different specialized ‘departments’, recruitment initiatives and opportunities for collaboration and trade. This structure can make crime syndicates and gangs highly responsive and adaptable to attempts at disruption by law-enforcement services.

That’s why police forces are keen to discover how these organizations are arranged: to map the networks that link individual members. This structure is quite fluid and informal compared to most legitimate businesses, but it’s not random. In fact, violent street gangs seem to be organized along rather similar lines to insurgent groups that stage armed resistance to political authority, such as guerrilla forces in areas of civil war, for instance in being affiliations of cells each with their own leaders. It’s for this reason that some law-enforcement agencies are hoping to learn from military research. A team at the West Point US Military Academy in New York has just released details of a software package it has developed to aid intelligence-gathering by police dealing with street gangs. The program, called ORCA (Organization, Relationship, and Contact Analyzer), can use real-world data acquired from arrests and questioning of suspects to deduce the network structure of the gangs.

ORCA can figure out the likely affiliations of individuals who will not admit to being members of any specific gang, as well as the sub-structure of gangs (the ‘gang ecosystem’) and the identity of particularly influential members, who tend to dictate the behaviour of others.

There are many reasons why this sort of information would be important to the police. The ecosystem structure of a gang can reveal how it operates. For example, many gangs fund themselves through drug dealing, which tends to happen by the formation of “corner crews”: small groups that congregate on a particular street corner to sell drugs. And having some knowledge of the links and affiliations between different gangs can highlight dangers that call for more focused policing. If a gang perpetrates some violent action on a rival gang, police will often monitor the rival gang more closely because of the likelihood of retaliation. But gangs know this, and so the rivals might instead ask an allied gang to carry out a reprisal instead. So police need to be aware of such alliances.

The roles of highly influential members of a social network are familiar from other studies of such networks – for example, in viral marketing and the epidemiology of infectious diseases. These individuals typically have a larger than average number of links to others, and their choices and actions are quickly adopted by others. An influential gang member who is prone to risky, radicalizing or especially violent behaviour can induce others to follow suit – so it can be important to identify these individuals and perhaps to monitor them more closely.

In developing ORCA, West Point graduate Paulo Shakarian, who has a doctorate in computer sciences and has worked in the past as an adviser to the Iraqi National Police, and his coworkers have drawn on the large literature that has grown over the past decade on the mapping of social networks. These studies have shown that the way a network operates – how information and influence spread through it, for example – depends crucially on what mathematicians call its topology: the shape of the links between people. For example, spreading happens quite differently on a grid (like the street network of Manhattan, where there are many alternative routes between two points), or a tree (where points are connected by the repeated splitting of branches), or a ‘small world’ network (where there are generally many shortcuts so that any point can be reached from any other in relatively few jumps). Many studies in this new mathematical science of networks have been concerned to deduce the community structure of the network: how it can be decomposed into smaller clusters that are highly connected internally but more sparsely linked to other modules. It’s this kind of analysis that enables ORCA to figure out the ecosystems of gangs.

One of the features of ORCA is an algorithm – a set of rules – that assigns each member of the network a probability of belonging to a particular gang. If an individual admits to this, the assignment can be awarded 100% probability. But if he will not, then any known associations he has with other individuals can be used to calculate a probable ‘degree of membership’. The program can also identify ‘connectors’ who are trusted by different gangs to mediate liaisons between them, for example to broker deals that allow one gang to conduct drug sales on the territory of another.

Shakarian and colleagues tested ORCA using police data on almost 1500 individuals belonging to 18 gangs, collected from 5418 arrests in that district over three years. These gangs were known to be racially segregated, and the police told the West Point team that one racial group was know to form more centrally organized gang structures than the other. ORCA confirmed that the latter, more decentralized group tended to be composed of more small modules, rather than larger, branched networks.

Although the West Point team can’t disclose details, they say that they are working with a “major metropolitan police department” to test their program and to integrate it with information on the geographical distributions of gangs and how they change over time. One can’t help suspecting that the developers of games such as Grand Theft Auto, which unfolds in a complex netherworld of organized crime gangs, will also be taking an interest to improve the realism of its fictional scenarios.

Reference: D. Paulo et al., preprint http://www.arxiv.org/abs/1306.6834 (2013).

Turning pearls

Here’s the previous fortnightly piece for BBC Future. That published today is coming up soon.

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Of all nature’s defence mechanisms, molluscs surely have the most stunning. If a foreign particle such as an abrasive sand grain or a parasite gets inside the soft body of a mollusc – most pearls are made by oysters, though clams and mussels will make them too – the organism coats it in nacre (mother of pearl), building up a smooth blob of this hard iridescent material. The mollusc is, of course, oblivious to the fact that this protective capsule is so gorgeous. Pearls can be white, grey, black, red, blue, green or yellow, and their attraction for humans has led to traditions of pearl-diving that are thousands of years old. Today pearls are harvested in oyster farms in the Indian Ocean, East Asia and all across the Pacific, in which pearl production is stimulated artificially by inserting round beads into the molluscs to serve an a seed.

Yet in spite of the commercial value of pearl production, the formation of pearls is still imperfectly understood. Only the most highly prized pearls are perfectly spherical. Many have other shapes: elongated and ovoid, say, or the teardrop shape that works well for earrings. Some, called baroque pearls, are irregular, like blobs of solder pinched off at one end into a squiggly tail. It’s common for pearls to adopt a shape called a solid of revolution, roughly round or egg-shaped but often with bands and rings running around them latitudinally, like wooden beads or bedknobs turned on a lathe. In other words, the pearl has perfect ‘rotational symmetry’: it looks the same when rotated by any amount on its axis. When you think about it, that’s a truly odd shape to account for.

It’s recently become clear that pearls really are turned. Pearl farmers have long suspected that the pearl might rotate as it grows within the pouch that holds it inside the soft ‘mantle’ tissue of the mollusc. In 2005 that was confirmed by a report published in an obscure French-language ‘journal of perliculture’, which stated that a pearl typically rotates once every 20 days or so. This would explain the rotational symmetry: any differences in growth rate along the axis or rotation get copied around the entire circumference.

But what makes a pearl turn? Julyan Cartwright, who works for the Spanish Research Council (CSIC), and his colleagues Antonio Checa of the University of Granada and Marthe Rousseau of the CNRS Pharmacologie et Ingénierie Articulaires in Vandoeuvre les Nancy, France, have now come up with a possible explanation.

Nacre is an astonishing material in its own right. It consists mostly of aragonite, a form of calcium carbonate (the mineral fabric of chalk), which is laid down here as microscopic slabs stacked in layers and ‘glued’ with softer organic membranes of protein and chitin (the main component of the insect cuticle and shrimp shell). This composite structure, with hard layers weakly bonded together, makes nacre extremely tough and crack-resistant, which is why materials scientists seek to mimic its microstructure in artificial composites. The layered structure also reflects light in a manner that creates interference of the light waves, producing the iridescence of mother-of-pearl.

The slabs of aragonite are made from chemical ingredients secreted by the same kind of cells responsible for making the mollusc’s shell. Several layers grow at the same time, creating terraces that can be seen on a pearl’s surface when inspected under the microscope.

Cartwright and colleagues think that these terraces hold the key to a pearl’s rotation. They say that, as new molecules and ions (whether of calcium and carbonate, or chitin or protein) stick to the step of a terrace, they release energy which warms up the surface. At the same time, molecules of water in the surrounding fluid bounce off the surface, and can pick up energy as they do. The net result is that, because of the conservation of momentum, the step edge recoils: the surface receives a little push.

If terrace steps on the surface were just oriented randomly across the pearl, this push would average out to zero. But for pearls with a solid-of-rotation shape, it’s been found that the terraces are arrayed in parallel like lines of longitude on a globe, creating a ratchet-like profile around the circumference of the pearl. Because of this ratchet shape, impart a preferred direction to the little impulses received by the pearl from molecular impacts on the vertical faces of the steps, causing the growing pearl to rotate. The researchers’ rough estimate of the size of this force during growth of a typical pearl shows that it should produce a rotation rate more or less equal to that observed.

In other words, the pearl can become a kind of ratchet that, by virtue of the unsymmetrical step profile of its surface, can convert random molecular motions into rotation in one direction.

The researchers can also offer an explanation for where the ratchet profile comes from in the first place. If by chance a growing pearl starts to turn in a particular direction, feedbacks in the complex crystallization process on the pearl surface will cause the step edges to line up longitudinally (that is perpendicular to the rotation), creating the ratchet that then sustains the rotation.

The researchers admit that there are still gaps to be filled in their argument, but they say that the idea might be applied to make little machines that will likewise rotate spontaneously powered only be ambient heat. But don’t worry: they haven’t invented perpetual motion. The rotation is ultimately powered by the heat released during the chemical process of crystallization, and it will stop when there is nothing left to crystallize – when the ‘fuel’ runs out.

Reference: J. H. E. Carywright, A. G. Checa & M. Rousseau, Langmuir, advance online publication doi:10.1021/la40142021 (2013).