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Home » Page 1417

What physicists do

By Margaret Harris

hands smll.jpg

Last week’s Facebook poll asked a pretty straightforward question:

If you have a physics degree, what do you do for a living?

The options we offered were engineering, finance, IT, research and teaching, and voters could also add their own choices. Among the 161 people who voted, “research” was by far the most popular category, accounting for 45% of the total (N.B. we went ahead and classed the three people who said they were graduate students under “research”). The runner-up was engineering, with 16% of the vote, closely followed by teaching (15%) and IT (13%).

The only user-generated option to attract more than two votes was “science communication”, which picked up six – just shy of 4%. That’s more than finance got, but maybe most physicists in finance are too busy dealing with the financial crisis to vote in Facebook polls.

One final note: could the person who said they were an “inflatable entertainment company owner” please e-mail us at pwld@iop.org? We publish a column in Physics World called Once a physicist that profiles physicists with unusual jobs and, frankly, you’re a shoo-in for a future edition.

Space Shuttle rap

By Michael Banks

It had to come didn’t it? With the launch of the last and final flight of the Space Shuttle Programme last month when NASA’s Atlantis shuttle landed back on Earth after an 11-day mission to the International Space Station, the rap video couldn’t be too far off.

So yesterday a tweet from @NASAKennedy – the official Twitter stream of NASA’s Kennedy Space Flight Center – allayed any fears that the rap wouldn’t emerge when it posted a link to the video saying “You know your curiosity will get the better of you so you might as well click.”

Featuring a group of youths dressed in NASA jump suits rapping about the history of the Space Shuttle Programme, I will leave it up to you to decide whether the rap beats the likes of the Climate Change Rap, the Hubble Rap or the Large Hadron Rap.

Cubic neutrons might find it hip to be square

Neutrons and protons may appear quite spherical here on Earth, but two physicists in Spain and Germany have suggested that, if squeezed under enough pressure, these subatomic particles might squish themselves into cubic shapes. Although no evidence for such cubic neutrons has yet been obtained, an unprecedentedly heavy neutron star that was discovered last year could potentially house these unusually shaped particles. The star in question, which inspired the study, is a rotating neutron star – or “pulsar” – with a mass twice that of our Sun.

Neutron stars are created when a star explodes in a violent supernova, shedding most of its matter and forcing the remaining 80–90% of the star’s mass to collapse in on itself. If that remaining star is more than 2.5 times the Sun’s mass, then it can collapse completely, forming a black hole. But lighter stars instead stabilize, crushing 1.3–2 times the mass of the Sun into a city-sized sphere with a radius of just 11–12 km. These stars are so dense that gravitational pressure forces the electrons in atoms to merge with protons – forming neutrons. The inside of the star ends up being composed almost entirely of neutrons, hence the name “neutron star”.

However, Felipe Llanes-Estrada, who is on leave at the Technical University of Munich, and Gaspar Moreno Navarro of the Complutense University of Madrid say that if the interior pressures are high enough, the neutrons could be squeezed into cubes. They could then pack more tightly, further reducing their total volume by about 24%. “It’s like stacking oranges in a supermarket – the oranges at the bottom of the stack are a little distorted because of the weight of the ones on top,” says Llanes-Estrada.

Cubic movement

According to the two physicists’ calculations, neutrons would become fully cubic at a density of some 1015 grams per cubic centimetre, which is equivalent to an energy density of 500 megaelectronvolts per cubic femtometre (10–15 metres) (MeV/fm3). Although the cores of most neutron stars are just less than that density, a new, heavy pulsar discovered last year has a central region that exceeds that density by a factor of two, according to Llanes-Estrada – meaning that it could potentially harbour cubic neutrons.

The discovery of the pulsar came as a surprise to astronomers, who had previously not thought that neutron stars could be so big. Yet, even “ordinary” neutron stars could contain partially cubic neutrons, starting at an energy density of 140 MeV/fm3. Paul Demorest of the National Radio Astronomy Observatory in Charlottesville, Virginia, who was a member of the team that discovered the heavy pulsar, says that the extra compressibility would allow heavy neutron stars to achieve higher densities. The snag, says Demorest, is that this “tends to lower the maximum allowed neutron-star mass”.

Llanes-Estrada acknowledges this problem; namely that the increase in density would turn the new pulsar into a black hole unless something else is pushing outward. However, he suggests that poorly understood interactions between the neutrons may be strong enough to counter gravity and stave off collapse.

Squaring up

Kai Hebeler of Ohio State University in Columbus offers a note of caution about the simulation. He points out that rather than fleshing out the complex interplay of the quarks and gluons inside it, the study instead models the neutron as if it were a bag of quarks. Still, while this estimation is limited, Hebeler still finds the team’s model to be reasonable. However, the extreme environment of a neutron star’s interior could make the quantum-mechanical identities of the neutrons bleed into one another, so he wonders whether identifying their geometric shapes is relevant, although he admits that he cannot make a “solid statement” about the problem.

The researchers have a couple of ideas about how astronomers could possibly look for evidence of cubic neutrons in the new pulsar. As segments of a neutron star that have settled into a crystalline lattice of cubes will be stiffer than expected, stellar seismologists could look for star quakes that produce types of waves that run through solids but not fluids. Another option is based on the fact that rotating neutron stars sometimes slow down or speed up abruptly, in an event known as a “glitch”. The researchers say that such glitches might indicate a change in the way that different layers of the star interact – possibly hinting at neutrons entering or leaving cubic configurations if they occur in the heavy pulsar.

Why philosophy matters to physics

“Philosophy is dead.” So say the venerable physicists Stephen Hawking and Leonard Mlodinow on the first page of their recent bestselling book, The Grand Design.

Physicists declaring philosophy to be lifeless is nothing new. In his 2010 book In Praise of Science: Curiosity, Understanding, and Progress, Sander Bias likened philosophers’ discussions of science to doctors who diagnose patients before considering symptoms. In a 2004 New Scientist article, Simon Singh said that scientists do not need philosophers any more than birds need ornithologists. Steven Weinberg’s chapter “Against philosophy”, in his classic 1992 book Dreams of a Final Theory, needs no explanation.

Why do physicists so often, and confidently, condemn a field that is not their own? Where are their instincts to be inquisitive, resist overstepping what they know, withhold judgment until certain and accompany claims with error bars?

The evidence that Hawking (using his name as shorthand) cites is essentially the following. Questions such as, “How can we understand the world in which we find ourselves?” and “How does the universe behave?” are traditionally considered to be philosophical. But because philosophers have not kept up with scientists in their answers, therefore “Scientists have become the bearers of the torch of discovery in our quest for knowledge.”

No. Philosophers (including myself) approach such questions differently.

What philosophy does

For philosophers, the world includes more than physical matter. As the Harvard University philosopher Steven Shapin writes in his book, Never Pure, “Plants photosynthesize, plant biochemists are experts in knowing how plants photosynthesize, [while] reflective and informed students of science are experts in knowing how plant biochemists know how plants photosynthesize.” In other words, the world studied by science researchers includes not just objects, but also connections between scientists and objects.

Human beings, after all, engage with the world in different ways. They seek wealth, fame, pleasure, companionship, happiness and other “good” things. They do this as children, adolescents, parents, merchants, athletes, teachers and administrators. All these methods arise through modifications of a matrix of ways by which human beings practically connect to the world that precedes any cognitive understanding. The technical term philosophers use for this matrix is the “lifeworld”. But scientists are not like plants whose product is knowledge. Plants do not plan to follow laws of nature and do not interpret themselves. Human beings, however, do interpret both the world and themselves. The technical term philosophers use for human self-interpretation is “hermeneutics”.

Understanding photosynthesis, for instance, is only one – rare – way for human beings to interact with plants. Hawking’s theoretical stance as an observer of fundamental structures, too, is only one way for humans to engage with the world, and not the default setting either. Humans are not automatic information absorbers; they must be trained to approach the world as he does. They have to pay a special kind of attention, pursue a special kind of inquiry and find that inquiry valuable.

The lifeworld is the domain to which philosophers bring their torch of discovery. They study similarities and differences between various modes of being in the world – their group structures, if you will – and how each arises out of the lifeworld. To study this is not to undermine or critique these activities, but to understand and help cultivate them.

But the lifeworld – a kind of horizon structured by powerful metaphors, images and deeply embedded habits of thought – has its own character that changes over time. Philosophers – and here they differ from other students of science – do not and cannot adopt a “view from nowhere”, in a phrase popularized by New York University philosopher Thomas Nagel, but seek to be reflective. When philosophers think about science, they struggle to be self-aware of that horizon and how it affects human self-interpretation. This is why the humanities matter, for they study and help reshape the lifeworld. Without ornithologists, wrote one astute respondent to Singh’s New Scientist article, many bird species, in these ecologically troubled times, are heading for extinction.

Why it’s misunderstood

It is easy to misunderstand what philosophy does, for several reasons.

First, like much of physics, philosophy often has a narrow focus and is concerned with special topics or technical issues, whose place in the big picture may not be easy for an outsider to see. Much philosophy of science is thin or even anorexic and does not flesh out all of the ways in which science is embodied in the world.

Second, the lifeworld – like any horizon – tends to drop out of view. It is overlooked in favour of the objects, plans and goals that appear in and thanks to that horizon. That is perfectly understandable; detecting the horizon and its impact on our lives is the philosopher’s special task.

Finally, the particular character of the modern world is that the very successes of science lead us to think that only the measurable is worthwhile. The messy, often inchoate lifeworld is bound to seem not only less distinct and valuable, but also less tangible and real, than the grandeur of the designs uncovered by Hawking and others.

The critical point

On the first page of his book Subtle is the Lord…, the physicist Abraham Pais reports a discussion with Einstein in which the latter asked Pais if he “really believed that the Moon exists only if I look at it”. One could hardly think of a deeper, more challenging question about the concept “to exist”. Yet Pais smoothly characterizes the conversation as “not particularly metaphysical”. Discussing the meaning of reality is okay, evidently, so long as it is done in an amateur way.

So is philosophy dead? No; it will live as long as science does. When will physicists stop misunderstanding it? Probably never. But those who are ignorant of philosophy are destined to commit a bad version of it.

The inside track on simulation software

It has been a busy summer for the folks at COMSOL. For those who do not know, COMSOL is the company behind the COMSOL Multiphysics software platform for modelling and simulation of all manner of physics-based systems – in fact, everything from optimization of wound treatment through energy-efficient lighting to the creation of award-winning fish dishes, and plenty more besides.

Along with preparations for its annual series of user conferences (being held in Boston, Stuttgart and Bangalore in October and November) and a packed schedule of tutorial webinars, the firm has begun shipping the latest iteration of COMSOL Multiphysics. Version 4.2 of the software is billed as a “major release”, combining three new application modules (microfluidics, geomechanics and electrodeposition), greater CAD interoperability, as well as enhanced geometry, mesh and solver functionality.

Clearly lots to talk about – and this special video feature lets COMSOL staffers do just that. In the first instalment (above), David Kan, COMSOL’s vice-president of sales, explains that while “simulation has become an integral part of what we do as scientists and engineers”, the key to success lies in simulations that mimic what happens in the real world. “The [whole] point”, he adds, “is to have a simulation that gets as close to reality as possible.”

In part two, Daniel Smith, lead developer of the COMSOL Multiphysics plasma module, provides an application-specific take on simulation, explaining why a multiphysics modelling approach is essential for the simulation of complex plasma processes in semiconductor manufacturing. “Plasmas are perfect examples of a multiphysics problem,” he says. “They’re conductive assemblies of charged particles, neutrals and electromagnetic fields – all [of which] interact with each other in a nonlinear and very complicated way.”

As for the bigger picture, it seems inevitable that as simulation software becomes ever-more sophisticated, the natural progression will see increasing numbers of scientists and engineers using it to shed light on real-world problems, as Kan describes in the final segment.

“We’re already seeing the benefits to our customers,” claims Kan. “People have discovered new physics phenomena. [They] have been able to innovate new medical treatments, make better products faster, and students have even been able to grasp theory faster using simulation.”

As he concludes, “The future’s bright for this technology. Our community [of users] is growing and we’re headed in the right direction.”

Rashba gets hotter and more pronounced

An international group of physicists has demonstrated an electron spin-splitting effect in a semiconductor that is far larger than has ever been seen before. The large Rashba effect – the phenomenon of spin splitting with an applied electric field instead of a magnetic field – could herald the room-temperature operation of spintronic devices.

Spintronics is expected to be one of the next revolutions in computing. The idea is to fabricate devices that operate using not just an electron’s charge, but also its spin. Because the spin of an electron can be switched more quickly than charge can be moved round, these spintronic devices should operate faster and at lower temperatures than their electronic counterparts.

Electron spins are tiny magnetic moments, so to manipulate them a magnetic field is needed. As magnetic fields are difficult to control on the small scales typical in computing, physicists tend to exploit the so-called spin–orbit interaction. In this phenomenon, an electron moving in an electric field “sees” a magnetic field, which interacts with the electron’s spin.

Towards a room-temperature Rashba effect

In an external electric field, this leads to the so-called Rashba effect – a splitting of the spin-up and spin-down states in energy and momentum that is crucial for proposed spintronic devices. In the design for spin transistors, for example, electrons of a single spin are injected and then – under an applied electric field – have their spins rotated. But the Rashba effect in well-established semiconductors, such as silicon and gallium arsenide, is so small that electrons have to travel large distances – perhaps several microns – before any spin rotation is noticeable. Such distances require ultrapure materials and low temperatures to ensure that the electrons are not knocked off course.

Now, Phil King and colleagues at the University of St Andrews in the UK, together with other researchers in Europe and China, have come up with a material that could make the Rashba effect, and spintronics in general, feasible at room temperature. Bismuth selenide – the researchers’ material of choice – is unusual in that its inner bulk structure behaves as a semiconductor while its surface behaves as a metal. Such materials, known as topological insulators, have been around for decades but it is only in recent years that their unique behaviours have been discovered.

King’s group dope the surface of bismuth selenide, which causes its electrons to become confined in 2D “quantum wells”. The researchers then use a technique called angle-resolved photoemission spectroscopy, in which a bright light of a single frequency displaces electrons from the surface of the sample via the photoelectric effect. By measuring the energy of these electrons and their incident angle, the researchers can record a snapshot of the sample’s electronic structure – one that reveals the Rashba effect, or the energy splitting of the spin-up and spin-down electrons.

At least 10 times better

The bismuth-selenide sample exhibited an amount of spin-splitting at least 10 times better than other semiconductors, and at temperatures above 100 °C. The results are due to be published in Physical Review Letters.

“The very large spin splitting that we see should allow the scaling of spintronic devices such as the spin transistor down to nanometre dimensions, thereby making it much easier to have the electrons travel from one side of the device to the other without scattering and flipping their spin,” says King. “This is also promising for room-temperature operation of these devices.”

Ulrich Zuelicke, a physicist specializing in spintronics at the Victoria University of Wellington, New Zealand, is impressed by the size of the Rashba effect, and says that it also has the advantage of being tunable in terms of the amount of spin splitting. However, he says that there may still be hurdles to overcome before an effective spin transistor is realized, such as the possibility of “spin relaxation”, which affects the rotation of electron spins.

Marco Grioni, a spintronics expert at the Ecole Polytechnique Fédérale de Lausanne in Switzerland, agrees that a reliable spin transistor will require more experiments. But he thinks that a working device may come sooner than we think. “Recent experience, namely with colossal-magnetoresistance devices [used in computer memory] has shown that industry can sometimes move extremely fast towards the practical application of a bright idea,” he says.

China–US neutrino facility opens

The biggest major science project in China that has been built through a genuine international collaboration has begun operation. Once fully complete next year, the Daya Bay Reactor Neutrino Experiment – a partnership lead by 19 Chinese and 16 US universities – will begin searching for the final undetermined neutrino “mixing angle”, known as θ13.

Neutrinos are difficult to detect because they interact weakly with matter. They come in three “flavours” – electron, muon and tau – that change or “oscillate” from one to another as they travel in space. The oscillation strength between different types of neutrino is characterized by three “mixing angles” – known as θ12, θ23 and θ13 – with Daya Bay designed to determine θ13 by measuring the disappearance of electron antineutrinos.

The US Department of Energy is providing about half of the cost of the $68m facility, with China paying for the other half and all of the civil-engineering costs. The Daya Bay experiment detects electron antineutrinos produced via nuclear beta decay at two neighbouring nuclear reactors – the Daya Bay and Ling Ao power plants, which are around 55 km north-east of Hong Kong.

The new neutrino facility will consist of three experimental halls that contain identical neutrino detectors, each filled with 20 tonnes of gadolinium-doped liquid scintillator. When a neutrino strikes the liquid, a flash of light is produced that is then picked up by a bank of photomultiplier tubes around the liquid.

The first experimental hall, which is around 300 m from the Daya Bay reactor, is now complete, while the second experimental hall – 500 m from the Ling Ao reactor – is expected to be finished in the next few months. Both of these stations, known as “near detectors”, are 100 m underground to help shield them against unwanted cosmic rays and each contains two detectors to characterize the beam of electron antineutrinos from the reactors.

A third hall, around 2 km away from both reactors and 300 m below ground, will be ready by June next year. Containing four neutrino detectors, it will measure the electron-antineutrino beam that has passed through the nearer detectors, so that any drop in the strength of the signal will be an indication of neutrino oscillation.

“Among the current generation of reactor neutrino-oscillation experiments for measuring θ13, Daya Bay has the best sensitivity,” says Daya Bay co-spokesperson Kam-Biu Luk, of the Lawrence Berkeley National Laboratory in California.

Measuring disappearance

The start-up of the Daya Bay experiment comes hard on the heels of two other neutrino successes. First, in early June, the Tokai-to-Kamioka (T2K) neutrino experiment in Japan for the first time measured muon neutrinos changing into electron neutrinos – a first step to determining θ13. A few weeks later, researchers at the MINOS experiment in the US detected a total of 62 electron neutrinos – 13 more events than the background of electron neutrinos.

At T2K, as well as similar planned experiments such as the NOvA facility being built at Fermilab, the probability of electron neutrino “appearance” depends on two unknown parameters: θ13 and the neutrino phase factor, δ, which is non-zero if neutrino oscillation violates charge–parity (CP) symmetry. Daya Bay, however, is blind to the neutrino-phase factor because the probability of disappearance of electron antineutrinos only depends on θ13, which means that researchers can focus on just its numerical value.

Yifang Wang, co-spokesperson for Daya Bay and a physicist at the Institute of High Energy Physics at the Chinese Academy of Sciences in Beijing, says that the three experiments will be complementary for searching for the phase factor. “If Daya Bay, NOvA and T2K find that θ13 is non-zero, then the CP phase can be jointly measured or strongly constrained,” he says.

New collaborations

As Daya Bay is the first major US–China scientific collaboration, Luk expects the facility will provide a good testing-ground for more partnerships between the two countries. “Daya Bay provides a unique opportunity to join forces to tackle a burning question in neutrino physics and, more importantly, to learn how to work together,” says Luk.

That view is shared by Wang, who says that Daya Bay will be important for both countries. “We believe we will have a better understanding of each other, and the experience will help us for future collaborations,” he says.

Dave Wark of Imperial College London and former international co-spokesperson for T2K says it is good news that Daya Bay has begun running, but warns that it could be some time before the experiment starts to get reliable measurements – given how difficult neutrino-disappearance experiments can be. “If θ13 is large, they have an easier target, but we are still talking about at most few per cent effects in a disappearance experiment so the measurements are tricky,” says Wark.

arXiv celebrates its 20th birthday

Paul Ginsparg.gif

By Tushna Commissariat

Yesterday, on 14 August, the arXiv preprint electronic server celebrated its 20th birthday. In 1991 physicist Paul Ginsparg (right), who had then just moved to the Los Alamos National Laboratory in New Mexico, set up the online physics archive, initially know as the Los Alamos Preprint Server (xxx.lanl.gov), as a place where high-energy physicists could share preprints of their upcoming work. The initial idea, according to Ginsparg’s recent comment piece in Nature, was for 100 full-text articles or so to be submitted every year, each of which would be stored for three months. “By popular demand, nothing was ever deleted” writes Ginsparg.

The server received close to 400 subscriptions in the first six months alone. By 1999 when xxx.lanl.gov had changed its name to arXiv, the repository was collecting almost two thousand new articles every month. In 2001, when the server turned 10, Ginsparg moved to Cornell University in Ithaca, New York and took the server with him. By 2008 the world’s favourite e-print server officially had half a million papers published on it.

In 2008, when Physics World celebrated its 20th anniversary, Ginsparg recounted the early days of the Web and looked at how it has changed scientific communication. You can read his thoughts on the subject here.

Over the years, the arXiv server has had a huge impact on physics and paved the way to open-access publishing for scholarly journals. Many scientific journals now publish their content with unrestricted online access, and this has allowed scientific information to become freely accessible to researchers and the public.

Now, the server contains “about 700,000 full texts, receives 75,000 new texts each year, and serves roughly 1 million full-text downloads to about 400,000 distinct users every week. It has broadened, first to cover most active research fields of physics, then to mathematics, nonlinear sciences, computer science, statistics and, more recently, to host parts of biology and finance infiltrated by physicists,” according to Ginsparg.

Early last year, librarians at Cornell University asked for extra external funding to support the server, as the running costs were “beyond a single institution’s resources”. Its budget – which covers personnel as well as operating expenses – was predicted to increase from $400,000 in 2010 to $500,000 in 2012. Ginsparg says that an international meeting of sponsor institutions will be hosted by the Cornell Library next month and will look into transforming the arXiv server into a more community-endorsed resource. “My hope is that the barrier to implementation of new ideas in this realm will remain low enough that, if all else fails, some young researcher elsewhere can launch another tiny ship on a fateful trip.”

Information paradox simplified

A black hole’s event horizon is the ultimate last-chance saloon: beyond this boundary nothing, not even light, can escape. But does this “anything” include information itself? Physicists have spent the best part of four decades grappling with the “information paradox”, but now a group of researchers from the UK thinks it can offer a solution.

The researchers have created a theoretical model for the event horizon of a black hole that eschews space–time altogether. Their work also supports a controversial theory proposed last year that suggests that gravity is an emergent force rather than a universal fundamental interaction.

Paradoxical history

The information paradox first surfaced in the early 1970s when Stephen Hawking of Cambridge University, building on earlier work by Jacob Bekenstein at the Hebrew University of Jerusalem, suggested that black holes are not totally black. Hawking showed that particle–antiparticle pairs generated at the event horizon – the outer periphery of a black hole – would be separated. One particle would fall into the black hole while the other would escape, making the black hole a radiating body.

Hawking’s theory implied that, over time, a black hole would eventually evaporate away, leaving nothing. This presented a problem for quantum mechanics, which dictates that nothing, including information, can ever be lost. If black holes withheld information forever in their singularities, there would be a fundamental flaw with quantum mechanics.

The significance of the information paradox came to a head in 1997 when Hawking, together with Kip Thorne of the California Institute of Technology (Caltech) in the US, placed a bet with John Preskill, also of Caltech. At the time, Hawking and Thorne both believed that information was lost in black holes, while Preskill thought that it was impossible. Later, however, Hawking conceded the bet, saying he believed that information is returned – albeit in a disguised state.

At the turn of this century, Maulik Parikh of the University of Utrecht in the Netherlands, together with Frank Wilczek of the Institute of Advanced Study in Princeton, US, showed how information could leak away from a black hole. In their theory, information-carrying particles just within the event horizon could tunnel through the barrier, following the principles of quantum mechanics. But this solution, too, remained debatable.

Tunnelling through the event horizon

Now, Samuel Braunstein and Manas Patra of the University of York in the UK think they have formulated a tunnelling theory that looks rather more attractive than Parikh and Wilczek’s theory. “We cannot claim to have proven that escape from a black hole is truly possible,” they explain, “but that is the most straightforward interpretation of our results.”

Normally, theorists dealing with black holes have to wrestle with the complex geometries of space–time arising from Einstein’s theory of gravitation – the theory of general relativity. In their model, Braunstein and Patra say that the event horizon is purely quantum mechanical in nature, with bits of quantum “Hilbert” space tunnelling through the barrier.

The theorists find that even such a heavily simplified tunnelling model can reconstruct the spectrum of radiation that is thought to emanate from black holes. This is unlike Hawking’s pair-creation model, which leads to the information loss and has always required many more theoretical details to work. Put simply, Braunstein and Patra say that tunnelling seems far more likely to be an intrinsic feature of black holes – so, probably, information is not lost after all. Their findings are published in the latest issue of Physical Review Letters.

Gravity’s depth

There is yet another twist to the researchers’ work. Last year, string theorist Erik Verlinde of the University of Amsterdam, building on work by Ted Jacobsen of the University of Maryland in the US, put forward a speculative idea for the origin of gravity. Under Verlinde’s proposal, gravity is not a fundamental interaction, but emerges from the universe trying to maximize disorder. Gravity is therefore an “entropic force” – a natural consequence of thermodynamics – much as one feels a force on a stretched rubber band as the molecules attempt to squiggle up into disordered states.

Braunstein and Patra believe that their black-hole model goes in favour of Verlinde’s proposal. If gravity – not to mention inertia or space–time – is an emergent force, then it would not be utilized to unravel the basic information-loss mechanism of black holes, which is what the York researchers have shown. “This doesn’t prove that Verlinde is correct, but that his proposal ‘has legs’,” Braunstein tells physicsworld.com.

Steve Giddings, a physicist specializing in quantum gravity at the University of California, Santa Barbara, does not think that Braunstein and Patra have addressed “the most central questions” of Verlinde’s proposal. However, he says they have put forward another hint of an important link between quantum information and gravity. “An important challenge is to figure out whether the ideas enunciated by Verlinde and others can be given a more concrete foundation,” he adds. “This may be one more piece of that puzzle, but we’re not there yet.”

Electron bunches keep their shape

Researchers in Australia have developed a new source of cold electrons that could be useful for imaging tiny structures at atomic-length scales. The source, which makes use of ultracold atoms, can deliver intense and coherent electron pulses with specific shapes – including the Batman motif shown above. According to the team, such pulses could be used in the diffraction imaging of biological molecules, viruses and nanostructures.

Robert Scholten and colleagues at the University of Melbourne begin with a cloud of about one billion rubidium atoms that are laser-cooled to a few millionths of a degree above absolute zero. The team then fires two laser pulses at the atoms. The first pulse puts the atoms in an excited electronic state. The second pulse provides just enough energy to liberate those electrons and create a pulse of cold electrons with a temperature of about 10 K. Electron pulses with complex shapes can be created by passing the first pulse through a spatial light modulator before it strikes the atoms.

The pulses are then accelerated to 1 keV using an electric field and then allowed to drift about 21 cm before being detected. Unlike pulses from a conventional, hot-electron source that blur rapidly from the random motion of the electrons, these pulses retain their shapes when detected.

High spatial coherence

Because the electron pulses retain their shape, they have a high degree of spatial coherence perpendicular to their direction of travel. This makes them ideal for diffractive imaging – which the researchers hope to carry out in the coming months. According to Scholten, the transverse coherence length is about 10 nm at the source, which is already good enough to do diffraction imaging of large biomolecules as well as small viruses.

“High spatial coherence means that [the electrons] propagate in a very parallel beam, so when they hit a target, we know where they came from,” explains Scholten. “If we then detect them after diffracting from the target, we know where they came from and where they were detected,” he adds. This information is used to infer the diffractive effect of the target, which is related to its structure. Such imaging systems would complement existing atomic-force microscopy (AFM) and electron-microscopy techniques.

Being able to shape the pulses should also help researchers get round the phenomenon of “Coulomb explosion”, which is a fundamental barrier to creating bright electron pulses. Because electrons have electrical charge, the particles repel each other, thereby causing the pulse to expand as it travels – reducing its intensity. However, if the pulse is created with a specific shape – a uniform-density ellipsoid – Scholten says that it can be refocused using standard electron optics to cancel out the effects of the Coulomb explosion.

“Leapfrog arrangement”

Scholten is quick to point out that the original idea for how to create shaped pulses of cold electrons came from Edgar Vredenbregt, Jom Luiten and colleagues at the Technical University of Eindhoven in the Netherlands. As well as setting out the theory, the Dutch researchers have also worked on electron sources. “We work closely with them and, indeed, they are now adopting the techniques we [have developed], and we are sending them engineering drawings of our system,” says Scholten. “It’s a leapfrog arrangement – we built on what they did using their experiences and suggestions to progress, and now we are returning the favour.”

Thomas Killian of Rice University in Texas tells that “This new work should be viewed as a potential source of electrons that would be used in something like a scanning electron microscope.” He describes the work as “a great leap forward” in the development of low source temperatures and long transverse-coherence lengths for the electrons. “I am hopeful that this will accelerate the development of practical tools based on this technology,” he adds.

The research is published in Nature Physics 10.1038/nphys2052.

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