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Switching oil type could cut emissions

The average greenhouse-gas emissions from the entire life cycle of shale and other “light tight” oils are two-thirds of those for heavy oil and bitumen resources, researchers in the US have found.

The result comes alongside an appeal to policy makers that we should be concerned not just with minimizing our consumption of oil, but on the type of oil that we do use, in order to reduce climate change.

“The main attention is on reducing oil consumption – which is [the] right direction,” said Mohammad Masnadi of Stanford University. “But … policy makers should pay equivalent attention to crude oil type and the corresponding greenhouse-gas emissions for designing future policies and strategies.”

Oils come in various forms, but broadly speaking there are three types: conventional crude, light-tight oil from low-permeability rocks such as shale, or particularly viscous heavy oil and bitumen from sands. In the early part of this century, investment in light-tight oil and heavy oil/bitumen rose dramatically as oil prices boomed.

That’s good news for meeting demand, but less good for the climate. Three years ago, Christophe McGlade and Paul Ekins at University College London in the UK estimated that to keep global temperature rise below 2 °C above pre-industrial levels, in accordance with the Paris agreement, humanity must leave over one-third of current oil reserves unburned.

Like most other climate researchers, Masnadi and colleagues believe that policy makers need to reduce our oil consumption as much as possible. But that is not enough, they say: given the excess of resources, we also need to choose wisely which oils we burn.

To assess the wisest choices, the researchers developed three oil life-cycle assessment tools to individually compute the greenhouse-gas emissions from oil extraction, oil refinement and final combustion or other usage. After integrating the tools into a single “well to wheel” tool, they applied it to data on 75 different oil fields worldwide.

The average (median) life-cycle emissions for light-tight oils were two-thirds of those for heavy oil/bitumen, a difference of some 200 kg of carbon dioxide equivalent per barrel. Opting for lighter oils could be a climate-change mitigation opportunity worth 10–50 gigatonnes of carbon dioxide equivalent by 2050, the team found.

“We know that we have abundant resources to address the supply,” said Masnadi. “Therefore, we need to select the crude types strategically in order to mitigate as much greenhouse-gas emissions as possible.”

Masnadi and colleagues’ work on this is not over yet. They want to expand their analysis from 75 sites to all the world’s oil fields, and understand the global climate implications. Then they want to connect the emissions information to cost indices, to explore the specific mitigation policies that would have most impact.

The study is published in Environmental Research Letters (ERL).

  • This article was updated on 11 December 2018 to correct the difference in emissions from “one and a half times lower” to “two-thirds”.

Swarms of robotic cockroaches navigate around obstacles

Robot barrier
Team effort: robots push a corral through an aperture. (Courtesy: A Deblais et al/Phys. Rev. Lett.)

Swarms of rod-like, self-propelled robots have been made to work together to navigate obstacles in a new study by Hamid Kellay of the University of Bordeaux and colleagues. Resembling cockroaches, the toy robots were confined to corrals where they self-organized into distinctive patterns. When the corrals were flexible and moveable, the robots could join forces and move the corrals around obstacles. The research could lead to practical applications including soft, agile robots that resemble living cells and can move through complex biological environments.

Large groups of rod-like particles that propel themselves forward are known to display a range of fascinating behaviours. Previous studies of the dynamics of these systems have typically focused on how unconfined self-propelled particles dissipate over time. However, little attention has been paid to the inertia of such systems, which defines their overall direction of motion.

To explore these dynamics, Kellay’s team use rod-shaped robot toys with asymmetrical legs. Each is about 4 cm long and contains a battery-powered vibrator, which drives it forward. When just a few robots are confined to a corral, the robots move around randomly and resemble molecules in a gas. However, when the robot density is increased, a certain fraction of the robots lined-up in large clusters against boundaries at either end of the corral. The rest of the robots behaved like a gas and as time progressed, a well-balanced equilibrium emerged as individual robots switched between the cluster and gas phases.

Turning corners

The team also used a flexible and light-weight corral that could be pushed around by the robots. In this case, the robots could join forces and push the entire corral thorough a gap between two barriers (see figure), turn corners and move the corral around an obstruction.

By tracking these motions, the team could quantify the inertial behaviour of the corral boundary in terms of several parameters related to robot motion. Using these values, they created simple yet accurate computer simulations of the system.

The discoveries made by Kellay’s team could aid the design of microscopic robots, consisting of many self-propelled particles confined in flexible, moveable boundaries. The devices would have highly tuneable and programmable properties. This could help them to navigate nimbly through complex biological systems, accurately mimicking the motions of microbes such as bacteria.

The research is described in Physical Review Letters and videos of the experiments can be viewed on Physics.

Mobile CT lines up for adaptive proton planning

CT is an integral part of adaptive image-guided proton therapy (IGPT). It is used to monitor changes in a cancer patient’s anatomy caused by weight loss and/or tumour shrinkage, as well as for treatment plan adaption. CT simulation scans are usually performed in imaging suites outside proton therapy treatment rooms. This set-up, however, can cause workflow inefficiencies and inconvenience for both staff and patients alike.

Compact mobile CT systems may change all this, by enabling scanning to be performed within proton treatment rooms. The Center for Proton Therapy at Orlando Health UF Health Cancer Center has successfully installed a mobile CT system for patient localization and set-up in its compact proton therapy vault. The research team has now described the commissioning process and the dosimetric implications of adaptive planning with the mobile system (J. Appl. Clin. Med. Phys. doi: 10.1002/acm2.12319).

The scanner (the AIRO Mobile CT System) being commissioned for use with the centre’s Mevion S250 proton therapy system is a 16-slice helical scanner that acquires images with 120 kV, 10-250 mA and a field-of-view (FOV) up to 51.2 cm. Designed for intraoperative surgery, the large FOV enables the scanner to capture the entire patient surface including immobilization devices and the treatment couch.

The commissioning process

A critical part of the commissioning process is setting up the stopping power curve for an in-room CT scan so that dose calculations on the scanner are dosimetrically matched to the treatment planning system.

“This groundwork is important so that if adaptive planning is performed on the in-room CT, one can be certain that the changes in dose are due to changes in anatomy and not to differences in the CT scanner or scanning protocols,” medical physicist Twyla Willoughby told Physics World. “This is very important in being able to make clinical decisions regarding adapting a treatment plan. When comparing two different CT scanners for dose comparison, any changes in CT values and in the calculated stopping powers can lead to changes in the dose along the proton path or to a change in the range of proton therapy.”

To do this, Willoughby and colleagues scanned an electron density CT phantom on a simulation CT scanner and the mobile CT, and compared the mean CT numbers to determine differences. They imaged a phantom containing 16 rods and 13 tissue substitute materials with varying plug patterns, table heights, and mA with fixed 120 kV. Images of plugs representing brain, lung 300, lung 450, cortical bone, adipose, breast, liver, solid water, and true water were analysed. They then determined the stopping power ratios (SPRs) by entering averaged CT numbers into a stoichiometric SPR calculation algorithm.

The last step of the commissioning process involved confirming dosimetric equivalence for dose calculated on CT scans from the two scanners. The researchers developed heterogeneous, single-field, non-robust plans on thorax, pelvis and head phantoms, to test the dose accuracy for proton beams traversing large areas of heterogenous media. They also generated five different clinically reasonable treatment plans on five different phantoms to test the accuracy of the adaptive system in common clinical scenarios.

Key findings

Lead author Jasmine Oliver and colleagues reported that proton dose calculations on CT image sets acquired by the mobile CT scanner could be used to calculate dose with relatively high accuracy, similar to the simulation scanner.

They cautioned that dosimetric equivalency testing, using visual display of isodose lines and water-equivalent thickness (WET) values between the planning and in‐room CT scanners, should be performed before any in-room CT system is deployed for adaptive planning purposes.

Test results showed that CT numbers differed between the scanners. Low-density plugs had a higher CT number in the mobile CT compared with the simulation scanner, while high-density plugs had a lower number.  Dose on the mobile CT extended deeper by about 5 mm compared with the original treatment plan.

To create equivalent dose distributions, it was necessary to adjust the SPR curve’s low-density data points of the mobile CT, to obtain better proton beam range agreement based on isodose lines. When the authors compared the stochiometric-based SPR curve and the “dose-adjusted” SPR curve, they observed slight improvement on gamma analysis between the treatment plan and the mobile CT plan for single-field plans at the 1%, 1 mm level. Clinical plans at 3%, 3 mm demonstrated equivalent dose.

“Our results demonstrated that performing the stoichiometric analysis for a given phantom and CT scan may not provide dose equivalence between two different CT scans… it was important to verify the dosimetric equivalence of the two CT image sets with their corresponding stopping curves,” wrote the authors. “To achieve this, it was necessary to directly map CT values and adjust them to yield better dosimetric comparisons at the end-of-range.”

The mobile CT system in the proton treatment vault is currently used to perform “re-simulations” for patients who may have anatomical changes due to radiation therapy. “It is used on all of our breast patients to monitor target swelling, on lung patients to monitor fluid in the lungs and tumour changes, and on head-and-neck patients to monitor tumour shrinkage,” Willoughby explained. “These things dramatically affect the proton range and modulation, and can cause significant changes in the treatment plan if they go unmonitored.”

The cancer centre does not offer pencil beam scanning (PBS) proton therapy. However, the authors believe that, based on their experience, the image quality of the mobile CT scanner is good enough for dose-recalculation on PBS as well as double-scatter systems.

A day of light

This month sees the first International Day of Light. Wednesday 16 May was chosen because it is the anniversary of the first successful operation of the laser, as demonstrated by the American engineer and physicist Ted Maiman in 1960.

It’s a good choice, because the laser is a perfect example of how a scientific discovery can yield revolutionary benefits to society in all sorts of areas, including communications, healthcare and manufacturing. However, when I read the words “first successful operation of the laser” on the International Day of Light website (lightday.org), I had to look further, as it sounded like there might be more to the story.

I have spent most of my career working in photonics, optical communications and lighting, so I was already somewhat familiar with the laser’s history. However, the details still interested me. It turns out that although Maiman did indeed demonstrate the first working laser on 16 May 1960, he is not the only person with a reasonable claim to have “invented” the laser. The other is Gordon Gould, another US physicist who described “Some rough calculations on the feasibility of a LASER: Light Amplification by Stimulated Emission of Radiation” in his lab notebook in November 1957.

Laser blues

This was the first recorded use of the famous acronym, and Gould had the foresight to get his notebook notarized. However, he clearly had poor advice in other areas, because he didn’t patent his idea, incorrectly believing that he needed to demonstrate a working device first. In his defence, in the early days of the laser, people were aware it had proved a theory described by Einstein in 1917 but felt it was an invention looking for a job.

Gould was forced to endure a 30-year‑long legal battle before he was eventually awarded a string of laser-related patents and millions of dollars in back royalties. This is a key point in commercializing technology: future inventors should make sure they get the kind of patent advice Gould received later in his career, rather than at the start.

There is, of course, a lot more to light than just the laser. I don’t think anyone has ever really worked out the full impact of light on society, but here are a few numbers:

  • The global lighting market (for lamps, street lights and so on) is worth about $120bn;
  • The global optical communications market, with the laser at its core, is worth about $15bn, and includes the network of optical fibre that underpins the high-speed Internet;
  • Firms in the UK Photonics Leadership Group contribute almost £13bn to the British economy and employ 65,000 people, making it larger than the UK pharmaceutical industry.

None of these markets developed overnight. When I started in this field in 2007, for example, everyone was getting very excited about LED lighting and were predicting efficiencies of more than 200 lm/W (where the lumen (lm) is the unit of luminous flux per watt that determines the efficiency of a light source). People were also talking about incredible lifetimes of 50,000 to 100,000 operating hours. Both figures were seen as a great improvement on incandescent lamps, which last for 1000–3000 hours and have efficiencies of 10–20 lm/W.

However, the first products were expensive, even if they did mostly work as promised (if engineered properly). That meant that early on, LED lighting was sold on a “total cost of ownership” basis and was adopted in areas where the cost of maintenance was high, such as tunnels, high ceilings and other inaccessible places where it costs a lot to change a light bulb. Only now – a decade later – are LED lamps heading past the 200 lm/W mark and becoming as cheap as any other lighting.

How did this happen? Well, one reason is the US Department of Energy (DOE), which committed to an ambitious roadmap between 2000 and 2020 for developing better light-emitting diodes, phosphors, packaging and drive electronics. Additional impetus came from a near-global phased removal from the market of inefficient incandescent lighting led by the EU, US and Japan. The goal was to improve the efficiency of LEDs from around 20 lm/W to at least 200 lm/W while also cutting costs dramatically, from $5 to pennies for one LED.

At the beginning, there was so much to do, but the goal was clear and a combination of funding, subsidies and visibility helped everyone get behind the plan. All the tricky technology issues were solved rapidly by a mix of academic research and commercial R&D. This progress got the attention of the global market. The Chinese government, in particular, saw an opportunity to make a big bet on LEDs, and as a result China has become one of the biggest manufacturer of LED devices, packages, lamps and lighting fixtures in the world.

Promises fulfilled

From a global perspective, though, what is important is that we really did get all the energy savings we were promised, along with excellent lifetimes. LEDs and associated cheap “everlasting” lighting products are therefore expected to replace all other light-source technologies almost entirely within 10 years.

Of course, this poses a problem for lighting companies, as the business model – selling a fixture and providing replacement light bulbs for it – has not changed since the days of Edison. The lighting industry has tried to “fix” this before, in the late 1920s and 1930s, when the Phoebus cartel limited light bulb lifetimes to 1000 hours. Members of that cartel included several companies that still exist, including Osram, General Electric (GE) and Philips.

However, those days are behind the lighting industry now, and today’s focus is on innovations such as the digitization of lighting and “human-centric” lighting designed to mimic the Sun and make all of us sleep, function and feel better. That seems like a more noble approach than planned obsolescence, and it’s certainly something to celebrate – on 16 May or any other day.

Light up your life

Cast your mind back to 2008 and think about what you remember of that year. Perhaps it was the global banking crisis and Gordon Brown’s alleged rescue of international finance. Maybe it was CERN switching on the Large Hadron Collider and then swiftly needing to repair its blown magnets. Or perhaps it was all the talk about high-temperature iron-based superconductors. But whatever your personal recollections, you surely won’t remember that the United Nations (UN) declared 2008 to be the International Year of the Potato.

Now it’s easy to laugh at the idea of celebrating potatoes, which is why I just did, but then the UN has been good for science too. In the decade since the potato was king of the crop, we’ve had international years devoted to astronomy (2009), chemistry (2011), crystallography (2014) and then light (2015). Physics, of course, took centre stage in 2005 to mark the centenary of Einstein’s papers on Brownian motion, the photoelectric effect, special relativity and E = mc2.

The International Year of Light was an apparent success, with organizers claiming more than 13,000 activities in almost 150 countries attended by an audience of more than 100 million. You might wonder, therefore, why we need an International Day of Light, the first of which is to be celebrated on Wednesday 16 May. Surely we’ve “done” light?

Not so, according to the organizers, who kick off with a launch event today at the headquarters of the UN Educational, Scientific and Cultural Organization (UNESCO) in Paris. The goals are ambitious: improve the public’s understanding of light; show the link between light and culture; highlight research and careers in light science; and seek ways to curb light pollution. But to me the most worthwhile effort is promoting alternative sources of light for people in developing nations not connected to the electricity grid, many of whom rely on dangerous kerosene lamps for light after dark.

My favourite is GravityLight – a lamp attached to a heavy 12 kg bag. Simply lift the bag up to a height of about 2 m with your hands and, as it descends slowly over the course of half an hour to the floor, gravitational energy is converted into electrical energy to turn on a light-emitting diode (LED) bulb. It’s a beautiful idea with some simple physics principles at its heart.

So my message is – don’t let the International Day of Light pass you by. There are lots of activities and events to get involved in. If nothing else, check out the Physics World website on 16 May for some light-themed treats in our special light-themed collection or read this month’s Focus on Optics and Photonics. It would be a shame if the International Day of Light went the way of the International Year of the Potato, which is to be well and truly forgotten.

Bioprinting builds 3D model of a brain tumour

Patients who are newly diagnosed with high-grade gliomas (GBMs), one of the most aggressive brain tumours, only have a median survival time of around 15 months – reducing to just 5–7 months for recurrent tumours. Glioma stem cells are thought to be at the root of these poor outcomes, so researchers are focusing on therapies that could target these cells. An important first step is to develop realistic models that will enable scientists to study the biology of glioma stem cells and to investigate the resistance of GBMs to chemotherapy.

To date, researchers have mainly exploited 2D monolayers of glioma lines as a model for the tumour, providing a way for studying how gliomas evolve and how they react to anti-cancer drugs. However, this model fails to take into account the 3D environment of the tumour, and it doesn’t allow researchers to study other significant factors such as cell–cell and cell–matrix interactions, spatio-temporal signalling and metabolic gradients. Unfortunatel,y this means that most anti-glioma drugs that proved to be effective in vitro have failed miserably in clinical trials.

Now, a team of researchers led by Tao Xu and Qin Lan from Soochow University, Tsinghua University and the Tsinghua-Berkeley Shenzhen Institute, all in China, and Medprin Biotech GmbH in Germany, have turned to 3D bioprinting to create a glioma stem-cell model. “Our work shows that we can use bioprinting technology to build 3D glioma models,” explains team member Xingliang Dai. “This is just beginning of our studies on the glioma microenvironment.”

Building a glioma stem-cell model

The advantage of bioprinting is that it can be used to fabricate complex 3D biological structures by building up layers of bioinks, essentially biomaterials mixed with cells. In the novel technique reported by Xu and his colleagues, which they reported in the journal Biofabrication, the researchers created a porous gelatine/alginate/fibrinogen hydrogel structure that mimics the extracellular matrix of glioma stem cells. They made the hydrogel more stable by adding the cross-linker transglutaminase (a non-toxic transferase that naturally exists in the human body) to reinforce the gelatine, which is the main component of the structure.

The team are now working to upgrade and improve their current system, and to optimize the bioinks used for printing. They also need to enhance the design the tumour model so that it more accurately mimics tumours in the body. “We have made much progress since the publication of the Biofabrication paper – including the fact that we observed differently expressed transcriptase profiles of the 3D bioprinted glioma stem cells compared to 2D-cultured ones,” says team co-leader Xu.

According to Xu, several other research teams have been in touch to request more details about the study. “We are also happy to say that we have also received funding support from the National Natural Science Foundation, the National High Technology Research and Development Program of China (863 Program), and the Suzhou Science and Technology Project,” he continues.

The researchers have also started to investigate the interactions between glioma stem cells and bone marrow mesenchymal stem cells, whih can be done by fusing these two types of cells together during the bioprinting process. “By applying the technology to glioma research, we have succeeded in shedding more light on glioma stem-cell behaviour, the glioma microenvironment, tumour-stromae interactions and glioma chemosensitivity,” concludes Xu.

  • Read our special collection “Frontiers in biofabrication” to learn more about the latest advances in tissue engineering. This article is one of a series of reports highlighting high-impact research published in the IOP Publishing journal Biofabrication.

Acoustofluidics separates circulating tumour cells

Circulating tumour cells (CTCs) are cancer cells that escape from primary tumour sites and enter the bloodstream. This metastasis is responsible for the majority of deaths from cancer. Monitoring the level of CTC levels in blood is thus important but has proved difficult to do. A team of researchers in China and the US has now developed a new way to isolate these cells using a technique called size-amplified acoustofluidics in which the CTCs selectively bind to microbeads.

The bound cancer cells are significantly different in terms of size and physical properties (they are stiffer, for example) compared to normal cells, explain the researchers led by Feng Guo of Indiana University in Bloomington in the US. This means that their acoustic radiation force is a 100-fold higher than that of bare CTCs or normal blood cells. They can thus be efficiently sorted from blood using microbeads (the “size-amplifiers”) in a travelling acoustic wave microfluidic device, and then released from the amplifiers by being degraded with enzymes.

The technique is 77% efficient and produces CTCs with a 96% yield.

Acoustofluidic devices

Acoustofluidic devices use ultrasonic waves within microfluidic channels to separate suspended micro- and nanoscopic entities (in this case, biological cells). They work thanks to a process called acoustophoresis, in which the suspended objects move when subject to sound energy.

The objects must be smaller in size than the incident wavelength of the sound and the width of the fluidic channels are usually tens to hundreds of microns across. This means that acoustofluidic devices generally make use of ultrasonic waves generated from transducers pulsating at high (megahertz) frequencies. At certain frequencies that depend on the geometry of the devices, it is possible to produce an acoustic field that shifts the moving trajectory of particles across streamlines that flow within the bulk of a fluid. In this procedure, the acoustic radiation force generated by the sound waves is key in sorting the complexes formed between the CTCs and the microbeads, say the researchers, since it is this force that pushes the complexes along in the fluid flow.

Selective capture

In their work, Guo and colleagues used 40-micron diameter silica microbeads coated with biodegradable gelatine and grafted with the capture agent EpCAM. These beads can selectively capture CTCs and can then be removed from them after they have been separated in the acoustofluidic device.

Not only can the microbeads efficiently and specifically bond onto CTCs to form CTC/size-amplifier complexes, they also enhance the differences in physical properties between the CTCs and normal blood cells, as mentioned.

Lab tests

The researchers tested out their platform on 1-millilitre-volume blood samples from eight colorectal patients and eight breast cancer patients. They collected the cell/size-amplifier complex using a centrifuge and then counted the number of cells obtained using an optical microscope. They found that the devices separated several to tens of CTCs respectively. They also calculated how pure the cells were (as the ratio of cells sorted with size-amplifiers versus all cells collected at the device outlet).

The technique could be a promising alternative to patient biopsies that are difficult or impossible to perform in some cases – for example, in some lung or breast cancers. It could also help us better understand the mechanisms involved in cancer metastasis, say Guo and colleagues, and so guide targeted cancer therapy.

The research is detailed in the IOP journal Nano Futures.

How clean is tropical hydropower?

Hydropower in the Mekong river basin in Southeast Asia is not a “categorically low-emission” energy source, according to researchers who studied its long-term greenhouse-gas output.

The study – the first to assess emissions on a large scale over a lifetime of 100 years – paints a mixed picture for hydropower in the tropics. Although many of the Mekong reservoirs have emissions comparable to renewable energy sources, a large portion have considerably greater emissions, with some even matching those from fossil-fuel power plants.

The researchers believe that if low emissions are to be considered important, many factors must be taken into account and the merits of hydroelectric reservoirs must be assessed on a case-by-case basis.

“Things are rarely simple,” said Timo Räsänen of Aalto University, Finland. “[… Our] understanding should be used for developing region- and location-specific energy alternatives from a mix of energy sources that minimize the total greenhouse-gas emissions.”

The Mekong region has undergone rapid social and economic development in recent decades. Since the 1960s, to meet rising energy demands, hydroelectric reservoirs have steadily multiplied, with a sharp increase this century.

Hydropower projects are often assumed to be a low-carbon source of energy, although this is not always backed up by what little data has been collected. In 2011, for instance, the consultancy Environnement Illimité in Canada found that emissions from 18 equatorial and tropical reservoirs ranged from 2 to 4100 kg of carbon dioxide-equivalent per megawatt-hour – compared with 380 to 1300 kg from typical fossil fuel plants.

Räsänen and colleagues from Aalto University and Leiden University in the Netherlands chose the Mekong region for their analysis because of its 141 existing and planned hydropower reservoirs. Data on reservoir emissions are lacking for the Mekong region, so the team borrowed data on carbon dioxide and methane emissions from other sources, and fed them into an existing statistical model.

The researchers also incorporated basic reservoir data they collected themselves from the Mekong region, such as age, size, net primary productivity, air temperature, erosion and annual energy production. “Our study is the first to make such [a] regional analysis which also considers the change of emissions in time,” said Räsänen.

Emissions ranged from 0.2 to 1994 kg of carbon dioxide-equivalent per megawatt-hour, with a median of 26 kg, the team found. Some 80% of general hydropower reservoirs and 45% of reservoirs also used for irrigation had emissions comparable to renewable sources, while the rest had greater emissions, some beyond those of fossil-fuel power plants.

According to Räsänen, the reasons for the variability include the amount of organic matter in the reservoirs, the warmth of the reservoir (which aids decomposition), and the shape of the reservoir – with relatively large surface areas also helping decomposition. But he stresses that educated guesses can only be so accurate, and many other specific factors need to be taken into consideration.

“The existing models are relatively simple and there is room for improvement,” he said. “For example, we should develop a typology of the physical characteristics of the reservoirs, and analyse the effect of reservoir characteristics on the emissions to make better predictions.”

The team published the findings in Environmental Research Letters (ERL).

Echoes of gravitational waves could point to quantum gravity

The first detection of gravitational waves in 2015 created huge excitement because it confirmed a long-standing prediction of Albert Einstein’s general theory of relativity and opened up a completely new way of observing the universe. Physicists have also been scrutinizing data from the growing number of gravitational-wave detections for “echoes” – the existence of which could mean that our understanding of relativity is incomplete. Physicists in Canada and Iran have found tentative evidence for such echoes gravitational waves from colliding black holes, and now say a stronger signal exists in data from colliding neutron stars.

Many physicists believe that general relativity is incomplete because it is at odds with quantum mechanics, leading to the information paradox when considering the extreme gravitational fields generated by black holes. Relativity tells us that whenever anything, including light, crosses a black hole’s event horizon the information it contains is lost to the rest of the universe forever. But quantum mechanics requires that information can neither be created nor destroyed. This is a problem given the existence of Hawking radiation, which implies that black holes can evaporate away to nothing and in the process erase all of the information that flowed into them.

If gravitational-wave echoes exist, it would suggest that black holes are not bounded by a classical event horizon but instead by a quantum-mechanical Planck-scale structure. One such structure put forward by theorists is the “firewall”, which would destroy any object passing through it but retain that object’s information and so keep it outside the black hole. Firewalls, however, are controversial. While physicists generally agree that quantum mechanics comes into play deep inside black holes – even though it is impossible to see its effects – they are largely sceptical about its role outside the event horizon.

Barrier bouncing

Gravitational-wave echoes would be created thanks to the presence of the Planck-scale structure, or “membrane”, and what is known as the angular momentum barrier. The latter is a boundary lying around 1.5 times as far as the event horizon (typically around 200 km from the centre of a black hole) that is predicted by relativity and which partially confines gravitational waves. Any outgoing wave generated between the event horizon and the barrier would normally bounce off the barrier and then pass through the horizon, never to be seen again. But the membrane, lying within a Planck length of the horizon, would instead reflect the wave back, allowing it to either bounce off the barrier again or, less likely, pass through the barrier into space.

As a result, the barrier can act like semi-reflective mirror that releases a small fraction of the gravitational-wave energy into space after each reflection from the membrane. This would appear as weak bursts of gravitational radiation – the echoes – separated by a well-defined time interval that depends only on the black hole’s mass and rate of spin.

This proposal is based on an idea originally put forward by Vitor Cardoso of the University of Lisbon in Portugal and colleagues in February 2016, just a couple of weeks after the LIGO collaboration in the US had announced the first detection of gravitational waves. Then in December that year, Niayesh Afshordi of the University of Waterloo and the Perimeter Institute for Theoretical Physics in Canada and colleagues said they had evidence to back up the idea, claiming to have found a 2.5σ signal for the echoes in gravitational waves from three pairs of merging black holes, including that seen in the first detection.

Consistent with noise?

That claim was met by scepticism from nine members of the LIGO collaboration, who did their own analysis of the data. They included more background than considered by Afshordi’s team and colleagues, and found a signal, but with less significance – about 2σ. The result, said the LIGO team, was “entirely consistent with noise”. They therefore concluded that the rival analysis did “not provide any observational evidence for the existence of Planck-scale structure at black hole horizons”.

Undeterred, Afshordi and his colleague Jahed Abedi of the Sharif University of Technology in Tehran looked for echoes in data from the merging neutron stars announced with much fanfare by LIGO and Virgo in Italy in October 2017. First, they calculated the range of expected echo frequencies and time delays between merger and echoing – 60-90 Hz and up to 1 s, respectively (the latter depending on whether the neutron stars collapsed directly to form a black hole or first produced a very massive neutron star). They then scanned the data set to find out whether there were waves matching those criteria. As they reported recently on the arXiv server, they did indeed find such a signal – at 72 Hz, around 1 s after the merger. What’s more, they found only a few similar repeating patterns at other times within the data. As such, they claim, the signal has a significance of 4.2σ.

Cardoso says it is “puzzling” that the neutron-star echoes should have a higher significance than those from the merging black holes – given that the latter signal was more intense. He also cautions that the repeating waves could be a consequence of conventional physics, such as “radiation from leftovers of the merger”. Nevertheless, he argues that the prospect of new physics makes such searches worthwhile. “It would be foolish not to dig deep into this,” he says.

Afshordi admits he was surprised to find such a strong signal in the neutron star data, and acknowledges that fresh observations from LIGO and Virgo will be needed to settle the issue. But he argues that the evidence is building, pointing out that another group, at the University of Toronto, has seen 3σ evidence for the echoes. “So far everyone who has looked for echoes has found them, including the LIGO group,” he maintains. “We have yet to have a group that doesn’t find anything.”

Recognizing mental health in the research environment

We are hearing more and more about the impact that poor mental health has on our working lives and environment and a recent review commissioned by the Royal Society and the Wellcome Trust highlighted the high incidence of mental health problems amongst researchers in the UK.

So as we start Mental Health Awareness Week 2018, last Friday’s one-day workshop exploring mental-health issues in the science, engineering, technology and mathematics (STEM) research environment was very timely.

The meeting was held at the Royal Society of Chemistry (RSC) in London, having been organized by the RSC with support from the Royal Society, the Royal Academy of Engineering, the Royal Society of Biology, the Wellcome Trust and the Institute of Physics, which publishes Physics World.

Opening the workshop, Susan Guthrie of RAND Europe, who carried out the review, said that one third of researchers in the UK reported they experienced unacceptable levels of stress and over three-quarters worked more than 48 hours per week. Alongside this, over half reported some degree of bullying and harassment. Academic burnout was on a par with other high-stress occupations such as teaching and social work. Guthrie estimated that the impact of the loss of productivity and talent to the UK’S research and science based through not addressing these issues ran to hundreds of millions of pounds per year.

Bringing a very personal perspective to the day was Joanna Waldie, a semiconductor-physics postdoc at the University of Cambridge, who talked courageously about her own experiences of coping with mental-health conditions. Dealing with isolation, setbacks in research and rejections from grant funders is all part of being a researcher but is particularly challenging when experiencing episodes of poor mental health. As Waldie reminded us, “It’s okay to take time out to look after your mental health.”

The third speaker was Sara Shinton, head of researcher development at the University of Edinburgh, who spoke about the challenges of working across a very complex institutional structure and getting the right messages across. She started by acknowledging that the academic research environment is challenging, that years of uncertainty takes its toll, and that the fragmentation of academic time all erode well-being. By addressing mental-health issues openly, the university wanted to encourage everyone to talk about their mental health, which would ultimately make it easier for people to access support. But one of the biggest challenges was engaging everyone to talk about these issues. “This could be the next revolution… having scientific people at scientific conferences talking about mental health” said Shinton.

During the workshop session, many ideas emerged about how the professional bodies can support everyone working in STEM to develop an environment that nurtures wellbeing, supports those who have mental health conditions and challenges stereotypes. Promoting mental health awareness week is just the start of the journey.

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