Page 1247 – Physics World

Home » Page 1247

Celebrating 25 years of Physics World

By Matin Durrani

Today marks the 25th anniversary of Physics World – the member magazine of the Institute of Physics (IOP) – which launched in October 1988. And to celebrate that fact, we’ve created a fantastic special issue of Physics World in which we look back at some of the highlights in physics of the last 25 years and also forward to where the subject is going next.

All members of the IOP can access the entire new issue right now via the digital version of the magazine or by downloading the free Physics World app onto your iPhone or iPad or Android device, available from the App Store and Google Play, respectively. The issue includes a stack of bonus audio and video content, including three short films we’ve specially made about some of the top spin-offs from physics.

We’ve split the bulk of the issue into five sections, each with five items (five times five being 25, of course):

• Find out our choice of the top five discoveries in fundamental physics over the last 25 years.

• See what five leading researchers have to say about Physics World‘s choice of the five biggest unanswered questions in physics right now.

• Enjoy our pick of the five top images from the last 25 years that have let us “see” a physical phenomenon or effect.

• Learn more about the five people who are changing the way physics is done.

• Gaze into the future as we disclose the five most promising spin-offs from physics.

We also have a set of fiendish physics-themed puzzles devised for you by staff at the UK’s Government Communications Headquarters (GCHQ) – the first is revealed in the special issue and on our blog, with the rest to be unveiled on physicsworld.com throughout October.

(more…)

What is the nature of the dark universe?

This year the Planck space mission released exquisite observations of the early universe, providing the strongest evidence yet that the universe we live in is very dark indeed. Its precise results show that our universe is composed of 26.8% dark matter and 68.3% dark energy, while less than 5% is made up of the stuff we are familiar with on Earth. With their long-standing quest to make these precision measurements essentially now concluded, cosmologists are rapidly turning their attention to a much bigger and further-reaching question: what is the exact nature of this dark universe?

Dark matter is exactly what it says on the tin: it is dark and comprised of a mysterious substance that does not emit or absorb light. We only know it exists because of its gravitational effects on the normal matter that we can see. Dark energy is less well described by its label, being an invisible source of energy that drives the post-Big-Bang expansion of the universe to mysteriously accelerate. Together, these two dark entities play out a cosmic battle of epic proportions. While the gravity of dark matter slowly pulls structures in the universe together, dark energy fuels the universe’s accelerating expansion, making it ever harder for those dark-matter structures to grow.

It is widely believed that to truly understand the dark universe, we will need to invoke some new physics that will forever change our cosmic view. As the conclusion of this dark quest could be so far reaching, astronomers are approaching the task with care, using a series of independent and meticulous observations. Efforts include the Canada–France–Hawaii Telescope Lensing Survey, which has directly mapped out the invisible cosmic web of dark matter by observing how its mass bends space and time, lensing the light of very distant galaxies. Projects such as the Sloan Digital Sky Survey are accurately charting the locations of billions of galaxies, which closely trace the distribution of dark matter because this gravitationally attractive substance dictates where and when galaxies form. Galaxies also carry with them a signal imprinted in the distribution of normal matter just after the Big Bang that can be seen in how galaxies cluster in the cosmos today.

Capturing dark matter

Astronomers have put their theories of dark matter to the test, finding that a very wide variety of observations all agree with a single theory, termed the “concordant cosmology”. This overwhelming body of evidence supports the theory that dark matter is made up of weakly interacting matter particles (WIMPs), and the challenge is now on for particle physicists to go out and catch or create one.

Several attempts have already been made to trap a dark-matter particle, but any hints of success have so far been controversial and open to interpretation. The next major leap in the search for a fleeting glimpse of a dark-matter particle in flight is taking shape not in space but nearly 1.5 km under the Black Hills of South Dakota. The LUX-ZEPLIN experiment will use nine tonnes of liquid xenon as its dark butterfly net. The hope is that a few of the trillions of WIMPs that pass through the Earth every second will be caught crashing into some of the xenon particles. How successful this new experiment will be in its quest to uncover the nature of dark matter will depend on just how much of a wimp the dark-matter particle turns out to be. An unquestionable direct detection of a dark-matter particle would be one of the most significant discoveries of this century, finally confirming Fritz Zwicky’s theory, which was ridiculed when he proposed it in 1933.

Exposing dark energy

While the astronomical community is now fairly united in postulating the existence of an invisible dark-matter particle, the same cannot be said about its support for the simplest explanation for dark energy. Observations that the expansion of our universe is accelerating are most easily explained by considering the extra energy associated with the vacuum that permeates the universe. According to quantum theory, empty space is filled with a swarm of virtual particles with a wide range of masses that can briefly pop in and out of existence. As mass and energy are equivalent, the growing vacuum within an expanding universe acts like a bank of unlimited energy, inflating the whole universe at an accelerated speed.

A dark-blue background featuring paler-blue amorphous blobs, some of which are so pale as to look white in places — The distortion of light from about 10 million galaxies, which is bent as it passes clumps of dark matter, has shown that dark matter in the universe is distributed as a network of gigantic dense regions (white), separated by a vast emptiness.(Courtesy: Van Waerbeke, Heymans, Canada–France–Hawaii Telescope Lensing Survey)

Unfortunately, there is a problem with this simple and elegant vacuum solution to the nature of dark energy. Particle physicists can make a theoretical estimate for the energy of a vacuum and they find that it is 120 orders of magnitude larger than the dark energy that the Planck results show. This wild discrepancy has opened up a wide range of exciting new dark-energy theories including exotic models such as a multiverse that resembles the middle of an Aero chocolate bar. Perhaps our universe is one Aero bubble being pulled by our neighbouring Aero-bubble universe?

Many cosmologists believe that the dark-energy phenomenon indicates that we need to look beyond Einstein’s theory of general relativity. By observing how dark-matter structures change over cosmic time, we can investigate how dark energy evolves and test gravity for the first time on cosmological scales. Just as Einstein revolutionized our understanding of Newtonian gravity, confirmed through observations of the solar system, so new observations of gravity on cosmological scales may bring about another revolution in our understanding of gravity.

Two major new international projects will lead our quest to discover what the dark-matter particle is and why the expansion of our universe is apparently accelerating. The Euclid satellite, due to launch in 2020, will image the full dark sky from above the Earth, while the Large Synoptic Survey Telescope, due to see first light in 2019, will image the full Southern sky from a mountain top in Chile. Both of these projects will chart the distant universe with exquisite precision, utilizing a diverse range of cosmological tools to map out the evolution of dark-matter structures and document the expansion and curvature of space and time from 10 billion years ago to the present day. Exciting times are ahead for our understanding of the fundamental physics that govern the dark side of the universe.

New ‘wagon-wheel’ molecules could make better OLEDs

A new giant organic molecule that is shaped like rotelle or wagon-wheel pasta has been created by an international group of researchers. The molecule is known as a “pi-conjugated spoked-wheel macrocycle” and emits light that has no overall polarization, according to the team. This feature could be useful for making more efficient organic light-emitting diodes (OLEDs) – however, the researchers say that much more work has to be done before there are any practical applications.

Currently, OLEDs are used in energy-saving light bulbs and as displays in televisions and mobile phones. They offer high pixel brightness, wide viewing angle, very high contrast ratio, fast response times and low power consumption. OLEDs have the potential to be even better but scientists have struggled to further boost their performance because of losses caused by the polarization of light as it passes through the devices.

Polarized loss

The problem is related to the difference in the refractive indices of the organic layer of an OLED and the adjacent glass layer. The result is a waveguide effect in the organic layer that prevents much of the light from escaping. This effect depends on the polarization of the light and this inspired John Lupton and colleagues at the University of Utah in the US along with other researchers in Germany to create wheel-shaped molecules to depolarize the light.

It’s just the beauty of making a perfectly symmetrical system and then watching how the symmetry is broken spontaneously
John Lupton, University of Utah

Lupton explains that current OLEDs are made up of spaghetti-shaped chain polymers that tend to emit polarized light. As a result, nearly 80% of the light generated remains trapped within the organic layer. In the spaghetti-shaped chain polymers, polarized light is produced by electrons that can only oscillate by moving up and down the chain. However, in the new wagon-wheel-shaped polymers the electrons can oscillate in all directions, creating light that is not polarized, reducing the losses associated with wave-guiding.

Perfect wagon wheels

The molecules were made by team member Sigurd Höger from Bonn University in Germany. Lupton told physicsworld.com that the polymers were challenging to make because they are so large – each one is 6 nm wide. He adds that the molecules retain their shape over time and there are no isomers. Indeed, it is their perfectly symmetrical shape that scrambles the polarization of the light. Lupton likens it to balancing a perfectly sharp pencil tip – in an ideal experiment, the pencil will fall in a different direction every time. “Every time you excite the ring, a photon is emitted from a different, random segment of the ring,” he explains.

The team then carried out single-molecule experiments, where it shone ultraviolet light on ring molecules to generate visible light photons. The photons had a “scrambled” polarization – one that changes randomly from photon to photon. The large ring-shaped molecules can “catch” other molecules, making them effective biological sensors. They also have potential use in solar cells and switches, according to the team.

Lupton is quick to point out that the new work is purely basic research so far – the team has only made the new ring-like molecules and has not yet tested them in an OLED. “The polarization scrambling is at present more of an academic curiosity. We cannot really claim whether this is terribly useful or not,” says Lupton. “It’s just the beauty of making a perfectly symmetrical system and then watching how the symmetry is broken spontaneously.”

The research is published in Nature Chemistry.

Reawakening the Kelvin wake

Wake angles: the black dashed line shows the outer edge of the wake and is at the angle predicted by Lord Kelvin. The red line shows the maximum amplitude of the wake and is at the angle predicted by Marc Rabaud and Frédéric Moisy. (Courtesy: A Darmon, M Benzaquen, E Raphaël)

By Hamish Johnston

Loyal readers may recall that earlier this year we published a news article entitled “Physicists rethink celebrated Kelvin wake pattern for ships” that reported on work done by two French physicists. While looking at Google Earth images of ships moving through the sea, Marc Rabaud and Frédéric Moisy noticed that some of the wakes did not conform to a prediction made years ago by Lord Kelvin, the renowned Victorian physicist, engineer and entrepreneur.

(more…)

NASA badly hit by government shutdown

NASA headquarters, Washington DC. (Courtesy: NASA)

US citizens woke up this morning to the unbelievable news that their federal government would be shutting down all its “non-essential” services after the two houses of Congress failed to reach an agreement on a new budget. What this means in practice is that hundreds of thousands of federal employees will now face unpaid leave – and NASA’s workforce is among the most badly affected.

A staggering 97% of NASA’s 18,134 employees have been granted leave of absence, according to the Office of Management and Budget, quoted in the New York Times. This is the highest percentage of all the federal departments and agencies to be affected by the shutdown. Other federal workers affected include 94% of the 16,205 employees of the Environmental Protection Agency, along with 69% of the 13,814 working within energy.

“Due to the gov’t shutdown, all public NASA activities/events are cancelled or postponed until further notice. Sorry for the inconvenience,” read a rather understated tweet from NASA earlier today. Within the past few hours, the NASA website has also shutdown indefinitely.

(more…)

Do ultracold neutrons get a kick from nanoparticles?

Physicists in France and Russia claim to have evidence that explains why so-called ultracold neutrons (UCNs) escape their traps. The evidence suggests that the neutrons are being kicked out of the traps by collisions with floating nanoparticles and could help explain discrepancies in measurements of the neutron lifetime. However, not all scientists in the field find the evidence compelling.

UCNs are neutrons that have been cooled to less than 2 mK above absolute zero. At these temperatures the neutrons are moving so slowly that they would easily be overtaken by someone running at a moderate pace. UCNs bounce off most surfaces they come into contact with, regardless of their angle of incidence. This has allowed physicists to trap large numbers of UCNs in oversized “bottles” made of materials such as copper or stainless steel – where the neutrons can be studied. Neutrons experience all four forces – electromagnetism, the weak force, the strong force and gravity – which make them a comprehensive laboratory for tests of the Standard Model of particle physics.

One quantity of great interest is the lifetime of the neutron, which is determined by the weak force. Although neutrons last for billions of years when bound within nuclei, free neutrons are known to decay into protons (with the emission of an electron and an electron-antineutrino) with a half-life of about 10 minutes. A precise value of the half-life is obtained by trapping UCNs in bottles and then counting how many of the particles are left after a certain time interval. Such measurements have been done by several different research groups and their results currently differ by about one second.

Vibrating nanoparticles

Some of the discrepancy could be caused by UCNs leaking from the rims of their bottles. The problem with this explanation is that once inside a bottle, a UCN should not be travelling fast enough to escape the downward pull of gravity. In 1999 physicist Valery Nesvizhevsky at the Institut Laue-Langevin (ILL) in Grenoble, France and colleagues suggested that the UCNs could collide with nanoparticles that are floating within a bottle and vibrating nanoparticles that stuck to the inner surfaces of the bottles. Unlike most collisions with the bottle surfaces, these collisions could give the neutrons enough energy to rise out of the bottle.

Now Nesvizhevsky and colleagues believe they have evidence that this is really the case. The researchers developed a theoretical model to predict the spectrum of UCN energies that would be generated if nanoparticles of different sizes were indeed supplying an extra kick at some surface collisions. Then the researchers looked to confirm their model by doing experiments at ILL, which produces UCNs. The experiment involves a 2-m-tall copper bottle that the team coated on the inside with nanoparticles of a known size distribution. The UCN energy spectra from these experiments appeared to match the theoretical predictions.

“This is indeed an interesting new idea for a possible loss mechanism of UCNs on the surfaces of material traps,” says experimental nuclear physicist Mike Snow at Indiana University Bloomington in the US. He believes Nesvizhevsky and colleagues’ mechanism might need to be taken into account in bottle experiments, although he points out that newer neutron-lifetime experiments are employing magnetic-field traps. These would probably be less susceptible to the mechanism, he says.

Does water do the same?

But not all UCN experts agree that Nesvizhevsky and colleagues’ mechanism is convincing. Stephan Paul and Erwin Gutsmiedl at the Technical University of Munich in Germany told physicsworld.com that they have estimated that water on the bottle walls could also reproduce the findings. “The feeling at present is that there’s no proof the scenario Nesvizhevsky proposes for his observation is the correct one,” says Paul.

Robert Golub at North Carolina State University in Raleigh, US agrees. He points out several potential issues, including that scientists routinely clean the surfaces of their bottles prior to performing experiments, a procedure that would remove any freely floating nanoparticles. If the nanoparticles are being generated on-site, he says, then Nesvichevski and colleagues would have to explain how they are generated. “The hypothesis that surfaces of materials are covered by a 2D gas of floating nanoparticles seems highly unlikely to say the least,” he adds.

The research is published in Crystallography Reports 58 743.

Physics World 2013 Focus on Big Science is out now

By Michael Banks

All eyes will be on Stockholm next week as the 2013 Nobel Prize for Physics is announced. One of the frontrunners for the prize in the minds of the Royal Swedish Academy of Sciences will surely be the discovery last year of the Higgs boson at CERN’s Large Hadron Collider (LHC).

But the LHC story is far from over and in the latest Physics World focus issue on “big science” find out how the LHC will hunt for new particles beyond the Higgs boson once the collider restarts in 2015 following an 18-month repair and upgrade programme at the Geneva-based lab.

All full members of the Institute of Physics will receive a print edition of the focus issue along with their copy of the October issue of Physics World, but everyone can access a free digital edition. The focus issue also looks at how particle physicists are already thinking about what could come after the LHC, with bold plans for a 80–100 km proton–proton collider. There are even plans for a collider based on lasers, with an international team looking at creating an array of “fibre lasers” to be used as a future “Higgs factory”.

(more…)

Ultrathin solar cell is efficient and easy to make

Researchers at the University of Oxford in the UK have made a thin-film solar cell with better than 15% light-conversion efficiency from an emergent class of semiconductors known as perovskites. The devices have a simple architecture and could easily be produced in large quantities because the vapour-deposition process used to make them is compatible with conventional processing methods for fabricating such solar cells.

Organometal trihalide perovskite semiconductors, which have the formula (CH₃NH₃)PbX₃ with X being iodine, bromine or chlorine, were first employed as the light-absorbing component in dye-sensitized solar cells in 2009. In these devices, the perovskites were coated onto the surface of a film made of titanium-dioxide (TiO₂) nanoparticles.

When the perovskite layer absorbs light, electrons and holes are generated. These charge carriers are subsequently transferred to different transport materials – TiO₂ for the electrons and to another material for the holes. The transport materials then carry the charges to separate electrodes and a voltage is produced. These solar cells have light-converting efficiencies of about 12–15% thanks to the large amount of perovskite packed into the TiO₂ film.

Simple structure

Now, two teams at Oxford led by Henry Snaith and Michael Johnston have joined forces to show that perovskites not only strongly absorb light, but also transport both electrons and holes. This new discovery means that the nanostructured architecture previously used in the dye-sensitized solar cells is no longer necessary, which simplifies the device structure greatly. Indeed, in the new device, the light-absorbing perovskites are simply sandwiched between electron- and hole-selective electrodes – a set-up that is, in fact, the same as that used in conventional planar solar cells.

“Our devices have a high solar-to-electric power efficiency of 15.4% and a large ‘open circuit’ voltage of 1.07 V – all in a solar cell in which the absorbing perovskite layer is only 330 nm thick,” explains Johnston. “This means that we only need a tiny amount of perovskite material to make a solar cell with good properties.” In contrast, conventional crystalline silicon cells are much thicker (0.15 mm wafers are typically used) and the voltage produced by these cells is only about 0.7 V under open-circuit conditions.

Photophysics still a mystery

“Little is known about the photophysics of these materials, which I think is quite exciting – this is a rapidly evolving field,” Johnston says. “The fact that we can make such good solar cells using a conventional planar p–i–n architecture indicates that the charge-carrier diffusion lengths (the distances electrons and holes travel before recombining) are long, and that these carriers survive a long time in perovskite. That we can fabricate an efficient device without complex mesostructuring – as was previously the case with solar cells made from this material – also shows that perovskite is very good at both absorbing light and transporting photogenerated charge.”

According to the researchers, these perovskite-based devices should be cheap to make using processes that are compatible with existing solar-cell manufacturing infrastructures. And since they absorb light in a different part of the electromagnetic spectrum to silicon, the two materials might be used together in so-called tandem cells in which a silicon device would be placed underneath a perovskite one. “Here, the perovskite top cell would absorb higher-energy photons and the lower-band-gap silicon the lower-energy ones,” explains Johnston. Such a cell could be more efficient that one made from either silicon or perovskite alone.

Richard Friend of the University of Cambridge, who was not involved in this work, says that this research began out of the Oxford team’s initial interest in dye-sensitized solar cells. These devices are considered to be “excitonic” photovoltaics that require a large surface area for charge separation between electron-accepting TiO₂ and the adsorbed dye layer. He says that the team’s new discovery is “remarkable” because it proves that these perovskites work as bulk semiconductors.

It is unprecedented to see such rapid progress in performance
Richard Friend, University of Cambridge

“Last year, this group already reported that the lead-iodide perovskite structure described in this work, formed with an organic semiconductor hole transporter, could produce a power-conversion efficiency above 10%. The new paper reports efficiencies of 15% in a straightforward layer-by-layer structure deposited by very simple evaporation and solution processing techniques,” says Friend. “It is unprecedented to see such rapid progress in performance – with less than a year of development, the material is now close to the efficiency of cadmium telluride (that has been studied for several decades).”

Spurred on by their initial results, the Oxford researchers are now busy optimizing film-deposition parameters and device design. “I think we will see the efficiencies of these devices climbing higher in the near future,” says Johnston. “Investigations into the fundamental photophysics of the perovskite layers will be particularly interesting and will also help us accelerate the optimization process.”

The present results are published in Nature.

This article first appeared on nanotechweb.org

Days out at CERN, serendipitous songs, shaken scientists and more

By Tushna Commissariat

A peek into the Red Folder this week brings up the CERN Open Days – the biggest particle-physics laboratory in the world will allow people from all over the globe to roam its hallowed halls freely for this weekend. While the most exciting part of the event will undoubtedly be visits into the underground caverns that host the Large Hadron Collider’s experiments, a whole host of other activities for researchers, science enthusiasts and children are available. Also this weekend, as a part of the European Researcher’s Night festivities, CERN will be hosting events in Paris, Geneva and Bologna for their Origins 2013 event that looks at two big scientific discovers made in the past two years: the discovery of the Higgs boson at CERN and the latest Planck mission data. For those of you attending, “Speed-dating – close encounters with researchers” definitely caught our eye. Those of us not fortunate enough to be in any of those places can watch many of the festivities via a live webcast. And lastly, you can explore CERN from the inside on Google Maps with Street View.

(more…)

Has a quantum computer solved the ‘party problem’?

A quantum computer made by the Canadian company D-Wave Systems has been used to solve a famous puzzle in mathematics known as the party problem – according to a team of physicists in Canada and the US that has done the work. D-Wave describes the result as one of the most significant achievements for its devices to date, but some physicists are being party poopers by remaining unconvinced there is anything to boast about.

Unlike classical computers, which store bits of information in definite values of 0 or 1, quantum computers store information in quantum bits (qubits) that exist as a fuzzy superposition of both. This mixed-up nature of quantum computing extends beyond individual qubits: multiple qubits can be entangled so that they work in unison. As a result, quantum computers should be able to solve certain problems – such as factorizing large numbers – much faster than their classical counterparts.

In principle, there are several ways that quantum computers can work. A more conventional approach is to perform a calculation by operating on the qubits one step at a time, so that in the final step the answer is encoded in the qubit states. Another way is called adiabatic quantum computing and involves letting all the qubits slowly evolve in carefully controlled conditions so that the problem is described by their web of interactions. Adiabatic quantum computing should still give the desired result in the final qubit states. However, when compared with more conventional approaches, it is less susceptible to external influences such as stray heat, which can destroy a quantum calculation.

Success and scepticism

Since 2004 D-Wave has been trying to build commercial adiabatic quantum computers with qubits made from superconducting rings. Founded in 1999 and based in the Vancouver suburb of Burnaby, the firm has published many results that it says provide evidence that its technology is capable of performing quantum calculations. D-Wave has supplied two of its computers to high-profile corporations, with one going to a consortium led by Google and the other going to the defence contractor Lockheed Martin. But despite this apparent success, there is still a significant amount of scepticism within the academic quantum-computing community about the company’s claims.

D-Wave’s latest results concern a well-known puzzle in mathematics about drawing up a guest list for a party. The party’s organizer wants to invite the minimum number of people such that there is a group of m guests that know one another or another group of n guests that do not know one another. The British mathematician Frank Ramsey was the first to prove that there is always a minimum number of guests, R(m,n), satisfying the criteria, although calculating this can be tricky as the guest list grows. Showing that R(3,3) is equal to six is straightforward, but the number for R(5,5) is currently unknown and the number for R(6,6) is supposedly beyond any realistically achievable classical computation.

This latest work was done by William Macready and colleagues at D-Wave, together with mathematician Lane Clark at Southern Illinois University and physicist Frank Gaitan at the Laboratory for Physical Sciences in Maryland. The team claims to have used a D-Wave adiabatic quantum computer to determine the numbers for R(3,3) and R(m,2), with m ranging from four to eight. Although these numbers were already known, the researchers claim that their quantum algorithm, which relied on 84 qubits, had a much greater chance of finding them than a classical algorithm in the same time period. “To the best of our knowledge,” the researchers write, “this is the largest experimental implementation of a scientifically meaningful adiabatic evolution algorithm.”

Mountains or molehills?

However, other researchers contacted by physicsworld.com were not convinced that D-Wave’s computer had achieved anything remarkable. Mathematician Greg Kuperberg at the University of California, Davis, in the US says that adiabatic computing is a generic strategy and “could be great or lousy” depending on how it is implemented. The results are “beyond easy” with any traditional strategy, he says. “The paper talks of mountains, and then climbs a few molehills,” he adds.

Colin Williams, director of business development and strategic partnerships at D-Wave, says there is “no question” of whether his company’s devices are quantum computers. He points to recent tests on a D-Wave computer at the University of Southern California in Los Angeles, US, which suggested that the device did indeed have a quantum nature. “If people would read our papers, they would see there is no doubt whatsoever,” he says.

One way D-Wave could convince sceptics is to make a discovery of an as-yet-unknown Ramsey number. According to Williams, the possibility of such a discovery may open up with a D-Wave computer containing 2048 qubits that is due to be released in 2015.

The research is published in Physical Review Letters.

Hamish Johnston, editor of physicsworld.com, visited D-Wave and interviewed its founder Geordie Rose. You can hear parts of that interview in the podcast “Quantum computing: Challenges, triumphs and applications”, which also includes contributions from several leading academics who work on quantum computers.

Browse articles by content type

Celebrating 25 years of Physics World

What is the nature of the dark universe?

Capturing dark matter

Exposing dark energy