Pale blue dot: From the solar system to the universe. (Courtesy: NASA)
Jillian Scudder tells the story of our universe’s history in Astroquizzical: a Curious Journey Through Our Cosmic Family Tree. Starting with the human race, she explores our perception of space and how we have been trying to explore and understand it from our “parent”, Earth. Along the way we are taken through several thought experiments and get to enjoy some beautiful images from space. She moves on to our “cosmic companion”, the Moon, briefly exploring its creation and effect on the Earth. Included here is a thought experiment about putting a wormhole between the Earth and Moon – which doesn’t seem like a good idea.
The next step is Earth’s immediate family, or “siblings”, in the form of the other planets in our solar system. We learn how the planets were formed from the leftover dust and gas as our Sun was forming. Scudder then starts considering exoplanets, with a thought experiment about the possibility of life on other worlds, and the weird places we may find it. Next along this family tree, at the level of “grandparent”, are stars, starting with our Sun. There is an entire chapter on stellar deaths. If they’re large enough, these stars will collapse into black holes, but there are plenty of other beautiful and violent ends to be explored. Extending the family tree even further, Scudder moves outwards to galaxies. These are a diverse bunch, with many different categories to choose from, and there is an all-too-brief look into my own favourite: supermassive black holes.
On the whole, Astroquizzical is a good read for those who want to know more about the universe, how we got to be here and how we fit in to the history of the cosmos. But ultimately, it is aimed at the astro-interested, not the astro-educated. The book ends with a look at the universe as a whole: how it is still expanding, and how the further away we are able to look, the further back in time we are able to see. Scudder does well in showing how little of this universe we know even today, and how tiny our place in it is.
The debate over the relative levels of carbon emissions from the various energy generation options has been quite long and tortuous. It seems clear that fossil fuels are bad news, but there is uncertainly about the relative merits of nuclear and renewables – most see the carbon footprint of renewables as very low, but what about nuclear?
A recent study by researchers at Potsdam Institute for Climate Impact Research, Germany, published in Nature Energy, estimates the 2050 full lifecycle greenhouse gas emissions of a range of sources of electricity in a 2 °C scenario. It claims that the carbon footprints of solar, wind and also nuclear power are many times lower than coal or gas with carbon capture and storage (CCS). This remains true after accounting for emissions during manufacture, construction and fuel supply, i.e. the so-called embodied energy.
Wind, photovoltaics (PV) and nuclear all do well, in the range 3.5–12 g CO2 equivalent/kWh, with solar at a mid-range 6 g CO2 eq/kWh, and wind and nuclear at 4 g CO2 eq/kWh. Fossil CCS is high, nearing 100 g. There are still upstream emissions from mining and not all the plants’ CO2 can be captured. Biomass and hydro are also high, around 100 g, but “highly uncertain”. Bio-energy with carbon capture and storage (BECCS) has the advantage of negative emissions – see my next post – but with hydro in some locations there can be methane production from trapped biomass.
Lifetime energy
In terms of the percentage of the lifetime energy produced by each plant, the energy needed for building wind turbines comes out by far the lowest, PV next best, with Energy Return on Energy Invested ratios (in effect the inverse of embodied energy) of 44:1 and 26:1, respectively. Nuclear comes out with an EROEI of 20:1. That seems high (it’s usually put at 15:1 for pressurized water reactors (PWRs)), given the energy/materials intensive nature of the complex plants and, crucially, the energy used in uranium mining and processing. What’s more, although new extraction techniques may help reduce the energy used for fuel production initially, longer term it may well rise, and EROEIs fall, since lower grade ores, harder to get and process, will have to be used in future as uranium reserves deplete. Certainly, an earlier meta review of multiple studies by Ben Sovacool suggested that the figure for nuclear was much lower.
…is there any point in trying to live on a polluted, radioactive planet with the ecosystem undermined in other ways – and draconian political regimes reinforcing social inequalities in response?
Dave Elliott
However, in the Potsdam study, all the rest are very much worse, including fossil fuels with CCS, but also hydro – surprisingly given its long operational life. But then it does involve a lot of concrete. Biomass also comes out poorly, including BECCS, but it’s a low energy-density fuel and you need more energy to collect it and get less energy out per unit of input than from fossil fuels.
The Potsdam paper’s estimates of full lifecycle greenhouse gas emissions for the different sources of electricity assume a 2 °C world in 2050, when global electricity supplies have been largely decarbonized. So the indirect emissions due to the electricity used for manufacturing systems are lower than now, for example, for a solar cell fabrication factory or for some parts of nuclear fuel production, though not presumably for uranium strip mining, if that is still used – it will need a lot of diesel diggers and trucks, unless they are all battery or syngas powered by then.
Overall the report concludes: “The indirect greenhouse gas emissions induced by upscaling wind, solar and nuclear power are small compared with other emissions sources, and thus do not impede the transformation towards climate-friendly power supply.” That is certainly interesting. For example, in a review of the report Carbon Brief notes that some earlier studies had suggested the opposite – renewables would need a lot of energy to build, and more than nuclear.
Wider metrics
Be warned though, this sort of analysis is tricky. There are lots of unknowns and assumptions about assessment boundaries. What’s more, although the Potsdam researchers say they have taken these into account in the ranges they offer, it’s hard looking to 2050. The technology is changing fast and market and regulatory pressures may shift priorities and valuations in unexpected ways. If by then we are running a basically low-carbon energy system, the main issues may be different – less to do with carbon, more to do with relative environmental and social impacts, land-use, water use and so on. For example, a recent report from University College London (UCL) has proposed new, wider metrics for sustainable development.
There are several candidates beyond just eco-impacts and costs. For example, employment creation is potentially a positive socio-economic impact, which some see as part of the case for adopting sustainable energy. There are now national campaigns for “One Million Climate Jobs”, e.g. in South Africa. And the EU has adopted job creation as a key metric in its approach to development aid. So we are moving beyond just carbon counting.
That is implicit in many recent studies that include wider social and economic costs and benefits: carbon saving is only one factor. Some interventions may deal with both carbon and other issues, for example, the adoption of renewables will also reduce air pollution. Some may deal with carbon but introduce other problems – for example, with nuclear power, the risk of incidental and accidental radiation release. Nuclear radiation hazards are, of course, not included in the Potsdam greenhouse gas analysis, nor in the UK government’s carbon accounting approach. It’s not easy to do that, given the uncertainties involved – for example, we are still debating the human cost of Chernobyl with, worryingly, a new UN report raising the thyroid cancer rate significantly.
Even leaving that aside, given the range of issues and options, strategically, in some cases, there may be conflicts over which focus should have priority. As the UCL paper says, “there are complex trade-offs between the natural resource dependencies of energy, food and water systems, and environmental threats including biodiversity loss, climate change and localized air and water pollution”. It concluded: “These synergies and trade-offs will manifest differently in different settings, and the impacts for different social groups will need to be understood and accommodated. Considerations of rights, justice and equity must be integrated into the exploration of solutions for these complex energy dilemmas to ensure we leave no one behind.”
Those worried mainly about climate change may object that everything else must be secondary, arguing that all other issues are irrelevant if the climate system is seriously disrupted. But equally, is there any point in trying to live on a polluted, radioactive planet with the ecosystem undermined in other ways – and draconian political regimes reinforcing social inequalities in response? Hopefully, extremes like this can be avoided, but as UCL says, “decision-makers can no longer think in silos, and will need to find ways of widening participation, creating collective ownership and building consensus.”
The trade-off issues can clearly be complex and that shows up when we look at specific sets of options in relation to carbon reduction, as I will explore in the next few posts.
Light that has travelled billions of light-years from now-ancient quasars has been used by physicists to close the “freedom-of-choice” loophole in Bell tests of quantum entanglement. Done on Spain’s Canary Islands, the experiment is a significant improvement over a similar test done in 2017 that used light from nearby stars. Meanwhile in China, an independent team has done a similar experiment using starlight that also closes the “fair sampling” loophole.
Entanglement is a curious consequence of quantum mechanics that allows two particles to be connected in a way that cannot be described by classical physics. It is observed as correlations between measurements made on two particles (such as their polarizations). In 1964 the Northern Irish physicist John Bell described his famous test of whether such correlations are stronger than those allowed by classical physics – as defined by a violation of what is now called Bell’s inequality.
Many Bell test experiments have since been done to confirm entanglement. However, no experiment is perfect and there are number of experimental “loopholes” that could allow purely classical phenomena such as faulty detectors to affect the outcome. In 2015, physicists simultaneously closed two important loopholes called “fair sampling” and “locality”.
Unknown correlations
Freedom of choice is another important loophole that involves how the measurements are done. In a Bell test on entangled photons, a large number of measurements are made on different entangled pairs in which the direction of the polarization measurement is selected at random. If, for some reason, the polarization selection is not random but correlated to other aspects of the experiment, then the outcome of the Bell test could be affected.
In 2017 Johannes Handsteiner and Anton Zeilinger of the University of Vienna and an international team used the random nature of starlight to close this loophole. Two telescopes at two locations in Austria separated by nearly 2 km were pointed at two different stars. The colour of the starlight changes in a random manner and this was used to decide how to set Bell test polarization detectors. The stars were chosen so that their light arrives at their respective telescopes first, before reaching other parts of the experiment. This, and the fact that the starlight light was created hundreds of years ago, very far away from Earth and in stars separated by a great distance allowed the physicists to conclude that there is no correlation between the choices of polarization measurement and the rest of the Bell test experiment.
Now, the team has joined forces with astronomers at two telescopes on the Canary Islands to do a similar Bell test using light that was first emitted from quasars billions of years ago. The experiment involved two measurement settings – one dictated by the colour of quasar light generated about 8 billion years ago and the other dictated by the colour light generated in a different quasar about 3 billion years ago. The result is a violation of Bell’s inequality by 9.3σ, which is well in excess of that usually needed to qualify a discovery-level measurement.
Fair sampling
The experiment is described in Physical Review Letters, where the team points out that their experiment does not close the fair sampling loophole. This is because the photon detection process was relatively inefficient and so many entangled pairs were not measured. As a result, there was no way of knowing if there is a bias in which photons were detected and which were not – and such a hypothetical bias could result in a false positive Bell test.
Also in that issue of the journal is a paper by Jian-Wei Pan of the University of Science and Technology of China and colleagues, who describe a similar experiment using starlight. In their experiment, however, they closed the fair sampling loophole by ensuring that their photon detectors operated at about 78% efficiency – high enough that a hidden bias would not affect the Bell test.
Unexpected hot spots in the cosmic microwave background (CMB) could have been produced by black holes evaporating before the Big Bang. So says a trio of scientists led by mathematical physicist Roger Penrose in a paper presenting new evidence that our universe is just one stage in a potentially infinite cycle of cosmic extinction and rebirth. Other researchers, however, remain sceptical that the microwave background really does contain signs from a previous “aeon”.
According to standard cosmology, the universe underwent a very brief but exceptionally intense expansion just after the Big Bang. This period of “inflation” would have ironed out any irregularities in the structure of the early universe, leading to the very uniform cosmos that we observe around us.
However, Penrose, based at the University of Oxford , has developed a rival theory known as “conformal cyclic cosmology“ (CCC) which posits that the universe became uniform before, rather than after, the Big Bang. The idea is that the universe cycles from one aeon to the next, each time starting out infinitely small and ultra-smooth before expanding and generating clumps of matter. That matter eventually gets sucked up by supermassive black holes, which over the very long term disappear by continuously emitting Hawking radiation. This process restores uniformity and sets the stage for the next Big Bang.
Losing mass
CCC has met with scepticism from many cosmologists since being put forward in 2005, not least because the matching up of an infinitely big universe in one aeon with an infinitely small one in the next requires that all particles lose their mass when the universe gets very old. However, in 2010 Penrose and Vahe Gurzadyan of the Yerevan Physics Institute in Armenia claimed that they had found evidence to support CCC in the form of rings of uniform temperature within the CMB. Those rings, the idea went, would be the signature in our aeon of spherically-emitted gravitational waves generated by colliding black holes in the previous aeon.
The pair found such rings in data from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), while at the same time claiming that they saw no such pattern in (standard) simulations of the CMB that they had carried out. Other groups, however, argued that simulations did indeed contain rings – once they had been modified to take account of the distribution of hot and cold spots at various angular scales that are seen in the real CMB and which are predicted by inflationary physics.
Undeterred, Penrose has now published a different kind of evidence in support of CCC. Rather than rings of near uniform temperature, he has instead identified patches within the CMB that are much hotter than the surrounding region. The idea is that these hot spots could be due to the (mainly electromagnetic) radiation given off during the Hawking evaporation of supermassive black holes in the previous aeon.
Hawking points
Penrose says that although originally very feeble, those emissions would have been concentrated in our own aeon into spots with huge amounts of energy that he and his colleagues call Hawking points. That concentration comes about, he explains, because “the universe loses track of how big it is at the transition between aeons”. The Hawking points would then have stretched during the early universe, forming circular patches with a diameter on the sky about five times that of the Moon.
In a preprint recently uploaded to the arXiv server, Penrose and two colleagues – Daniel An of the SUNY Maritime College in the US and Krzysztof Meissner at the University of Warsaw in Poland – report scouring CMB data from the European Space Agency’s Planck satellite for hot spots of various sizes and analysing how quickly the microwave temperature drops off around them compared to spots in 1000 simulated maps of the CMB. They found that in and around small spots, not a single simulated map had higher temperature gradients than the real cosmos – with the temperature variations in the latter case being about an order of magnitude higher (some 3×10-4 K) than the CMB average.
Strong backing
According to Penrose, this disparity between real and simulated data provides strong backing for CCC over inflation. “We certainly welcome attempts to explain these observations in terms of currently accepted models,” he says, “but we think this will be hard unless radically new ideas come forth”.
Some other physicists, however, remain unconvinced. James Zibin of the University of British Columbia in Canada points out that scientists have been scrutinising the CMB for years and have found no evidence for particularly hot spots (although they have identified one anomalous cold patch). He also reckons that Penrose and colleagues have failed to account for the “look elsewhere” effect, arguing that because they found the hottest spots in the real as opposed to simulated data in just 2 out of 40 tests (focusing on different sizes of spot and CMB border region each time) the chances of having been the victim of a statistical fluke drop from 1 in 1000 to as low as 1 in 50.
Douglas Scott, a colleague of Zibin at British Columbia, is also sceptical. Describing the paper as “very muddled and hard to follow”, he is wary of what he sees as a potentially never-ending series of attempts to find unusual features in the CMB. “Obviously, if someone could show that some specific pattern on the microwave sky was a proof that the universe underwent a series of cycles then that would be spectacularly exciting,” he says. “But this paper falls very short of doing that.”
Switching from gasoline-fuelled to electric-powered vehicles can reduce local levels of air pollution, particularly in cities with lots of traffic. It’s a swap that many countries are keen to encourage, but what’s the best way to nudge vehicle owners in this direction?
“A rebate targeted at affordable battery electric vehicles (BEVs) combined with early investments in charging infrastructure along roadways where EVs would most need them is likely to increase EV adoption,” says Easwran Narassimhan of Tufts University, US, who carried out the analysis with Caley Johnson from the US National Renewable Energy Laboratory.
Narassimhan and Johnson’s results show that a $1000 incentive increase given as a rebate can raise EV sales by 4.8% compared with just 2.3% when the saving is provided as a tax credit. The observation tallies with earlier work, which found that incentives closer to the point of sale tended to be more attractive to potential customers than rewards that arrived later.
Early investments in infrastructure get the thumbs up from the team as public charging points are likely to incentivise early adopters, which can provide a multiplying effect on electric vehicle sales. There are other options too.
One of the most cost-effective policies was an exemption from high-occupancy lane rules – a relatively inexpensive incentive that boosted sales of battery electric vehicles by 15%.
Also, there are signs that rising environmental awareness could be as strong a factor as the availability of tax incentives — especially in promoting sales of plug-in hybrid electric vehicles — at least in states with a good track record in communicating pollution issues.
With more data being added all the time, policy-makers are likely to be even better equipped in future to reduce the number of gas-guzzling vehicles on our roads. Narassimhan and Johnson are keen to expand their analysis as new figures become available.
“This includes looking more closely at demographic factors such as vehicle miles travelled per capita, environmental awareness, and unemployment,” says Narassimhan. “We’re also interested in incentives for home charging and home electric vehicle supply equipment, which intuitively should have a strong relationship since a majority of electric vehicle owners charge at home.”
Quantum simulators can be thought of as specialised kitchen equipment that is exceptionally good at producing one type of food – perfectly cooked toast, for example. That is the analogy used by Lincoln Carr of the Colorado School of Mines in this video for our 100 Second Science series.
My CV was once described by a head-hunter as “interesting”. Although I’m not sure that was meant as a compliment, I can hold my hands up to having been an opportunist and a little fortunate. After completing my PhD in optoelectronics at University College London (UCL), I was walking past the offices of a small computational fluid-dynamics consultancy in London, when I decided to go in and find out a bit more about what they did. They offered me a job as a project engineer and I worked with them for clients including NASA. I’ve since made rather too frequent use of the phrase “it’s not rocket science – I know, because I was a rocket scientist”.
Admittedly that was a rather serendipitous introduction to the world of work, but it wasn’t such an unusual first career move for a graduate physicist. But then followed a more unusual stream of roles: aviation-operations research analyst; KPMG Consulting first as a process analyst, then a mergers and acquisitions adviser and an implementation programme director for large IT systems. I have been a chief people officer for an international media and advertising giant; a professional leadership coach; a director at the BBC; and a director of human resources and organizational change in the civil service. Along the way I studied drama; designed, coded and sold iOS apps in popular psychology; earned money as a guitarist in an indie band; and as a semi-professional – and most definitely only semi-talented – artist.
Today, I am a coach and interim manager, and an adviser to government on diversity and inclusivity; though I can see myself taking on another permanent job soon. Perhaps my most ironic role has been as a consultant and graduate/staff course designer and lecturer at UCL in learning and facilitation, psychology and winning funding bids.
Looking back, I find it hard to believe that I followed this meandering path
Looking back at that list, I find it hard to believe that I followed this meandering path. I left mainstream physics quite early and there is every indication that many physics graduates do the same, and this means being prepared for a different world. With that in mind, I want to share three key concepts – to be aware, willing and able – which I now understand but wish I had taken the time to delve into while I was completing my university studies.
Aware
First is the importance of awareness. There are many types of awareness, and foremost is self-awareness. I coach some very senior people in science and also in business, and I marvel at how little many of them understand about their own skills and personalities before making career decisions. Moreover, evaluation tools for skills such as verbal and numerical reasoning, spatial resolution (and recruitment simulations based on these) are frequently used in hiring so it’s a good idea to evaluate your aptitudes before you even write a job application.
Awareness of the job market is also important, and it is sensible to form a view on whether career opportunities in a given sector are likely to increase or decrease in the medium to long term. Establishing relationships with a few well-chosen recruitment consultants or market analysts at the start of a career can help – after all they are the experts, it’s their job. They see lots of people and have long memories – and your initiative will keep you in their minds for future opportunities. I’ve had two major roles come out of the blue from headhunters I’d made contact with years previously.
Using psychometrics – the objective measurements of mental and psychological abilities – can be invaluable in raising self-awareness, but they come with caveats and need balanced interpretation. The psychologist Carl Jung postulated that the brain has a dominant preference to perceive information in one of two ways (“sensing” or “intuition”) and to make decisions in one of two ways (“thinking” or “feeling”). Personality type psychometrics can illuminate personal preferences toward these mental functions and provide insights into whether, for example, a theoretical or practical choice of career (or indeed final-year physics project or PhD thesis) might suit a person better. Having the desire theoretically to conceptualize the Higgs boson is one thing, and being drawn to carrying out experiments to find it is quite another, which is why different physicists pursued those two goals. The same principle applies to pursuing careers.
Willing
The second concept is the importance of being truly motivated to do what you do – not just doing it because you need a job, but because it is something you will find personally fulfilling as a completely willing participant. There’s extrinsic motivation (to avoid punishment or earn a reward) and there’s intrinsic motivation (enjoying an activity for its own sake) – and the latter is more powerful. It’s the type of motivation that will get you out of bed in the morning without need for an alarm clock or the fear of an exam deadline.
Be truly motivated to do what you do – not just because you need a job
Research carried out by psychologists Edward Deci and Richard Ryan in the 1980s and 1990s found that there are three main drivers of intrinsic motivation: increasing one’s ability in an area where one has an interest; having an appropriate degree of control about what one does; and being given the opportunity to genuinely relate to the people with whom one works. In addition, everyone has their own specific intrinsic motivators: things a person finds enjoyable, or important beliefs such as moral, ethical or political views.
Examining the opportunities for all your personal intrinsic motivators in a particular career or organization will lead to a more fulfilling career path and so can help you before you initiate job applications. Making career choices that align with your motivators will go a long way to ensuring you are on the right path for you. On the other hand, a negative correlation between a career choice and your intrinsic motivators could see you professionally unfulfilled and back on the job market sooner than you might like.
Able
The third concept – and the one on which most people focus their efforts to the exclusion of the first two – is the importance of knowing your abilities, or what you need to be good at in a particular job. Ability includes so-called “soft” skills (sometimes not soft at all) relating to teamwork and personal interactions. Ask yourself what transferable skills you have from your education and life so far. If you are lacking in the abilities or skills you need, find out how you can develop them. Last, but not least, you need the ability to present yourself to a potential employer in a way that they will find attractive enough to interview and employ you. This means writing a standout CV and developing your interview skills – but these are topics for another day.
Autism spectrum disorder (ASD) is a condition comprising behavioural and developmental problems that affect the communication, social and interaction skills of the patient. According to the World Health Organization (WHO), this pathology affects one in 160 children worldwide, with three times more males affected than females, and it’s been increasing globally over the last 50 years.
From a genetic perspective, different gene modifications exist in ASD patients, but only 25% of them have been identified. To develop a therapy, however, we need to identify targets – the genes involved in the disease. As a consequence, more studies are essential to understand and develop a cure for this disease.
With this aim, Maria Rita Passos-Bueno and her team at the University of São Paulo investigated a protein named actin, which is the most abundant protein in our cells and is involved in many structural and functional roles, including cell motility and cell division for cell proliferation (Scientific Reports 8 11138).
Actin is present as filaments in the cytoskeleton of cells. To achieve their functions, these actin filaments are subject to polymerization and depolymerization processes. In the brain, the correct regulation of this polymerization/depolymerization dynamic is essential to sculpt neuronal connectivity.
First author Karina Griesi Oliveira.
In this study, the group investigated actin filament reconstitution under stimulation of RhoGTPase (the main group of molecules involved in actin polymerization), in stem cells obtained from the exfoliated deciduous teeth (SHEDs) of 13 ASD patients and eight controls (non-ASD patients).
The researchers used an inhibitor of RhoGTPase to induce actin depolymerization, and promoted repolymerization using a direct activator (DA) that simultaneously activates three molecules involved in repolarization: Cdc42, Rac and RhoA. Those proteins were also activated separately using epidermal growth factor (Cdc42 and Rac) or calpectin (Rho) in order to activate upstream signals.
From the 13 ASD patients studied, only a subgroup of seven patients’ cells behaved differently to the control group, with different responses depending upon the pharmacological treatment. This group of seven ASD patients showed a lower percentage of cells with reconstituted actin filaments.
Schematic showing possible responses depending on the alteration in the actin polymerization pathway (red cross), with abnormal regulation represented by red arrows and normal by green arrows. (Courtesy: Scientific Reports8 11138/CC BY 4.0)
The researchers studied the possible genetic alterations in downstream or upstream molecules using the four different pharmacologic treatments. Out of the seven patients, two were presumed to have a genetic alteration in an upstream molecule and two were presumed to have a downstream genetic alteration. The origin of alterations in the three other patients couldn’t be determined. This means that, in patients with alteration in actin dynamics, the genetic origin of this alteration isn’t always the same.
SHEDs might provide a useful model to study genetic alteration, as they may reflect altered neuronal phenotypes. Prior clustering of patients was essential in this study as only a subgroup of ASD patients had an altered actin polymerization dynamic. This paper emphasizes the multiple possible genetic alterations in ASD patients. Further investigations to identify targets that could benefit many patients would be of great interest.
Protons in neutron-rich nuclei have a higher average energy than previously thought, according to a new analysis of electron scattering data that was first collected in 2004. The research appears to refute the conventional description of a nucleus in which neutrons and protons move independently of one another in a mean field. The results could have important implications for our understanding of nuclear structure and could also impact several other areas – including the physics of neutron stars.
Developed in the second half of the last century, the independent particle shell model of the nucleus assumes that nucleons (protons and neutrons) move independently in the mean field created by their mutual strong nuclear interaction –with negligible interactions between individual nucleons. Electron scattering experiments in the 1990s provided the first hints that this picture was inadequate and physicists have subsequently realized that nucleons can momentarily form high-energy pairs whose mutual interaction dominates over their interaction with the remaining nucleus.
Theory suggests a high-energy pair is much more likely to form between a neutron and a proton than between identical nucleons. This is backed-up by experimental work on light nuclei done by the CLAS collaboration – based at the Thomas Jefferson National Accelerator Facility in the US – and others. However, light nuclei normally contain almost equal numbers of protons and neutrons and the picture was murkier in heavier nuclei, which generally have a significantly more neutrons than protons.
Neutron outsiders
“You could have a core of protons and neutrons with correlations and some extra neutrons on the outside that don’t do anything,” explains Or Hen of Massachusetts Institute of Technology, a senior CLAS member, “or you could say these guys from the outside actually reach inside, find protons and correlate with them.” Different models gave different predictions: “Whenever there’s a big calculation of a nucleus like lead, these correlations are completely ignored,” says Hen.
The problem is that what we think is not interesting today might be fascinating tomorrow
Or Hen
More experimental input was needed and the process of designing, building and analysing a new experiment would have been costly and time consuming. But Hen and his colleagues came up with a better plan: “The CLAS spectrometer records literally every single interaction of an electron that hits the detector,” says Hen. Almost uniquely, all these data are retained. “In big, particle physics detectors like the LHC, people are experts at deciding in real time whether an interaction was exotic and interesting enough to be recorded on the computer for further analysis,” he explains. “The problem is that what we think is not interesting today might be fascinating tomorrow.”
The researchers have now re-analysed data from a CLAS experiment originally run for completely different purposes in 2004. They looked at the momenta of electrons that had scattered off targets made from various elements. The targets ranged from carbon (whose nucleus contains six neutrons and six protons) to lead (82 protons and around 125 neutrons). The momenta of the proton or neutron ejected in each collision was also recorded, allowing the team to work-out the momentum of that nucleon just before the collision occurred.
The conclusion of the study was clear: a nucleus contains almost identical numbers of high-momentum protons and high-momentum neutrons, regardless of its neutron/proton ratio. This means that adding extra neutrons to a nucleus increases the fraction of protons with high momentum.
Neutron stars
The team is preparing new experiments to explore these nucleon interactions in more detail. “We’re interested in understanding how you move from a quark-gluon picture to protons and neutrons and on to a full atomic nucleus,” says Hen. This could lead to a better understanding of neutron stars, which contain about 5% protons and could also impact how the next generation of neutrino experiments are interpreted.
Commenting on the research, Willem Dickhoff of Washington University in St Louis, Missouri says: “What they document is not necessarily surprising, but it’s very useful to make the data quantitative at this stage,” says theoretical nuclear physicist. “There is a fraction of the community that prefers not to think about nucleons having high momentum.” Whether or not the results will have observable consequences for neutron star modelling, he says, remains “an open issue, but an interesting one – especially now that neutron star mergers have been observed with gravitational waves.”
Researchers have succeeded in programming the thermal conductivity of a material for the first time. The work, which was carried out on a squid-inspired protein containing multiple DNA string repeats, could help in the development of better thermal switches, regulators and diodes to solve thermal management problems in modern technologies such as refrigeration, data storage, electronics and textiles.
“From bitcoins to cloud servers, sportswear to protective suits, medical devices to vaccine stability, thermal management is a key challenge in our modern world,” says Melik Demirel of Penn State University, who co-led this research effort with Patrick Hopkins at the University of Virginia. “The synthetic squid-inspired biomaterials that we are working on have low thermal conductivity under ambient humidity conditions. We can engineer them, however, so that their thermal conductivity increases dramatically by increasing the number of tandem repeats (repeating strings of DNA) in the protein when hydrated.”
Network topology
Conventional solid-state devices conduct or control thermal energy by phonons (vibrations of the crystal lattice) or by scattering of random vibrations in an amorphous material, he explains. “In soft matter, there is a third parameter – network topology – that comes from interactions of long polymeric chains.
“Structural proteins like the ones we are studying contain both crystalline domains that are linked by amorphous ones. Together with colleagues at the University of Virginia, the University of Maryland and NIST, we discovered that we could programme the thermal conductivity between these domains by controlling their network topology.”
The researchers did their experiments on synthetic proteins that mimic squid ring teeth. “We patterned these proteins on tandem repeating sequences and were able to choose the number of repeats to study how the proteins react under different experimental conditions.”
This is not the first time that squid proteins have been used to inspire new technologies, he says. They have already led to the development of camouflage coatings, self-healing materials, soft actuators and renewable bioplastics, to name but a few.
Programming the amount of thermal conductivity
“We found that under ambient conditions – less than 35% humidity – the thermal conductivity of films made from these proteins do not depend on the number of repeat units and have similar thermal conductivities to disordered polymers and water-insoluble proteins,” says Demirel. “However, when we engineer the materials to have an increased number of tandem repeats, their conductivity jumps when they become wetter. Indeed, the greater number of repeats, the greater the thermal conductivity.
“Since the thermal conductivity is linearly related to the number of repeats, we can programme the amount of thermal conductivity into the materials,” he tells Physics World.
The researchers measured the thermal properties of the materials using sub-picosecond optical pump-probe and inelastic neutron scattering techniques.
The material returns to its original level of thermal conductivity under normal humidity conditions. “Such a switch could be used to make better regulators and diodes, similar to high-performance solid-sate devices, to solve the thermal problems in modern technologies, such as refrigeration, data storage, electronics and textiles,” says Demirel.
“For example, the material could become more thermally conducting when it absorbs the sweat produced by an athlete and so remove excess heat from her or his body,” he says. “We are indeed now in the process of developing some small and large prototypes to test this concept in sportswear textiles.”
The researchers, reporting their work in Nature Nanotechnology 10.1038/s41565-018-0227-7, say they are also testing out the technology for heat dissipation applications in electronics devices.
