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Take off for partial gravity rehabilitation project

A joint project between the European Space Agency (ESA) and the University of Bath will use computer simulations and anti-gravity devices to assess the movement forces on human lower limbs in low-gravity settings. The findings will inform the ESA in developing exercise countermeasures for astronauts. The study will also help improve and refine rehabilitation programmes for patients recovering after a sustained period of bed rest, following surgery or orthopaedic injuries, for example.

“People who go through a period of unloading of the leg from to bed rest due to surgery or, in extreme cases, astronauts who have been to the International Space Station, [afterwards] their body is no longer fit to function for normal life,” explains James Cowburn, whose PhD is funded by the programme. “It’s important to understand how we can rehabilitate those people optimally so they can transition back into their normal life.”

ESA has a long-term interest in human missions to the Moon, both to explore the surface itself and as an operational testbed for future planetary explorations to Mars and beyond. To date, however, little is known about the physiological and biomechanical effects of life in low gravity and whether Lunar (0.16G) and Martian (0.38G) gravity is sufficient to maintain the long-term integrity of important physiological systems, such as muscles, bones and the cardiovascular system.

The newly launched project aims to estimate the forces experienced internally by lower limbs when the body is subject to different gravity environments. Specifically, it will map the variation in external loads, muscle strength and the reduced gravity effect to see how these interact when it comes to different forms of human locomotion. “This is a particularly exciting project to be involved in, with the potential for really significant impact in the years to come,” says Cowburn.

The project will be overseen by Aki Salo, Dario Cazzola and Steffi Colyer from the University’s Department for Health. “Anti-gravity devices are used by endurance runners and people in rehabilitation after lower limb injuries. We need to understand better how much strain can be reduced by these devices and what rate the load can be increased during the rehabilitation,” explains Salo.

“To better understand the effects of the space environment on human physiology and the challenges that ESA astronauts are likely to face during future exploration missions, we are actively engaging with academic institutions from across ESA’s Member States,” adds Jon Scott, Medical Projects Team Lead at the ESA’s Space Medicine Office. “We look forward to a productive co-operation with the University and are confident that the findings generated by the project will make a significant contribution to both terrestrial rehabilitation and human spaceflight knowledge.”

Measuring local invariants in topological materials

Topological materials – materials whose surface properties are very different to those in their bulk – have come to the fore in recent years and are currently revolutionizing modern condensed matter physics thanks to their unique properties.

Topological phases of matter are so-called because they are described by global invariants that are not at all affected by imperfections, such as defects or other variations, in a material. Mathematically, these invariants are constructed as integrals of a local property over a closed parameter space. And although they show much promise for use in a host of applications, including error-resistant spintronics and quantum computation, they have been considered as global, and not local, quantities thus far.

Existing picture may be wrong

A team of researchers at the University of Chicago in the US has now succeeded in measuring no less than three topological invariants of a quantum Hall type of material system, all with spatial locality. “Normally, topological invariants are quantities that require us to know details about the entire system in question,” explains study lead author Nathan Schine of the James Franck Institute and the Simon Lab at the Department of Physics. “For instance, you cannot know the knottiness of a loop of string without first seeing the entire string. This is how we think about topological invariants in physical materials, but recent research suggests that this picture may be wrong.”

The first topological invariant that the researchers measured is the Chern number, which directly provides a value for the electrical conductance (that is, how much electric current flows in response to an applied electric field). The second is the mean orbital spin, a bulk invariant that quantifies the magnetic-like coupling of a particle to curvature and which is related to the Hall viscosity and Wen–Zee shift. The third is chiral central charge, which is abstractly related to gravity, is equal to the total number of edge modes (neutral and charged) in integer quantum Hall and Laughlin states, and gives rise to the thermal Hall conductance. The mean orbital spin and the chiral central charge are similar to the Chern number in the way they are defined.

Very difficult to access

“The topological invariants we have measured are typically very difficult to access because just making a clean, low-disorder quantum Hall material is a big challenge in itself,” says Schine. “So, rather than fabricating a solid-state device, we built our material out of photons that we can trap and imbue with an effective mass by placing them inside an optical resonator.”

The resonator is made from high-reflectivity mirrors pointing at each other, he explains. Inside this structure, light can bounce back and forth thousands of times before being absorbed or scattered out.

“By engineering the geometry of the resonator, we make the trapped photons behave like electrons in a quantum Hall material,” he tells Physics World. “This means that these photons have the same sorts of topological structure that electrons would have in a regular quantum Hall material.

Unusual mathematical description

“However, because we use photons, we can go further and actually engineer the space through which the photons move, for example, so that it is curved like the surface of a cone. We then look at how much light transmits through this resonator as a function of the frequency of the light and where the light hits the resonator. This data contains information about the topological invariants since introducing a conical geometry (and then threading the cone tip with a small tube of magnetic flux, as in our experiments) produces a localized density response that strongly depends on both the mean orbital spin and the chiral central charge.”

The researchers say they made use of an unusual mathematical description of the Chern number in their work that goes back over a decade in a paper published by Alexei Kitaev. “This description is directly related to the types of local measurement that we perform in our lab,” says Schine.

3D topological insulators go photonic

Magnetic lengths

The Chicago team also discovered that the three invariants it measured converge to their global values when probed over increasing length scales – over several “magnetic lengths” in fact. This result agrees with what is expected for a topological insulator.

“Magnetic lengths are an important length scale in quantum Hall materials and in our system, one magnetic length is approximately 53 microns,” explains Schine. “Researchers usually describe quantum Hall systems as being comparable to or larger than this length and such a system has well defined and constant topological properties. The idea of convergence describes how topological invariants take on their expected values as the system size increases from smaller-than to greater-than the magnetic length.”

This idea is not very different to that of a solid crystal having the length scale of the crystal lattice spacing, he says. “It does not make sense to talk of a diamond crystal containing only one carbon atom, but once you have a system size of many lattice spacings, you have many carbon atoms and can so describe a diamond crystal – and start to think about its properties.”

The researchers, reporting their work in Nature 10.1038/s41586-018-0817-4, say they are now busy building more complex topological materials out of light. “These materials should have some very interesting and unique properties and the measurement techniques we have employed so far will be critical to measuring these and understanding their topological order.”

Trends, events and predictions for 2019

In the first episode of the year of Physics World Weekly, our journalists gaze into their crystal balls to predict the future. Hamish Johnston, Matin Durrani and Susan Curtis forecast the key trends in physics and related areas of science, including the unabating rise of quantum-based technologies. They also preview some of the year’s key events including the International Year of the Periodic Table (IYPT2019), a United Nations-backed initiative to celebrate the 150th anniversary of the table’s creation by Dmitri Mendeleev. Readers of Physics World magazine can also discover some of the highlights to look out for in the monthly publication during 2019.

If you enjoy what you hear, you can subscribe to Physics World Weekly via the Apple podcast app or your chosen podcast host.

Crystal ball image courtesy Christian Schnettelker (www.manoftaste.de)

Optical technology watches nerve cells fire

A research team from the Palanker Lab at Stanford University has developed a new technique for visually monitoring nerve cells firing. The technology could one day allow doctors to observe nerve activity in the eye (Light: Science & Applications 10.1038/s41377-018-0107-9).

The method relies on shape changes that occur when a nerve cell fires or “spikes”. During a spike there is a change in the potential across the cell membrane, which increases its surface tension, resulting in the cell temporarily becoming more spherical. This means that light passing through the cell exhibits phase changes after the spike, which the researchers can then detect using a technique called quantitative phase microscopy.

Previously, watching the electrical activity of cells was limited to using electrical recordings or fluorescent probes. Electrical recordings require placement of invasive electrodes adjacent to cells and have poor spatial resolution, while fluorescent probes are susceptible to phototoxicity, photobleaching and heating. In contrast, the new optical method is ideally suited for potential application in patients.

Developing new techniques

The researchers used cells engineered to spike in a similar way to nerve cells and examined these using an interferometric microscope. To confirm that what they were seeing was a spike, the researchers matched the images from the microscope with signals from an electrode array, which is a proven technique to identify nerve cells firing. They saw that when there was an electrical signal, there was also a change in the phase of the cells, attributed to the cells becoming more spherical.

The phase change, however, was smaller than the noise in the images, which made it impossible to detect a single spike. To circumvent this, the researchers used an ultrafast camera that collects 50,000 frames per second. They then combined 50 frames into one, which averaged out the noise and enabled them to see the cell deformation. The team also developed an algorithm — based on the basic template of a spike that they recorded previously using the electrode array — to identify regions where the signal was strongest and further increase the signal-to-noise ratio.

Using this method, the researchers could determine the extent of cell deformation and provide valuable insight into mechanical changes. They found that the cells deformed up to 3 nm and were able to test theories on cell deformation. As well as the clinical possibilities of the work, the researchers also hope that this study will provide a solid reference for understanding the mechanical effects in cells when they fire.

Eye opening possibilities

One key advantage of this technique is that it could be used in patients to image light-accessible parts of the body, such as the eye. This work is part of a larger collaborative project that aims to use the technology to detect signals passing through the optic nerve, or signals from individual nerve cells in the retina.

In the future, the team aims to use this technique in conjunction with optical coherence tomography — an imaging method commonly used to visualize the back of the eye. They hope that direct monitoring of individual cells will provide more information and allow researchers to better design new therapies for retinal diseases.

“These developments give promise for a day when we can study retinal diseases in humans on a cellular scale and evaluate the treatments to cure them,” comments the principal investigator of the collaboration, Austin Roorda from the University of California, Berkeley.

An anxious descent

Strong conclusions: Eric Johnson covers Ludwig Boltzmann’s science well, but veers into dangerous territory by pathologizing him. (Courtesy: Photo Researchers / Science Photo Library)

Eric Johnson’s beautifully titled Anxiety and the Equation: Understanding Boltzmann’s Entropy is a conundrum. The writing in this text, which doubles as an introduction to the life of Ludwig Boltzmann and his entropy formula, is unusually literary for a popular-science book. Johnson clearly has a knack for prose, and his passion – for both Boltzmann and the equation S = k log W that is considered to be his greatest accomplishment – shines through. For the reader who enjoys popular science that is a little technical in nature, this could be an enjoyable and, at 170 pages, fast read.

Yet I found myself struggling, greatly, with it. My deepest struggle was that I wanted to love the book because the structural ideas that underpin it are so fun. I love the idea of a biography that goes in reverse, and this is of course a poignant nod to the fact that the arrow of time is both irreversible and deeply enmeshed with the second law of thermodynamics, which the entropy formula illuminates. As time marches forward, entropy can only increase and the past is irretrievable. So the book begins with Boltzmann’s suicide and promises to work backward from there. It does, sort of. There are indeed episodes from Boltzmann’s life, but by the end I could not understand the exact thread that tied it all together. Johnson – a chemistry professor at Mount St Joseph University in Cincinnati, US – doesn’t tell us much about Boltzmann’s origins, and I would have preferred some nod to the traditional biography, even with this new fascinating twist.

As a working physicist, I’m not exactly the target audience, and I tried to be conscientious about this as I read. But I am also a theoretical physicist who thinks that statistical mechanics is extremely undervalued in the physics curriculum, which is why I agreed to review this book. My hope was that this would be a text I might give to undergraduates, to give them a helpful qualitative discussion of the entropy formula. Indeed, Johnson seems to love explaining the entropy law, which quantitatively gives the relationship between the entropy of an ideal gas and the number of possible microstates – configurations of particles – for that gas. Indeed, arguably, this is the equation that allows us to have a sense of what entropy even is: a quantification of the number of microstates, given a macrostate, or the large-scale characteristics of the gas.

Johnson undoubtedly faces a daunting task: how to explain a fundamentally mathematical concept and equation without resorting to derivations that are written in equations? In many ways he succeeds – by relying on our intuition in an unusual way. Often we teach physics by hoping that our student will have an intuitive sense of how things usually work. Johnson flips this by beginning with a physical scenario that – based on everyday experience – we all have a strong intuition is highly unlikely or impossible. What is the likelihood that all of the air in a room is in the left half of the room or the right half, instead of evenly distributed across both? From here, Johnson is able to lead the reader to intuition about both microstates and macrostates, and he does it better than any textbook I’ve ever read.

Having read the book, I think I could give this book to undergraduates, but would only recommend it to them with a caveat: ignore nearly everything that Johnson has to say about anxiety and mental health because it’s just one man’s opinion, which he never backs up.
Chanda Prescod-Weinstein

Even so, he ultimately relies on equations to explain the logarithm that appears in the formula, and after such a creative attack on microstates, this is a bit disappointing. In either case, this is not a book I would give to someone who is uncomfortable with maths, and I won’t be giving it as a present to family. Having read the book, I think I could give this book to undergraduates, but would only recommend it to them with a caveat: ignore nearly everything that Johnson has to say about anxiety and mental health because it’s just one man’s opinion, which he never backs up.

Boltzmann displayed behaviours that have, in hindsight, been pathologized as signalling mental illness, possibly bipolar disorder. In his text Johnson argues that it was more likely an anxiety disorder. Although he caveats his comments with a reminder that he’s not a health professional, Johnson draws some rather strong conclusions, both about the possibility of an anxiety disorder and the nature of anxiety, which he calls a “21st-century disease”. He goes on to explain that by this he means that one can only feel anxious in a context where one’s basic needs are already taken care of, and that in fact, anxiety is irrational. Along with his casual use of “crazy”, this explication of anxiety disorder troubled me. An alternative interpretation that he does not grapple with at all is that in the 21st century, we are becoming more conscious of how the brain works, rather than that we are becoming more sensitive. Does this mean that those of us who have experienced poverty could not be diagnosed with anxiety disorders because we only felt fear in those moments? Many racialized people experience racism-related anxiety, even if they are from comfortable income backgrounds. Do the conclusions that Johnson draws apply outside of the Global North? If not, does this mean that the Global South is not in the 21st century?

My wish is that Johnson had chosen – or been encouraged to – engage with disability justice literature, which problematizes the common use of “crazy” because of the way casual uses of this language actually harms disabled people. My worry about sharing this with undergraduates is that we know they experience significant amounts of anxiety, and I would be concerned that this text could make them feel bad about it, although that’s clearly not Johnson’s intention. Johnson in fact ends his book by saying Boltzmann was “a kind man with a generous mind”. I wish I had gotten this sense from the book, but these qualities were never emphasized to provide context for what anxiety might have really meant in Boltzmann’s life.

Dying star sheds light on spin of supermassive black hole

A surprising X-ray signal coming from a supermassive black hole at the centre of a distant galaxy could provide crucial information for determining how these behemoth objects are formed.

The two most fundamental properties of a black hole are its mass and its spin. If you know these, “then you know everything there is to know about a black hole,” says Dheeraj Pasham of the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology, who led the observation.

Measuring the mass of a supermassive black hole at the heart of a galaxy is relatively straightforward. Astronomers just determine the velocity of stars in orbits around the object, and the more massive the black hole, the faster the stars are moving. Measuring how a black hole is spinning, however, is more complex because the spin only affects space-time very close to the black hole’s event horizon.

X-ray flashes

On 22 November 2014, the All-Sky Automated Survey for Supernovae (ASASSN) detected a massive flash of X-rays from a galaxy 300 million light-years away. Called ASASSN-14li, the event proved not to be a supernova, but the death cry of a star pulled apart by the tidal gravitational forces of a supermassive black hole with a mass about a million times greater than the mass of the Sun.

A trio of X-ray satellites – the Chandra X-ray Observatory, XMM-Newton and Swift – quickly followed up, and while pouring through the data collected, Pasham and his colleagues in the United States and Europe found an additional, mysterious but periodic, X-ray signal that persisted for the 450 days of observations made by the three satellites.

The signal brightens and fades every 131 s and, at its peak, is 40% stronger than the average X-ray brightness of the black hole. The short period implies that it comes from a source that is orbiting very close to the black hole, in what is known as the innermost stable circular orbit (ISCO). This is the smallest orbit that an object can take around a black hole without falling through the event horizon. The faster the black hole is spinning, the closer the ISCO is to the event horizon. Based on the 131-second periodicity of the orbit, Pasham’s team calculate that the event horizon of the black hole is spinning at half of the speed of light.

Spaghetti and a white dwarf

Some of the debris from the dead star, which produced the original burst of X-rays when it was “spaghettified” by the black hole’s gravitational tidal forces, should still be circling around the black hole, but the debris alone would not produce the periodic X-ray signal. Instead, the best explanation, says Pasham, is that there is an unidentified object also in orbit around the black hole. This object must be denser than the star that was torn apart, otherwise it too would have been destroyed by gravitational forces – and this leaves few options.

Pasham’s theory is that the object in the ISCO is a white dwarf, which is the dense core remnant of another star. The stellar debris accumulates onto the white dwarf, creating a hotspot on its visible surface. As the white dwarf orbits the black hole, we see the X-ray signal from the hotspot periodically brighten and fade as it moves behind the black hole.

Pasham is cautious about this theory. “While this model can explain the observation, it is a very rare situation,” he tells Physics World. This is because the white dwarf should not linger in the ISCO for more than a few hundred years. Although the orbit is gravitationally stable, other forces are at work, These include the viscosity and turbulence of the stellar debris through which the white dwarf is moving, magnetic fields around the black hole, radiation pressure and even the emission of gravitational waves, all of which will eventually see the white dwarf’s orbit decay into the black hole. Therefore, the likelihood of one star being ripped apart at the same time as a white dwarf being present around the black hole seems slim.

Constant supply of stars

However, the centres of galaxies are hotbeds for star formation, so there is a constantly replenished reservoir of stars for supermassive black holes to gobble up. Furthermore, there are enough supermassive black holes in the universe that all-sky searches should be able to detect more similar events – called “tidal disruption flares” – which could lead to some exciting possibilities.

Understanding how supermassive black holes form and grow remains a longstanding puzzle and knowing how fast the objects are spinning will reveal their formation history. The fastest spinning black holes are rotating at close to the speed of light. For example, the black hole at the centre of the galaxy NGC 1365 has been measured to rotate at more than 80% of the speed of light. Black holes with such rapid rotations are thought to form mostly from the accretion of large amounts of gas, whereas black holes formed mostly from the mergers of smaller black holes, where angular momentum is being added to the system from random directions, end up spinning more slowly. The high spin rate of the black hole in the ASASSN-14li event indicates that it may have formed mostly from gas accretion.

By measuring the spins of many black holes from future observed tidal disruption flares, “we can build spin distributions of supermassive black holes at various redshifts, which would directly constrain models for the growth of supermassive black holes,” says Pasham.

The research is described in Science.

A swansong from the Energy Technologies Institute

Set up in 2007, the Energy Technologies Institute (ETI) brings together researchers at Loughborough, Nottingham and Birmingham universities and elsewhere to look at system-level energy issues in a £400m UK industry–government partnership. That scheme finishes at the end of this year.

So, although we will no doubt hear more from the ETI in its last year, and from the various Catapult groups that have emerged, the Institute has brought together some of its conclusions into something of an early “goodbye” overview of the various options. The report is based on the ETI’s Clockwork and Patchwork scenarios, now revised since their original publication three years ago, for example taking account of the downward trends in energy service demand, which the ETI says “tend to make carbon targets easier to meet”.

The high-level conclusion is that “a balanced, multi-vector approach can deliver an affordable, low carbon UK energy transition, with costs rising to around 1% of GDP by 2050”. But it says “without certain key technologies, meeting carbon targets would be much harder, jeopardizing industry and severely limiting lifestyle choices”. Although it warns that, given the potential for innovation across a range of technologies, “we cannot be prescriptive about the precise mix over a 30-year period”, it does push some ideas forward, and adopts quite a challenging approach.

For example, the ETI notes that “sustainably grown biomass has the potential to become a critical resource for the UK energy system”, since it can be burned directly for heat and power, or converted into low-carbon gases and liquid fuels to decarbonize hard-to-treat sectors. That’s not a popular view, given the land-use constraints and biodiversity issues. It’s more usual these days to look to farm, food and municipal bio-wastes as a bio-energy source. The ETI also says that “carbon capture and storage (CCS) offers a versatile solution with applications across power, industry and hydrogen production”, arguing that “without CCS, UK carbon abatement costs could be double by 2050”. Given the low state of CCS work at present that’s also provocative.

However, the ETI overview’s major theme is that “bioenergy and CCS are especially valuable in combination”. It says that, “together, they offer the potential for negative emissions to counterbalance the continued use of fossil fuels in difficult sectors like aviation. Without negative emissions generated in the UK, achieving a ‘net zero’ emissions target will require the prohibition of certain industrial activities and lifestyle choices or reliance on imported carbon credits from other countries.” So, like the IPCC, the ETI backs BECCS. But with CCS at a low ebb, that seems optimistic.

ETI seems to be on safer ground when it says “system flexibility requirements will change, and new approaches will be needed. Storage of electricity, heat and gas (including hydrogen) will all have a role to play, along with backup generation in power and hybrid systems for heat”, although the institute’s approach to green heat is quite challenging. “Low carbon heat solutions exist but consumer experience is key,” it says. “Most UK households have relied on gas boilers for more than a generation. Low carbon alternatives will require powerful consumer propositions that match, if not exceed, current experiences”. That could be taken as a challenge to the government’s initial focus on fossil gas decarbonization via electric domestic heat pumps, although in the ETI’s Clockwork 2050 scenario, heat pumps make up 20% of all building heating capacity, and supply over 50% of UK space & heat production. So the “electrification” approach is still in there.

Heat time

However, the ETI also backs heat networks. In its Clockwork scenario, large district heat networks are also rolled out from 2030 onwards in urban areas, with a range of technologies deployed to supply the heat to these networks, beginning with smaller gas combined heat and power (CHP) plants, while medium-term, low-carbon sources include heat recovery from large-scale thermal electricity plants. But, more dubiously, it says “in the longer term, new and extended networks are fed by heat offtake from small modular CHP nuclear reactors”. Though the institute also adds “commercial-scale marine heat pumps also make a sizable contribution by 2050”.

The ETI continues “in those areas where heat networks are not economic, there is a recognition that full electrification — to the extent required to cope with extreme cold weather — would place undue stress on electricity networks. For this reason, existing gas networks in these areas are maintained but energy throughput is much reduced, with gas boilers now playing a supplementary role in support of electric heat pumps, as part of a hybrid solution.” That is very much the CCC’s line, heat pumps backed up by gas boilers, some of the latter running on hydrogen.

So ETI says “some areas undergo conversion of the gas distribution network to deliver hydrogen to buildings. In areas anticipating hydrogen conversion, advanced notice ensures end-of-life gas boilers are replaced with hydrogen-ready models, minimizing the risk of stranded assets”. It adds “thanks to a concerted effort by government and gas network operators, around one third of the remaining gas network in 2050 is fully converted to hydrogen distribution, delivering over 55 TWh of hydrogen annually. The other two-thirds of the remaining distribution network continue to supply natural gas, but reduced gas boiler operation means that annual delivered energy is actually lower than from hydrogen”.

Finally, in addition to industrial take-up of green energy and hydrogen, the ETI says electrification of transport can begin to deliver significant carbon dioxide cuts from 2020 onwards. The speed of transition is uncertain, but it says whole-system coordination can ensure we make best use of existing electricity system capacity, minimizing the need for investment in grid upgrades to support mass electric vehicle (EV) adoption. Well, we’ll see.

Growth patchwork

In the ETI’s Patchwork scenario, in which growth and wealth are at higher levels, there is much less government co-ordination, slower decarbonization, but more diversity. Less CCS, hydrogen, and nuclear, but more CHP and local photovoltaics (PV), with electric heating and heat storage being popular. Lots to debate about that! And, indeed, about the whole ETI approach. Its acceptance of renewables as major players is welcome and its commitment to hydrogen is interesting, but its promotion of nuclear and CCS, including BECCS, seems a little surprising. More understandable is its hedging of bets on green heat – the “pipes versus wires” debate continues!

The ETI concludes by saying “we have refreshed Clockwork and Patchwork as part of the ETI’s legacy of insights and evidence. The scenario team now takes this work forward within the Energy Systems Catapult. We hope that this publication will once again stimulate discussion with stakeholders across the energy system as we continue to learn together about the key options and choices facing decision-makers across the UK”. Amen to that.

Looking inside a proton therapy system

In this short video, Imran Patel explains the processes involved in proton therapy treatment at the Christie Hospital in Manchester, UK. Patel, head of proton therapy physics at the hospital, points out the different features of the equipment, including the accelerator, rotating beam delivery system and the treatment gantry. Patel explains how different medical professionals work together to create a treatment plan for patients.

Why solid-state physics is like a Star Wars plot

In a daring image at the beginning of Solid State Insurrection: How the Science of Substance Made American Physics Matter, science historian Joseph D Martin compares the rise of solid-state physics to the plot of the original Star Wars movie. Both, he points out, involve “a ragtag band of misfits, many of whom are adept at manipulating a force pervading everyday matter, who ally to mount an insurrection against the established order and help destroy a giant, partially built beam machine”.

At first the image seems over the top, the hyped image of a popularizer. But what follows over the next thoroughly documented 200 pages is a dramatic story, by a skilled historian, of just that.

Martin’s tale begins in the 1940s, with Bell Labs’ characterization in 1946 of “solid-state physics” as a research area, and the American Physical Society’s creation of a division of solid-state physics the following year. These steps gave the field institutional recognition. Throughout the next few decades however it struggled, lacking not only prestige but also solidarity, so to speak. While nuclear and particle physics were more or less unified, their solid-state sibling was “balkanized” into subgroups, “analogous to a disorganized system beginning to self-organize”, as Martin puts it.

For the next few decades, solid-state physics continued to lack status and voice, and was famously dismissed by Murray Gell-Mann as “squalid state physics” and by Wolfgang Pauli as Schmutzphysik (literally, dirty physics). In the 1960s, it acquired its first big user facility, the National Magnet Laboratory at the Massachusetts Institute of Technology. Sceptics began to challenge contentions by particle physicists that their field was the frontier – not only the most fundamental branch of physics but also the one responsible for applications and spinoffs. Resistance leaders included Alvin Weinberg, director of Oak Ridge National Laboratory, who in the 1960s challenged particle physicists about the meaning of “fundamental physics” and Bell Labs theorist Philip Anderson with his 1972 Science article “More is different”.

In the 1970s solid-state physics began to mature as a discipline, but continued to suffer an identity crisis from its interdisciplinary nature and inevitable unfavourable comparisons with its wealthier, prominent and more unified sibling – particle physics. Martin is engrossing in tracing the sometimes bumpy reframing during the years that followed, of solid-state physics morphing into materials science (a more interdisciplinary field), then into condensed-matter physics – a European concept that included liquid matter.

Fighting the death star

While all this was going on, US high-energy physicists continued to monopolize the discipline’s prestige, funding and facilities. In 1982 they hatched a plan for their biggest facility yet: a giant beam machine. The Superconducting Super Collider (SSC), was to be built in Texas, targeting discovery of the Higgs particle or whatever other key physics phenomenon was behind the secret of mass. High-energy physics seemed to be not only the frontier but entering an era of “Megascience”, with projects of ever bigger scale, collaborations and funding.

But directors of existing high-energy labs feared the SSC would divert badly needed money from their projects; materials scientists feared that funding for their fields would dry up. In 1986, to head off growing opposition, the director of the Office of Energy Research at the Department of Energy, Alvin Trivelpiece, devised a plan whose impact, in my opinion, is insufficiently recognized by Martin.

Trivelpiece decided to award each of four national laboratories a large facility. Three – the Advanced Light Source at Berkeley, the Advanced Photon Source at Argonne, and the Advanced Neutron Source at Oak Ridge – would serve materials scientists, while the fourth – the Relativistic Heavy Ion Collider – exploited infrastructure that Brookhaven National Laboratory had previously built for an accelerator that had been axed to support the SSC.

The SSC’s promoters, however, made two errors. First, they underestimated its cost – partly to kill Brookhaven’s accelerator, partly from technical over-optimism and partly from mismanagement. Second, they misjudged the impact of materials scientists’ newfound strength. In Martin’s words, the SSC crystallized “the boiling over of tensions that had strained the American physics community for half a century”.

The ensuing controversies in scientific literature and the halls of Congress finally exposed the weak underpinnings of claims that physics at high energies was the font of applications and spinoffs and also the field’s true frontier. The notion of a single intellectual frontier – a “reductionist worldview,” Martin says, that amounted to “pure-science fundamentalism” – was challenged practically by the excitement surrounding such endeavours as the Human Genome Project. It was also challenged theoretically by serious philosophical examination of Anderson’s point that “more is different”.

Anderson’s view was that the behaviour of metals could not be understood simply by adding particles together; instead it had to be approached with unique sets of concepts and tools. When the SSC – overbudget and lacking support across the full physics community – was terminated by the US Congress in 1993, the four components of the Trivelpiece plan were under way. Condensed-matter physics was thereby not only “left standing”, as Martin puts it, after the US Congress’s vision of advanced physics was shot down, but well equipped to advance. The material – in the sense of facilities – for materials science was solidly in place.

The critical point

The transformation of solid-state physics into condensed-matter physics, and its support by a wide variety of facilities in the wake of the destruction of the Death Star, is part of a bigger story that involves the morphing of physics into a different, more pluralistic kind of science during the past three decades in particular. Unlike Star Wars, though, this tale doesn’t take place a long time ago and far, far away. Instead, it is still unfolding right in front of us. In a future column I’ll provide an illustration.

3D human atrial model enables study of heart function

More than 33 million people worldwide suffer from atrial fibrillation, an irregular and often rapid heart rate that can increase risk of stroke, heart failure and other cardiac complications. Studying atrial biology may offer insight into treatment development.

One obstacle to the development of anti-arrhythmic drugs is the difficulty of isolating and maintaining human atrial cardiomyocytes (cardiac muscle cells). Atrial cardiomyocytes, together with the ventricular cardiomyocytes, form the muscular walls of the heart — the myocardium — and make an important contribution to the refilling of ventricles with blood, which enhances the subsequent ejection of blood from the heart. Because animal models do not accurately represent human cardiac physiology, cardiomyocytes derived from human induced pluripotent stem cells (hiPSC) may represent a solution for evaluating potential drugs.

With this in mind, scientists from Germany and the UK have evaluated the suitability of hiPSC-derived atrial-like cardiomyocytes (hiPSC-CMs) as a 3D model of the human atrium. A 3D model offers a physiological cell environment and allows the study of heart function parameters. The findings of this study offer a new platform for the investigation of heart function parameters and pharmacological responses (Stem Cell Reports 10.1016/j.stemcr.2018.10.008).

Model creation

To generate a 3D model of the human atrial heart muscle, the authors induced an atrial phenotype in hiPSC-CMs using previously established retinoic acid (RA) protocols. RA is a vitamin A metabolite involved in the switch between cell proliferation and differentiation.

The RA treatment caused the hiPSC-CMs to achieve properties characteristic of the atrial heart muscle: a decrease in cell size, together with an increase in gene expression and protein level of atrial-specific markers (especially MLC2A, a protein that modulates cardiac development and contractility) and ion channels (potassium channels). These markers indicate that RA promotes atrial-like cardiomyocytes to develop instead of ventricular-like cardiomyocytes. Moreover, the authors noticed that the hiPSC-CMs beat and contracted faster upon RA treatment.

Electrophysiological characteristics

The team used electrophysiological characteristics, such as the action potential duration and the repolarization fraction, to distinguish between atrial and ventricular-type functionalities. RA treatment resulted in a reduced action potential duration and an increased repolarization fraction in hiPSC-CMs, which correspond to an atrial-like electrophysiological phenotype.

In the human heart, two potassium currents are predominantly expressed: the acetylcholine-activated potassium current and the ultrarapidly activating delayed rectifier potassium current (I_Kur). When the researchers added carbachol, a drug that activates and binds acetylcholine receptors, they observed a shortening of action potential duration in the RA-engineered heart tissue. In addition, when they applied 4-aminopyridine (the inhibitor for I_Kur), the expected changes in action potential morphology were observed in the RA-engineered heart tissue. These findings suggest that following RA treatment, a more atrial-like electrophysiological phenotype was obtained for the engineered heart tissue.

This study indicates that RA-treated hiPSC-CMs develop an atrial phenotype and form spontaneously beating engineered heart tissue. This tissue shows characteristic atrial heart muscle features, in terms of gene expression, contraction kinetics, action potential features, and pharmacological responses to potassium current blockers and activators. The 3D engineered atrial heart may serve as a useful model in preclinical drug development.

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