Antimatter  

The ALPHA experiment at CERN has reported the first ever measurement on the optical spectrum of an antimatter atom. This achievement, 20 years in the making features technological developments that open up a completely new era in high-precision antimatter research.

The ALPHA collaboration at CERN particle physics laboratory near Geneva has reported the first ever measurement on the optical spectrum of an antimatter atom. This achievement features technological developments that open up a completely new era in high-precision antimatter research. It is the result of over 20 years of work by the CERN antimatter community.

The results were published in the journal Nature.

In the experiment, the researchers trained an ultraviolet laser on antihydrogen, the antimatter counterpart of hydrogen. They measured the frequency of light needed to jolt a positron — an antielectron — from its lowest energy level to the next level up, and found no discrepancy with the corresponding energy transition in ordinary hydrogen.

Atoms consist of electrons orbiting a nucleus. When the electrons move from one orbit to another they absorb or emit light at specific wavelengths, forming the atom's spectrum. Each element has a unique spectrum. As a result, spectroscopy is a commonly used tool in many areas of physics, astronomy and chemistry. It helps to characterise atoms and molecules and their internal states.

Hydrogen's spectrum has already been measured to very high precision. Antihydrogen atoms, on the other hand are poorly understood. Because the universe appears to consist entirely of matter, the constituents of antihydrogen atoms – antiprotons and positrons – have to be produced and assembled into atoms before the antihydrogen spectrum can be measured. Any measurable difference between the spectra of hydrogen and antihydrogen would break basic principles of physics and possibly help understand the puzzle of the matter-antimatter imbalance in the universe.

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The ALPHA result is the first observation of a spectral line in an antihydrogen atom, allowing the light spectrum of matter and antimatter to be compared for the first time. The result shows no difference compared to the equivalent spectral line in hydrogen. This is consistent with the Standard Model of particle physics, the theory that best describes particles and the forces at work between them, which predicts that hydrogen and antihydrogen should have identical spectroscopic characteristics.

The ALPHA collaboration expects to improve the precision of its measurements in the future. Measuring the antihydrogen spectrum with high-precision offers an extraordinary new tool to test whether matter behaves differently from antimatter and thus to further test the robustness of the Standard Model.

ALPHA is a unique experiment at CERN’s Antiproton Decelerator facility, able to produce antihydrogen atoms and hold them in a specially-designed magnetic trap, manipulating antiatoms a few at a time. Trapping antihydrogen atoms allows them to be studied using lasers or other radiation sources.

“Moving and trapping antiprotons or positrons is easy because they are charged particles,” said ALPHA spokesperson, Jeffrey Hangst. “But when you combine the two you get neutral antihydrogen, which is far more difficult to trap, so we have designed a very special magnetic trap that relies on the fact that antihydrogen is a little bit magnetic.”

Antihydrogen is made by mixing plasmas of about 90,000 antiprotons from the Antiproton Decelerator with positrons, resulting in the production of about 25,000 antihydrogen atoms per attempt. Antihydrogen atoms can be trapped if they are moving slowly enough when they are created.

Using a new technique in which the collaboration stacks anti-atoms resulting from two successive mixing cycles, it is possible to trap on average 14 anti-atoms per trial, compared to just 1.2 with earlier methods. By illuminating the trapped atoms with a laser beam at a precisely tuned frequency, scientists can observe the interaction of the beam with the internal states of antihydrogen. The measurement was done by observing the so-called 1S-2S transition. The 2S state in atomic hydrogen is long-lived, leading to a narrow natural line width, so it is particularly suitable for precision measurement.

SOURCE  CERN

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Artificial Intelligence  

Stephen Hawking – one of the world’s most brilliant thinkers, and a man who rarely gives interviews – joins Larry to discuss the greatest issues facing the planet, where artificial intelligence is headed (and what he makes of Kurzweil’s singularity theory), and what still mystifies him about the universe.

Larry King recently interviewed physicist Stephen Hawking for RT. The video below also features  astrophysicist Garik Israelian on creating the Starmus Festival, which celebrates the intersection of science and art and is this year dedicated to Hawking.

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In the interview, when asked about the dangers of artificial intelligence, Hawking explained that increases in technology may be coming at a steep cost, saying: “Governments seem to be engaged in an AI arms race, designing planes and weapons with intelligent technologies. The funding for projects directly beneficial to the human race, such as improved medical screening seems a somewhat lower priority.”

That does not mean that artificial intelligence may not come without a cost.

“Artificial intelligence has the potential to evolve faster than the human race. Beneficially AI could co-exist with humans,” but there must be a line drawn, Hawking said. “Once machines reach the critical stage of being able to evolve themselves, we cannot predict whether their goals will be the same as ours.”

"I don't think advances in artificial intelligence will necessarily be benign."

When asked about Ray Kurzweil and the Singularity, Hawking responds, "I think his views are both too simplistic and too optimistic."

"Exponential growth will not continue to accelerate," says Hawking. "Something we don't predict will interrupt it, as has happened with similar forecasts in the past. And I don't think advances in artificial intelligence will necessarily be benign. "

Hawking was at the Starmus Festival in the Canary Islands where, this year, the festival is dedicated to the lifelong researcher. Hawking has been a large presence in science and mathematics, and his reputation precedes him. One of the many unique things about Hawking is how well he has beaten the odds. Having lived with amyotrophic lateral sclerosis (ALS), Hawking has become gradually paralyzed over the decades. The majority of ALS patients die of respiratory failure within three to five years from the onset of symptoms. However, Hawking has made it to 50 years and counting.

SOURCE  RT

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Gravity  

A research paper has proposed, with supporting mathematical proof, a device with which to create detectable gravitational fields. Put into working practice the theory would mean scientists could manipulate gravity the same way they do magnetic fields today, which could produce whole new scientific breakthroughs.

Today, researchers study gravitational fields passively. They observe and try to understand existing gravitational fields produced by large inertial masses, such as stars or the Earth, without being able to change them as they can do with magnetic fields, for example.

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This frustration that led André Füzfa, from Namur University in Belgium to attempt a revolutionary approach: creating gravitational fields at will from well-controlled magnetic fields and observing how these magnetic fields could bend space-time.

In his article, Füzfa has proposed, with supporting mathematical proof, a device with which to create detectable gravitational fields. This device is based on superconducting electromagnets and therefore relies on technologies routinely used, for example, at CERN or the ITER reactor.

Although this experiment would require major resources, if conducted, it could be used to test Einstein’s theory of general relativity. If successful, it would certainly be a major step forward in physics: the ability to produce, detect and, ultimately, control gravitational fields. People could then produce gravitational interaction in the same way as the other three fundamental interactions (e.g. electromagnetic and strong and weak nuclear forces).

Füzfa concludes in his paper: "The generation of artificial gravitational fields withelectric currents could be in principle detected through the induced change in space-time geometry that results in a purely classical deflexion of light by magnetic fields. This effect does not invoke any new physics, as it is a consequence of the equivalence principle."

"It could lead to amazing applications like the controlled emission of gravitational waves with large alternative electric currents."

"Would this technology be developed, it could lead to amazing applications like the controlled emission of gravitational waves with large alternative electric currents," states Füzfa.  "Gravity would then cease to be the last of the four fundamental forces not under control by human beings."

This could usher gravitation into a new experimental and industrial era.

Until now, a scientific advance like this was a dream of science fiction, but it could open up many new applications tomorrow, for example in the field of telecommunications with gravitational waves: imagine calling the other side of the world without going through satellite or terrestrial relays.

SOURCE  Numur University

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Physics  

Rumors are swirling that the Laser Interferometer Gravitational-Wave Observatory experiment has actually observed gravitational waves predicted by Einstein's General Theory of Relativity over 100 years ago for the very first time and may be close to formally announcing the findings.

A major cosmological experiment designed to hunt for gravitational waves—ripples in the fabric of spacetime first predicted by Albert Einstein—has observed them directly for the very first time and may be close to formally announcing the findings. If confirmed, this would be one of the most significant physics discoveries of the last century. While no official announcement has been made, physicist Lawrence Krauss has posted a few Tweets that amount to a scientific spilling of the beans.

According to Einstein's general theory of relativity, gravity is how mass deforms the shape of space: near any massive body, the fabric of space becomes curved. But this curving does not always stay near the massive body. In particular, Einstein realized that the deformation can propagate throughout the Universe, just as seismic waves propagate in Earth's crust. Unlike seismic waves, however, gravitational waves can travel in empty space — and they do so at the speed of light.

My earlier rumor about LIGO has been confirmed by independent sources. Stay tuned! Gravitational waves may have been discovered!! Exciting.

— Lawrence M. Krauss (@LKrauss1) January 11, 2016

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The Laser Interferometer Gravitational-Wave Observatory (LIGO) has been hunting for gravitational waves since 2002 with no luck. But a more powerful, advanced LIGO that's about three times more sensitive than the original detector started operating in just last fall.

"Gravitational waves may have been discovered!! Exciting."

LIGO is designed to open the field of gravitational-wave astrophysics through the direct detection of gravitational waves. The multi-kilometer-scale gravitational wave detectors use laser interferometry to measure the minute ripples in space-time caused by passing gravitational waves.

These phenomena could have been created from cataclysmic cosmic sources like the merging of pairs of neutron stars or black holes, or by supernovae. LIGO consists of two widely separated interferometers within the United States—one in Hanford, Washington and the other in Livingston, Louisiana—operated in unison.

The confirmed discovery of gravity waves would further support the theory of inflation — the idea that in the first few moments the universe existed, it underwent a rapid and incredibly massive expansion. That kind of rapid expansion would almost certainly leave behind ripples through spacetime and imprinting the cosmic background radiation.

The most important thing about this discovery, if proven, is that it could be a way to link up quantum and classical physics—a step to a Theory of Everything for physics.

SOURCE  Tech Insider Video Source: Nature

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Physics  

Physicists have found a radical new way confine electromagnetic energy without it leaking away, akin to throwing a pebble into a pond with no splash.The theory could have broad ranging applications from explaining dark matter to nanotechnology to combating energy losses in future technologies.

Physicists have found a radical new way confine electromagnetic energy without it leaking away, akin to throwing a pebble into a pond with no splash. The theory could have broad ranging applications from explaining dark matter to combating energy losses in future technologies including nanotechnology.

The new theory, which is part of work published in the journal Nature Communications, appears to contradict a fundamental tenet of electrodynamics however, that accelerated charges create electromagnetic radiation, said lead researcher Dr Andrey Miroshnichenko from The Australian National University (ANU).

"This problem has puzzled many people. It took us a year to get this concept clear in our heads," said Dr Miroshnichenko, from the ANU Research School of Physics and Engineering.

The fundamental new theory could be used in quantum computers, lead to new laser technology and may even hold the key to understanding how matter itself hangs together.

"Ever since the beginning of quantum mechanics people have been looking for a configuration which could explain the stability of atoms and why orbiting electrons do not radiate," Dr Miroshnichenko said.

The absence of radiation is the result of the current being divided between two different components, a conventional electric dipole and a toroidal dipole (associated with poloidal current configuration), which produce identical fields at a distance.

If these two configurations are out of phase then the radiation will be cancelled out, even though the electromagnetic fields are non-zero in the area close to the currents.

"You can also convert this energy to electricity in more efficient ways because it will be held in small volumes. So other applications could be to electrical ones such as computers, and smartphones . We could make all of this communication faster."

Dr Miroshnichenko, in collaboration with colleagues from Germany and Singapore, successfully tested his new theory with a single silicon nanodiscs between 160 and 310 nanometres in diameter and 50 nanometres high, which he was able to make effectively invisible by cancelling the disc's scattering of visible light.

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This type of excitation is known as an anapole (from the Greek, 'without poles').
Dr Miroshnichenko's insight came while trying to reconcile differences between two different mathematical descriptions of radiation; one based on Cartesian multipoles and the other on vector spherical harmonics used in a Mie basis set.

"The two gave different answers, and they shouldn't. Eventually we realized the Cartesian description was missing the toroidal components," Dr Miroshnichenko said.

"We realized that these toroidal components were not just a correction, they could be a very significant factor."

Dr Miroshnichenko said the confined energy of anapoles could be important in the development of tiny lasers on the surface of materials, called spasers, and also in the creation of efficient X-ray lasers by high-order harmonic generation.

"You can also convert this energy to electricity in more efficient ways because it will be held in small volumes," Dr Miroshnichenko told the Sydney Morning Herald.

"So other applications could be to electrical ones such as computers, and smartphones . We could make all of this communication faster."

SOURCE  Australian National University via EurekAlert Top Image - Andrey Miroshnichenko
 

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Physics  

Summary Dark energy is cosmology's biggest mystery—an anti-gravitational force that confounds the conventional laws of physics. It makes up more than two-thirds of the cosmos, but science is still grappling to explain what dark energy actually is. In this program, top physicists search for clues to this mystery in both the earliest moments of the universe and far into the future of the cosmos.

SOURCE  World Science Festival

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 Physics  

Using a scanning tunneling microscope scientists have created a minute transistor consisting of a single molecule and a handful of atoms. The transistor action is markedly different from the conventionally expected behavior and could be important for future device technologies as well as for fundamental studies of electron transport in molecular nanostructures.

Physicists from the Paul-Drude-Institut für Festkörperelektronik (PDI) and the Freie Universität Berlin (FUB), the NTT Basic Research Laboratories (NTT-BRL), in Japan, and the U.S. Naval Research Laboratory (NRL), United States, have used a scanning tunneling microscope (STM) to create a minute transistor consisting of a single molecule and a small number of atoms.

The image above features an STM image of a phthalocyanine molecule centered within a hexagon assembled from twelve indium atoms on an indium arsenide surface. The positively charged atoms provide the electrostatic gate of the single-molecule transistor.

"In our case, the charged atoms nearby provide the electrostatic gate potential that regulates the electron flow and the charge state of the molecule."

The observed transistor action is markedly different from the conventionally expected behavior and could be important for future device technologies as well as for fundamental studies of electron transport in molecular nanostructures. The complete findings are published in the journal Nature Physics.

Transistors have a channel region between two external contacts and an electrical gate electrode to modulate the current flow through the channel. In atomic-scale transistors, this current is extremely sensitive to single electrons hopping via discrete energy levels. Single-electron transport in molecular transistors has been previously studied using top-down approaches, such as lithography and break junctions. But atomically precise control of the gate – which is crucial to transistor action at the smallest size scales – is not possible with these approaches.

The physicists used a highly stable scanning tunneling microscope to create a transistor consisting of a single organic molecule and positively charged metal atoms, positioning them with the STM tip on the surface of an indium arsenide (InAs) crystal. Kiyoshi Kanisawa, a physicist at NTT-BRL, used the growth technique of molecular beam epitaxy to prepare this surface. Subsequently, the STM approach allowed the researchers, first, to assemble electrical gates from the +1 charged atoms with atomic precision and, then, to place the molecule at various desired positions close to the gates.

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Stefan Fölsch, a physicist at the PDI who led the team, explained that “the molecule is only weakly bound to the InAs template. So, when we bring the STM tip very close to the molecule and apply a bias voltage to the tip-sample junction, single electrons can tunnel between template and tip by hopping via nearly unperturbed molecular orbitals, similar to the working principle of a quantum dot gated by an external electrode. In our case, the charged atoms nearby provide the electrostatic gate potential that regulates the electron flow and the charge state of the molecule”.

There is a big difference between a conventional semiconductor quantum dot – comprising typically hundreds or thousands of atoms – and the present case of a surface-bound molecule: Steven Erwin, a physicist at NRL and expert in density-functional theory, pointed out that “the molecule adopts different rotational orientations, depending on its charge state. We predicted this based on first-principles calculations and confirmed it by imaging the molecule with the STM”.

This coupling between charge and orientation has a dramatic effect on the electron flow across the molecule, manifested by a large conductance gap at low bias voltages. Piet Brouwer, a physicist at FUB and expert in quantum transport theory, said that “this intriguing behavior goes beyond the established picture of charge transport through a gated quantum dot. Instead, we developed a generic model that accounts for the coupled electronic and orientational dynamics of the molecule”.

This simple and physically transparent model entirely reproduces the experimentally observed single-molecule transistor characteristics.

The perfection and reproducibility offered by these STM-generated transistors will enable the exploration of elementary processes involving current flow through single molecules at a fundamental level. Understanding and controlling these processes – and the new kinds of behavior to which they can lead – will be important for integrating molecule-based devices with existing semiconductor technologies.

SOURCE  AlphaGalileo

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 Quantum Mechanics  

Light behaves both as a particle and as a wave. Since the days of Einstein, scientists have been trying to directly observe both of these aspects of light at the same time. Now, scientists have succeeded in capturing the first-ever snapshot of this dual behavior.

Quantum mechanics tells us that light can behave simultaneously as a particle or a wave. However, there has never been an experiment able to capture both natures of light at the same time; the closest we have come is seeing either wave or particle, but always at different times. Taking a radically different experimental approach, Ecole polytechnique fédérale de Lausanne (EPFL) scientists have now been able to take the first ever snapshot of light behaving both as a wave and as a particle.

The breakthrough work is published in Nature Communications.

When UV light hits a metal surface, it causes an emission of electrons. Albert Einstein explained this “photoelectric” effect by proposing that light – thought to only be a wave – is also a stream of particles. Even though a variety of experiments have successfully observed both the particle- and wave-like behaviors of light, they have never been able to observe both at the same time.

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A research team led by Fabrizio Carbone at EPFL has now carried out an experiment with a clever twist: using electrons to image light. The researchers have captured, for the first time ever, a single snapshot of light behaving simultaneously as both a wave and a stream of particles particle.

"This experiment demonstrates that, for the first time ever, we can film quantum mechanics – and its paradoxical nature – directly. Being able to image and control quantum phenomena at the nanometer scale like this opens up a new route towards quantum computing."

The experiment is set up like this: A pulse of laser light is fired at a tiny metallic nanowire. The laser adds energy to the charged particles in the nanowire, causing them to vibrate. Light travels along this tiny wire in two possible directions, like cars on a highway. When waves traveling in opposite directions meet each other they form a new wave that looks like it is standing in place. Here, this standing wave becomes the source of light for the experiment, radiating around the nanowire.

This is where the experiment’s trick comes in: The scientists shot a stream of electrons close to the nanowire, using them to image the standing wave of light. As the electrons interacted with the confined light on the nanowire, they either sped up or slowed down. Using the ultrafast microscope to image the position where this change in speed occurred, Carbone’s team could now visualize the standing wave, which acts as a fingerprint of the wave-nature of light.

While this phenomenon shows the wave-like nature of light, it simultaneously demonstrated its particle aspect as well. As the electrons pass close to the standing wave of light, they “hit” the light’s particles, the photons. As mentioned above, this affects their speed, making them move faster or slower. This change in speed appears as an exchange of energy “packets” (quanta) between electrons and photons. The very occurrence of these energy packets shows that the light on the nanowire behaves as a particle.

“This experiment demonstrates that, for the first time ever, we can film quantum mechanics – and its paradoxical nature – directly,” says Fabrizio Carbone. In addition, the importance of this pioneering work can extend beyond fundamental science and to future technologies. As Carbone explains: “Being able to image and control quantum phenomena at the nanometer scale like this opens up a new route towards quantum computing.”




SOURCE  EPFL

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Physics  

Most of the laws of nature treat particles and antiparticles equally, but stars and planets are made of particles, or matter, and not antiparticles, or antimatter. That asymmetry, which favors matter to a very small degree, has puzzled scientists for many years. Physicists offer a possible solution to the mystery of the origin of matter in the universe.

New research by UCLA physicists, published in the journal Physical Review Letters, offers a possible solution to the mystery of the origin of matter in the universe.

Alexander Kusenko, a professor of physics and astronomy in the UCLA College, and colleagues propose that the matter-antimatter asymmetry could be related to the Higgs boson particle, which was the subject of prominent news coverage when it was discovered at Switzerland's Large Hadron Collider in 2012.

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Specifically, the UCLA researchers write, the asymmetry may have been produced as a result of the motion of the Higgs field, which is associated with the Higgs boson, and which could have made the masses of particles and antiparticles in the universe temporarily unequal, allowing for a small excess of matter particles over antiparticles.

The Higgs field "had to descend to the equilibrium, in a process of 'Higgs relaxation."

If a particle and an antiparticle meet, they disappear by emitting two photons or a pair of some other particles. In the "primordial soup" that existed after the Big Bang, there were almost equal amounts of particles of antiparticles, except for a tiny asymmetry: one particle per 10 billion. As the universe cooled, the particles and antiparticles annihilated each other in equal numbers, and only a tiny number of particles remained; this tiny amount is all the stars and planets, and gas in today's universe, said Kusenko, who is also a senior scientist with the Kavli Institute for the Physics and Mathematics of the Universe.

The research also is highlighted by Physical Review Letters in a commentary in the current issue.
The 2012 discovery of the Higgs boson particle was hailed as one of the great scientific accomplishments of recent decades.

The Higgs boson was first postulated some 50 years ago as a crucial element of the modern theory of the forces of nature, and is, physicists say, what gives everything in the universe mass. Physicists at the LHC measured the particle's mass and found its value to be peculiar; it is consistent with the possibility that the Higgs field in the first moments of the Big Bang was much larger than its "equilibrium value" observed today.

The Higgs field "had to descend to the equilibrium, in a process of 'Higgs relaxation,'" said Kusenko, the lead author of the UCLA research.


SOURCE  UCLA

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Cosmology  

Most of us understand the Big Bang as the concept that our entire universe came from a single point, what astrophysicists call a 'Singularity.' A new model however suggests that we might not need a singularity to have a Big Bang.

T
he question of cosmology of how do you get something from nothing has confounded philosophers for millennia. The question's accepted answer for the better part of recent history (outside of religious conceptions) has been the Big Bang.

The University of Lethbridge's Saurya Das and Ahmed Farag Ali – who was a PhD student at the U of L before taking a faculty position at Benha University in Egypt, recently put forth a mathematical model that assumes the Big Bang never happened and the universe has simply been eternal.

The paper explaining the model was recently published in the journal Physics Letters B, has received a lot of attention in the cosmology community for convincingly contradicting the conventional wisdom that the universe originated from a Singularity about 13.8 billion years ago.

While it explains a lot of what we have so far observed about the universe, the Big Bang theory and the Singularity in particular, leads to some conclusions which are very difficult to model and explain.

"The Singularity [in cosmology] is basically where all theories and all physics breaks down, so nobody really likes that,” Das said.

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The best current theories can only explain what happens immediately after the Big Bang, not during or before it. Even the idea of “before” the Big Bang is itself a conundrum, as the Singularity is thought to be the origin of space and time.

"We need more confirmations and more careful study of our model, for sure. We have to understand the mathematics better and whatever predictions it makes has to be tested with observations."

The new model developed by Das and Ali gets around that problem, as it simply states there was no singularity and the universe’s existence has been in a more or less steady state of existence.

Physicists have long believed that a quantum version of gravity would include a hypothetical particle, called the graviton, which generates the force of gravity. In their new model, Ali and Das propose that such gravitons could form this quantum fluid.

The new model moves away from the “expanding universe” theory. It also gets away from the matter of the universe’s previous infinite size and density, which the paper calls the “smallness problem,” by relying on a “cosmological constant” term that puts the universe at a finite size.

Speaking to Nature Middle East last month, Ali said the theory helped unify quantum mechanics and general relativity: "Our theory serves to complement Einstein’s general relativity, which is very successful at describing physics over large distances…But physicists know that to describe short distances, quantum mechanics must be accommodated."

Das said he’s been encouraged to see such interest in the model but cautioned there’s still plenty more work do to.

“We need more confirmations and more careful study of our model, for sure,” he said. “We have to understand the mathematics better and whatever predictions it makes has to be tested with observations.”

The new model also takes dark matter into account. So far, dark matter has only been perceptible through its gravitational effect on visible matter such as stars. When Das and a colleague set the mass of the graviton in the model to a small level, they could make the density of their fluid match the universe’s observed density of dark matter, while also providing the right value for dark energy’s push.

“This is the first time that anyone has shown that these two major problems in cosmology can be solved simultaneously by the quantum Raychaudhuri equation,” says Ali.

“We feel a deep sense of satisfaction that this model may resolve some of the most important cosmological issues in one stroke,” adds Das.

If his work turns out to fundamentally alter our understanding of where everything comes from, Das also suggests it would be far from the final answer.

“Of course, it brings new problems, new questions, like: What was the universe before what we think is the Big Bang?” he said. “That’s great, because a lot of us would be interested in investigating those things further and hopefully getting answers to them, in due course.”


SOURCE  Metro News

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 Graphene  

Physicists have found a mehtod to induce magnetism in graphene while also preserving graphene’s electronic properties. The finding has the potential to increase graphene’s use in computers, as in computer chips that use electronic spin to store data.

Graphene, the one-atom thick sheet of carbon atoms arranged in a hexagonal lattice, has many desirable properties. Magnetism, however, is not one of them. Magnetism can be induced in graphene by doping it with magnetic impurities, but this doping tends to disrupt graphene’s electronic properties.

Now a team of physicists at the University of California, Riverside has found an ingenious way to induce magnetism in graphene while also preserving graphene’s electronic properties. They have accomplished this by bringing a graphene sheet very close to a magnetic insulator – an electrical insulator with magnetic properties.

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The study results have been published in Physical Review Letters.

"The magnetic graphene acquires new electronic properties so that new quantum phenomena can arise. These properties can lead to new electronic devices that are more robust and multi-functional."

"This is the first time that graphene has been made magnetic this way," said Jing Shi, a professor of physics and astronomy, whose lab led the research. "The magnetic graphene acquires new electronic properties so that new quantum phenomena can arise. These properties can lead to new electronic devices that are more robust and multi-functional."

The finding has the potential to increase graphene’s use in computers, as in computer chips that use electronic spin, or spintronics, to store data.

The magnetic insulator Shi and his team used was yttrium iron garnet grown by laser molecular beam epitaxy in his lab. The researchers placed a single-layer graphene sheet on an atomically smooth layer of yttrium iron garnet. They found that yttrium iron garnet magnetized the graphene sheet. In other words, graphene simply borrows the magnetic properties from yttrium iron garnet.

Magnetic substances like iron tend to interfere with graphene’s electrical conduction. The researchers avoided those substances and chose yttrium iron garnet because they knew it worked as an electric insulator, which meant that it would not disrupt graphene’s electrical transport properties. By not doping the graphene sheet but simply placing it on the layer of yttrium iron garnet, they ensured that graphene’s excellent electrical transport properties remained unchanged.

In their experiments, Shi and his team exposed the graphene to an external magnetic field.  They found that graphene’s Hall voltage – a voltage in the perpendicular direction to the current flow – depended linearly on the magnetization of yttrium iron garnet (a phenomenon known as the anomalous Hall effect, seen in magnetic materials like iron and cobalt).  This confirmed that their graphene sheet had turned magnetic.


SOURCE  University of California, Riverside

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 Materials  

A new way to calculate the electrical properties of individual components of composite materials could open a path toward more energy-efficient medical refrigerators, air-conditioned car seats and more.

If you’ve ever gone for a spin in a luxury car and felt your back being warmed or cooled by a seat-based climate control system, then you’ve likely experienced the benefits of a class of materials called thermoelectrics.

Thermoelectric materials convert heat into electricity, and vice versa, and they have many advantages over more traditional heating and cooling systems.

Recently, researchers have observed that the performance of some thermoelectric materials can be improved by combining different solid phases -- more than one material intermixed like the clumps of fat and meat in a slice of salami. The observations offer the tantalizing prospect of significantly boosting thermoelectrics’ energy efficiency, but scientists still lack the tools to fully understand how the bulk properties arise out of combinations of solid phases.

"Effective medium theory is pretty old. What’s new about what we did is we took a composite, and then backed-out the properties of each constituent phase."

Now a research team based at the California Institute of Technology (Caltech) has developed a new way to analyze the electrical properties of thermoelectrics that have two or more solid phases. The new technique could help researchers better understand multi-phase thermoelectric properties – and offer pointers on how to design new materials to get the best properties.

The team describes their new technique in a paper published in the journal Applied Physics Letters.

Because it’s sometimes difficult to separately manufacture the pure components that make up multi-phase materials, researchers can’t always measure the pure phase properties directly. The Caltech team overcame this challenge by developing a way to calculate the electrical properties of individual phases while only experimenting directly with the composite.

“It’s like you’ve made chocolate chip cookies, and you want to know what the chocolate chips and the batter taste like by themselves, but you can’t, because every bite you take has both chocolate chips and batter,” said Jeff Snyder, a researcher at Caltech who specializes in thermoelectric materials and devices.

To separate the "chips" and "batter" without un-baking the cookie, Snyder and his colleagues turned to a decades old theory, called effective medium theory, and they gave it a new twist.

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“Effective medium theory is pretty old,” said Tristan Day, a graduate student in Snyder’s Caltech laboratory and first author on the APL paper. The theory is traditionally used to predict the properties of a bulk composite based on the properties of the individual phases. “What’s new about what we did is we took a composite, and then backed-out the properties of each constituent phase,” said Day.

The key to making the reversal work lies in the different way that each part of a composite thermoelectric material responds to a magnetic field. By measuring certain electrical properties over a range of different magnetic field strengths, the researchers were able to tease apart the influence of the two different phases.

The team tested their method on the widely studied thermoelectric Cu1.97Ag0.03Se, which consists of a main crystal structure of Cu2Se and an impurity phase with the crystal structure of CuAgSe.

Thermoelectric materials are currently used in many niche applications, including air-conditioned car seats, wine coolers, and medical refrigerators used to store temperature-sensitive medicines.
“The definite benefits of using thermoelectrics are that there are no moving parts in the cooling mechanism, and you don’t have to have the same temperature fluctuations typical of a compressor-based refrigerator that turns on every half hour, rattles a bit and then turns off,” said Snyder.

One of the drawbacks of the thermoelectric cooling systems, however, is their energy consumption.
If used in the same manner as a compressor-based cooling system, most commercial thermoelectrics would require approximately 3 times more energy to deliver the same cooling power. Theoretical analysis suggests the energy efficiency of thermoelectrics could be significantly improved if the right material combinations and structures were found, and this is one area where Synder and his colleagues’ new calculation methods may help.

Many of the performance benefits of multi-phase thermoelectrics may come from quantum effects generated by micro- and nano-scale structures. The Caltech researchers’ calculations make classical assumptions, but Snyder notes that discrepancies between the calculations and observed properties could confirm nanoscale effects.

Snyder also points out that while thermoelectrics may be less energy efficient than compressors, their small size and versatility mean they could be used in smarter ways to cut energy consumption. For example, thermoelectric-based heaters or coolers could be placed in strategic areas around a car, such as the seat and steering wheel. The thermoelectric systems would create the feeling of warmth or coolness for the driver without consuming the energy to change the temperature of the entire cabin.

“I don’t know about you, but when I’m uncomfortable in a car it’s because I’m sitting on a hot seat and my backside is hot,” said Snyder. “In principle, 100 watts of cooling on a car seat could replace 1000 watts in the cabin.”

Ultimately, the team would like to use their new knowledge of thermoelectrics to custom design ‘smart’ materials with the right properties for any particular application.

“We have a lot of fun because we think of ourselves as material engineers with the periodic table and microstructures as our playgrounds,” Snyder said.


SOURCE  American Institute of Physics via Newswise

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