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Home » Page 1296

Philosophical about space–time

Conceptual illustration showing a clock suspended in space, surrounded by stars and nebulae
Ways of thinking: Tim Maudlin’s new book looks at space and time from a philosophical, as well as a technical, point of view. (Courtesy: Mike Agliolo/Science Photo Library)

Canonical works in the philosophy of physics tend to begin with the Greeks. This practice stems from the philosopher’s need to flag the intellectual tradition from which current physics and its philosophy are derived – albeit with plenty of ruptures, fractures and slippages through the centuries. Even with these bumps in the road, there appears to be a more distinct connection between Greek science, early modern science and the discipline we know today than there is between modern science and the scientific schools of, say, the ancient Indus Valley or Babylon.

The first source of this connection is a set of logical and mathematical formalisms. The second source consists of specific concepts – space, time and matter – that interest modern physicists as much as they did ancient philosophers. The offspring of these two sources is geometry, the flexible mathematical tool that allowed some of humanity’s earliest preoccupations to be translated into tangible scrawls and shapes, ready for thought experiments and demonstrations.

It makes sense, then, for the philosopher Tim Maudlin to begin his latest book, Philosophy of Physics: Space and Time, with geometry – and, more specifically, with that classical Greek-inspired version called Euclidean geometry, which describes the 3D space of our mundane world. Maudlin is also careful to note the physical heritage that accompanied geometry’s introduction, via a brief foray into Aristotle’s Physike Akroasis. There, Plato’s most famous student laid the foundation for thinking about the nature of an object’s motion – something Maudlin returns to later in the book with his discussion of Newton’s laws. By providing such background, he ensures that the reader knows the prehistory of many terms used throughout the book.

Although Maudlin mentions matter in his discussion of the Greek philosophers’ atomistic postulate, and then later, in his account of the philosophical debate between Newton’s supporter, Samuel Clarke, and his great rival, Gottfried Leibniz, he ultimately decided to reserve much of this weighty topic for the forthcoming second volume of the book. In the current volume, he concentrates instead on giving a comprehensive historical overview of the development of the concept of space and time, beginning with an expostulation of Newton’s three laws of motion and continuing as far as Einstein’s special and general theories of relativity.

One of the book’s underlying themes is the way certain aspects of physics have been subject to misconceptions over the years. For example, the book explains how the theory of relativity became a potent source of confusion for those physicists (including, at one point, Richard Feynman) who attempted to frame the theory in terms of the symmetry between the actions of two objects interacting with one another, and external forces that interact with the motion of the objects. The confusion, in Maudlin’s view, comes from a common misunderstanding, even among latter-day scientists, over the question of absoluteness in motion and space, and of the inertial reference frames pertaining to the coordinate system used.

Such confusion has a long history, and in the book, Maudlin describes an interesting early conflict between the claim that all motions are relative and the possibility that absolute motion could exist. This is Newton’s famous “bucket argument”, in which he described a thought experiment featuring a bucket filled with water and hung from the ceiling by a strongly twisted rope. When the rope is released, the water will slosh in the bucket – evidence, in Newton’s view, that a force is acting on the bucket in relation to absolute space. In another thought experiment, Newton imagined two globes rotating about their centre of gravity and connected by a cord. In his view, the tension in the cord is an effect of the absolute rotation of the spheres, since their position in relation to each other is constant. Hence, the globes and the bucket experiments were seen as confirmation of Newton’s theory on absolute space and motion. Newton’s beliefs have not persisted to this day as scientists have stopped believing in the idea of absolute motion.

Modern physics texts re-interpreted Newton’s early geometrical conception of his three laws through the rules of arithmetic, hence stripping them of the form familiar to their inventor. Since the theory of relativity takes Newtonian mechanics as its starting point, Maudlin argues that much confusion would be avoided by using geometrical spatial coordinates, rather than the more often used inertial frame of reference, to explain relativistic effects such as the famous “twin paradox”.

Throughout the book, Maudlin takes much pain to correct what he considers to be misconceptions and confusions about space and time. He may have adopted this strategy because his book is targeted at the non-physicist, and therefore at an audience with no specific investment in other explanations. However, even though the book is meant for the non-specialist, its subject matter is highly technical, and reading it requires constant switching between a philosophical and technical way of thinking. A non-expert reader would have to work patiently through some non-intuitive and potentially confusing points that are not so clearly set out by the author.

Although Maudlin concentrates on pre-quantum physics in the book, he connects some of the questions arising in relativity, such as observability and a discussion of the light-cone, as a way of visualizing events within temporal space to certain experimental conditions in particle physics. This seems like a smart move, given the recent media interest in accelerator physics and experiments at CERN, and the corresponding greater familiarity with these topics among a lay audience. He also touches on the problem of interatomic forces raised in Bell’s Theorem, which builds a discursive bridge between classical determinism and some types of quantum “hidden variables” known as non-locally observable properties. (Maudlin discusses this in more detail in his book Quantum Non-Locality and Relativity: Metaphysical Intimations of Modern Physics.) Such extensions allow the reader a peek at why physical ideas hundreds of years old are still relevant to today’s most cutting-edge problems.

  • 2012 Princeton University Press $29.95pb 200pp

Digital files stored and retrieved using DNA memory

Forget hard disks or DVDs. If you want to store vast amounts of information look instead to DNA, the molecule of which genes are made. Scientists in the UK have stored about a megabyte’s worth of text, images and speech into a speck of DNA and then retrieved that data back almost faultlessly. They say that a larger-scale version of the technology could provide an extremely dense and long-lived form of digital storage that is particularly well suited to data archiving.

As ever-greater quantities of electronic data are produced, the problem of how to store that data becomes more acute. There are many options for archiving data but all have their drawbacks. For example, hard disks used in data centres are expensive and need a constant source of electricity, and magnetic tape, while requiring no power, starts to degrade after a few years.

Neanderthal bones

In the latest research, Nick Goldman and colleagues at the European Bioinformatics Institute near Cambridge have stored digital information by encoding it in the four different bases that make up DNA. While the storage technique does not offer the convenience of random access or being rewriteable, it does have a couple of major advantages. One is its extremely high density – as a result of the information being stored at the molecular level – and the other is its durability. As Goldman points out, intact DNA has been extracted from Neanderthal bones tens of thousands of years old. “Nature has discovered that this molecule is very stable,” he says. “And we are piggy-backing on nature.”

The group used DNA that was produced in the lab rather than from inside living organisms, since the latter is vulnerable to mutation and hence data loss. But in choosing this approach the researchers had to overcome a couple of significant hurdles. One was the fact that using current technology it is only possible to make, or “synthesize”, DNA in short strings – and the shorter a string the lower is its information-carrying capacity. To get round this problem, Goldman and colleagues devised a coding scheme in which a fraction of each string is reserved for indexing purposes, specifying which file the string belongs to and at what point in the file it is located, so allowing a single file to be made up of many strings.

Encoding trits

The second challenge was how to avoid errors that occur during both writing and reading, a particular problem when neighbouring bases are of the same variety. The solution was simply to encode data in trits – digits with the values 0, 1 or 2 – and stipulate that a given trit is represented by one of the three bases not used to code the trit immediately preceding it. An additional measure was to copy the final 75% of each string into the start of the successive string.

The team tested the scheme by encoding five data files into single DNA sequences and then split those sequences up into roughly 150,000 individual strings, all 117 bases long. Fittingly, one of the files was a PDF of Watson and Crick’s famous double-helix paper – successfully encoded into double helices. The text of Shakespeare’s sonnets and an audio recording of 30 s of Martin Luther King’s “I have a dream” speech were also stored in MP3 format. The team then uploaded the encoded files to a private webpage to enable Agilent Technologies in California to synthesize the DNA. This involved using a sophisticated kind of inkjet printer to fire chemical reagents onto a microscope slide in such a way as to add one molecule at a time to a growing string of DNA, and then repeating the process to produce the thousands of strings required.

Sent as a tiny quantity of powder at room temperature and without specialized packaging, the DNA arrived in Heidelberg, Germany, at the main site of the European Molecular Biology Laboratory, of which the European Bioinformatics Institute is a part. After being put into solution the DNA was read, or “sequenced”, using a now fairly standard laboratory machine, and the resulting series of bases was then decoded on a computer to reproduce the five files. Four of the files were identical copies of the originals, while the fifth required some minor adjustment to recover its full set of data.

Video in a teacup

Goldman and colleagues claim to have achieved a density of 2 petabytes (1015 bytes) per gram of DNA which, they calculate, would allow at least 100 million hours of high-definition video to be stored in a teacup. Their DNA sample was therefore very small. “In our test tube the DNA looks like a speck of dust,” says Goldman. “In fact the sample is so small that when it arrived it looked like the test tube was empty.”

Currently the technology is too expensive to be competitive for all but the most long-term archiving. But Goldman is confident that prices will come down, given the continuing interest in DNA research. If the cost of synthesizing DNA falls by a factor of 100 over the next decade, which he says is possible, he says the technique will be as cheap as magnetic tapes for archives extending over at least 50 years. This is because unlike tapes, which need to be periodically rewritten, DNA remains unchanged as long as it is stored somewhere that is cold, dry and dark.

The current work follows similar research done last year by a team that included Sriram Kosuri of Harvard Medical School. His group used an encoding scheme that involved bits rather than trits and which included relatively little redundancy. However, he says that the two techniques are nevertheless “similar approaches to the same concept,” adding that both sets of research show DNA storage to be “approaching scales that should be of interest to investors”.

The latest research is published in Nature.

All together now

How do you tackle the world’s biggest problems such as making sure everyone has enough food, clean water, a secure energy supply and access to proper medicine and healthcare?

According to Shirley Ann Jackson, president of Rensselaer Polytechnic Institute in the US, the answer lies in a novel kind of research endeavour, which she dubs “the new polytechnic”.

Speaking at the 2013 ERA Foundation international lecture at the Royal Academy of Engineering in London last night, Jackson spelled out the principles of such an endeavour, which would essentially involve bringing researchers from different subjects, countries, cultures and sectors together to work on important multidisciplinary problems.

Exploiting computer technology, the Web and big data sets would be the key to tackling such challenges, she reckons.

Jackson, who trained as a particle theorist and is also a member of Barack Obama’s science advisory council discussed three interesting fields that, she thinks, could – indeed, already do – benefit from such an approach. They are using tissue-regeneration techniques to heal injured patients, incorporating solar panels and other forms of energy-saving devices into buildings, and exploiting the “data trails” we leave when we use social media.

I found Jackson a polished speaker, no doubt honed by her years in top positions in the US. Apart from being one of Obama’s science-policy wonks, she was boss of the US Nuclear Regulatory Commission between 1995 and 1999 and is a former president of the American Association for the Advancement of Science. Before that Jackson was part of the faculty at Rutgers University, spent 16 years at Bell Labs and had spells as a postdoc at Fermilab and CERN. Her impressive CV also includes a string of directorships at the likes of IBM and FedEx.

Jackson was less detailed on the nitty-gritty of setting up her “new polytechnic” or explaining who would fund such an enterprise. Last night was perhaps not the forum for those questions. But to me the elephant in the room was the whole concept of multidisciplinarity itself, which surely can only work if you have strong, vigorous disciplines in the first place. Jackson pointed to the likes of Cardinal Newman as advocates of the need for a broad education as long ago as the mid-19th century, but there are probably very valid reasons why so many of us prefer to hone our talents in a particular discipline and why the goal of multidisciplinarity can be so hard to put into practice.

Jackson hinted that leadership is the key in her new vision, highlighting Nelson Mandela as an example of the kind of bold, visionary thinker who is needed to get her blueprint off the ground. I found Mandela on odd choice given that he is not the first person you would associate with revolutions in higher education, but Jackson was right that managing, leading and encouraging multidisciplinary teams – particularly if they are spread over different continents and different time zones – is crucial.

Next stop for Jackson on her European tour is the World Economic Forum in Davos, Switzerland, this week. Whether she’s got time to pop in on her old haunt – CERN – I’m not sure.

Take photos for our 25 year anniversary

Camera phones at the ready! 2013 marks the 25th anniversary of Physics World and we want you to be a part of the celebrations this year. To kick things off we would like you to submit photos containing the text “PW25” to our Flickr group, a selection of which we will then publish later in the year.

To give you an idea of the sort of photos we are looking for, we created the scene above using copies of Physics World magazine. If you’re looking for inspiration, think about your working environment. You might consider using laser writing, empty drinks cans, lines in the sand, basically use anything you can find in your vicinity. Happy snapping!

Single molecules detected with plasmonic metamaterial

A new technique that could lead to chemical sensors that detect single molecules has been unveiled by researchers in the UK and France. It involves creating arrays of tiny nanostructures that interact strongly with light at a specific wavelength. When as few as one molecule of interest attaches to a nanostructure, the optical properties of the material change dramatically – signalling the presence of that chemical species.

The technique involves using “plasmonic metamaterials”, which are arrays of tiny metal structures that can be fine-tuned to interact with light in very specific ways. With these materials, it is possible to tweak the shape, size and arrangement of the structures to support collective oscillations of conduction electrons – called plasmons – at specific frequencies.

In this latest work, Sasha Grigorenko and colleagues at the University of Manchester and Aix-Marseille University have made metamaterials that are arrays of gold structures on a glass substrate. Each structure is about 90 nm tall and 100 nm across and separated by about 300 nm. The most basic array comprised a single gold pillar as the unit cell, but other arrays had double pillars or dumbbell shapes as unit cells – while others were indentations in sheets of gold.

Zero reflection

The arrays are designed to reflect no red light at a wavelength of about 710 nm and it is this property that changes if a tiny number of target molecules stick to the gold nanostructures. To make the array sensitive to a specific molecular species, it is coated with a tiny amount of a special chemical that will grab onto the molecules of interest. Each nanostructure in an array has about 2000 such binding sites.

The team used its device to detect the presence of streptavidin – a particular protein that will bind to the nanostructures. This involved firing 710 nm light at the surface of the array at an incidence angle of 53 degrees before measuring the intensity of the reflected light along with the relative phase of its S and P polarization – the phase being extremely sensitive to tiny changes in the reflective properties of the array. When the array was exposed to a dilute solution of streptavidin for just a few minutes, the team saw a jump in the amount of light reflected accompanied by a sharp phase shift.

Detecting one molecule per nanostructure

The team reckons that the devices have a detection sensitivity of about 1–4 streptavidin molecules per nanostructure. But the researchers say that by using advanced phase detection techniques, this could be improved to as little as 0.004 molecules per nanostructure. This is less than one molecule per square micron of the device and between two and three orders of magnitude better than plasmonic sensors based on changes in light intensity rather than phase.

One interesting aspect of the work is that the response of the arrays was calibrated by overlaying them with graphene – a sheet of carbon just one atom thick. Once the graphene is in place it is bombarded with hydrogen, which can then bind to nanostructures. The presence of hydrogen on the graphene can be measured using Raman spectroscopy, which provides an independent calibration of the number of molecules bound to the nanostructures.

Grigorenko told physicsworld.com that the team is now looking at how the arrays could be produced more cheaply. The first devices were made using the expensive process of electron lithography, while the team is now working on much cheaper self-assembly techniques. Grigorenko and his colleagues are also looking at employing better techniques for phase measurement with a long-term goal of creating prototype sensors that could be used as analytical tools for detecting trace amounts of chemicals.

The devices are described in Nature Materials.

Lasers could chill antihydrogen

A method for laser-cooling magnetically trapped antihydrogen atoms to temperatures of about 20 millikelvin has been proposed by a team of researchers from Canada and the US.

The team claims that cooling the antihydrogen would make it much more stable and so easier to study in experiments. In particular, it could lead to better spectroscopic analysis of antihydrogen, so that its properties can be compared with those of hydrogen.

Antihydrogen is an atomic bound state of a positron and antiproton that was first produced at CERN in 1995. Over the past few years, physicists working on the ALPHA experiment at the Geneva lab became the first to capture and store a significant amount of the stuff, holding a total of 309 antihydrogen atoms for 1000 seconds in 2011. In early 2012 the team then showed that it is possible to probe the internal structure of an antihydrogen atom by carrying out the first tentative measurements of the antihydrogen spectrum. By improving such measurements, researchers hope to determine what structural differences, if any, antimatter has compared with ordinary matter. This, they hope, could eventually explain why the universe currently contains much more matter than antimatter.

Trapping dimensions

Most experiments such as ALPHA create antimatter by injecting positron and antiproton plasmas into a magnetic trap, where the antimatter is then held for some time and studied. But the antihydrogen atoms have relatively high energies compared with the depth of the trap. This causes distortions in the spectral analysis of the sample because of effects such as line-broadening.

To ensure that these effects do not affect experiments, Francis Robicheaux of Auburn University in the US (also a member of the ALPHA collaboration) and colleagues have suggested using “Doppler cooling” to reduce the energy of the trapped antihydrogen. The method uses laser light that is at a slightly lower frequency than a transition in an atom to slow the atom down. “As the antihydrogen cools, it moves more slowly and it stays near the centre of the atom trap. If the atoms are moving more slowly, the frequency shift because of motion – the Doppler shift – is reduced,” says Robicheaux, as he explains the benefits of the proposed cooling process. “Atoms travelling towards the laser would ‘see’ the light shifted up slightly and are more likely to absorb it than if they are travelling away from the laser. Thus, the laser tends to oppose the motion, which leads to slowing the atom down.”

The researchers need to use laser light at a wavelength of 121 nm (as it is slightly below a transition in antihydrogen), but creating a source that is intense enough is a challenge in itself. “It is not trivial to make the necessary amount of laser light at a specific wavelength of 121 nm,” says Robicheaux.

Three dimensions in one

The nature of the trap also poses a challenge. Robicheaux explains that most Doppler-cooling experiments use laser light coming from six directions in 3D, so that any atom in any direction has a laser that opposes its motion. But the ALPHA experimental apparatus only allows for laser light from one direction (so the cooling would be only in 1D). “Our calculation was meant to show whether the ALPHA experiment would get 1D or 3D cooling. The neat result from our simulation is that the atom motion is complicated enough that directly cooling from one direction still leads to cooling in 3D,” says Robicheaux. According to the team, the cooling could be carried out using either a continuous-wave Lyman-α laser (generally considered the laser of choice for antimatter cooling) or a pulsed laser, “as long as the intensity is high enough and the laser line width is small enough”.

Through a series of computer simulations, the team showed that antihydrogen atoms could be cooled to around 20 millikelvin, making the effort worthwhile. Currently, trapped antihydrogen atoms have energies up to 500 millikelvin.

Over the years, some other experiments and research groups have suggested similar cooling methods for antimatter. But the new proposal is the only one that uses one laser and the time required to do the cooling would be quite short – a matter of a few minutes. The researchers also say that the cooling should have no negative effects on the trapping efficiency. “There are good reasons to believe that cooling will increase the lifetime of the antihydrogen in the trap, but we have not yet done detailed calculations to prove it,” explains Robicheaux.

Optical versus magnetic trapping

Yasunori Yamazaki of the RIKEN laboratory in Japan, who was not involved in the work, feels that the new proposal contains important results and implications. “It is demonstrated by the simulation that the pulsed laser can cool antihydrogen down to 20 millikelvin, which is about a 10th of the trapped-hydrogen temperature now. In other words, the bad influence from the non-uniform magnetic field that is unavoidable when magnetically trapping antihydrogen can be reduced by a factor of 10, and so the spectroscopic accuracy can be improved by a factor of 10, which is great,” says Yamazaki. But he cautions that optical trapping of antihydrogen is essential to carry out very accurate spectroscopic studies, and how the cooling applies to that remains to be seen.

The researchers are now working on estimating just how much laser power would be needed to successfully carry out their suggested experiment.

The research is published in Journal of Physics B: Atomic, Molecular and Optical Physics.

Nanowires make good solar cells

Researchers in Sweden and Germany say they have made an important breakthrough in the development of highly efficient solar cells based on nanowires. They have shown that cells made from tiny wires of the semiconductor indium phosphide (InP) have efficiencies as high as 13.8% while covering only about 12% of the surface of a device. While an efficiency of 13.8% is not as good as the best commercial silicon devices, the team believes that it could be improved significantly by further research.

Nanowires – tiny semiconductor wires with a thickness of just a few hundred nanometres or less – show great potential for making solar cells that are more flexible, lightweight and cheaper than conventional planar devices. Nanostructures such as wires are efficient absorbers of light and can act as “antennas”, harvesting much more light than a device with a planar surface. This is thanks to collective oscillations of charge carriers – called plasmons – that interact strongly with light. “One consequence of this strong absorption on nanowires is that we observe high light-absorption efficiencies even though only a small part of the device’s surface is covered by the nanomaterials,” explains team member Magnus Borgström of Lund University.

Millions of wires

The devices made by the team measured about a square millimetre and each contains about four million InP nanowires. The researchers grew their nanowires using an established technique called “vapour solid growth”. “Our nanowires needed to be uniform, having a certain diameter and length in a certain pitch. From our first working p–n InP junctions, it has taken us four years to reach this result,” Borgström explains.

The team, which includes scientists from Solid State Physics in Lund, Fraunhofer ISE in Freiburg, the University of Kassel and the start-up company Solvoltaics, also in Lund, chose InP because it has a direct band gap of 1.34 eV, which means that it can absorb light over a range of solar-spectrum wavelengths.

In this latest work, the team was able to identify the ideal diameter of the nanowires – which turned out to be about 180 nm. “The right size is essential for the nanowires to absorb as many photons as possible. If they are just a few tenths of a nanometre too small, their function is significantly impaired,” says Borgström.

Proof of principle

The cells made by the team have efficiencies as high as 13.8% – which is promising but still significantly less than the best commercial silicon devices, which operate at 15–22%. “Our findings are the first to show that it really is possible to use nanowires to manufacture solar cells,” says Borgström. However, the highest efficiency ever reported for a conventional InP solar cell is 22% and the team admits that it still remains to be seen whether that record can be broken using smaller amounts of nanowire material.

The researchers also believe that the way forward for nanowire-based solar cells is the multi-junction approach, which boosts efficiency by using several different structures tuned to different wavelengths of light from the Sun. “We believe that the road ahead for solar-cell application involves multi-junction technology with nanowires, for which the record is 44% in thin films,” says Borgström.

The work is reported in Science.

Physicists create SQUID-like Bose–Einstein condensate

Physicists in the US have developed an analogue of a superconducting quantum interference device (SQUID) that replaces the superconductor with a Bose–Einstein condensate and measures rotation rather than magnetic flux. They hope that the research will lead to the development of new, ultra-sensitive gyroscopes.

The SQUID is a well-established and extremely sensitive device for measuring magnetic fields that has found a range of commercial applications. At its heart is a loop of superconductor broken by one or two Josephson junctions. These are thin barriers of non-superconducting material that superconducting pairs of electrons are able to tunnel across. SQUIDs rely on the fact that superconducting electrons are all represented by the same wavefunction, which extends around the loop and includes the junctions. This means that the current that flows around the loop – and therefore the magnetic flux through the loop – is quantized at discrete values. If the magnetic flux in the loop increases or decreases, there is an oscillation in the voltage across the Josephson junctions every time the magnetic flux changes by one quanta. These quanta are very small and therefore an extremely small change in magnetic flux can be measured by counting the voltage oscillations.

A Bose–Einstein condensate (BEC) resembles a superconductor because it is a collection of ultracold atoms that is represented by the same wavefunction and flows as a superfluid without mechanical resistance. If a BEC is confined in a loop, the superfluid can flow around forever – at least in principle – and the angular momentum of the BEC is quantized much like current is quantized in a superconductor loop.

Torus of atoms

In this latest work Gretchen Campbell and colleagues at the University of Maryland and the National Institute of Standards and Technology used two infrared laser beams to confine a BEC of sodium atoms in a torus. They used a third green laser beam – which repels the condensate – to cut the ring at one point (see figure). This cut is analogous to a Josephson junction in a SQUID – the wavefunction remains continuous but the atoms must tunnel across.

The team then rotates the laser cut around the torus, which allows the researchers to “stir” the condensate. When the laser is moved slowly through the BEC, the atoms tunnel “backwards” through the cut so that the BEC’s angular-momentum state does not change. But once the laser goes above a certain speed, the tunnelling current is not sufficient to conserve angular momentum and the condensate undergoes a “phase slip”. This involves atoms tunnelling through the beam in the opposite direction, which causes a change in the angular-momentum state. Similar phase slips occur again as the laser beam is speeded up further and the BEC shifts between angular-momentum states.

Measuring rotation

“At the end of the day, the laser beam is playing the same role as the Josephson junction in a SQUID,” explains Campbell. “The only difference is that since rotation is analogous to having a magnetic field, we actually have to rotate the barrier.” As such, if the set-up itself is rotated, the effect is similar to exposing a SQUID to an external magnetic field and therefore the system could be used to make very sensitive measurements of rotation.

This is not the first time that a superfluid has been used to measure rotation. Quantum gyroscopes use liquid helium to make incredibly sensitive measurements of, for example, the rotation of the Earth. Campbell believes the new set-up has a significant advantage over superfluid-helium gyroscopes. “Since our junctions are just made with laser beams, we can tune those junctions,” she says. This could make a gyroscope more sensitive, she suggests, because altering the power of the green laser beam affects the critical angular velocity at which a phase shift occurs. The researchers are now studying in more detail precisely how the condensate behaves in the optical trap to allow them to tune it effectively.

Condensed-matter physicist Augusto Smerzi at the University of Florence in Italy agrees that the work could produce interesting applications, but he is more intrigued by the researchers’ observations about the breakdown of superfluidity when the condensate is stirred too quickly. There are several competing theories about how this happens, he explains, but researchers suspect that it involves the formation of vortices in the condensate’s structure called topological excitations. “This work gives one of the first demonstrations of the formation of these vortices when the superfluidity breaks down,” he says.

The research is described in Physical Review Letters.

Physicists seek cosmic domain walls

Exotic structures known as cosmic domain walls could be observed from Earth by measuring the subtle effect of their magnetic-like fields as they pass through our galaxy. That is the conclusion of a team of physicists in the US, Canada and Poland that has proposed a new way of probing the nature of the mysterious dark matter and dark energy thought to permeate the universe.

The current standard Big Bang model of cosmology assumes that much of the energy in the universe is contained within two mysterious substances – dark matter and dark energy. Dark matter explains anomalies in the motion of galaxies and is thought to account for about 20% of the universe’s energy. Dark energy is invoked to explain the universe’s accelerating expansion and is reckoned to make up about 75%. Most direct searches assume that dark matter consists of some kind of particle, while dark energy is often taken to exist in the form of a “cosmological constant” that is added to the field equation for general relativity. A number of other possibilities have been put forward, however.

Walls and axions

One is the idea that dark matter and dark energy are instead contained within objects known as “domain walls”. These structures would form within an exotic kind of force field mediated by as-yet-undiscovered sub-atomic particles known as axions, which were originally proposed back in the 1970s as a way of accounting for the fact that the universe appears to contain much more matter than it does antimatter. In the hot early universe the strength of the field would have varied randomly in space, but as the cosmos expanded and cooled, the field would have settled down to single values within extended regions. The boundaries between these different regions would be the domain walls, with the sudden jump in field across the walls endowing them with energy.

In the latest work, a collaboration headed by theorist Maxim Pospelov of the University of Victoria in British Columbia and experimentalist Dmitry Budker of the University of California, Berkeley, set out to establish whether or not such walls could be detected using instruments on Earth. The researchers’ idea is to use magnetometers, devices made up of atoms whose spins are initially lined up and can then be rotated by an external magnetic field. An axion-like field would be “scalar”, which means that, unlike a magnetic field, it does not have a preferred direction in space and so ordinarily would not affect the output of a magnetometer. However, a change in the field strength, as would occur at a domain wall, would affect the spins of the atom in the device.

To work out whether or not this effect could be measured, Pospelov and colleagues assumed that domain walls would store a considerable fraction of either the universe’s dark matter or dark energy. On this basis they worked out both the effective magnetic field generated and the time it would take for the Earth to pass through a wall, assuming it to be moving relative to the network of domain walls at a typical galactic speed of one thousandth of the speed of light. The researchers found that for quite a range of possible values of the axion mass and coupling between the associated field and ordinary matter, both the effective magnetic field strength and the interaction time would be within the sensitivities of modern magnetometers. They also established that such interactions would take place at least once every few years.

Relatively rare sightings

As the researchers point out, however, such relatively rare sightings would be difficult to identify amid continuing background noise from the magnetometer itself, its shielding and a host of external sources such as power lines, passing cars or even magnetic storms in the Earth’s atmosphere. The solution they propose is to create a network of at least five such devices. Four would establish the speed and direction of travel of a passing wall. These data would be used to calculate the wall’s time of interaction with the fifth device. If prediction and measurement match up then, says Budker, “you can be more confident that you have seen a domain wall”.

The researchers have been assessing the performance of two prototype devices, one located at Berkeley and the other at the Jagiellonian University in Krakow. They have shown that they can correlate the signals from the two machines and that they can reject a significant fraction of the noise. They now hope to obtain up to about $10m of new funding to build the full-scale network, with other devices potentially located at California State University in East Bay and elsewhere in the US, as well as in other countries.

Not seeing not a failure

Budker concedes that the idea of domain walls is “a little bit exotic” and outside the mainstream when it comes to searching for dark matter and dark energy. He also acknowledges that the theoretical uncertainties make it hard to know what the chances of detection might be. But he maintains that detection should not be taken as the only measure of success. “It is very important to realize in the search for exotic physics that not seeing something is not a failure,” he says. “If instead you rule out a whole class of possible models then that is a success.”

Joana Oliveira of the University of Porto in Portugal cautions that a network of domain walls could only contribute significantly to dark energy if it were “frustrated”. This would mean the walls being almost static relative to one another, their only movement being the stretching caused by the universe’s expansion. “The difficulty in achieving this configuration is similar to that which exists in preventing foam from dissolving in a glass of beer,” she says.

Pospelov acknowledges the difficulty in devising a model that could tie up significant amounts of dark energy in domain walls. He points out that simple models proposed previously contained too few walls to achieve this, and as such is looking to develop more elaborate models. “To become a legitimate theory, the model has to be consistent with what we know about the evolution of the universe,” he says.

The research is published in Physical Review Letters.

Are patents hampering the commercialization of graphene?

It has become almost a cliché to call graphene the “wonder material”, but this super-thin 2D honeycomb array of carbon atoms boasts some enviable electronic and mechanical properties. Apart from being the strongest material ever measured, graphene is also the stiffest and has an electrical current density a million times that of copper. Hardly surprising then that companies and institutes around the world have been stumbling over themselves to carry out research into this material, which was first isolated through Nobel-prize-winning work at the University of Manchester in the UK in 2004.

But a new report from the intellectual-property consultancy CambridgeIP suggests that the UK might be losing out in the quest to commercialize this material. By the end of last year, companies and institutes in China had apparently applied for or won a total of 2204 graphene-related patents – more than any other nation – ahead of the US, with 1754, and South Korea with 1160.

The most prolific firm in the patent-filing business is the South Korean electronics giant Samsung, with 407 patents and patent applications, followed by the US tech company IBM in second, with 134. The whole of the UK, in contrast, has filed and applied for just 54 graphene patents, with only 16 of those coming from the University of Manchester. UK science minister David Willetts complained that this was “the classic problem of Britain inventing something and other countries developing it”.

But do patents tell the whole story? After all, not all patents applied for actually get granted and many graphene patents may be merely speculative applications either made as a kind of insurance policy, or as shots across the bow to ward off rival businesses from entering the same territory. And even if a company or institute has a particular patent granted, the technology still has to be exploited – plus there is always the danger of having to defend one’s patent, often at great cost.

In the University of Manchester’s case, it is therefore focusing its patent efforts on areas that are likely to be “most useful”, such as scalable manufacturing techniques, coatings and composites, and is seeking only a few patents related to applications of graphene, such as graphene-polymer composites and fluorographene. Continued research is the key, the university claims, because by the time reliable methods for making graphene have been developed, today’s patents may have in any case expired.

But is the huge number of patents on graphene a positive sign that this material could soon find its way into real products that will revolutionize our lives? Or is the fact that big business is snapping up patents likely to hamper the commercialization of graphene?

Let us know what you think by taking part in this week’s Facebook poll

Are patents hampering the commercialization of graphene?

Yes
No

Please feel free to explain your answer by posting a comment on the poll

In last week’s poll we asked you if you felt that university professors have one of the least stressful jobs. The question was inspired by a ranking exercise on the website careerscast.com, which suggested that being an academic researcher is one of the cushiest jobs around. That conclusion had got quite a few scientists pretty steamed up, so we wanted to find out what you thought.

Physics World‘s Facebook followers proved to be fairly evenly split, with 47% of respondents agreeing that being a prof is an easy number and 53% saying no. One poll respondent – Leonardo Paulo Maia – felt our question was too broad-brush. For him, university professors who don’t actually do any research – and presumably are more involved in teaching and admin – definitely do have a relaxing time, even if they might be busy. He felt the really stressed-out people are the active researchers.

But another respondent – Lois Hoffer – was quite clear on her views. In a magnificent 250-word diatribe on the reality of a typical academic’s lot, Hoffer painted a picture of a life with far too much teaching, not enough money to hire a postdoc or student for research, no departmental administrators, complicated European grant applications, no office, plus poor students who jabber away during lectures, don’t know how to take exams and yet still have to be taught, marked and graded.

“Yes the money is good, and the job is for life,” Hoffer concluded. “But lack of stress?? You gotta be kidding.”

Thank you to everyone for taking part and we hope to hear from you again this week.

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