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Laser recreates X-rays emitted by a black hole

Physicists have used high-power lasers to recreate X-ray spectra emanating from some black holes and neutron stars. Conclusions drawn from the experiment appear to conflict with previous interpretations of astronomical data, suggesting that we may have to rethink our view of the structure surrounding black holes and neutrons stars.

Large quantities of X-rays are produced when a black hole or neutron star sucks in matter from a companion star, creating a ring of matter known as an accretion disc. As matter spirals into the black hole or neutron star, gravitational energy is converted into kinetic energy and heat. The intense radiation that is released travels outwards (in the form of photons) and ionizes material closer to the outer edge of the accretion disc – creating an X-ray emitting plasma.

Interpreting the X-ray spectrum of such a plasma is key to understanding the physics of such systems, because it is impossible for astronomers to directly measure its temperature, density and pressure. It has also proven very difficult to recreate such a “photo-ionized” plasma here on Earth because it requires an extremely hot source of radiation.

But now researchers in Japan, Korea and China are helping to address this weakness by studying the spectra of plasmas created in the lab. Such spectra are very similar to that produced by Cygnus X-3, a black hole and a companion star with highly ionized silicon ions on its surface. A similar X-ray spectrum has also been recorded from Vela X-1, a neutron-star binary system.

Pump up the power

The researchers produced their X-ray spectra at the GEKKO-XII laser facility, which is located at Osaka University, Japan. The system combines a 10 TW laser that is capable of producing nanosecond pulses from twelve beams with a 10 PW laser that can deliver four picosecond beams.

“We used 12 nanosecond laser beams with wavelength, energy and pulse duration of 0.53 µm, 4 kJ in total and 1.2 ns [respectively],” explained Shinsuke Fujioka from Osaka University, who proposed and organized the experiment.

The beams are fired at a tiny plastic capsule, causing it to implode. “As it shrinks, a hot and dense plasma core forms inside the capsule,” says Fujioka. The radiation produced then photo-ionizes a nearby sample of cold silicon gas.

Similar, but different

Fujioka says that the shape of their X-ray spectra is quite similar to that recorded by astronomers. However, interpretations of the origin of characteristic lines emissions differ.

Astrophysicists claim that an X-ray peak at 1.84 keV stems from a forbidden transition of silicon ions. But Fujioka says that calculations performed by his team – which consider experimental measurements of the temperature and density of the plasma – suggest that the peak is associated with a different resonance transition of silicon ions.

However, the researchers admit that they cannot provide a definite explanation for the origin of this peak. That is because the radiation flux produced in the laboratory lasts for tiny fractions of a second, while that produced by compact astrophysical objects is continuous.

The work is reported in Nature Physics and, writing in a companion piece, Paul Drake of the University of Michigan described the technique as having “great potential for further development,” because it allows the energy of the photon source to be varied over a wide range while allowing a great deal of control over the photo-ionized material. However, Drake also cautions that more work must be done in terms of characterizing the physical properties of the resulting plasmas.

Fujioka says that the team may now turn its attention to investigations of the absorption of intense beams of X-rays. It is widely believed that the X-ray absorption rate in materials and plasmas is independent of the intensity of the beam, but they suspect that a plasma may become transparent in incredibly intense X-ray beams. If this is the case, it will modify our understanding of how plasmas behave in supernovae.

Television series tackles 'science's last taboo'

Did it matter that Einstein was of northern European descent?

By James Dacey

This thought-provoking image forms part of the advertising campaign for a new UK television series that will look at the controversial history of science and scientists addressing the issue of race.

Race: science’s last taboo has been created by Channel 4 and will be focused around five documentaries, each one engaging in a different aspect of the debate.

The season kicks-off tonight with a programme about race and intelligence, which includes the controversy surrounding James Watson’s cancelled UK lecture tour of 2007.

In case you missed it at the time, the Nobel Laureate — who co-discovered the double-helical structure of DNA — was quoted as saying that there is scientific evidence to suggest that black people are less intelligent than people of other races. People were so incensed that Watson was forced to abandon his tour and leave the UK early.

For more details about the new series, check out the related website. Amongst other features, you can offer your own definition of race — though bear in mind you’ve only got 140 characters!

BRIC to prop up developing-world science

Africa and parts of Latin America still lack a strong scientific foundation

By James Dacey

In economic circles, the notion of there being a clear divide between the “developed” and the “developing” world has long been a discredited one. Indeed the economies of the BRIC countries (Brazil, Russia, India and China) are now widely recognized as rapidly growing with the potential to compete with the richest economies in the world within the next few decades.

Now, it seems that the folks interested in scientific development are also starting to recognize the role of these “middle” world nations.

This week, the developing world’s academy of sciences (TWAS announced that it was looking to double its endowment fund to US$25 million to support science and scientists in the developing world. It hopes to raise the funds by seeking donations from the more successful developing world countries such as the BRIC nations. The announcement was made at the academy’s 11th general meeting in Durban, South Africa.

When I heard this, it struck me as an echo of something Fernando Quevedo, new director of the International Centre for Theoretical Physics (ICTP, had said to me a couple of weeks ago. If you’re not familiar with the ICTP, it was founded in 1964 as the inspiration of the Pakistani Nobel-prize-winning theorist Abdus Salam, who wanted to establish an international research centre for young scientists from the developing world. The ICTP has its headquarters in Trieste, Italy, where it also houses TWAS.

Quevedo said that he will be using his presidency to try and build more networks across the developing world. His idea being that the nations like the BRIC group, which are starting to churn out more science year-on-year, can help give a leg-up to the nations that are still struggling to get their science bases off the ground. He cites academic networks and improved internet connectivity as key tools.

“The developing world has changed a lot in the past 40 years. Some of the larger countries like Mexico, Brazil, China and India are now offering a significant contribution to international science,” he said. Adding, “But there are also smaller countries, particularly in South America and Africa that have been slower to develop, which people still don’t like to talk about”.

Bacteria power micro-ratchet

In contrast to the random motion of tiny objects placed in a bath of molecules at thermal equilibrium, miniature asymmetrically shaped cog wheels rotate continuously in one direction when exposed to swimming bacteria. That is the finding of a group of Italian scientists who built ratchets just a few hundredths of a millimetre across and filmed their motion when immersed in solutions containing E. coli. The researchers say that such bacteria-propelled ratchets could be used to power micro-machines or potentially even provide a new macroscopic source of energy.

Bacteria swim through fluids by using a number of rotary motors embedded in their cell walls to drive thin helically shaped filaments in a clockwise or anticlockwise direction. Researchers have used this motion to propel micro-structures but, as the bacteria movement is random, it is difficult to control without the addition of chemicals.

Now, Roberto Di Leonardo and colleagues at the University of Rome “La Sapienza” have shown how to exploit the motion of E. coli without controlling the bacteria in any way. They exploit the fact that a collection of bacteria immersed in a liquid represents a non-equilibrium thermodynamic system by virtue of the self-propulsion of the bacteria. This non-equilibrium system, they say, should confer ordered motion on an asymmetrically shaped object, in contrast to the random, Brownian motion, that thermal gas molecules would generate.

Using electron-beam lithography they etch out 10,000 saw-tooth gear wheels from the polymer SU8, each just 48 µm across and 10 µm thick (a volume equivalent to that of about 20,000 bacteria). They then dispersed the gear wheels in a suspension of E. coli, and hung a droplet of this liquid from the underside of a glass slide. This allowed the gear wheels to accumulate on the liquid-air interface, where they were held by capillary forces and free from the strong adhesion they would experience next to a solid surface.

The researchers observed that bacteria would strike the long-edge of each tooth and either bounce off or have their movement blocked by the next short-edge (see diagram). Given that bacteria do not stop swimming when they meet an obstacle or blockage, this caused them to push continuously against the short edge, with the combined motion of many such bacteria causing the gear wheel to rotate in one specific direction (clockwise in this case).

By filming the motion of the gear wheels, the researchers found that the gears rotated at a rate of about 1 revolution per minute, and that this value and variations in it caused by fluctuations in the number and arrangement of bacteria at the surface of the wheels closely matched the predictions of a computer simulation they carried out last year.

According to Di Leonardo, the performance of these bacteria-propelled ratchets could be improved tremendously. He notes that the linear velocity of the edge of the gears in their experiments is only about 2.5 µm per second whereas E. coli swim at about 20 µm per second. Getting closer to this second figure will, he says, involve experimenting with the shapes and sizes of the gear wheels as well as using other types of bacteria with different shapes and different swimming techniques.

Di Leonardo and his colleagues note in their paper that they can already foresee a completely new technology where “passive micro-devices can be fabricated and simply actuated by immersion in an active liquid”. Applications could include the powering of autonomous micrometre-sized machines that do not need to be powered by external electrical, magnetic or optical fields, as well as pumps or valves in microfluidic devices. This conversion of chemical energy into mechanical energy also suggests that the technology might be used as a power source, although Di Leonardo points out that the low-power density of a bacterial suspension – about 1 Wm^–3 – makes this “very challenging”.

Howard Berg, a biophysicist at Harvard University in the US, says that the research is intriguing but not ultimately convincing. “If I were serious about this business, I would fabricate electromechanical propulsive devices out of silicon,” he says. “These are much more robust.”

This research is published on the arXiv preprint server.

Spintronics made easy

The second instalment in our series of videos features two leading physicists in the field of spintronics – Albert Fert and David Awschalom. Both were interviewed at the Royal Society‘s recent discussion meeting The Spin on Electronics, where we also recorded a selection of video “vox pops” with speakers and attendees.

The first interview is with David Awschalom of the University of California, Santa Barbara. If you are new to spintronics – or if you are wondering what all the excitement is about – Awschalom provides a fantastic introduction to the field and explains how electron spin could be harnessed to create very dense computer memories and even quantum computers.

Awschalom also outlines the challenges that must be overcome before we see the next generation of spintronics devices and explains how he is addressing some of these in his lab.

Nobel laureate looks to the future

The second interview is with Albert Fert of the Université Paris-Sud, Orsay, who shared the 2007 physics prize for his discovery of giant magnetoresistance. Not content to rest on his laurels, Fert spoke about his current research projects, which include the development of spintronics-based tuneable microwave sources that could someday be exploited in mobile phones and other consumer electronics.

If you store lots of multimedia on your computer, Fert is one person you should thank. He also told me how his research in pure and applied physics was picked up and commercialized by IBM into highly sensitive read heads for hard drives.

Vox pops

Last, but by no means least, we made a “vox pops” video of short interviews with a wide range of people at the meeting.

For example, spintronics guru and meeting organizer Stuart Parkin of IBM Almaden describes how a spintronics racetrack memory works; Ian Appelbaum of the University of Maryland explains why humble silicon could be the material of choice for future spintronics circuits; and Theo Rasing of Radboud University in the Netherlands talks about his lab’s recent successes in flipping spins very quickly using laser light.

And if you are considering a career in spintronics, you can hear several PhD students explain why they find the field so exciting.

Nobel laureate looks to the future of spintronics

What do you do after winning the Nobel Prize for Physics for pioneering a technology that has touched billions of lives? You might take it easy or, if you are Albert Fert, you would try to come up with a completely new spin on that technology – which could find its way into billions of mobile phones and other communications devices.

Fert shared the 2007 prize for the discovery of giant magnetoresistance – if you store lots of multimedia on your computer, he is the one person you should thank. In this interview, he tells us what he is up to in his lab at Université Paris-Sud, Orsay and also how his prize-winning research was picked up and commercialized by IBM into highly sensitive read heads for hard drives.

Vox pops: the spin on electronics

The physicsworld.com camera crew was at the Royal Society in London for a discussion meeting on spintronics. We asked a wide range of attendees what excites them most about electron spin.

In the video, spintronics guru and meeting organizer Stuart Parkin of IBM Almaden describes how a racetrack memory works; Ian Appelbaum of the University of Maryland explains why humble silicon could be the material of choice for future spintronics circuits; and Theo Rasing of Radboud University in the Netherlands talks about his lab’s recent successes in flipping spins very quickly using laser light.

They’re just a few of the experts in the video – and if you are considering a career in spintronics, you can hear several PhD students explain why they find the field so exciting.

Spin doctors caught on film

By Hamish Johnston

Three new video interviews with top physicists are now available on our multimedia page. This month’s theme is spintronics and I had the pleasure of speaking with two leaders in that field — Albert Fert and David Awschalom — at the Royal Society’s recent “The Spin on Electronics!” discussion meeting. We’ve also produced a selection of video “vox pops” with physicists at the meeting.

If you are new to spintronics — or if you are wondering what all the excitement is about — David Awschalom of the University of California, Santa Barbara provides a fantastic introduction to the field and explains how electron spin could be harnessed to create extremely dense computer memories and perhaps even quantum computers.

Awschalom also outlines the challenges that must be overcome before we see the next generation of spintronics devices and explains how he is addressing some of these in his lab.

Albert Fert of Université Paris-Sud, Orsay shared the 2007 Nobel Prize for Physics for his discovery of giant magnetoresistance and is not content to rest on his laurels. He tells me about his current research projects, which include the development of spintronics-based tuneable microwave sources that could someday be exploited in mobile phones and other consumer electronics.

If you store lots of multimedia on your computer, Fert is one person you should thank. He also explains how his research in pure and applied physics was commercialized by IBM to create highly sensitive read heads for hard drives.

Vox pops

And last, but by no means least, we’ve made a “vox pop” video of short interviews with a wide range of people at the meeting.

Finally, a plug for the Royal Society, which kindly allowed us to film in its fantastic London premises.

If you are in easy reach of London, I would keep an eye on the Royal Society’s series of Discussion Meetings. I have been to two so far – the first was on the cross fertilization between cosmology and condensed matter physics, and the most recent on spintronics. Both meetings included talks by top physicists from around the world — and best of all, anyone can attend for free (but you must register online ahead of time).

The next physics-related meeting looks like a real humdinger: The detection of extra-terrestrial life and the consequences for science and society on 25-25 January 2010. Confirmed speakers and chairs include Lord Martin Rees, Catherine Cesarsky, Paul Davies and Colin Pillinger.

Hmm, I might go to that one myself!

‘Mini lasers’ illuminate dark molecules

A new microscopy technique that turns molecules into “mini lasers” has been developed by researchers in the US. The new method could help scientists to study biological samples containing “dark molecules”, which are invisible to today’s advanced fluorescence microscopes.

Fluorescence microscopy is the technique of choice for obtaining high-resolution images of biological samples. It works by tagging molecules in the sample with fluorophores – molecules that emit light shortly after being illuminated with light of a shorter wavelength. However, some important biological molecules such as haemoglobin cannot be tagged in this way, rendering them invisible to such microcopes.

The new method is called stimulated emission microscopy and was developed at Harvard University by Wei Min, Sijia Lu, Sunney Xie and colleagues. It is a classic “pump-then-probe” measurement that involves firing two different laser pulses at the sample. Each pulse is about 200 femtoseconds long and the two are separated by less than a picosecond.

Energy from the first (pump) pulse is absorbed by a molecule of interest, placing it in an excited energy state. The energy of the photons in the second (probe) pulse is set at precisely the difference between the molecule’s excited and ground states. This stimulates emission of photons from the excited molecules, which boosts the amplitude of the probe pulse by a factor of 1+10^–4 to 1+10^–8.

Mini lasers

Xie told physicsworld.com that this is the same process involved in the production of laser light – in effect the molecules are acting as mini lasers.

To extract this tiny signal, which is much smaller than noise in the probe laser, the team switched the train of pump pulses on and off at about 5 MHz and used a lock-in amplifier to eliminate low-frequency noise. An image can be built up in a matter of minutes by scanning the pulses across the sample and repeating the measurement.

By adjusting the energies of the pump and probe lasers, the Harvard group were able to image a number of biological samples containing hitherto dark molecules. They could, for example, see individual red blood cells in a sample of mouse tissue as well as measure the distribution of a certain drug in a similar sample.

Watching quantum dots

In addition to biological samples, Xie believes that the technique could prove useful for characterizing a wide range of organic and inorganic materials with so-called “dark states” – including quantum dots.

Xie said that he hopes to “test the interest of microscope manufacturers” with regards to the commercialization of the technique.

The research is reported in Nature and, in a commentary in the same issue of the journal, Stefan Hell and Eva Rittweger of the Max Planck Institute for Biophysical Chemistry in Heidelberg describe the work as “a bold step towards unveiling details of live cells and tissues that would otherwise be left uncharted”.

String theorist takes over from Hawking

String theorist Michael Green has been announced as the next Lucasian Professor of Mathematics at Cambridge University. Green, 63, will succeed Stephen Hawking who held the chair from 1980 before retiring last month at the age of 67 and taking up a distinguished research chair at the Perimeter Institute for Theoretical Physics in Canada.

Green is currently the John Humphrey Plummer Professor of Theoretical Physics at Cambridge, a position he had held since 1993. Green obtained his BA and PhD from Cambridge and later spent time as a postdoc at Princeton, Cambridge and the University of Oxford. In 1978 he joined Queen Mary, University of London before heading back to Cambridge in 1993.

The Lucasian chair was created in 1663 as a result of a gift from the then Member of Parliament for the university, Henry Lucas. Green is the 18th person to hold the chair and follows a long line of influential figures including Isaac Newton, who held the post from 1669 to 1702, and Paul Dirac, who was Lucasian professor form 1932 to 1969. Hawking stepped down from the position last month because the post must be vacated when the holder is 67, according to Cambridge rules.

Superstring revolution

Green, a fellow of the Royal Society, is regarded as one of the founding fathers of string theory. In 1984, in what is deemed the first superstring revolution, he developed the Green–Schwarz mechanism – a major discovery that led to the realization that string theory may be able to describe all the elementary particles and the interactions between them – together with John Schwartz from the California Institute of Technology. String theory became the first theory in physics to predict the number of space–time dimensions and in 1984 went from being a fringe activity to mainstream theoretical physics

Green has been awarded the Dirac and Maxwell Medals of the Institute of Physics, UK, and the Dirac Medal from the International Centre for Theoretical Physics in Trieste, Italy.

Green takes up the position of Lucasian professor on 1 November.

“Professor Green is certainly a worthy successor to Hawking,” says Robert Bruen, who maintains the website lucasianchair.org and has studied the history of the professorship. “[This is] a great day and well deserved.”

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