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Magnetic fields near the Milky Way’s black hole seen for the first time

The first direct evidence of magnetic fields near a black hole has been found by researchers using the Event Horizon Telescope (EHT), who have seen such fields around Sagittarius A* (Sgr A*) – the black hole at the centre of our Milky Way galaxy. While such magnetic fields have long been predicted, this is the first time that they have been detected close to the “event horizon” of the black hole. Magnetic fields near a black hole are thought to power the huge relativistic jets that emerge from many accreting black holes and blast across thousands of light-years – shaping entire galaxies along the way.

Global effort

Astrophysicists have long been keen to understand how relativistic jets form and where they get their colossal power. However, detecting magnetic fields at or near the event horizon – the boundary at which even light cannot escape a black hole’s gravitational pull – has been an impossible task until very recently. The Event Horizon Telescope (EHT) – a global network of radio telescopes that are linked together to function as one giant, Earth-sized telescope – now makes it possible to resolve features at the horizon of Sgr A*. This is the Milky Way’s 4.5 million solar-mass black hole that is hidden behind colossal veils of gas and dust some 26,000 light-years away from Earth.

The EHT uses a radio-astronomy technique called very-long-baseline interferometry (VLBI), and its network of receivers extends from Hawaii to Spain (illustration below). It is still being built but when it is fully functional, it should be able to resolve features as small as 15 micro-arcseconds – an angular resolution that would allow it to see an orange on the surface of the Moon.

Map showing all 13 radio telescopes, past, present and future, involved in the Event Horizon Telescope
Cross-continental network (click to enlarge)

To make its measurement, the EHT team connected together four telescopes: the Submillimeter Array and the James Clerk Maxwell Telescope, both in Hawaii; the Submillimeter Telescope in Arizona; and the Combined Array for Research in Millimeter-wave Astronomy (CARMA) in California. The team observed the polarization of the incoming radio waves on different angular – and therefore spatial – scales. The orientation of this polarization is dependent on the orientation of the local magnetic fields in the vicinity of Sgr A*.

Polarization data

EHT researcher Michael Johnson, from the Harvard-Smithsonian Centre for Astrophysics, explains that each telescope records two data streams that capture both the right- and left-handed circular polarization of the incoming radio waves. The team then compares streams from different telescopes using a supercomputer. In the past, all of the data used by the EHT compared streams that were of the same handedness: right versus right and left versus left. The cross-hand signals are much weaker, but they carry information about the linear polarization. “Because of technical advances with the EHT, we can now detect this polarization information. And because the emission near Sgr A* is synchrotron radiation, the direction of linear polarization traces the magnetic fields. So that’s how we can go from measuring polarization at the telescope to understanding magnetic fields near the black hole,” says Johnson.

EHT physicist Avery Broderick, of the University of Waterloo and the Perimeter Institute for Theoretical Physics in Canada, told physicsworld.com that this is the first time that researchers have detected horizon-scale order in the magnetic fields in the immediate vicinity of a black hole and not thousands of Schwarzschild radii away. “By measuring high-polarization fractions on horizon scales, compared with low polarizations on scales 5–10 times larger, we have shown that the polarization patches, and thus magnetic fields, are organized on horizon scales,” says Broderick.

The researchers were “shocked to see how strong the polarization signal was on our longest interferometric baseline”, says Johnson, adding that the polarization fraction on the longer baseline was a factor of 10 higher than what the shorter baselines detect. “Such a dramatic difference in how the polarization appears when you observe with the highest angular resolution really took us by surprise, and it comes about because of this mix of ordered and tangled magnetic fields near the black hole. The ordered fields are good news for theories of jets, since ordered fields are thought to play a central role in the formation and collimation of these jets.”

The other surprise that lay in wait for the team was just how active the polarization was and how it changed on a daily basis. “On one day, the polarization traced out a clean ‘loop’ over the two-and-a-half-hour observation, possibly from orbiting material very close to the event horizon,” says Johnson. The researchers also saw fluctuations in the strength and direction of the horizon-scale linear polarization from Sgr A* over a 15 min timescale, revealing the extreme dynamics at play at the heart of the black hole. Indeed, the innermost orbits near Sgr A* have timescales of about 30 min, so the fluctuations most likely arise from turbulent fields in the innermost accretion disc. Johnson points out that “it is very difficult for material to fall into a black hole without turbulence, so these first direct observations of the turbulence are finally placing these accretion theories on firm observational footing”.

Once the EHT is functioning with its full complement of telescopes, the researchers will study these magnetic fields repeatedly, as they try to understand what drives accretion onto black holes and what powers their jets. While Sgr A* does not currently have a jet, the team’s other candidate – the supermassive black hole at the centre of the M87 galaxy – has a prominent one, and the researchers hope to understand what role the magnetic fields play in launching and powering that jet. “All of that means that we are one step closer to understanding, and ultimately removing, the uncertain astrophysics that complicates the forthcoming precision tests of Einstein’s general relativity in the strong-gravity limit,” adds Broderick.

The research is published in Science.

Hawaiian court throws out Thirty Meter Telescope building permit

Hawaii’s Supreme Court has ruled that the construction permit for the $1.4bn Thirty Meter Telescope (TMT) on top of Mauna Kea mountain is invalid. Construction of the telescope has been on hold since March, when hundreds of native Hawaiians and their supporters protested and prevented building crews from entering the site. The court ruling is a fresh blow to the project, with the telescope’s backers now having to restart the permit process.

With a 30 m primary mirror made of 492 hexagonal segments, the TMT will be the largest and most powerful instrument on Mauna Kea, which is already home to 13 others. The TMT will sit on a plateau about 150 m below the summit – a location picked to reduce the telescope’s visibility from the majority of the island.

In its ruling on 2 December, the court claimed that Hawaii’s Board of Land and Natural Resources (BLNR) should not have approved the permit in 2011 because it failed to follow due process when awarding it. “Quite simply, the board put the cart before the horse when it issued the permit before the request for a contested case hearing was resolved and the hearing was held,” the ruling notes. “Accordingly, the permit cannot stand.”

The court decision now requires the TMT to go through the process again that will first involve a “contested case hearing”, which must be heard first before a permit can be issued. The decision by the court will result in delays beyond the targeted 2023 construction date, although how long is currently unknown.

“I’m not surprised the court looked at the permitting process itself, since the actual case for building the TMT – that it will be an environmentally and culturally responsible observatory that benefits the educational and economic future of Hawaii – is and remains exceptionally strong,” says Thayne Currie, an astronomer with the Subaru Telescope on Mauna Kea. “Considering the tremendous scientific discoveries and opportunities that the TMT will afford, the wait will be worth it.”

Respecting the decision

In a statement, Henry Yang, chair of the TMT International Observatory Board of Directors, noted that the TMT collaboration is now considering its next steps. “We thank the Hawaii supreme court for the timely ruling and we respect their decision. TMT will follow the process set forth by the state, as we always have,” says Yang. “We appreciate and thank the people of Hawaii and our supporters from these last eight-plus years.”

Groups opposed to the TMT’s construction have, however, welcomed the decision. Kamahana Kealoha, head facilitator of Sacred Mauna Kea Hui, says the group is “elated” at the move to annul the permit, which he says has been acquired “through an immorally and unethically manipulated process that has been negligently overseen by the BLNR”. “Our goal has always been to systematically secure the Mauna,” Kealoha told physicsworld.com. “The revoking of the construction permit is essential for us to succeed in protecting the mountain summit and the endangered-species environment from further development by this 18-storey monstrosity.”

Kealoha adds that they now hope that TMT’s backers will “see red flags and pull out”. “Quite frankly, the resistance is here to stay, and we plan to see to it that the TMT is not built,” he says. “This could be more costly than the $1.4bn already appropriated and, speaking for myself and my group, we will continue to make sure the TMT being built on our sacred summit is not a profitable venture for investors, as we already have.”

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. Two months after the protests in March, the governor of Hawaii, David Ige, noted that the TMT had the right to proceed, and that all of the necessary permits for the observatory had been obtained. However, he criticized how the University of Hawaii has managed the land on Mauna Kea, outlining that a quarter of the telescopes on the summit should be completely dismantled and each site returned to its natural state by the time the TMT is operational.

Top physics books of 2015

By Margaret Harris

PW book logo 2015Well written, scientifically interesting to physicists, and novel: these are the criteria used to select Physics World’s annual list of the year’s top physics books. We’ve done this every year since 2009, and over the past few weeks, we’ve been at it again, sifting through the 52 books reviewed in the magazine in 2015 and separating the best from the rest (most of which, I should add, are also very good – we try not to review bad books).

This is never an easy task, and as usual, the selections in our shortlist have been influenced by the views of external experts: the physicists, science writers and science historians who read and reviewed books for Physics World magazine throughout 2015. Their reviews (and, in a few cases, their private opinions) helped us decide which books deserved a closer look, and we thank them all for their contributions.

Deciding on a winner, however, is a privilege we reserve for ourselves, and with so many great books to choose from, it has been both a privilege and a challenge this year. We’ll be announcing our “Book of the Year” in a special edition of the Physics World podcast later this month, but in the meantime, here are the candidates:

(more…)

Dripping with science

Pick up a newspaper, and chances are it will have a story about water in it. From news of floods and droughts to the discovery of water flowing on Mars, water is embedded in our surroundings and in our culture (“Water, water everywhere”). Yet despite its ubiquity, we don’t really understand it. From the forces that bind it into a solid, to the way it cools and even how it sustains the cells that build our bodies – the humble H2O molecule still retains many of its mysteries.

In The Water Book, science journalist Alok Jha endeavours to connect each of us through the intrigues of this deceptively simple substance “to everyone and everything else and the rest of the universe”. The book is told in the form of a story that chases the enigmatic water molecule from the far reaches of space to the Atacama Desert and back again. But it isn’t just us, the readers, who get to go on a journey, for Jha is on the move as well: as the book begins, he is boarding the vessel MV Akademik Shokalskiy, bound for Antarctica on a research trip.

The voyage doesn’t start well, but it does give Jha an excuse to treat us to an extensive description of the trials and tribulations of a journey through the “roaring forties” to the icy Southern continent. As the ship progresses, he describes the larger context of its voyage, elegantly moving from historic to present-day scientific impacts. In doing so, Jha sets the tone for the book, weaving tales from his journey to Antarctica in with the twists (and a few red herrings) in the meandering story of water.

The scope of the task Jha sets himself is challenging, as there are few scientific topics that water does not seep into. To tackle this, Jha loosely separates his treatise into sections discussing water on Earth in liquid (“The Hydrosphere”) and solid (“The Cryosphere”) forms before heading off our planet to hunt for places where water hides out in the rest of the solar system. This sectioning of the book serves him well as he winds descriptions of various research on the water molecule into stories of his own journey south.

When discussing water here on Earth, Jha is unapologetic about refusing to separate the fundamentals of water science (particularly as it is applied to our environment) from the pressing issue of climate change. This issue crops up a number of times, and although he states that a discussion of climate change is beyond the scope of the book, many of the topics he chooses to cover stray unavoidably into the Earth’s gloomy future. This is particularly stark as he travels across the Antarctic ice in search of the now ice-locked hut that sheltered the Australian explorer and scientist Douglas Mawson in 1912–1913. On arrival at the hut, Jha pauses his story to discuss the effect of the changing landscape on the nearby colony of Adélie penguins, followed swiftly by a description of research on ice-sheet dynamics and ice-core isotopes, and then by a summary of results from the Intergovernmental Panel on Climate Change. This rather breathless coverage of topics is somewhat a feature of the book.

As for water as a chemical, I particularly liked the book’s account of the “Victorian controversy” over who actually discovered that water molecules consist of two hydrogen atoms attached to an oxygen. Was it the aristocrat Henry Cavendish? The engineer James Watt? Or the French tax collector Antoine Lavoisier? Jha includes a deft description of the polar nature of the water molecule and its implications, and he delves particularly into the impact of polarity on our understanding of biochemistry, pointing out the usefulness of the ubiquitous hydrogen bond in water. In doing so, he certainly fulfils the brief to connect water to everyone in the most intimate sense. Most crucial, he explains, is the generation of “proton currents” (thanks to water’s ability to carry charge momentarily) for storing energy in our bodies and indeed those of most other living organisms. Paired with water’s extraordinary solvent properties, which enable it to transport elements where needed, it’s easy to see why we are made of more than 70% water.

In tackling the physics behind water, Jha takes in both the big and the small. He describes the compound’s astrophysical beginnings in chance encounters of oxygen and hydrogen atoms, then moves on to discuss the diversity and complexity in the structure of water. To learn about the many solid forms that water can take, he consults with a number of scientists who explain how 16 different solid structures of ice can form, depending on pressure and temperature. Jha then applies all of this knowledge to the question of where water might exist elsewhere in our solar system – and perhaps beyond.

The subject matter in The Water Book is admirably diverse, but in its quest to use water to connect such a wide range of scientific topics, the book does come across as a little rushed in places. Some topics that Jha promises to return to are barely covered later in the text, while various intriguing aspects of water research are introduced but not dwelled on – meaning that at some junctures, the reader is only given a tantalizing glimpse into possible depths.

To my mind, there also seemed to be a number of exciting, topical water quests that went under-explored. What about all the water in the interior of the Earth? Surely the ability of water to store energy, or the physics of quasi-liquid layers, also has an impact on our lives? I also found it surprising that some historic water controversies, such as the “polywater” debacle of the 1960s and early 1970s, were not covered. But perhaps this is churlish. Overall, I would venture to say that even the most general of “water experts” is likely to learn something new in this book. In addition to the breadth of topics covered, I genuinely enjoyed Jha’s descriptions of the voyage on the MV Akademik Shokalskiy. He brings to life the progress of the voyage in a way few Antarctic explorers could. The dramatic conclusion of the journey, and of the book, serves as the perfect testament to humankind’s struggle with water – a substance that never quite does what we expect.

  • 2015 Headline £20.00hb 384pp

Launch of LISA Pathfinder probe heralds new era in search for gravitational waves

The European Space Agency (ESA) has launched a concept mission to test the technology required for a space-based gravitational-wave observatory. Sent into space today at 04:15 GMT (05:15 CET) on a Vega rocket from Kourou in French Guiana, LISA Pathfinder will now make its way to “Lagrange Point 1”. Lying about 1.5 million kilometres from Earth in the direction of the Sun, this point in space provides a very stable environment to control the precision of the instruments on the satellite. The launch of LISA Pathfinder marks the 100th anniversary of Albert Einstein’s general theory of relativity.

Gravitational waves are distortions of space–time that occur when massive bodies, such as black holes, are accelerated. Ground-based detectors – such as the Advanced Laser Interferometer Gravitational-Wave Observatory located in Hanford, Washington and Livingston, Louisiana – are already trying to identify high-frequency gravitational waves in the 100–500 Hz range, but they have so far turned up empty-handed. As for finding lower-frequency waves, these are inaccessible on Earth because ground-based interferometers would be required to have impossibly long arms. A space-based mission, however, could pick up gravitational-waves with frequencies between 10–4–10–1 Hz from, for example, the coalescence of supermassive black holes.

Trillionth of a metre

LISA Pathfinder will use two 2 kg test masses made of gold and platinum that will float freely inside the craft and be separated by 38 cm. It will have a 20 × 20 cm optical bench – containing 22 mirrors and beamsplitters – to measure with a laser the deviations in their movements to an accuracy of a trillionth of a metre.

LISA Pathfinder will not be able to detect gravitational waves directly because their impact is so tiny that the test masses would need to be millions of kilometres apart. The experiment will, however, demonstrate that the two independent masses can be monitored as they free fall through space. The goal is to minimize external and internal disturbances to the point where the position of the test masses would be more stable than the expected change caused by a passing gravitational wave – a change in distance equal to much less than the size of an atom.

We will learn a huge amount about operating such instrumentation in space Martin Hewitson, Max Planck Institute for Gravitational Physics

“As well as testing the technologies needed for future space-based observatories, LISA Pathfinder will also allow us to characterize most of the sources of disturbance that we expect to face with such an instrument, allowing us to optimize the design of any future observatory,” says Martin Hewitson of the Max Planck Institute for Gravitational Physics in Hannover, Germany, which is a leading partner in the mission. “We will learn a huge amount about operating such instrumentation in space, so that we expect to have demonstrated the ability to place test masses in free fall at the level needed to routinely observe gravitational waves in a full-scale space-based observatory.”

If the mission succeeds, it could pave the way for a future space-based gravitational-wave observatory. One such proposal is eLISA, which would measure gravitational waves using laser beams bounced off test masses inside three identical spacecraft placed at the vertices of a virtual equilateral triangle with sides a million kilometres long. A passing wave would change the distance between the masses by just a few trillionths of a metre, causing the beams’ interference pattern to vary. Such a mission is unlikely to be launched before the early 2030s at the earliest.

Exciting time

“It is an exciting time in gravitational-wave astronomy. The flight of LISA Pathfinder will be a major milestone in the endeavour to observe gravitational waves from space by testing much of the critical technology needed to build a full-scale observatory, such as eLISA,” adds Hewitson. “The team has waited for this moment for a long time, and we can’t wait to begin the experimental campaign and get our hands on the data.”

Physicist Monica Colpi of the University of Milano Bicocca, Italy, who is also a member of the eLISA consortium board, says the launch of LISA Pathfinder will “mark the beginning of a new era for astrophysics, cosmology and fundamental physics”. “The gravitational-wave sky is clearly an unexplored frontier, and for this reason it holds great potential for discovery and, equally important, the promise for understanding one of the most mysterious interactions of nature: gravity,” she adds.

The main scientific mission for LISA Pathfinder will begin on 1 March 2016 and last for at least six months.

  • Nergis Mavalvala of the Massachusetts Institute of Technology explains how we can detect gravitational waves in the 100 Second Science video below

The great high-energy write-off

The US Congress wrote off $2bn and 10,000 person-years of effort in 1993 when it cancelled the giant, high-energy particle accelerator project known as the Superconducting Super Collider (SSC), approved in 1987. The repercussions of this decision have been severe and long-lasting. Five years later, when I interviewed one of the abandoned project’s keenest advocates, particle physicist and Nobel laureate Steven Weinberg, he was still mourning its loss. “In a way, the vote that cancelled it was democracy in action,” Weinberg told me. “The public has always been interested in things that are directly important to them – medical cures, national defence – and they have a certain general interest in cosmology. Our big failure was that we did not succeed in making the public feel excited about learning the laws of nature.” This was true despite Weinberg’s own general-interest book, Dreams of a Final Theory, which was conceived as an inspiring argument for the SSC and published in 1992. “They felt excited about putting a man on the Moon,” he reflected ruefully.

But it was not only the public and its political representatives in Washington DC who failed to support the completion of the SSC. Many US physicists, too, had reservations about the importance of its scientific agenda, its military-industrial organization and, especially, its enormous and ever-growing price tag. The last of these had the inevitable knock-on effect of reducing the funding for other fields of science. In 1989 Weinberg’s fellow physics Nobel laureate, Philip W Anderson, testified against the SSC before a Senate committee as follows: “Scientists like myself in the fields of condensed-matter physics…are caught between the Scylla of the glamorous big-science projects like the SSC, the genome and the Space Station, and the Charybdis of programmed research with ‘deliverables’ aimed at some misunderstood view of ‘competitiveness’ or at some unrealistically short-term goal.” This emboldened other condensed-matter physicists, including two Nobel laureates (Nicolaas Bloembergen and J Robert Schrieffer), to speak out against the SSC. Indeed, in 1990 feelings were running so high that condensed-matter physicists threatened, as a community, to leave the American Physical Society because of its unequivocal support for the project.

The Anderson quote comes from the brilliantly titled Tunnel Visions, an anatomy of the SSC’s failure that its authors describe as “three decades in the making”. Michael Riordan, Lillian Hoddeson and Adrienne Kolb are experienced US historians of science; the latter two recently collaborated on a history of Fermilab, the flagship US particle-physics laboratory (see “From prairie to energy frontier”). Their book is based partly on oral interviews with more than 100 participants in the SSC project, including politicians, political advisers, physicists and science journalists (but not including former presidents George H W Bush and Bill Clinton, or, surprisingly, Anderson). Other facts are drawn from published statements dating from the 1970s to the present, or from the many archives of unpublished evidence. It is not the first history of the SSC, but it is likely to be the last word on the subject. Although too lengthy and detailed for a general reader, and sometimes needlessly repetitious, Tunnel Visions will unquestionably be vital reading for anyone interested in the complications of funding “big science”, especially projects requiring international contributions.

The authors identify five chief factors directly responsible for the SSC’s cancellation, if we leave aside the project’s underlying failure to inspire the public. The first was beyond the control of the SSC’s supporters. After the end of the Cold War in 1991, the incoming Clinton administration shifted the government’s decades-long support for physics (and its possible military spin-offs) towards other kinds of science, such as genetics and climate science. The second factor was the rhetoric of the Reagan administration, which approved the SSC as an essentially national project, unlike its lower-energy European equivalent at CERN. This, combined with the subsequent failure of the first Bush administration to attract a substantial contribution to the project from any foreign government (despite Bush’s public commitment to do so and his wooing of the Japanese) meant that few non-Americans had much invested in its completion. The third factor was the choice of an unprepared site in Texas, far from any centres of high-energy physics, rather than a site in Illinois, where the project could have benefited from Fermilab’s long experience. The fourth was the poor management of the construction phase, in which there was no single project manager. Instead, a dysfunctional clash between academic physicists inexperienced in project management and engineers habituated to a military-industrial ethos produced chaos on site.

Finally, and probably most fatally, there was the escalating cost. The finished project was projected to cost $4.4bn in 1987, but by 1993 the revised estimate was running at over $10bn and heading, some feared, for $15bn – all this at a time of government cutbacks in science funding. Because of its cost, the authors report, “the SSC had crossed an invisible line beyond which sole-sourcing its management contract was politically impossible”. Its construction had become “more like building an aircraft carrier than a high-energy physics laboratory”.

Why did the later Large Hadron Collider (LHC) at CERN succeed, where the SSC failed? Parts of Tunnel Visions, especially its epilogue (“The Higgs boson discovery”), address this important question in considerable and revealing detail. In the first place, the management of CERN was not subjected to direct political interference by the European Union or national governments. Second, the LHC benefited from the contributions of more than 20 nations worldwide. Third, it was built in the same tunnel as the previous Large Electron–Positron Collider, so lessons could be learnt from the latter’s construction and operation. Fourth, it was project-managed from 1993 until its completion in 2008 by a single physicist, Lyn Evans (the son of a Welsh coal miner), who was assisted by the burgeoning World Wide Web platform invented at CERN. Finally, although the LHC certainly suffered from cost overruns – and eventually cost more than $10bn – its physicists and engineers enjoyed the strong support of CERN’s management.

As Tunnel Visions is driven to conclude: “pure-science projects at the multibillion-dollar scale should henceforth be attempted only as international enterprises involving interested nations from the outset as essentially equal partners” – as with the LHC. “Nations that attempt to go it alone on such immense projects are probably doomed to failure like the Superconducting Super Collider.”

  • 2015 Chicago University Press $40.00/£28.00hb 480pp

Universe’s missing mass found in the cosmic web

The best estimate yet of how much mass is contained within the long, tenuous threads of hot gas thought to span the vast distances between galaxy clusters has been made by a team of astrophysicists in Europe. The researchers used the XMM-Newton X-ray satellite to characterize three “filaments” of plasma extending from the galaxy cluster Abell 2744. Such filaments are believed to make up a cosmic web that permeates the universe, and the team says that the filaments are likely to contain much of the universe’s ordinary or “baryonic” matter.

Observations of the afterglow of the Big Bang known as the cosmic microwave background (CMB) suggest that protons, neutrons and other (three-quark) baryon particles only account for about 5% of the universe’s energy density – the rest is believed to consist of enigmatic dark matter and dark energy. However, the combined mass of all of the stars within a radius of about a billion light-years from Earth only amounts to about 2.5% of the energy density within that region. Computer simulations predict that the missing baryons instead exist within low-density plasma filaments millions of light-years long.

Indeed, in regions of the sky containing two galaxy clusters, smaller groups of galaxies can be seen tracing out a line between the clusters. These filaments are thought to permeate the universe, creating a “cosmic web” of galaxy clusters that is surrounded by an extremely low-density “void”. The seeds for this web can be seen in the tiny fluctuations within the CMB; as the universe expanded, gravitational attraction caused slightly denser regions to accumulate mass, while less-dense regions lost mass.

Good target

To measure the baryonic mass of several filaments, Dominique Eckert of the University of Geneva in Switzerland and colleagues looked towards Abell 2744, which is a vast galaxy cluster with a mass 1000 trillion times that of the Sun lying about four billion light-years from Earth. Like other clusters, its mass consists of galaxies (about 2%), gas (around 15%) and dark matter (80–85%). It is a good target, say the researchers, because its composition suggests that it is located at the intersection of several filaments of the cosmic web.

Although the gas in the filaments would be cooler than that in the cluster – at around several million degrees, as opposed to some 100 million degrees – it would still be hot enough to radiate at X-ray wavelengths. Eckert and his colleagues turned XMM-Newton towards Abell 2744 for 30 h last December, and studied the X-ray emission from the cluster and from a large volume of space around it.

The researchers identified three structures of interest, each of which is several tens of millions of light-years long. A close match between the position of these structures and the location of galaxy concentrations away from the centre of the cluster allowed Eckert and colleagues to conclude that they had observed filaments of the cosmic web.

Hot gas

“Where we see hot gas, we see galaxies,” says Eckert. “This is telling us that these galaxies are embedded in this cosmic web, and that we are seeing the gas within the filaments.”

The team worked out the temperature and density of the plasma in each of the filaments using the X-ray spectra and a model that simulates emission from very dilute plasmas. This revealed that each cubic metre of filament typically contains no more than a few tens of particles. While this may seem paltry, it is some 200 times greater than the average density of baryons in the universe.

Trillions of solar masses

The researchers then established what fraction of the filaments’ total mass the gas represents. To do so, they studied images from the Hubble Space Telescope and ground-based telescopes of galaxies lying behind Abell 2744, and worked out how much the light from those galaxies is bent by the gravitational pull of the intervening matter. They concluded that the filaments each weigh in at a few tens of trillions of solar masses. In other words, the researchers say, gas makes up roughly 10% of each filament by mass, with most of the rest being dark matter.

According to Eckert, galaxy surveys and numerical simulations show that most of the universe’s galaxies and dark matter lie in the filaments of the cosmic web. As such, he says, if the filaments contain significant amounts of hot gas, then that gas would contain a sizeable proportion of all baryons – about half, he estimates. “Our findings strengthen evidence for a picture of the universe in which a large fraction of the missing baryons resides in the filaments of the cosmic web,” he and his colleagues wrote in a paper published in Nature.

Who will win the Physics World 2015 Breakthrough of the Year?

Glittering gong: who will be taking home this year's Breakthrough of the Year award?
Glittering gong: who will be taking home this year’s Breakthrough of the Year award?

By Hamish Johnston

This week marks the beginning of awards season here at Physics World and we have been polishing the 2015 Breakthrough of the Year trophy in anticipation of presenting it to the winner on Friday 11 December.

The winning research must have been published in 2015 and also has to meet four criteria:
• fundamental importance of research;
• significant advance in knowledge;
• strong connection between theory and experiment; and
• general interest to all physicists.

Last year’s ESA’s Rosetta mission was our winner for the remarkable feat of landing a spacecraft on a comet while acquiring a wealth of scientific data. In 2013 the IceCube South Pole Neutrino Observatory won for making the first observations of high-energy cosmic neutrinos. But please don’t think that all the winning research is done by large collaborations. Aephraim Steinberg and colleagues from the University of Toronto were winners in 2011 for their bench-top experimental work on the fundamentals of quantum mechanics, while the inaugural prize in 2009 went to Jonathan Home and colleagues at NIST for creating the first small-scale device that could be described as a quantum computer.

We also commend nine runners-up each year who we believe deserve recognition for their contributions to physics.

So who do you think should win this year?

New laser creates a swirling vortex of light

A new laser based on a swirling vortex of light has been created by physicists in the US. The “topological-defect laser” could be a useful addition to lab-on-a-chip devices, where it could manipulate fluids and tiny particles. The design could also be modified to create beams of light with orbital angular moment (OAM).

Conventional lasers confine light by bouncing it back and forth in an optical cavity made of two opposing mirrors. Hui Cao and colleagues at Yale University and the Joint Quantum Institute at the University of Maryland have taken a new twist on this design by making an optical cavity that confines light by having it swirl around in a vortex. They made their optical cavity within a photonic crystal, which is a material containing a regular array of elements which are separated by distances on par with the wavelength of light. Light at certain wavelengths and travelling in certain directions will pass freely through a photonic crystal, whereas light not meeting these criteria will be diffracted into a new trajectory.

Topological vortex

The team’s photonic crystal comprises an array of holes in a thin sheet of gallium arsenide. Each hole is elliptical and the rotational orientations of the individual ellipses are set to create a vortex-like “topological defect”. The laser’s optical cavity lies at the centre of the defect and is a solid square of gallium arsenide with no air holes. The structure is designed so that light inside the cavity is reflected from its walls, thereby causing the light to flow around the cavity in a vortex.

Computer simulation of a resonance mode in the topological-defect laser that produces a vortex flow of light
In a whirl

The gallium-arsenide sheet also contains three layers of quantum dots made of indium arsenide. To operate their topological-defect laser, the researchers first “pump” their device using pulses from an external laser. This puts the quantum dots into an excited state, which then decays with the emission of laser light. This emission is stimulated by the trapped vortex of light and the device behaves like a laser, but with the light travelling in a circle rather than bouncing back and forth.

Twisted light

One potential application of the laser is to manipulate fluids of tiny particles in a lab-on-a-chip device. “I have been talking to people who are experts on particle sorting,” Cao told physicsworld.com. Cao also said that her team is working on a new topological-defect laser that is designed to emit “twisted light” that carries OAM. Such light could have a wide range of uses including optical telecommunications, quantum computing and new analytical techniques for chemistry and biology.

The new laser is described in a paper in Journal of Optics that can be read free of charge.

Between the lines: Christmas books

Terrible tech

A lot of ink has been spilled trying to explain how to get good ideas out of the lab and into commercial technologies. In his book Monsters, the veteran science writer Ed Regis is interested in precisely the opposite challenge: how to prevent monumentally awful ideas from getting off the drawing board. Regis’ prime example of what he terms “pathological technologies” is the hydrogen airship, or zeppelin, which enjoyed remarkable popularity in the early 20th century despite its appalling safety record. The most famous zeppelin disaster was, of course, the Hindenburg, which caught fire and crashed during a landing attempt in 1937, killing 36 of the 97 people on board. But as Regis shows, at least nine earlier zeppelins – eight German and one British – met almost identical fates, and their American counterparts were no safer despite being filled with non-flammable helium. Regis’ tone is a lively mixture of exasperation, comic understatement and black humour, and his inventiveness in coming up with fresh denunciations for zeppelins (“a technology very much worth abandoning”) is frequently a delight. Some physicists will, however, feel their hackles rising when Regis turns his rhetorical fire on the never-completed particle accelerator known as the Superconducting Super Collider (SSC). Though the SSC was not dangerous, Regis argues that it fulfilled other criteria of a pathological technology: it was physically huge; it inspired deep, almost spiritual devotion among its proponents (some of whom still mourn its loss); the scientific risks associated with it were played down; and its enormous financial cost outweighed the benefits. This verdict seems a trifle harsh, but few will question Regis’ stance on another cancelled physics experiment: Project Plowshare. The idea behind this US-based effort was to use nuclear weapons for “peaceful” purposes, such as carving out new harbours or canals. Eventually, Regis reports, the sheer lunacy of doing civil engineering with hydrogen bombs proved too much to ignore – but only after 17 years and hundreds of millions of dollars had been poured into Plowshare and its offshoots. Now that’s pathological.

  • 2015 Basic Books £19.77/$29.99hb 352pp

Quantum physicists are Go

“Astrophysics is easy,” quips a grinning Brains on the cover of Brains Explains Quantum Physics – a slim, illustrated volume in which the cerebral character from the 1960s TV series Thunderbirds takes readers on a whirlwind tour of the quantum world. The book is actually written by Ben Still, a physicist at Queen Mary University of London who works on the T2K neutrino experiment in Japan, and he/Brains begins this light-hearted chronicle by explaining that quantum mechanics did not emerge fully formed from the brain of one individual. Instead, it developed gradually as a response to contradictory and downright bizarre experimental results of the late 19th and early 20th centuries. There are good descriptions about how early modern physicists scratched their heads over blackbody radiation, the photoelectric effect and Rutherford scattering as the ideas of quantum mechanics were forming. The book also explains how quantum theory revolutionized the way chemists think about bonding – something that many physicists may not fully appreciate. Next on the agenda is a nice explanation of how the atomic measurements of Pieter Zeeman led Wolfgang Pauli to the concept of spin and his famous exclusion principle. Only at this point does the book return to the astrophysics promised on the cover, as Brains takes off in a Thunderbirds ship for the nearest neutron star, where he explains how quantum mechanics alone prevents some stars from collapsing into black holes. Much of the book is written in narrative form, and at times it reads like a “who’s who” of quantum physicists, emphasizing the human aspect of science. The only point where it falls down is when Brains presents a chart showing all the particles in the Standard Model of particle physics, with one very notable exception: for some reason, the Higgs boson is not on the chart. This seems an odd editorial decision for a book published well after the Higgs’ discovery in July 2012.

  • 2015 Cassell £10.00hb 96pp

Cooking with maths

What does “beauty” mean in mathematics? Jim Henle, a mathematician at Smith College in the US, describes mathematical beauty as “an elusive concept, subtle, abstract and intellectual” and admits that he’s “struggled for years” to grasp it. In time, though, the answer came to him: cheesecake. He isn’t joking. In fact, one of the most endearing characteristics of Henle’s book The Proof and the Pudding: What Mathematicians, Cooks and You Have in Common is just how earnest it is. The book is a decidedly strange dish, one that combines such diverse ingredients as mathematical puzzles, recipes, homespun philosophy and a dash of self-help. By rights, it shouldn’t work. Somehow, though, it does, thanks to some fun mathematics (including a proof that the 13th of the month falls on a Friday more often than any other day) and Henle’s gently encouraging tone. In mathematics, as in cooking, he writes, “stumbling around is a method”, and the best results come to those with the right combination of confidence and humility. Sometimes unexpected combinations (such as Cambozola ice cream with salted caramel sauce) work. Often, they don’t. This is okay. Keep trying.

  • 2015 Princeton University Press £18.95/$26.95hb 176pp

Feynman’s words of wisdom

So much has been said by and about the charismatic physicist Richard Feynman that it is no surprise to find that his witticisms fill a book nearly 400 pages long. The Quotable Feynman was edited and compiled by Feynman’s daughter Michelle, and it includes some 500 quotations sourced from letters, lectures, books, articles and interviews. Divided into 27 sections with labels such as “youth”, “how physicists think”, “Challenger” and, of course, “humour”, the quotations range from pithy one-liners to ponderous paragraphs. A few, such as “The thing that doesn’t fit is the most interesting”, are relatively well known, but some of the less-famous ones are just as delightful. For example, when the young Feynman discovered that Santa Claus was not real, he was apparently relieved “that there was a much simpler phenomenon to explain how so many children all over the world got presents on the same night”. Most of the quotes are not in context, and this is definitely a reference book rather than a piece of light reading. However, it does boast a wonderful selection of photographs, plus a gushingly over-the-top foreword by the particle physicist and science communicator Brian Cox. (A second foreword, by the cellist Yo-Yo Ma, is both more personal and less effusive.) All in all, the book is a good choice for those interested in the history of science, as well as a fun present for any Feynman fanboys and fangirls in your life.

  • 2015, Princeton University Press, £16.95hb, 405pp

Johannes Kepler, faithful son

In December 1615, when the astronomer Johannes Kepler should have been enjoying the modest fame that followed the publication of his treatise Astronomia Nova, he was instead confronted by a family emergency. His aged mother, Katharina, stood accused of witchcraft, and powerful enemies were plotting to have her imprisoned and perhaps even killed. Ulinka Rublack’s book The Astronomer and the Witch is a detailed portrait of Katharina’s trial, the circumstances surrounding it and the role her famous son played in defending her. Both the trial and the events that led up to it are remarkably well documented and Rublack, a historian at the University of Cambridge, mines this rich seam of primary sources to place the Kepler family’s situation in context. Within the book are descriptions of the official correspondence associated with the trial; the scientific patronage system that Kepler depended on to earn his living; the outcomes of other witchcraft trials, and many other topics. Even apparently tangential aspects of Katharina’s case are given plenty of careful attention. All of this context does, however, mean that Rublack doesn’t begin to describe the actual steps that Kepler took to defend his mother until page 119, and readers without a deep interest in history (as well as science) are unlikely to get that far. Those who stick with her story, however, will gain a much deeper understanding of the life and work of one of the most influential and revered scientists of the European Renaissance.

  • 2015 Oxford University Press £20.00hb 400pp

Loads of prezzies

How many gifts are given, in total, in the traditional song “The 12 Days of Christmas”? To find the answer, you could, of course, count up all the various calling birds, French hens and turtle doves by hand. However, if you and your true love have better things to do this holiday season, you might prefer to pick up Arthur Benjamin’s book The Magic of Maths: Solving for x and Figuring out Why. In a chapter on “the magic of counting”, Benjamin presents a general formula for this type of problem: on the nth day of Christmas, he explains, your true love will fork over n(n+1)/2 presents. But Benjamin, a mathematician at Harvey Mudd College in California, doesn’t stop there. Next, he shows how the gift-counting problem relates to patterns in a mathematical construct called “Pascal’s triangle” – a triangular table of numbers in which each number is the sum of the two numbers diagonally above it. And before you know it, he’s on to another, even more beautiful construction: the fractal pattern known as Sierpinski’s triangle. As soon as the reader has absorbed one “trick”, Benjamin is already moving on to the next one – and each is more dazzling than the last.

  • 2015 Basic Books £9.99pb 336pp
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