Page 1343 – Physics World

Home » Page 1343

Portrait of our planet – readers’ pictures

To celebrate the Earth’s spectacular geology, we asked readers to submit photos to our Flickr group on the theme of “portrait of our planet”. Thank you to everyone who took part. Below is a selection of the images we received.

The tallest volcano in Europe, Mt Etna is an active volcano on the island of Sicily. On 8 October 2011, Flickr user Andrea Rapisarda took this spectacular photo of gas and volcanic ash spewing out from the side of this dark mountain. “A great show…but it was very cold and windy today over there!” Rapisarda remarked.

Photo of multicolour sunset due to volcanic ash from an eruption of Chile's Puyehue volcano

Volcanic ash sunset

Volcanic ash can have far-reaching effects, from wreaking havoc with aviation to influencing global weather and climate systems. The ash can alter the way sunlight is scattered when it passes through the Earth’s atmosphere, leading to spectacular sunsets, such as this one photographed by Peter Shanks. This multicoloured sky above Mt Wellington in the Australian island state of Tasmania was caused by an eruption of Chile’s Puyehue volcano in June 2011.

Photo of lightning striking Sandton City in South Africa

Electric storm over Sandton City

Another time when the atmosphere can produce spectacular light shows is during electric storms, when bolts of lightning can emerge from clouds before shooting down towards the ground. These weather events can be menacing and dramatic, such as this one experienced by Flickr user MGCarter88 who took this shot of lightning striking Sandton City in South Africa.

Icicle growing upwards

You’ve seen fire, so now for some ice. Flickr user PipeSmoke submitted this intriguing image of an icicle, which appeared growing upwards from a bird bath. PipeSmoke speculates that the icicle formed due to the accumulation of a thin surface layer of oil in the middle of the water bowl. As the water froze from the edges of the bowl, the increased volume eventually forced water up through the oil, causing it to freeze as well.

Sampling the Earth

In addition to appreciating the beauty of the physical planet, scientists also see Earth as a puzzle to be solved. One way to do this is to collect samples in the field and return these to the lab where they can be examined in closer detail. This was the case for this zoisite mineral sample, photographed by James Gehrt on the premises of the Alumnae Association of Mount Holyoke College, in Massachusetts, US.

Shell suite

James Gehrt describes himself as a fine art photographer and that is evident in this image of Liocancha castrensis shells, which form part of the teaching collection at the Mount Holyoke College facility. “I captured the items as I would in my personal art work, and not as a scientist. They tend to have a portrait quality to them, showing the uniqueness in the objects,” Gehrt told physicsworld.com.

Navajo Sandstone cliffs

Fractures and planes are present in rocks and minerals across a range of different scales. Laura Tucker submitted this close-up of Navajo Sandstone cliffs, a geologic formation in Glen Canyon, Arizona. Tucker says that this area would have been a vast quartz sand desert in the Jurassic period, but today the red colours are due to the iron oxide hematite, and the black colours are due to the iron oxide magnetite. “Our guide for the trip told us people often see what looks like a girl on a swing on this section of the rock,” she adds.

Ammonite in detail

This next image takes us back in closer to the fine detail of rocks. It reveals the crystal structures inside a fossilized ammonite, a type of invertebrate marine animal that existed 400–65.5 Ma. The image was submitted by Ian Jacobs, an adviser to the National Science Museum in Thailand, who says he collected the ammonite in Lyme Regis, a coastal town in the south-west of England noted for its large range of fossils, particularly from the Jurassic period (approximately 200–145 Ma).

Photo of volcanic rock formation along the Na Pali Coast in Kauai, Hawaii

Alien landscape on Earth

Finally, we see a rather fairy-tale vision of the planet in this submission from Christine Kingsley-Kozima. This volcanic rock formation, along the Na Pali Coast of the Hawaiian island of Kauai, looks like something you might imagine in JRR Tolkien’s Lord of the Rings.

There were plenty of other striking images, which you can view in our Flickr group, the Physics World photo challenge. For more spectacular images of Earth, and for a range of feature articles, you can also download a free copy of Physics World‘s March issue, a special edition on the earth sciences

The theme for our next photo challenge is “doing physics”. We want you to submit your pictures of the process of science. It could be you or your colleagues working on an experiment in the laboratory, or perhaps out in the field collecting data, or maybe looking at the heavens through a telescope. Or perhaps you are more theoretically minded and you want to send us an image of your paper-littered office, or the chalkboard detailing the calculation that has been keeping you awake for the past two weeks. Be as creative as you like.

Please add your photos to our Flickr group by Tuesday 8 May and then after this date we will choose a selection of our favourite images to be showcased on physicsworld.com.

Rapidly spotting major earthquakes using GPS

Researchers in California have developed a system that can rapidly determine the size of an earthquake and the extent of its impacts within a fault zone, including its potential for triggering a devastating tsunami. The researchers have used the system – which is based on GPS measurements – to accurately model two historic earthquakes in Japan and northern Mexico.

The 2011 Japanese earthquake disaster showed that the first few minutes after an earthquake are critical. When the Tōhoku earthquake struck, it took geophysicists more than 20 min to compute that the earthquake was magnitude 9.0 on the Richter scale. Had the authorities known the full extent of the earthquake sooner, it would have given them valuable time to activate early-warning systems to help prepare people for the large tsunami that would inevitably follow.

The speed of response in current systems is constrained by the fact that instruments in seismic stations close to major earthquakes tend to be saturated by periods of intense shaking. Therefore, to determine the size and extent of earthquakes, seismologists need to look at data from a range of stations further afield. What is more, because instruments in seismic stations cannot measure the full range to which tectonic plates are displaced, earthquake magnitudes are often underestimated in the first minutes following an earthquake.

The view from above

An alternative approach that has been developed over the past two decades is to use GPS data to monitor the Earth’s crust from space. The basic principle is to create a regional network of GPS stations where geoscientists can then track the position of the stations within a given geographical area. Following an earthquake, scientists can examine the movements of these GPS stations relative to each other in order to work out the degree to which the land has shifted.

Brendan Crowell, Yehuda Bock and Diego Melgar at the Scripps Institute of Oceanography at the University of California, San Diego have now developed this approach into a system for modelling the extent of earthquakes in detail. The system is based on a mathematical model that enables the researchers to use regional GPS data to recreate fault planes and to characterize the activity within these zones during an earthquake. They claim that their system can ascertain the magnitude of an earthquake significantly faster than is possible with traditional seismic methods. It would have enabled seismologists to identify the magnitude-9 Japanese earthquake within 2 to 3 min, Crowell told physicsworld.com.

Crowell and his team say that the model can be run in two different ways. The first is to apply the model to a region where the network of faults is already well known to geoscientists. In the second approach, the model is applied to a region where the details of the fault zone are not so well established. In this second instance, the model is capable of using the GPS data to recreate the network of faults beneath the Earth’s surface by using a mathematical function known as a centroid moment tensor.

Recreating major quakes

Publishing their findings in a paper to appear in Geophysical Research Letters, Crowell’s team demonstrates its model for two large earthquakes. The first example used GPS data from 356 GEONET stations to build a picture of the magnitude-8.3 Tokachi-oki earthquake that struck 100 km offshore from Japan’s Hokkaido island. The second case used GPS data from 95 stations in the California Real Time Network (CRTN) to recreate the magnitude-7.2 El Mayor-Cucapah earthquake that struck the Baja California region of northern Mexico in 2010.

In both cases Crowell’s group was able to determine the earthquake magnitudes in less than 2 min, improving on traditional seismic methods by a factor of 10. The researchers found that in the case of the Japanese quake, the method of predefining the faults worked better because the fault system gets more complicated with depth. In the Mexican case study, the method of recreating the fault zone worked better too, partly because the dip angle of the fault does not change much with depth, allowing for a simple representation of the fault. Crowell told physicsworld.com that his group has also used this GPS method to model the 2011 Tōhoku earthquake and it is currently working on a separate paper to describe this work.

The research is funded in part by NASA, and Crowell says that his group is developing a prototype working system that will be deployed within the next year and monitored from the Scripps Institute. He cautions that the system should not be viewed as a true early-warning system, because that would require a response within seconds of an event. “It can, however, assist first-responders to locate the regions of greatest damage more accurately. For tsunami modelling, this method would work perfectly and could speed up the current modelling by tens of minutes,” he says.

In which TV show should Stephen Hawking make his next cameo appearance?

By James Dacey

Stephen Hawking appearing on the Big Bang Theory

Sheldon meets one of his heroes. Courtesy: CBS

Later today, fans of the hit comedy series The Big Bang Theory are in for a real treat, because the show includes a special guest appearance from Stephen Hawking. The 70-year-old theoretical physicist will be playing himself in an episode called “The Hawking Excitation”, which will be aired at 1900 (CST) on the CBS television channel. Details of the plotline are scarce, but this picture released by the CBS network shows the programme’s chief protagonist (or leading geek, I should say) Sheldon, meeting Hawking in a library.

These days Hawking is just as famous in popular culture as the epitome of intelligence as he is as a physicist. This rise to pop-culture icon was fuelled by the phenomenal popularity of Hawking’s popular-science book A Brief History of Time, which has sold more than 10 million copies worldwide since it was first released in 1988. But in recent years, Hawking has also made several appearances on television. In addition to appearing in science shows, he has also starred in Star Trek, as well as several episodes of The Simpsons, the ever-popular animated sitcom.

So here at the Physics World HQ, it’s left us speculating as to where Hawking might crop up next. After much agonizing and heated debate, we’ve managed to draw up a shortlist of four popular TV shows. In this week’s Facebook poll we want you to choose the one that you think would make the most entertaining viewing.

In which TV show should Stephen Hawking make his next cameo appearance?

Doctor Who
Glee
How I Met Your Mother
Red Dwarf

Have your say by casting your vote on our Facebook page. As always, please feel free to explain your response by posting a comment.

In last week’s poll we addressed the topic of alien life, as we asked you “How common is life in the Milky Way?”.

The question was prompted by recent results from the European Southern Observatory’s High Accuracy Radial velocity Planet Searcher (HARPS) instrument, revealing that our galaxy could be awash with rocky super-Earths orbiting within the habitable zones around faint red stars. The international team of researchers claims that there may be tens of billions of such planets in the Milky Way alone, and probably about 100 in the Sun’s immediate neighbourhood.

62% of respondents chose the option “We are by no means the most intelligent civilization in the galaxy”. 29% opted for “The galaxy is teeming with primitive organisms”, and just 9% believe that “We are alone in the galaxy”.

One voter, Peter Frederick Woolman, wrote “A galaxy teeming with primitive organisms is almost a certainty. I’d find it very surprising if we were the most intelligent civilization in the galaxy, but the existence of intelligence is still far less certain than the existence of primitive organisms.”

Taking a more hard-line stance was Dale Who, a voter who wrote “You’re not even the most intelligent civilisation on the planet”, which also raised some interesting questions about Dale himself.

Thank you for all your participation and we look forward to hearing from you in this week’s poll.

Quantum man, revisited

By Margaret Harris

Lawrence Krauss

Yesterday’s edition of the Physics World online lecture series saw the cosmologist Lawrence Krauss hold forth on one of his favourite subjects: the life and science of his intellectual hero, Richard Feynman.

Krauss has won awards for his work in science communication, and his biography of Feynman, Quantum Man, garnered Physics World‘s own Book of the Year gong for 2011, so it was no surprise to “see” almost 300 of you tuning in yesterday to learn more. But if you weren’t able to watch the lecture live, don’t worry: you can still watch Krauss’s talk on demand here, complete with images of Feynman’s calculus notebooks (a real highlight for me, personally) and Krauss’s eloquent explanation of how Feynman’s beloved first wife, Arline, shaped his way of thinking.

With Feynman as the subject, there were sure to be plenty of questions from audience members at the end of the lecture, and inevitably there wasn’t time for all of them. However, Krauss has now sent us written answers to a few of the most interesting ones, and I’ve pasted his replies below. Enjoy!

Audience member: Do you feel that only scientists like Feynman and Sagan who have an “outgoing” nature that augments the brilliant science they do will be remembered or revered in this day and age?
Lawrence Krauss: Ultimately, I think not. Their names will be most recognized by laypeople in the near term perhaps, but in the long run I believe scientists are remembered for their contributions to changing the way we think about the universe. It takes time for that historical perspective to be obtained, but I believe it arises eventually.

What would you regard as Feynman’s most negative characteristic?
I explain this in more detail in Quantum Man, but I think his persistent desire to redo everything himself (a plus) also meant that he did not follow the work of others as well as he should have. As the American theoretical physicist Sidney Coleman put it, “the other people are not all jerks”, and had Feynman been more aware of this other work, in a number of key areas, he could have had more breakthroughs than he did.

Feynman expressed regret at not reconsidering his involvement with the Manhattan Project after Germany was defeated, but do you think he felt guilt about the technology he helped to create?
I think he ultimately decided he was not responsible for the ills of the world, or what people did with his work. John von Neumann convinced him of this.

Did Feynman believe in any particular physical interpretation of quantum mechanics?
He developed his own, and as for philosophical questions, he avoided them as he got older – rightly, I believe. Amusingly, he said he never really understood quantum mechanics, which is one of the reasons he was hoping for a quantum computer, as that might reveal the quantum world in a way that would have given Feynman a more intuitive understanding.

As with most scientific presentations, it seems as if this one was preaching to the converted. How can we best reach out to a non-scientific audience?
I find it helps to use hooks that relate to things people are already interested in. I used Feynman the “character” as a hook to learn about his science; I used Star Trek as a hook to get people interested in modern physics in my first book; and most recently I have used the religious question of why there is “something rather than nothing” as a hook to teach about modern cosmology.

What area of physics did you talk about when you met Feynman? Also, can we have more details of the weekend you spent with him as an undergraduate?
I was talking about his lecture, which was about the theory of the strong interaction, quantum chromodynamics. I have written about the weekend a bit in the book, and I will leave it at that. My favourite Dirac joke is in the book, too.

Lawrence Krauss’s book Quantum Man: Richard Feynman’s Life in Science (2011 W W Norton) is available now in hardback

Split decision for SKA?

Artist’s impression of the dishes for the €1.5bn Square Kilometre Array.
(Courtesy: SPDO/Swinburne Astronomy Productions)

By Michael Banks

A decision on who will build the €1.5bn Square Kilometre Array (SKA) will have to wait after the SKA Organisation announced yesterday that no outcome had been achieved.

SKA is a massive next-generation radio-astronomy facility consisting of around 2000–3000 linked antennas that will probe the first 100 million years after the Big Bang for clues about galaxy evolution, dark matter and dark energy.

Two rival bids are going head-to-head to host the telescope: one led by Australia and the other by South Africa.

The eight members of the SKA Organisation – including China, Italy and the UK – have the final say in who will host the telescope. They met yesterday at Amsterdam’s Schiphol airport in the Netherlands to discuss the site selection advisory committee’s report, which according to leaks suggested that southern Africa had got the nod.

After the meeting the SKA Organisation issued a press release that gave no indication of a site choice, only saying that it “wished to move ahead with the site selection process”.

However, instead of going for a single winner, rumours on the blogs suggest that the SKA Organisation may opt for splitting the SKA antennas between Africa and Australasia. Indeed, this is already happening on a smaller scale via the two SKA prototypes: the Australian Square Kilometre Array Pathfinder in mid-west Australia and the MeerKAT array in the Northern Cape province of South Africa.

Yesterday’s press release alluded that the SKA Organisation may be heading in this direction. The statement says that the members “recognised that it is desirable to maintain an inclusive approach to SKA”, adding that “it is important to maximize the value from the investments made by both candidate host regions”.

The SKA Organisation has now set up a scientific working group to “explore possible implementation options that would achieve this”. The working group will report back to the SKA Organisation at a meeting in mid-May, when perhaps a final decision will be made.

Nanomachines could benefit from superlubricity

Researchers in China and Australia have observed superlubricity – the dropping of friction to near zero – on length scales much larger than before. They say that the phenomenon, which they measured in sheared pieces of graphite, could find applications in sensitive microscopic resonators or nanoscale gyroscopes.

Superlubricity is sometimes used to mean simply very low friction, but the original meaning is that the friction between two surfaces disappears almost completely. Proposed in the early 1990s by Motohisa Hirano, then at the Nippon Telegraph and Telephone Corporation in Tokyo, Japan, and others, it relies on a special arrangement of atoms on a material’s surface. In graphite, for instance, the surface atoms have a bumpy hexagonal arrangement like egg-boxes. In certain orientations, two surfaces of graphite can mesh in such a way that the “bumps” can slide past one other effortlessly – and friction drops towards zero.

Since it was first proposed, superlubricity has been observed on the nanoscale, mostly under high-vacuum conditions. Now, however, Quanshui Zheng at Tsinghua University and others have observed the phenomenon on the microscale, in ambient conditions.

“Big advance”

“This is a big advance beyond the nanometre-scale superlubricity experiments,” says Hirano, who was not involved with the latest study. “It could lead to implementing superlubricity [as a] lubricant for future practical use in mechanical engineering, including [devices for] saving energy.”

In its experiments, Zheng’s group used pyrolitic graphite, a type of graphite manufactured under high temperature that has particularly well-aligned crystal planes. Using lithography, the researchers made square columns – or mesas – of graphite up to 20 µm wide and up to 400 nm in height. They transferred these mesas to a scanning electron microscope or an optical microscope and, with a tungsten probe, sheared them into flakes, which they could rotate into different orientations.

Zheng’s group found that the flakes orientated symmetrically with respect to the underlying mesa stayed still, even when poked with the probe. However, when the researchers misorientated the flakes and poked them, the flakes retracted to their original, lowest energy position. This could only happen because of the extremely low friction, the researchers say – that is, because the surface “bumps” could mesh together and allow superlubricity.

Easy and practical

“The ultimate significance of these results is they imply that the conditions for superlubricity are more easily created, and more reproducible, than previously supposed,” says Zheng. “This implies a much wider practical significance for the phenomenon of superlubricity, for example in nano- and micromachines.”

Other specialists in superlubricity seem to agree. “I think this is very interesting and promising work, which could lead to a breakthrough in the field of superlubricity and more generally in the control of friction properties,” says Michael Urbakh of Tel Aviv University in Israel, who had previously published a theory suggesting microscale superlubricity is possible. “This work may open a new way for preparation of graphite lubricants with improved lubrication properties.”

Zheng points out that his group’s results may also be applicable to graphene – a single layer of graphite with superlative properties that was the subject of the 2010 Nobel Prize for Physics. However, for applications with regular graphite, he points to high-frequency microscopic resonators and nanoscale gyroscopes, to which superlubricity could offer reduced wear and lower actuation energies.

“The conventional wisdom so far has been that friction is a major hurdle to shrinking mechanical systems to the micro- and nanoscale,” he says. “This is because of the increasing surface-to-volume ratio of smaller components, which favours friction – a surface-dependent force. [Our work] provides new avenues to produce practical micro- and nano-scale mechanical devices that rely on the ultra-low friction of superlubricity.”

The paper is due to be published in Physical Review Letters.

‘Superradiant’ laser created for first time

A new type of laser in which light is emitted collectively by several atoms at the same time – rather than just one – has been unveiled by physicists in the US. Their “superradiant” laser, which is based on a concept first described in 1954 by the US physicist Robert Dicke, has the potential to be up to 1000 times more stable than the best conventional visible lasers. It could therefore be used to boost the performance of the most advanced atomic clocks, although much more development work will be needed before that goal is fulfilled.

At the heart of a conventional laser is a mirrored cavity in which light bounces back and forth to create a standing wave that is related to the wavelength of the laser’s output. The cavity contains atoms or molecules that have been “pumped” into a high-energy state. When a photon interacts with an atom or molecule, an exact copy of the photon is produced and these two photons can go on to create four photons, and so on. If one of the mirrors is thin enough to allow some of the light to pass through it, a coherent beam of identical photons will stream out of the cavity to make the laser beam.

In principle, all these photons should have exactly the same wavelength, which is defined by the distance between the mirrors. But in a real laser, the mirrors tend to vibrate, which means that the distance varies in time about an average value, thus broadening the distribution of wavelengths of the light produced. Stabilizing the length of the cavity is therefore essential to create lasers with very fixed frequencies that can be used for atomic and optical clocks.

Standing wave of atoms

The new laser, which has been built by James Thompson and colleagues at the University of Colorado, Boulder, avoids the problem of unstable cavities because the coherence of its emitted light is not maintained by the light in the standing wave (and hence the length of the cavity) but rather by a standing wave in the electrical polarization of the atoms. Indeed, nearly all of the light produced in the cavity escapes without ever bouncing off a mirror, which means that the width of the emitted light is not affected by mirror vibrations. Remarkably, the atomic standing wave can be maintained even when there is – on average – less than one photon bouncing back and forth in the cavity.

To create their laser, Thompson and colleagues began with an ultrahigh-vacuum chamber containing a vapour of rubidium atoms. The atoms were laser cooled to about 20 µK and then held between two mirrors by creating a 1D optical lattice using a conventional laser operating at a frequency different to that of the superradiant laser. Separate lasers at the sides of the cavity were then used to pump the atoms.

The excited atoms begin to emit photons with wavelengths tuned to the length of the cavity. But while most of these photons escape immediately to produce the laser beam, an extremely small number bounce back and forth in the cavity to create a standing wave in the electrical polarization of the rubidium atoms. Amazingly, the laser can operate even when there are, on average, only 0.2 photons trapped in the cavity.

The team found that the laser was extremely stable with respect to the frequency of the laser used to place the atoms in the pumped state – although in absolute terms, the device is nowhere near as stable as the best cavity-stabilized lasers. In addition, the team was only able to run the superradiant laser for about 140 ms.

Potential “game-changer”

For practical metrological applications, a superradiant laser would have to operate continuously for hours or days, according to Mark Oxborrow of the National Physical Laboratory in the UK. However, Oxborrow told physicsworld.com that the laser has the potential to be a “game-changer” in terms of how extremely stable lasers are made. He believes that the superradiant laser could become very useful if its stability was able to compete with the best cavity-stabilized devices, which would require more than a thousand-fold improvement.

Ultimately, Thompson and colleagues believe the technology could be 1000 times more stable than conventional lasers, which would require a whopping million-fold improvement in stability. While this would be a remarkable improvement over the current design, Oxborrow points out that such a huge improvement in performance is not unprecedented when a new way of doing things is introduced.

Thompson says that the team is now examining how to make a superradiant laser using atoms in which the excited state has a longer lifetime. An example of such an atom is strontium, which is used in optical clocks. Superradiant lasers could even be used in long-baseline interferometry experiments that search for gravitational waves, which rely on light with a very precise wavelength. Thompson also believes that the laser could be used in large-aperture telescopes, in which light captured in different parts of the world is combined to create an image.

The laser is described in Nature.

Quantum man: Richard Feynman’s life in science

It took a man who was willing to break all the rules to tame a theory that breaks all the rules. This lecture is based on Lawrence M Krauss’ new book Quantum Man: Richard Feynman’s Life in Science. Krauss presents a scientific overview of the contributions of Feynman, as seen through the arc of his fascinating life.

From quantum mechanics to antiparticles and from Rio de Janeiro to Los Alamos, this whirlwind tour provides insights into the character, life and accomplishments of one of the 20th century’s most important scientists, as well as an object lesson in scientific integrity.

Date: Wednesday 4 April 2012

Speaker: Lawrence M Krauss, Foundation Professor in the School of Earth and Space Exploration, Physics Department, Arizona State University, US
Lawrence M Krauss is a cosmologist at Arizona State University and the author of nine popular-science books, including the bestselling The Physics of Star Trek and his most recent work A Universe from Nothing, in addition to Quantum Man. He has won numerous international awards, is a commentator and essayist for newspapers such as the New York Times and the Wall Street Journal, and has written regular columns for New Scientist and Scientific American. He serves as co-chair of the board of sponsors of the Bulletin of the Atomic Scientists, is on the board of directors of the Federation of American Scientists and is one of the founders of Science Debate 2012.

Moderator: Dr Margaret Harris, reviews and careers editor, Physics World

Entropy hits the right notes

By minimizing the entropy in the sound waves produced by a musical instrument it might be possible to use an electronic device to tune that instrument as well as is possible with the best human ear. So says a German physicist who has found that seemingly random fluctuations in the pitch difference between successive keys in a tuned upright piano may in fact be crucial to a harmonious sound.

The system of tuning used in most Western musical scales is known as “equal temperament”, which means that the ratio of frequencies of successive notes in the scale is a constant. Since the pitch doubles every 12 notes, the frequency ratio between neighbouring notes is 2^1/12. Intuitively, it might be expected that tuning a piano or other keyboard instrument is then simply a question of ensuring that this ratio holds for every pair of adjacent notes. However, this would only be true for pianos with perfect strings – those that have no stiffness.

In practice, the higher-frequency modes, or “harmonics”, that always accompany a note of a particular fundamental frequency, and which provide an instrument with its characteristic sound, deviate from their theoretical frequency values. This means that where they ought to coincide and produce a harmonious sound, harmonics from different notes that are played together are instead out of step and produce a series of unpleasant beats.

Stretching intervals

Professional aural tuners overcome this problem by “stretching” intervals – slightly increasing the pitch of higher notes to ensure their harmonics are never too far below those of lower-frequency notes while marginally decreasing the pitch of the lower notes. This can also be done using electronic devices rather than the human ear, although this approach is complicated by the fact that “inharmonicity” varies from instrument to instrument. Some devices simply use predefined stretch factors depending on the type and size of instrument, while more sophisticated appliances record the spectrum of harmonics from each note for a particular instrument and then use these data to calculate how the stretch should vary as a function of frequency.

Haye Hinrichsen, a statistical physicist at the University of Würzburg, wondered whether this approach could be improved upon, after measuring the frequencies of the notes produced by a piano that his family had recently bought and which had been tuned aurally by a technician. Rather than finding that the stretch varied smoothly with frequency, as is the case when pianos are tuned using electronic devices, Hinrichsen found it varied in quite an irregular way. He wondered whether these irregularities were random and the result of the limitations of human hearing or whether they might, on the contrary, be vital for good tuning. He also speculated that entropy was the key to reproducing these fluctuations systematically.

Here, entropy refers to the amount of information needed to describe a physical state. When two harmonics from different notes overlap, less information is needed to describe their combined state than if they were to remain distinct, which means they have a lower entropy. So Hinrichsen hypothesized that maximizing tuning means minimizing entropy.

Fourier transforms

He began by recording the waveforms produced by each of the 88 keys on his piano. He used a computer program to apply a Fourier transform to each waveform, giving him a series of peaks on a plot of intensity versus frequency. He then “detuned” the resulting spectrum so that it formed a scale of equal temperament, with the fundamental frequency of each note being 2^1/12 times higher than the preceding one, and then he added all 88 plots together. Next, he applied an algorithm to work out the spectrum’s entropy, then randomly increased or decreased the pitch of one of the 88 notes by a small amount. If as a result of this change the spectrum’s entropy dropped, then the change was kept, otherwise it was rejected. The pitch of another note chosen at random was then changed and the cycle repeated until the entropy could be reduced no more.

Hinrichsen was able to produce tuning curves very similar to those from the aural tuning. As he points out, not only does it reproduce the overall shape of the curve but it also recreates many of the individual fluctuations. He believes that it might therefore be possible to produce a new kind of hybrid electronic device that uses conventional harmonics-matching to generate a smooth tuning curve and that then uses entropy minimization to produce the all-important detail. He explains that the entropy technique is probably not suitable for use on its own because it yields local rather than global minima, potentially causing the system to get locked into higher or lower notes than it should. He also points out that the approach remains unproven, because he has so far tested it on just one piano and because he has not shown it to work using small subsets of notes, which is the approach taken by professional tuners.

Yuriy Ushakov of Nizhny Novgorod State University in Russia, part of a team that carried out research showing how neurons fire in response to harmonious sets of notes, believes that the latest work makes a “noticeable contribution” to our understanding of sound perception, as well as that of instrument tuning, by highlighting the role of entropy in that process.

Brian Foster of Hamburg and Oxford universities, a particle physicist who is also a keen violinist, says that although the latest research is interesting, he believes “there is nothing very profound in it”, pointing out that Hinrichsen’s entropy is not the thermodynamical quantity with which most physicists would be familiar. He also thinks that the research leaves a number of questions unanswered, including the significance of the fluctuations in the tuning curve, adding that it should be a trivial matter to test whether or not these are random. He believes the research could lead to a commercial tuning device but doubts that it would have a huge market. “Musicians, not being a very technologically aware bunch,” he says, “will pretty much stick to the human tuner, in my humble opinion.”

The research is described in a preprint on the arXiv server.

Have superluminal claims been put to rest?

OPERA physics co-ordinator Dario Autiero resigned on Friday. (Courtesy: CERN)

By Tushna Commissariat

Following my blog last Friday afternoon about the resignation of OPERA spokesperson Antonio Ereditato, it emerged later that evening that OPERA’s physics co-ordinator Dario Autiero, from the Institute of Nuclear Physics in Lyon, France, had also resigned. Nature reported that Autiero felt that tensions within the OPERA collaboration that had always existed were becoming impossible to resolve and that the media attention about superluminal neutrinos added fuel to the fire.

On Saturday, Ereditato broke his silence and wrote a long public statement about his resignation in a letter to the editor of Le Scienze, the Italian edition of Scientific American. In it, he says that words such as “‘errors’, ‘mistakes’ and ‘flop’ were bandied about regarding what in actual fact is standard scientific procedure in experimental work” and that “it is no accident that the word ‘error’ has a completely different meaning in scientific method than it does in common parlance”. He too points towards media attention, saying that “the message [of the results from the first press conference] was excessively sensationalized and portrayed with not always justified simplification” and that the “enormous media interest” put unexpected pressures on the entire collaboration. You can read his entire statement here.

Coincidently or not, the OPERA collaboration held a mini workshop on Friday evening at the Gran Sasso lab that was streamed live online. The “LNGS results on the neutrino velocity topic” workshop included a further analysis of the two errors that led to the superluminal results.

Slides and PDFs of some of the presentations are available online. One of the talks, entitled “Measurements and cross checks on OPERA timing equipments”, was given by G Sirri from INFN Bologna on behalf of the OPERA collaboration. His slides indicate that a connector for a fibre-optic cable that was incorrectly plugged in definitely contributed towards the error. But the cable error alone would have been much larger than the observed error, which perhaps would have led the researchers to find the result implausible. The other error that occurred – a problem with one of OPERA’s oscillators that led to a “time-stamp drift” – caused the neutrino time of flight that was recorded to be longer than the actual travel time. The unfortunate combination of these two “opposing” errors meant that the final result of the neutrinos travelling at a velocity 20 parts per million above the speed of light was an almost believable result. While there has been no official statement from OPERA regarding this just yet, it seems that the mystery of the superluminal neutrinos has been put to rest.

Browse articles by content type

Portrait of our planet – readers’ pictures