By Margaret Harris at the APS March Meeting in Baltimore
With so many sessions taking place at the APS March Meeting, finding time to write about them is almost impossible. However, now that I’m waiting for my flight from Baltimore back to the UK, I’ve got all the time in the world – so here’s my list of five conference highlights.
The best measurement yet of the photonic spin Hall effect (SHE) has been made by researchers in the US. The team amplified the normally extremely weak and difficult to detect photonic SHE by measuring polarized light incident on a specially designed metamaterial – a 2D sheet of gold nanoantennas. The researchers suggest that the observed polarization-dependent deflection of the photons could be useful in sensing and communication applications.
Photonic SHE is an optical phenomenon of quantum mechanics that is similar to the more familiar SHE observed for electrons. The spin possessed by an electron is a property related to magnetism – it is a quantized angular momentum that gives rise to a magnetic moment. The electron also has an orbital angular momentum relative to the fixed atoms in the solid, which also results in a magnetic moment. The interaction between the spin and orbital moments – the famous spin–orbit interaction – causes spins of opposite sign to be deflected in opposite directions so that they follow curved paths through a semiconductor. Put simply, the electron’s spin is deflected by an electric field and the SHE describes the curved path that spinning electrons follow through a semiconductor. The curved movement arises from the interaction between the physical motion of the electron and its spin.
Photons also possess spin and so exhibit a similar SHE. But the effect is extremely weak thanks to the fact that photons have a very small momentum compared with electrons. “The photonic SHE is very weak because the spin angular momentum of photons and spin–orbit interactions are basically negligible,” says Xiang Zhang of the Lawrence Berkeley National Laboratory’s Materials Sciences Division, who is one of the authors of the new research, published in Science. He points out that while other experiments have tried to detect the photonic SHE using techniques such as weak measurements, the effect observed has always been very small.
Zhang designs and studies metamaterials – artificial materials specially engineered to have properties not found in nature – and he wondered whether such a material could be used to enhance this weak optical effect. “We looked at specifically making a 2D metamaterial with optical resonators that are very small compared with the wavelength of the incident light and we engineered it to change the gradient of the light to greatly enhance a naturally weak effect to the point where it was directly observable with simple detection techniques,” explains Zhang.
In the new work, Zhang and colleagues’ metamaterial surfaces are about 30 nm thick and were constructed from V-shaped gold nanoantennas with a geometry that could be configured by adjusting the length and orientation of the arms of the Vs. “We chose eight different antenna configurations with optimized geometry parameters to generate a linear phase gradient along the plane,” says Xiaobo Yin, a member of Zhang’s research group and the lead author of the Science paper. “This enabled us to control the propagation of the light and introduce strong photon spin–orbit interactions through rapid changes in direction. The photonic SHE depends on the curvature of the light’s trajectory; so the sharper the change in propagation direction, the stronger the effect.”
Zhang told physicsworld.com that the researchers had to manoeuvre the light into a “dramatic turn” while incident on the metasurface so that a “giant” photonic SHE would occur. “It’s like riding a bicycle and being forced to make a very sharp and sudden turn,” he says. “It’s the key concept of our idea – to change the direction of the light dramatically, and the V-shaped nanoantennas do that,” he explains. Since the entire metasurface sample measured only 0.3 mm, a 50 mm lens was used to project the transmission of the light through the metamaterial onto a charge-coupled device (CCD) camera for imaging.
Yin adds that their metamaterial allows the researchers to control not only the incident light’s propagation, but also its circular polarization. “This could have profound consequences for information encoding and processing,” he says. He explains that information could be encoded and manipulated into the spin–orbit interaction as well as the polarization of light, “much like the 0 and 1 of today’s electronic devices”. Also, Yin claims that the ability to control left- and right-circular polarization of light at the metamaterial surface should allow the researchers to form optical elements, like highly coveted “flat lenses”, or to manage light polarization without using wave plates.
According to researcher Miles Padgett, of the University of Glasgow in the UK, who was not involved in the work, the new results have some other interesting effects. “The surface ‘Vs’ that the researchers make act as tiny resonators where the emitted light has a different phase to the illuminating light. Changing the angle of the V allows them to change the phase delay and this allows one to make a phase grating that is very thin…less than the wavelength,” he says. He further explains that the new research exploits a difference in behaviour for the two polarization states and so their “grating” imposes a polarization splitting. “What was also interesting to me was that the effect seems to be broadband rather than restricted to a monochromatic wavelength,” says Padgett.
“One of the most important things that our new work shows is that you can manipulate the spin–orbit interaction in photons too…spintronics may not just be for electrons now,” says Zhang.
The research is published in Science.
By James Dacey
Richard Feynman – undoubtedly one of the greatest scientists of the 20th century – died 25 years ago this year. To mark the passing of this physics and cultural icon, the BBC and the Open University have teamed up to produce two television programmes about Feynman’s life and work. The first programme aired in the UK on Monday, a docudrama called The Challenger portraying the role Feynman played in the investigation into the causes of the Challenger disaster. Readers in the UK can watch the programme here. Later this year, the BBC will broadcast a documentary about Feynman’s life.
I enjoyed Monday’s drama. I thought William Hurt did an excellent job of playing a smart and humane Richard Feynman, without over-cooking the “eccentric bongo-player” aspects of Feynman’s personality. Hurt certainly earned his wages, as the plot focused almost exclusively on how the Nobel laureate navigated his way through the alien world of high-level US politics, with all its game-playing and vested interests. My only criticism would be that because the film was so intensely focused on Feynman’s moves and responses, we didn’t really get to know any of the supporting characters.
Participants include John Martinis of the University of California, Santa Barbara; Raymond Laflamme of the University of Waterloo in Canada; John Preskill of the California Institute of Technology; and Charles Marcus – who was at Harvard when the recording was made but who is now at the Niels Bohr Institute in Denmark.
While most experts agree that practical quantum computers are some way off in the future, I also spoke to Geordie Rose, who is co-founder of Canada’s D-Wave Systems, which claims to have already built – and sold – quantum processors. While Rose says that the firm’s processors are currently being used to develop practical commercial applications, he also thinks that ultimately they may even have more artistic uses.
By Hamish Johnston
The string theorist Alexander Polyakov has won the 2013 Fundamental Physics Prize. The $3m prize is awarded by Milner Foundation, which is funded by the Russian entrepreneur Yuri Milner and was inaugurated last year.
An innovative new system at Massachusetts General Hospital (MGH) is helping to reduce the uncertainties in proton therapy. In this short film, researchers at the MGH explain how they can fire a beam of protons at a tumour then check whether their beam has hit the intended target. The team had to devise a way to quickly transfer a patient to a PET scanner after proton therapy, without leaving them feeling seasick.
If you enjoyed this video, then you may want to watch this film about the history of the MGH and its approach to cancer therapy.
After more than two years of painstaking analysis, cosmologists working on the €700m Planck space mission have announced their first results. Speaking today at the headquarters of the European Space Agency (ESA) in Paris, the researchers have released the most precise measurement of the cosmic microwave background (CMB) radiation – a remnant of the Big Bang – to date.
The results revise downwards the proportion of the universe made up by dark energy from 74% to 68.3%, while dark matter accounts for 26.8% of the total (up from 22%) and ordinary matter 4.9% (up from 4%). Planck also reveals that the universe is some 80 million years older than thought, to put the age of the universe at 13.8 billion years old. Planck scientists also say there is no evidence from the data of an additional fourth type of neutrino, which had been hinted at by NASA’s Wilkinson Microwave Anisotropy Probe (WMAP).
“The progress made in understanding the origin of the universe is an order of magnitude better compared with what has been done before,” says ESA director general Jean-Jacques Dordain. “This [the data] is what they call perfect; but as scientists got much more than they expected, so it is almost perfect.”
In July 2010 ESA released Planck’s first all-sky survey of the CMB showing tiny temperature fluctuations thought to have been produced by the same irregularities in space that led to the formation of galaxies. However, ESA researchers deliberately scrambled the survey image that was released to the public while they spent the next two years carrying out a full scientific analysis. In the new results, released today, cosmologists have used some 15 months’ worth of Planck data.
Launched by ESA in 2009, Planck uses two instruments to measure the CMB at frequencies between 27 GHz and 1 THz. It takes these measurements at a point in space that is some 1.5 million km further out from the Sun than the Earth. Known as Lagrange point L2, Planck hovers there, barely disturbed by stray signals from Earth and without needing to use much fuel to stay in position.
Cosmologists believe that the nascent universe underwent a period of extremely rapid growth – a period that began 10–35 s after the Big Bang – during which the universe is thought to have undergone enormous expansion called inflation.
The CMB was born about 380,000 years after the Big Bang, when primordial protons, neutrons and electrons formed neutral atoms that allowed photons to “decouple” and finally move freely. Photons could then suddenly travel unhindered through space, their wavelengths being stretched by the expansion of the universe to leave a haze of microwave radiation in every direction.
As with WMAP, the previous CMB space-based mission, Planck has found almost perfect agreement with inflationary models and the standard model of cosmology. Known as “lambda-CMD” (lambda cold dark matter), this model describes a flat, homogenous universe dominated by dark matter and dark energy. “There is little doubt that we have now uncovered a fundamental truth of the universe,” says George Efstathiou of the University of Cambridge, speaking at the ESA press conference.
David Spergel, a theoretical astrophysicist from Princeton University who has worked on the WMAP data, told physicsworld.com that the Planck results are “a great triumph” for experiment and theory. “With even higher precision than WMAP, the [Planck] data fits the standard model,” he says.
There are, however, some hints of physics beyond the standard model of cosmology in the new Planck data. Efstathiou showed that fluctuations in the CMB temperatures at large angular scales do not match those predicted by the standard model, in addition to an asymmetry in the average temperatures on opposite hemispheres of the sky. Such deviations were hinted at by WMAP but were largely ignored because of doubts over their origin.
“Such features are not caused by galactic emission or instrumentation,” says Efstathiou. “This is exotic physics – there seems to be some memory that has been retained on the largest scales from previous phases of the universe.” One possible explanation for this is that the universe is not the same in all directions on a larger scale than we can observe.
Cosmologist Joanna Dunkley from the University of Oxford says that the large angular scale anomalies are “tantalising” and could point to new physics. “It needs some more thought about what kind of theoretical models could produce this sort of signal,” she says.
Another big aim of the Planck mission is to detect a so-far-unobserved type of polarization known as “B-modes”, which date back to the period of inflation and are determined by the density of primordial gravitational waves. If such waves could be detected, they might tell us what mechanism generated them in the universe’s first moments, what caused inflation, and even if there was something before the Big Bang. However, Efstathiou says that the Planck team has not yet exploited those data.
The Planck material released today represents only half the results expected to come from Planck over its lifetime.
The CMB was first discovered in 1964 by the US radio astronomers Arno Penzias and Robert Wilson, earning the pair the 1978 Nobel Prize for Physics. However, it was NASA’s Cosmic Background Explorer (COBE) that set the field of cosmology alight in 1992, when it revealed that the CMB is not uniform but has slight variations that carry information about the early universe.
The launch of WMAP in 2001 and its study of the CMB proved to be huge vindication for the standard model of cosmology. A few years after launch, WMAP returned the first all-sky survey of the CMB and revealed the temperature of this background radiation in exquisite detail. In 2006, after three years of data-taking, the WMAP team measured the incredibly weak polarization signal of the photons, allowing cosmologists to infer how much the fluctuations are cuased by the distorting effects of matter and how much they are down to gravity waves in the infant universe. WMAP placed strong constraints on models of inflation, showing that the first stars formed when the universe was 400 million years old.
Some 30 papers based on Planck’s findings will be released on arXiv tomorrow.
Doctors at Massachusetts General Hospital (MGH) are pioneering an exciting new approach to proton therapy. This short film introduces the pencil-beam scanner, a way of firing protons at tumours with impressive precision. The pencil beam allows more precise shaping of the beam’s range, as doctors can adjust the beam throughout the duration of the therapy as if they were painting the tumour.
If you enjoyed this video, then you may want to watch this film about the history of the MGH and its approach to cancer therapy.
The “maverick genius” referred to in the title of Phillip Schewe’s book is Freeman Dyson: a truly great mathematical physicist, bestselling author, longest-serving member of the US military’s JASON advisory group, and occupant of the “fourth chair” when the Nobel Prize for Physics was awarded for quantum electrodynamics (QED) – among many other distinctions. Indeed, a biography of Dyson was long overdue, even though his own autobiographical writings are extensive and so beautifully written that no ordinary author could match them, Schewe included.
Why, in that case, should we bother with this biography? Because, as the author makes clear, there are many Freeman Dysons, and how they developed (evolved?) into each other, and what their relationship is, are both relevant parts of his story – as is some kind of appraisal of what one is to make of the final individual.
My own contacts with Dyson have been indirect. Of course, I tried to understand the fundamental QED papers of 1949 that revised all our views of quantum field theory, and I used the techniques presented in them to help solve a puzzle in solid-state physics. Then, in 1958 I was chosen as a substitute for Dyson after he was enticed away from the University of California, Berkeley – where he had spent three summers researching condensed-matter problems with Charles Kittel – to work at General Atomics in La Jolla, California. There, for much more money than Kittel could command, Dyson helped design the safe reactor TRIGA and the Orion spaceship. (I had a marvellous summer at Berkeley, though my papers were crude compared with Dyson’s.) But we did not meet until the first energy crisis, when we both attended a workshop on energy that was sponsored by the American Physical Society. Afterwards, we met at disarmament seminars at Princeton University in New Jersey, which is where I first sensed his ambiguity about conventional liberal positions on subjects such as the “Star Wars” defence initiative – most of which I hold unambiguously.
This is not an authorized biography, so Schewe did not have access to any private letters in his research. However, he is a well-known popularizer of physics (being employed in that capacity by the American Institute of Physics) and he has done a meticulous job of finding all of the relevant sources available. He has researched the course of Dyson’s life in detail, beginning with his privileged and precocious childhood at Winchester and foreshortened Cambridge years, which were overshadowed by the approach of the Second World War. Dyson spent the war years doing operations research for Bomber Command, and his determinedly itinerant graduate years with Hans Bethe and Richard Feynman culminated in the great breakthrough of QED. After his relatively brief, but scientifically fertile, junior faculty years at Birmingham and Cornell, he settled permanently at the Institute for Advanced Study (IAS) at Princeton in 1953, at the age of 30.
“Settled”, however, is hardly the word for it: the liberal vacations and relaxed leave policies of the IAS have enabled Dyson to become the epitome of the “have briefcase, will travel” scientist, bringing him several further careers. The one that seemed to leave the strongest impression on him was his involvement with the nuclear world and particularly the Orion project, which foreshadowed major themes of his later career. Orion was a nuclear-powered spaceship that he, Edward Teller and Ted Taylor designed in 1959 and advocated thereafter, and this experience seems to have left him with a visionary predilection for thinking the unthinkable in terms of the long-term future of the human (or other intelligent) race in space. He also became a major influence in the effort to achieve some measure of nuclear disarmament; after initially opposing the test ban treaty, as a JASON consultant he co-wrote an influential report opposing the employment of nuclear weapons in Vietnam.
Until the 1980s Dyson kept up a continuous and active career in mathematical physics, with occasional forays into broader interests such as condensed matter, biology (particularly studies on the origins of life) and astronomy. Around that time, he discovered his second métier as a writer of extraordinarily readable prose. A number of well-received essays were followed by his first autobiographical book, Disturbing the Universe (1979), which was nominated for the US National Book Award. Then came Weapons and Hope (1984), which captured the public’s interest in the Reagan administration’s Strategic Defense Initiative (the aforementioned “Star Wars”). He continues to publish a book every few years as well as many articles and book reviews. Partly thanks to his prolific writing, but also because he seems to have something inspiring and beautifully phrased to say for any occasion, he has become a popular lecturer and maintains a frighteningly full travel schedule. Most recently he has delighted in maintaining minority views on a number of topics such as climate, religion (his Christianity places him in the minority for his profession) and genetic modification.
Did he ever have time for a private life? Schewe’s book records Dyson’s claim (perhaps a dubious one) to have had two principles in his relations with women: he did not allow himself to become interested if he didn’t have marriage in mind; and he intended to have six children. He proposed to his first wife, Verena – a bright, glamorous mathematician and single mother at the IAS – almost on meeting her, and wooed her by mail for over a year throughout his continual travels until (with some reluctance on her part) they married in 1950. She bore him two children before a miscarriage, but theirs was a somewhat stormy marriage, noteworthy for the fact that her thesis and mathematical notes were deliberately burned in the interest of domesticity. Both children, Esther and George, became well-known figures, she as a journalist-entrepreneur and he as an author. Dyson’s second wife Imme, formerly his children’s au pair, produced four more daughters. Friends of his children report that Dyson is a kindly, avuncular figure, though a rather strict father.
A more important question, though, is whether Dyson is the important world figure that Schewe makes him out to be. In his career, we can see traces of the mathematical physicist’s reluctance to tackle the ambiguous or deeply puzzling question, or to go out mathematically even a little bit on a limb – something that contrasts sharply with his joyful interest in bizarre futurology. Perhaps this is the source of Dyson’s dreadful misjudgment on the climate question: he sees that the possible errors are large, but does not factor in that they are likely to be large in the wrong direction, and does not credit obvious qualitative arguments from simple laws of physics.
One could wish, as in many biographies of scientists, that the scientific contributions were more critically presented and contextualized. Sometimes the hype goes too far, as when Schewe compares Dyson’s popularity as the guru of QED in the 1950s with the Beatles’ “conquest of America” in the 1960s. Dyson’s very elegant arguments do not always have much to say about how things work in the real world, and the author makes little effort to distinguish whether they do. He did not, for one thing, participate in any of the revolutionary events that created the Standard Model. However, my own preference is for the sloppy and practical rather than elegant and precise, so I am prejudiced.
It is natural for biographers to fall a little in love with their subjects, but on balance, this book leaves the reader intrigued but a bit unsatisfied. Dyson is a superbly able man and has done so much, but what if he had focused on one career? Perhaps the career he really wanted was scotched, as Schewe suggests, by the fallout problems of Orion? In any case, he is worth reading about and marvelling at.
The gradual sliding of tectonic plates across the Earth’s surface may be lubricated by a layer of partial melting, according to researchers from the US. Their study involved conducting magnetotelluric imaging across the Middle America Trench, off the shore of Nicaragua – where the Cocos plate is subducting under the Caribbean plate – and it revealed a far-reaching, high-conductivity layer at a 45–70 km depth in the upper mantle.
“Scientists have known for some time that Earth’s tectonic plates are able to slide across the mantle because they are underlain by relatively low viscosity material in the asthenosphere, but the processes that decrease this viscosity are debated,” explains lead author Samer Naif, a PhD student at the Scripps Institution of Oceanography in San Diego. “Our observations show that the asthenosphere beneath the Cocos plate contains a partially molten channel. Since small amounts of melt in the asthenosphere will significantly lower its viscosity, we infer that this layer could be facilitating the motion of the Cocos plate over the mantle, much like a lubricant.”
The structures and forces that enable tectonic plates to move across the mantle have long been debated. Previous studies have suggested that the low viscosities found in the asthenosphere – the ductile part of the Earth just below the lithosphere, within the upper mantle – that are needed for such movement might be caused by the presence of small quantities of dissolved water. The partially molten channel the team has discovered, however, supports an alternative solution – acting to decouple the motion of the tectonic plate above from the convecting mantle below.
“Our data tell us that water can’t accommodate the features we are seeing,” Naif told physicsworld.com. “The information from the new images confirms the idea that there needs to be some amount of melt in the upper mantle – and that’s really what is creating this ductile behaviour for plates to slide.”
Natural low-frequency electromagnetic-field variations at the Earth’s surface induce secondary electromagnetic fields in the conducting Earth. Taken on the sea floor, magnetotelluric imaging uses measurements of the strengths of these induced fields, as a function of frequency, to generate images of electrical conductivity within the crust and mantle. Naif explains that this technique of exploring the inaccessible mantle is particularly sensitive to the presence of conductive materials – such as the ionic fluids in molten rock. The newly found layer – which extended beyond the sampling region in the direction of the Cocos plate interior – stopped short of the subduction zone. The team believes that here the melt’s buoyancy may keep it from sinking with the descending plate edge – instead pooling under the plate.
The melt also has a notable characteristic – being 1.5 to 2 times more conductive in the direction of plate motion, compared with parallel to the trench axis. “Shearing of this melt-rich layer at the base of the plate could explain both the anisotropy of conductivity and why oceanic plates seem to move over the underlying mantle with little resistance,” comments Donald Forsyth, a marine geophysicist at Brown University in the US, who was not a part of the team. “A sheared melt layer could lower the viscosity of the mantle in the direction of plate motion, so that there would be little drag at the base of the plate.”
The team had not set out to uncover this feature of the upper mantle. Indeed, the researchers originally intended to study the fluid cycle around the Middle America Trench, then extended their planned array of sea-floor magnetotelluric stations towards the Cocos plate interior with the idea of defining a baseline crust/mantle structure for comparison with the fluid structures at the trench. In total, 50 measuring stations were placed along a 280 km profile of the sea floor. “We went out looking to get an idea of how fluids are interacting with plate subduction [and] we discovered a melt layer we weren’t expecting to find at all – it was pretty surprising,” says another team member, Kerry Key, also from the Scripps Institution of Oceanography.
Two possibilities are being proposed to explain the origin of this layer of partial melt. In one scenario, the melt occurs and is retained beneath the crust as a result of the mantle upwelling and decompression melting that forms the newly emerging plate material. Another theory proposes that the melt is generated in small amounts throughout the upper mantle and – as a result of buoyancy – rises up to the asthenosphere to pool out in such a layer underneath the cold lithospheric plate above.
“This study shows rather convincingly a strong electrical resistivity contrast between [the] lithosphere above and partial melt beneath,” comments Gregory Houseman from the University of Leeds in the UK, who was not involved in the work. “The authors’ explanation of the low resistivity band makes sense for oceanic lithosphere that is only about 20 million years old – although it implies either a hotter mantle or more water present than is usually quoted in order to get melting at 45 km.” He adds that it would be important to get seismic data for the boundary and check for consistency with the electrical measurements.
In search of more information on the formation of the melt layer, the researchers are now expanding their area of investigation. “We would like to extend the survey to younger regions of the Cocos plate in order to determine if the melt layer exists there as well,” says Naif. “This may help us to determine its origin and also to understand how prevalent this layer is beneath the plate.”
The work is published in Nature.
