Page 1346 – Physics World

Home » Page 1346

Do you think the Large Hadron Collider will discover new physics beyond the Standard Model?

By James Dacey

You can be taken on a fascinating journey through space–time if you read this month’s issue of Physics World. It contains a feature by the theoretical physicists Henrik Melbéus and Tommy Ohlsson that describes how particle-physics experiments such as the Large Hadron Collider (LHC) are being used in the hunt for extra dimensions. Melbéus and Ohlsson trace the history of theories of extra dimensions, which are known collectively as KK theories after the physicists Theodor Kaluza and Oskar Klein who first proposed the idea in the early 20th century.

KK theories represent science that goes beyond the Standard Model of particle physics. The Standard Model has been incredibly useful, but as Melbéus and Ohlsson point out, it does have a few shortcomings. For instance it cannot be used to explain dark-matter particles, which have been predicted to exist in order to explain the observations of how galaxies move under the influence of gravity. Some of the KK theories predict the existence of particles that could prove to be these elusive dark-matter particles. In their feature, Melbéus and Ohlsson describe how particle collisions at high-energy accelerators could lead to the creation of KK particles.

So far, however, the LHC has not revealed any signs of these particles, but this “no show” has helped theoretical physicists to constrain the scale of these extra dimensions – if they do indeed exist. Over the next few years, physicists will continue to analyse the abundance of data from LHC collisions in the search for KK particles. They will also be looking for other signs of physics beyond the Standard Model such as the “sparticles” predicted by supersymmetry theories.

In this week’s Facebook poll we want to know what you think will come of this hunt. Please let us know by answering the following question.

Do you think that the Large Hadron Collider will discover new physics beyond the Standard Model?

Yes
No

Have your say by visiting our Facebook page, and please feel free to explain your response by posting a comment below the poll.

Last week’s poll was the concluding part of a three-week series dedicated to careers in physics. We asked you to select the action that you think would be most helpful to benefit the career prospects of physics postdocs. Out of a choice of five options the most popular by a large margin was “Longer-term contracts (e.g. three years rather than one)”, which attracted 73% of responses.

The second most-popular option with 11% of votes was “More training in transferable skills”. In joint third place, with 6% each, were “Better advice on career options outside academia” and “Creating more mid-level ‘permanent postdoc’ jobs”. In last place was “Improved support for postdocs with spouses and families”, collecting just 4% of responses.

Thank you for all your responses and we hope to hear from you again in this week’s poll.

Complex quasicrystals created using new nanofabrication technique

Researchers in the US have invented a new nanofabrication technique that can generate 2D patterns with very high rotational symmetries over large areas. Until now, only spatially repeating structures – which have sixfold or less rotational symmetry – could be patterned over such large areas using industrial photolithography techniques.

Dubbed moiré nanolithography, the technique can produced patterns with rotational symmetries as high as 36-fold – something that has never been observed in nature. Such high rotational symmetries could prove useful for a huge range of applications, from making better photonic crystals to boosting the performance of photovoltaic devices.

Until the 1980s most researchers thought that long-range order in physical systems was impossible without spatial periodicity. They believed that atoms were packed inside crystals in symmetrical patterns that were periodically repeated over and over again, and that this repetition was necessary to obtain a crystal.

Quasicrystals break the mould

In 1984 Daniel Shechtman of the Technion-Israel Institute of Technology discovered quasicrystals – materials that have ordered but not periodic structures. Shechtman made his discovery while studying samples of an aluminium–manganese alloy and found that the atoms in these crystals were packed in an icosahedral pattern that could not be repeated but which had “10-fold” rotational symmetry.

A system is said to possess n-fold rotational symmetry if it looks the same after it has been rotated through 360/n degrees. A sample with 10-fold rotational symmetry therefore remains unchanged after being rotated through 360/10 = 36 degrees. Before Shechtman’s discovery, a 2D ordered system was only supposed to have either one-, two-, three-, four- or sixfold rotational symmetry, with anything else being forbidden by the laws of crystallography.

Since 1984, scientists have discovered hundreds of different quasicrystals – some of which have properties of technological interest – and Shechtman bagged the 2011 Nobel Prize in Chemistry for his efforts.

36-fold rotational symmetry

Now, a team led by Teri Odom at Northwestern University has created 2D quasicrystal nanostructures with a staggering 36-fold rotational symmetry using a new moiré nanolithography technique. Moiré patterns have been known for a long time and can be seen in the everyday world by placing two pieces of fine mesh one on top of the other and then rotating them to create new, more complicated patterns. As you keep on turning, the patterns change as in a kaleidoscope.

Moiré nanolithography relies on the interference of two repeating patterns overlapped at a specific angle, explains Odom. 2D periodic patterns can be routinely fabricated by photolithography over large areas, but these arrangements have sixfold rotational symmetry at most – like a hexagonal lattice. Other high-rotational-symmetry quasicrystals have become popular in recent years (the most well known and highest being the 12-fold symmetry ones), but these must be patterned through serial lithography methods, such as focused-ion-beam milling and electron-beam lithography, which are time-consuming and expensive.

Moiré exposures

“We succeeded in making nanopatterns with rotational symmetries higher than any quasicrystals previously reported by performing two or more exposures through patterned poly(dimethylsiloxane) (PDMS) elastomeric masks,” Odom explains. “Because we first make the patterns in a photoresist, we can then transfer the moiré pattern onto a wide range of materials, from silicon to metals. We can then fabricate omnidirectional reflectors or electrodes, for example, using these structures fairly easily.”

Photograph of a wafer-scale moir&eacute; pattern

Quasicrystal on a wafer

One area in which such high-symmetry moiré nanopatterns might have an impact is in photovoltaics, Odom adds. Thanks to their high rotational symmetry, these patterns can trap light with nearly the same efficiency at all angles. This could come in handy for making solar panels that would not need sophisticated trackers to follow the position of the Sun during the day, for example.

And that is not all, because the distances between the features in these high-rotational-symmetry lattices are on the length scale of the wavelength of visible light – about 500 nm – the patterns have the potential to manipulate the flow of light in new and exciting ways. “For instance, we are currently transferring these patterns onto metallic substrates that can trap, concentrate and slow down light via so-called surface plasmon waves,” says Odom. “We are also looking at how nanohole arrays patterned with this moiré technique in metallic sheets can selectively transmit light at specific energies in the optical regime.”

The research is reported in Nano Letters.

Crackpots and consequences

The letter arrived in Physics World‘s in-tray last summer. Written in blue ink, with occasional recourse to red for especially important points, it claimed to predict the date of “The Second Coming of the Lord Jesus Christ” using images from the Hubble Space Telescope. The specified date came and went without incident, and we were somewhat surprised to receive another letter from the same source. This one was labelled “The Second Coming of the Lord Jesus Christ (Correction)”, and it posited a new, later date for Jesus’s arrival. The revised date was calculated using the rotation period of the planet Venus, plus a simple mathematical identity that, according to our correspondent, “angels know and use”. Two subsequent letters contained further revisions, and the current predicted date is 22 September 2012. But in the letter-writer’s words, “only GOD knows whether the new presented date is the correct one”, so mere mortals like us will just have to wait and see.

Nearly all professional physicists receive at least one letter like this a year. In most cases, such letters go straight into the recycling bin, although a handful of the best – including Physics World‘s “Lord Jesus Christ” correspondence – end up in semi-permanent storage. But these ad-hoc attempts at crackpot curation pale in comparison with those of the science writer Margaret Wertheim. For the past 15 years, Wertheim, a physics graduate, has assiduously collected all the letters, books, glossy brochures and poems she receives from people peddling their own theories of the universe. After accumulating two long shelves full of such material, and investigating a few of the authors, she has distilled her experience into a fascinating and occasionally frustrating book called Physics on the Fringe: Smoke Rings, Circlons, and Alternative Theories of Everything.

Wertheim’s book is fascinating in large part because she has boldly gone where few trained scientists have gone before. Rather than ignoring the people she calls “outsider physicists”, she has engaged with their ideas, attended annual conferences organized by their professional body (yes, they have one) and even driven 1000+ miles to visit one of them. All of this qualifies her to draw conclusions about who outsiders are and what makes them tick, but for the most part, she avoids sweeping generalizations. The truth, she writes, is that outsider theorists are a varied bunch, and as for their ideas, “there is very little that unites the disparate range of theories in my collection except for the sense that mainstream physics is badly off course”. They also display what Wertheim calls “a startling inattention to copy-editing”.

A few common threads do emerge. For example, Wertheim observes that the men (and they are almost all men) who create alternative theories of physics frequently have some kind of technical background, such as engineering. Despite this, they reject mathematics as a means of understanding the world, and rely on words rather than equations to explain their ideas. They write in a kind of scientific pidgin, they describe their work as “revolutionary” – and, of course, they view the establishment’s failure to acknowledge their brilliance with dismay and befuddlement.

In the middle part of her book, Wertheim turns away from studying outsider physics as a whole, and instead focuses on one particular outsider, whom she dubs “the Leonardo of the field”. For me, this is where the book’s frustrating aspects begin. The object of Wertheim’s journalistic interest and affection is Jim Carter, a successful inventor who lives in a trailer park in rural Washington State. It is not hard to see why she has singled him out. For starters, Carter is obviously sane and, as a theorist, he is clearly operating on a more sophisticated level than Physics World‘s “Lord Jesus Christ” correspondent. Carter’s “circlon” theory of atomic structure is unusually coherent, lacking the non-sequiturs and characteristic rambling style so beloved of outsiders and, like many mainstream physicists, he is motivated by a deep desire to understand how the universe works. To this end, he has spent more than 50 years developing his theories. Remarkably, one of them even makes a prediction that could be tested – albeit only during a space mission.

And therein lies the nub of Wertheim’s argument. In 2010 she attended the annual conference of the outsiders’ professional body, the Natural Philosophy Association, which that year featured 121 outsider physicists presenting 121 different theories of the universe. This might sound bizarre, but in Wertheim’s opinion it was beaten hands down by the string cosmology conference she attended in 2003. At this star-studded event, she writes, participants were “fired up” by the idea that there could be as many as 10⁵⁰⁰ different variants of string theory, each of them totally different – and all utterly unsupported by a shred of evidence. So why are string theorists respected physicists with television programmes and book contracts, while Jim Carter and his brethren are regarded as a bunch of cranks?

One answer is that the string theorists have gone through a lengthy training process. This is rather a straw-man argument, though, and Wertheim is accordingly quick to demolish it. Some fields of endeavour, she reasons, should indeed be restricted to trained and accredited personnel: brain surgery, for example. But others, such as art and lovemaking, are rightly left open to pretty much anyone, regardless of their qualifications or ability. The question, she suggests, is whether theoretical physics is more like brain surgery or more like sex.

In case this argument fails to convince, Wertheim has another. Outsider theorists draw meaning and personal satisfaction from their work, she observes. For this reason, she writes, “Might we not simply enjoy their alternative narrative arcs?”

Physics on the Fringe is a great read, and Wertheim a persuasive writer with a knack for challenging unspoken assumptions. But there are two significant problems with her thesis. The first is that most theoretical physics is not like string theory. Wertheim sort of acknowledges this when she discusses string theory’s critics within the physics community, but I wish she had said more about the role of experimental evidence, since many areas of theoretical physics are supported by truckloads of it. One does not need to be an elitist or a fan of mathematical elegance to believe that quantum mechanics – to take just one example – is “true”, while alternative theories are “false”, in the scientific senses of these words.

The second problem is that most outsider physicists are not like Jim Carter, and unfortunately this is something Wertheim addresses only obliquely. Carter comes across as an intelligent and likeable eccentric, and I wouldn’t mind visiting him, as Wertheim did, in his remote forest idyll. But some outsiders are not so amiable. There is a distinct streak of anger and grievance running through their letters, and while Wertheim claims that mental illness is rare in the alternative-physics community, I am not so sure. In any case, even if the self-regard of outsider theorists falls short of pathology, the sheer chutzpah required to persevere in the face of universal condemnation is not always benign. People who fervently believe their theories are correct sometimes react badly to criticism, and if they have enough money and power, they can do real harm.

That statement might sound like hyperbole, but I know of at least one person who learned, to his cost, just how true it was. I would like to name him, but unfortunately, some cranks have a penchant for making legal threats, and England’s strict libel laws mean it is safer not to. So you will just have to trust me when I say that this particular “alternative narrative arc” was not, in fact, very enjoyable for any of the people who found themselves enmeshed in it.

Delving into extra dimensions

We experience three spatial dimensions in nature: length, width and height. In addition, we perceive time as a fourth dimension. But some theoretical physicists have speculated that “extra” spatial dimensions could exist in addition to the four ordinary space–time dimensions, although they are too small to see with the naked eye. Now, with the advent of new data from the Large Hadron Collider (LHC) at CERN near Geneva in Switzerland, as well as other particle- and astroparticle-physics experiments, it might be possible to answer the fundamental question of whether these extra spatial dimensions exist.

The idea that space–time could have more than four dimensions was first proposed by the German mathematician and physicist Theodor Kaluza and the Swedish theoretical physicist Oskar Klein in the early 20th century. In 1921 Kaluza published an article in which he extended Einstein’s theory of general relativity (which is still the best known description for gravitation) from four to five dimensions, and in 1926 Klein assumed that an additional fourth spatial dimension is curled up into a circle with an extremely small radius – the extra dimension bends round on itself and is said to be “compact”. Indeed, it can be shown that 5D space–time can be separated into Einstein’s theory of gravitation in four dimensions and Maxwell’s theory of electromagnetism. For this reason, a physical model that tries to unify the fundamental forces of gravitation and electromagnetism is known as a Kaluza–Klein theory (KK theory), although nowadays the phrase is used to refer to any theory with extra spatial dimensions.

Trying to imagine extra spatial dimensions is not at all intuitive. How could there be anything other than front-to-back, side-to-side and up-and-down. One analogy to help understand this is to consider a tightrope (figure 1). Seen from a distance, such as from the eyes of a human being balancing on the rope, it might seem possible to move only forwards or backwards. However, on closer inspection, such as from the perspective of an ant, it is possible to move along the rope and also around it. In the same way, extra dimensions could be hidden for someone who is looking at them from a large distance compared with their size.

Left: person balancing on a tightrope. Right: ants crawling around all sides of a rope

1 Ant’s-eye view

As for how extra dimensions could reveal themselves, the experimental study of fundamental particles is the most promising route. The energy of a particle in a 3D space consists of the rest energy of its mass, E = mc², and the kinetic energy of its motion. If extra dimensions exist, the particle will have more freedom to move and so could obtain an additional, independent contribution to its kinetic energy. Since we do not observe the motion of the particle along the extra dimension, we would interpret this kinetic energy as part of its rest energy, or in other words the mass of the particle. To us, the particle would not look like one particle, but a set of particles – all with different masses. The faster the particle moves along the extra dimension, the larger this apparent mass seems to be. It turns out that the mass of each particle is related to the mass of the particle at rest in the extra dimension. We assume that the Standard Model particles are at rest in the extra dimension, and that for each of these known particles there could exist heavier versions that have not yet been discovered. Known as “KK particles”, we can arrange each set of them in a schematic “KK tower” with KK (particle) numbers n = 1, 2, 3… and masses m_n = √(m² + n²/R²), where m is the mass of the Standard Model particle (n = 0) and R is the characteristic size of the extra dimension. For example, one can imagine an electron, which is a Standard Model particle, and a corresponding KK tower of heavier KK electrons.

KK theories are interesting for several reasons. Perhaps most importantly, they can be used to address several of the shortcomings of the Standard Model of particle physics, which is currently our best description of the subatomic world. Although the Standard Model is very successful, it does have a number of problems that point towards the need for it to be extended. KK theories are one possible such extension; others include, for example, supersymmetry, which predicts that for each of the Standard Model particles there exists a heavier “sparticle” sibling, and grand unified theories, in which the strong, weak and electromagnetic interactions are regarded as different aspects of a single force.

Standard Model shortcomings

Among the most famous problems with the Standard Model is that observations of how galaxies move give us a large amount of evidence for there being more matter in the universe than we can see. This additional matter has been termed “dark matter”, although “invisible matter” would perhaps be a better term, since it is not visible (not “luminous”) as ordinary matter is.

The current most plausible solution to this problem is that dark matter is made up of particles that interact very weakly with light. From the particle physics point of view, however the problem with this solution is that none of the known particles in the Standard Model could make up the dark matter. But in some KK theories, it turns out that some of the KK particles that are predicted to exist could be this elusive dark matter as they would not interact with light and would have other characteristics that we expect of dark matter.

Another peculiarity in particle physics is the fact that gravity is much weaker than all of the other fundamental forces. This force is nevertheless important because, unlike the strong and weak forces, it has an infinite range, and most macroscopic objects are electrically neutral and so are not strongly affected by the electromagnetic force, leaving gravity as the only important force for them. From the point of view of quantum physics, the weakness of gravity is a puzzling fact that would seem to require a very precise fine-tuning of the parameters in nature. But in KK theories, the weakness of gravity might be fundamental.

A third problem relates to the properties of neutrinos in the Standard Model. Being electrically neutral and interacting very weakly with other matter, neutrinos are very difficult to detect. In fact, the Standard Model says that all neutrinos are massless. However, observations of neutrinos turning from one form into another – a phenomenon known as “neutrino oscillation” – strongly indicate that neutrinos do have mass. We would therefore have to extend the Standard Model to take this fact into account, but the problem is that neutrinos are much lighter than all other known (Standard Model) particles. In KK theories, natural mechanisms that generate these small neutrino masses can be obtained.

Universal pictures

The bottom line is that to have avoided detection thus far, any extra dimensions would have to be compact and small. Indeed, one might naively imagine that extra dimensions must be so small that we will never be able to observe them, at least in the foreseeable future. However, in recent years different models have been proposed that avoid such stringent constraints, allowing models of extra dimensions to be tested in high-energy physics experiments.

There are three main models that describe extra spatial dimensions. In the scenario of universal extra dimensions (UED model), which was proposed in 2001 by the theoretical physicists Thomas Appelquist, Hsin-Chia Cheng and Bogdan Dobrescu (Phys. Rev. D 64 035002), all of the particles of the Standard Model are allowed to propagate in the extra dimensions, with each fundamental particle having its own KK tower. In this model, there is an extra number (comparable to a particle’s mass or charge) attached to each particle known as “KK parity”, which is assumed to be conserved in particle reactions, meaning that the product of all particles’ KK parities before a reaction should be equal to the product of all remaining particles’ KK parities after the reaction. The KK parity +1 is assigned to all even-numbered KK particles, which includes all Standard Model particles as they have a KK number of 0, whereas odd-numbered KK particles have a KK parity of –1. The consequence of conservation of KK parity is that odd-numbered KK particles can only be produced in pairs, which has a low probability of happening despite being the only allowed option. Furthermore, a second-order KK particle (with parity +1) can decay into two Standard Model particles – because (+1) = (+1)(+1) – but a first-order KK particle (with parity –1) cannot decay into any number of Standard Model particles because –1 never equals (+1)ⁿ.

The UED model would give rise to observable effects at the LHC such as the production of two Standard Model particles or more complicated signals, as would the ADD model described below. Another important implication of KK parity is that it ensures that the so-called lightest KK particle (LKP) is stable and hence could be a possible dark-matter candidate. This kind of dark matter is known as Kaluza–Klein dark matter.

Going extra large

The second significant model describes “large extra dimensions”, which were first proposed in 1998 by Nima Arkani-Hamed, Savas Dimopoulos and Gia Dvali, in what is known as the ADD model (Phys. Lett. B 429 263). The novel feature of this model is the assumption that the Standard Model particles are confined to a so-called brane, which is identified with ordinary 4D space–time, but residing in a larger space–time (figure 2). An analogy is the surface of the Earth being a 2D layer that resides in a larger 3D world. In this case, the surface of the Earth would be the brane.

Schematic figure illustrating an extra spatial dimension

2 On the brane

Since Standard Model particles only “live” in the usual dimensions, not in the extra dimensions, they will not help us in setting limits on the size of extra dimensions. On the other hand, the particle responsible for gravity, the graviton, is not a Standard Model particle and is therefore allowed to propagate in the extra dimensions. In principle, the assumption that gravity lives in a larger space–time leads to deviations from Newton’s gravitational law at short distances. However, because of its weakness relative to the other fundamental forces, gravity has only been tested down to distances of the order of microns, and thus the experimental constraints are still quite weak.

Warped thinking

In addition to the ADD model, the theoretical physicists Lisa Randall and Raman Sundrum proposed a similar model in 1999 (Phys. Rev. Lett. 83 4690). This is normally called the Randall–Sundrum (RS) model but is sometimes called the 5D warped geometry theory, and it assumes that the real world is a higher-dimensional universe described by a warped geometry instead of a flat geometry as in the case of the ADD model. (A warped geometry is one in which space is curved, as in Einstein’s general relativity.)

In the ADD and RS models, the weakness of gravity compared with the other fundamental forces is related to the geometry of space–time. Simply speaking, gravity is spread out in a larger space–time than the other forces, which act only in the 4D brane. In fact, this means that all of the four forces could have similar strengths and gravity only appears weaker as a result of this geometric dilution.

One final framework that implies extra dimensions is string theory, though this is very different from and more hypothetical than typical KK theories. This mathematical approach attempts to describe particles as vibrations of tiny strings and predicts that the number of space–time dimensions has to be 10 or 11, depending on the exact formulation of the theory.

The hunt is on

The search for extra dimensions is not just something for the future but is currently ongoing at the LHC. If extra dimensions exist, one could hope to produce KK particles by colliding ordinary particles (protons in the case of the LHC) at very high energies. So far neither ATLAS nor CMS – the LHC’s two main experiments – has found any signs of extra dimensions. However, these non-observations are still useful. The fact that nothing has been observed places stronger constraints on the size of the extra dimensions: if they exist and have not yet been seen, they must simply be even smaller than was previously thought to be necessary.

Photo of the CMS detector at CERN: a huge machine with brightly coloured metals and wires

Probing the unknown

So far, new constraints have been placed on the ADD and RS models by the ATLAS and CMS experiments. For the ADD model, they have found that the so-called effective extra-dimensional Planck scale, which is the highest possible energy scale that makes physical sense within the given model, must be larger than 2–4 TeV, whereas for the RS model, the graviton needs to be heavier than 1–2 TeV. As the energy scale of a KK particle, i.e. its mass, is essentially directly proportional to the inverse size of the extra dimension, this equates to finding a maximum length scale for the extra dimension.

On 4 July this year the ATLAS and CMS collaborations at CERN announced that they had discovered a new particle with a mass of around 125 GeV, which most likely is the famous Higgs particle. If it is the Higgs, then the characteristic size of the extra dimension needs to be smaller than about 1.8 × 10^–18 m – about a thousandth of the proton radius. But it should be noted that the LHC has really only just started collecting data, and much could remain to be discovered. Previously, other, less powerful machines have been used to restrict extra dimensions in a similar way, most notably the former accelerators the Large Electron–Positron collider at CERN and the Tevatron at Fermilab in the US.

Limits on extra dimensions could also be identified by the many dark-matter experiments, such as the XENON detector at the Gran Sasso underground laboratory in Italy, the space-borne Fermi and PAMELA satellites, and the IceCube neutrino telescope at the South Pole. Indeed, this year the Fermi collaboration put strong limits on the Planck scale in the ADD model that are compatible with the LHC results: it has to be larger than 230 TeV for two extra dimensions, 16 TeV for three and 2.5 TeV for four. If dark matter is made up of KK particles – i.e. Kaluza–Klein dark matter – then these experiments could prove useful in the study of extra dimensions.

Although neither the LHC nor any dark-matter experiments has yet found conclusive signs of physics beyond the Standard Model, over the next few years the LHC will produce numerous new data that will hopefully show whether extra dimensions exist or not. We may know the answer before the end of this year. Even if we do not, other experiments will continue the hunt, during what is proving to be a very interesting time for physics research at its most fundamental.

SKA names director-general

The British astronomer Philip Diamond has been appointed the first permanent director-general of the Square Kilometre Array (SKA) – the world’s biggest and most sensitive radio telescope. Diamond, who has spent the last two years in Australia as head of astronomy and space science at the CSIRO, will be moving back to the UK in October to head-up the SKA Organisation, which is based at the Jodrell Bank Centre for Astrophysics near Manchester. Diamond had been director of Jodrell Bank before joining the CSIRO in 2010. He replaces the Dutch astronomer Michiel van Haarlem, who has been interim SKA boss since December 2011.

SKA will be a €1.5bn ground-based radio-astronomy telescope used to probe the early universe for clues on galaxy evolution, dark matter and dark energy by looking as far back into time as the first 100 million years after the Big Bang. SKA will consist of more than 3000 radio dishes that will be spread out across thousands of kilometres in both Australia and southern Africa. The dishes will have a total collecting area of more than a square kilometre – hence the facility’s name.

Co-location surprise

The decision to co-locate the telescope on two different continents, which was announced in May 2012 after a six-year-long site-selection process, came as a surprise because the SKA Organisation had been expected to choose either Africa or Australasia to host the facility. SKA is currently in a “pre-construction” phase, where its engineers are testing out components that will make a fully fledged SKA possible. As well as the South Africa and Australia, other members of SKA include Canada, China, Italy, the Netherlands, New Zealand, Sweden and the UK.

“The SKA telescope is now moving from technology concept to the final, detailed design,” says Diamond, who has been involved with the project since 2000. Construction of the first phase of SKA is expected to start in 2016 and last for three years. This will involve building 50 low-frequency antenna stations, 60 mid-frequency dishes in Australia and 190 mid-frequency dishes in South Africa. About 100 existing dishes in both countries will also become part of SKA. The second phase should begin in 2018, with SKA being complete by 2023.

Nitrogen vacancies detect magnetic fields in fluids

A new electron spin resonance (ESR) technique involving tiny diamonds and optical tweezers has been developed by physicists in the US. The method measures local magnetic fields in liquid environments and could be used to monitor a range of phenomena that occur in fluids – including processes in biological cells and electrochemical devices. Indeed, the team believes that it could use the technique to image electromagnetic fields around neurons in the brain.

The diamonds contained nitrogen vacancies (NVs), which are defects that occur when two neighbouring carbon atoms are replaced by a nitrogen atom and an empty lattice site. An NV has an electronic spin that is extremely well isolated from the surrounding lattice, which means that if the NV is placed in a certain spin state it will remain in that state for a relatively long period of time – even at room temperature. What is more, the spin states can be read out reliably and reinitialized when needed. The structures can therefore be used to store quantum information or as quantum probes to detect magnetic fields in their surroundings.

In this latest work, David Awschalom and colleagues at the University of California, Santa Barbara looked at NV centres in “nanodiamonds” just 100 nm in diameter. He explains that “The nanodiamonds themselves can also be placed with nanometre precision wherever we wish in a sample and be moved around at will, something that has potential applications in sensing, tracking and tagging in submicron biophysical systems.”

The team’s nanodiamonds are trapped using optical tweezers, which involves using a single laser beam that is so tightly focused that dielectric particles such as diamond are pulled to the beam focus rather than being pushed forwards by the beam. The particles are therefore held in the focus, optically levitated and trapped. “By moving the laser focus with respect to the fluidic environment, we can choose where to position the particles using an all-optical technique (no wires or physical contacts needed),” says Awschalom.

Monitoring magnetic fields

Awschalom and colleagues used nanodiamonds that had been commercially irradiated to create more than 500 NV centres in every nanodiamond particle. The researchers then use ESR to measure the energy-level structure of the NV centres. ESR occurs when the electron spins are subjected to a magnetic field, which creates an energy difference between different spin orientations. If the sample is exposed to microwaves with energy equal to this difference, the spins will resonate between the two energy levels. Therefore by measuring the resonant frequency, the magnetic field strength can be determined. “We take advantage of the so-called Zeeman effect, which shifts the spin energy levels of the NV centre, to monitor the magnetic fields detected by the NV sensors in the nanodiamonds,” explains Awschalom.

“Being able to measure the local magnetic field at a chosen location in a fluidic environment using a laser to position the nanodiamond sensor may have multiple applications,” he claims. “For example, it could help improve our understanding of biological cellular processes, electrochemical cells, surface catalysis or lipid membranes. It also offers us a new way to visualize important biological and chemical structures that may be difficult to probe with conventional techniques.”

The team says that it is now interested in using functionalized nanodiamonds that have chemical groups attached to them so that they are attracted to certain molecules. These could be used in microfluidic channels in combination with optical trapping and electron spin resonance to maximize the potential for nanodiamond sensing and on-chip sorting to identify and quantify specific targets.

The work is described in the Proceedings of the National Academies of Science.

Ultracold fermions simulate spin–orbit coupling

Two independent groups of physicists are the first to use ultracold fermionic atoms to simulate “spin–orbit coupling” – an interaction that plays an important role in the electronic properties of solid materials. Both experiments were done by firing laser beams at the atoms, which caused their momentum to change by an amount that depends on their intrinsic spin. Because the interactions between atoms in such simulations can be adjusted with great precision, the breakthrough could shed further light on a range of physical phenomena, including magnetism, topological insulators and Majorana fermions.

Spin–orbit coupling describes the interaction between the intrinsic spin of an electron in a material and the magnetic field induced by the electron’s movement relative to its surrounding ions. As well as playing a key role in the magnetic properties of materials, spin–orbit coupling also influences the performance of “spintronic” devices – those that exploit the spin, rather than the charge, of electrons and that could one day lead to faster and more energy-efficient computers.

Quantum simulators

Because of its fundamental nature, physicists are therefore very keen to use clouds of ultracold atoms to simulate spin–orbit coupling. Such “quantum simulations” are carried out by subjecting the gas to laser light and magnetic fields, which lets researchers create interactions between atoms that are similar to those experienced by electrons in a solid. The advantage of these simulations is that – unlike in a solid – the strength of these interactions can be easily adjusted, allowing physicists to test theories of condensed-matter physics.

In 2011 Ian Spielman and colleagues at the National Institute of Standards and Technology (NIST) in Maryland were the first to simulate spin–orbit coupling in an ultracold gas of bosonic atoms. Now, two independent groups – one in China led by Hui Zhai of Tsinghua University and Jing Zhang of Shanxi University, and the other in the US headed by Martin Zwierlein and Lawrence Cheuk at the Massachusetts Institute of Technology (MIT) – have extended Spielman’s technique to fermions. As electrons are fermions and not bosons, the new work is much more relevant to electron physics.

Using potassium-40…

The Chinese team began with about two million potassium-40 atoms that are held in an optical trap and cooled to well below the ensemble’s Fermi temperature. This means that nearly all the atoms in the gas are in the lowest possible energy state, like the conduction electrons in a metal. The team focused on two closely spaced magnetic energy states, which are used to simulate the spin of the electron – one state corresponding to spin up and the other to spin down.

The team then fired two laser beams into the gas from opposing directions. The laser light is set to resonate with a transition between the two spin states – a process that involves the atoms continuously absorbing and emitting photons. As these photons carry momentum, if an atom absorbs a photon moving in one direction and then re-emits it in the same direction, the atom’s momentum will not change. However, an atom can also be stimulated by the opposing beam to emit the photon in the opposite direction – thus changing the atom’s momentum. Such an interaction involves a change in the direction of the atom’s spin and is therefore analogous to spin–orbit coupling – albeit in 1D.

The Chinese team used its system to study several aspects of spin–orbit coupling. In one experiment, the researchers began with a state in which all the spins are initially pointing in the same direction. They then turned on the spin–orbit interaction by pulsing the lasers for a very short time – just a few hundred microseconds. They found that the spins began to point in different directions in a process known as “dephasing”. This is expected from fermions because atoms with the same spin cannot have the same momentum and therefore each atom will be affected differently by the spin–orbit interaction.

Understanding dephasing is important because it has a detrimental effect on technological applications of spin such as spintronics and quantum computing. The team also looked at several other effects related to spin–orbit coupling, including its effect on the momentum distribution of the atoms.

…and lithium-6

The MIT physicists, meanwhile, used a gas of lithium-6 atoms, which meant that their realization of spin–orbit coupling was more difficult than for the Chinese team. The problem is that lighter atoms such as lithium are more prone to heating via the resonant absorption of light. So to get round this problem, the MIT team kept most of its atoms in “reservoir states” in which they do not interact with the light and stayed cool – using radio waves to drive a small number of atoms into the spin–orbit coupling states.

The MIT team focused on showing that ultracold atoms can be used to simulate a “spin diode” – a device that is likely to play a key role in the development of spintronic circuits. It allows spin-up atoms to flow forwards but not backwards, and spin-down atoms backwards but not forward. “The gas acts as a quantum diode, a device that regulates the flow of spin currents,” says Cheuk.

Simulating band structure

By applying radio-frequency radiation to the gas, the MIT physicists were also able to simulate a periodic potential similar to that found in a 1D lattice. As expected for real materials, the periodic potential led to the existence of spin-dependent energy bands. According to the team, the ability to create spin-dependent band structures in this way could lead to the simulation of topological insulators.

The spin–orbit coupling simulated by both teams occurs only in 1D and therefore cannot be used to simulate the 2D and 3D systems found in most real-life electronic devices. However, there are several interesting scenarios that can be investigated in a 1D system. For example, it could be used to simulate the behaviour of electrons in semiconductor/superconductor nanowires. Such systems are believed to harbour quasiparticles that resemble Majorana fermions – long sought-after particles that are also their own antiparticle.

Both experiments are described in Physical Review Letters.

The September 2012 issue of Physics World is out now

By Matin Durrani

Ask a non-scientist what a theoretical physicist does and you’re likely get a shrug of the shoulders along with a guess such as “Scribbles equations all day?” Even most physics students probably don’t know what theorists really do.

In an attempt to shed light on how theoretical physicists work, the September 2012 issue of Physics World, which is now out, contains the first of an occasional series exploring the emotional challenges behind some of the most elegant, ingenious or important calculations in physics

Our plan is to look at calculations that theorists consider their own favourite or that represented a personal triumph – a reward for years of study or a moment of clarity into what science is all about. This month we examine the work of Peter van Nieuwenhuizen, Daniel Freedman and Sergio Ferrara in 1976 on the theory of “supergravity”, which combines supersymmetry with gravity.

Although there is not yet any experimental proof that supergravity is a valid description of the real world, the tale of how the theory was created – as told by science writer David Appell – is fascinating and gripping. You can read Appell’s feature “When supergravity was born” by clicking here.

Elsewhere in the issue, Magdolna Hargittai from Budapest University of Technology and Economics examines the long-standing question of whether the physicist Chien-Shiung Wu should have received a share in the 1957 Nobel Prize for Physics – or whether she missed out to theorists Tsung-Dao Lee and Chen Ning Yang as a result of gender discrimination. Meanwhile, Henrik Melbéus and Tommy Ohlsson from the KTH Royal Institute of Technology in Sweden look into whether CERN’s Large Hadron Collider could find evidence for “extra dimensions”. Plus reviews, careers, lateral thoughts, feedback and much more.

Members of the Institute of Physics (IOP) can access the entire new issue online through the free digital version of the magazine by following this link or by downloading the Physics World app onto your iPhone or iPad or Android device, available from the App Store and Google Play, respectively.

For the record, here’s a rundown of highlights of the issue:

• Support mounts for ‘honeytrap’ physicist – Michael Banks looks at the physics community’s attempts to support 68-year-old particle theorist Paul Frampton, who is languishing in an Argentine jail on drug-smuggling charges

• Delivering on a promise – Shiraz Minwalla says India’s education needs to be reformed before the country can realize its full scientific potential

• Critical point: One amazing moment – Robert P Crease wonders why physicists are not doing more to celebrate the centenary of one of the most important events in science – the discovery that crystals diffract X-rays

• Credit where credit’s due? – Magdolna Hargittai asks if physicist Chien-Shiung Wu should have received a share in the 1957 Nobel Prize for Physics – or whether she missed out to theorists Tsung-Dao Lee and Chen Ning Yang as a result of gender discrimination

• Delving into extra dimensions – Henrik Melbéus and Tommy Ohlsson describe three different theories of extra dimensions – universal, large and warped – and how these unseen dimensions could be observed, if they exist at all

• Crackpots and consequences – Margaret Harris reviews Physics on the Fringe: Smoke Rings, Circlons, and Alternative Theories of Everything by Margaret Wertheim

• Science in a dictatorship – Gordon Fraser reviews The German Physical Society in the Third Reich: Physicists between Autonomy and Accommodation edited by Dieter Hoffmann and Mark Walker

• Speak up – The role of spokespeople on international physics collaborations is important, complex and, as David Wark explains, requires skills that nobody ever taught you during your PhD

• Once a physicist: Ralph Palmer – Meet the 12th Baron Lucas – a Conservative member of the House of Lords

• Fiddling around with physics – In this month’s Lateral Thoughts column, Nicole Yunger Halpern muses on what would happen if great physics-loving musicians were to meet

If you’re not yet a member, you can join the IOP as an imember for just £15, €20 or $25 a year via this link. Being an IOPimember gives you a full year’s access to Physics World both online and through the apps.

Between the lines: innovation and creativity special

Light bulb made from screwed up paper — That’s the idea: Three books explore scientific innovation, from DNA to zero-gravity chemistry. (iStockphoto/xefstock)

Teaching innovation

How do you define innovation? For Roberta Ness, it is simply “creativity with a purpose”, and the aim of her book Innovation Generation is to teach people how to think more creatively. To this end, she identifies a series of discrete stages in the innovation process, from “phrasing the question” properly to generating ideas, incubating them and eventually disseminating them to an appreciative world. Ness’s background is in public health, so many of her examples naturally lean towards the life sciences. Her book is also strongly US-centric, and UK readers will get a good laugh at Ness’s suggestion that they – unlike their benighted American counterparts – do not have to apply for financial support via an “arduous and lengthy…process of peer-reviewed grant submissions” for each experiment they wish to do. But the biggest problem with Innovation Generation is that the book itself is not terribly innovative. In fact, a fair amount of it feels like it has been recycled (with attribution) from earlier books in the science-lite genre, such as Malcolm Gladwell’s The Tipping Point, Steven Levitt and Stephen Dubner’s Freakonomics and James Surowiecki’s The Wisdom of Crowds. Readers who enjoyed those books will enjoy this one too – but only if they can overcome their sense of déjà vu at seeing familiar anecdotes repurposed.

2012 Oxford University Press £18.99/$29.95hb 272pp

No easy answers

If Innovation Generation‘s “let’s break this down into steps” approach to scientific creativity strikes you as overly simplistic, you might want to pick up Drive and Curiosity instead. In it, chemist-turned-science-historian Istvan Hargittai profiles 15 innovative scientists and tries to explain how (and why) they made their career-defining discoveries. His chosen 15 include: the DNA pioneer James Watson; Watson’s sometime rival, Linus Pauling; and two physicists, Peter Mansfield and Rosalyn Yalow, who made Nobel-winning breakthroughs in biomedicine. After analysing the character traits and personal circumstances of all 15, Hargittai concludes that they do not, in fact, have much in common. The only unifying traits he can find are the “drive and curiosity” of the book’s title and, as he puts it, “Drive and curiosity do not always yield discovery, and when they do, the discovery is usually a minor one.” Moreover, a factor that benefits one person might well be disastrous for another. For example, Hargittai suggests that ignorance was a vital ingredient in Watson’s DNA breakthrough, because if Watson had understood the limitations of X-ray crystallography at the time, he might never have tried to probe the molecule’s structure. On the other hand, Pauling was successful precisely because he was, in Hargittai’s words, a “walking data bank” of structural chemistry. For him, ignorance would have been disastrous. Similarly, Pauling benefited from competition, but for Yalow, life in a scientific backwater proved advantageous, as it meant that she and her chief collaborator “could work on problems of their own choosing and develop their work at their own pace”. Hargittai’s insights are not the type that would catapult him into a lucrative third career as a motivational speaker. But after the glib grandiosity of Innovation Generation, the limited conclusions offered in Drive and Curiosity feel both thoughtful and refreshingly honest.

2011 Prometheus Books £22.95/$26.00hb 328pp

A lot of silly ideas

The “Daedalus” column of scientific tomfoolery made its first appearance in New Scientist in the mid-1960s and was later a regular feature in Nature and the Guardian. To write it, author David Jones had to conjure up one silly-but-interesting idea every week for well over 30 years. He also performed regular scientific demonstrations on British and German regional television. That is an admirably long track record of innovative thinking, and fans of Jones’ Daedalian alter ego will be delighted to hear that he has now written an entire book about scientific creativity. The Aha! Moment is actually more like two books in one: its first chapters describe Jones’s theory of creative thought, while later ones give examples of his own creative ideas in action. Jones has undoubtedly had some very good ideas in his career, including an unrideable bicycle, a series of zero-gravity chemistry experiments that flew on the Space Shuttle and an artificial geyser made from a tea kettle, a glass tube and a metal bin lid. However, as he states on several occasions, at least 80% of all ideas turn out to be bad, and unfortunately there is some evidence for this in the book. The author’s less-great ideas include an eyebrow-raising division of creativity into allegedly “male” and “female” forms, the idea that a woman can subconsciously select which of her eggs will be released for fertilization and even suggestions that certain people can psychically influence the outcome of Schrödinger’s-cat-type experiments. True, some of these bad ideas are attributed to Daedalus, rather than to Jones himself, but it is sometimes hard to tell where one begins and the other ends. Those who never read Daedalus in his prime will struggle to understand why some of his jokes were funny, and will gain little from the book’s frequent references to Jones’s past work.

2012 Johns Hopkins University Press £13.00/$25.00pb 264pp p

Geoengineering is ‘comparatively inexpensive’

Researchers in the US have estimated that modification of stratospheric albedo – a widely discussed geoengineering technique to counteract some of the effects of climate change – could cost as little as $5bn a year. Although this is just a small fraction of the gross domestic product (GDP) of most western countries, the team stresses that there are many potential risks of geoengineering the planet in this way.

Geoengineering aims to mitigate man-made climate change by making large-scale modifications to the Earth’s surface or atmosphere. One of the main proposals discussed by scientists is stratospheric albedo modification: changing the reflective power of the atmosphere 10–50 km above the Earth’s surface so that more solar radiation is reflected back into space. Such a modification would be achieved by pumping tiny particles known as aerosols into the upper atmosphere.

Price on pumping

Now, technology and policy expert Jay Apt of Carnegie Mellon University in the US and colleagues have put a price on such aerosol pumping. Using statistical methods, they estimated the cost for various systems of delivering between one and five million tonnes of particles such as sulphur compounds to altitudes of 18–30 km. These systems included the use of existing aircraft, the procurement of new aircraft or airships, the procurement of rockets and guns, and suspended gas pipes.

Apt’s group found that the basic technology exists today to deliver enough aerosols into the stratosphere to offset the man-made warming expected over the next 50 years. In fact, several of the systems under consideration could perform the feat for less than $5bn a year – just 0.03% of US GDP.

Some of the cheapest systems would involve airships, the researchers found, although the development cost and high-altitude performance of these is uncertain. Gas pipes would offer low recurring costs, although their development cost would be high and they would need to demonstrate very high tensile strength. Rocket and gun systems, according to the calculations, would be the most expensive.

Environmental and political risks remain

Apt’s group is quick to point out that the estimated costs should not be seen as a green flag for stratospheric albedo modification, since the researchers have considered neither its potential environmental risks nor its political or moral strains. Their study “simply means that an attribute of [albedo modification] is that it is comparatively inexpensive”, they say.

Indeed, scientists and policy experts have uncovered many disadvantages of stratospheric albedo modification. One problem is that different regions of the world might need different amounts of modification, since global warming is not expected to occur evenly. Another issue is that altering albedo could affect other aspects of the climate, such as rainfall. In fact, some climate models suggest that albedo modification could hasten the droughts that climate change is expected to induce.

Worst, however, is the knowledge that, once begun, albedo modification must be maintained indefinitely. “Abrupt stopping of the delivery of particles to the stratosphere would cause very rapid climate changes,” says Apt.

The research is published in Environmental Research Letters.

Browse articles by content type

Do you think the Large Hadron Collider will discover new physics beyond the Standard Model?