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Home » Page 1645

BEC bubble could measure tiny forces

Two physicists in the US have come up with a way to use a “bubble” of ultracold atoms to measure extremely small forces. The scheme, which has yet to be tested experimentally, involves monitoring the motion of a bubble of one type of atomic gas that is surrounded by another atomic gas. The physicists claim that accelerations as small as 10-10 m/s2 could be detected — allowing the system to be used to perform new tests of the gravitational inverse-square law or to study the forces on individual atoms.

The measurements would take place in a Bose-Einstein condensate (BEC) — a gas of bosons (atoms of integer spin number) that are cooled to such low temperatures that they fall into the same quantum state. Over the past decade physicists have perfected the creation of BECs using crisscrossing laser beams to trap the ultracold atoms and using applied magnetic fields to finely tune the interactions between the atoms.

More recently, physicists have worked out ways to make BECs that contain mixtures of two different types of atoms — say “A” and “B”. By adjusting the magnetic fields such that A and B tend to repel each other, the atoms can be separated into two different phases.

‘Buoyancy’ force

Now, Satyan Bhongale of Rice University and Eddy Timmermans at Los Alamos National Laboratory have proposed a system that involves a phase-separated mixture in which one component (say B) forms a bubble in the other (Phys Rev Lett 100 185301).

Timmermans told physicsworld.com that the bubble would be subject to a force that tended to push the bubble from the middle of the trap to the edge. This “buoyancy” force, he explained, is similar to the force that causes an air bubble to rise in water. The team believes that by adjusting the laser beams that are trapping the BEC, this buoyancy force can be exactly cancelled, causing the bubble to float at the centre of the trap.

The position of the bubble could be monitored by shining two relatively weak laser beams through the bubble such that they intersect at its centre. The slightest movement of the bubble by an external force could be detected by carefully monitoring the laser beams. In some ways, the system is similar to a spirit level, which offsets the buoyancy of a bubble against the force of gravity, leading Bhongale and Timmermans to dub their system a “BEC level”.

Measuring gravity over micrometres

According to Timmermans, the level could measure gravitational acceleration to about one part in 10 billion. While this is on par with existing schemes to test gravity such as the torsion pendulum, the BEC level could reach this precision on length scales as small as a micrometre – much smaller than the millimeter distances probed by the best existing experiments. As a result, the BEC balance has the capability to reveal deviations from the familiar inverse-square law of gravitation, which could ultimately help physicists overcome one of the outstanding challenges of physics — how to unify gravity with the three other fundamental forces of nature.

In addition, Timmermans believes that the BEC level could be used to study the Casimir-Polder forces that are experienced by atoms that are near to a surface. Such forces have proved very difficult to measure but are of interest to nanotechnologists because they appear to play an important role in how atoms are organized on a surface.

While Timmermans is not aware of any experimental groups that are currently trying to build a BEC level, he believes that several groups around the world have the required experimental expertise.

One such group is led by Nobel laureate Carl Wieman at the University of Colorado and recently demonstrated that a BEC of rubidium-85 and rubidium-87 atoms could be separated into two different species (arXiv:0802.2591). Team member Scott Papp (now at Caltech) told physicsworld.com that it is “plausible that phase-separated BECs could be used for force detection”. He added that the Colorado team has already shown that such BECs are sensitive to external forces such as gravity.

However, he also observed that realizing Bhongale and Timmermans’s design in the laboratory would be “difficult, but not impossible”.

Willis Lamb: 1913-2008

Willis Lamb, who won the 1955 Nobel Prize in Physics “for his discoveries concerning the fine structure of the hydrogen spectrum”, died last week at the age of 94.

In 1947, Lamb discovered the famous “shift” in the hydrogen spectrum that bears his name. The Lamb shift provided important experimental evidence for the then emerging theory of quantum electrodynamics (QED).

Lamb was born on 12 July, 1913 in Los Angeles, California and like many physicists of his generation, he worked on radar technology during the Second World War. After the war, he turned his microwave expertise to the study of the hydrogen atom.

While working at Columbia University in New York, Lamb found that the 2S1/2 electron energy level in hydrogen was slightly higher than the 2P1/2 energy level. This shift was not predicted by relativistic quantum mechanics, which had been used two decades earlier by Paul Dirac to explain the fine structure of the hydrogen atom.

Instead, the Lamb shift provided crucial evidence for the new theory of QED, which describes the interactions between charged particles in terms of the exchange of photons. Ten years later, Julian Swinger and Richard Feynman of the US and Sin-Itiro Tomonaga of Japan shared the 1965 Nobel Prize in Physics for their work on QED — and, in particular, its use in explaining the Lamb shift.

Lamb spent his formative years in California and in 1938 he gained PhD in nuclear physics from the University of California at Berkeley under the supervision of Robert Oppenheimer. Lamb then joined the physics department at Columbia University, where he did his Nobel-prize work at the Columbia Radiation Laboratory.

Lamb shared the 1955 Nobel Prize with his Columbia colleague Polykarp Kusch, who won for his independent work on using microwave techniques to determine the magnetic moment of the electron.

Lamb left Columbia in 1951 for Stanford University in California and over the next 22 years he held positions at Harvard, Yale and Oxford. In 1974, Lamb joined the School of Optical Sciences at the University of Arizona, where he remained until his retirement in 2002.

Lamb died on 15 May, 2008 in Tucson, Arizona and is survived by his wife Elsie and brother Perry.

Information ‘not lost’ in black holes

The “information paradox” surrounding black holes has sucked in many noteworthy physicists over the years. For more than three decades Stephen Hawking of Cambridge University in the UK insisted that any information associated with particles swallowed by black holes is forever lost, despite this going against the rule of quantum mechanics that information cannot be destroyed.

When four years ago Hawking famously made a volte-face — that information can be recovered after all — not everyone was convinced. “The general view is that [Hawking’s] argument is not sufficiently detailed,” says Abhay Ashtekar at Penn State University in the US.

Now, Ashekar and colleagues at Penn State claim to have more reliable mechanism that can preserve information dragged into the shadows of black holes.

Not so black after all

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

Although Hawking famously conceded his bet, he left his original argument for information loss orphaned but alive Steven Giddings, University of California in Santa Barbara

Quantum entanglement demands that the trapped particle would have negative energy and, because of Einstein’s mass-energy equivalence E = mc2, negative mass. With each successive negative-energy particle the black hole would therefore steadily lose mass or “evaporate”. Hawking argued that even after a black hole has totally evaporated it would leave behind its central, infinitely dense point known as the singularity, in which information would be lost forever.

The significance of the information paradox came to a head in 1997 when Hawking, together with colleague Kip Thorne at Caltech, US, put this argument forward as a bet with John Preskill, also at Caltech. Preskill believed that, in accordance with quantum mechanics, information loss is impossible because it prevents the equations governing the process from being reversible. But in 2004 Hawking conceded the bet, saying he now believed that information is returned, although in a disguised state.

Sticking points

Hawking’s revised stance failed to sway other theorists. Aside from the fact that his new theory was based on mathematics that is not obviously relevant to physical space–time, it did not directly address his original argument about the singularity.

The Penn State group, which includes Ashtekar as well as Victor Taveras and Madhavan Varadarajan, claims to have overturned this argument by performing calculations of a black hole model in two dimensions: one space and one time. “In my opinion this remains a very important question to settle,” says Steven Giddings at the University of California in Santa Barbara. “Although Hawking famously conceded his bet, at the time he left his original argument for information loss orphaned but alive. This new work appears to have found improved control over the calculations.”

The advantage of working in two dimensions is that it has allowed Ashtekar’s group to write down exact quantum equations governing the gravity at a black hole, which they can evaluate using two approximations. The first is a “bootstrapping” process, essentially reaching a solution for the equations using a series of better-informed guesses. “Bootstrapping serves to demonstrate that quantum geometry can be perfectly regular even when the classical geometry acquires singularities,” explains Ashtekar.

Second is a “mean field” approximation that finds a solution for the region away from the centre of the black hole. It was using this approximation that Ashtekar’s group discovered the inner region approaching infinite density is much larger than previously thought using classical arguments — large enough to allow the recovery of information (Phys. Rev. Lett. in publication; preprint at arXiv:0801.1811).

It strongly suggests that black hole evaporation does not destroy information Seth Lloyd, Massachusetts Institute of Technology

‘Not convinced’

Preskill, who accepted Hawking’s 2004 concession even though he was doubtful of his theory, is also “not convinced” of the Penn State research — though he notes that he has not yet studied it carefully. “I thought we made a pretty strong case back in 1994 that models of this type exhibit information loss…I don’t see how the observations by Ashtekar et al. change that conclusion, but I may be missing something.” Thorne, who was also dubious of Hawking’s concession at the time, did not want to comment because he is not familiar with this particular field of research.

Other theorists think Ashtekar’s group have made an important development, though they add that the debate is still not over. “After some extended discussions with Abhay, I am not yet convinced that they have shown the information comes out,” says Giddings.

“It is indeed very interesting,” says Seth Lloyd of the Massachusetts Institute of Technology. “It strongly suggests, although it does not prove, that black hole evaporation in one-plus-one dimensions does not destroy information: all information escapes as the black hole evaporates…[but] it is not clear that the derivation would work in three-plus-one dimensions.”

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

DustyGalaxy.jpg

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

PieChart.jpg

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.

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