Researchers at CERN are renowned for their musical side-projects. Notable examples include the album released by scientists at the ATLAS detector in 2010, and the “Large hadron rap“, which currently has almost 8 million hits on YouTube. And of course don’t forget the pop-star-turned-physicist Brian Cox who had the UK chart-topping hit “Things can only get better” in the 1990s with his band D:Ream.
Following in this musical tradition, a duo of Mexican researchers has invented a “Cosmic Piano” inspired by the technologies used at the ALICE particle detector at the Large Hadron Collider (LHC). The instrument’s inventors Arturo Fernández Téllez and Guillermo Tejeda Muñoz hold positions at CERN and the University of Puebla in Mexico. They hope the device can demonstrate both the science and the art of the work being carried out at particle-physics facilities.
I’ve always wondered what triggered the great ideas. When I was a child, I often imagined that somewhere out there, there was a superior being whispering secrets to the chosen few – to those humans lucky enough to gain some special new insight into the world. Indeed, this view seemed to fit with stories such as Newton’s apple or Archimedes’ “Eureka!” moment, in which a serendipitous event precipitates some sudden epiphany. So notorious are these tales that they’ve become myths, used as a shorthand for those moments when our understanding of the world advances to a new level.
When we tell and retell the fall of that fateful apple, or of Archimedes’ bathtub revelation, it sometimes feels as if we are saying that this is how all scientific discoveries take place. But is it really? The truth is that science is built upon many untold smaller discoveries. But what do these smaller discoveries look like, and what leads each individual scientist to their own revelatory moment?
The nuclear question
These questions re-surfaced for me a few years ago during a conversation with artist Ana Sousa Carvalho. We were talking about scientific curiosity and its consequences. At a certain point in our discussion, she asked me if knowing everything we know now – and if given the chance – I would have helped to build the nuclear bomb. This was, of course, a hypothetical question, and my initial response was, “Yes, just out of curiosity.” My gut feeling was that I’d want to show myself that I could crack the problem.
On reflection, however, I became annoyed with myself for having had such a reaction and so I decided to research the history of the atomic bomb to find out more about the decisions the physicists involved faced at each step. While going over a few books, my attention drifted to the technical details, and I began to think that similar ideas could be used in gravitational physics. Well, this incursion eventually gave rise to new work and a scientific paper on instabilities in black-hole systems. This was also the first time in my role as a scientist that I found myself thinking in a serious manner about the way in which ideas come to life.
At the same time, Ana and I had been talking about the similarities between creativity in art and science. After some back and forth, we hit upon the idea of creating a repository of essays written by scientists describing the genesis of their ideas. It was our hope that these texts would eventually grow into an archive that would, with time, take the shape of an informal history of contemporary science, captured from a personal perspective. We felt that the project could be of interest not only to the scientific community, but to students and the general public as well. We called the project The Birth of an Idea) and set about asking the science community to share stories with us.
Our approach was very simple: to write to colleagues and ask them to write a short essay on the genesis of their ideas. It was important for us to emphasize that we weren’t interested solely in tales about breakthrough discoveries. We were also after the trivial and the ordinary, the small victories and the long struggles. As a physicist, I am well aware that this is the territory where most of us toil. It took a while for the project to get off the ground, but we put our focus on the physics community and so far the response has been amazing – we now have a collection of 64 essays in total, from physicists and astronomers in a diverse range of research fields and from countries the world over. We usually send out five requests a month and right now we have a success rate of roughly 30%. Unquestionably, most of the work lies on the contributing authors, who have been extraordinarily generous with their time and experiences.
Just have a bath
Reading the essays has been a rewarding experience – going over these first-hand accounts is like being granted a behind-the-scenes view of physics. From Roberto Emparan, we learn how the association between black-hole entropy and entanglement entropy was born while he put his son to sleep. Pauline Gagnon reveals in her account “Only women could think of it” the surprising way in which she and a female colleague managed to fix a detector at CERN. In some other great examples, Michele Vallisneri recounts how the work of Richard Feynman helped him to optimize gravitational-wave detection, and Andreas Warburton describes the discovery of the top quark. Two examples are given in full below.
Above all, these essays give the reader the human perspective. Each, in its own way, offers a glimpse into an incredible world that usually remains unseen. For instance, Eric Poisson’s contribution does a beautiful job of rendering the restlessness that comes with the struggle to understand a problem: “That night I walk the streets of downtown Milwaukee in a state of agitation. There is something wrong somewhere, and I have to get to the bottom of it.”
We hear from colleagues that they have their best ideas walking home or in bed or in the shower, and we can’t help but nod in agreement while reading Masaru Shibata when he tells us, with the concision of a haiku: “In my experience, ideas often come to me when I am taking a bath. Thus, I recommend taking a bath every day.” Perhaps there was something to Archimedes’ approach after all.
There are moments of elation, as in this experience described by Shahar Hod: “It was incredible to know that, at that moment, I was the only person in the Milky Way who knew the simple truth about the quantization of the black-hole horizon area: k = 3!” But most often, and perhaps by virtue of what Paolo Pani identifies in his essay as the sadistic nature of science, we watch these scientists as they confront the despair of witnessing their ideas die many deaths before they are finally allowed to get their hands on the reward.
Collective creativity
I had many personal conversations with colleagues who did not contribute because they claim that none of their ideas are original but stem from other ideas already in existence. These people make up a considerable fraction of the community and say that their work relies on conversations with colleagues, meetings and workshops. In fact, even some of the colleagues who did send in their contributions assert that the majority of their work gets done in groups, by talking to other colleagues. I find this fascinating, because it is a testament to the existence of a kind of collective creativity – a spontaneous brainstorming that happens when scientists meet.
This concept contradicts the widespread notion that science advances only in big leaps made by an individual, through new big ideas that materialize out of the blue before instantly crystallizing into their final form. As Nicolas Yunes puts it in his essay: “The image of the lonely genius with her or his ‘Aha!’ moment is an illusion. The birth of an idea is much more of a community activity than we sometimes care to acknowledge.”
Courtesy: Adoplhe Millot, Nouveau Larousse Illustré (1897–1904)
An idea is a neat little thing. Ideas can easily show up, uninvited, and disappear without warning. They are of such a fleeting nature that sometimes they seem to lead a life of their own.
For me, more interesting even than an idea, is the glimpse of an idea. Some ideas are big and clumsy, and what they lack in subtlety they compensate for with persistence. They look at us in the face until we look back. But others are far more elusive; we only feel they should be there. We look at something and think “well, that’s funny” or “this was not supposed to happen”, and frown or make a funny face. For a brief moment, the chaos of the world seems not to enter our ears as loudly as before, and we hear little gears grind in our head. When that happens we are at one end of a thread that, once followed, will take us to the feet of a new idea.
But too often we are too busy to care. Or simply forget, somehow, that an idea is something worth searching for, and only get the big, loud and persistent ones. My advice on the subject is: practise finding new ideas. Next time you feel the funny feeling and hear your gears grinding, focus. Do not let the moment fly away, unnoticed and unused. The thread is before you.
Do not wait for the ideas to come, go after them, and with a club. You will miss the first few, but that doesn’t matter. Experience will render your senses more acute, and practice will sharpen your hunting. Learn to examine your ideas against the background of evidence you have gathered, so that you can tell good from bad, useful from useless. As you do this, you will find not only a way of collecting good ideas, but the person you are will be changed by the quest.
And when you are struggling, remember: the truth will set you free, but first it will piss you off.
Pedro Figueira is a researcher at the Centro de Astrofísica da Universidade do Porto, Portugal
A journey by Djordje Minic
(Courtesy: Shutterstock/Redshinestudio)
I would like to recall the foggy and emotional beginnings of an ongoing journey regarding the foundations of quantum gravity and string theory.
The idea that quantum gravity, in the guise of a novel formulation of string theory, should represent a new framework for physics that goes beyond (and also sheds light on) the current framework based on quantum theory (as well as its puzzling relation to the classical world), presented itself to me in a rather vague form in the fall of 1997 as I was moving from Chicago to State College.
I still remember the initial, almost tactile, sensation of excitement and elation, as well as the feeling of dread, of profound fear at being completely wrong and deluded. These initial contradictory emotions have since then become almost an obsession as well as a concrete research programme.
The most important aspect of the flowering of this initially misty notion is that many friends and collaborators have provided at least partial sanity checks to the original intuition. Perhaps the most exciting and concrete realization of the idea that quantum gravity/string theory is a new framework for physics has been realized in my recent work with two dear friends: Laurent Freidel and Rob Leigh.
It is still not clear where this journey will take us, but it has so far been a wonderful example of Antonio Machado’s “Traveller, there is no path/The path is made by walking.”
Djordje Minic is a professor of physics at Virginia Tech, US
The first “loophole-free” measurement of the violation of Bell’s inequality by a quantum system has been claimed by physicists in the Netherlands, Spain and the UK. Their experiment involves entangling spins in diamonds separated by 1.28 km and then measuring correlations between the spins. The large separation between the diamonds and the relative ease with which the spins can be measured ensures that the experiment is performed properly and its result confirms the existence of the seemingly bizarre concept of quantum-mechanical entanglement.
The idea of entanglement first arose back in 1935, when Albert Einstein, Boris Podolsky and Nathan Rosen pointed out that two quantum particles such as electrons can be in a state in which a measurement on one particle instantaneously affects the other – no matter how far apart they may be. This apparent paradox upset the trio because, in the world of classical physics, it would require information to travel faster than the speed of light. This relationship between particles was later dubbed entanglement and subsequent work showed that entanglement can be determined by looking at correlations between measurements made on the two particles, such as the direction in which the two electrons are spinning. Entangled particles have much stronger correlations than are allowed in classical physics – a property that can be exploited in quantum computers and other quantum technologies.
Upper limit
In 1964 the Northern Irish physicist John Bell famously calculated an upper limit on how strong these correlations could be if they were caused by classical physics alone – what has become known as Bell’s inequality. Correlations stronger than this limit, Bell reasoned, could occur only if the particles were entangled. Experiments using photons, ions and other entangled particles have confirmed that Bell’s inequality is indeed violated. However, these experiments are plagued by one or more loopholes that allow unforeseen effects of classical physics to cause the violation.
In this latest work, Ronald Hanson and colleagues at the Delft University of Technology, along with researchers at the Institute of Photonic Sciences in Barcelona and the diamond-maker Element Six in Oxford, have eliminated what they consider to be the two most significant loopholes that can arise in Bell-violation experiments. Crucially, they have done so simultaneously in one experiment, which had not been done before.
Channels unknown
One is the “locality” loophole, whereby information about the measurements is exchanged between detectors via unknown classical communication channels – thereby increasing the apparent correlation between the particles. Because this communication is classical, it cannot be transmitted faster than the speed of light and therefore this loophole can be closed by increasing the separation distance between the particle detectors and/or reducing the time it takes to make the measurement so that communication is impossible.
The second is the “detection” loophole, whereby an experimentalist is fooled into thinking a large correlation exists because an unknown aspect of the experiment causes it to favour the detection of particles with large correlations over those with small correlations.
Best of both particles
The locality loophole is easily eliminated by using photons as quantum particles, because photons are able to travel many kilometres without being scattered or absorbed. However, it is very difficult to detect each and every photon in such an experiment, which leaves it open to the detection loophole. Conversely, experiments involving electrons suffer from locality problems because they cannot be done over large distances. However, electron experiments can beat the detection loophole because electrons can be more reliably detected. What Hanson and colleagues have done is to use both photons and electrons in their experiment.
Their set-up consists of two diamonds separated by 1.28 km. Each diamond has a single nitrogen vacancy (NV) centre, which is essentially an electron spin. The measurement process begins with each NV centre emitting a photon that is entangled with its parent NV electron. Both photons travel to a third location that is hundreds of metres away from both diamonds. There, the photons are detected and when this measurement occurs, the NV electrons become entangled in a process called “entanglement swapping”. The next step is to quickly measure the spin states of the two electrons, which is done using a very efficient fluorescence technique.
The team ran 245 trials of the Bell test over a total measurement time of 220 h and found a very strong violation of Bell’s inequality. Furthermore, the team calculates that the large separation between the two diamonds and the rapid readout time of the spins closes the locality loophole, while the high efficiency of the spin readout technique closes the detection loophole.
No more freedom of choice
While the team points out that no Bell experiment can be free of every conceivable loophole, the researchers say that their experiment places the strongest restrictions to date on classical theories of quantum entanglement. They also say that their experiment could be modified to close more exotic loopholes such as “freedom of choice”, whereby unbeknown to the experimentalist the design of the experiment is somehow limited in a way that boosts the measured correlations.
The experiment is described in a preprint on arXiv. Update: The research was published in the journal Nature on 21 October 2015.
A new technique that accelerates positrons much more efficiently than conventional particle accelerators has been unveiled at the SLAC National Accelerator Laboratory in the US. The technology has the potential to make future positron accelerators more powerful yet more compact, and could also be used to boost the maximum collision energy of existing electron/positron colliders.
Some particle physicists believe that the next big facility after the Large Hadron Collider (LHC) should be a high-energy lepton collider that smashes electrons and positrons (antielectrons) together. Such a machine would produce cleaner, easier-to-interpret collisions than a hadron collider, and would create a far greater proportion of new particles per collision. To reach a high enough energy, such an electron/positron collider would have to run in a very long, straight line. This is because conventional accelerator technology using radiofrequency electromagnetic field cavities has a maximum energy gradient of about 100 MeV/m. The proposed 0.5 TeV International Linear Collider (ILC), for example, is expected to be about 30 km long, including two 11 km accelerator sections. For this reason, accelerator physicists are trying to develop new ways to accelerate electrons and positrons so the particles reach higher energies over shorter distances.
One such method is “plasma wakefield acceleration”, which was first demonstrated in 2007 and involves firing bunches of electrons into a plasma. An initial “drive” bunch repels the free electrons in the plasma, and this creates a charge-density wave. A second, trailing bunch of electrons “surfs” this wave and gains energy very rapidly. In 2014 Sebastien Corde and researchers at the SLAC National Accelerator Laboratory in California, and international colleagues, accelerated electrons through a gradient of 4.4 GeV/m using this method. Unfortunately, this technique cannot be applied directly to the acceleration of positrons because there is no practical way of creating an “anti-plasma” containing free positrons.
One bunch, not two
In this latest research, the same team has modified its technique to allow positrons to be accelerated. The process begins with a single bunch of positrons from a conventional accelerator that is injected into a lithium plasma. Under the right conditions, the positron bunch will interact with the plasma, causing the front portion of the bunch to behave like the drive bunch in an electron accelerator. The front portion of the bunch will slow as it feeds energy into the plasma electrons. Meanwhile, the back end of the same bunch plays the role of the trailing bunch, drawing energy back out of the plasma and being accelerated.
“The overall energy of the bunch is obviously not going to be increased, because energy must be conserved,” explains Corde. “We are just transferring energy from the front to the tail. What’s important for particle colliders is that each particle has a very large energy.” The team has dubbed the technique “self-loaded plasma wakefield acceleration”. The team boosted the energy of the positrons at the end of the bunch by 5 GeV/m.
Multi-stage acceleration
Today, the researchers believe the plasma-wakefield technique could double the energy of particles in a conventional accelerator, allowing particles in the ILC to reach 1 TeV before collision. Further optimization may boost this multiplication factor, perhaps as high as five. Ultimately, it may be possible to construct a multi-stage plasma wakefield accelerator, in which the same bunches of particles could be accelerated multiple times. However, as only a fraction of the positrons are accelerated in each stage, simply separating out the accelerated positrons over and over again would rapidly produce a very small bunch. The researchers therefore aim to separate the accelerated trailing bunch from one stage and manually load it into the back of a fresh bunch of positrons. “We have a good idea that it could work but it’s also a technically challenging experiment,” says Corde. “We will work on that.”
“The electron was relatively easy to accelerate, but the positron was actually a big deal,” says beam physicist Philippe Piot of Northern Illinois University in the US, who says the research is “the first experimental proof” that plasma wakefield acceleration can accelerate positrons. He says that more research is needed into the scattering of both electrons and positrons during this type of acceleration before wakefield acceleration can be applied in any accelerator. “There are still a lot of issues, but given the progress that has been made over the last decade in this type of acceleration, I personally would be optimistic,” he says.
The acceleration technique is described in Nature.
Humans have been trying to keep things cold for a very long time. As far back as the 18th century BC, the Sumerian ruler Zimri-Lim was ordering subordinates to build him an icehouse. The story of Zimri-Lim’s chilly construction (and the finicky tastes of one of his rivals, who demanded that ice be washed “free of twigs and dung and dirt” before being added to drinks) is just one of many engaging anecdotes in Tom Jackson’s book Chilled: How Refrigeration Changed the World and Might Do So Again. As Jackson explains, there is some fascinating science as well as history in the various mechanical, chemical and physical methods that people employed to make objects cooler before the advent of modern refrigeration. Jackson’s integrated approach to his subject is as refreshing as a cool beverage on a hot summer’s day; however, a few of his tales are rather tangential to the main story, and at times it also seems that he has bitten off more than he can chew. The chapter on the complex chain of technologies required to keep cold food flowing to Western supermarkets is fascinating, but it could have been a whole book in itself. Meanwhile, some more recent milestones in the history of cold are covered too briefly to do them justice (Bose–Einstein condensation, for example, is dealt with in a mere four pages). There are a couple of factual lapses too, as when Jackson repeats the myth that ordinary wine and beer are alcoholic enough to kill “most” of the germs in them. (Try leaving a pint of beer out for a few days, uncovered, and watch what happens to it.) These criticisms aside, though, his tale of how scientists both famous (Isaac Newton) and less well known (William Cullen) have grappled with the nature of cold and temperature makes enjoyable and not-too-heavy reading.
2015 Bloomsbury Sigma £16.99hb 272pp
Security breach
In the early hours of 28 July 2012, an unlikely trio of saboteurs – two men in their late 50s or early 60s and, most famously, an 82-year-old nun – broke into Y-12 National Security Complex in Oak Ridge, Tennessee and walked unhindered up to the building that houses America’s stockpile of weapons-grade uranium. How did they get there? The question has both a philosophical answer and a practical one, and in Gods of Metal, the investigative journalist Eric Schlosser begins with the former. The three saboteurs were, he explains, heirs to a decades-long tradition of anti-nuclear activism among a small but dedicated group of radical US Catholics. They hoped that their protest would help hasten the end of the US nuclear-weapons programme. As for the practical answer, Schlosser details how the trio’s entry to the so-called “Fort Knox of Uranium” was made possible by months of careful planning, a pair of bolt cutters and a cavalcade of security lapses that would be laughable if the implications were not so serious. Schlosser’s analysis of these lapses makes up the heart of his story (which was originally published as an article in the New Yorker and has been extended only slightly in book form). Security at Y-12 has been tightened considerably since the incident, and the private companies responsible for some of the worst failings have not had their contracts renewed. That, however, is small comfort to those whose nuclear-security concerns centre on terrorism rather than war. As Schlosser warns, “If terrorists manage to steal weapons-grade uranium or plutonium from a Department of Energy facility because of a contractor’s mistakes, the firm responsible for the security breach stands to lose its contract. The United States could lose a city.”
2015 Penguin £1.99pb 128pp
Nerding out
For evidence that science is having a “moment” in pop culture, one need look no further than the Festival of the Spoken Nerd. This comedic trio – Helen Arney, Steve Mould and Matt Parker – have been touring the UK with their brand of science comedy for five years, and in the DVD version of their show (titled, drolly enough, Full Frontal Nerdity) they attempt to answer a time-honoured question: what is a nerd? Is it someone who does experiments? Someone who devotes a lot of time and effort to mastering an obscure skill? Or is it someone who is entirely too fond of Excel spreadsheets? In the DVD (recorded in early 2015 during two of their live shows in London), each member of the group takes one of these definitions and runs with it. Arney’s speciality, for example, is scientific song parodies, while Mould does mildly unsafe-looking experiments with household objects and Parker gets enthusiastic about graphs. It’s all good fun, and considerably less laboured than it sounds on paper, although (as with a lot of science outreach events) one does get the impression that their audience has self-selected from a fairly geeky segment of the population. Even if they are preaching to the choir, their message is fun to hear – and if you live in the UK, you can catch it in person this autumn as their new live show, Just For Graphs, begins touring.
How and where do new ideas in physics emerge? We often think they arise serendipitously, which is why we love stories like Newton discovering gravity after seeing an apple fall. The reality, though, is often very different.
Writing in the September 2015 issue of Physics World magazine, which is now out, theoretical physicist Vitor Cardoso from the University of Lisbon explains his efforts to find out how breakthroughs – both big and small – really emerge. As he discovered through his project The Birth of an Idea, it turns out that how new thoughts arise is often much more of a communal activity than we might think.
In the dry scrubland of eastern Washington State, a few miles from what was once America’s premier plutonium factory, sits a massive laboratory, its two long arms stretching off into the distance. On windy days, tumbleweeds roll by, piling up against the arms’ concrete housing and creating headaches for the lab’s maintenance workers. Inside, though, there is a buzz of activity as scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) prepare for the most exciting period in the facility’s 14-year history. Later this month, they will begin observations with an upgraded machine, new instruments and a corresponding sense that this time, when they go on a gravitational-wave hunt, they’re going to catch a big one.
Gravitational waves were predicted by Albert Einstein in 1916 as a consequence of the field equations of his general theory of relativity. These 10 coupled nonlinear equations formulate the universe as the dynamic interplay between mass–energy and space–time. As the physicist John Wheeler put it, “Matter tells space how to curve, and space tells matter how to move.” One prediction of the general theory is that when massive bodies move around, they cause the fabric of space–time to warp, generating ripples that propagate outwards at the speed of light. These ripples are known as gravitational waves, but they are not the familiar sinusoids found in electromagnetism. Instead, they stretch space in one direction perpendicular to the line of travel, while simultaneously compressing it in the other – a bit like lips puckering up and down for a kiss.
No prediction made by Einstein’s equations has ever been proved wrong, and in the 1970s observations of a binary pulsar – a rapidly rotating neutron star in orbit around another neutron star – strongly suggested that gravitational waves do indeed exist (see “Pulsar detectives” below). However, nobody has ever detected such waves directly, despite decades of trying.
LIGO was built to change that. From 2002 until 2010, laser beams travelled from the lab’s hub down its two long, perpendicular arms, where they reflected off huge, hanging masses and were recombined back near their origin. The idea was that a passing gravitational wave would cause the masses to move enough for the length of the arms to change and produce a detectable phase shift in the interference pattern of the recombined laser beams. On a handful of occasions during LIGO’s first period of experimental operations, researchers thought they had spotted such a shift – only for the supposed signal to be revealed as noise or, in one case, a deliberate fake generated by researchers within the collaboration, as a test of their internal data-checking procedures.
Now, however, the LIGO facility near Hanford, Washington – along with its twin in Livingston, Louisiana – is entering a new era. In March contractors completed a $221m upgrade of the dual facilities that improved their ability to detect the feeble waves of gravity by a factor of 10. Thanks to this upgrade – known as Advanced LIGO, or aLIGO – researchers should be able to detect gravitational waves that originate anywhere within a sphere of about 420 million light-years in radius, centred on the Earth. That is still only a small fraction of the total universe, but it’s a thousand-fold increase (by volume) on what was possible before the upgrade. As the upgraded system kicks into high gear and achieves its designed sensitivity in 2016 or 2017, the scientists at LIGO are quietly confident that they will see something real.
Only an attometre
The recently completed aLIGO upgrades all have the same goal: reducing noise. Noise poses challenges for many physics experiments, of course, but for the dual LIGO machines and their interferometric kin, the problem is particularly acute. Although gravitational waves come from some of the most massive and energetic systems in the universe (such as a pair of black holes or neutron stars orbiting one another) their amplitudes are exceedingly small by the time they reach Earth. In fact, a passing gravitational wave is expected to change the length of LIGO’s 4 km-long arms by only a few attometres (10–18 m) – around 1000 times less than the diameter of a proton (see “How LIGO works” below).
To ensure that the observatory can detect such a tiny change, almost every aspect of LIGO has been upgraded. For starters, a new isolation system has been installed to keep seismic noise (caused, for example, by passing trucks or tiny earthquakes) negligible across the range of frequencies that interest would-be wave observers. The dual US facilities are vital here, since noise seen in one facility but not the other can be ruled a local hiccup, not a passing gravitational wave.
Quiet room Putting the new seismic isolation system in place. (Courtesy: LIGO Laboratory)
At higher frequencies, though, the performance of the LIGO detector is limited by shot noise, which arises from the quantum nature of light. Basically, the number of photons produced by the laser fluctuates with time, creating a degree of uncertainty in the amplitude and phase of the beam. Increasing the laser’s power mitigates this problem somewhat, because the signal produced by a passing gravitational wave varies in proportion to the power, while the shot noise is proportional to the square root of the power. Accordingly, aLIGO boosts the power of the facility’s laser by more than an order of magnitude, from an initial input of 10 W to about 200 W.
That, however, creates a new problem. Each of those laser photons packs a tiny momentum punch and, collectively, they generate enough radiation pressure to make the masses at the end of each arm twitch ever so slightly. To counteract this, the masses have been beefed up: the aLIGO masses are both larger in diameter (34 cm instead of 25 cm) and more massive (40 kg instead of 11 kg) than before. That reduces their radiation-pressure-induced motion down to a level comparable to the thermal noise in the wire that suspends them – noise that has itself been reduced by replacing the old steel wire with fused silica fibres.
Putting the squeeze on
To really get a handle on shot noise, though, you need ingenuity as well as more powerful lasers and bigger masses. This is where the expertise of physicists like Sheila Dwyer comes in. A postdoctoral researcher at LIGO’s Hanford site, Dwyer began working at the lab in 2010, when she was a PhD student at the Massachusetts Institute of Technology. Her graduate work (conducted with the astrophysicist and gravitational-wave detection specialist Nergis Mavalvala) spanned quantum optics, quantum measurement theory and gravitational-wave detection, and it prepared her to play a central role in one of the most important aLIGO upgrades: the switch to a laser that emits light in an exotic form called a “squeezed state”.
Like all particles, the photons in squeezed light obey the uncertainty principle: the product of the uncertainties in any two complementary properties (such as amplitude and phase) always equals or exceeds ℏ/2. What makes squeezed light special is that the uncertainty in one of these variables has been “squeezed” down, while the uncertainty in the other is correspondingly allowed to balloon upwards. At LIGO, the fluctuations in phase are squeezed, so that the phase shift of the recombined beam can be measured more precisely. Of course, this means that the fluctuations in the beam’s amplitude become relatively large – making it all the more important that the mirrored masses used in aLIGO be as heavy as is reasonably possible.
Dwyer explains that LIGO is not the first gravitational-wave interferometer to use squeezed light. That honour belongs to the GEO600 experiment in Sarstedt, Germany, which began working with squeezed light in 2011 and now uses squeezing in its normal operations. Physicists there found that at certain frequencies (around 3 kHz), squeezing reduced quantum noise by a third, increasing the machine’s detection rate of gravitational waves in that frequency band by a factor of (3/2)3 (about 3.4). For LIGO, Dwyer showed in her doctoral thesis that using squeezed light would increase detector sensitivity by 80%, leading to an improved detection rate of almost a factor of six.
Knowing what to look for
While experimentalists have focused on upgrading LIGO’s physical components, theorists have been improving their understanding of what a gravitational-wave signal might look like. To help identify a passing wave, numerical relativists have calculated the waveforms that several of the most likely sources of gravitational waves are expected to generate. Thanks to their efforts, any signal registered at LIGO can be compared with about 10,000 expected waveforms for gravitational waves created by binary neutron stars, around 100,000 created by binary black holes and on the order of a million waveforms from neutron-star–black-hole binaries. This last number is larger than the others because the black hole’s angular momentum can couple to the orbital angular momentum of the binary pair, producing a much more complicated waveform (see “Relativity’s new revolution“).
In addition to building the database of expected waveforms, some researchers have been developing algorithms that look for gravitational-wave signals from other, unimagined sources. This type of search (known as a “burst” search) does not make any assumptions about the waveforms it is looking for, explains Duncan Brown, a gravitational-wave astronomer at Syracuse University in New York. “That way, when the universe goes bump in the night, LIGO will feel it,” he adds.
Hanging out Testing the suspension of one of the inner optical cavity mirrors. (Courtesy: LIGO Laboratory)
Astronomers have also worked out how many events aLIGO might expect to see as it approaches its design sensitivity. Binary neutron stars are considered the most promising source of gravitational waves, and (based on observations made with conventional telescopes) researchers estimate that aLIGO could see up to three binary-star coalescences – events in which two stars merge to form a single body – in its first year of operations. In its second year, as further technical improvements expand the fraction of the universe being observed, it might see a further 20 coalescences, and perhaps as many as 200 after four years. But it could also see none at all. Although that would be a surprise, it is possible that noise could, despite the upgrades, drown out a gravitational-wave signal; that binary-star coalescences could occur less often than astronomers think they do; or even that the strong-field, nonlinear gravitational regime, where coalescences finish, lies beyond general relativity.
The collapse of a star’s core as it becomes a supernova would also generate gravitational waves, unless the collapse is spherically symmetric. However, the maximum amount of gravitational-wave energy expected in such an event is much smaller than from binary coalescences (by a factor of at least 107). This means that gravitational waves from a core collapse may be seen only if the collapse happens in our cosmic backyard: within the Milky Way or its smaller, satellite galaxies – the Large and Small Magellanic Clouds.
The era of ‘multimessenger astronomy’
Whatever its cosmic source, the first direct detection of a gravitational wave will be big news. It will confirm a prediction from general relativity, but more importantly it will also give astronomers, astrophysicists and gravity theorists entirely new information about the objects they study. Indeed, astrophysicists hope that gravitational-wave observatories will someday operate as routinely as optical telescopes do today. If that happens, gravitational waves could fundamentally alter our picture of the universe, just as radio-wave and X-ray astronomy altered it from the placid, silent galaxies Edwin Hubble observed at visible wavelengths to the raucous universe we know today, full of quasars and pulsars, black holes and neutron stars. At some point, it may even be possible to observe cosmic events such as supernovae with light-based telescopes, neutrino detectors and gravitational-wave observatories – a new type of science dubbed “multi-messenger astronomy”.
More prosaically, a detection at LIGO or another ground-based observatory could also pave the way for a more ambitious successor facility. Dwyer, the LIGO postdoc, is part of a small group working on plans for the next generation of detectors. An underground observatory with arms 40 km long could bring another factor of 10 in sensitivity, she says, and it could, in theory, detect gravitational waves generated a mere billion years after the Big Bang – corresponding to a region of space–time far bigger than aLIGO’s current sphere of sensitivity.
Still more ambitious are plans to put a gravitational-wave observatory in space. Designs for the European Space Agency’s eLISA project, for example, call for three satellites arranged in an “L” shape so that each “arm” – consisting of empty space – is 1 million km long. The separation distances would be monitored to detect gravitational waves at frequencies between 0.03 mHz – below which the spacecraft are buffeted by fluctuations in solar radiation pressure, solar wind and cosmic rays – and 100 mHz. Within this range, expected gravitational-wave sources include galactic short-period binary stars and supermassive black-hole binaries. Once gravitational-wave detection is routine on Earth, new ideas, even beyond eLISA, will surely abound. The sky – no, the universe – is the limit.
Pulsar detectives
Pulsar pair Artistic illustration of two rotating neutron stars beaming out radiation. (Courtesy: Michael Kramer, Jodrell Bank)
The existence of gravitational waves was implied in a beautiful series of observations made more than 30 years ago by Joseph Taylor Jr and Joel Weisberg, utilizing a new type of pulsar discovered by Russell Hulse and Taylor in 1974. The pulsar PSR B1913+16 is a rapidly rotating neutron star that emits electromagnetic radiation, in orbit around a neutron star that was not seen to pulse. The general theory of relativity predicts that such a system will radiate energy, E, at a rate of
where m1 and m2 are the masses of the two bodies orbiting one another in circular orbits a distance r apart. (The calculation and expression for an elliptical orbit is a bit more complicated.) Note that the radiated power, P, is negative because the system is losing energy as the two masses spiral in towards one another.
For the Earth–Sun system, this energy loss rate comes to a feeble 200 W (less than used by a toaster), and a related calculation shows that the distance between them changes by only 400 fm (10–15 m) per year. But for a binary pulsar system such as PSR B1913+16, the rate of energy loss is almost 1025 W – equivalent to about 2% of our Sun’s output of electromagnetic radiation. By monitoring the system over several years, Taylor and Weisberg found that the stars’ separation was shrinking rapidly, by about 2 cm per day. More importantly, their observations showed that the cumulative shift of the stars’ orbital periastron (the point where the stars are closest together) decreased in a way that almost exactly followed the predictions made by general relativity. Subsequent work found an even tighter agreement.
Hulse and Taylor were jointly awarded the 1993 Nobel Prize for Physics for their discovery of the first binary pulsar. The Taylor and Weisberg result, more than anything, convinced physicists that gravitational waves exist.
How LIGO works
Inside LIGO Schematic of LIGO Laboratory. (Courtesy: IOP Publishing)
LIGO and other interferometric gravitational-wave observatories (such as VIRGO in Italy, GEO600 in Germany and KAGRA in Japan) consist of two long arms built at a right angle to each other. At the end of each arm hangs a highly polished “test mass” that acts as a mirror for a laser beam that is split at its source, with separate beams reflected down each arm. If a gravitational wave from a distant source washes through the detector, it will change the distance between space–time points in the detector ever so slightly, producing an alteration in the length of the interferometer’s arms.
The magnitude of this length change ΔL will be the arm length (4 km in LIGO’s case; 2 km for VIRGO and KAGRA; 600 m for GEO600) multiplied by a dimensionless strain factor h ∼ (GM/Dc2)(v2/c2), where M is the mass of the system generating the gravitational wave, v the characteristic velocity of the system’s components (such as two black holes orbiting one another) and D its distance from the detector. The value of h varies inversely with the distance of the source, but with the most likely sources – coalescences of binary stars in nearby galaxies and superclusters – h is expected to be of the order 10–21. Hence, to detect a gravitational wave, LIGO needs to be able to measure a change in the length of its arms of about 4 × 10–18 m.
To accomplish this remarkable feat, gravitational-wave astronomers rely on some sophisticated optics. In each of LIGO’s two arms, the beam is reflected up to 400 times in a Fabry–Perot cavity, travelling a total distance many times the facility’s arm length. The two beams are then recombined at a photodetector, which measures the phase difference of the two beams. The change in the light travel time for each beam will be Δt = 2(ΔL/c) = 2hBL/c, where B is the number of bounces, creating a phase shift ΔΦ = (2π)f Δt = 4πhBL/λ – about 10–9 radians, where f is the laser frequency and λ its wavelength.
In its initial phase, LIGO was designed to detect gravitational waves with frequencies from about 40 Hz to the highest gravitational-wave frequencies expected, 10,000 Hz, with its strain sensitivity lowest at about 100 Hz. The aLIGO upgrades improve on this by shifting the observatory’s lowest detectable frequency down to 10 Hz, and boosting its strain sensitivity by a factor of 10, to values of h below 10–22. (Below about 1 Hz, even small seismic vibrations and inhomogeneities in the Earth’s gravitational field from atmospheric fluctuations create insurmountable noise.) As LIGO scientist Rick Savage puts it, “We’re far beyond splitting hairs here.”
An international research team has used carbon nanotubes to enhance the efficiency of laser acceleration, bringing table-top sources for carbon-ion therapy a step closer to reality. Therapeutic ion beams are currently delivered using large, expensive particle accelerators. Laser-driven ion acceleration may one day provide a compact, cost-effective alternative – but current techniques cannot match the energy and quality of beams created by conventional accelerators.
Laser-driven ion acceleration typically works by firing high-intensity laser pulses at ultrathin diamond-like carbon foils. The light pulses strip electrons from atoms in the foil, generating a negatively charged electron plasma. This plasma creates an electric field that then accelerates positively charged carbon ions stripped from the foil.
Now, a team led by Jörg Schreiber at LMU Munich has calculated that the energy of the resulting carbon ions could be boosted by using laser pulses with a steep (few-femtosecond) rising edge. Such pulses would allow an efficient process called radiation pressure acceleration (RPA). Generating such pulses experimentally, however, is a formidable challenge. “RPA is the most efficient way to accelerate ions,” says Schreiber. “In particular, RPA promotes substantially more ions to high energies as compared to other schemes, and eventually even allows for non-exponential energy distributions.”
Pulse shaping
To create optimally shaped laser pulses, Schreiber and colleagues coated one side of a 10 nm-thick diamond-like carbon foil with a foam of carbon nanotubes. When the laser irradiates the nanotube foam, a near-critical-density plasma is formed, which acts like a lens and focuses the traversing laser pulses. “The CNT foam provides a plasma that acts as a nonlinear medium to shape the laser, both temporally and spatially, to become better suited for RPA,” explains Schreiber.
To test their approach, the researchers carried out experiments using femtosecond pulses from the Gemini laser at the Rutherford Appleton Laboratory. Comparing the temporal shapes of an incident laser pulse and a pulse transmitted through the carbon nanotube layer revealed significant pulse steepening, with the pulse rise time reduced to about 4 fs by the nanotubes. The laser intensity is also increased and reaches peak values of more than 10 times the peak vacuum-focused intensity. This extremely steep-rising edge accompanied with much higher peak intensity provides ideal conditions for RPA to occur.
The researchers recorded the ion spectra generated upon firing circularly polarized laser pulses onto diamond-like carbon films with different carbon-nanotube foam thicknesses. They found that ion energies increased with increasing carbon-nanotube foam thickness. The best performance was observed for the thickest layer (5 µm), which increased the maximum energy of accelerated carbon ions by approximately a factor of three over an uncoated diamond-like carbon foil – from 80 to almost 240 MeV.
Boosted output
This maximum energy (20 MeV per nucleon) is significantly higher than previously attained by laser-driven ion acceleration, and makes experiments on cells with beams of carbon ions feasible for the first time. However, energies of at least 1 GeV will be required for clinical applications – about five times higher than that attained in this work.
According to the researchers, boosting power output to this level is not impossible. Future experiments will exploit the 3 PW ATLAS-3000 ultrashort pulsed laser, which will be located at a new Centre for Advanced Laser Applications (CALA) being built in Garching. Combined with the energy enhancement from the nanotube-coated foils, this system could help make laser ion acceleration a more viable tool.
The team also plans to advance from proof-of-principle experiments demonstrating the creation of 20 MeV/atomic-mass-unit carbon ions in a few shots, towards experiments with ion bunches. This will include cell experiments and, in the near future, also small-animal studies.
“In parallel, we are scattering the globe to advertise the state-of-the-art of laser acceleration, to raise awareness among potential applicants in various fields of science,” adds Schreiber. As for whether laser-driven ion acceleration will ultimately enable a low-cost particle-therapy system, Schreiber says that this is a tough call to make. “The challenge is not simply to make a cheaper accelerator – laser acceleration should provide some new quality that is not or hardly accessible by other means,” he says. “This feature is certainly the bunched nature and the synchronicity to other laser-driven sources of radiation. Even the multi-ion species available in one shot could turn out to be a benefit. The next years will be exciting as we approach medically relevant energies and exploit the first applications that utilize these special features.”
Cocoa conch: a chocolate’s distinctive flavour and texture comes from “conching”. (Courtesy: iStock/deyangeorgiev)
By Michael Banks, Tushna Commissariat and Matin Durrani
Chocolate, the food of the gods, is more popular now as a sweet treat than ever before. And while more and more people know their 70% cocoa from their truffles, “lecithin” still isn’t a word that pops up often. It is an ingredient that plays a key role in chocolate-making and other foods. But this fatty substance has long confounded food-scientists and confectioners alike – we don’t know how this ingredient works on a molecular level and confectioners have had to rely on observations and trial-and-error methods to perfect recipes.
Now, though, chocolatiers have had help from an unexpected field – that of molecular biology – to figure out chocolate “conching” – the part of the chocolate-making process where aromatic sensation, texture and “mouthfeel” are developed. In a special issue on “The Physics of Food” published in the Journal of Physics D: Applied Physics, Heiko Briesen and colleagues at Technische Universität München, Germany, use molecular dynamics to model and simulate how lecithin molecules, made from different sources, attach to the sugar surface in cocoa butter. “I’m quite confident molecular dynamics will strongly support food science in the future” says Briesen.
The “wonder material” graphene has another significant quality to add to its impressive list of electrical and mechanical properties: superconductivity. Physicists in Canada and Germany have shown that graphene turns into a superconductor when doped with lithium atoms – a result that could lead to a new generation of superconducting nanoscale devices.
Graphene exhibits a range of remarkable properties, thanks to its special structure – a one-atom-thick hexagonal lattice of carbon atoms. It is far stronger than steel while also flexible, and is an excellent conductor of both electricity and heat. In its pristine form, however, it is not a superconductor.
Coupling Cooper pairs
Neither is pure graphite, but in 2005 physicists showed that graphite could be made to superconduct when chemically treated, so as to create bulk materials consisting of graphene alternated with one-atom-thick layers of another element. The best performing material thus created, calcium graphite (CaC6), has a superconducting transition temperature of 11.5 K. Theorists identified the underlying mechanism for that superconductivity as electron–phonon coupling. Phonons are vibrations in a material’s crystal lattice that bind electrons together into “Cooper pairs” that can travel through the lattice without resistance – one of the hallmarks of superconductivity. It was then realized that such electron–phonon coupling might occur not just in bulk graphite compounds but also by depositing atoms of a suitable element on to single layers of graphene.
In 2012 Gianni Profeta of the University of L’Aquila in Italy and colleagues used computer modelling to predict that lithium ought to be a particularly good candidate for such doping. This came as a surprise, given that bulk LiC6 had not been shown to superconduct, but the researchers nevertheless found that the monolayer structure should promote superconductivity in two ways. The additional lattice vibrations generated by the lithium atoms should yield a high density of phonons, they said, while lithium’s donation of electrons to the graphene should strengthen overall electron–phonon coupling.
Lithium decorations
That prediction has been borne out by the latest work, which has been carried out by Andrea Damascelli at the University of British Colombia in Vancouver, together with colleagues in Europe. Damascelli and co-workers prepared their samples by growing layers of graphene on silicon-carbide substrates, and then very precisely depositing lithium atoms onto the graphene – a process known as “decorating” – in a vacuum at 8 K.
The team then studied the properties of the samples using angle-resolved photoemission spectroscopy, which exploits the photoelectric effect to measure the momentum and kinetic energy of electrons in a solid. The researchers found that the electrons were being slowed down as they travelled through the lattice, an effect that they attributed to enhanced electron–phonon coupling. Crucially, they also showed that this greater coupling leads to superconductivity by identifying an energy gap between the material’s conducting and non-conducting electrons – which is the energy needed to break Cooper pairs. At 0.9 meV, the measured value of this gap implies a transition temperature of about 5.9 K – as compared with Profeta and colleagues’ prediction of up to about 8 K.
Further checks
According to Damascelli, this result enhances graphene’s utility as model system for studying quantum phenomena, as well as showing how a wide range of electronic devices could be connected to one another via a single substrate. Indeed, Patrick Kirchmann and Shuolong Yang of the SLAC National Accelerator Laboratory in California, who were part of a group that last year demonstrated the phonon basis of CaC6 superconductivity, believe the work might eventually lead to the production of nanometre-sized superconducting quantum interference devices and single-electron superconductor quantum dots, for example. They add, however, that the result must first be confirmed, via the observation of two additional effects: graphene’s complete loss of electrical resistance and its expulsion of external magnetic fields – the Meissner effect – when cooled below the transition temperature. These measurements, says Kirchmann, “are needed to confirm superconductivity and pin down the transition temperature”.
Damascelli says that carrying out these measurements will require a new way of preparing the decorated graphene – one that allows the material to remain stable at ambient conditions while exhibiting macroscopic superconductivity. “We are looking at different elements,” he says, “and at different substrate–graphene combined systems that could aid the retention of the decorating atoms.”
A separate group of researchers, including Hyoyoung Lee, Tuson Park and colleagues at Sungkyunkwan University in South Korea have observed superconductivity in samples consisting of several layers of graphene doped with lithium. The group records a transition temperature of 7.4 K, obtained via observation of the Meissner effect.
“The next milestone would be to demonstrate this signature of superconductivity in a single layer of graphene,” says Kirchmann.
As this article went to press, a preprint of a paper looking at “Superconductivity in Ca-doped graphene”, based on work carried out by Andre Geim – who in 2010 won the Nobel Prize for Physics together with Konstantin Novoselov for their work with graphene – and colleagues appeared on arXiv
