An international team of astronomers has found a potential explanation of how red-giant stars loose the bulk of their mass towards the end of their lives – a process currently not fully understood. Using new observational techniques, the researchers looked at the dust shells surrounding these dying stars, which gave them information about what causes the powerful “superwind” of dust grains that leads stars to lose their mass. Much of this star dust comprises of silicates, which go on to form planets such as the Earth.
Celestial lifestyles
Towards the end of their lives, intermediate-mass stars – stars with masses ranging from 0.6–10 solar masses – eject the bulk of their outer envelope in a slow, dense wind. This “superwind” occurs over a period of 10,000 years, is a 100 million times stronger than the solar wind and removes almost half the mass of the star, leaving behind a fading stellar remnant.
The problem with this model, though, is explaining just how so much mass is lost in the “superwind”. This is because it is difficult to observe gas and dust that is very close to its parent star. Current theory suggests that the “superwinds” occur because of the acceleration of the minute dust grains in the shells surrounding the stars. These grains are said to absorb starlight, which transfers momentum to them and causes the dust to blow away from the star. The problem with this model is that at the grain size estimated, light from the star would cause the dust to sublimate before it could be pushed away.
Unmasking stellar dust
In the new work published in Nature, the team, led by Barnaby Norris from the University of Sydney, Australia, looked at three red-giant stars and their dust shells using the European Southern Observatory’s Very Large Telescope in Chile. The researchers used a technique known as “aperture-masking polarimetric interferometry” to look at the red giants plus other dust-free stars to verify their detection methods. Team member Albert Zijlstra, of Manchester University’s Jodrell Bank Observatory in the UK, explains that using an “aperture mask” inside an infrared instrument along with a polarimeter is a combination that had not been previously used for this purpose. “The aperture mask turns a single telescope into a collection of much smaller telescopes, which can then be used as an interferometer. This gives excellent, reproducible image quality at the cost of reduced sensitivity,” he explains. Using the observed data, the team developed a model to determine the dust-shell radius and the amount of light scattered by the shell at each wavelength.
The researchers found that the dust exists a lot closer to the stars than previously thought – less than two stellar radii. They also found that the grains are much larger in size than expected, being almost a micrometre across or about 300 nm in radius – this is quite large for stellar-wind particles. At these sizes, the grains are transparent to starlight, and so would not be sublimated by the intense radiation from the light. Although transparency suggests that the grains would again not be propelled away to form the wind, the researchers say that the acceleration occurs as a result of photon scattering rather than absorption. These large grains are driven out at speeds of 10 km s–1, creating a virtual “stellar sandstorm”.
Zijlstra says that the work provides new insights into “superwinds” and the process of stellar evolution. “The dust and sand in the superwind will survive the star and later become part of the clouds in space from which new stars form. The sand grains become the building blocks of planets. Our own planet was formed from star dust. We are now a big step forward in understanding this cycle of life and death,” he says.
Replacing roofs and pavements with more reflective versions could lower global temperatures by up to 0.07 °C, equivalent to a reduction in carbon-dioxide emissions of about 150 billion tonnes. That is according to researchers in Canada who used a global climate model to look at the effects of such albedo changes in urban areas.
“Scientists have been proposing novel ideas – mostly untested – for the geoengineering of global climate,” says Hashem Akbari of Concordia University. “But humans have had experience with white buildings and reflective pavements for thousands of years without any unknown negative side effects. Hence, cool urban surfaces should be our geoengineering 101.”
From Lyon to Dunedin
Akbari and colleagues from Concordia used the University of Victoria Earth System Climate Model to investigate the effect of albedo increases of 0.1 until 2300 over all land between latitudes of ±20° (i.e. roughly from Mexico City and Hanoi in the north to Bulawayo, Zimbabwe, in the south), and between ±45° (approximately from Lyon in France and Portland in the US to Dunedin in New Zealand). The team used both a business-as-usual emissions scenario and an aggressive mitigation scenario.
The albedo increase on all land between ±20° latitude would decrease temperature by roughly one degree over 20 years, while the 45° latitude case would double this decrease. After 200 years, the decreases would be 1.3–3 °C. The scientists estimated that urban areas make up roughly 1% of the total land area in these regions; increasing albedo by 0.1 only in urban areas would be equivalent to a global change in land-surface albedo of 0.001.
“Increasing albedo of urban areas by about 0.1 – increasing flat-roof albedo by 0.4, increasing sloped-roof albedo by 0.25 and pavement albedo by 0.15 – cools the globe equivalent to offsetting more than 100 billion tonnes of carbon-dioxide emissions,” says Akbari. “This is equivalent to offsetting the emissions for all the cars in the world for the next 20 to 30 years.”
In order to firm up their calculations, the researchers employed two estimates of urban area: one from the Global Rural and Urban Mapping Project (GRUMP), and another from an analysis based on MODIS satellite data. The GRUMP results suggest that global urban areas are more than five times larger than the MODIS data set indicates.
Urban-only cooling significant
The climate model revealed that increasing albedo by 0.1 only in GRUMP-designated urban areas would produce long-term cooling of 0.07 °C, equivalent to 130–150 billion tonnes of carbon. Using the MODIS data for urban areas, in contrast, would cool the Earth by 0.01 °C, equivalent to 25–30 billion tonnes of carbon.
According to Akbari, albedo increases could lead to air-conditioning savings of about 20% for space under roofs. “This is a saving of about $50bn per year and carbon-dioxide savings of about 0.4 billion tonnes per year; over the next 100 years; that is an emission reduction of 40 billion tonnes, “he says. “The direct cooling of the Earth by reflecting radiation back into space is an added bonus that actually counters global warming while putting dollars in our pocket.”
The researchers found that the effect of albedo change did not depend to a large extent on the carbon-dioxide emissions scenario. That said, aggressive mitigation appeared to produce a roughly 10% larger temperature decrease, which the team ascribed to stronger snow-albedo feedback.
“We should develop policies for no-regret, no-cost global-cooling measures,” says Akbari. “Cool cities will save all the people in the world equally and the value of the dollar saved is significantly higher in developing countries than the developed country (e.g. $1 saved in the US pays for 10 min of a labourer in the US; in the developing countries that pays for a day of labourer).”
Stringed Relief, reproduced by permission of The Henry Moore Foundation
By James Dacey
My first experience of Henry Moore’s sculptures came from several visits to the Yorkshire Sculpture Park, located near where I grew up in the north of England. As a kid on a day trip with my parents, I was no art critic. But I was always fascinated by Moore’s looming bronze figures dotted across the rolling Yorkshire hillside. Within the works, I could see both the abstract body parts of a giant metal person, but also what appeared to be stark geometric shapes.
So, I was interested to hear the news that a new exhibition in London is celebrating Moore’s fascination with mathematics. The exhibition is being held jointly by the Royal Society and the Science Museum, and it showcases some of Moore’s lesser-known sculptures that were directly inspired by maths, including the work above, Stringed Relief. According to the exhibition catalogue, Moore (1898–1986) stated on several occasions that the use of string in his sculpture, which he started in 1937, was influenced by seeing models at the Science Museum in London.
I was fascinated by the mathematical models I saw there, which had been made to illustrate the difference of the form that is halfway between a square and a circle. One model had a square at one end with 20 holes along each side…Through these holes rings were threaded and lead to a circle with the same number of holes at the other end. A plane interposed through the middle shows the form that is halfway between a square and a circle…It wasn’t the scientific study of these models but the ability to look through the strings as with a bird cage and see one form within the other which excited me.
Moore is by no means the only artist to have drawn inspiration from the ideas of science. One high-profile contemporary example is the British artist Anthony Gormley, who has created several sculptures inspired by the theory of quantum mechanics. It has also been suggested that Picasso’s development of the Cubist style of painting was informed by a similar line of thinking to Einstein’s during the formulation of the theory of relativity.
Two teams developing rival designs for an international linear collider will continue with their own separate blueprints – even though both teams are joining forces at the organizational level. Barry Barish, head of the design effort for the International Linear Collider (ILC), told physicsworld.com that both the ILC and the rival Compact Linear Collider (CLIC) will remain distinct projects despite the recent creation of a Linear Collider Board that will govern the development of the two designs. But given the costs of building such a machine, which will be the successor to CERN’s Large Hadron Collider (LHC), only one such design is likely to be built and the new organizational structure will not result in one joint proposal.
Most of the R&D design for CLIC, which could measure precisely any of the new particles that the LHC might discover, is being carried out at CERN. Both it and the ILC will collide electrons with positrons, but while the ILC will use superconducting technology to collide particles with energies of about 500 GeV, CLIC will collide particles at 1 TeV or more using a novel “two beam” acceleration technique.
The ILC design is more mature than CLIC’s and is, in principle, ready for construction whereas some of CLIC’s concepts have not yet been proven and would need to be demonstrated over the coming 5 to 10 years. “The two groups already work together on mutual technical problems, but this will enable joint planning, including preparations for any comparisons in the future,” says Barish. “There can only be one possible linear collider in the world, preparations are expensive and need to be co-ordinated.”
Joint leadership
Under the new structure, the directorate of the two design teams will report into a linear collider director, a new position that has yet to be appointed. The director will then report into the Linear Collider Board, which will oversee the preparation of a collider proposal that will then report to the International Committee for Future Accelerators chaired by Fermilab boss Pier Oddone. The Linear Collider Board will consist of 16 members – a chair plus five representatives each from Europe, Asia and the Americas – who have not yet been chosen. “Clearly, joint leadership can help bring this all together as a global project best matched to the science,” says Barish.
“I think this is an excellent development, which I have been supporting for some time,” says theoretical physicist John Ellis from Kings College London, who is on the CLIC steering committee. “There are many technical issues in common between CLIC and the ILC such as civil engineering, beam delivery and detector design, that it makes great sense to work together”
Indeed, deciding which design to go for will depend on what physics the LHC discovers over the coming years. “If the ILC parameters match the science coming from CERN, it will be the obvious choice; but if it is crucial to go well beyond 1 TeV, then the ILC is impractical and CLIC represents a possible solution on a longer timescale to go as high as maybe 3 TeV,” says Barish.
The ILC team is expected to release its technical design report by the end of the year.
In the latest episode of the Physics World books podcast, released yesterday, we look at the enduring appeal of quantum mechanics in popular-science books. I presented the programme along with Physics World‘s editor Matin Durrani and the magazine’s reviews editor Margaret Harris, and we were joined by several authors of these books. In the podcast we explore the question of why it is that so many popular-science books have been written on the topic over the years, and why the public has such a strong fascination with the ideas of quantum physics. You can find more details and listen to the podcast here.
One of the aspects we discuss is the counterintuitive nature of the quantum world. On the one hand, this weirdness of quantum mechanics can be intriguing because it describes a world that is so different from our everyday experiences. But on the other hand, some of the concepts and the mathematics of quantum mechanics can be quite mind-boggling, and it can be difficult to explain these ideas in basic terms. I know from the experience of writing news articles about quantum mechanics that it can sometimes be very challenging to find everyday analogies to describe the quantum world while remaining faithful to the underlying physics.
But I want to know what you think about this, via this week’s Facebook poll question:
What is the trickiest feature of quantum mechanics to get your head round?
Wave–particle duality The Heisenberg uncertainty principle Entanglement, aka “spooky action at a distance” The Pauli exclusion principle Superposition
Have your say by casting your vote on our Facebook page. As always, please feel free to explain your response by posting a comment.
In last week’s poll we embraced Stephen Hawking-mania. The 70-year-old theoretical physicist appeared in an episode of the hit TV show The Big Bang Theory, which aired on CBS in the US last Thursday. Unfortunately, I haven’t been able to see the show yet here in the UK, but I have seen Hawking in some of his earlier cameos, including his several appearances in The Simpsons and his role in Star Trek: The Next Generation. We asked you “In which TV show should Stephen Hawking make his next cameo appearance?”.
The most popular choice by a country mile was Doctor Who, which picked up 70% of the votes. In second place with 18% of the vote was the slightly more leftfield choice that Hawking should appear in the US sitcom How I Met Your Mother. Just 7% of voters opted for the musical comedy-drama Glee, while only 4% opted for the cult UK sci-fi comedy Red Dwarf.
So, Stephen Hawking; producers of Doctor Who. Make this happen. For the sake of our Facebook fans, please.
Thank you for all your participation and we look forward to hearing from you in this week’s poll.
It was in 1935 that the Austrian physicist Erwin Schrödinger proposed his now-famous cat image to comment on what he thought was the irresponsible failure of his colleagues to think through quantum mechanics. He could hardly have imagined that the cat, which he introduced half-jokingly, would still be discussed almost 80 years later – nor that it would have become permanently ingrained in popular culture. So why does the image still seem as packed with creative force as ever?
One recent example crops up in Will Grayson, Will Grayson, a young-adult novel published by John Green and David Levithan in 2010. In the book, Will asks Jane – a girl for whom he has mixed and unexpressed feelings – about Schrödinger’s cat. Jane describes the physicist’s famous thought-experiment, before adding that Schrödinger “was not endorsing cat-killing or anything…just saying that it seemed a little improbable that a cat could be simultaneously alive and dead”.
Will ponders that for a moment. Thinking of his own mixed emotions – though attracted to Jane, he once declined her offer of a kiss – he doesn’t think it strange that something can be real and not real at the same time. “[A]ll the things we keep in sealed boxes are both alive and dead until we open the box,” he broods to himself. “[T]he unobserved is both there and not.”
A completely different Schrödinger’s cat image is found in Blueprints of the Afterlife, an apocalyptic science-fiction novel by Ryan Boudinot, published this year. It features a character named Abby Fogg, who shows up both dead and alive at the same time after being programmed to infiltrate another reality. In a morgue one day, she creepily stares at two naked and dead identical versions of herself. “[Y]our selfhood, Abby, has gone into superposition,” the forensics director tells her. “It’s as if you are both alive and dead simultaneously, and this simultaneity is a self-replicating system in which there are various ‘snapshots’ of your dead self. Which makes an autopsy pretty dang hard, let me tell you.”
Weird stuff
Quantum mechanics describes the world as the product of two ingredients. The first is an information function, the ψ-function described by Schrödinger’s equation, which is a classical wave that expands outwards and overlays, or “superimposes”, many possibilities. The second ingredient is something that befalls this function, causing it to disappear and one of its possibilities to appear. If this sounds odd to you, you are not alone: even the pioneers of quantum mechanics struggled to connect this strange picture with the familiar world.
Niels Bohr and Werner Heisenberg said the world is divided into two separate domains: quantum and classical. The quantum domain is governed by the unobservable ψ-field and when this encounters something in the classical domain, through measurement or other interactions, the encounter evaporates, or “collapses”, the function. One hitherto only probable state becomes “real” and all other possibilities are eliminated.
This idea was sufficiently weird that it sparked opposition. Einstein led the attacks, which culminated in the famous “EPR” paper of 1935 co-authored with Boris Podolsky and Nathan Rosen, entitled “Can quantum-mechanical description of physical reality be considered complete?”. Published in May of that year (Phys. Rev. 47 777), the paper’s answer to the rhetorical title question was a clear “no”. There must be elements independent of processes of measurement, the EPR trio argued. Our common-sense experience – and the very definition of reality – depends on elements the existence of which is independent of observation and measurement.
Schrödinger was thrilled, and wrote to his friend Einstein expressing his delight. “You have evidently caught dogmatic q.m. by the coat-tails,” he declared. By “dogmatic q.m.”, Schrödinger meant quantum mechanics as espoused by Bohr and Heisenberg that denied the reality of certain properties such as position and momentum apart from in measurement situations.
Einstein replied equally enthusiastically, and elaborated on his intuitions: physics describes reality, but not all descriptions are complete. He imagined having two boxes with lids you can open to peer inside, and there’s a ball in one. Before you “make an observation” by looking inside the first box, how do you describe the situation? We say, quite correctly, that the probability that the ball is in the first box is ½, or 50%. But is that a complete description? Of course not, Einstein answers. It characterizes only our knowledge of the situation, not reality itself. Really, the ball is in the first box or it isn’t. Yet according to “dogmatic q.m.”, it can in principle be a complete description to say the chance of it being in that box is 50%. So quantum mechanics seems to be saying that the ball is not in one or the other box, but first exists in a box only when you peer inside. In the Bohr–Heisenberg account, Einstein wrote incredulously, “The state before the box is opened is completely described by the number ½.”
The year of the cat
Two months later, Einstein sent Schrödinger another analogy. Suppose a pile of gunpowder has a probability of exploding in a year, he mused. Its ψ-function is therefore a superposition of exploded and unexploded gunpowder. In Einstein’s view, this was nonsense. He felt that quantum mechanics, thanks to its ψ-function, is an incomplete and inadequate description of reality. Einstein’s letters inspired Schrödinger to set down an informal account of his own views, which he published in October 1935 as “The present situation in quantum mechanics” (Naturwissenschaften 23 807). This was the first appearance of Schrödinger’s cat.
Schrödinger began the paper by saying that the classical world bequeathed us the idea that nature can be exactly described. Sure, experimental data may not – in practice – allow this to be carried out in complete detail, but they do let us model phenomena that we can compare with reality and modify when necessary. These models describe states, which are specified by what Schrödinger calls “determining parts” or variables. A small set of variables uniquely determines all others in a state, though different sets can be used.
Yet this is impossible in quantum mechanics, which says that not all variables can be “co-determined”. The obstacle is not any practical limitation but Heisenberg’s uncertainty principle; when you measure some variables, others become uncertain. What about those other variables? “Have they then no reality, perhaps (pardon the expression) a blurred reality; or are all of them always real and is it merely…that simultaneous knowledge of them is ruled out?” puzzles Schrödinger.
In philosophical language, Schrödinger is asking whether the probabilities affect the “ontology” of the variables – whether the quantities to which they refer exist or not – or merely their “epistemology”, that is, our ability to know what they are.
In thermodynamics, Schrödinger continues, probabilities affect only epistemology. Scientists model systems containing billions of billions of molecules by treating them as if they involve single states arbitrarily chosen from ensembles of many possible states. This is convenient but not strictly correct. In thermodynamics you don’t care how a system behaves exactly – indeed, you aren’t even interested – only how it behaves for the most part.
But in the quantum domain, some variables remain indeterminate or blurred when others are exact. Perhaps if we knew more about the underlying situation, Schrödinger said, we would find it more complex than we thought, and causality might reappear. Still, as long as the ψ-function is confined to the subatomic domain, the indeterminacy is harmless. “Inside the nucleus,” argued Schrödinger, “blurring doesn’t bother us [but] serious misgivings arise if one notices that the uncertainty affects macroscopically tangible and visible things, for which the term ‘blurring’ seems simply wrong.”
Schrödinger now conjures his famous image. In his words:
“One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with a Geiger counter, which must be secured against direct interference by the cat. The Geiger counter contains a tiny bit of radioactive substance, so small that perhaps in the course of an hour one of the atoms decays, but also, with equal probability, perhaps none. If an atom does decay, the counter tube discharges and – through a relay – releases a hammer that shatters a small flask of hydrocyanic acid. But if no atom decays after an hour, the cat still lives. The ψ-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.”
Later in the article Schrödinger describes the implication – the nonseparability of previously interacting quantum states even after the interaction – with the now-famous neologism “entanglement”.
Border lines
Science historian Stephen Brush has remarked that the cat image “captures the spirit of Einstein’s critique better than the published EPR paper”. Yet when Schrödinger’s paper came out, the cat provoked little discussion. For Bohr, Heisenberg and company, cats are too complicated to have ψ-functions – they inhabit classical territory. For Einstein and Schrödinger, the image showed that, just as we are not content to accept a “blurred model” to represent reality in the macroworld, we should not in the microworld. Things are different on the other side of the boundary, but not that different. For both sides, the cat symbolized nonsense.
The boundary dispute, however, did not vanish. It got worse. To the consternation of Einstein and Schrödinger, no way was found to reformulate the rules of the quantum domain so that “determining parts” could all co-exist. Entanglement did not go away, and remained an ontological, not just epistemological, disturbance. Bohr and Heisenberg were disappointed that no way was found to pin down the classical–quantum boundary, which implied that entanglement reached into more territory than they imagined. The price of eliminating superposition collapse is alternate worlds, which does not eliminate but multiplies the cat.
By the time popular-science writing took off in the 1970s, the cat image had become an accessible and accurate illustration that vividly captured the weirdness of entanglement, superposition, the measurement problem and the ψ-function – the cat was the symbol of the challenge posed to conventional realism by quantum mechanics. In The Dancing Wu Li Masters (1979), an over-the-top book about alleged connections between quantum mechanics and Eastern mysticism, Gary Zukav declared that “Schrödinger’s cat has long illustrated to physics students the psychedelic aspects of quantum mechanics”. The cat appeared increasingly often, not only in science fiction, but also in fringe and mystical fiction, amateur philosophy and self-help literature. There is even a Wikipedia page about “Schrödinger’s cat in popular culture”.
The critical point
Physicists do not seem to care much about Schrödinger’s cat any more except as a label: “cat-states” is sometimes used to refer to large coherent quantum systems, though nothing near as complex as a cat.
The rest of the world, however, seems to care a lot, for different reasons. For teenagers such as Will and Jane, the “entanglement” issue surrounding the cat is a great metaphor to express the reality of their mixed feelings, conflicting identities and unexpressed passions. “[K]eeping the box closed doesn’t actually keep the cat alive-and-dead,” Jane tells Will later in Will Grayson, Will Grayson. “Even if you don’t observe the cat in whatever state it’s in, the air in the box does. So keeping the box closed just keeps you in the dark, not the universe.” Will gets that – and gets as well that they aren’t talking about physics, but about their relationship.
For science-fiction writers such as Boudinot, the cat story makes weird and otherwise magical plots plausible, however remotely. For science writers, it symbolizes what is wrong with common-sense realism. For philosophers (amateur and professional) who seek to understand how quantum mechanics connects with the everyday world, it captures the idea of an “intermediate level of reality” that Heisenberg said was the price we had to pay for quantum mechanics.
Technically speaking, the application of the image of Schrödinger’s cat outside the quantum domain is a “fail”, to use the slang of Will and Jane and their friends. But in the real world, its persistence demonstrates how a tool developed in one human domain can, in unpredictable ways, be meaningful and useful in others.
An international team of physicists has simultaneously measured the angular momentum and torque exerted by acoustic waves for the first time. It found that this ratio agrees exactly with the predicted theory for acoustic and optical waves. According to the researchers, their techniques may also have potential in medical imaging and treatment.
A fundamental principle of optics and acoustics is that waves carry momentum, and can therefore exert a force. Equally important is the notion that they can also carry angular momentum and exert a torque. The ratio between these two quantities – the push and the twist – is central to the physics of waves and has long been taken for granted without direct experimental proof of its validity.
Difficulties with optics
The concept of radiation pressure has traditionally been explored and exploited in optics – for example, it is the basis of the “optical tweezers” used to grab and manipulate microscopic objects in microbiology and nanotechnology. The force of a light beam is equal to its power divided by the speed of light. Its torque is proportional to the radiation pressure, which depends on the varying properties of acoustic and optical beams. Because the speed of light is extremely large, the force and torque exerted by a light beam are very small and therefore difficult to measure. To complicate matters further, it is hard for scientists to work out precisely how well an object absorbs linear and angular momentum from a light beam, and hence to calculate the forces and torques exerted.
Sound versus light
For these reasons, nobody has ever managed to measure simultaneously the force and torque of a light beam on an object. Fortunately, the same equations apply in acoustics, where the speed of light is replaced by the much smaller speed of sound. A sound beam of the same power therefore exerts both a stronger push and twist, making it easier to measure the ratio between the two.
Now, ultrasound physicist Christine Demore, biophotonics researcher Mike MacDonald and colleagues from the Institute for Medical Science and Technology at the University of Dundee in the UK, together with Gabriel Spalding from Illinois Wesleyan University in the US, have levitated and twisted a rubber puck in water by bombarding it with a “vortex beam” of ultrasound – a twisted coil of sound shaped a bit like a DNA double helix. This was done to verify experimentally the angular momentum to torque ratio and directly prove this fundamental theory. The researchers found – as expected – that the ratio of the torque to the linear force on the puck was equal to the ratio of the number of intertwined helices per wavelength.
Applications beyond physics
“The key part of the paper is the fact that we’ve demonstrated that ratio,” explains MacDonald, “but the advance that we had to make to get that result was the level of control over the ultrasound beams, which hasn’t been possible before.” Demore adds that developing focused ultrasound has applications well beyond pure research. “There’s a whole field developing of using ultrasound to kill tumours completely non-invasively,” she says. There is also a project to develop “sonotweezers” that are based on optical tweezers but which are able to move larger and heavier objects.
Optical physicist Miles Padgett from the Glasgow University in the UK describes the work as “a beautiful experiment”. He feels that “people active in the field won’t be surprised by the ratio because essentially the results show are as one would expect”. “But if you never checked the things that we know, you’d never find the things that we don’t,” he says.
The paper has been accepted for publication in Physical Review Letters.
As usual, the podcast is hosted by James Dacey, who is joined by Physics World‘s editor Matin Durrani and the magazine’s reviews editor Margaret Harris. The first part of the podcast addresses the question of why so many authors decide to write these books. The Physics World hosts are joined by physicist Chad Orzel, author of the bestselling book How to Teach Quantum Physics to Your Dog, which was released in 2010.
The middle section of the podcast looks in more detail at the process of writing these books. It features the established popular-science writer Marcus Chown, who describes his experience of writing the book Quantum Theory Cannot Hurt You, which was published in 2007. Chown admits that he found the Pauli exclusion principle to be the most challenging aspect of quantum mechanics to explain in everyday language. This leads on to an interesting debate about the pros and potential pitfalls of using metaphors to describe complex science and mathematics.
If scientists and science writers go through such pain to describe these features of the quantum world, then surely somebody without a scientific background should run a mile. But they don’t, instead they keep buying these books. In the final section of the podcast, the historian and philosopher Robert P Crease shares his thoughts on why the counterintuitive nature of quantum physics holds such a fascinating appeal for readers.
Researchers in the US and South Korea have for the first time managed to image the process of nanocrystal growth at the atomic scale. Their technique, which involves placing the crystals inside a liquid cell bound by graphene sheets and imaging them with a transmission electron microscope, has revealed new and unexpected growth stages as they were occurring. The method might be used to study a variety of nanomaterials in solution, and even biological samples in their natural liquid environments.
The transmission electron microscope (TEM), which was first introduced in the 1930s, produces images at a significantly higher resolution than an optical microscope as it works using electron beams instead of light. However, liquids are notoriously difficult to image with a TEM because they need to be hermetically encapsulated in a solid material (usually silicon nitride or silicon oxide) to prevent them from evaporating, since the microscope operates under vacuum conditions. Such capsules, or liquid cells as they are known, can have membranes that are up to 100 nm thick. This is far too thick to penetrate successfully using an electron beam and means that objects can only be imaged with a spatial resolution of a few nanometres at best.
Now, Jungwon Park at the University of California, Berkeley and colleagues at the Lawrence Berkeley National Laboratory and KAIST in South Korea have shown that capsules fabricated from graphene can be used as see-through “windows” for liquid cells. The sub-nanometre walls of the capsule are effectively transparent because graphene is a sheet of carbon just one atom thick. Therefore, the graphene does not scatter the electron beam but instead lets it pass through. Graphene is also very strong and impermeable, as well as being chemically non-reactive, and so helps protects the sample in the liquid cell from the high-energy electrons in the microscope beam.
The researchers filled the graphene capsules with a solution containing platinum nanocrystals and studied the capsule using an aberration-corrected form of TEM. Park explains that his group was able to see particle nucleation and growth on the very high-resolution angstrom-scale (0.1 nm). He says that his team also observed new and unexpected stages of nanocrystal growth as they happened, in particular how certain nanocrystals frequently coalesce along the same crystallographic direction, modify their shape and form facets on their surfaces.
Park believes that electron-microscopy experiments using graphene liquid cells could be used to image a wide range of nanomaterials, such as nanoparticles, nanostructures and even biological samples in liquid. But the group’s next step will be to use its graphene liquid cells to study how nanocrystals other than platinum grow. “Watching real-time chemical reactions in liquids is a dream for chemists and physicists, and we now hope to study a variety of nanoparticles growing in solution using our liquid-phase electron-microscopy technique,” says Park.
Thank you to everyone who took part in our last Physics World photo challenge. We asked readers to submit photos to our Flickr group relating to the theme of “portrait of our planet”. We had some great submissions, a selection of which are showcased in this article.
The theme for our new photo challenge is “doing physics”. We want you to submit your pictures of the process of science. It could be you or your colleagues working on an experiment in the laboratory, or perhaps out in the field collecting data, or maybe looking at the heavens through a telescope. Or perhaps you are more theoretically minded and you want to send us an image of your paper-littered office, or the chalkboard detailing the calculation that has been keeping you awake for the past two weeks. Be as creative as you like.
Please add your photos by Tuesday 8 May and then after this date we will choose a selection of our favourite images to be showcased on physicsworld.com. And feel free to write a caption to share the story behind the image. We look forward to your submissions.
