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Home » Page 1147

Between the lines

Stump a physicist

Think back to the last time you took a physics exam. Was your heart racing? Did you gulp when you realized that you had no idea – really no idea at all – how to solve the first problem? And then, just as the fear threatened to engulf you, did you hear a tiny, confident voice in the back of your head saying, “Hold on a minute, what if I do this…”? If any of these experiences sound familiar to you – and if they inspire a certain wry fondness rather than a panic attack – then you need to add Physics on Your Feet to your library. Subtitled “Ninety Minutes of Shame but a PhD for the Rest of Your Life,” the book is a collection of the best questions posed to physics students at the University of California, Berkeley, during the oral exams that were, until 2010, part of the standard postgraduate training programme. The curators of this collection, Dmitry Budker and Alexander Sushkov, have been on both sides of the examiner/examinee divide (both of them earned their PhDs at Berkeley, and Budker is now a physics professor there), and the questions that appear in their book are both fiendish and fascinating. A few can be solved using only high-school-level physics, carefully applied. But be warned: some problems that seem simple actually contain hidden depths, and a few of them might even lead to heated debates in departmental tea rooms and company cafes as senior physicists argue over the answers. (Try the one about tides during a solar eclipse on an unsuspecting friend.) Budker and Sushkov have supplied their own solutions to each of the problems, but they do not claim that these are the only answers; some of the questions are, in any case, open-ended enough to permit several possible approaches. Readers who work their way through all of these questions won’t get a PhD for doing so, but they will get a much better appreciation for the richness of physics as a discipline.

  • 2015 Oxford University Press £19.99pb 224pp

Going deep underground

The inside of the Earth is intimately connected to the universe outside it. Many of the treasures hidden there originated in distant stars and galaxies, and theories of our solar system’s very early years, when the Earth and its neighbours coalesced, can also tell us much about our planet’s present composition. This deep connection helps explain why an astronomer, David Whitehouse, has chosen to base his latest book on the Earth’s interior. Journey to the Centre of the Earth takes its title from Jules Verne’s novel, which celebrates its 150th anniversary this year, and, like Verne, Whitehouse has a magpie’s eye for interesting facts and stories. Perhaps the oddest of these little nuggets is the revelation that the brand name Bovril comes from Vril, an early science-fiction novel about a subterranean race of super-humans. None of these tales of the deep Earth is told in very much detail, though, which is sometimes a pity. Humankind’s various attempts to drill through the Earth’s crust and into the mantle, for example, occupy a mere eight pages in Journey, yet it is clear from Whitehouse’s description that an entire book could be written about the Kola Superdeep project alone. This effort lasted for 25 years and produced a 12 km-deep borehole before technical and monetary difficulties ended operations, and Whitehouse calls it “one of the great scientific projects of the 20th century”. Perhaps someone will write a book about it one day, but in the meantime, there is plenty to enjoy in this one. As Whitehouse puts it, “if you want strangeness and surprises, look below”.

  • 2015 Weidenfeld & Nicholson £20.00hb 288pp

The June 2015 issue of Physics World is now out

For nearly three decades, physicists have been unable to answer a seemingly simple question: where does proton spin come from? Adding up the spins of the three quarks that make up the proton seems, in principle, straightforward, but physicists have been struggling with a strange problem: the sum of the spins of its three quarks is much less than the spin of the proton itself.

Physics World June 2015 cover

Known as the “spin crisis”, the topic appears as the cover story of the June 2015 issue of Physics World, which is out now in print and digital formats. In the feature article, science writer Edwin Cartlidge examines the origins of the problem – and whether new experiments could mean we are about to solve it at last.

If you’re a member of the Institute of Physics (IOP), you can get immediate access to the feature with the digital edition of the magazine on your desktop via MyIOP.org or on any iOS or Android smartphone or tablet via the Physics World app, available from the App Store and Google Play. If you’re not yet in the IOP, you can join as an IOPimember for just £15, €20 or $25 a year to get full digital access to Physics World.

The issue also includes a great Lateral Thoughts article by Felix Flicker that’ll have you twisting and bending your arms as you try to follow what he’s on about.

(more…)

Big bucks from The Big Bang Theory, the good, bad and ugly of physics writing and more

Jim Parsons and Mayim Bialik on set
Jim Parsons and UCLA alumna Mayim Bialik from The Big Bang Theory TV show. (Courtesy: Warner Bros Entertainment Inc.)

It’s not often that one can say that watching TV may help your future career as a scientist, but today, after the hit US TV show The Big Bang Theory announced a scholarship for STEM students at the University of California, Los Angeles (UCLA), it may be possible. The show, revolves around a group of young scientists – mainly physicists, but also an engineer, a microbiologist and a neuroscientist – making it a science-heavy show. Indeed, we at Physics World have delved into the secrets of the show’s success and talked to one of its scientific advisers. Now, the sitcom’s co-creator, cast and crew have announced a scholarship fund at UCLA to provide financial aid to undergraduate students pursuing degrees in science, technology, engineering and mathematics. The show’s executive producer, Chuck Lorrie, told the Deadline website that “when we first discussed it, we realized that when Big Bang started, this freshman class were 10 year olds”, adding that  “some of them grew up watching the show, and maybe the show had influence on some of them choosing to pursue science as a lifetime goal. Wouldn’t it be great if we can help.” For this academic year, 20 “Big Bang Theory scholars” will be picked to receive financial assistance, with five new scholars each year from now. You can read more about it on the BBC website.

(more…)

Infrared detector to free up Internet of tomorrow

A new kind of silicon photodetector invented by physicists in Canada and the UK should help to ensure that the Internet does not grind to a halt, as people share and download ever-more online data. The device, which could be built using existing chip-fabrication techniques, would ease the pressure on the Web’s data centres by opening up a new frequency range in optical communications.

Ever-greater use of social media, video streaming and other data-heavy applications is causing a rapid rise in Internet use. US technology company Cisco Systems says that worldwide Internet traffic increased more than five-fold between 2008 and 2013, and is set to rise by another factor of three by 2018. This apparently insatiable appetite for online services puts pressure on the hardware that makes up the Internet, and in particular on the huge data centres that route bits and bytes between computers across the globe.

Rising demand

Until now, the companies operating such data centres have tended to use electrical wiring to link the thousands of servers that they employ, but many are now replacing that wiring with fibre-optic cables – these have a number of advantages over the copper variety, including lower losses over long distances, the fact that they do not pick up electromagnetic interference and, crucially, their far greater bandwidth. However, says Andrew Knights of McMaster University in Ontario, even current fibre technology might be swamped by rising user demand within a decade or two.

In the latest work, Knights, together with Jason Ackert and colleagues at McMaster and also the University of Southampton in the UK, have built and tested a photodetector – a device that converts the pulses of light sent down a fibre-optic cable into electrical signals that serve as input to computer processors – that is designed to meet that demand by increasing the range of wavelengths that can be used to send data. Most current cables work at bands centred on 1.3 or 1.5 μm, and transmit data over about 100 different channels within each band – with each channel corresponding to a slightly different wavelength. The new device would, the researchers say, help to open up at least another 100 channels by operating at longer wavelengths, exploiting a band lying in the mid-infrared region at around 2 μm.

Photodetectors operating at mid-infrared wavelengths have already been built from materials other than silicon, notes Knights. But these rival devices, he says, suffer from drawbacks. Germanium–tin detectors, for example, operate too slowly, and produce quite small electrical currents, while detectors based on III–V semiconductors are difficult to integrate into silicon circuitry.

Ion defects

The new device consists of a 3 μm-wide strip of silicon that has a thickness of 220 nm at its centre and just 90 nm at its edges, deposited on a substrate of silicon dioxide. Ordinarily, such a device would serve as a waveguide that is transparent to mid-infrared radiation, but the Anglo-Canadian group inserted ions into the silicon to introduce defects into the lattice structure. These defects result in electronic states within the silicon’s band gap, which allow low-energy mid-infrared photons to excite electron–hole pairs and so generate a measurable photocurrent.

Knights and co-workers have shown that their device generates a reliable electrical response at wavelengths between 1.96 and 2.5 μm, albeit with a lower sensitivity than similar silicon detectors operating at 1.55 μm. They also exposed the device to the output of a 1.96 μm laser diode, showing that at this wavelength it could operate without errors at speeds of greater than 20 gigabits per second – more than any other photodetector at this wavelength.

Easy fabrication

The researchers have also shown that their design could be realized using standard CMOS fabrication, having had their prototype device manufactured at a commercial foundry in Singapore. This is crucial, they say, because it means being able to take advantage of the existing vast infrastructure used to fabricate microchips, so lowering costs significantly. Indeed, waveguides, multiplexers and most of the other components needed for optical-fibre communication in this new frequency band have already been built from silicon by other research groups. “We have shown that our device is fast, that it operates at the right wavelength, and that it is compatible with silicon fabrication,” says Knights. “If you put these things together, that is a major breakthrough.”

Knights points out that there is unlikely to be a mass market for this device until Internet traffic has grown accordingly. But he is confident that the market will materialize. “We can place this in the toolbox and it will be ready when needed,” he says.

The work is published in Nature Photonics.

Preparing for DEMO

Laying the groundwork: engineers are rebuilding the Mega Amp Spherical Tokamak. (Courtesy: Michael Banks)

Yesterday I took the train from Bristol and headed to the Culham Centre for Fusion Energy (CCFE) in Oxfordshire.

Owned and operated by the United Kingdom Atomic Energy Authority, the CCFE is already home to the Joint European Torus (JET) tokamak, which in 2011 underwent a £60m upgrade programme that involved replacing the carbon tiles in the inner reactor wall with beryllium and tungsten. The purpose of this retrofit was to test the materials that are to be used in the ITER fusion experiment, which is currently being built in Cadarache, France.

Culham is also home to the Mega Amp Spherical Tokamak (MAST). MAST has a spherical plasma, shaped much like a cored-out apple, whereas JET (and ITER) has a doughnut-shaped plasma. A spherical tokamak allows for a much more compact – and cheaper – device and it is hoped that this kind of tokamak could one day be used as a potential fusion reactor.

MAST is currently half-way through a £45m upgrade of its own that will be complete sometime next year. The upgrade is a major overhaul of the facility that will see the tokamak given a new “divertor”, which extracts the waste fuel from fusion. Called “Super-X”, it is hoped that the new divertor could even be used in a future demonstration fusion plant – dubbed DEMO.

Yesterday I was given a tour of the MAST construction site, which involved donning the customary hard hat and overalls. The level of the overhaul is impressive, and it becomes immediately clear that this is not just a minor upgrade, but almost like building a new machine.

Before the MAST tour I spoke to Steve Cowley, director of the CCFE, who says that an actual fusion reactor delivering energy to the grid could even be in some way a combination of a standard fusion tokamak like ITER and a spherical tokamak. This, according to Cowley, makes the experiments that will be run with the MAST upgrade important. “This is a world-class experiment,” notes Cowley, “and one of the biggest physical-sciences facilities in the UK.”

Be sure to keep an eye out for a piece about the MAST upgrade in an upcoming issue of Physics World.

Go-ahead for protest-hit Thirty Meter Telescope, but with fewer future sites on Mauna Kea

Construction of the Thirty Meter Telescope (TMT), which has been delayed for months following protests by native Hawaiians, has taken a key step towards restarting. At a press conference on 26 May, governor of Hawaii David Ige noted that the TMT – to be built on the country’s highest peak, Mauna Kea – has the right to proceed, and that all of the necessary permits for the observatory have been obtained. Yet, he criticized how the University of Hawaii has managed the land on Mauna Kea, outlining 10 improvements – some recommended and some required – for how the university uses the mountain.

Since 1968 the University of Hawaii has leased more than 44.5 km2 of land on Mauna Kea from the Hawaiian Department of Land and Natural Resources (DLNR) for scientific purposes, with the highest 2.1 km2 devoted to astronomy research. The top of Mauna Kea is already home to 13 telescopes, and the TMT will be the largest and most powerful instrument when it is operational in 2023. The telescope’s 30 m primary mirror will be made of 492 hexagonal segments, and a structure 66 m wide and 56 m tall will house the telescope. The TMT will sit on a plateau about 500 feet below the summit, a location picked to reduce the telescope’s visibility from the majority of the island.

Fewer telescopes

Construction of the TMT had been halted in early April following protests by native Hawaiians, who see its construction on Mauna Kea as desecration of their spiritual and cultural pinnacle. Over the past eight weeks, Ige has mostly stayed quiet regarding the protests, but now, along with giving permission for construction to restart, he requests that the university returns all of the land not used for astronomy to the jurisdiction of the DLNR. He also says that the University of Hawaii should begin decommissioning one telescope later this year with at least one-quarter of the remainder to be completely dismantled by the time the TMT is operational, with each site to be returned to its natural state.

One of those affected could be the Caltech Submillimeter Observatory. It was already slated to be dismantled, starting in 2016, but that may now be brought forward. Yet according to cosmologist Asantha Cooray of the University of California, Irvine, decommissioning a telescope is far from simple, and takes at least a couple of years. He says that the University of Hawaii – rather than the astronomical community – will likely decide which other three telescopes will be removed.

Cultural contribution

Ige also announced that the state government would work to change how the mountain is managed, and that it will form the Mauna Kea Cultural Council. This group will review the subleases to observatories, all proposed rules, and any preparatory work regarding the environmental impact of telescopes on the mountain. Thayne Currie, an astronomer with the Subaru Telescope on Mauna Kea who has also worked at other observatories on the site, says that this new council is a step in the right direction, provided that it “is tasked with not just simply receiving messages from the native Hawaiian community, but really considering their mission to try as much as they can to make sure their concerns are reflected in action”.

While the new rules will hit facilities on Mauna Kea, Ige, however, still believes that both science and culture should co-exist on the mountain. “Science has received most of the attention and has gotten way ahead of culture in our work on the mountain,” he says, adding that “the proper balance” between the two had been lost. In a statement, TMT members noted that “We are grateful to governor Ige for his leadership and his statement of support for TMT’s right to proceed. We will work with the framework he has put forth.”

Latin America’s scientific ‘magic’

Latin America is a diverse continent, interconnected by its common Iberian heritage. Its pre-Columbian societies were quite sophisticated, as can be seen in the Teotihuacan pyramids (see “The pyramid detectives” December 2014 pp24–27) near Mexico City, or the remains of the city of Machu Picchu in Peru, or even by a visit to the Museo del Oro in Bogotá, Colombia. Nevertheless, this part of the world always has, nowadays, the adjective “developing” associated with it, or even (at least until recently) the politically unsavoury term “underdeveloped”.

Beyond Imported Magic explores the science, technology and society of the Latin American countries in a collection of essays. Its title seems to be inspired by Gabriel García Márquez’ wonderful novel One Hundred Years of Solitude and its magical-realist set-up, but upon reading the opening remarks of the editors, I found that it actually refers to the way that engineering students in Rio de Janeiro in the early 1970s supposedly talked about computers as “imported magic”. Since I was a student in São Paulo at that time, it strikes me as quite improbable that this was the case. Nevertheless, one of the book’s editors (and the author of the introduction), Ivan da Costa Marques, is well known in Brazil for leading an attempt to set up a computer industry there. After a period in government, private industry and state industry, he came back to academia, focusing his interest on the interactions of science, technology and society. One presumes, therefore, that he knows what he is talking about.

The essays in this book are written by (and for) sociologists of science who have an interest in Latin America, rather than for scientists. They cover a wide array of topics, ranging from provocative subjects such as “Who invented Brazil?” (a title taken from a famous carnival samba lyric), to accounts of scientific expeditions that brought medical care to isolated populations and studied unknown diseases. It was on one such expedition that Chagas disease, which affects many of the poorest populations in the world, was identified. Another essay describes the invention, in late 19th-century Argentina, of fingerprinting as a tool to help solve criminal cases.

Two of the essays touch directly on the role of physicists in the promotion of nuclear energy in Argentina and Mexico, and by extension in Brazil as well. The first – entitled “Bottling atomic energy in Argentina” – describes how, in 1951, Juan Perón, the charismatic president of Argentina, announced at a press conference the success of Proyecto Huemul. This was an atomic fusion research programme that would, he claimed, bring cheap energy to every household in the country. The improbable character behind this announcement was an Austrian-German physicist called Ronald Richter, who had done his research on an isolated island in a gorgeous Andean lake, just across from the city of Bariloche. The amount of money invested in Richter’s enterprise is estimated to have been around $150m at today’s prices, but the whole operation came apart when a group led by another physicist, José A Balseiro, used concealed gamma-ray detectors to reveal the fraudulent nature of the project’s experiments.

That a programme like Proyecto Huemul could have been supported, despite the solid tradition of physics already present in Argentina at the time, is a reflection of a phenomenon seen again and again in Latin America, whereby populist leaders such as Perón tend to distrust their countries’ scientific establishments. But the article points out that despite the disaster of this enterprise, it nevertheless became the seed of what is now one of the most productive research centres in physics in Latin America: the Instituto Balseiro, a top-class research and educational establishment that lies just across the channel separating the island and Bariloche.

The book’s other essay on nuclear energy is “Peaceful atoms in Mexico”. Written by Edna Suaréz-Díaz and Gisela Mateos – two historians of science based in Mexico – it emphasizes the role that programmes such as Atoms for Peace (an initiative by US president Dwight D Eisenhower) had in establishing nuclear activity in Latin America. It also highlights how physicists worked to encourage the establishment of peaceful nuclear research in Mexico. In particular, the main force behind the Mexican effort was Manuel Sandoval-Vallarta, a physicist who worked at the Massachusetts Institute of Technology before the Second World War and is best known for identifying the effect of latitude on the flux of cosmic rays.

Suaréz-Díaz and Mateos argue that the distinctive feature of the Mexican nuclear programme was its civil, non-military character, in contrast to the Argentinian and Brazilian programmes. To me, though, this distinction seems to be exaggerated. While it is true that the Brazilian Navy was interested in developing nuclear submarines for defensive purposes, there was never any serious attempt there to develop nuclear armaments. Also, both the Argentinian and the Brazilian nuclear programmes were actually developed by civilian institutions: the CNEA (Comisión Nacional de Energía Atómica) in Argentina and CNEN (Comissão Nacional para Energia Nuclear) in Brazil.

Further evidence of the pacifist character of the use of nuclear energy in this part of the world is shown by the Tlatelolco Treaty, which forbids nuclear weapons in Latin America and the Caribbean and was signed by all the countries in the region. In addition to this treaty, Argentina and Brazil have an even more stringent agreement for mutual verification of all nuclear facilities, one that came about after strong pressure from the physics communities of both countries.

The essays in Beyond Imported Magic focus mainly on frustrated attempts to develop science and technology in the Latin American continent. But while it is true that many such attempts have fallen short, the book fails to recognize the immense advances that the region has seen, in part as an indirect consequence of peaceful nuclear-energy initiatives. One result of cross-border collaboration on nuclear science was the creation, in 1962, of the international Centro Latino Americano de Física (CLAF) under the auspices of the United Nations Educational, Scientific and Cultural Organization (UNESCO). The CLAF came about after the First Latin American School of Physics, an initiative by three leading figures in the region (Juan José Giambiagi from Argentina, José Leite Lopes from Brazil and Marcos Moshinsky from Mexico) that influenced a whole generation of Latin American physicists. Today, the CLAF remains very active in promoting co-operation among the Latin American countries and giving support to initiatives to establish physics facilities in the region.

Physics research is also linked with other Latin American success stories. Brazil is home to a very successful aircraft manufacturer, Embraer, which is smaller only than Boeing and Airbus. The boom in agrobusiness in Brazil has been supported by research conducted at EMBRAPA, a company that is developing ways to improve the productivity of Brazilian agriculture; one of EMBRAPA’s main research centres (located in São Carlos, in the state of São Paulo) is dedicated to the application of physics to agriculture. But regardless of whether the focus is on “atoms for peace” or on “atoms for peas”, physics in Latin America has been much more than “imported magic” for many decades now. I wish this book had done more to reflect that.

  • 2014 MIT Press £24.95/$35.00pb 410pp

Web life: The Conversation

So what is the site about?

The Conversation‘s stated aim is to provide “informed news analysis and commentary that’s free to read and republish”. In other words, it’s a blog. It is, however, a very big blog, with lots of expert authors, a prestigious team of editors and some very deep-pocketed sponsors.

Who is behind it?

That depends on which edition you’re asking about. The Conversation was founded in Australia in 2011, but it has since spread to more northerly reaches of the English-speaking world, gaining a full UK edition in May 2013 and a pilot US version in October 2014. In Australia and the UK, its financial backers are mostly universities and government bodies, such as the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and Research Councils UK. In the US, it’s currently supported by an array of private charities, including the Bill and Melinda Gates Foundation.

Who’s doing the writing?

Each edition has its own set of editors, who are largely drawn from the (decimated) ranks of broadsheet newspaper journalists in Australia and the UK. Most of the articles, though, are written by academic experts from The Conversation‘s supporting universities. Within the science and technology section, for example, top columnists include Monica Grady, a planetary scientist at the UK’s Open University; Simon Redfern, an earth scientist at the University of Cambridge in the UK, and Matthew Bailes, an astrophysicist who is also a pro-vice-chancellor at Swinburne University of Technology in Australia.

What are some of the topics covered?

All three national editions feature stories on arts, business, culture, economics, education, energy, the environment, health, medicine, politics, society, science and technology – so pretty much everything, in other words. Each edition also features country-specific “hot topics” that change over time. As of mid-April, for example, Australian conversationalists were busy debating taxation and private health insurance, while the Americans were keen on vaccines and cybersecurity. In the UK, meanwhile, the conversation was focused on the forthcoming national elections and the digital economy.

These national differences persist within The Conversation‘s science and technology section, and the divisions are not always logical: at the time of writing this, the top physics story on the UK site was about the US space programme, while the Australian edition featured an article about Albert Einstein and Leó Szilárd’s famous 1939 letter to US president Franklin Roosevelt, in which they urged him to consider building nuclear weapons. But with so many stories to choose from, it’s fair to say that Physics World readers are sure to find something, somewhere, that interests them.

Can you give me a sample quote?

From a 28 March post by Nate Szewczyk (University of Nottingham, UK) and Tim Etheridge (University of Exeter, UK) about NASA’s plans to send astronauts to the International Space Station for a full year: “Several strong and valid arguments have been put forward to justify the significant public expenditure that this research involves. These include the notion that the survival of humankind ultimately centres on our ability to inhabit other planetary bodies…More immediate benefits may present themselves, though. The muscle problems from spaceflight closely resemble those caused by numerous conditions on Earth, including long periods of bed rest, muscular dystrophies, cardiovascular diseases and type-2 diabetes. In particular, the ageing process also displays a striking similarity with the changes that occur in space, albeit over a more prolonged timeframe…In short, the unique stresses imposed by living in space provide an opportunity to study, understand and develop countermeasures to some of the most prominent health challenges faced by the human race. The question should therefore not be ‘why should we continue exploring space’, but rather ‘why wouldn’t we?’ “

Nascent ‘Kuiper belt’ seen encircling nearby star

A debris disc just discovered around a nearby star bears an uncanny resemblance to a young version of our solar system’s Kuiper belt, according to an international team of astronomers. This extrasolar disc shares many similarities with our ring, and it could be the key to better understanding the interactions between debris discs and planets, as well as how our solar system evolved early in its lifetime.

Analogue rings

The Kuiper belt is a ring of icy and dusty debris that lies just beyond Neptune’s orbit, and it is home to Pluto, several other known dwarf planets, and thousands of other remnants of the early stages of icy-planet formation in our solar system.

Understanding how this disc was formed, and how it gained its current structure and composition, is crucial to understanding the birth and evolution of our solar system. The best way to do so would be to witness a similar debris disc in an earlier stage of its life. Until now, astronomers had discovered several other discs around nearby stars but none of the systems were similar to our own: the rings are typically too large, the central star too massive, or the stars exist in regions very unlike what we think our Sun’s birthplace was like. Furthermore, the observations made of these discs were limited in their quality, and they did not include spatially resolved spectra that could provide clues about the composition of the discs.

Now, though, a team of astronomers led by Thayne Currie from the University of Toronto in Canada and the National Astronomical Observatory of Japan has changed this using the Gemini Planet Imager (GPI), an instrument on the Gemini South Telescope located in Chile. The researchers discovered a debris disc of roughly the same size as the Kuiper belt orbiting the star HD 115600, which is located a mere 360 light-years from Earth. The star is only slightly more massive than our Sun, and sits in a star-forming region similar to that in which we believe the Sun was born. But HD 115600 is different in one key way – it is only 15 million years old, compared with our Sun’s age of 4.6 billion years. This means that observing it gives us the perfect opportunity to see how our solar system might have behaved when it was much younger.

“To be able to directly image planetary-birth environments around other stars at orbital distances comparable to the solar system is a major advancement,” says team member Nikku Madhusudhan of the University of Cambridge in the UK. “Our discovery of a near-twin of the Kuiper belt provides direct evidence that the planetary-birth environment of the solar system may not be uncommon.”

This spectroscopy, combined with measurements of the reflectivity of the disc, has led the team to suspect that the disc might be composed partly of water ice, just like our own Kuiper belt. The disc also shows evidence of having been sculpted by the motions of giant planets orbiting the central star, in much the same way that the outer planets in our solar system may have shaped the Kuiper belt.

Next-generation planet hunters

This discovery has been made possible thanks to the new generation of extreme adaptive-optics systems, such as those used on the GPI, in which the optics actively correct – in real time – for the distortion caused by effects such as atmospheric turbulence. This results in images with reduced glare and higher levels of sharpness than were previously possible at the same wavelengths. “In about 50 seconds of exposure time on GPI, we can see things (like this debris ring) that we cannot see in 50 minutes of time with conventional systems,” Currie explains. The GPI also boasts another unique advantage – it records spatially resolved spectra while it images, thus giving astronomers additional information about the dust in the disc, such as its composition.

The research will be published next month in Astrophysical Journal Letters. A preprint is available on arXiv.

Do atoms going through a double slit ‘know’ if they are being observed?

Does a massive quantum particle – such as an atom – in a double-slit experiment behave differently depending on when it is observed? John Wheeler’s famous “delayed choice” Gedankenexperiment asked this question in 1978, and the answer has now been experimentally realized with massive particles for the first time. The result demonstrates that it does not make sense to decide whether a massive particle can be described by its wave or particle behaviour until a measurement has been made. The techniques used could have practical applications for future physics research, and perhaps for information theory.

In the famous double-slit experiment, single particles, such as photons, pass one at a time through a screen containing two slits. If either path is monitored, a photon seemingly passes through one slit or the other, and no interference will be seen. Conversely, if neither is checked, a photon will appear to have passed through both slits simultaneously before interfering with itself, acting like a wave. In 1978 American theoretical physicist John Wheeler proposed a series of thought experiments wherein he wondered whether a particle apparently going through a slit could be considered to have a well-defined trajectory, in which it passes through one slit or both. In the experiments, the decision to observe the photons is made only after they have been emitted, thereby testing the possible effects of the observer.

For example, what happens if the decision to open or close one of the slits is made after the particle has committed to pass through one slit or both? If an interference pattern is still seen when the second slit is opened, this would force us either to conclude that our decision to measure the particle’s path affects its past decision about which path to take, or to abandon the classical concept that a particle’s position is defined independent of our measurement.

Photon first

While Wheeler conceived of this purely as a thought experiment, experimental advances allowed Alain Aspect and colleagues at the Institut d’Optique, Ecole Normale Supérieure de Cachan and the National Centre for Scientific Research, all in France, to actually perform it in 2007 with single photons, using beamsplitters in place of the slits envisage by Wheeler. By inserting or removing a second beamsplitter randomly, the researchers could either recombine the two paths or leave them separate, making it impossible for an observer to know which path a photon had taken. They showed that if the second beamsplitter was inserted, even after the photon would have passed the first, an interference pattern was created.

The wave–particle duality of quantum mechanics dictates that all quantum objects, massive or otherwise, can behave as either waves or particles. Now, Andrew Truscott and colleagues at Australian National University carried out Wheeler’s experiment using atoms deflected by laser pulses in place of photons deflected by mirrors and beamsplitters. The helium atoms, released one by one from an optical dipole trap, fell under gravity until they were hit by a laser pulse, which deflected them into an equal superposition of two momentum states travelling in different directions with an adjustable phase difference. This was the first “beamsplitter”. The researchers then decide whether to apply a second laser pulse to recombine the two states and create mixed states – one formed by adding the two waves and one formed by subtracting them – by using a quantum random-number generator. When applied, this final laser pulse made it impossible to tell which of the two paths the photon had travelled along. The team ran the experiment repeatedly, varying the phase difference between the paths.

Double pulse

Truscott’s team found that when the second laser pulse was not applied, the probability of the atom being detected in each of the momentum states was 0.5, regardless of the phase lag between the two. However, application of the second pulse produced a distinct sine-wave interference pattern. When the waves were perfectly in phase on arrival at the beamsplitter, they interfered constructively, always entering the state formed by adding them. When the waves were in antiphase, however, they interfered destructively and were always found in the state formed by subtracting them. This means that accepting our classical intuition about particles travelling well-defined paths would indeed force us into accepting backward causation. “I can’t prove that isn’t what occurs,” says Truscott, “But 99.999% of physicists would say that the measurement – i.e. whether the beamsplitter is in or out – brings the observable into reality, and at that point the particle decides whether to be a wave or a particle.”

Indeed, the results of both Truscott and Aspect’s experiments shows that a particle’s wave or particle nature is most likely undefined until a measurement is made. The other less likely option would be that of backward causation – that the particle somehow has information from the future – but this involves sending a message faster than light, which is forbidden by the rules of relativity.

Aspect is impressed. “It’s very, very nice work,” he says, “Of course, in this kind of thing there is no more real surprise, but it’s a beautiful achievement.” He adds that, beyond curiosity, the technology developed may have practical applications. “The fact that you can master single atoms with this degree of accuracy may be useful in quantum information,” he says.

The research is published in Nature Physics.

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