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Watch out for the space junk

spacejunk.jpg
Computer-generated images of objects in orbit around the Earth. These are objects, not shown to scale, that are large enough (at least 5 cm across) to be tracked by the US Space Surveillance Network. Some 95% are junk, i.e. not functioning satellites, with most occupying a low Earth orbit up to an altitude of 2000 km (left). Those items of junk in a geostationary orbit some 36 000 km high (right) form a clear ring, since they are located directly over the equator and have the same orbital period as the Earth. (Courtesy:NASA)

By Hamish Johnston

It’s finally happened — a very expensive telecommunications satellite has been destroyed by a piece of space junk. On Tuesday, one of 66 satellites that cover the Earth for the phone company Iridium was taken out by a defunct Russian satellite.

Although space is a big place, humans have managed to put lots of junk up there since the first satellite was launched in 1957 — as you can see in the illustrations above.

The pictures come from an article by Edwin Cartlidge — “Our orbiting junk-yard” — that appeared in the October 2007 issue of Physics World. I’m afraid that the full article is not available on physicsworld.com, but if your library subscribes to the Physics World Archive, you can read Edwin’s article here

Members of the Institute of Physics can read an online version of the Physics World. Simply login here and follow the Physics World link.

Visiting Fermilab

Fermilab's Wilson Hall.jpg
Fermilab’s Wilson Hall

By Margaret Harris

“I know it’s kind of a busman’s holiday for you, but do you want to visit Fermilab?”

The AAAS conference doesn’t officially kick off until tomorrow, so I was supposed to spend today de-jet-lagging myself while visiting my uncle west of Chicago. Now, a rainy February day in the Chicago suburbs is not everyone’s idea of great holiday material, but one of those suburbs happens to host the world’s biggest operational particle accelerator…and several of its scientists have prominent slots on the conference schedule…so…

Fermilab’s striking Wilson Hall atrium is open to the public from dawn to dusk most days, and you can hike in the surrounding prairies, too (just watch out for the resident bison herd — now down to 20 head due to budget cuts). But on Wednesday and Saturday mornings they also run guided tours, so my uncle and I joined the small group of curious local residents following science historian (and UK native – between that and the mist, I felt right at home) Yvonne Twomey around the linear accelerator building.

The ongoing Higgs boson hunt means that the Tevatron is nearly always running, so there’s a limit to what you can see at Fermilab on a public tour. But we poked our heads into the auditorium, peered through glass at the giant Cockcroft-Walton generator and the first few feet of the linear accelerator beam line, and learned a little about the great astrophysicist office-space takeover (they used to be confined to the third floor, but as the lab’s particle physics mission winds down, other sub-disciplines have picked up territory) before going back to the high-rise’s 15th floor to gaze out at the lab’s other buildings. And the mist. And the bison.

And, of course, to the distant skyline of Chicago proper, where I’ll be reporting on the conference from tomorrow on. Until then…

Bouncing atoms take a measure of gravity

Physicists in the US have devised a new way of making very precise measurements of gravity by bouncing atoms up and down off a laser beam. Unlike comparable techniques that involve dropping atoms about 10 cm, this new method only needs a drop of about 20 µm. The team has also modified the experiment to perform atom interferometry, whereby quantum interference between atoms can be used to measure tiny accelerations.

Because of its compact size, the team believes the technique could be used to make precise accelerometers that could be used in navigation systems for aircraft, submarines and even spacecraft. The technique may also find use in experiments that look for deviations from Newton’s laws of gravitation.

Over the past two decades physicists have become adept at trapping and manipulating small numbers of atoms using lasers and electromagnetic fields. Like all matter, these atoms fall towards Earth, which has allowed scientists to make very precise measurements of how the gravitational force acts on very small objects and over relatively short distances.

10,000 ultracold atoms

The new bouncing technique was developed by Cass Sackett and colleagues at the University of Virginia in the US, who begin their experiment with a collection of about 10,000 rubidium-87 atoms cooled to micro-Kelvin temperatures. Using such ultracold atoms is important because they have little thermal energy and therefore are nearly motionless when they begin to fall within a vacuum chamber (arXiv:0902.0109).

Initially held up by a magnetic field, the atoms fall when the field is switched off. As they drop down, most of the atoms collide with photons emitted by a laser diode placed directly under the trap, which emits pulses of light in an upwards direction about once every 2 ms. When the atoms collide, they each receive a precise amount of momentum, which knocks them back up. These atoms then fall down again, only to collide with the next laser pulse.

If the colour of the laser light and the frequency of pulses are set correctly, the atoms will be set bouncing and the acceleration due to gravity can be deduced from the experimental parameters and Planck’s constant. The team managed to sustain this bouncing for about 100 cycles, which they say is the equivalent of dropping the atoms about 2 cm in a standard experiment. After correcting for the effect of stray magnetic fields on the atomic spins, the team obtained a gravitational constant of 9.814 ± 0.008 m/s2, which agrees with the expected value.

“Our technique provides a way to apply an extremely well-known force to the atoms”, explained Sackett. “By knowing the force needed to balance gravity, we get a precise measurement of gravity”, he added.

Stanford University’s Mark Kasevich, who pioneered the use of ultracold atoms to measure gravity, told physicsworld.com. “It’s a nice paper — it certainly indicates that it would be interesting to do follow up experiments to push the limits of sensitivity.”

Room for improvement

Sackett told physicsworld.com that the team plans to improve the precision of the method by preparing the atoms in a state that is not as sensitive to magnetic fields. The team also plans to reduce the number of atoms that are lost to the experiment — currently about 0.1% per bounce. Sackett believes this can be done by improving their control of the intensity of the laser pulses.

The team has also worked out a way to use bouncing atoms to do atom interferometry. If atoms are sent along different paths and then combined at a detector, the resulting interference pattern depends on any differences in the accelerations experienced by the atoms. If built into a portable device, such accelerometers could be used as part of highly-accurate navigation devices that could work independently of GPS systems — making them attractive for submarine and other military applications.

To do atom interferometry, the team takes a slightly different approach and use laser pulses to split the atoms into two different groups, which are then independently bounced for many cycles before being recombined. The resulting interference pattern depends on any differences in the accelerations experienced by the atoms.

Although this technique gave a less precise value for gravitational acceleration than just bouncing, the team believe that they can improve the precision by about a factor of ten. If built into a portable device, such accelerometers could be used as part of highly-accurate navigation devices that could work independently of GPS systems — making them attractive for submarine and other military applications.

Slight imbalance

With regards to the interferometer, the current scheme is limited by a slight imbalance between the forces on the two groups of atoms — and Sackett says that improving this is possible, but will be difficult.

Sackett believes that an early application of the technology could be in navigation aids for military vehicles, vessels and aircraft. Such a device would keep track of its position by monitoring its acceleration — without the need of a GPS signal, which could be jammed.

Such devices could also be used in oil and mineral exploration, says Sackett, because they would be small enough to lower into bore holes to measure the mass density of the earth as a function of depth.

In the future, Sackett believes that the technique could be used to make very precise measurements of gravity over very short length scales — which could reveal deviations from Newton’s law as predicted by some theories of particle physics. However, he cautions that even bouncing over a few micrometres could be too great a distance for such measurements.

Top scientists join Hawking at Perimeter Institute

Nine leading researchers are to join Stephen Hawking as visiting fellows at the Perimeter Institute for Theoretical Physics in Ontario, Canada. The researchers, who include string theorists Leonard Susskind from Stanford University and Asoka Sen from the Harisch-Chandra Research Institute in India, will each spend a few months of the year at the institute as “distinguished research chairs”. They will be joined by another 30 top scientists to be announced at a later date.

The Perimeter Institute was founded in 1999 by Mike Lazaridis, chief executive of Research in Motion — the company that makes Blackberry wireless handheld devices. Home to more than 60 resident researchers, the institute focuses on areas such as cosmology, particle physics and quantum gravity. Hawking was made the institute’s first distinguished research chair last November, while his Cambridge University colleague Neil Turok was appointed as the executive director of the institute last May.

There has been criticism that the institute focuses too heavily on particle physics and cosmology and does not carry out any research in theoretical condensed-matter physics. However, one of the new chairs — condensed-matter physicist Xiao-Gang Wen from the Massachusetts Institute of Technology — thinks that the subject will increase in prominence at the Perimeter Institute.

“The methods used in condensed-matter physics and other areas in the institute such as quantum gravity are very similar,” says Wen, who will spend every summer at the institute. “I get the impression the institute is looking for people in condensed-matter physics whose work overlaps with the research already performed there.”

The other new chairs, who will all keep positions at their home universities, include Yakir Aharonov from Chapman University and Tel Aviv University, who shared the Wolf Prize in 1998; Juan Ignacio Cirac, director of the Max Planck Institute for Quantum Optics in Germany; and Harvard University condensed-matter theorist physicist Subir Sachdev.

They will be joined by string theorist Nima Arkani-Hamed of the Princeton Institute for Advanced Study; the Princeton University cosmologist Neta Bahcall at Princeton University; and Gia Dvali at the New York University Center for Cosmology and Particle Physics.

How can a theory of physics possess beauty?

By Hamish Johnston

Paul Dirac famously said “A physical theory must possess mathematical beauty”.

This morning Radio 4’s Today programme asked Dirac’s biographer Graham Farmelo “How can a theory of physics possess beauty?”.

Farmelo was joined on-air by Jim Al-Khalili, professor of physics at the University of Surrey, and together they discussed if aesthetics and logical thinking can go together.

You can listen to the conversation here — scroll right down to the last item of the programme. The audio clip will be available for about one week.

Although Al-Khalili and Farmelo did their best to explain, Radio 4’s normally razor-sharp John Humphrys seemed humbled by the concept.

The piece ended with Humphrys asking Al-Khalili for a concrete example of how the beauty of mathematics led Dirac to ground-breaking physics.

Al-Khalili recounted how Dirac used mathematics to predict the existence of antimatter several years before the stuff was actually seen in an experiment.

“…and antimatter is?”, replied Humphrys.

Chameleon particle blends into the background

Researchers at Fermilab in the US have carried out the first laboratory experiment to look for a hypothetical form of matter known as “chameleon” particles. Although it failed to find any conclusive evidence for the existence of the particles — which may offer an explanation for dark energy — the team managed to set a lower limit on their mass.

Chameleon particles were first proposed in 2003 by Justin Khoury and Amanda Weltman at Columbia University in New York as a possible explanation for dark energy — the mysterious entity that accounts for 70% of the mass-energy content of the universe and is causing the expansion of the universe to accelerate. Chameleon particles are so named because like the lizards, they adjust their properties according to their local environment.

In places where the density of matter is relatively high, chameleon particles interact very weakly with other matter and only over very short distances, which could explain why we have yet to spot them here on Earth. However, in inter-galactic space where matter density is extremely low, the particles interact much more strongly with other matter and over very large distances. This means that the particles could be exerting a force that is pushing the universe apart.

Weltman (now at the University of Cape Town) told physicsworld.com that there are several ways to test for the presence of chameleons. Physicists could send two masses made of two different materials into space and watch if they are accelerated by gravity in different ways. The particles could also be spotted by how they affect light travelling to Earth from distant galaxies, although this approach is dependent on a number of unrelated assumptions.

Looking here on Earth

Weltman has now joined forces with Aaron Chou and colleagues at Fermilab to carry out the first search for chameleons here on Earth. The team used the lab’s GammeV experiment, which was originally built to search for the axion — another hypothetical particle that was introduced in the 1970s but has yet to be spotted (Phys. Rev. Lett. 102 030402).

GammeV is a steel-walled vacuum chamber that is a few centimetres in diameter and several metres long. If chameleon particles could be created within the chamber, many of them would be unable to escape. That is because the mass of a chameleon is proportional to the local density, which means that if one of the particles ventures near to the chamber walls, it becomes very massive. This mass gain would be offset by a loss of kinetic energy, meaning the particle would find it hard to pass through the chamber walls.

The team sought to create chameleons by firing a laser into a region of the chamber containing a very high magnetic field. The particles are expected to be created in such a region when two photons interact. After switching the laser off, the team use sensitive photomultipliers to look for the “afterglow” as the chameleons decay into photons.

Although GammeV did not detect any sign of chameleon particles, their absence in the experiment enabled the team to put a lower limit on the mass of the chameleon (as it exists in the vacuum chamber) at about 1 meV. This limit comes about because the light detectors are sensitive to thermal noise within the vacuum chamber, which was at room temperature.

Pressure drop

Chou said that the team is now looking at how to investigate greater masses — which he says could be done by cooling the experiment to cryogenic temperatures, which would have two positive effects. First, it would reduce the thermal noise, allowing lower levels of light to be detected. Second, cooling the chamber would also allow the experiment to be carried out under a much higher vacuum (about 10-10 Torr), which would boost the difference in density between the chamber and its walls by a factor of about 10,000. This should mean that fewer chameleons can escape the chamber, leading to a stronger signal.

Astrophysicist Malcolm Fairbairn of King’s College London told physicsworld.com that physicists have found it “difficult to come up with a working model for a chameleon field which can explain dark energy using either quantum field theory or string theory”. “This experiment is important for those people who want to create such models and will focus their ideas in the future”, he added.

LHC’s biggest collisions on hold until after 2010

Maximum-energy collisions will take place at the Large Hadron Collider (LHC) only after 2010.

Following the recommendations of LHC staff given at a workshop last week in Chamonix, France, CERN management has decided upon a restart schedule that will see the accelerator collide protons at record energies of 10 TeV towards the end of this year. However, collisions at the maximum collision energy of 14 TeV will have to wait until at least 2011.

CERN says that it has made the decision to ensure that there is enough data produced next year for theorists to search for new physics.

“The schedule we have now is without a doubt the best for the LHC and for the physicists waiting for the data,” said CERN director general Rolf Heuer in a prepared statement. “It is cautious, ensuring that all the necessary work is done on the LHC before we start-up, yet it allows physics research to begin this year.”

Now there is cautious but real optimism for the start-up. There is also great belief

Steve Myers, CERN

The schedule specifies that the LHC will have protons re-enter its ring at the end of September — six weeks later than previously estimated — and will see its first collisions a month later. It will operate through winter, perhaps without stopping for any of the usual four-month maintenance period, while working up to 10 TeV. During this time staff will also try injecting lead ions into the rest of CERN’s accelerator complex, which feeds the LHC. This will open up the possibility of ion-ion collisions in late 2010.

Real optimism

The LHC circulated its first protons on 10 September last year to a global audience, but its commissioning was brought to an abrupt halt nine days later when an electrical fault caused a huge magnet “quench” which evaporated some 6 tonnes of liquid helium into the underground tunnel. The force of the helium leak was such that it broke anchors in the concrete floor and ruptured connections between the magnets.

Since then CERN has been assessing the LHC’s safety and conducting repairs, which could total as much as CHF 40m (£23m). The repairs include the replacement of 53 magnets, the installation of additional pressure-relief valves, and the laying of hundreds of kilometres of new cable to monitor electrical resistance in an improved “early warning” system.

CERN says that the improved early-warning system will be in place before the LHC’s restart. However, the lab also says that it has identified two more “suspect” connections, which are being addressed.

“For the three golden days [after 10 September] the atmospheric was electric,” Steve Myers, CERN’s director of accelerators and the chair of the Chamonix group, told physicsworld.com. “Immediately afterwards there was disappointment and deception. Now there is cautious but real optimism for the start-up. There is also great belief.”

Moth eyes inspire more efficient solar cell

Photovoltaics, which convert sunlight into electricity, have long been touted as one of the most promising solutions to our energy needs. Unfortunately, today’s devices reflect a lot of solar energy as heat, which means that solar power is currently not as cheap as other forms of energy. Now, however, researchers in the Netherlands have developed an anti-reflective coating based on the nanostructure of a moth’s eyes, which could reduce the reflection from photovoltaic cells and thereby make them more efficient.

Jaime Gomez Rivas and colleagues at the AMOLF institute in Eindhoven say that their “moth-eye” technology is superior to other known anti-reflection measures. Additionally, they have developed a new eco-friendly production technique that can apply the coating with high precision.

Attracted to the light

Everyone knows that moths seek light in the dark, but it seems that they really can’t get enough of those rays. To maximize the amount of light entering their eyes, to help them see at night, the insects’ eyes are covered in tapered nanostructures. This creates an “effective medium” where the refractive index gradually increases as light travels from air through to the insects’ optical nerve. The resulting effective index is graded from close to one at the top to close to 3.4 at the bottom, which means that very little light is reflected out of the eye.

Inspired by these biostructures, Gomez Rivas and colleagues have mimicked the effect by growing nanowires of different lengths, creating a metamaterial with optical properties that change gradually as a function of distance. The group has been reporting, over the past few years, a significant reduction in reflection over a broad range of colours and angles of incidence. Until now, however, it has not been clear whether this effect is the result of increased light transmission, or just scattering and absorption in the nanowires.

This time, the team used gallium phosphide (GaP) nanorods on top of a GaP substrate, then measured reflection and transmission simultaneously (Adv. Mater.:2009.21.1 ). “We showed for the first time that light transmission was dominant, with only a minor part of the [reduced] reflection linked with scattering losses and absorption” said Rivas.

Green coat

Early versions of a “graded index” nanostructure have been created from etching a silicon substrate with different kinds of nanostructure. A major drawback of this approach, however, is that the antireflection layer is also absorbing. Two years ago, the first “bottom up” graded index nanostructure was produced by combining silicon dioxide with titanium dioxide. Now, this new research offers a significantly more energy efficient approach, by creating a graded index in a single material.

“The idea of slowly changing the impedance of air to that of the material is well known, but the method by which they do it is rather elegant,” said Pete Vukusic, who studies the optical properties of natural materials at the University of Exeter, UK.

Rivas told physicsworld.com that his team’s long-term goal is to turn this into a product but, for the next few years, they will continue to seek even lower reflection. “In theory we could get 99% transmission; the difficulty is that our nanowires will need to get longer and thicker and then scattering come into play,” he said.

Francisco J Garcia-Vidal, an optics researcher at the Autonomous University of Madrid, said, “This bottom-up technique will very flexible because, allowing us to choose almost independently the material for the nanowires and the substrate.”

Entanglement goes mechanical

Physicists in the US are the first to demonstrate the quantum-mechanical phenomenon of entanglement in a common physical system known as a mechanical oscillator. The team has entangled the vibrational states of two pairs of atomic ions in an experiment that is another step forward in our understanding of the boundary between the quantum and classical worlds.

Since the birth of quantum mechanics in the 1920s and 1930s, physicists have argued about the relationship between the probabilistic domain of atoms and molecules and our everyday world of concrete objects. The problem was most famously highlighted by Erwin Schrödinger when he imagined the fate of a cat enclosed in a sealed box with a tiny quantity of radioactive substance placed near a Geiger-counter-enabled hammer and a glass flask of poison. Quantum mechanics suggests that after a certain period of time — at which point there may or may not have been a radioactive decay, causing the poison to be released — the cat is in a superposition of alive and dead states.

No one, however, has ever observed a cat or any other large object in such a superposition. It is possible that this is simply a technical issue, and that we are not yet able to isolate large objects from the environment for long enough to observe their fragile quantum states. Alternatively, there could be a fundamental, but as-yet undiscovered, mechanism that restricts quantum-mechanical effects to systems below a certain size.

Quantum effects in bigger systems

In the last decade or so, researchers have seen quantum effects in systems approaching the size of objects that normally obey classical physics. These have included quantum interference between beams of carbon-60 molecules, and a superposition of electrical currents circulating in opposite directions around a ring of superconducting material. However, no one has thus far observed entanglement — which gives two or more entities a much closer relationship than allowed by classical physics — in mechanical oscillators.

These are vibrating systems that include an oscillating mass on a spring, a plucked violin string and the vibrations of atoms in a molecule. When the energy of oscillation is extremely low the system’s energy levels become quantized and the motion is then described by the superposition of different quantum states. The simultaneous superposition of states from several oscillators leads to so-called entangled states, in which the properties of the constituent oscillators are correlated in ways not described by classical physics.

Now, John Jost and colleagues at the US National Institute of Standards and Technology (NIST) in Boulder, Colorado have entangled two oscillators by placing four ions — two beryllium-9 ions and two magnesium-24 ions — in the potential well of a single electrode (arXiv:0901.4779).

They then entangled the internal states of the two beryllium-9 ions and separated the four ions into two pairs, each containing one ion of each kind. This was done by placing the pairs next to two different electrodes. In this way, each pair acted as if it were a distinct mechanical oscillator, composed of two masses connected by a spring about 4 μm, with the two pairs separated by a distance of 0.24 mm.

’Non-locality’ could be studied

To entangle the mechanical oscillators, the team used laser beams to transfer the entanglement from the beryllium-9 ions’ internal states to the motion of the separated ion pairs (with the ions in each oscillator vibrating out of phase with one another). They were able to maintain this entanglement for around 100 μs.

Jost and team report that their work extends “the regime where entanglement has been observed in nature”, and that it could be used to investigate “non locality”, the interdependence of measurements carried out at two different places simultaneously. They also claim that their experimental technique “represents an important step towards large-scale trapped-ion quantum information processing”.

Now you see it… now you don't

By Hamish Johnston

UPDATE 11 February: Things have moved on in Washington and according to ScienceDebate2008 the US Senate has restored $3.1 billion in proposed cuts to science-related spending in the president’s stimulus package. You can see a comparison of the Senate’s proposal and the House of Representatives’ spending plans for science here.

I just received an email from the folks at ScienceDebate2008, claiming that a US senate committee has suggested significant cuts to Barack Obama’s $825bn economic stimulus package.

And it doesn’t look good for science — with all the extra money for the National Science Foundation (NSF) and the Department of Energy Office of Science included in the cuts.

ScienceDebate2008 (they really need to update their name) says the cuts are as follows, the percentage referring to the amount first proposed by the president.

NASA exploration $750,000,000 = 50%
NSF $1,402,000,000 = 100%
NOAA $427,000,000 = 34.94%
NIST $218,000,000 = 37.91%
DOE energy efficiency & renewable energy $1,000,000,000 = 38%
DOE office of science $100,000,000 = 100%

The political website Talking Points Memo has posted a detailed list of all the proposed cuts here apparently they total $77.9bn.

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