White dwarfs forming in extreme magnetic fields could be stabilized, allowing them to get bigger before they explode, leading to a brighter bang when they finally do, according to a team of researchers in India. Type Ia supernovae, caused by exploding white dwarfs, are often used by astronomers as “standard candles” to calculate the distance to a point in space because they are extremely bright and usually have similar luminosity. But some anomalously bright type Ia supernovae that scientists could not explain have been observed recently and the new work could provide an explanation for them.
A white dwarf is a star that has used all its hydrogen and helium and is too cool to burn carbon. It has therefore collapsed into a highly dense state. With no source of energy, it glows only because of residual heat, and over billions of years it will cool down and become a black dwarf, if it is left undisturbed.
Limits on stellar evolution
In 1935 the Indian astrophysicist Subrahmanyan Chandrasekhar famously showed that a star would not form a white dwarf if its mass was greater than 1.44 solar masses because the core temperature would be sufficient to ignite carbon fusion. If a star’s mass increased beyond this “Chandrasekhar limit” of 1.44 solar masses after it has collapsed to form a white dwarf, the star shrinks still further. The loss in gravitational potential energy causes an increase in temperature, and a runaway fusion process begins, creating a massive thermonuclear explosion that obliterates the star in seconds.
Because type Ia supernovae are almost always formed by the thermonuclear explosion of an object with about the same mass, they almost always have about the same brightness. Observations of distant type Ia supernovae proved the expansion of the universe was accelerating, a discovery rewarded with the 2011 Nobel Prize for Physics. However, there have been a small number of troubling observations recently of nearby type Ia supernovae that are abnormally bright, and which appear to have been formed by the detonation of a white dwarf well above the Chandrasekhar limit. The absence of a satisfactory model for how these could be produced has placed a question mark over the use of type Ia supernovae as standard candles for observing distant galaxies.
Super-sizing
In the new research, Upasana Das and Banibrata Mukhopadhyay of the Indian Institute of Science in Bangalore suggest that these “super-Chandrasekhar” white dwarfs might occur in very high magnetic fields. Such fields, they reason, could stabilize a white dwarf of mass up to 2.58 solar masses by a process known as Landau quantization. This would increase the stellar remnant’s resistance to gravitational collapse, allowing it to continue accreting mass until it reached a higher limit. At this point it would detonate with an even bigger bang than would otherwise have been possible.
But how might such a field be generated? Das and colleagues point out that magnetic fields of 107–108 G can be detected in about 25% of accreting white dwarfs. If such a star collapses, the magnetic flux is conserved, while the radius is reduced dramatically. The magnetic fields therefore become orders of magnitude stronger.
Mukhodpadhyay now believes that the team needs to focus on observing a larger sample of highly magnetized white dwarfs in the hope of observing this spike in field as one collapses. “You start with an observed white dwarf of field 109 G,” says Mukhopadhyay, “Later on, it should pass through an intermediate phase when the field increases to 1011 G – those we have not seen yet.” He cautions, however, that an increase in field might not be detectable if the matter accreting onto the white dwarf caused magnetic shielding.
Impressive increase
Mukhopadhyay believes that it is too early to say whether the model has any direct implications for the expansion rate of the universe. He does, however, tell physicsworld.com that “the existence of super-Chandrasekhar white dwarfs is a major paradigm shift in our understanding of white dwarfs and several of the related results may have to be examined in this light”.
Jeffrey Silverman, an astrophysicist at the University of Texas at Austin, says the paper presents “a pretty impressive increase in the maximum white dwarf mass which, as they point out, matches some of these recent observations – or at least gets closer to them”. He is more sceptical, however, about the researchers’ claims of a paradigm shift: “We’ve seen very few of these super-Chandrasekhar objects. They’re very rare, they’re pretty distinguishable from more typical type Ia supernovae that we use for cosmology. It’s highly unlikely that our calculations of the history of the universe would have many – if any – of these objects contaminating them.”
The first search for magnetic monopoles in mantle-derived polar igneous rocks – thought to be likely to contain a higher ratio of monopoles to matter – has been conducted by researchers in Switzerland. The team analysed 23.4 kg of samples from Arctic and Antarctic regions. While no monopoles were found, the monopole to nucleon ratio in the search samples was constrained, with a 90% confidence level, to an upper limit of 1.6 monopoles per 1028 nucleons. The team claims that its study, which adds to existing matter-bound monopole searches, has comparable or better sensitivity than the most extensive meteorite search to date.
Magnetic monopoles were famously predicted by Paul Dirac in 1931 as a way of explaining electric charge quantization. Their existence is also predicted by a number of grand unification theories, but the much-anticipated particle has so far remained elusive.
“The magnetic monopole is a truly fascinating hypothetical object – it explains electric charge quantization and it is needed in theories that unify the fundamental interactions,” says lead researcher Philippe Mermod, from the University of Geneva. “This makes us wonder why it has never been found in nature, as its non-existence would be a complete mystery.”
Cosmic origins?
It is believed that monopoles might be created within high-energy particle accelerators, such as the Large Hadron Collider. However, if monopoles are very heavy, the beam energy could be insufficient to create them in particle collisions. In this case it could be that the only events energetic enough to produce monopoles happen in the cosmos, or occurred just after the Big Bang.
“In such a case, monopoles should be around us: either travelling freely through space or bound to matter,” explains Mermod, “but [past] experiments were unable to observe any in hundreds of kilograms of terrestrial matter, and even in asteroids and rocks from the Moon, showing that if they are there, they are extremely rare.”
Mermod and his team believe that the key to finding monopoles might lie not with more powerful accelerators or the analysis of larger amounts of source material, but with novel search designs.
During its formation, the Earth was molten, and has since differentiated into a number of chemically distinct layers – the crust, the mantle, and the inner and outer core. During this stage, any monopoles bound to the matter that formed the Earth would likely have sunk towards the core. The crust, therefore, is expected to be depleted in such stellar monopoles – monopoles already trapped in stardust before the formation of our solar system.
In-depth look
Monopoles in the solid mantle, however, would be restricted in their movement and, regardless of their polarity and the magnetic-field direction, would be slowly moved along in line with mantle convection. On reaching the liquid core, their mass would pull them towards the Earth’s centre, before being attracted pole-ward by their magnetic charge.
Mermod and colleagues therefore predict that monopoles might be distributed throughout the mantle, up to a distance of 3400 km – the core radius – from the Earth’s magnetic axis. Rocks from polar mantle-derived sources are therefore potential candidates in the search for monopole-bearing material. Samples selected for this study were mainly confined to mantle-derived rocks from high (greater than 63 degrees) latitudes.
The team searched for monopoles in the samples by looking for the signature of a persistent current in a loop of superconducting material – a SQUID-based rock magnetometer – that is housed in a shielded room at the Laboratory of Natural Magnetism in Zurich.
Probing polar rocks
“Polar volcanic rocks were never probed for monopoles before,” says Mermod. He goes on to explain that since his calculations showed that the gravitational/electromagnetic force balance was advantageous for a range of monopole masses and charges, and he had access to a magnetometer that was able to detect the signature of a monopole in rock samples, he decided “that this needed to be done. There was truly a chance that monopoles would be spectacularly discovered”.
In addition, the team also analysed rocks with chemical compositions that hint at deep mantle origins. These included: basaltic rocks extruded above hotspots where local heat increases in the mantle cause an upwelling plume; samples from large igneous provinces – areas of massive flood basalt deposits that have been connected to continental break up and mantle plume activity; lava containing lherzolite nodules that have been transported, unaltered, up from the mantle; and basaltic lava from Coleman Nunatak, a ridge of rock located at the head of the Berry glacier in Antarctica, the high 206Pb/204Pb ratios of which indicate a low extent of melting and deep origin.
Control samples for the study were taken from crust-derived lava from an Antarctic subduction zone and from low-latitude hotspots (Hawaii) and mid-ocean ridges (the Mid-Atlantic Ridge and the East Pacific Rise), as these should be depleted of monopoles.
Continuing the hunt
The monopole hunt in matter, however, will not end here. “The future of searches for monopoles trapped in matter depends on possible access to better instruments based on new technologies, or to exotic material samples,” Mermod says. “As soon as new material becomes available – for instance, returned samples from asteroid or comet survey missions – it will be important to probe it for monopoles.”
A pre-print of the research is available on the arXiv server.
Physics and medieval history don’t overlap that often. I should know: I got an undergraduate minor in medieval and renaissance studies in part because I wanted a break from doing physics. So the fact that this arXiv paper and this documentary have both come out in the past 10 days is about as unusual as – well, finding a medieval king buried in a car park.
Fascinating as the discovery of Richard III’s skeleton is, though, I’m going to write instead about the arXiv paper, which proposes something even more remarkable: a possible link between space weather and episodes of famine in late medieval Europe.
The paper focuses on the years 1590–1702, a period during which Europe’s population suffered repeatedly from famine. Over the same period, the Sun was experiencing a decades-long lull in activity, known as the Maunder minimum. Might there be a connection?
To answer this question, the paper’s authors – physicist Lev Pustilnik and economist Gregory Yom Din – begin by summarizing the evidence for a connection between space weather and local weather. Overall, this appears fairly convincing, if a bit circumstantial. For example, a 1997 study found a link between cosmic rays and cloud cover, while a 2004 paper demonstrated a similar correlation between global atmospheric circulation and level of activity in the Earth’s magnetosphere.
With the principle of a connection thus established, Pustilnik and Yom Din go on to suggest three conditions under which space weather could lead to famine:
• Local weather has to be in a “threshold state” such that it is sensitive to space weather. For example, if there is no water vapour present, clouds won’t form even if space weather is “seeding” the Earth’s atmosphere with lots of extra ions.
• Harvests must be sensitive to weather anomalies. This is more likely in areas of so-called “risk farming”, where conditions are marginal enough that a few days of bad weather can completely wipe out a crop.
• The area has to be economically isolated, such that local shortages cannot be ameliorated by buying grain from elsewhere.
To test these hypotheses, Pustilnik and Yom Din begin by comparing levels of solar activity with grain prices in 17th century England. Between 1590 and 1700, the price of grain in England and the abundance of 10Be isotopes (a proxy for solar activity) in Greenland ice cores exhibit an almost exactly inverse relationship. High prices correspond to periods of low solar activity and vice versa. Several other European markets that the authors studied also showed strong correlations between grain prices and solar activity, but in southern Europe, where crops are more likely to suffer from drought than from excess rain, prices tended to spike during solar maxima rather than minima.
Things get a bit shakier when the authors turn their attention to 19th century Iceland. In this case, famines seem to correlate with both minima and maxima in solar activity. Pustilnik and Yom Din claim this is what they expected to see, but don’t really say why; in particular, they don’t explain why the Icelandic pattern should differ so markedly from the English one.
Still, it’s an interesting study, and reading it stirred up some memories from my brief foray into medieval studies. In particular, I thought of a book called Lost Worlds whose author, a Swiss historian called Arthur Imhof, makes unusually good use of hard data in analysing what life was like for an ordinary person in early modern Europe. Might his book have something to add to the famine/space weather debate?
I skimmed my copy of Lost Worlds a couple of times before I located the bit where Imhof writes about famine. Tree-ring data and written sources from the 16th and 17th centuries, he notes, indicate a long series of harsh winters and summers with too much rain, resulting in exceptionally bad growing conditions. As a result, he adds, “our ancestors had more reason to beg for their daily bread between 1550 and 1700” than they did at almost any point before or since.
This is, of course, almost exactly the same period that Pustilnik and Yom Din studied, and it’s nice to see that Imhof’s sources corroborate their grain-price data. But Imhof wasn’t interested in climate for climate’s sake. Instead, he was trying to demonstrate that populations in areas prone to famine, plague and war became traumatized by their repeated misfortunes. You’d have to read the book to appreciate Imhof’s argument in full, but among other things, he suggests that people in these “unlucky” areas developed fatalistic attitudes to life, death and birth. These attitudes show up not only in religious beliefs, but also in data on infant and maternal mortality. For example, even in peaceful, plague-free years, more than one-third of babies born in the plague-prone and war-torn German village of Gabelbach died in infancy. In “luckier” villages, the comparable figure was one in eight.
Where does this leave us regarding space weather? Well, if we add Imhof’s conclusions to Pustilnik and Yom Din’s, it seems that the behaviour of heavenly bodies could have influenced not only the viability of medieval grain crops, but also the habits and attitudes of the people who tended them – perhaps even to the extent of determining whether their children were likely to live or die. That might not be very surprising to the peasants of 17th century Gabelbach, who lived in a more religious age (and, according to Imhof, believed fervently in astrology). But to me, it’s absolutely mind-blowing – and a whole lot more interesting than England’s “Tricky Dick” turning up in a car park.
Physicists in the US are the first to make an integrated device that extracts photons from a tiny piece of diamond before the light is sent through a waveguide to the outside world. The photons all have the same frequency and originate in a nitrogen vacancy (NV), which is a defect that occurs in diamond when two neighbouring carbon atoms are replaced by a nitrogen atom and an empty lattice site. According to the researchers, the chip could be used to create quantum-information technology such as quantum repeaters.
For anyone trying to build a quantum computer NVs are useful because they have an electronic spin that is extremely well isolated from the surrounding lattice – so if an NV is placed in a certain spin state then it will remain in that state for ages, even at room temperature. An NV can also emit just a single photon if excited by a laser of the right wavelength. Taken together, these properties mean that NVs allow data to be stored for long times in a defect, before being read out as a single photon.
Researchers are particularly interested in extracting photons that do not interact with the surrounding lattice because these “zero phonon line” (ZPL) photons have a well defined frequency. Unfortunately, one challenge in building NV-based quantum systems is how to reliably get ZPL photons out of the diamond and into an integrated optical system, where it can be processed further. What Andrei Faraon and colleagues from Caltech, Hewlett Packard and the University of Washington have managed to do is to create an integrated optical system that does just that.
Matching frequencies
At the heart of their device is a ring of diamond that is just 4.5 μm in diameter and contains NV centres. The ring sits next to a waveguide that is about 10 μm long (see figure). The device is cooled to below 10K and the ring is scanned with a green laser until a NV centre with a resonant frequency close to that of the ring is located. The team then introduces a noble gas into the cryostat and some of it condenses on the ring – changing its resonant frequency. More gas is added until the frequencies of the NV centre and the ring exactly match.
The ZPL photons are created by firing the green laser at the NV centre. The photons first circulate around the ring before jumping into the wave guide. They then travel to either end of the waveguide, where a diffraction grating scatters them out of the device, where they can be observed with a microscope connected to a spectrometer and a photodetector.
The researchers found that they collected about 25 times more ZPL photons from these devices than were collected from NV centres in similar samples of diamond that were not part of integrated devices.
Faraon sees this work as an important step towards creating integrated circuits in which ZPL photons carry quantum information from one NV centre to another. “We demonstrate that photons – the information carriers – from a single NV centre can be coupled to an optical resonator and then further coupled to a photonic waveguide,” he says. “We hope that multiple devices of this kind will be interconnected in a photonic network on a chip.”
What Faraon and colleagues want to do now is to develop devices that include more than one NV centre and show that photons emitted by two NVs can be made to interfere – a pre-requisite for entangling NV centres. Once entanglement has been achieved, the devices could then be used as quantum repeaters, which absorb and re-emit entangled photons without disturbing the entangled state – something that is necessary if quantum information is to be transmitted over large distances. Faraon told physicsworld.com that his colleagues at Hewlett Packard are now working on entangling NV centres on the same chip.
Will the universe go on expanding forever? Why should we care about climate change? Can we make objects invisible?
These are Big questions with a capital B, which individually could occupy the mind of a scientist for an entire academic career. In fact I am sure they have.
But yesterday at Imperial College in London we asked a bunch of physicists to tackle questions of this size and stature and to answer them in 100 seconds or less – using nothing more than a white board and a few marker pens. It was a seriously tough challenge in the overlapping arts of brevity and clear communication.
The presentations were filmed as part of our 100 Second Science video series and they will be joining the existing batch of these “mini lectures”. The picture above shows the PhD student and radio DJ Martin Archer preparing for his moment in the spotlight during which he tackled several questions on the fundamentals of quantum mechanics.
One of the questions Martin addressed related to one of the seemingly paradoxical implications of quantum mechanics: “Is Schrödinger’s cat dead or alive?”. I won’t spoil Martin’s 100 Second Science video on this famous thought experiment, but let us know your thoughts on this question by visiting our Facebook page and taking part in our poll where we ask:
Is Schrödinger’s cat dead or alive?
It must be one or the other at any given time
It exists in a superposition of dead and alive
It could be dead and alive in separate universes
Another outcome is possible
We look forward to your responses. Look out for more in this series of films over the coming months.
A team of researchers in France has found that applying a voltage across a 100-nm-thick cylindrical soap film causes the fluid inside it to flow upwards. If the voltage is increased, the film thickens and the flow rate increases significantly. The researchers say this could be useful in microfluidic systems and to stabilize liquid foams.
The field of microfluidics involves the manipulation of minute amounts of liquids – generally picolitre (10–12) quantities – within micron-width channels. This allows scientists to work with substances that are expensive to produce in large quantities. Also, it allows many different liquids to be used simultaneously, creating a “lab on a chip” platform for the study of many chemical processes at once.
Soft channels
Anne-Laure Biance, Oriane Bonhomme and colleagues at the University of Lyon in France have been studying nanochannels in their lab, and became interested in producing soft nanochannels, as compared with hard ones, because they felt that such soft channels would be easier to make and use, and would cost less. So the researchers looked at soap films – a soft deformable channel that consists of two sheets of ionic surfactant molecules that enclose a layer of water. Biance also feels that soap films are convenient as their thickness is usually at the nanometre scale and they are easy to make.
The team used two platinum-covered plates (the electrodes), separated by about 0.5 cm, facing each other. The researchers then created a bubble out of a soapy liquid made of water, a surfactant and potassium chloride that produces the free ions, and trapped this bubble between the plates.
“We then applied different voltages to the plates, and observed that the liquid inside the bubble flows from the bottom to the top,” explains Biance. The walls of the bubble act as the channel for the flow. As the surfactant molecules are positively charged and the chloride ions are negatively charged, the molecules attract and this causes the drag. That is, the surface ions are pulled by the electric field and they drag the fluid inside the channel along with them.
Drag and flow
This behaviour, known as an “electro-osmotic” flow, can be described as the motion of liquid induced by an applied potential across a porous material, capillary tube, membrane, microchannel or any other fluid conduit. The effect of this type of flow becomes more pronounced at the micro- and nanoscale, where there is a high surface-to-volume ratio. While this has been studied with solid nanochannels, Biance says that not enough is known about the behaviour in soft channels, where the ratio can change.
When Biance and her colleagues measured the magnitude of the flow in their experiment, they also found that the soap film itself had thickened. The researchers could distinguish between the current caused by surface ions and the current caused by the flow of the liquid being dragged (bulk flow). They found that as they increased the voltage, the bulk flow increased at a faster rate than expected. “This was because our film was actually thickening. We could see a change of colour of the film too, and this is only seen when there is a thickness modification,” explains Biance. “We measured the growth of the film’s upper meniscus, where it touches the plate, and we measured the growth rate,” she says, which proved that a wider channel allows the rate of flow to increase quickly.
Thickening effects
Biance told physicsworld.com that this thickening effect is comparable to another effect that explains why a solid plate lifted out of a liquid bath is never dry. Instead, it lifts up a film of fluid thanks to viscous forces that come into play because the plate is being pulled and is known as the Landau–Levich flow. She also points out that the flow and the applied voltage have a nonlinear relationship. “This is interesting because it almost has diode-like properties,” says Biance. The team is also interested in seeing whether any of these effects would help in stabilizing liquid foams that collapse because of the effects of gravity on the liquid. “We might be able to use a voltage to stabilize them,” says Biance.
Currently, Biance and her colleagues are looking at how their experiment would change if they used two bubbles, placed side by side or on top of each other. “Other factors come into play, such as the border between the bubbles and the flow there. In the end, we would like to do this with many bubbles,” says Biance.
“Can’t we make science more efficient?” a physicist asked me recently. Here’s what often happens, he said, especially in speculative areas. Someone proposes a theory and – to ground it in reality and attract interest – makes a prediction. Experimentalists then set to work.
Guess what usually happens next? A banquet of interpretations! The experimental findings are judged inconclusive. The theorist reworks the theory or calls for more experiments. Other experimentalists jump in with new experiments that disprove the first, but with flaws of their own. Theorists refine the theory to incorporate the initial findings, rethink the theory or produce rival theories.
My physicist-friend cited half a dozen or so recent episodes. “End the interpretations!” he demanded. “Wouldn’t it be more efficient to get everyone in a giant room to thrash out specific predictions and exactly how to test them?”
Against interpretation
I sympathized. Yet I had to tell him that an affirmative answer would be possible only if science were akin to that erstwhile TV game show Concentration, in which contestants make clear-cut guesses regarding the answers to well-formulated questions – and no sooner is a guess formulated than offstage technicians activate a video screen to reveal the equally clear-cut answer. Science, in this idealized view, always consists of well-formulated theories and perfectly set up experiments.
Theories, however, do not necessarily make clear-cut predictions. As I’ve written before in this column, theories organize what we’ve learned, frame the present and orient us by indicating important areas for future research. They come in many forms and play many roles. Some make exact predictions, while others – evolution, for instance – are open-ended.
Experiments, meanwhile, do not necessarily have clear-cut outcomes. They are conceived, staged and performed without knowing beforehand exactly what will happen – that’s why we do them! Nature is always deeper and richer than the concepts through which we understand it, and the possibility is always open that experiments will disappoint, confuse or mystify us. When they do, it initiates an interpretive process in which we try to adapt theory and experiment to understand the findings.
Let me illustrate with what happened to a theory – proposed in the 1960s by my colleague Fred Goldhaber and a collaborator – of how protons scatter at small angles off heavy nuclei. An experiment gave a result that could not be explained without making absurd assumptions about the shape of the nucleus. Discussions between participants revealed that the experimentalists had analysed their data by taking account of additional pre- and post-scattering interactions at very small angles of a proton beam on other nuclei in the target, which involved using a recipe that incorporated only Coulomb interactions and ignored nuclear interactions. The nuclear interactions, however, proved to be important in just the extremely small angle data addressed by the experiment and related theory in this case. When the very small-angle Coulomb-plus-nuclear scatterings were taken into account, the theory turned out to work fine.
This story illustrates, writ small, a process that commonly occurs in science. The theorists were not slipping up. Nor were the experimentalists. They were trying to frame what was significant about their findings: the principal scattering component. “You put in all the things you know are important at the time,” Goldhaber says, “but these may not be all the things you need later on.” Both theory and experiment turned out to be right, after additional discussion, exposure of untested assumptions and the incorporation of new distinctions to make explicit what was implicit in the findings.
Why, then, does the game-show view of science persist?
One reason is that scientists may be reluctant to admit the pervasive presence of interpretation for fear it makes science seem a matter of judgment, as if it is one person’s view over another’s – just another way of talking about the world. Philosophers who seek a merely formalistic approach to truth rather than one rooted in interpretation can also buy into the game-show model of science. The wish to shun interpretation is understandable but unnecessary, for we cannot perform experiments any way we like, and the experimental process exposes us to the stubbornness of nature – to something that our concepts and theories do not create but to which they must be accommodated.
A second reason is that, every so often, something like the game-show model actually seems to occur. A classic example is Einstein’s theory of general relativity. This story is messier in detail, but superficially enough like game-show science to satisfy those who yearn to believe.
A third reason is that both theorists and experimentalists tend to write up their work to fit the game-show mould, which among other things makes it more attractive to funding agencies. The media, meanwhile, love the game-show format because it is understandable, explainable, full of clear winners and losers, and makes science as exciting as…a game show.
In short, we are all co-conspirators in promoting the game-show model of science.
The critical point
My physicist friend’s lament, however, was partly driven by the desire to protect science.
For what happens when, inevitably, science doesn’t behave game-show-like? Then it looks uncertain or broken, beset by inefficiency and incompetence – and maybe even malpractice or fraud. Politicians who control science budgets may retaliate, especially if findings imply the need for costly political action. This happened three years ago, for instance, when publication of stolen e-mails of climate scientists provoked some US legislators to seek to slash science budgets.
So I answered my friend that we do get everyone in a giant room to thrash out questions and answers. That room is called science. But there are more than scientists present, and the challenge is to explain to everyone what is happening – without resorting either to the fiction of science as an ideal process, or to science being just another way of talking about the world.
Without carbon, life as we know it on Earth would not exist. But carbon itself would not exist in sufficient quantities for life were it not for a strange feature of its nuclear structure, which has yet to be fully understood. This mystery surrounds one particular excited state of the carbon-12 nucleus – a special arrangement of its protons and neutrons – which our basic models say should not even be there, but is. An essential step in carbon creation within the depths of red giant stars, the nature of this excited state is one of the most intensely researched questions in nuclear physics.
Only three elements were produced in the Big Bang – lots and lots of hydrogen and helium, plus a tiny amount of lithium. Since the 1950s we have known that heavier elements, such as carbon, oxygen and silicon, are produced through nuclear fusion in stars. But as the scientific understanding of these fusion processes began to evolve, scientists also wanted to know why and how the elements form in the abundances in which we find them.
We now know that stellar nucleosynthesis begins with four hydrogen nuclei (four protons) fusing together to form a helium nucleus (two protons and two neutrons). Heavier elements are then created when these alpha particles fuse at high temperatures in massive stars, leading to nuclei such as beryllium-8, carbon-12 and oxygen-16, which have twice, three times and four times the number of protons and neutrons as alpha particles. However, there is a problem. The first step in any of these reactions involves two alpha particles fusing to create beryllium-8, but this nucleus is extremely short-lived, with a lifetime of only 8 × 10–17 s. It therefore does not usually hang around long enough for a third alpha particle to fuse together with it to form carbon-12. Indeed, the probability of carbon-12 forming should be tiny and could not account for the observed abundances.
To try to explain this mystery, in 1953 the British astronomer Fred Hoyle reasoned that because we exist and carbon manifestly exists, there must be something that makes the production of carbon in stars proceed at a much faster rate than would appear likely at first glance. To understand what that something is, recall that nuclei – like atoms – have a discrete set of energy levels consisting of a ground state plus many excited states. Hoyle predicted the existence of a very short-lived excited state in carbon-12 that serves as an intermediate step in the synthesis of carbon.
This fleeting “Hoyle state”, which would decay by emitting gamma rays to reach the stable ground state of carbon-12, would serve as a “resonance” that would accelerate production of carbon-12 by seven orders of magnitude. However, for this to work the Hoyle state would need to have certain properties, including a specific excitation energy.
To understand why, recall that some nuclei have larger binding energies than others. In particular, carbon-12 has 7.28 MeV more binding energy than an alpha particle and a beryllium-8 nucleus combined. So when an alpha particle and a beryllium-8 nucleus, both at rest, fuse they form a carbon-12 nucleus with an excitation energy of exactly 7.28 MeV – because energy has to be conserved. In stars, however, nuclei are not at rest but have some thermal energy, which they bring with them when they fuse, resulting in a slightly higher excitation energy. Hoyle calculated that the excitation energy of this short-lived state had to be 7.68 MeV, plus or minus a few per cent. The Hoyle state should also have a nuclear spin of zero and positive parity.
Hoyle badgered his experimental colleague, Ward Whaling from the Kellogg Radiation Laboratory at the California Institute of Technology, to search with his team for such a state, which they did by bombarding a nitrogen-14 target with deuterons (which contain one proton and one neutron). Using a magnetic spectrometer, they then measured the energy spectrum of the alpha particles produced alongside carbon-12, which – using energy conservation – revealed a state at the precise excitation energy suggested by Hoyle.
Sir Fred Hoyle The astronomer whose theories concerning the origin of elements are arguably his most lasting contributions to science and a plaque placed at his former school, Bingley Grammar School, near Bradford, by the Institute of Physics Yorkshire Branch, which mentions his achievement in discovering the origin of carbon. (Courtesy: By permission of the Master and Fellows of St John’s College, Cambridge; Richard de Grijs)
So had the carbon-production conundrum now been solved? In a way, yes – scientists could now rationalize that, via the Hoyle state, carbon can be formed fast enough to overcome the fact that beryllium-8 is so short-lived, resulting in the abundance of carbon we see around us. However, a new puzzle immediately emerged that, 60 years on, has still not been resolved. Quite simply, the Hoyle state cannot be described by any known models of atomic nuclei. Given how important carbon is for life to form, this situation is deeply unsatisfying, though it is not for want of effort. Indeed, trying to work out the precise nature of the Hoyle state remains one of the most intensive theoretical and experimental efforts in nuclear physics.
Not your normal nucleus
So what picture do we have of nuclei? Our understanding of the nucleus is underpinned by the “nuclear shell model”, which was developed in 1949 – four years before the discovery of the Hoyle state. The model, which was a triumph in our understanding of nuclear structure, says that protons and neutrons act like independent particles that fill up shells rather in the same way that electrons do in atoms, with simple nuclear states constructed from configurations of protons and neutrons in different shells. This model is remarkably successful in describing ground states and excited states, especially in light nuclei. It also does a reasonably good job of describing most of the lowest excited states in carbon-12. The nuclear shell model is, however, hopelessly inaccurate for the Hoyle state. Quite simply, none of the energy levels that the model calculates are remotely close to the energy of the Hoyle state. Indeed, there are similar states in other light nuclei where the shell model also fails badly.
But could it be that the Hoyle state is better described in terms of clusters of alpha particles rather than in terms of nucleons as independent particles? Indeed, in 1956 – only three years after the prediction and discovery of the Hoyle state – Haruhiko Morinaga, who was then at Purdue University, Indiana, conjectured that the Hoyle state could be thought of as a linear chain of three alpha particles. Similar states composed of four, five and six alpha particles, he argued, should also exist in oxygen-16, neon-20 and magnesium-24.
From the beginning, it was clear that the static, linear-chain structure proposed by Morinaga had to be a gross over-simplification. The alpha particles would have to be in continuous motion so as not to violate Heisenberg’s uncertainty principle: since they are confined within a small distance – a nucleus no larger than a few femtometres – their momentum and therefore speed must be above a certain value. The alpha particles might even exchange nucleons between one another. In other words, Morinaga’s alpha-cluster model merely represented the time-averaged picture of a dynamic system.
Our current “best understanding” is that the Hoyle state is a gas-like system of weakly interacting alpha particles that move almost freely over distances that are large on nuclear scales. Moreover, within the past decade, it has become clear that the Hoyle state is likely to exist as a superposition of states: in the same way that Schrödinger’s cat is 50% dead and 50% alive, the Hoyle state is about 70% a state of three alpha particles and 30% a shell-model-type state (in which individual nucleons fill up quantized energy levels).
The only other strong contender for a competing theory is an evolution of the alpha-cluster model in which the Hoyle state could be a Bose–Einstein condensate (BEC). This idea arises because alpha particles have zero spin, which means that they are bosons and obey Bose statistics. A system of alpha particles with a low internal temperature might therefore form a BEC, similar to the atomic condensates that are nowadays routinely made and manipulated in laboratories. This idea has attracted much attention but also faces many difficulties. Nuclear condensates, if they exist, would differ from their well-established atomic counterparts in many respects, and it is not clear how, or if, the theory of atomic condensates can be adapted to the nuclear domain.
Recent revelations
Current-day Hoyle-state physicists are trying to get to the bottom of this mystery once and for all. Theorists dream of being able to predict the properties of nuclei from first principles, which requires two ingredients: a theory of how the protons and neutrons in the nucleus interact and a method to solve the many-body Schrödinger equation numerically. The first ingredient was provided by Steven Weinberg in 1991 in the form of the “chiral effective field theory”. For many years the second ingredient seemed a distant prospect, but the enormous growth in computer power has recently allowed nuclear physicists to make some hugely exciting theoretical simulations.
In 2011, for example, a team led by Evgeny Epelbaum, from the Institute of Theoretical Physics II at Ruhr-Universität Bochum in Germany, performed the first calculation of the Hoyle state from first principles, using a computational method known as Lattice Monte Carlo simulation and a nuclear interaction derived from quantum chromodynamics – the theory of quarks and gluons – using Weinberg’s approach. Remarkably, they found a state with zero nuclear spin and positive parity about 7 MeV above the ground state that appears to be the Hoyle state. And only a few months ago, Epelbaum and colleagues published new improved calculations for the Hoyle state (2012 Phys. Rev. Lett.109 25201) in which the alpha-cluster structure emerges naturally from their calculations rather than having to be “put in by hand” as is usually the case. According to Epelbaum’s work, the alpha particles in the ground state of carbon-12 appear to be arranged in a compact triangle, whereas in the Hoyle state, they are in a bent-arm configuration (figure 1).
1 Carbon configurations Physicists have been trying for 60 years to figure out the nuclear structure of the Hoyle state, an excited state of carbon-12. (a) The earliest model, proposed in 1956, comprises a linear chain of three alpha particles. (b) In 2001 it was suggested that the Hoyle state can be viewed as a Bose–Einstein condensate, i.e. rather than being treated as three separate entities the alpha particles are described by one single wave function. Most recently, in December 2012 the ground state and Hoyle state of carbon-12 were calculated from first principles. In the ground state the alpha particles were found to be arranged in a compact triangle (c) whereas in the Hoyle state they were in a bent-arm configuration (d). (Courtesy: IOP Publishing)
As well as advances in theory, there have been important strides in experiment. Unfortunately, as we cannot use a “nuclear microscope” that lets us see directly what is going on inside the Hoyle state, we have to rely on indirect experimental evidence of its structure. Physicists have, for example, determined the excitation energy of the Hoyle state to a precision of 0.05% and they have measured a very rare decay mechanism seen in only seven in one million decays of the Hoyle state, where it relaxes into the carbon-12 ground state by simultaneously emitting an electron and its antimatter counterpart, the positron.
Nevertheless, 60 years on from the verification of the existence of the Hoyle state, experiments are only just beginning to provide discriminating tests of its true nature. By studying its properties in detail, physicists hope to figure out if the alpha-cluster description is the best model for the Hoyle state, and if so whether the alpha particles behave like a gas or a BEC.
One promising line of experimental inquiry relates to rotational excitations of the Hoyle state. This story dates back to 1956, when Morinaga conjectured that the Hoyle state might consist of a linear chain of three alpha particles, and that – just like a chemical molecule – it should therefore be possible for this structure to rotate. If this were to happen, it would lead to a set of quantized energy levels with spin and parities of 2+, 4+, 6+, etc. The spacing of these levels is related to the moment of inertia associated with the Hoyle state, which would tell us something about how compactly the alpha particles are arranged.
This was a tangible prediction and experimentalists quickly began looking for these “rotational excitations” of the Hoyle state. Sadly, more than half a century later the search is still on – showing just how hard the task is and the importance of the result in pinning down the structure of the Hoyle state. But efforts have not been abandoned; most people think the states exist but are just incredibly difficult to identify experimentally. In fact, the search is being conducted more vigorously now than ever before, with as many as 10 experiments performed in the past decade. An important recent result has been a candidate 2+ state identified by Martin Freer of the University of Birmingham in the UK and his team in an experiment conducted at the iThemba Laboratory for Accelerator-Based Sciences in South Africa.
A second strand of experimental endeavour has been to study the break-up of the Hoyle state into its constituent alpha particles. Recently, one of us (OK) led an experiment to scrutinize the break-up mechanism. It was already known that the Hoyle state preferentially decays to the ground state of beryllium-8, which subsequently breaks up into two alpha particles. We wanted to see if, on rare occasions, the decay to three alpha particles would proceed differently, bypassing the ground state of beryllium-8. We found that, at least 99.5% of the time, the Hoyle state decays via the ground state of beryllium-8. If an experiment with higher sensitivity could be designed, it might be possible to discover other, very rare, decay mechanisms. This could shed light on the current controversy regarding the nature of the Hoyle state – BEC or “ordinary” alpha-particle gas?
Understanding the origin and structure of the Hoyle state continues to pose a strong challenge to theory and attracts considerable experimental efforts. At last year’s international Cluster12 nuclear-physics conference, for instance, discussion of the nature of the Hoyle state was a major focus of the meeting and dominated the first day. Curiosity still abounds 60 years on from Fred Hoyle’s prediction of his eponymous nuclear state, as we strive to finally understand how carbon, and thus life, came to be.
Anthropic musings
Were it not for a particular excited state of the carbon-12 nucleus known as the Hoyle state, the universe would look very different. In particular, there would be essentially no carbon and hence life as we know it would not have evolved, nor would other heavy elements exist. In hindsight, it is remarkable that the laws of nature have conspired to create an excited state in carbon-12 that allows carbon to be produced in significant quantities in stars.
Some people have discussed this situation in terms of the anthropic principle. This is the point of view that the physical parameters of the observed universe must be compatible with our existence as conscious observers; if the physical parameters had been different in a way that prevented life from forming, we simply would not be around in order to measure them.
The situation becomes more remarkable still when one considers that the rate at which three alpha particles fuse into carbon is incredibly sensitive to the energy of the Hoyle state. The Hoyle state is 7.65 MeV above the ground state of carbon-12, but if it were a mere 0.06 MeV more or less, the abundance of carbon in our universe would be very different.
In 2010 Sylvia Ekström, an astrophysicist at the Observatory of Geneva, and colleagues used this property of the Hoyle state to test whether the strengths of electromagnetic and nuclear interactions might vary in time and space, rather than be constant as assumed. Ekström and her team observed abundances of carbon and oxygen on the surfaces of very old stars in the galactic halo, which were formed from interstellar dust only 200–300 million years after the Big Bang. This allowed them to determine by how much the energy of the Hoyle state could have been different back then. Since the energy of the Hoyle state ultimately depends on the interplay between the electromagnetic and nuclear interactions, they were able to relate this to a change in the interaction strengths. Their conclusion: there is no evidence that the strengths of the electromagnetic and nuclear interactions were any different in the early universe, and the most they can have changed by is 0.001% and 0.1%, respectively.
Cellular communication networks can be used to accurately predict large-scale rainfall distribution patterns in real time, according to researchers in the Netherlands. The team created rainfall maps for the whole of the Netherlands by using data from a mobile network in the country. This was based on measurements of the attenuation of microwave signal levels across 2400 network links over a four-month period. The resulting maps had a strong correlation with the same measurements taken by the conventional techniques of weather radar and rain gauges.
Predicting precipitation
The research was carried out by Aart Overeem and colleagues, from both Wageningen University and the Royal Netherlands Meteorological Institute. “Microwave links are used in telecommunication networks to transmit signals from the antenna of one telephone tower to the antenna of another telephone tower,” says Overeem, who led the study. “When it rains the signal is attenuated, which is noticed as a decrease of the received power measured by these telephone towers. The average rainfall intensity…can be computed from the decrease in power during rainy periods with respect to the power during dry periods.”
The team looked at the minimum and maximum received signal power at each telephone tower over 15 minute periods. Passing through falling raindrops absorbs part of the incident microwave transmission and causes minor beam scattering, lowering the power that ultimately reaches the receiving tower. The more raindrops in the beam’s path – or the larger the drops are – the more signal power is lost.
By comparing received powers for each network link with reference values for known dry periods – and factoring in accounts for humidity and the water films that can develop on the communications antennae – rainfall densities along each path can be calculated. These values are then treated as point measurements at the centre of each network link and used to extrapolate the larger rain distribution maps. In the frequencies employed in these links, attenuation caused by raindrops is the main source of power reductions, beyond free space losses. Typically, the microwave network links operate at least 10 m off the ground and use frequencies between 13 and 40 GHz.
Reasons for rain reporting
Measurements of precipitation rates are essential for agriculture, weather forecasting and the management of water resources and, in the long run, for climate-based studies. Also, rapid rain reporting can be vital in minimizing the loss of life in flood-prone regions. The researchers point out that their new technique is based on measurements that telecommunication companies often already take to monitor the stability of the microwave links in their networks. The team suggests that this approach – using pre-existing resources – could thus help to ease the reliance on the rapidly falling number of rain gauges worldwide.
It is estimated that the total number of rain gauges in Europe, Africa and South America has fallen by around 50% in the last 17 years. In contrast, the spread of mobile networks continues to increase and, while they are more comprehensive in urban areas, it was estimated that in 2007 they covered around 20% of the land-based surface of the earth.
Scarcity of measurements
“More than 25 years ago it was estimated that if you put all the world’s rain gauges together in a single place they would barely fill a football pitch,” explains Dominic Kniveton, a professor of climate science at the University of Sussex. He goes on to say that globally, the area covered by surface-based rainfall radar measurements would only be approximately 4%. “I can confidently state that little has changed since in this scarcity of measurements: this remains one of the major challenges to providing high-quality climate services.”
Kniveton told physicsworld.com that “the technique shown by Overeem et al. presents an exciting addition to efforts to measure rainfall. By virtue of coming from the cellular-phone networks these measurements potentially can be harnessed using fast hydrological models. With such techniques, and the rise of mobile networks, this opens a range of possibilities to help build the resilience of many to increasing climatic hazards with climate change”.
In addition, weather radar readings are often fine-tuned using local readings from gauges. This is especially useful at greater distances from the radar. In countries where radar is frequently used, communications network data thus have the potential to be combined with radar measurements for increased accuracy.
The team is to continue its research, Overeem reports. Future work aims to improve on the algorithms used to calculate the rainfall distribution by looking at communications network data over longer periods, such as across one whole year.
Physicists in the UK have come up with a new way of storing a handful of photons in an ultracold atomic gas, in which strong interactions between neighbouring photons can be switched on and off using microwaves. The team believes that the technique could be used to create optical logic gates in which single photons could be processed one at a time. The method could also prove useful for connecting quantum-computing devices based on different technologies.
Optical photons make very good “flying” quantum bits (qubits) because they can travel hundreds of kilometres through fibres without losing their quantum information. However, it is very difficult to get such photons to interact either with each other or with “stationary” qubits such as those based on trapped ions or tiny pieces of superconductor. Exchanging quantum information between such devices can therefore be tricky.
What Charles Adams and colleagues at Durham University have now done is come up with a way of storing individual optical photons in highly excited states of an atomic gas. Once stored, the photons can be made to interact strongly, before being released again. An important feature of the technique is that it uses microwaves, which are also used to control some types of stationary qubit.
Rydberg polaritons
The Durham experiment involves holding up to 100 rubidium atoms in an optical trap created at the focus of a laser beam, before two pulses of light are fired at the trapped atoms. One pulse is “signal” light that is to be stored and the other is “control” light. The control light allows 10 or so neighbouring rubidium atoms to absorb a signal photon, creating a collective state called a “Rydberg polariton”. Such a state is similar to that of a Rydberg atom, which has an electron in a highly excited state – in this case, with a principal quantum number of 60.
When the control pulse is switched off, the photon remains “stored” in a Rydberg polariton for as long as 1 μs. But if the control light is switched on again, the Rydberg polariton is converted back into light, re-emitting the photon that it had held.
Adams told physicsworld.com that the team used Rydberg polaritons – rather than Rydberg atoms – because there is strong coupling between a photon and a Rydberg polariton. This is because the polariton contains many atoms rather than just one. It is therefore much more likely that the photon will be captured and stored in their set-up. Another benefit of a Rydberg polariton is that it will only absorb one photon – and no more.
Micro-spheres in a row
Each Rydberg polariton can be thought of as a 7 μm diameter sphere. The atomic cloud, meanwhile, is only about 30 μm long and has a diameter of about 6 μm – which means that it contains a line of about three Rydberg polaritons in a row. In practice, however, not all of these polaritons will contain a photon, and photons are able to hop between the polaritons. Adams explains that in this case, photons can be lost and are therefore not recovered when the control pulse is switched on again.
However, if a microwave signal is applied to the cloud it creates an interaction between neighbouring polaritons that prevents hopping from occurring – and therefore photons are not lost but rather are recovered when the control pulse is switched back on.
Next step, logic gates
The team believes that this ability to control interactions between adjacent polaritons could be used to create logic gates for single photons. Instead of making three polaritons in a row, this could involve making a Y-shaped junction in which the output of one polariton would be determined by the presence of absence of photons in the other two polaritons.
According to Adams, this would require a new experimental set-up with two focused laser beams and a larger atom cloud – something that the team is looking at creating.
Alex Kuzmich of the Georgia Institute of Technology in the US says that the Durham team’s ability to create strong interactions between single photons makes the work “an important advance”. He adds that the research “breaks new ground on the way towards realization of quantum logic for photons”.
The experiment will be described in an upcoming issue of Physical Review Letters and a preprint is available on arXiv.
