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Radio telescopes could give us a new view of gravitational waves

The cosmic microwave background (CMB) is a rich source of information about the early universe, and now physicists in Switzerland and Germany reckon it could also serve as a detector of high-frequency gravitational waves, which are ripples in space–time. Indeed, the researchers have used pre-existing radio observations of the CMB to calculate new upper limits on the size of high-frequency primordial gravitational waves.

The best developed technique for detecting gravitational waves, and the one used to discover them in 2015, relies on interferometry. In LIGO and other observatories, laser beams are deflected between mirrors at the ends of long (several kilometres) evacuated pipes and then interfere with one another. When a gravitational wave travels through the Earth it causes tiny changes in the distance between the mirrors, which is observed as changes in how the light interferes.

The size of interferometers like LIGO makes them most sensitive to gravitational waves within a certain frequency band – from about 10 Hz to 10 kHz – meaning that much of the gravitational-wave spectrum remains unexplored. While the planned space-based LISA observatory will target lower frequencies in the millihertz range to detect waves from supermassive black holes, observations at megahertz, gigahertz or even higher frequencies could provide a window on exotic phenomena in the very young, hot universe. Detecting these high frequencies could also provide new insights into the fundamental constituents of nature, by allowing tests of the Standard Model of particle physics at energies beyond the most powerful particle colliders.

The Gertsenshtein effect

To observe these higher frequencies, physicists have investigated a range of alternative approaches. This latest effort relies on the Gertsenshtein effect, which involves gravitational waves converting into electromagnetic waves (or vice versa) in the presence of a magnetic field.

While other researchers have looked for this effect in the results of pre-existing terrestrial experiments, Valerie Domcke at the CERN laboratory in Geneva and Camilo Garcia Cely at DESY in Hamburg have come up with a way for detecting the effect at cosmic scales. The idea is to scrutinize the spectrum of the all-pervasive CMB, which was produced about 400,000 years after the Big Bang when electrons combined with protons to form neutral hydrogen. Whereas today’s leading cosmological model tells us that this spectrum should be that of a black body, significant cosmic conversion of gravitational to electromagnetic radiation at megahertz to gigahertz frequencies would instead raise the intensity of the CMB’s low frequency “tail”.

The researchers specifically looked for distortions in the CMB spectrum generated before the first stars formed and hydrogen started reionizing, some 150 million years or so after the universe came into being. During these “dark ages” there were few free electrons to scatter photons, so the probability of oscillations occurring between gravitational and electromagnetic waves was higher than it would otherwise have been.

EDGES and ARCADE2

To set new limits on the size of gravitational waves at high frequencies, Domcke and Garcia Cely analysed data from two radio telescopes designed to peer far back in time. One, EDGES, consists of two dipole antennas and a dish located in the desert of Western Australia. The other, ARCADE2, was a balloon experiment flown over Texas.

The researchers found they could indeed use the data to set new limits, although they did have to make an assumption about the strength of cosmic magnetic fields. With the fields set low, their results were less stringent than those from putative terrestrial oscillations – the maximum amplitudes at 78 MHz (EDGES) and 3-30 GHz (ARCADE2) coming in at one part in 1012 and 1014 respectively. But with the fields set high, those limits dropped to one part in 1021 and 1024 respectively, the latter being seven orders of magnitude lower than limits imposed by the most sensitive laboratory experiment.

Domcke and Garcia Cely argue that their new approach to gravitational-wave detection could improve substantially as radio telescopes become more sensitive – particularly as scientists develop new facilities to measure the 21 cm line in neutral hydrogen, which is central to studies of reionization. More sensitive telescopes would set tighter limits on primordial gravitational waves or could even reveal their existence.  They say that this radiation could in principle be produced by sources such as merging light black holes or from clouds of dark matter around spinning black holes.

They add that excess photons with frequencies below 10 GHz have been observed by both EDGES and ARCADE2. However, they point out that this excess would imply that gravitational waves have far more energy than that inferred from other cosmological observations. As a result, they say that astrophysical sources, “are a more likely explanation for the excess radiation observed”.

A paper describing the work has been accepted for publication in Physical Review Letters.

Cryomech makes its name as a problem-solver

The Gifford-McMahon Cryocooler may be synonymous with Cryomech, but the company’s legacy isn’t defined by product. It’s far more about problem-solving and the company’s consistent success doing so since it was founded in 1963

“We understand that in science and in emerging technologies, the customer’s approach to their research is unique and we have to always be prepared to come forward with a unique product or solution for that,” says Cryomech’s business-development manager Tabitha Sebastino.

“That’s something we’re immensely proud of: that we’re able to think so creatively, consistently and reliably when it comes to solving customer problems.”

We’re able to think so creatively, consistently and reliably when it comes to solving customer problems

Tabitha Sebastino

That consistent creativity drives the routine release of pioneering technology, from the legendary Pulse Tube Cryocooler to more recent solutions like laboratory-scale helium-recovery systems. This Cryomech quest for perfection spans all its products, from cryocoolers to cryostats.

“Today researchers are very much focused on their area of expertise and they’re expecting their scientific equipment to be more reliable and just a little bit more autonomous,” Sebastino adds. “The market demands confidence that a product works as promised out of the box, and for a very long time.”

New challenges

Cryomech hq lores
Home values: Cryomech recently moved to a 76,000-square-foot, state-of-the-art building near Syracuse, New York. Courtesy: Cryomech

Cryomech evolves quickly, releasing new products almost annually. The company’s flexible nature also equipped it well for succeeding even amid a pandemic that brought production to a standstill at first.

“What Cryomech found was how to take the adaptive and quick-on-your-feet problem-solving mentality we use for product development, and shifted it to our business,” Sebastino says. “We adopted a multi-shift, seven-day-a-week operations model that not only allowed us to get our employees back to work faster in a safe and socially-distant environment, but it also allowed us to continue to fulfill customer orders. We were back shipping product again probably within two weeks of the shutdown.

“And then in September, we were able to take that manufacturing model and shift it into our new space,” she says.

That new Syracuse-based facility brings all Cryomech operations under one state-of-the-art roof. Because of unprecedented growth over the last decade or so, the old campus just a few blocks away ended up including multiple buildings. Some employees had never actually worked together.

“There’s been an interesting time of re-introduction to one another,” Sebastino says. “It leads to stronger collaboration. It also leads to stronger information and knowledge sharing which is at the foundation of what makes Cryomech great.”

That collaboration is essential at Cryomech, where they take an integrated approach to R&D.

“Our R&D process sits right next to our engineering and operations. It’s not something separate that develops in isolation that then presents products to the company, it’s a group that actually solves the company’s problems and the customer’s problems in real time,” Sebastino says. “If we can’t pull something off the shelf, that team springs into action to develop a solution for them.”

The sales and customer-service team asks a lot of questions of their cutting-edge customers. They have a deep bank of product options available and want to ensure the customer is provided the best solution for their application. If those established solutions don’t fit, R&D and engineering staff at Cryomech will look into providing a new configuration.

One example was when the company first commercialized the Pulse Tube Cryocooler in 1999, initially offering a heat lift – the amount of heat a cryocooler can remove at a certain temperature – of 0.5W@ 4.2K. It subsequently introduced a 0.75W, 1W, 1.5W and now a 2W version. Today, Cryomech offers the world’s largest selection of two-stage Pulse Tube Cryocoolers available anywhere.

Driving innovation

This air of creativity comes from all over the company. It’s by the design of the late Peter Gifford, the second-generation owner of the company who grew it from a niche operation to legitimate world leader over the course of 40 years.

“Peter took so much pride in taking people off the street right from here in Syracuse,” Sebastino says. “When you create the right environment, you can take just about anybody and really teach them the science and teach them what an important role they can play in the scientific community.

Peter Gifford
Founding father: Peter Gifford took over full leadership of Cryomech in 1980, growing it from a niche operation to world leader. (Courtesy: Cryomech)

“We really live that model,” she says. “And the ability to collaborate and the ability to share information is at the foundation of it.”

Cryomech converted to an employee stock ownership plan (ESOP) before Gifford’s death in 2017.)

“Employee ownership really was Peter’s parting gift,” Sebastino says. “He knew he was leaving his company and leaving his technology in very good hands.

“So, in selling his company to the employees, he really passed the baton to us to take this company he created and move it forward into the future,” she says.

A seat at the table

With a healthy mix of direct-sale and original equipment manufacturers, Cryomech feels uniquely positioned heading into the future. As it continues to develop new products on a customer-by-customer basis, it also strives to serve its larger partners with higher volumes of equipment linked to emerging technologies like quantum computing, fusion and large-scale magnet systems.

“We always want Cryomech to be the name on people’s minds. When they have new technology that is being developed, we want to be at that table,” Sebastino says. “We want to be the people that are really enabling these pioneering technologies of the future.”

No matter where the future takes Cryomech, it will always enhance a legacy of problem-solving that’s been more than 55 years in the making. It’s this no-matter-what spirit that drives its workforce and ensures their seat at the proverbial table of emerging technology.

“Everybody gets a solution,” Sebastino says. “When we take on a challenge, we always deliver.”

Where many have gone before

As a consummate fan of televised science fiction, it was inevitable that the task of reviewing a book entitled Space 2069 would first bring to my mind the cult 1970s series of almost the same name (Space 1999). Having finished journalist David Whitehouse’s latest book – which bears the subtitle After Apollo – Back to The Moon, to Mars and Beyond – I found myself musing, however, on the opening narration of a quite different franchise. Space, as so many episodes of TV’s Star Trek have reminded us, is the final frontier; one in which we have unparalleled potential to go “where no-one has gone before”. In this reviewer’s opinion, the same, alas, can no longer be said of publishing on the subject. When it comes to writing about our past and future exploration of the solar system in particular, the pioneer days are over, the wagon trains have arrived en masse, and to stand out on the now crowded shelves, newcomers really need to show they have access to an untapped motherlode.

It is for this reason, I think, that I found myself – while not disappointed – generally unaffected by Whitehouse’s serviceable new offering, a sequel to his widely acclaimed bestseller, Apollo 11: the Inside Story. As the US renews its interest in picking up crewed lunar exploration where the Apollo programme left off, and China continues to build the steady momentum of its own space programme, Whitehouse knowledgeably (if predictably) imagines our next 50-ish years of humans travelling back to the Moon; on to the red planet; and beyond. The book alternates between imagining the voyages of future space missions, and an encyclopaedic account of the past missions that will eventually make such journeys possible.

The book includes a number of fun titbits. For example, I was relieved to hear that coffee and hops would grow well on Mars, and was intrigued by the notion that the Moon could be used to house a backup seed bank and master library, in the event the Earth is struck by a global cataclysm. I was also amused to learn that astronauts refer to the impact of low gravity as “puffy face, bird legs”.

But some other areas were missed possibilities for engaging tangents, and left me wanting more. For example, I was keen to hear details about the “mutiny” of the Apollo 7 crew – surely it did not just involve “talking back” to ground control, as suggested? And I wanted a real discussion of the experiments that have been conducted into the psychological impacts of a simulated Mars voyage. Whitehouse mentions the longest such test – Russia’s 520-day-long Mars500 – but neglects the more captivating Sphinx-99 mission, whose chaos peaked with a fist fight and one “astronaut” locking herself away from most of the rest of the crew out of sheer frustration.

Whitehouse certainly has a formidable command of the history of space exploration, but in parts Space 2069 lets its overarching narrative slip into obscurity, behind the intense recitation of historical particulars. The most prominent example comes early on in the work, in a chapter that opens with the observation that, unlike the Earth, the geography of the Moon is unfamiliar. “At the time of Apollo many offices and children’s bedrooms had a poster of the Moon. They are seldom seen now,” Whitehouse writes, adding that “the names of the major craters and ‘seas’ are a mystery to many”.

As if to prove this point, the following 11 pages take the reader on a journey across the lunar landscape, from one such unheard-of locale to the next. With, perhaps, the aid of a map and plenty of pictures, this gambit might have worked well to engage the reader with the Moon’s alien geographies. Instead, the frenetic array of lunar placenames – Langregous, Medii, Aristarchus, Riccioli, Morteus and so on  – is overwhelming and self-defeating without visuals to help. In a similar vein, a later chapter called “22 images” is curious in showing exactly none.

In contrast, the more futurist sections of the work suffer from almost the opposite problem. The voyages of such imagined craft as the James Caird II – the first crewed vessel to orbit Mars in 2039 – are engaging (to the extent that the book could easily have been hung on such pegs alone) but would have benefited from more explanation as to the research grounding such narratives.

Space 2069 is rich, topical and informative. But so are the three other books on the same topic on my bookshelves

I should confess that a small portion of my mild disaffection with Space 2069 began even before the book’s preface, and manifests from how Whitehouse chooses to present himself in the acknowledgement’s section: as someone with personal relationships with Arthur C Clarke, Carl Sagan and Patrick Moore. Perhaps this was meant to establish his credentials, but for me, this name-dropping was ostentatious and unrelatable. I should prefer to have been informed of his background in astrophysics (a fact that I do not recall being mentioned at any point in the work) or instead introduced to his personality and humour as a writer.

This returns me to my opening point. Space 2069 is a perfectly agreeable book. It is rich, topical and informative. But so are the three other books on the same topic on my bookshelves. Without that special something – whether it be a fresh perspective, an unexpected framing device, or a particularly compelling or accessible authorial voice – there is little to commend one over the others.

  • 2020 Icon Books £16.99hb 304pp

Spin-enhanced nanodiamonds could improve disease diagnosis

Fluorescent nanodiamonds can increase the sensitivity of paper-based medical diagnostic tests, according to a proof-of-concept study from researchers in the UK. Replacing the gold nanoparticles widely used in lateral flow tests with nanodiamonds that contain nitrogen-vacancy centres led to a dramatic improvement in test sensitivity. Taking advantage of the quantum sensing abilities of these nanodiamonds could enable earlier detection of diseases such as HIV, the study authors say, improving outcomes for patients.

Lateral flow tests, such as home pregnancy tests, for example, work by soaking a paper test strip in a fluid sample. If the hormone, protein or DNA being detected is present, the test is positive and the paper changes colour, normally with a coloured line appearing. These tests are cheap, easy to use and provide rapid results, making them perfect for home-testing and use in low resource settings. But they lack the sensitivity of other diagnostic tests and are often unable to detect low levels of biomarkers in early stages of infection.

Most lateral flow tests use what is known as a sandwich assay. If you are trying to detect a viral particle – an antigen – you attach antibodies that target it to visual tags, usually gold nanoparticles, and a specific point on the test strip, the test line. If the antigens are present, they bind to both the visual tags and the test line, immobilizing themselves and the gold nanoparticles in one place on the strip. This creates a visible line that indicates a positive test.

In new research published in Nature, Ben Miller of University College London and his colleagues found that if they replaced the gold nanoparticles with spin-enhanced nanodiamonds they could make the tests many thousand times more sensitive.

These nanodiamonds have a very precise imperfection in their crystal lattice known as a nitrogen-vacancy centre. The energy structure of this point defect causes the nanodiamonds to fluoresce. And the intensity of this fluorescence can be modulated by using an electromagnetic field to manipulate the electron spin of the nitrogen-vacancy centre. This fluorescence, and the ability to manipulate it, makes these nanodiamonds attractive as potential biomarkers.

To investigate whether these nanodiamonds could work in lateral flow tests, Miller created two tests for detecting the vitamin biotin: one using gold nanoparticles as the visual tag and the other using fluorescent nanodiamonds. The researchers then tested the assays using increasing dilute solutions of biotin. They found that the nanodiamond tests were 100,000 more sensitive than those that used gold nanoparticles, and could detect concentrations as low as 0.5 molecules per microlitre, or 27 particles in a 55 µl sample.

The researchers also created a lateral flow test for detecting HIV RNA. They found that after a 10-minute amplification step, which creates multiple copies of the RNA, they were able to detect HIV RNA in a sample that contained just a single molecule.

Reading the nanodiamond test strips, however, was not as simple as just looking at them. To check the results, the researchers imaged the paper strips using a fluorescence microscope while using a microwave field to modulate the nanodiamonds’ fluorescence.

Although you could use laboratory equipment to measure lower levels of gold nanoparticles than are visible with the naked eye, Miller tells Physics World that fluorescent markers are more sensitive, as fluorescence is easier to detect at lower levels than the change in light absorption caused by the gold.

However, when researchers previously tried to use fluorescent markers in similar assays, they struggled due to the background fluorescence from the test strips. This is where the nanodiamonds come in. Miller explains that you can modulate their fluorescence at a specific frequency and then filter to detect fluorescence at that frequency, separating their signal from the background fluorescence.

“We need to do more work to make [our tests] more applicable to low resource settings,” Miller tells Physics World. He says that the team is currently developing a prototype hand-held, low-cost portable reader, based around a smartphone. This will hopefully replace the microscope, he explains, making the test more suitable for primary care settings.

Happy new year! The January 2021 issue of Physics World magazine is now out

Physics World January 2021 cover
Deep questions: Is physics linked to consciousness?

Wishing all Physics World readers a very happy and prosperous new year…let’s hope it’s better than the last one!

Now we all know that predicting the future is a mug’s game – even if it’s just trying to imagine what might happen in physics over the next 12 months.

But at a deeper level, do we even have our destiny in our hands? The notion of “free will” worries many physicists, who feel that our actions are the result of deterministic physical laws that govern the behaviour of particles, over which we have no control. Though if that’s the case, why bother with anything?

In the new issue of Physics World, science writer Philip Ball explores the debate about free will, while elsewhere there’s a great feature by Betony Adams and Francesco Petruccione on the links between quantum physics and consciousness. There may be nothing in it, but thinking about it surely beats worrying about what might happen in the real world over the next 12 months.

If you’re a member of the Institute of Physics, you can read the whole of Physics World magazine every month via our digital apps for iOS, Android and Web browsers. Let us know what you think about the issue on Twitter, Facebook or by e-mailing us at pwld@ioppublishing.org..

For the record, here’s a run-down of what else is in the issue.

• Silicon breakthrough bags award – The Physics World 2020 Breakthrough of the Year goes to researchers who have created a silicon-based material that emits light at practical wavelengths, as Hamish Johnston reports

• Astronomers mourn Arecibo collapse – The devastating collapse of the iconic Arecibo Observatory last month has left a large hole not just in astronomy but with the people of Puerto Rico too, as Liz Kruesi reports

• China space mission retrieves lunar samples – Chang’e-5 aims to return lunar material for the first time in 45 years to study the evolution history of our closest neighbour, as Ling Xin reports

• Why free will is beyond physics – Philip Ball argues that “free will” is not ruled out by physics – because it doesn’t stem from physics in the first place

• Why breadth beats depth – Niki Bell says that subject-matter experts do not necessarily make the best teachers

• Not over yet – The recent US presidential election doesn’t necessarily herald a new day for science, cautions Robert P Crease

• Powering the beast – The Internet will use a fifth of all the world’s electricity by 2025 –  and that’s no bad thing, says James McKenzie

• The light of the mind – Do quantum effects play a role in consciousness? Or are the two areas being linked simply because they are both difficult to understand? Betony Adams and Francesco Petruccione explore this developing, and contentious, field of quantum biophysics

• CERN’s new era for calorimeters – The new calorimeter for CERN’s CMS experiment is one of the most challenging engineering projects in particle physics of all time. Dave Barney explains how it will be pivotal to the success of the High-Luminosity Large Hadron Collider

• The 10 greatest predictions in physics – Over the centuries there have been many theoretical physics predictions that have rocked our understanding of how the world works. David Appell highlights what he thinks are the top 10 of all time

• Intertwined entities – Tushna Commissariat reviews Entanglements: Tomorrow’s Lovers, Families, and Friends edited by Sheila Williams

• Where many have gone before – Ian Randall reviews Space 2069: After Apollo – Back to the Moon, to Mars, and Beyond by David Whitehouse

• From physicist to patent attorney – Monifa Phillips beat the odds, becoming the first Black woman to graduate with a PhD in physics from the University of Glasgow. She describes her pathway into physics, her successes and struggles in academia, and her future in patent law

• Ask me anything – Erik Bakkers is professor of advanced nanomaterials and devices at the Eindhoven University of Technology (TU/e) in the Netherlands.

• Doomsday numbers – Peter Wright explains why R isn’t the only number to worry about

 

How the Arecibo Observatory created a scientific legacy for Puerto Rico

On 1 December 2020 disaster struck on the island of Puerto Rico. A few minutes before 8 a.m. local time the iconic Arecibo Observatory collapsed, devastating the radio-astronomy community and planetary-radar scientists. The radio telescope’s suspended platform – with its Gregorian dome focus and a plethora of instrumentation – fell after multiple suspension cables failed. The 900-tonne platform crashed into the 305 m dish lying 137 m below. It was the ending that many had feared would meet the legendary telescope, leaving a community of researchers, staff and the people of Puerto Rico in mourning.

Astronomers hold their telescopes in high regard and Arecibo was instrumental in understanding compact objects like pulsars and other remnants of once-massive stars. It was a crucial tool in studying the surfaces of solar system objects and especially for learning about potentially hazardous objects to Earth. And it was an icon in the search for extraterrestrial intelligence, known as SETI. The collapse now leaves a gaping hole in astronomy and atmospheric geoscience.

Arecibo was an engineering and architectural marvel. The dish itself, made of nearly 40,000 perforated aluminium panels, each about 2 m by 1 m, sat in a natural sinkhole in the Puerto Rican jungle. Suspended like a bridge 137 m above a canopy of trees was a platform where antennas, reflectors, receivers, platform motors and other instrumentation sat. The telescope first came online in 1963 and an upgrade in 1974 added a radar transmitter.

In the 1990s the telescope and its instrumentation were further upgraded – including the addition of the three-storey-tall Gregorian dome. In that upgrade, 12 new auxiliary cables were added to the six main ones strung between three reinforced concrete towers situated around the dish to help support the additional weight. “It is a magical place,” recalls astrophysicist Laura Spitler from the Max-Planck Institute for Radioastronomy in Bonn, Germany. “It’s greener than you can imagine. In the evening you get hit by the humidity and the sound of all the frogs chirping.”

Yet over the past 15 years, Arecibo suffered from budget reductions and in 2017 a category-4 storm – Hurricane Maria – slammed into Puerto Rico, damaging and flooding the site. In early 2018, with the observatory at risk of closing, a new collaboration – the University of Central Florida; Metropolitan University in San Juan, Puerto Rico; and Yang Enterprises in Oviedo, Florida – stepped in to manage the observatory and increase funding. But it was not enough. On 10 August 2020 an auxiliary cable slipped out of its socket on “tower 4” and swung down, carving a 30 m gash into the dish below. Once the auxiliary cable failed, the other cables had to take on more tension.

The management team brought in multiple consulting engineering firms to assess the structure and the tension on those cables, and they found that the remaining cables should hold it in place. But on 6 November, one of the four main cables attached to tower 4 snapped when it was carrying some 60% of its designed load. The environmental factors of the site over the years – from constant moisture, storms and earthquakes – had degraded the cables faster than expected. Some reports also suggest that poor maintenance may have accelerated the wear on the facility. The same independent engineering firms re-evaluated the site after the second cable failed and came to a different conclusion.

Speaking at a press conference two weeks later, Ralph Gaume, director of the division of astronomical science at the National Science Foundation (NSF), said that engineers had advised them that “the loss of one more cable on tower 4 will likely result in a catastrophic uncontrolled collapse”. Gaume and colleagues announced at that point that Arecibo would be decommissioned and safely dismantled – fixing it was no longer an option given the structural integrity and safety considerations for workers onsite.

To track any new wire breaks – each main cable was made up of about 170 wires – Arecibo staff used cameras in the main operations room and drones to monitor the situation every few hours. Further wire breaks were spotted on cables at tower 4, then on 1 December at about 7:55 a.m. local time, one of the main cables snapped, with others quickly following. The platform swung and pulled cables out of the other towers. And then the 900-tonne platform came crashing down onto the 305 m dish. Arecibo Observatory staff who live onsite heard the collapse and news spread quickly through social media. “It was just such a shock when it actually happened. I think many of us are still really taking it in,” says Robert Minchin, a radio astronomer from Universities Space Research Association, who between 2005 and 2018 was a staff astronomer at Arecibo.

A legend falls

Arecibo was a multifaceted research instrument. Its size made it the most sensitive radio telescope in the world for decades, peering into space to collect faint signals. Its observations led to the 1993 Nobel Prize for Physics being awarded to Russell Hulse and Joseph Taylor Jr for the discovery of a new type of pulsar that led to new ways to study gravity. The dish was the largest single-dish telescope in the world until 2016 when China’s Five-hundred-meter Aperture Spherical Radio Telescope (FAST) came online. One of Arecibo’s most important contributions was its series of sky surveys of compact stellar objects, mysterious pulsing radio signals and the diffuse gas between galaxies.

Arecibo’s SETI work was also iconic. Scientists would look through Arecibo data for narrow-band radio signals, such as a navigation beacon, accidental radar, television broadcasts or some other form of radio leakage from another civilization. Dan Werthimer, a researcher at the University of California, Berkeley’s SETI Research Center, and colleagues developed a “piggyback” technique at Arecibo. “We figured out a way to use the telescope at the same time that other astronomers were using it to do their sky surveys,” he says. “We would just go along for a ride.” Werthimer was also one of the developers of the SETI@home project, which launched in 1999 and used processing power on personal computers around the world while the machines were asleep to analyse data from Arecibo’s SETI search. After more than 20 years, in March 2020 the volunteer aspect of the initiative “went into hibernation”.

In 1974 Frank Drake and the late Carl Sagan used the just-installed Arecibo transmitter to send a message toward globular cluster M13, which hosts thousands of stars. Encoded in that beamed “Arecibo Message” – a message to another intelligent civilization, if they intercepted it – were graphics of DNA, a sampling of biochemicals of Earth-based life, the solar system, a stick-figure human, and a drawing of the telescope. Lasting just under three minutes, the pictorial message contained 1679 bits, arranged into 73 lines of 23 characters.

That transmitter, used for the Arecibo Message, was also what enabled planetary radar work at Arecibo. It would send radio waves toward planetary objects, such as just-discovered near-Earth objects, planetary moons and asteroids. “The signals scatter from the surface of those objects, and then we detect the echo,” explains Anne Virkki, who has led the planetary radar group at Arecibo. They compare the transmitted and received signals to learn about the object’s distance, motion, size and surface. A large portion of this work involves characterizing potentially hazardous-to-Earth objects, and Arecibo took on much of this planetary defence work. The next-best facility is some 20 times less sensitive. It is also part of the Deep Space Network, with a priority of communicating with spacecraft. The planetary defence work at Arecibo, funded by NASA, is part of this idea that the telescope could continue to evolve. “Arecibo was a telescope that kept reinventing itself,” says Werthimer. “It wasn’t the telescope that was born in the 1960s; it kept getting better and better.”

The NSF says it is not closing Arecibo, which also houses an education centre, a 12 m radio telescope and a LIDAR facility

Arecibo was doing science right up until 10 August 2020, when that auxiliary cable failed. Minchin and his colleagues had collected sky survey data the day before. Spitler, who studies transient radio sources including millisecond-long flashes of radio waves called fast radio bursts, had been monitoring one as recently as 8 August.

The National Science Foundation (NSF) says it is not closing Arecibo, which also houses an education centre, a 12 m radio telescope and a LIDAR facility. But questions remain whether the observatory can still have the same draw without the 305 m telescope. There have been calls to rebuild, producing a more advanced structure in the same location. But it’s not a simple task. “The NSF has a very well-defined process for funding and constructing large-scale infrastructure including telescopes,” says Gaume. “It’s a multi-year process that involves congressional appropriations and the assessment and needs of the scientific community. So it’s very early for us to comment on the replacement.” A petition to rebuild the telescope has had more than 100,000 signatures.

Arecibo’s legacy – a symbol of inspiration

Arecibo was more than a metal dish in the jungle. it was a place that inspired generations of people to study the universe and our place within it. “We recognize the significance of this loss to Puerto Rico and the significance of this loss to so many who have called the observatory home,“ notes Ashley Zuaderer, Arecibo programme director at the National Science Foundation. Indeed, Arecibo’s legacy is the community it built and inspired. The scientists it helped forge, the people who turned to it to understand our place in the stars, and the local Puerto Ricans who it filled with pride.

On 19 November, after the National Science Foundation announced it would decommission Arecibo Observatory, planetary scientist Edgard G Rivera-Valentín from the Lunar and Planetary Institute in Houston, Texas, who grew up in Arecibo, Puerto Rico, started the hashtag #WhatAreciboMeansToMe on Twitter, which received hundreds of responses. “Arecibo is more than an icon in Puerto Rico. It is part of our culture,” Rivera-Valentín told Physics World. “Over the past 57 years, it has woven itself into the Puerto Rican culture and has become a symbol of science and excellence in Puerto Rico. A symbol of our hopes and dreams to improve, to grow and to achieve. A symbol of inspiration.”

The 10 quirkiest physics stories of 2020

Despite the impact of the COVID-19 pandemic, physicists have still found time to carry out research that touches on the quirkier side of science. Here is our pick of the 10 best, not in any particular order.

Low-temperature LEGO

Condensed-matter physicists like to put materials under extreme conditions to tease out their interesting properties. So why not see if anything interesting happens with everyday objects? That is exactly what physicists at Lancaster University did when they put four LEGO bricks into their dilution refrigerator and cooled them to a chilly 70 mK. LEGO bricks are made from a material called Acrylonitrile Butadiene Styrene (ABS). The team found that this acts as a better insulator than many well-known bulk-insulator materials such as the glass-ceramic Macor or the plastic-based Vespel . Given that ABS is relatively cheap, the team says that 3D-printed scaffolds of the material could be an alternative to current materials used to build low-temperature equipment. Being a playful group of physicists, they also put a LEGO minifigure into the refrigerator, which not only survived the deep freeze but has also become the “world’s first LEGO cryonaut”.

Sag, settle, curl

Dried spaghetti has long perplexed physicists due to its tendency to break into three pieces when bent, rather than two. Nathaniel Goldberg and Oliver O’Reilly of the University of California, Berkeley, tackled another puzzle related to the stringy pasta. When placed in a pot of boiling water, a spaghetti strand will first sag and then settle on the pot’s bottom before finally curving in on itself into a U-shape. The researchers identify these three separate states as sagging, settling and curling, with Goldberg and O’Reilly creating a model that could predict the mechanical behaviour of the pasta as it cooks. The duo says that the model could be useful for the food-production industry and that future research could focus on the deformation of other types of pasta such as rigatoni or lasagne. It seems like the pasta-bilities are endless.

Grinding it out

You’d think that making an espresso is easy – just force hot, high-pressure water through a bed of coffee grounds. Some of us have become coffee connoisseurs during lockdown, but even for skilled baristas operating top-of-the-range coffee machines, creating cups of espresso that taste consistent from one to the next is tricky. So to discover the secret of the perfect espresso, a team of researchers led by mathematician Jamie Foster from the University of Portsmouth created a model to describe the espresso-making process. Espresso coffee normally comes in a fine grind, which maximizes the surface area of coffee that is in contact with the water. That, in theory, should result in a strong cup of coffee that gets the most flavour out of the grounds. But by examining different grinds of coffee, Foster and pals discovered that if the coffee is too fine, the water can be prevented from reaching all of the coffee grounds. The researchers’ theory is currently being tested at a coffee shop in Eugene, Oregon – which makes a mocha-ry of the notion that Americans aren’t interested in a decent brew.

Parking theory, part 2

Finding the best place to park your car might be the least of your worries during the current global COVID-19 pandemic. But when life does finally return to normal and you’re searching for that perfect car-park spot, US physicists Paul Krapivsky and Sidney Redner have some advice. Last year they examined whether it’s better to park far from your destination (which should be easy but then require a long walk) or to park nearby (which should be harder but require only a short walk). Their conclusion was that a “prudent” strategy is best, in which you park in the first gap of cars you come across but take the spot nearest to the venue. This year the duo formulated a general rule for this prudent strategy. According to Krapivsky and Redner, drivers who observe a particular rule will have a probability of finding the best spot that can be as high as 25%. Trouble is, their calculation depends on every driver observing the rules, which as we know isn’t guaranteed. And it assumes a 1D parking lot, which is the kind of place that only exists in a physicist’s imagination.

(Courtesy: Perimeter Institute)

Backgrounds to the fore

With video-conference services like Zoom all the rage this year, you still might want to impress your friends and colleagues with a background from the Perimeter Institute for Theoretical Physics (PI) in Canada. As one of the world’s top centres for physics research, it released nine science-themed images for use as a backdrop when you don’t want colleagues seeing the dirty-laundry basket, rioting kids or peeling wallpaper behind you. The images include an artist’s impression of two colliding black holes, a dark-matter map of the cosmos as well as an exterior shot of the PI building in Waterloo, Ontario. Of course, PI isn’t alone – other Zoom backgrounds are available too, including images of the ATLAS detector at the CERN particle-physics lab and shots taken by the Hubble Space Telescope.

Friday on my mind

Come Dine with Me is the brilliant British TV show in which amateur home cooks take turns hosting a dinner party for each other throughout the course of a week. Each dinner is scored in private by the other contestants on the evening it’s served and whoever racks up the highest score at the end of the week wins a cash prize. Two physicists from the University of Hamburg analysed results from 2268 episodes of the German version of the show – Das Perfekte Dinner – and concluded that you have more chance of winning if you host a dinner later in the week. Cook on a Monday and you might as well just open a jar of sauerkraut. Peter Blum and Marc Wenskat say that their finding is an example of the “secretary problem”, which arises when things are rated consecutively using the same criteria. Apparently, the application of those criteria change as each scoring occurs, skewing the results. “When competing, one should always carefully choose when to compete,” the authors write. So if you want perfection, go last.

The bottom line

Researchers revisited a classic problem in the animal kingdom this year: why can penguins poo such large distances? In 2003 researchers from Germany, Finland and Hungary – who bagged an IgNobel prize in 2005 for their efforts – found that some penguins can fire their excreta as far as 40 cm, allowing these aquatic birds to continue to nurture their eggs without sitting in a sea of faeces. Researchers in Japan modified this pooping model to calculate the maximum distance that a penguin could manage to fling their dung when at a certain height. Thanks to the penguins’ “strong rectal pressure”, which they calculate to be higher than previous work, the team finds that the maximum distance is 1.34 m. The team says this information could be “useful” for zookeepers, who would know just how far away to stand to avoid getting hit by the firing faeces. So next time you visit the penguin enclosure – do keep your faecal distancing.

Cubic earth

We’ve long dismissed the notion developed by the Greek philosopher Plato that the universe is made of five types of matter: earth, air, fire, water and the cosmos. Each was described with a particular geometry: a cube in the case of earth. However, new research shows that Plato might have been onto something after scientists in the US and Hungary measured and analysed fragmentation patterns in around 100 rocks that they collected as well as thousands from datasets collected by others. To their surprise, they found that the resulting shape of the fragments is indeed a cube. “It turns out that Plato’s conception about the element earth being made up of cubes is, literally, the statistical average model for real earth,” says geophysicist Douglas Jerolmack from University of Pennsylvania. “And that is just mind-blowing.”

Fair slices

Watermelon being sliced
(Courtesy: Timoteo Carletti)

What’s the fairest way to slice up a watermelon? Physicists in Belgium, France and Italy tackled the problem using geometry and calculus. After cutting the whole watermelon in half along its length and then in the middle to yield four equal quarters, the researchers discovered that this “half rule” fell away when then trying to slice the watermelon up into equal thin portions. Instead, they found a “2/3 rule”. So for a spheroid of length 10 cm, for two slices, the first slice should be made at 3.5 cm along the length but for three slices, the first slice should be 2.1 cm and the second 4.2 cm. After doing the calculations, the researchers tested them on a 4 kg watermelon and used Archimedes’ principle to confirm that the slice volumes were equal. Eureka!

Borderline collider

And finally, in 1977 the Nobel-prize-winning physicist Leon Lederman published a tongue-in-cheek proposal to build a collider using existing subway tunnels in New York. The city was then in financial crisis and Lederman, who died in 2018, reckoned that physicists could acquire the tunnels for a knock-down price. It never happened, of course, but his proposal has now inspired Caltech physicist David Hitlin to propose building another collider to address the politically controversial wall along the US–Mexican border. In a preprint paper, Hitlin describes how long, straight sections of the border between the states of Sonora and Arizona could be blocked by a huge linear particle collider. Stretching some 300 km long, he reckons the machine could achieve a centre-of-mass energy of 5   TeV. The proposed International Linear Collider (ILC) in Japan, in contrast, would be a mere 20 km long and have an initial energy of just 250 GeV. And what would Hitlin call the facility if it was built? The TrumpILC, of course.

You can be sure that next year will throw up its fair share of quirky stories from the world of physics. See you in 2021!

Medical physics highlights of the year

This year has been a year like no other. In 2020, many physicists turned their research efforts towards tackling the pandemic. Within medical physics, researchers worked to develop improved diagnostics and potential treatments for COVID-19, as well as coming up with innovative technologies and devices to help healthcare workers.

Alongside, the medical physics community continued to innovate in the more traditional areas of research – from new approaches to delivering radiotherapy to novel diagnostic imaging devices and techniques. Here are a few of the highlights that caught my eye.

Tumour hypoxia tracked in real-time

Hypoxia, or lack of oxygen, in a tumour can make it resistant to radiotherapy. But currently, there’s no method to monitor tumour oxygenation noninvasively or without averaging across the whole lesion. In January, a team headed up at Dartmouth-Hitchcock’s Norris Cotton Cancer Center demonstrated that Cherenkov excited luminescence imaging (CELI) could be used to non-invasively image oxygen distribution in tumours during radiation delivery.

Tracking hypoxia
Oxygen partial pressure in radiosensitive (top row) and radioresistant (bottom row) tumours during radiotherapy. (Courtesy: CC BY 4.0/Nature Commun. 10.1038/s41467-020-14415-9).

Brian Pogue and his team irradiated tumours in mice with X-rays, generating Cherenkov light that served as an internal source to excite a phosphorescent probe. They examined two tumour lines, one that’s responsive to radiation and one that’s radioresistant, and saw differences in tumour oxygenation that reflected their differences in response.

The researchers concluded that time-resolved CELI offers high spatial resolution and could be easily added to clinical protocols to evaluate tumour oxygenation at the time of radiation delivery – a long-sought goal in cancer therapy.

Real-time dosimetry for FLASH radiotherapy

Ultrahigh dose rate radiation therapy (FLASH) is a hot topics in radiotherapy research. Animal studies have shown that FLASH can destroy tumours while vastly reducing damage to normal tissue, and clinical trials are just beginning. But with such high dose rates, it is vital to continuously monitor dose deposition in the patient. Researchers at the University of Michigan have proposed a new technique, ionizing radiation acoustic imaging, that can measure dose while simultaneously obtaining images of the radiation target and surrounding tissue.

FLASH dosimetry
Combined ultrasound and ionizing radiation acoustic images of a rabbit liver phantom. (Courtesy: Med. Phys. 10.1002/MP.14358)

The method is based on a thermoacoustic effect: as ionizing radiation deposits energy in the patient, the tissue temperature increases, causing expansion and a propagating pressure wave. In conventional radiotherapy, these waves are extremely weak. But with FLASH dose rates (40 Gy/s or more), the signals can be detected by ultrasound probes on the patient’s skin.

First author Ibrahim Oraiqat and colleagues showed that this acoustic imaging signal increased linearly with the dose-per-pulse and that measurements at different depths agreed with those from commercial film dosimeters. They also demonstrated simultaneous acoustic dosimetry and ultrasound imaging in a moving rabbit liver phantom.

Imaging airflow through the lungs

Cystic fibrosis (CF) is a hereditary disease that causes sticky mucus to build up in the lungs, hindering breathing and leading to lung infections. The lung damage caused by CF is often non-uniform, and treating the patches of damaged tissue can slow disease progression. To quantify and localize such damaged regions, researchers in Australia have developed a novel tool to measure regional lung function.

The team, led by Freda Werdiger from Monash University, used X-ray velocimetry (XV) to non-invasively generate high-definition images of real-time airflow through the lungs of mice. XV combines high-speed imaging – in this case, propagation-based phase-contrast X-ray imaging – and post-processing analysis to produce a detailed ventilation map of the lungs. The researchers tested the technique in mice with CF-like lung disease and their healthy littermates. Maps of regional lung expansion clearly showed the presence and locations of areas of airflow deficits in the diseased animals. Following its recent commercialization, this technology could help improve the length and quality-of-lives of people with CF and other respiratory diseases.

Non-invasive skin cancer detection

Suspicious skin lesions are usually identified by dermatologists using a handheld optical magnifier, and subsequently diagnosed via pathological analysis of a tissue biopsy. This process, however, is invasive, costly, time-consuming and dependent upon the skill of the physician. To address these shortfalls, Abraham Katzir of Tel Aviv University and co-researchers are creating an accurate, affordable clinical system that can identify skin cancers in near-real-time.

Skin cancer diagnosis
When the U-shape in the centre of the fibre touches a skin lesion, the system measures its characteristic mid-IR absorption spectrum. (Courtesy: Abraham Katzir)

The team has developed a fibre-optic evanescent wave spectroscopy system based on a long, U-shaped, mid-IR transmitting fibre connected to a mid-IR spectrometer. By simply touching suspicious regions for 30 s with centre of the fibre, the system could identify cancerous lesions – including melanoma, basal cell carcinoma and squamous cell carcinoma – on patients’ skin.

Katzir suggests that in the future, this non-invasive “spectroscopic pathology” could replace standard invasive biopsies.

A portable brain MRI scanner

As 2020 drew to a close, we reported on a low-cost, portable brain MRI scanner being developed by researchers at Massachusetts General Hospital/Harvard Medical School. MRI is the standard modality for assessing neurological disorders, but conventional high-field scanners are expensive, immobile and require dedicated power and cooling infrastructure. As such, MRI is unavailable to critically ill patients who cannot be safely transported to the scanner or patients in low-resource settings.

Portable brain MRI
A low-cost, portable brain MRI scanner could be mounted in an ambulance or installed in small clinics around the world. (Courtesy: Massachusetts General Hospital)

To address this, Clarissa Cooley and colleagues used an array of neodymium rare-earth magnets to generate an 80 mT static field. This permanent magnet does not require external power or cryogenic cooling, allowing the researchers to build a truly portable MRI brain scanner that can be wheeled around on a cart and operated from a standard power outlet. They demonstrated that the system could perform standard brain scans employed for diagnosing and monitoring clinically important brain pathology. Such a portable scanner could be used in many locations, such as a patient’s bedside, at the scene of an accident or in a rural clinic – expanding access to MR neuroimaging.

Global map of tiny ‘third-degree tides’ made using satellite observations

Third-degree tides – tiny sea-level fluctuations once known only locally from measurements made by tidal gauges – have been mapped out across the globe by geophysicist Richard Ray at NASA’s Goddard Space Flight Center in Maryland, US. As well as helping to refine ocean tidal predictions, the work could find application in geodesy and in understanding the behaviour of the Earth’s crust and mantle, which tides affect.

In simple terms, tides occur because the Moon’s gravitational tug causes the oceans to bulge out in two places – one positioned beneath the Moon and the other on the opposite side of the Earth. The gravitational potential can be expressed mathematically in terms of latitude and longitude using spherical harmonic functions.

To model the Earth’s dominant tidal patterns, explains Ray, “it suffices to use just the three spherical harmonic functions of degree two. However, that is only a first approximation.” In reality, the tidal bulges are ever-so-slightly asymmetrical, with one side larger than the other.  “To express that mathematically, we need higher degree spherical harmonic functions. Specifically, here we use functions of degree three,” says Ray.

Swamped in the data

The “third-degree” tides that arise from the tiny asymmetry are very small and their signal is easily swamped in tidal data – both by measurement noise and genuine non-tidal oceanographic signals.

“The tides are fairly obscure, because they are so small, but they can be detected in coastal tide gauges if the time series is long enough, say 10 years or longer,” Ray explains. “Away from these isolated tide gauges, little information about them has existed; no one had ever seen a global map of these waves from direct measurements.”

Today, maps of second-degree tides on the open ocean are made today using satellite altimetry observations. Records have been collected for some three decades, so researchers like Ray can search for third-degree tides. “The nice thing about going fishing for a time-coherent signal – like tides – is that after lots of averaging, even a tiny signal can eventually begin to emerge from the background noise.”

In his study, Ray worked with altimetry data collected by the TOPEX/Poseidon and Jason satellites, which operated from 1992–2005 and 2001–present, respectively. Ray used an averaging process that involved using a least-squares fit to sine waves of known frequency, which revealed the tidal amplitude and phase at each ocean location.

Distinctive pattern

The findings revealed a pattern of third-degree tides that was quite distinct from their second-degree counterparts. Ray says that this is a result of the spherical harmonic forcing being so different in each case. “Depending on the tidal forcing, its frequency, the ocean depth, and the shape of a basin, tidal waves can have certain resonances, some with unusually large amplitudes,” he adds.

The maps revealed that in the Atlantic Ocean, for example – where regular, second-degree diurnal (daily) tides are relatively suppressed – third-degree tides are relatively large. They reached even greater amplitudes in the Indian Ocean. In the South Pacific, meanwhile, third-degree tides were suppressed and barely reached 2 mm in height.

Philip Woodworth, a researcher at the UK’s National Oceanographic Centre in Liverpool says, “The largest third-degree tide, called M1, has been mapped previously around parts of the global coastline and at islands using tide gauges, but this is the first time they have been mapped over the whole ocean”.

Edward Zaron at Oregon State University adds, “It is truly astounding when you consider that the millimetre signals are here being extracted using data from satellites flying above the Earth at about 1300 km elevation, and it is a testimony to the precision of the satellite orbit determinations and geodetic sciences in the current age”. He concludes, “Richard is a master of these analysis techniques and their application to satellite altimetry”.

With his initial study complete, Ray is now looking to refine the accuracy of his tidal maps. “The results are still fairly noisy,” he says. “I’d like to apply formal data assimilation methods, which combine theory from fluid dynamics with measurements, to get the best of both.” He adds that the collection of more altimetry data in the future will also yield better results.

The research is described in Science Advances.

Nanotechnology and materials highlights of 2020

Regular readers of Physics World know that we have a penchant for materials and nanotechnology research. I developed my fondness for materials physics when I did a PhD many years ago on the magnetic properties of ultrathin films and my interest has never waned. So here are five of my favourite materials and nanotechnology stories of 2020.

Snake vision inspires pyroelectric material design

Bioinspiration and biomimicry involve studying how living organisms do something and using that insight to develop new technologies. Pit vipers have two special organs on their heads called loreal pits that allow them to “see” the infrared radiation given off by their warm-blooded prey. Now, Pradeep Sharma and colleagues have worked out that the snakes use cells that act as a soft pyroelectric material to convert infrared radiation into electrical signals that can be processed by their nervous systems. As well as potentially solving a longstanding puzzle in snake biology, the work could also aid the development of thermoelectric transducers based on soft, flexible structures rather than stiff crystals.

Perovskites could be platforms for exciton condensates

Is there anything that perovskites cannot do? This family of crystalline materials is usually associated with high-performance solar cells, but perovskites are true wonder materials that are finding increasingly exotic applications. Now, researchers have shown that certain perovskites could be ideal platforms for creating Bose–Einstein condensates (BECs) of excitons. Excitons are quasiparticles that comprise an electron–hole pair (and we do love a quasiparticle at Physics World). BECs are normally made from atomic gases that must be chilled to near absolute zero. However,  Kai Chang and colleagues reckon that exciton BECs in perovskites could exist at a balmy 77 K.

Rippling graphene harvests thermal energy

There is no shortage of clever applications for graphene – a sheet of carbon just one atom thick – but research really fired my imagination this year. The rippling thermal motion of a tiny piece of graphene has been harnessed by a special circuit that delivers low-voltage electrical energy. The system was created by  Paul Thibado and colleagues, who say that if it could be duplicated enough times on a chip, it could deliver “clean, limitless, low-voltage power for small devices”.

Supercurrent goes to the edge

The topological properties of matter have been a very hot topic over the past few years, so it is no surprise that in 2020 physicists have observed “topological superconductivity” for the first time. Nai Phuan Ong and colleagues have measured a robust supercurrent at the edge of a superconductor that is very different to the supercurrent in the material’s bulk. The team does not yet fully understand the reason for why the edge supercurrent remains independent of bulk supercurrent, but they believe it could come from the topologically protected edge states in the material.

Tipsy sludge worms simulate active polymers

Polymer strands are often described as worm-like, so why not use living worms to gain insights into polymer materials? And if you want to alter the behaviour of the worms, there is no better way than to give them a stiff drink. That is exactly what Antoine Deblais and colleagues did – using worms to gain new insights into the properties of poorly understood “active polymer” materials by measuring the viscosity of clusters of sludge worms as they were subjected to shear forces. The wriggling activity of the worms was controlled by adjusting their temperature, and the creatures were temporarily knocked out using alcohol.

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