Page 1516 – Physics World

Testing for no dark matter

You might recall a while back physicsworld.com reported on a prediction for peculiar event that takes place on the two equinoxes. On the 20 March and the 22 September (or thereabouts) at two places on the Earth’s surface, many of the gravitational forces in the Milky Way should cancel out.

Such a quiet time in the turmoil of our galaxy provides an ideal opportunity for a ruthless test of Newton’s laws of motion. Some physicists think that if there were any deviation in the laws at very low accelerations it would mean dark matter — the elusive substance thought to make up around 95% of the universe’s mass and the dream catch of experiments worldwide — does not exist. Instead, all the phenomena associated with dark matter could be explained by a slight alteration in the laws known as modified Newtonian dynamics (MOND).

When Alex Ignatiev from the Theoretical Physics Research Institute in Melbourne, Australia, came up with the idea for the equinoctial experiment, there were a couple of problems with his proposal. First, there was a worry that stray icebergs at high latitudes where one of the experiments would have to be performed might give a false gravitational signal. Second, Ignatiev did not know the exact time that the desired signal would occur.

Now, in a new paper, he has resolved both of these. He has shown that even the biggest icebergs would not produce a signal big enough to confuse the data. And he has also shown how to predict the exact signal times.

One of the referees for Ignatiev’s paper has given a rich endorsement to the proposal: “MOND is the leading alternative to cosmic dark matter. It has passed a surprising number of astronomical tests and is desperately in need of laboratory tests. The author’s idea for testing MOND in a terrestrial setting is the only viable suggestion I’ve ever heard for such a possibility. This is an incredibly important problem, and deserves to be explored just as much as CDMS and the many other dark matter search experiments.”

‘Polaritonics’ forges ahead

Researchers in Greece have taken an important step towards the creation of practical optoelectronic devices that use “polaritons” — quasiparticles that are part matter and part light. The team has made a polariton light emitting diode (LED) that works at near room temperature, rather than at the extremely low operating temperatures of previous polariton devices. What’s more, the LED was made using conventional semiconductor technology and runs on a simple battery rather than being powered by a laser like its predecessors.

A polariton is a particle-like entity that can be used to describe how light interacts with semiconductors and other materials. It is an amalgam of two different ingredients: an electron-hole pair (or “exciton”) and a photon, which is emitted when the electron and hole recombine. When a photon is emitted, it remains trapped in the material and creates another exciton, so the cycle is repeated. This continuous exchange of energy between photons and excitons can be described in terms of polariton states.

Now, Pavlos Savvidis of the University of Crete and colleagues have made diodes from the semiconductor gallium arsenide (GaAs) that emit light directly from such polariton states at temperatures of 235 K, which is around 60 K below room temperature (Nature 453 372). This is the first time that GaAs-based polariton devices have been operated anywhere near room temperature.

Quantum wells

The researchers made their LED using molecular beam epitaxy – which is a standard technique for creating gallium-arsenide based devices. Photons are trapped in a tiny “microcavity” that is created between two highly reflecting “Bragg mirrors”. Excitons are trapped in quantum wells placed at the two ends of the microcavity to enhance their coupling to the photons. Finally, metal contacts were formed at the bottom and on top of the device to inject electrons and holes respectively.

According to Savvidis, the team’s success in using standard semiconductor fabrication processes to make a polariton LED that operates at near room temperature “raises hopes” that polaritons could be used in a wide range of devices.

Future polariton devices include polariton lasers, LEDs, amplifiers, switches and polarization modulatorsPavlos Savvidis, University of Crete

“Future polariton devices include polariton lasers, LEDs, amplifiers, switches and polarization modulators,” Savvidis told physicsworld.com. A particularly attractive feature of a polariton laser is that coherent monochromatic light is produced by the single lowest state of the system at the bottom of the polariton trap, he added. Here a Bose-Einstein condensate of polaritons is formed that requires no “population inversion”, as in conventional solid-state laser, so the lasing threshold is reduced by several orders of magnitude. “This makes these lasers extremely promising as ultra-low threshold lasers, or as low-power sources of coherent and non-classical light.”

In general, polariton devices will have very low power consumption and be ultrafast, he added. They could also be assembled in parallel into large matrices of thousands of devices each, which would dramatically reduce the cost per device.

According to the researchers, there is no fundamental reason why the devices should not work at temperatures even closer to room temperature and their next goal is to make such a device.

The new LED is a step further towards fulfilling the promise of room-temperature, electrically driven polariton devices, writes Benoît Deveaud-Plédran of the Ecole Polytechnique Fédérale de Laussane, Switzerland, in a related article (Nature 453 297).

Einstein letter fetches record amount at auction

A letter written by Albert Einstein the year before his death was sold for the staggering amount of £170,000 at an auction in London yesterday. The previously unrecorded letter, which has spent the past 50 years in a private collection, contains a discussion of Einstein’s views on religion, bringing new material to the debate about whether or not he believed in God. It was expected to fetch between £6000–8000.

The little-known letter, which is handwritten in German, was penned by the eminent physicist in 1954, at the age of 74, while he was living in Princeton in the US. It is addressed to his Jewish philosopher friend Eric Gutkind, in response to Einstein reading Gutkind’s newly published book Choose Life: The Biblical Call to Revolt.

As such, it reveals some of his thinking on religion. He states in the letter, for example, “The word god is for me nothing more than the expression and product of human weaknesses, the Bible a collection of honourable, but still primitive legends which are nevertheless pretty childish.” And, although from a Jewish background, he writes, “For me the Jewish religion like all others is an incarnation of the most childish superstitions. And the Jewish people to whom I gladly belong and with whose mentality I have a deep affinity have no different quality for me than all other people.”

Famous references to religion

Einstein’s other references to religion, such as the famous, “Science without religion is lame, and religion without science is blind,” and his assertion that God does not play dice with the universe, collectively paint a confusing and ambiguous picture of his beliefs, and this letter, which is not listed in the source material of Max Jammer’s Einstein and Religion, the most authoritative text on the subject, will provide new fuel for the long-running debate about whether or not Einstein was an atheist.

According to Richard Caton at Bloomsbury Auctions the winning bidder is, “a private buyer with a passion for theoretical physics and all that that entails.” In this case it clearly entails parting with a lot of cash, as once the auction fees are added on, the buyer will actually be paying £207,600. This smashes the previous record of around £30,000 for the price of a letter by a physicist.

Canadian firm shelves isotope reactors

The sad saga of the MAPLE nuclear reactors may have finally come to a close with today’s announcement from Atomic Energy of Canada (AECL) that the firm will no longer try to get the pair of reactors licensed to produce medical isotopes.

MAPLE was conceived in the 1980s as a replacement for AECL’s ageing NRX and NRU research reactors at Chalk River, Ontario. “M” stands for “multipurpose”, and the MAPLE was intended for both basic research as well as the commercial production of radioactive isotopes for medical and other applications.

Two MAPLE reactors were finally built at Chalk River in 2000, but it soon became apparent that they both suffered from serious safety problems associated with shoddy workmanship. As a result the facilities have never been granted full operational licences by the Canadian nuclear regulator.

AECL has also had safety problems with the 50-year old NRU, which had to be shutdown unexpectedly for about a month in 2007, leading to an international shortage of medical isotopes.

In the case of NRU, the Canadian government stepped in to restart the reactor — overruling its own regulator. AECL may be gambling that its move to scrap MAPLE may cause the government to pressure the regulator into approving the reactors.

The dusty cosmos

Astrophysicists have a better idea of how dust obscures the light from galaxies, according to a paper published in Astrophysical Journal Letters.

It is already well known that dust, which permeates all galaxies, attenuates the light reaching Earth from the cosmos. It absorbs light of most wavelengths and then re-emits it as a blanket of infrared radiation. Now, Simon Driver of St Andrews University in the UK and colleagues have produced the first model that accounts for this absorption.

One of the model’s implications — that dust absorbs just under half the radiation produced by stars — will not be a surprise to astronomers. They already know this, having compared the average magnitude of the infrared radiation in the sky with the magnitude of the radiation from pinpoint sources like stars and galaxies. But what might be of interest is that Driver and colleagues can show how the dust affects the light output of galaxies depending on their orientation.

I spoke with Alastair Edge of Durham University, who is familiar with Driver’s team’s work, and he was pleased that that the researchers have managed to model the dust successfully. He followed up our conversation with an email: “The authors have made an important link between the observed properties of the galaxies we see from the light coming directly from their stars to the amount of long wavelength radiation we see coming from the dust within the galaxies. Obtaining a match between the energy absorbed and that re-radiated allows us to understand the global properties of galaxies in a more holistic fashion.”

Eccentric pulsar could be part of a threesome

An international team of astronomers has spotted a binary pulsar that appears to behave very differently than most known binary systems in our galaxy. The most likely explanation, says the team, is that the formation of the pulsar involved three stars rather than two — the first such system to be spotted.

Pulsars are rapidly spinning neutron stars that earn their name from the beams of radiation they emit, which appear as pulses to an observer on Earth. Many pulsars orbit a companion star and such objects are called binary pulsars.

David Champion of the Australia Telescope National Facility and colleagues used the Arecibo radio telescope in Puerto Rico to study pulses coming from PSR J1903+0327, a binary pulsar located in the disk of our galaxy and discovered by the team in 2005 (Science DOI: 10.1126/science.1157580) .

Highly eccentric

The pulses arrive at Earth about once every 2.15 ms and by watching the pulsar very carefully over 1.5 years the team determined that the binary orbit is highly eccentric. This came as a surprise, because other binary pulsars with periods less than about 10 ms — called “millisecond pulsars” — have nearly perfect circular orbits. And to make matters even more confusing, the companion star in all known millisecond pulsars is a white dwarf, while PSR J1903+0327 appears to involve a star much like our Sun.

Astronomers believe that a normal millisecond pulsar is formed from a binary system comprising a large star (greater than about eight solar masses) and a star about the same size as the Sun. The large star explodes in a supernova, leaving a spinning neutron star with a period greater than 10 ms and knocking the pair into a highly eccentric orbit.

As time progresses the companion star expands to become a red giant and the neutron star begins to suck in (or accrete) material from its bloated companion. This causes the neutron star to rotate more rapidly — eventually reducing its period below 10 ms — and makes the binary orbit more and more circular. Eventually, the companion becomes a white dwarf, the accretion stops, and the pair settle down into a millisecond binary pulsar with a nearly perfect circular orbit.

Three possible explanations

However, PSR J1903+0327 does not fit this mould and the team have put forth three possible explanations. The first and least satisfactory explanation is that object is simply a very young — yet fast spinning — binary pulsar. However, the age of a pulsar can be determined by the rate at which the its period is increasing, and such measurements suggest that this binary is much too old for this explanation.

The second possibility is that the pulsar was formed in a conventional manner in a globular cluster — a region with a relatively high density of stars — where the gravitational pull of other nearby objects caused the spinning neutron star to be ejected from the cluster, taking a Sun-like companion with it. However, there is no evidence of a nearby globular cluster and Champion believes that there is at most a 10% chance that this is what happened.

The most likely explanation, according to Champion, is that the pulsar was born within a trio of stars. Two of these stars formed a millisecond pulsar, while the gravitational field of a third — and more distant Sun-like star — conspired to make the orbit highly eccentric. A variation on this explanation is that the white-dwarf companion has since been completely destroyed by the neutron star, leaving the neutron star in a highly eccentric orbit around the Sun-like star.

The team now plan to make further observations to get a better understanding of the system. One crucial measurement that must be made, according to Champion, is whether the spinning neutron star is orbiting the Sun-like star, or another object such as a white dwarf. This requires the use of a large optical telescope such as Gemini or the VLT.

US public say yes to science debate

I’m sorry to say that, having taken a day’s leave on Monday, this snippet of news (above) about ScienceDebate 2008 escaped my attention. According to a poll conducted by Harris Interactive on behalf of ScienceDebate 2008 and Research!America, 85% of US adults think agree that the presidential candidates should participate in a debate on science in the run up to the November election.

(For those of you who have missed the protests of the 37,000 signatories of ScienceDebate 2008, see my last news story on their progress.)

Shawn Otto, CEO of Science Debate 2008, gave the following statement in a press release:

“This topic has been virtually ignored by the candidates, but this poll shows that Americans of all walks know how important science and technology are to our health and way of life. We’ve heard a lot about lapel pins and preachers. But tackling the big science challenges is critical to our children’s future — to the future of the country and the future of the planet. Americans want to know that candidates take these issues seriously, and the candidates have a responsibility to let voters know what they think.”

The poll also shows that:

67% of adults think scientific research has contributed either “a lot” or “a great deal”
67% think that scientific evidence, rather than personal belief, should influence science policy
69% rate alternative energy as one of the most serious long-term issues
53% rate climate change as one of the most serious long-term issues

You can read more here.

Oh brother, where art thou?

I wonder if many other scientists under the wing of the Holy See agree with Jose Gabriel Funes, the head of the Vatican observatory, or whether he’s something of a radical.

In an interview in yesterday’s edition of the Vatican newspaper L’Osservatore Romano, Funes not only admits that he believes in the Big-Bang model of the universe’s creation, he states that humans should be open to the possibility of alien life. “Just as there’s a multiplicity of creatures on earth,” he says, “there can be other beings, even intelligent, created by God.”

To be clear, Funes is in no way dismissing the first two chapters of Genesis. In fact, he sees “no contrast” between the notion of aliens and the Catholic faith. The other beings might also be worshipping God, he says.

The interview is headlined “The extraterrestrial is my brother”.

Shrimp see a polarized world

Put on a pair of polarized sunglasses and your view of the world around you can change dramatically. A bright sky will darken, giving you a better view of objects on the horizon, and the glare from the surface of a pond will vanish, allowing you to peer into its depths. Your vision is improved because the sunglasses filter out much of the scattered sunlight that has become polarized as it travels through the atmosphere and reflects off objects around us.

Scientists have known for at least 50 years that some insects, crustaceans, spiders and other anthropods can “see” light of different polarizations — without the need for any artificial aids. These animals can even use this ability to navigate, find food and evade predators. But biophysicists had thought that this ability was only true for linearly polarized light, in which the electric-field vector points in a fixed direction.

Such animals, it was thought, could not detect circularly polarized light, in which the electric field vector rotates in a clockwise or anticlockwise direction. This belief was based mostly on the observation that as there are very few sources of circularly polarized light in nature, detecting it would not be of much use and it would be unlikely that any animal had evolved the ability to do so.

Simultaneous detection

Now, Sonja Kleinlogel of the Max Planck Institute for Biophysics in Frankfurt and Andrew White of the University of Queensland have carried out experiments on the eyes of mantis shrimp that show how the crustacean can detect simultaneously both circular and linear polarization of light (PLoS ONE doi:10.1371/journal.pone.0002190).

This new analysis backs up a report made earlier this year by Kleinlogel and colleagues in Australia and the US that live mantis shrimp behave differently when exposed to light of different circular polarization (Current Biology 18 429).

In their new experiments, Kleinlogel and White made a series of measurements on eyes that had been removed from mantis shrimp. These creatures have two spherical compound eyes, each of which comprises hundreds of smaller eyes organized into three substructures — two hemispheres separated by a mid-band.

The team shone light of different linear and circular polarizations on the three substructures and measured the electrical response of the photoreceptors within. They discovered that the photoreceptors in the hemispheres respond to light with different linear polarizations, while the mid-band was sensitive to circularly polarized light.

An ‘artificial mantis shrimp eye’ would give us a great opportunity to figure out what these little critters really perceive in their environmentSonja Kleinlogel, Max Planck Institute for Biophysics

Stokes parameters

As a result, the team claims that the shrimp can detect any possible combination of linear and circular polarization. Mathematically, this means that the eye is sensitive to all of the six parameters that are required to define the polarization of light — the so-called Stokes parameters.

White told physicsworld.com that the mantis shrimp detects circular polarization in this mid-band region by first passing light through a biological cell that appears to act as a “quarter wave plate”. This is an optical device that converts circularly polarized light to linearly polarized light. The team believe that this light is then detected by cells sensitive to linearly polarized light.

This ability to visualize all types of polarization allows the shrimp to perceive very subtle changes in the polarization, claims the team, which could improve their view of their environment. “Some of the animals that the [mantis shrimp] like to eat are transparent, and quite hard to see in sea-water — except they’re packed full of polarizing sugars — which makes them light up like Christmas trees as far as these shrimp are concerned”, explained Kleinlogel.

Another team of researchers at Queensland have recently shown that light reflecting from mantis shrimp is circularly polarized, suggesting that the polarization vision is involved in the mating process.

Nick Roberts, who is a biophysicist at the UK’s University of Manchester, described the work as “persuasive”. He also pointed out that the team’s novel measurement and analysis technique, which involves using six measurements to define uniquely the response of different parts of the eye, was an important contribution to the understanding of polarized light vision.

Shrimp’s-eye view

Kleinlogel said that the team now plan to build an “artificial mantis shrimp eye” — a camera that would have a shrimp’s-eye view of Australia’s Great Barrier Reef, which is home to the crustacean. “This would give us a great opportunity to figure out what these little critters really perceive in their environment”, she said.

Ozone could clean up biological-warfare agents

The succession of letters containing anthrax sent to US media outlets in the wake of the 11 September 2001 terrorist attacks was a grave warning of the potential impact of biological warfare. Five people died, more than 20 were infected and estimates for the total clean-up bill stretched as high as $1bn.

The main reason for the huge cost was that entire office buildings had to be fumigated with chlorine dioxide. Although this bleach is effective at killing microbes like anthrax, it is also toxic and so in the end must be neutralized. It would seem prudent, therefore, to use sterilizing agents that are harmless to humans. One possibility is ozone (O₃) gas which inactivates dangerous microbes by oxidizing their cell structure. Unfortunately, ozone dissolved in water decays quickly into oxygen rendering it useless.

Now, however, scientists in the Republic of Korea have learnt how to extend the lifetime of ozone by making the water acidic. “The acidic ozone water can be produced abundantly from tap water or fresh water at any place on Earth,” explains Han Sup Uhm at Ajou University. “For example, a moderate size of system can produce the acidic ozone water at one or two tons per minute which can be sprayed over the contaminated area.”

Long lasting

In the past scientists have suspected that the reason for ozone’s accelerated decay in water might result from its interaction with OH^– ions. Because acid affects the way water dissociates so that there are fewer OH^– ions compared with H⁺ ions, acidifying ozone water should make the ozone last longer. Uhm and colleagues have proved that ozone lasts longer in acidic water and have come up with an analytical expression that predicts how many microbes can be killed with ozone of a certain acidity, or pH (Appl. Phys. Lett. 92 174102).

The researchers found that in neutral water at pH 7, the “decay time” of ozone — the time for half of it to turn into oxygen — was just 23 minutes. However, adding just a small amount of hydrochloric acid to reduce the pH to 4 upped the decay time to three hours.

This extended decay time not only means that there is more time to use ozone water, it means that ozone water can kill more microbes. Their analytical expression predicted that pH 4 ozone water should kill around 25 times more microbes than neutral ozone water. This prediction was backed up with experiments using both kinds of ozone water with bacterial spores.

Uhm says that his team developed the ozone water for use in the defence against biological warfare agents like anthrax, though there could be many other applications. “Acidic ozone water may be applicable to the agriculture, seafood and livestock industries to prevent spoilage and to prolong the shelf life of products,” he says. “It may also prevent or control diseases and pests in agriculture.”

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Testing for no dark matter