A new optical clock that is insensitive to an important source of noise has been developed by physicists at the National Institute of Standards and Technology (NIST) in the US. The researchers believe the new design, which allows the clock to reach its peak precision much more quickly than before, could provide a step towards allowing optical clocks to be used in a wider range of applications than is possible today.
Optical lattice clocks trap atoms in a standing-wave potential created by two counter-propagating laser beams. A third laser is used to repeatedly excite and de-excite a specific atomic transition, which gives the “ticks” of the clock. Such a clock’s principal advantage over a similar timekeeper based on trapped ions is that technical difficulties currently prevent more than one ion being used at a time. This makes ion clocks prone to the inherent quantum randomness in the way the ion behaves when excited by the laser – called the quantum projection noise. In contrast, thousands of neutral atoms can be used in the same trap at once and this greatly reduces the quantum projection noise.
In 2013, Andrew Ludlow and colleagues at NIST in Boulder, Colorado, demonstrated two optical lattice clocks that are stable to within a record-breaking half a second in the age of the universe. Physicists have proposed that, if such clocks could be made robust enough to be taken out of the laboratory, the precise measurements of time dilation taken at various points on Earth could give important insights into the internal composition of our planet. Taking such clocks to space could allow physicists to look for deviations from Einstein’s general theory of relativity and quantum effects in gravity.
Billiard balls
“Dick noise” is an important effect in optical clocks and it arises because the atoms cannot be monitored continuously. “They’ll typically stay around [in the trap] for a few seconds before molecules of background gas bump into them and knock them out like billiard balls,” explains Ludlow, “and so we have to get some more.” During the “dead time” while they do this, the laser frequency can vary slightly. The effect of these random variations can be averaged down by measuring for many hours, but this is experimentally cumbersome.
Researchers have tried to minimize the problem using ultra-stable clock lasers. “The clock laser is becoming the most difficult part of the experiment,” explains team member Marco Schioppo, who is now at Heinrich Heine University of Düsseldorf in Germany. “The laser cavity must be as isolated as possible from the environment, both thermally and vibrationally. The clock laser is definitely the one piece of equipment it is extremely difficult to move anywhere.”
Schioppo, Ludlow and colleagues have now produced a clock containing two trapped atomic ensembles – essentially a timekeeper comprising two optical clocks. While one trap is being refilled and its atomic state prepared and measured, the laser is locked to the other trap. This has been proposed before, says Ludlow, but the researchers are the first to implement it successfully in an optical lattice clock: “To be able to do it with just two clocks, you need to make sure that the amount of time you can coherently interact with the atoms is at least the same as the amount of dead time,” he explains. “For a long time, the dead time would be much larger than the spectroscopy time.”
Simple, robust laser
The new clock reaches extreme stability 10 times faster than the team’s 2013 clock. “As soon as you improve the instability, you decrease the timescale for your measurement, and then you’re really able to pin down systematic effects more effectively,” explains Schioppo. The researchers suggest that, as the laser is permanently locked to one cavity or another, a simpler, more robust laser system could also be used.
“I think it’s a big step forward,” says optical-clock specialist Helen Margolis of the National Physical Laboratory in Teddington, UK. Quantum metrologist Piet Schmidt of Germany’s Leibniz University Hannover agrees, although he adds that the numerous difficulties the researchers had to overcome leave him wondering if the work provides a plausible route towards a simpler or more portable clock: “You need to have your two clocks synchronized extremely well to not lose a cycle of your clock laser. If you can come up with a way of producing a more stable laser source you could possibly have the same gain for less effort, but that remains to be seen.”
When the BepiColombo spacecraft begins its journey to Mercury in 2018, it will carry a unique payload: the first X-ray telescope bound for orbit around another planet. In the past, such telescopes have been too unwieldy for interplanetary travel thanks to the massive nature of traditional X-ray reflecting optics. With BepiColombo, however, a revolutionary type of optic means that this joint European Space Agency (ESA) and Japan Aerospace Exploration Agency mission could usher in a fundamental change to the way we do X-ray astronomy.
The high energies of X-ray photons mean that they penetrate into and are absorbed by traditional telescope mirrors rather than reflecting off them. To avoid this problem, X-ray telescopes use mirrors aligned along the photons’ direction of travel, which corral these high-energy particles and deal them a glancing blow that sends them towards the telescope’s focus at a shallow angle. Traditionally, these mirrors have consisted of chunky rings of glass or metal shells, but they add substantially to a spacecraft’s mass. Since interplanetary missions have strict mass constraints, X-ray telescopes simply haven’t been able to fly on them.
The Mercury Imaging X-ray Spectrometer (MIXS) aboard BepiColombo gets around this problem with a new design featuring micro-pore optics. Instead of one large glass ring, optics of this type are composed of a series of micro-channel plates: thin glass wafers covered in myriad square holes, or pores, just a few dozen microns across, and coated in reflective iridium. These plates are so light that the entire MIXS instrument has a mass of less than 10 kg – a sharp contrast to the optical system aboard NASA’s Chandra X-ray Observatory, which comes in at a whopping 1000 kg.
Direct comparisons are perhaps a bit unfair, since the two instruments have very different scientific goals. Nevertheless, “The fact that the optics are so lightweight is the only way you can get this type of technology onto an interplanetary spacecraft,” says Adrian Martindale, instrument scientist for the MIXS and a researcher at the University of Leicester, UK. Although an X-ray collimator previously featured on NASA’s MESSENGER mission to
Mercury, Martindale explains that BepiColombo, which is due to enter Mercury’s orbit in 2024, will be the first to carry a bona fide X-ray telescope to another planet. Its job will be to map the composition and elemental abundances of the Mercurian surface, and measure the planet’s auroral zone, where cosmic rays excite elements in the surface to release X-rays.
Lobster eyes consist of many square-shaped cells that reflect light to a focus along shallow grazing angles
The idea for developing X-ray optics along these lines dates back to the 1970s, when Roger Angel, an astronomer at the University of Arizona, US, was inspired by the eyes of lobsters and other crustaceans. While their geometry is not exactly the same as the MIXS system, lobster eyes also consist of many square-shaped cells that reflect light to a focus along shallow grazing angles. However, the lobster-eye technology did not really come of age until the 21st century, when a University of Leicester team led by the late George Fraser began developing it for X-ray observations.
X-ray vision
“George was one of the most brilliant physicists I have ever known,” says Emilie Schyns, micro-channel plates product manager at PHOTONIS France SAS, which manufactures the plates used in the MIXS. Schyns explains that the company’s longstanding relationship with Leicester dates back to the 1990s, when Fraser, who died in 2014, was working in the ultraviolet domain using PHOTONIS’ custom-made micro-channel plates. The micro-pore optics technology on MIXS “evolved out of that working experience”, Schyns says.
The plates manufactured for X-ray astronomy differ from standard micro-channel plate technology in several ways. One is that standard plates are active charge amplifiers with 1000 V running through them, whereas micro-pore optics contain no electronics. Another key difference is the shape of the holes. These are normally round, but to ensure a point-like image at X-ray wavelengths, rather than a diffuse glow caused by photons reflecting in different directions off a curved edge, the holes in MIXS are square.
Currently, PHOTONIS is the world’s only manufacturer of square-pore micro-channel plates. Paul O’Brien, head of Leicester’s Department of Physics and Astronomy, describes the 40-step process of making the plates as “optical cookery”, noting that getting the right curvature for the required focal length involves a proprietary method of heating and moulding the specially chosen glass. The finished plates are then tested using Leicester’s X-ray beam line – the only facility of its kind in the UK, according to O’Brien.
Despite this challenging manufacturing and testing process, micro-pore optics are garnering a lot of attention. Already there are plans for them to appear in the X-ray telescope onboard the French-led Space Variable Objects Monitor (SVOM) mission, which will track down the sources of gamma-ray bursts (GRBs). Although the micro-channel plates don’t have the spatial resolution to pinpoint exactly where on the sky a GRB is, these stellar explosions will be the strongest X-ray emitting objects in that part of the sky, so such accuracy isn’t required.
Meanwhile, a team from NASA’s Goddard Space Flight Center hopes to use the micro-pore optics on a proposed mission that would fly to the International Space Station and hunt for the sources of gravitational waves from a perch on the station’s robotic arm. Similarly, a Chinese proposal called the Einstein Probe hopes to use the optics to find gravitational-wave sources and track down X-ray flares from black holes.
The hunt is on
All three of these missions take advantage of the fact that micro-pore optics can be optimized for wide-field as well as narrow-field observing. This is an important new development for X-ray astronomy, since instruments such as Chandra or ESA’s XMM-Newton satellite only have fields of view of half a degree (about the size of the full Moon in the night sky). The ability to search large swathes of the sky for the sources of gravitational-wave mergers or the afterglows of gamma-ray bursts is “the next big thing in astronomy”, O’Brien says, adding that it is “nice to see technology developed for a planetary mission being used in very different ways”.
The wide field-of-view and limited spatial resolution is not to the detriment of MIXS, since the resolution depends on the number of X-rays detected. However, a different type of optics is required in situations where good angular resolution is important. This is the case for ESA’s upcoming Athena (Advanced Telescope for High-Energy Astrophysics) mission, which is scheduled to launch in 2028 with a next-generation technology called silicon-pore optics on board.
Although the design of silicon-pore optics draws on the same mass-saving principles as the micro-pore optics aboard BepiColombo, O’Brien explains that in other respects, they are very different technologies. Instead of carefully prepared glass plates, the reflecting optics on Athena will be made from silicon wafers, the smoothness of which (down to a few Ångstroms) makes them perfect for reflecting X-rays. Using this existing technology means that “we can leverage the large investments that the silicon wafer industry has already made”, says Giuseppe Vacanti of Cosine Measurement Systems, a Netherlands-based company tasked with developing prototype optics and, potentially, putting them into full-scale production.
Novel optics Examples of the silicon-pore optic blocks being designed for the Athena mission. Each block contains 35 plates and by stacking the plates, the ribs sawn into them create pores that reflect X-rays. (Courtesy: Cosine Research)
To make the silicon-pore optics, silicon wafers are cut into plates and a series of ribbed channels about 1 mm across are sawn into each one. The sides of the plates opposite the ribs are then coated in highly reflective layers of materials such as iridium and boron carbide, which improve reflective efficiency at higher energies. When these coated plates are stacked together, the ribs act like the holes in micro-channel plates. A silicon mandrel shapes the plates to the required curvature, after which they are stacked together, 35 plates per block, with each block combined with others to form mirror modules. “The difficulty comes in making sure that when you stack them up, you maintain the optical quality and don’t introduce any distortions,” says Vacanti. “It’s more a matter of being careful than the process being complex.”
The prototypes currently have angular resolutions of around eight arcseconds, but Cosine hopes that further refinements to the manufacturing process will reduce this to four or five arcseconds. This would be considerably short of Chandra’s 0.5 arcsecond resolution, but enough to resolve almost all of the X-ray sources that Athena can detect, and with a much larger collecting area. The company aims to demonstrate a finished prototype with four to five arcsecond resolution by 2019; move into mass production the following year; and complete all of the mirror modules by 2025, three years before launch, to allow time for assembly and testing. These are ambitious goals, however, and to meet the deadline Vacanti says that they will have to stack one plate every two minutes – a task that currently takes up to 20 minutes. “The challenge is to carefully place the plates in their stacks fast enough but to not lose any of the precision that we have achieved,” explains Vacanti.
Mission possible
Like the micro-pore optics, silicon-pore optics are receiving interest from elsewhere. NASA is considering a silicon-pore optics-based mission called Arcus, which would use X-rays to study everything from interstellar dust to black holes and galaxy formation. The team at Leicester is also working on proposals for additional missions featuring micro-pore optics. However, their future involvement on missions outside of Europe is currently uncertain. “The Chinese would like us to be involved with their missions, but that requires the UK Space Agency to fund the instrument that we would build,” says O’Brien. “But of course they’re focusing on European missions, so it’s hard to get money for that sort of thing.”
New applications for both types of optic will likely continue to be developed, opening up fresh possibilities for X-ray astronomy. For example, lightweight optics could be used for novel pulsar-based navigation systems. By detecting X-rays from these spinning neutron stars, spacecraft could become self-guiding, eliminating the need to triangulate with Earth-based listening stations. Scientists in other fields may also take advantage of the small, lightweight micro-channel plates to build portable, tabletop devices that were impossible or too expensive before. “Over the next decade or two there’s going to be a spate of instruments using these optics,” enthuses O’Brien. “All that hard work is now paying off for what is becoming a very successful story.”
Abstract, mathematically complex and (so far) unsupported by direct experimental evidence, string theory attracts plenty of criticism. Yet it remains an incredibly active area of research, with thousands of physicists and mathematicians around the world working on strings and related ideas. The reasons for its continued popularity are eloquently presented in Joseph Conlon’s book Why String Theory? – Physics World‘s Book of the Year for 2016.
“String theory is much more than just a candidate theory of quantum gravity – people can use it for all sorts of reasons,” Conlon says. “Whatever your interests in physics are, it gives you things to think about.” To explain why this is, Conlon – a string theorist at the University of Oxford – begins the book by describing the origins of string theory and showing how it has changed over the years. Later chapters address the chief reasons why string theory continues to be a popular research topic. These include the theory’s status as a candidate theory of quantum gravity and the interest it poses to mathematicians, but also its applications to quantum field theory, cosmology and particle physics.
Conlon’s emphasis on string theory as something that is useful, even if it is not the ultimate “theory of everything”, is unusual in popular writing about string theory – or indeed any physical theory. This clear-eyed and distinctive approach helped Why String Theory? stand out in a strong shortlist of books that are all novel, well-written and scientifically interesting to physicists – the criteria used to determine Physics World‘s Book of the Year.
Conlon told Physics World that he wrote the book in order to “let the other side of my brain loose” after time spent writing formal scientific papers, and his dry, often acerbic wit is sometimes aimed at string theory’s advocates as well as its detractors. At one point, he describes multiverse-based thinking as “incontinence of speculation joined to constipation of experiment,” while at another he advises those who seek “proof” of a certain string theory principle that “physics is not mathematics, and those with scruples on this matter can be well advised that the math department on campus is generally in the next building down the street”.
Why String Theory? had stiff competition from the other nine books on the 2016 shortlist. Unusually, we have singled out one of these books for special recognition. Cosmos: the Infographic Book of Space has a very different format from the other books on our list, conveying information about space exploration, planetary science, cosmology and more via a series of colourful and elegant infographics. Written by Stuart Lowe and Chris North, with input from designer Mark McCormick, the book is both visually stunning and packed with fascinating ideas, and its innovative and well-executed approach earned it Highly Commended status in our competition.
To hear more about these books, you can listen to the latest Physics World podcast, in which our book experts – reviews editor Tushna Commissariat and past reviews editor Margaret Harris – discuss the winner and three other books on the 2016 shortlist with science communicator Andrew Glester, creator of the Cosmic Shed podcast.
This is the eighth year the magazine has picked a Book of the Year. Previous winners include Trespassing on Einstein’s Lawn, Amanda Gefter’s personal quest to understand the meaning of “nothing” (2015); Stuff Matters, Mark Miodownik’s salute to everyday materials science (2014); and The Strangest Man, Graham Farmelo’s landmark biography of Paul Dirac (2009).
We love talking about great physics books. In fact, we could go on about them for hours. The sparkling writing and deft analogies. The precise explanations that draw out the essence of complex concepts. The humorous anecdotes that make the research process come alive. We love it all, and as usual at this time of year, we’re sharing our thoughts on a few of the year’s best popular-physics books in a special edition of the Physics World podcast.
As with last year’s Book of the Year announcement, we teamed up with local science communicator Andrew Glester to record the 2016 edition in his garden shed, where he can often be found musing about “science fiction, science fact and everything in-between” for his podcast the Cosmic Shed. It was a trifle chilly in the shed this year, but thanks to hot drinks and some lively conversation, the time flew by as Glester quizzed Physics World’s current reviews editor, Tushna Commissariat, and her predecessor Margaret Harris about their favourite books of the year.
The decision about which of these books should be Physics World’s 2016 Book of the Year was an unusually tough one, for reasons you’ll hear about in the podcast. We congratulate all of the shortlisted authors on their fantastic books, and we hope that everyone will find something to appreciate on this list.
Shortlist for Physics World Book of the Year 2016 (alphabetical by author)
Xavier Barcons will lead European Southern Observatory
The Spanish astronomer Xavier Barcons will take over as director general (DG) of the European Southern Observatory (ESO) in September 2017, replacing the current DG Tim de Zeeuw who completes his mandate. Barcons is a professor at the Spanish Council for Scientific Research in Madrid and is an expert in the field of X-ray astronomy. He served as ESO council president in 2012–2014 and is currently chair of the organization’s Observing Programmes Committee. Based in Garching, Germany, the ESO has three observing sites in Chile. “I look forward to seeing the European Extremely Large Telescope (E-ELT) come to fruition and overseeing the further development of the Very Large Telescope, Atacama Large Millimeter/submillimeter Array (ALMA) and many other projects at ESO,” said Barcons.
Fountain gives physicists time to study molecules
A molecular fountain has been created that allows molecules to be observed for very long times as they free fall. Created by Hendrick Bethlem and colleagues at Vrije University in the Netherlands, the technique involves cooling ammonia molecules to milliKelvin temperatures and then launching them upwards at about 1.6 m/s. The molecules can then be studied in free fall for as long as 266 ms. This set-up is similar to atomic fountains, which allow very precise measurements to be made of atomic energy levels and form the basis for atomic clocks. A molecular fountain has proven much more difficult to create because molecules can vibrate and rotate – and this makes it very difficult to cool and manipulate them using conventional laser techniques. Bethlem and colleagues overcame this problem by using electric field gradients to exert forces on ammonia, which is a polar molecule. The team says that its new molecular fountain could be used to look for tiny deviations from the Standard Model of particle physics – which could be revealed by tiny shifts in molecular energy levels. Tests of the equivalence principle of Einstein’s general theory of relativity could also be done by measuring the acceleration due to gravity experienced by different types of molecule. The fountain is described in Physical Review Letters.
X-ray imaging technique could improve cancer treatment
Images of breast tissue taken using conventional X-ray imaging (left) and X-ray phase-contrast imaging (XPCI). The white flecks are calcifications, which are much better resolved by XPCI (Courtesy: EPSRC)
An X-ray imaging technique that could only be done at large synchrotron facilities has been adapted for widespread use by Sandro Olivo at University College London and colleagues. Called X-ray phase-contrast imaging (XPCI), the method involves measuring changes in the phase of an X-ray beam as it travels through a sample. This is unlike conventional X-ray imaging, which measures the attenuation of the X-ray beam. The technique is better able to distinguish structures in living tissue, making it ideal for medical imaging. XPCI is also better at finding tiny cracks and defects in materials and could also be used to detect the presence of weapons and explosives in baggage. However, XPCI could only be done using the laser-like X-ray beams produced by synchrotrons – which are huge electron accelerators. Now, Olivo and colleagues have developed a technique that allows XPCI to be performed using X-rays generated by conventional medical sources. It involves first passing the X-rays through a “mask” containing an array of apertures to create a number of beams. These then interact with the sample before passing through a second mask to a detector. This configuration converts differences in phase to differences in measured intensity. “We’ve now advanced this embryonic technology to make it viable for day-to-day use in medicine, security applications, industrial production lines, materials science, non-destructive testing, the archaeology and heritage sector, and a whole range of other fields,” says Olivo. The technology has already been licensed to Nikon Metrology UK for use in a security scanner and UCL and Nikon are currently developing a medical scanner.
You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today for the announcement of the Physics World Book of the Year for 2016 .
Stephen Wolfram was a child prodigy, receiving his PhD in particle physics at the age of 20. His subsequent achievements include significant work on cellular automata, the creation of the computer-algebra system “Mathematica” and the computational knowledge system “Wolfram Alpha”. He also wrote a large and controversial book, A New Kind of Science (2002), which argues that computation is key to understanding our universe. But you don’t need to know about his achievements to read Wolfram’s latest book, Idea Makers: Personal Perspectives on the Lives and Ideas of Some Notable People, not least because they are rather frequently mentioned in the text.
Idea Makers is exactly what the subtitle suggests: it presents a very personal view of a number of people who have interested Wolfram, who is himself a significant scientific thinker. Some are major past figures from the history of science, such as Gottfried Wilhelm Leibniz, Ada Lovelace and Alan Turing. Others the author himself knew and worked with, including the likes of Richard Feynman – one of Wolfram’s PhD examiners, whom he later came to know well – Steve Jobs and Solomon Golomb. I knew of Golomb as the inventor of “pentominoes” (a generalization of dominoes, which involves joining five equal squares edge-to-edge) but his work, I found out from the book, is of fundamental importance in electronic engineering. Most of the people featured in the book only appear briefly, as their entries are written to mark an anniversary, an event or a death. Many of the entries do focus on Wolfram’s personal connections with his subjects, but this is understandable given the premise of the book.
While most of the subjects are well known, the name of Russell Towle was new to me. Towle, a US mathematician who died in a road accident in 2008, used Mathematica to explore zonohedra, a special kind of polyhedra, and corresponded with Wolfram about his work. The chapter on Towle includes some of his fascinating images, and I am delighted to have learned about Towle and his work.
Wolfram does not attempt to provide a complete portrait of his subjects: instead, he is interested in particular aspects of their work (naturally, those which relate to his own ideas), and there is a diversity of approach. The very interesting chapter on Feynman is largely personal reminiscence, while the section on Kurt Gödel (written on the centenary of his birth) argues that Gödel’s “abstruse theorem of mathematics” has set the agenda for 21st-century science.
Wolfram’s first-hand research into archival material, coupled with his enthusiasm for investigating the past, comes to light in the three more comprehensive chapters, on Lovelace, Leibniz and Srinivasa Ramanujan. The extended essays were, for me, particularly intriguing. Wolfram quotes extensively from original documents in arguing for Lovelace’s importance. When I first heard of Lovelace, there was a view that, while she undoubtedly played an important role in helping Charles Babbage develop his calculating engines, there were doubts as to the extent of her personal technical contribution. Wolfram, along with other recent researchers, convincingly dispels any such doubts. Happily, following last year’s bicentenary of her birth, Lovelace’s reputation has never been higher, and Wolfram puts forth even more convincing evidence of her abilities.
The chapter on Ramanujan delves into his unique way of doing mathematics, as well as his extraordinary insights into mathematical patterns. Wolfram goes as far as recommending the reader adopt a similarly adventurous and experimental attitude in mathematics. Personally, I’m enough of a traditional “pure” mathematician to want proof to go along with experimentation, and in some ways, I found this the least convincing section of the book.
In writing about Leibniz, Wolfram analyses Leibniz’s thinking about computation. He concludes that Leibniz didn’t take discrete systems seriously enough to anticipate modern ideas of universal computation; but he is interested in Leibniz’s use of binary and in his calculating machine. Although Wolfram’s approach is not entirely that of a historian – he is comfortable drawing conclusions from the perspective of our present-day thinking – the chapter is nevertheless illuminating and thought-provoking.
Wolfram writes very well: he is always entertaining and his ideas are interesting. While it is somewhat natural that he relates his subject’s ideas to his own, reading the frequent speculations about how these great thinkers might have appreciated Wolfram’s work became slightly overwhelming for me. There are sections with headings such as “What if Ramanujan had Mathematica?”, many comments along the lines of “I’m sure he [John von Neumann] would have been a big Mathematica user today”, and “perhaps long ago he [Turing] would have campaigned for the creation of something like Wolfram|Alpha”. The book discusses why John von Neumann did not anticipate Wolfram’s insight that cellular automata with simple rules can generate very complex behaviour. Despite this, I don’t think Wolfram intends to be self-congratulatory. To his credit, in the chapter on Benoit Mandelbrot, he quotes the fractalist’s highly unfavourable opinion of A New Kind of Science. Reading this book, one wonders whether his description of Mandelbrot as “constantly seeking validation and constantly fighting to get his due”, might also apply to himself.
Because of the personal nature of the book, Wolfram provides a very partial view of his subjects, in more than one sense of the adjective. The reader will meet some fascinating characters who deserve to be better known, and will gain insights into some of the major figures in the history of science and technology, both recent and from the more distant past. They will also learn a lot about the author and his own thoughts – more explicitly than most scientific biographers would allow themselves. Whatever you think of Wolfram’s big ideas, his thoughts and perspectives are illuminating and are worth careful consideration.
Milky Way meets bioluminescent phytoplankton along the Tasmanian coast, in Arwen Dyer’s “Illuminated”. Such a large bloom points to an unusually warm ocean – a sign of climate change. (Courtesy: Arwen Dyer)
For the past eight years, astrophotographers from all corners of the globe have sent in their best and more exquisite images of our cosmos, to participate in the Astronomy Photographer of the Year award, run by the Royal Observatory in Greenwich, UK. This year’s submissions were a bumper crop with nearly 4500 images entered into the competition, from photographers in 80 countries.
In Astronomy Photographer of the Year: Collection 5, the Royal Observatory has put together all the winning and shortlisted images from the 2016 competition. The coffee-table book contains 140 large glossy photographs of a variety of celestial objects, all photographed from the Earth. While the Royal Observatory hosts a free exhibition of the winners each year, this book offers readers world-over a chance to view these breath-taking images.
The ‘Man on the Moon’ – photographed by Dani Caxete at Cadalso de los Vidrios in Madrid, Spain – has never been quite so clear. This image was the runner-up in the People and Space category. (Courtesy: Dani Caxete)
The competition has eight different categories including skyscapes, auroras, galaxies, people and space. There is also a separate competition for entrants aged 15 and under, where the subject can be anything astronomical. The competition now also boasts two special prizes – the Sir Patrick Moore Prize for Best Newcomer, for first-time entrants who have become involved in astrophotography only for the past year; and the Robotic Scope Prize, which is given to photographs taken with a remotely controlled telescope.
Star-gazing from the bowels of a big city is a tough task. But a long exposure of 10 seconds and an interesting perspective gave ‘City Lights’ – taken at Quarry Bay, Hong Kong by Wing Ka Ho – a winning edge in the People and Space category. (Courtesy: Wing Ka Ho)
The entries are judged by a panel that includes astronomers, photographers and artists, with the book containing a poignant foreword penned by judge and Turner-prize-winning fine-art photographer Wolfgang Tillmans. “Astronomy transcends borders and cultures,” he writes, recounting how European astronomers in the 18th century were given special “safe passage” to observe a transit of Venus, despite the fact that France and Britain were then at war. “In today’s divided world, maybe astronomy can still help bring us together.”
Eclipse chasing: poster for the August 2017 total solar eclipse in the US. (Courtesy: Tyler Nordgren)
On 21 August 2017 a total solar eclipse will cast its sweeping shadow across the US. Starting around 10 a.m. in Oregon on the west coast, it will end all too soon, a mere 90 minutes later in South Carolina in the east. An awe-inspiring cosmic display awaits those who are either lucky enough to live along that swath of land, or who make the effort to get there. Depending on where it’s observed from along its path in the US, totality will last from anywhere between 1 and 2.5 minutes, but for the rest of Northern and Central America, the eclipse will be partial. Nonetheless, the sight of the new Moon forming a dark crescent shadow on the solar surface will be a celestial spectacle for any viewer. In his book Sun Moon Earth: the History of Solar Eclipses from Omens of Doom to Einstein and Exoplanets, author and astronomer Tyler Nordgren charts the evolving history and science of the natural phenomenon that is a solar eclipse.
Through a narrative that flows effortlessly between personal experiences and scientific facts, Nordgren – a professor of physics at the University of Redlands in California, US – engages the reader with fascinating scientific discoveries that this cosmic coincidence makes possible. Eclipses are a good reminder that planetary motions follow well-defined laws of physics, for the most part. It is these latter exceptions, when the laws are broken, that have driven some of the most significant discoveries of our time, including the modifications to Newton’s laws of gravitational motion in the form of Albert Einstein’s theory of relativity, which was substantiated thanks to Arthur Eddington’s observations of starlight during the eclipse of 1919.
Nordgren also describes just how addictive it can become once you witness a total solar eclipse – see one and you are already planning for the next. Eclipse chasing can become a costly affair as you often travel to remote locations. As a scientist who has been leading a team to observe total solar eclipses since 1995, I understand all too well the eclipse-addiction syndrome. For my group – the Solar Wind Sherpas – an eclipse offers us the unique opportunity to probe a small section of the sun’s corona (of just a few solar radii) that is closest to its surface, which currently cannot be observed by any other instrument. With each eclipse, opportunities arise for testing new ideas and new instrumentation.
For the most part, it is the fleeting beauty of this event that makes the experience so compelling. Lasting a maximum possible seven minutes, the magic is always over too soon. The prospect of bad weather – that sometimes looms as a literal dark cloud – is omnipresent and makes observing solar eclipses even more of a challenge. The Solar Wind Sherpas have experienced the entire spectrum of emotion, from utter disappointment when the view was clouded out, to extreme joy when clear skies prevailed. But the addiction persists as we continue to strive to uncover some of the secrets of the Sun and its corona.
Astronomers and non-scientists are often equally obsessed with eclipses. But for the former, it is the unique opportunities that a solar eclipse offers – to test certain theories or trial new technologies – that is tempting. Beyond the sheer visual awe of an eclipse, the celestial setting that comprises the Sun, Moon and Earth serves as an excellent laboratory tool. Light is a ubiquitous astronomical signal that can be detected using everything from a telescope to a spectrograph to the naked human eye and has been studied through the centuries.
According to Nordgren, the world’s first eclipse-chaser happened to be a scientist – Jacques d’Allonville, or Chevalier de Louville, a member of the Royal Academy of Sciences in Paris – who travelled to London to see the total solar eclipse of 22 April 1715. This eclipse had been predicted by astronomer and mathematician Edmund Halley, using his friend Isaac Newton’s laws of motion. Incidentally, this event was also one of the first times that the public had been asked to engage in the science being done. Halley distributed posters across the country asking people to record the time and duration of the event using their pendulum clocks and to mail him their results. Halley’s aim was to make better measurements of the Moon’s orbit, thereby improving his ability to predict future eclipses.
A strikingly similar example of citizen science followed some 200 years later, when a total solar eclipse was predicted to pass over New York City on 24 January 1925. A number of scientists urged the public to witness totality and record the eclipse’s time and duration, in what the New York Times deemed “cosmic detective work.”
The book ends by offering the author’s insight into the evolution and the ultimate fate of our Sun and solar system. Nordgren teases us with the basic question, and worry, of whether eclipses will “forever” be present for us to marvel at. Currently, no other planet in our system has the privilege of experiencing a total solar eclipse. Our Sun, Moon and Earth can continue to boast of their unique alignment, but for how long? We can breathe a sigh of relief for now, as this fortuitous combination of sizes, distances and orbits that allows for a total solar eclipse to occur will last for at least the next few hundred million years.
Nordgren captures the scientific significance of total solar eclipses in a manner that is readily accessible to most readers. Certain concepts mentioned in the book – including the rather complicated story behind the modification to Newtonian gravity, which could not account for discrepancies in the orbit of Mercury, and eventually led to Einstein’s theory of relativity – might be difficult for some to follow. However, this does not deter the reader from carrying on. Nordgren’s book is extremely timely, and hopefully many of its readers will be compelled to witness the beauty of the corona next year.
Yesterday we announced the winner of the Physics World 2016 Breakthrough of the Year, which went to the LIGO Scientific Collaboration for its revolutionary, first ever direct observations of gravitational waves. I caught up with six LIGO scientists in the above video Hangout and asked them what it was like when they first realized that they had detected gravitational waves emanating from two coalescing black holes 1.3 billion light-years away.
“Weightless” experiments that compare the gravitational acceleration of two different quantum objects have been performed by physicists in France. Carried out in free fall on board an aircraft undergoing a parabolic trajectory, the tests were far too insensitive to test the long-held idea that all bodies fall at the same rate (in a vacuum) in a given gravitational field. However, the research could lead to far more powerful space-based experiments and might also result in the development of new navigational aids.
The universality of free fall is a consequence of the equivalence principle, which lies at the heart of Einstein’s general theory of relativity. It states that inertial and gravitational mass are equal, which means that a body’s mass – or indeed its internal structure – has no bearing on its acceleration in a gravitational field. Therefore two bodies with different masses or compositions will accelerate at the same rate.
Universality has been tested to ever greater precision since Galileo’s mythical experiment at the Leaning Tower of Pisa – and, so far, has never failed. The most precise experiment to date was carried out in 2008 by researchers at the University of Washington in Seattle, who found that universality held to one part in 1013.
Microscope in space
Physicists would like to boost precision by at least a factor of 100, since it as this level that some theories beyond the Standard Model of particle physics predict that the universality of free fall will break down. In fact, one space mission already in orbit around the Earth – the Micro-Satellite à traînée Compensée pour l’Observation du Principe d’Equivalence (Microscope), developed by the French National Centre for Space Studies (CNES) – is designed to reach a sensitivity of about 10–15 and could produce its first significant results early next year.
Microscope takes advantage of the fact that orbiting satellites are in free fall towards the Earth. Therefore objects inside the satellite are themselves in free fall for far longer than any mass dropped on Earth. This means that acceleration measurements can in principle reach very high sensitivities.
Microscope, like the University of Washington experiment, studies the free fall of large “classical objects”. In contrast, the latest work, carried out by Philippe Bouyer and Brynle Barrett of the LP2N laboratory in Bordeaux and colleagues, uses “quantum objects”. These are extremely cold clouds of two types of atom: rubidium-87 and potassium-39. Atomic systems have a number of advantages over macroscopic objects, according to Bouyer, including the fact that there is no possibility of contamination by unknown quantities of impurities. Also, spin and other quantum-mechanical properties of the atoms can be varied to see if this causes a violation of the equivalence principle.
Struck by lasers
The rubidium and potassium atoms are allowed to fall under the influence of gravity. As they drop they are struck by lasers, which acting as a beam splitter for matter, causes the atoms’ wave packets to split and follow two vertical paths at the same time. At the end of their trajectory, the two states interfere with one another, producing an interference fringe. Comparing the position of the fringes produced by the rubidium and potassium then allows the researchers to establish whether the two different types of atom have undergone different relative phase shifts and hence experienced very slightly different accelerations.
Physicists have previously used cold-atom interferometers to investigate the universality of free fall, having achieved sensitivities of around 10–8, but these experiments were performed on the ground. As with classical tests, the ultimate aim is to go into space. Bouyer and colleagues haven’t yet managed that, but have instead taken advantage of the near weightless conditions on board a specially adapted Airbus aeroplane owned by French company Novespace. The “zero-G” aircraft undergoes free fall for about 20 s at a time by climbing at an angle of around 45° and then cutting its engines just enough to cancel its air drag, so that it traces out a parabola as it accelerates downwards under gravity. The plane then drops, noses up again and traces out another free-fall parabola, repeating the cycle many times over.
Bouyer and colleagues have carried out nearly 10 years of painstaking work on many parabolic flights to stabilize their complex equipment in the noisy environment of the aircraft. This allowed them to perform tests on rubidium-87. Now, the team has compared the behaviour of two different types of atom over the course of six flights last year.
Suitable for space
Barrett says that the work relies on a number of technical innovations to reduce the effects of on-board vibrations, which can reach about 0.01 g, and the aircraft’s rapid rotation, which can get up to one revolution per minute during a parabola. Noting that he and his colleagues tested the universality of free fall with a modest sensitivity of just 3 ×10–4, he says that the importance of the work was in showing the suitability of their set-up for space-based tests. “The techniques we developed here could be exploited by many experiments over the next few years,” he predicts.
The team’s next step is to carry out new tests early next year to show how single atoms could be used for “inertial” navigation, which involves continually monitoring a body’s acceleration and rotation over time. Beyond that, some group members are also working to exploit the interferometer technology on a mission known as the Space-Time Explorer and QUantum Equivalence Principle Space Test (STE-QUEST). But according to Bouyer, the roughly €500m satellite will not launch until at least 2025. “It is a big, long term project,” he says.
