In less than 100 seconds, Simon Buckle shares his thoughts on a question that can rouse strong feelings.
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Kepler – it’s not all doom and gloom just yet
By Tushna Commissariat
To much general dismay, earlier this month NASA officials announced that their Kepler space telescope had gone into a self-imposed “safe mode”, something that the telescope is programmed to do if one of its primary systems is not fully functional. Although the telescope was then rebooted, it shut down again this week and it seems that all is definitely not well with our favourite exoplanet spotter: the mission collaboration announced that the instrument has suffered a critical failure and may never be fully operational again.
‘Ghostly’ 3D images taken without a camera
A simplified 3D imaging system that does not require a conventional camera has been developed by researchers in the UK. The computational imaging technique uses information from single-pixel detectors to create an image, can be used over a range of wavelengths and is cheaper than other 3D methods. The researchers claim that, in addition to taking images, their system could be used as a detector in oil and gas exploration as well as in medical and biological imaging systems.
Most imaging systems – from cameras to the retinas in our eyes – capture images in 2D and then process the information to create a 3D image. While plenty of 3D imaging techniques exist – including stereoscopic, holographic and volumetric imaging – they are expensive and require bulky and specialist equipment, such as lenses and lasers. And despite all this advanced technology, the techniques only work for light at specific wavelengths.
Illuminated imaging
Baoqing Sun, Miles Padgett and others from the University of Glasgow, along with other UK-based colleagues at Cambridge University, set out to create a simple system that can deliver 3D imaging without a camera or any other lenses. Sun told physicsworld.com that the team’s new technique involves using nothing more than a light projector, four single-pixel detectors and a computational imaging technique known as “ghost imaging.”
Computational ghost imaging creates images using “intelligent illumination”. The object to be imaged is lit with a specific, known light pattern (such as a speckle pattern) and the reflected light is detected by a single-pixel photodetector. This device has no spatial resolution and merely collects all the light that is incident on it. The multiple signals of the varying light intensity from the detector are then processed, along with the knowledge of the lighting pattern, to build up the final image. Until now, however, this technique has only been used to build 2D images.
Chequered patterns
In this latest work, the team used a simple light projector to illuminate a polystyrene model of a human head with computer-generated random binary speckle patterns. The light reflected from the head was collected by four single-pixel detectors, which are placed at different angles.
Thanks to the random binary speckle patterns – which illuminate the object with a “chequered” pattern – and the varying angles of the detectors, the team was able to see a clear shading profile in the images.
“For each detector, we saw a 2D image that appeared to be illuminated from a different direction, even though we used only one projector to illuminate the head,” says Sun. The individual images were reconstructed using an “iterative algorithm” that the team created. From the shading of the images, the researchers measured the surface reflectivity and the varying depth of surface features could be derived. The 3D head was reconstructed by integrating more than a million iterations of the images. Sun also explains that different speckle patterns are projected one after another and that the greater the number of patterns used, the shaper and more highly resolved the final image.
Across the range
In terms of adapting the new technology to real-world applications, Sun points out that all of the experiments to date have been carried out in a lab. “So if this was to be used outside in bright sunlight, or any such environment, we will have to account for the high noise levels, and this is something we are working on,” he says. But at the same time, he explains that the system works well as a cheap “3D camera” and can be used across a wider wavelength range – from ultraviolet to infrared – compared with normal cameras. This could allow it to be used as a detector in oil and gas exploration, where infrared remote-sensing technologies are used to “see” oil reserves.
The team is also keen to collaborate with other researchers to extend the 3D computational ghost imaging to the terahertz scale, so that it could be used to carry out medical as well as other biological imaging, and this is something the researchers are currently in the process of doing.
The research is published in Science.
What is M-theory?
In less than 100 seconds, Leron Borsten explains how M-theory has the potential to unify the various forms of string theory with the theory of supergravity.
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BBC radio celebrates 101 years of cosmic rays
By Hamish Johnston
The BBC’s Melvyn Bragg has lots to talk about. Over the past few months he has chatted about the Icelandic sagas, water, Gnosticism, and much more on his Radio 4 programme In Our Time. So he can be forgiven for missing a centenary and celebrating cosmic rays 101 years after they were discovered by the Austrian physicist Victor Hess.
An afternoon of quantum theory
By Louise Mayor
Yesterday I had an exciting trip out of the office.
Earlier this week, one of Physics World’s freelance writers, Jon Cartwright, told how me he’d been invited to the Bristol University theory department’s weekly seminar. Felix Flicker, a 2nd-year PhD student who organizes these events, had seen Jon’s article “The life of psi” in this month’s Physics World, which discusses a theorem published in Nature Physics. The theorem is interesting because if its assumptions hold, it rules out one of the four interpretations of quantum mechanics and leaves us with three.
I wanted in on the seminar action!
Last year when I was planning the Physics World special issue on quantum frontiers (which was out in March and is still available as a free PDF download), I had approached Jon to ask whether he’d like to tackle a quantum topic, and he let me know he was interested in covering the paper by Matthew Pusey, Jonathan Barrett and Terry Rudolph. Jon had seen the story reported elsewhere but had found these accounts were light on the details and didn’t get to the bottom of the science. I liked the idea and Jon went ahead. Once the story arrived in my inbox I was hooked! I found it to be one of those stories that covers some tricky concepts but if you let yourself become immersed in the story and think through what’s being explained, is very rewarding.
Do you try to pronounce physics terms as they sound in their language of origin?
By Hamish Johnston
Like many disciplines, physics incorporates words from a number of different languages – and this can often leave a physicist tongue-tied.
How should a native English speaker pronounce Einstein, for example? Should it be the Germanic “Ein-shtein” or the anglicized “Ein-stein”? How should one say De Broglie, Raman or Bernoulli? Should a native English speaker even attempt zitterbewegung, or translate it to “trembling motion”?
I’m sure that some physics terms of English origin are tricky for native speakers of other languages, and their pronunciations are sometimes adjusted accordingly.
Some believe that making an effort to use the original pronunciation shows respect and knowledge of the origin of a word. Others are happy to use the pronunciation they are most comfortable with.
Bose–Einstein condensate is in the can
Calculating the properties of a quantum particle in a box is something most physics students have to do as part of their degree course – but actually creating such a simple system in the lab can be an experimental challenge. Now, however, physicists in the UK are the first to create a Bose–Einstein condensate (BEC) in a 3D optical-box trap, which resembles a tin can. The breakthrough could allow physicists to study a range of multi-body physics phenomena in controlled conditions.
The first BEC was made in 1995 in Nobel-prize-winning work that involved cooling a cloud of rubidium-87 atoms down to temperatures of near absolute zero. The atoms settle into a quantum state that extends over a macroscopic volume, which means that the BEC behaves like a superfluid. In addition to being fascinating in their own right as a new state of matter, BECs are interesting because they are created under very controlled conditions, which allows them to be manipulated to resemble a variety of quantum phenomena.
Earlier this year, for example, physicists carried out an experiment in which a BEC behaved much like a Josephson junction – a device that is normally make from a superconductor. As such, BECs can be used as “quantum simulators” to gain a better understanding of less-accessible quantum systems, ranging from magnets to superconductors and neutron stars. Unfortunately, physicists have so far only been able to create BECs in traps where the trapping potential – and so the atomic density – varies harmonically, which is no good for anyone simulating, say, electrons in solids, as these systems tend to have homogenous particle densities.
Lids on a tin can
Now, however, Zoran Hadzibabic and colleagues at the University of Cambridge are the first to create a BEC in a 3D trap that – for most practical purposes – has a constant potential in all three directions. “Our trap is an optical box made out of green light: a dark region of empty space is surrounded by thin walls of light that repel the atoms and keep them confined inside the box,” says Hadzibabic.
His team created the box by imprinting a phase pattern onto a conventional laser beam. The result is a hollow tube of green laser light and two “sheet-shaped” laser beams lying perpendicular to the tube. “The beams form the end lids that close up our optical tin can,” says Hadzibabic. The effects of gravity, which would otherwise distort the box, were eliminated by suspending the atoms using a magnetic field.
Before creating the BEC, Hadzibabic’s team had to cool down a cloud of rubidium-87 atoms to nanokelvin temperatures. This involved first gradually lowering the harmonic trapping potential so that faster-moving hot atoms escape the trap, leaving only cooler atoms behind. As it undergoes this process of “evaporative cooling”, the cloud shrinks until it is small enough to fit inside the optical box. The box is then switched on and the harmonic trap is turned off slowly.
The next step involves subjecting the atoms to a final round of evaporative cooling to get the gas to a temperature of below about 90 nK, where it turns into a BEC. This was done by adjusting the intensity of the laser light that creates the walls of the trap. Faster-moving hot atoms were able to penetrate the walls and exit the trap, while the cooler atoms cannot – causing evaporative cooling.
Einstein’s right again
To confirm that they actually had a BEC in a uniform potential, the researchers turned off the box trap and let the gas expand freely while measuring the velocity distribution of the atoms. A large peak at very low velocity confirmed that a BEC had formed and the shape of the peak contained information about the shape of the box trap. The velocity distribution also revealed the temperature below which the atoms condensed into a BEC. This temperature was first predicted by Einstein in 1925 and Hadzibabic says their analysis is the best experimental confirmation so far.
While the box trap is a good approximation to a constant potential in 3D, it is not perfect, although Hadzibabic argues that it is good enough for most applications. “In our trap,” he says, “we can estimate that more than 80% of the atoms live within the region where the density deviates by less than 10% from the average value, so these atoms should heavily dominate all experimental signals.” In contrast, less than 20% of the atoms in a conventional harmonic trap lie in a region that is representative of the average density.
Focus on phase transitions
Now that they have created a near-homogenous BEC, the researhcers are keen to use it to simulate a range of quantum systems. In particular, the set-up should be good for studying how a system makes the phase transition from a cold gas to a BEC. The team’s first target is to study the effects of inter-particle interactions on Bose–Einstein condensation of a homogeneous gas or fluid. This problem was first proposed in 1957 by Chen Ning Yang and Tsung-Dao Lee – Chinese-American physicists who also won the Nobel prize that year for unrelated work on particle physics.
“This problem has been studied in liquid helium, but many questions remain open and the agreement between theory and experiment has not been reached,” explains Hadzibabic. The team is also looking at doing other experiments in which the interactions between atoms can be fine-tuned. This will involve modifying the experiment to use potassium-39, which is more difficult to trap but better for creating tuneable interactions.
The results are described in Physical Review Letters and a preprint is available on arXiv.
Google and NASA acquire a D-Wave quantum computer
By Hamish Johnston
Canada’s D-Wave Systems is installing one of its quantum computers at NASA’s Ames Research Center in California. The new 512-qubit system – dubbed D-Wave Two – will be used by NASA, Google and the Universities Space Research Association (USRA) to investigate how quantum computers could be used to solve a range of different problems. According to Vancouver-based D-Wave, the computer will be available for use in the third quarter of this year.
Consciousness from the ground up
The book Physics in Mind: a Quantum View of the Brain certainly aims high. Written by the eminent biophysicist Werner Loewenstein, its goal is nothing less than a theory that explains our sense of conscious existence, built from the bottom up. Remarkably, Loewenstein’s arrows of explanation hit their target almost all the time, even though the promise implied in the book’s subtitle remains tantalizingly just out of range.
This book is a fantastic journey for any reader, but especially for a physicist. In Loewenstein’s account, life is a delicate dance between the bits of information and quantized chunks of energy that drive all biological processes. Accordingly, he takes us on an intellectual rollercoaster ride through the microscopic world of signalling molecules, autocatalytic sets, DNA, RNA, natural selection and the electromechanics of cell membranes – before culminating in an account of quantum computing and the role of quantum mechanics in the brain.
The book begins with the relationship between the human sense of time and the abstract concept of time in physics. An exposition of the various “arrows of time” – perceptual, thermodynamic and cosmological – leads up to a discussion of the basic mechanisms of molecular signalling in living systems. Loewenstein takes as his central model Maxwell’s famous demon (see April pp36–39), which gets information about the microscopic motions of molecules and uses that information to reduce entropy. The author encourages the reader to think of all molecular information processing in demonic terms, since a wealth of biological processes – the function of ion channels, photosynthesis, the detection of light in the eye, the absorption of scents by receptors in the nose and many more – all operate via the interplay between information and energy at the microscopic scale.
Loewenstein’s choice to describe biological information processing in terms of “demons” that deal in information and energy is a wise one. By providing a central metaphor for the microscopic mechanisms of biological signalling, he supplies a unified picture of how the brain gathers to it the material of sensation. Our eyes and nose, for example, offer us very different senses of our surroundings; but when one describes them in terms of the dance of information and entropy, it becomes clear that they function in very similar ways. A single photon passing through the lens of the eye excites a molecule of rhodopsin, pushing open an ion channel that excites a neural signal. A single molecule locks into an odour receptor in the nose, allowing charge to flow in a way that again excites a neural signal. The steps in each dance are different, but the results are the same: a slight difference in our surroundings is amplified into a perceived difference in our brain.
At this point, the book transits from the solid ground of molecular biology to the more speculative territory of human perception. Here, too, Loewenstein exhibits sure hands. He admits that we don’t know just how conscious awareness works, but gives hints and clues to the evolutionary origins of perception, and to the way in which the brain integrates the products of the sensory apparatus. He draws on the recent comprehensive investigations of how neural circuitry functions to recognize patterns and extract correlations from sensory data. He speculates on the beginning of self awareness and on the role of perception of space and time. His arguments on language, thought and computation are fascinating, and almost persuade the reader that the age-old problems of human consciousness might be solvable.
The dazzling diversity of topics and the rapid pace of the brilliant exposition are overwhelming: at the end of every chapter, the reader has to stop to catch breath. In general, the author’s skill in unifying the array of molecular mechanisms under the single theme of information processing pulls us through. However, by the end of the book, so much knowledge has been imparted that our brains would indeed need to be quantum computers to assimilate it all.
Yet ironically for a book whose subtitle is “a quantum view of the brain,” it is just at the application of quantum mechanics to neural processes that the book stumbles. The author provides a spirited exposition of quantum mechanics and quantum computing, but when it comes to applying quantum theory to the brain itself, Loewenstein hems and haws. While he speculates on the possible role of quantum weirdness such as quantum superposition and entanglement in consciousness, in the end he is too good a scientist to be taken in by claims that wavefunction collapse plays a role in mental processes. Indeed, in the final chapter he admits that quantum mechanics doesn’t seem to play much of a role in the brain at all. It’s a relief not to be subjected to nonsensical claims of entangled neurons, but it does reveal the book’s subtitle to be nothing more than a tease.
Loewenstein is an engaging writer, one who spices his prose with elaborate wordplay, assonance, internal rhymes, puns, metaphors and quotations. All those verbal high jinks go to good use, put into the noble service of communicating hard stuff in a comprehensible fashion, but it does make for a book that could do with a few more monosyllables. Still, in the final analysis, this is a ripping good read. Each chapter brings novel insights into the fundamental workings of life. Those who buy their ticket and take the ride will emerge breathless, but enlightened.
- 2013 Basic Books £19.99/$28.99hb
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