About 10 years after I left university, I went to a reunion of my former classmates. When we talked about our jobs, they were stunned when I told them I was still doing physics pretty much every day. “But I thought you worked on baby nappies!” I confirmed this was accurate, but added that I had just hired a theoretical physicist to help me develop the differential equations for urine transport through nappies.
My classmates thought I was being funny, but I was just telling the truth. I work in the research and development (R&D) division at Procter & Gamble (P&G) and we joke that what we do is not rocket science – it’s harder. The fact is that many commercially available software codes for simulating fluid flow and mechanics do not readily work for nappies. As a result, we continuously face situations where either the theory that describes relevant phenomena does not exist or the simulations are numerically unstable owing to challenges that are specific to our systems.
For example, unlike in geological materials, where the pore structure is typically relatively stable during fluid flow, the materials used for consumer products such as nappies are soft and can deform because of wet collapse or external conditions. In addition, the swelling of super-absorbent materials produces large changes in the dimensions of the pore structure. This poses a range of challenges when developing methods of physically characterizing the materials and theories of how they behave. In short, the stereotypical view that “consumer products are simple to use – therefore they are simple to understand” is almost completely wrong. After 23 years “in baby diapers” (as the Americans would phrase it), I still find myself using physics every day and having a lot of fun too.
An accidental entry
Joining P&G was one of the best decisions of my life, but it happened more or less by chance. I studied physics at the University of Leipzig in what was then East Germany and my diploma thesis focused on developing simulations to compute the molecular orientation of nematic liquid crystals in electrical fields. For my PhD, I extended these simulations to include birefringent optics, making it possible to predict the behaviour of liquid-crystal displays based on the display’s design and material properties.
When I graduated in 1991, I was only 26 years old, but I had met my wife at university and (as was common in East Germany at the time) we decided to have children early. Our daughter was born before I earned my diploma and our son a few years later. My original plan was to stay at university as an assistant after my PhD, working my way towards a tenured professorship. However, while I was working on my thesis the Berlin wall came down and after Germany reunited the university system changed almost overnight. Permanent assistant positions like the one I had hoped for were no longer available; instead, it became common to accept temporary assistant or postdoc positions, with the hope of eventually becoming a professor.
I decided that this new path would be irresponsible for our family and that working in industry would provide a safer and more stable environment. Then I spotted an advertisement in a newspaper; a company called Procter & Gamble was soliciting applications from graduate students to attend a seminar for “technical management”. I thought P&G must be a consulting company (it was unknown in East Germany and this was before the Internet became popular) and I had no idea what technical management was, but I was curious, so I sent in my application.
A couple of weeks later I received an invitation to an interview. When I arrived, the recruiter informed me that P&G was a consumer-goods company and that I would be interviewed for an actual job, since I was already too advanced for the seminar. I didn’t know any more about consumer goods than I did about technical management, but again, I was willing to find out and later that day I was offered a starting position in material development for Pampers, P&G’s brand of nappies.
A physicist among chemists
My first assignment at P&G’s centre in Schwalbach, Germany, was to develop an upgrade for the absorbent gelling material (AGM) in nappies that absorbs and “locks in” urine to keep the baby’s skin dry. AGMs are hydrogels that are made of partially neutralized polyacrylate polymer networks and they were originally invented for agriculture as a means of improving the water-holding capacity of soil. P&G had introduced AGMs into Pampers in the mid-1980s and now my boss was convinced that they could be improved.
AGM development was seen as the domain of chemists, and as the only physicist working on it within P&G and our material suppliers, I was really pushed out of my comfort zone. I had to learn a lot more about polymer chemistry and materials science. But I was also able to ask physics questions such as “How does liquid transport happen in nappies?” and “How can we understand how the swelling of the AGM changes this?” A few of these things were known qualitatively, but to my surprise there was no detailed understanding and no predictive model to guide me. I had to develop models and characterization-test methods on my own and the breadth of the task – which also included working with material suppliers and even supervising some consumer testing – was very new to me. However, I found it exciting and within a few years I led the first AGM upgrade in P&G’s European Pampers plants.
This success encouraged me to push my role as a physicist further. In addition to AGM development, I started programmes aimed at improving absorbent-core technology, and our modelling and simulation programmes. In a way, I think that being a physicist among non-physicists was one of the reasons for my success because my different point of view helped spur us along.
Becoming an ‘expert generalist’
P&G has a dual career system, with management and technologist tracks. I went for the latter, progressing from principal scientist in 1995 to research fellow in 1999. In 2006 I was inducted into the company’s Victor Mills Society, which is the top rung of the technical career track. In fact, it is a bit like being a professor because I get to lead major R&D programmes and help develop new ways of educating and nurturing young innovators. I also collaborate with a range of companies, universities and institutions, and frequently present at conferences.
The term “expert” is usually associated with deep knowledge in one particular field. However, I find it more useful to view myself as an “expert generalist” or “master integrator” – someone with deeper-than-average knowledge and experience of multiple fields. My physics education taught me that “if it is the same equation, then it is the same problem”, and I think this has helped me to think and act like a master integrator because I can find connections between areas that appear unrelated on the surface. This is especially useful at the fuzzy front end of innovation, when the uncertainties surrounding what is needed and what is possible are both very high.
One thing that my physics education did not teach me, however, was the role of emotions and perceptions in the decision-making process. When I started working at P&G, one of the first things I learned was that “perception is reality” for consumers; a product may work fine, but if it does not look that way, consumers will not accept it. It took me much longer to learn that “perception is reality” also applies much more generally in decision making. For example, I sometimes give a talk called “Can I trust your model?” that highlights the challenges of status-quo bias and human behaviour as it applies to innovation. Learning more about how to influence people has become a hobby for me, so I read a lot of books about behavioural science in addition to keeping up with new topics in physics, chemistry, materials science and engineering.
Overall, I have found that working as a technical expert at P&G has given me plenty of new insights, as well as tremendous opportunities for learning. I enjoy tasks that take me outside my comfort zone and being “in nappies” means there is also the sense of excitement that comes with getting new technologies into consumers’ hands – and onto the bottoms of their babies.
Two independent teams of physicists in Germany have created the first high-gain optical transistors that can be switched using a single photon. Based on ultracold atomic gases, the devices make use of the “Rydberg blockade”, whereby the creation of an atom in a highly excited state has a huge effect on the ability of the surrounding gas to transmit light. The research might lead to the development of all-optical logical circuits that could operate much faster than conventional electronics. The transistors could also find use in photon-based quantum-information systems of the future.
Communications and computing systems that use only light to transmit and process information have the potential to be faster and much more energy-efficient than those that use electronic signals. While optical-fibre communications is already widespread, the switching and processing of optically encoded data is usually done by converting light pulses to an electronic signal, which can then be easily processed. The electronic signal is then converted back to a light pulse.
Making photons interact
This time-consuming and energy-hungry process is necessary because photons do not readily interact with each other, which makes the design of all-optical components a major challenge that is currently being addressed by physicists and engineers. During the past few years, several research groups have made important breakthroughs in this area by showing that photons can be made to interact with each other in specially prepared samples of ultracold atomic gas.
Now, two independent teams led by Sebastian Hofferberth of the University of Stuttgart and Stephan Dürr of the Max Planck Institute of Quantum Optics near Munich have created devices in which a single “gate” photon can switch off a stream of as many as 20 photons. This gain of 20 is a huge improvement on previous attempts at optical switches, which either needed pulses of several gate photons to achieve gains greater than one or offered gains of much less than one for single-gate photons.
Both teams based their gates on gases of rubidium atoms that were cooled to temperatures below 1 mK. Normally, the gas is transparent to a beam of “source” photons, which can travel through the device and emerge via the “drain” – gate, source and drain being terms used to describe the control, input and output channels, respectively, of a conventional field-effect transistor.
Blocking the drain
When a gate photon is fired into the gas, it is absorbed by one atom, which puts that atom into a highly excited Rydberg state with one electron in an extremely large orbital. The large distance between this electron and the nucleus gives the atom a very large electric dipole moment, which shifts the energy levels of nearby atoms. This shift causes the gas to become opaque to light from the source, effectively switching the transistor off. The Rydberg state endures for about 1 μs, which is a surprisingly long time for an atomic system. This allowed Dürr and colleagues to use their transistor to switch off a stream of 20 source photons, while Hofferberth’s team prevented 10 photons from reaching the drain of its device.
“This effect should make it possible – at least in principle – to cascade such transistors to solve complex computational tasks,” says Dürr. He also points out that the experiments offer physicists a new and non-destructive way of studying the physics of Rydberg states. The ability to operate at the single-photon level also means that the transistors could find use in quantum-information applications such as secure quantum-communication systems or powerful quantum computers.
Another interesting aspect of the devices is that the gate photon is re-emitted by the gas when the Rydberg states decay – an effect that has been observed in other experiments. In principle, this means that the transistors could also be used as storage devices for quantum information.
Researchers in the US have used a self-assembly method based on viruses and DNA to position almost 200 fluorescent molecules to within a few nanometres of a tiny gold nanoparticle. This accurate positioning of the molecules boosts their fluorescence output and the method could have applications in information processing, sensing and energy technologies.
Electrons in a metallic nanoparticle undergo collective oscillations known as a surface plasmon resonance when exposed to certain frequencies of light. The nanoparticle then behaves as a tiny antenna, concentrating light within a few nanometres of the nanoparticle surface. If a fluorescent molecule – or fluorophore – is placed within this region, the amount of light captured by the molecule can be boosted significantly.
“This provides a means of creating an intense electromagnetic field near a light-absorbing centre, thus allowing us to greatly increase the capture of light,” explains James De Yoreo of the Pacific Northwest National Laboratory, who was part of the research team. Boosting the number of fluorophores surrounding the nanoparticle helps increase the amount of light captured even further. However, the process is very sensitive to the distance between the nanoparticles and fluorophores, making it difficult to take advantage of the effect in complicated arrangements of nanoparticles and fluorophores.
First, they used a virus “capsid” to form a container for the fluorophores. The capsid, which would normally encapsulate a virus, is made of a protein that self-assembles with many copies of itself to form a shell. The researchers modified the inner surface of the shell with fluorophore attachment sites, capturing almost 180 fluorophores and giving a density of one fluorophore per 14 nm2. The capsid was further modified so that the outside was coated with DNA strands.
The second step involved a process called “DNA origami”, whereby a collection of several hundred synthetic DNA strands self-assemble into shapes that are around 100 nm in size. The researchers formed a tile to act as a “molecular breadboard”, with two binding locations to allow objects to be attached. Finally, the team modified gold nanoparticles with a set of DNA strands that recognized a location on the origami. The DNA sequences on the outside of the capsid were designed to bind to a different location.
Adjustable origami
Mixing the three components together – the fluorophore-loaded capsid, the origami tile and the gold nanoparticle – resulted in the capsid and nanoparticle being held nanometres apart on the origami. By adjusting the origami design, the capsid–nanoparticle separation distance could be adjusted.
The researchers confirmed that the system had formed as designed by using atomic force microscopy and electron microscopy. First they investigated how the capsid–nanoparticle separation affected the fluorescence characteristics by studying a sample using confocal microscopy. The separation distance was then determined using atomic force microscopy. The combined measurements demonstrated increased fluorescence intensity for a number of separation distances.
To better understand the experimental results, the researchers modelled the interactions of the fluorophores with the nanoparticle, demonstrating that the system behaved as they expected. For larger sizes of gold nanoparticle, the model showed that the fluorophores would undergo significant increases in fluorescence.
Mix and match
“The model enabled us to explore changes to the nanoparticle size, choice of fluorophore, arrangement of fluorophores and even the capsid shape to optimize the performance,” says De Yoreo.
The primary interest of the team is to create technologies that mimic some of the highly efficient processes that living organisms use to harvest energy from the Sun. “Our use of the effect is directed towards energy harvesting for solar-energy applications,” explains De Yoreo. “When one looks at light-harvesting complexes in biological systems, they often utilize a similar architecture.”
The effect of gravity on virtual electron–positron pairs as they propagate through space could lead to a violation of Einstein’s equivalence principle, according to calculations by James Franson at the University of Maryland, Baltimore County. While the effect would be too tiny to be measured directly using current experimental techniques, it could explain a puzzling anomaly observed during the famous SN1987 supernova of 1987.
In modern theoretical physics, three of the four fundamental forces – electromagnetism, the weak nuclear force and the strong nuclear force – are described by quantum mechanics. The fourth force, gravity, does not currently have a quantum formulation and is best described by Einstein’s general theory of relativity. Reconciling relativity with quantum mechanics is therefore an important and active area of physics.
An open question for theoretical physicists is how gravity acts on a quantum object such as a photon. Astronomical observations have shown repeatedly that light is attracted by a gravitational field. Traditionally, this is described using general relativity: the gravitational field bends space–time, and the light is slowed down (and slightly deflected) as it passes through the curved region. In quantum electrodynamics, a photon propagating through space can occasionally annihilate with itself, creating a virtual electron–positron pair. Soon after, the electron and positron recombine to recreate the photon. If they are in a gravitational potential then, for the short time they exist as massive particles, they feel the effect of gravity. When they recombine, they will create a photon with an energy that is shifted slightly and that travels slightly slower than if there was no gravitational potential.
Irreconcilable differences
Franson scrutinized these two explanations for why light slows down as it passes through a gravitational potential. He decided to calculate how much the light should slow down according to each theory, anticipating that he would get the same answer. However, he was in for a surprise: the predicted changes in the speed of light do not match, and the discrepancy has some very strange consequences.
Franson calculated that, treating light as a quantum object, the change in a photon’s velocity depends not on the strength of the gravitational field, but on the gravitational potential itself. However, this leads to a violation of Einstein’s equivalence principle – that gravity and acceleration are indistinguishable – because, in a gravitational field, the gravitational potential is created along with mass, whereas in a frame of reference accelerating in free fall, it is not. Therefore, one could distinguish gravity from acceleration by whether a photon slows down or not when it undergoes particle–antiparticle creation.
An important example is a photon and a neutrino propagating in parallel through space. A neutrino cannot annihilate to create an electron–positron pair, so the photon will slow down more than the neutrino as they pass through a gravitational field, potentially letting the neutrino travel faster than light through that region of space. However, if the problem is viewed in a frame of reference falling freely into the gravitational field, neither the photon nor the neutrino slows down at all, so the photon continues to travel faster than the neutrino.
Two neutrino pulses?
While the idea that the laws of physics can be dependent on one’s frame of reference seems nonsensical, it could explain an anomaly in the 1987 observation of supernova SN1987a. An initial pulse of neutrinos was detected 7.7 hours before the first light from SN1987a reached Earth. This was followed by a second pulse of neutrinos, which arrived about three hours before the supernova light. Supernovae are expected to emit large numbers of neutrinos and the three-hour gap between the second burst of neutrinos and the arrival of the light agrees with the current theory of how a star collapses to create a supernova.
The first pulse of neutrinos is generally thought to be unrelated to the supernova. However, the probability of such a coincidence is statistically unlikely. If Franson’s results are correct, then the 7.7-hour gap between the first pulse of neutrinos and the arrival of the light could be explained by the gravitational potential of the Milky Way slowing down the light. This does not explain why two neutrino pulses preceded the light, but Franson suggests the second pulse could be related to a two-step collapse of the star.
Scepticism needed
Nevertheless Franson is cautious, insisting that “there are very serious reasons to be sceptical about this and the paper doesn’t claim that it’s a real effect, only that it’s a possibility.” He is also pessimistic about the prospects for the idea being proven or refuted in the near future, saying that the chances of another supernova so close are very low, and other possible tests do not presently have sufficient accuracy to detect the effect.
Raymond Chiao of the University of California, Merced, agrees with Franson that, observationally and experimentally, “there are a lot of caveats that need to be clarified,” most notably, that if Franson’s hypothetical interpretation of SN1987a is correct, there are two clear neutrino pulses separated by five hours, but little evidence of two corresponding pulses of light. Nevertheless, he says “There is a deep seated conceptual tension between general relativity and quantum mechanics…If, in fact, Franson is right, that is a huge, huge step in my opinion: it’s the tip of the iceberg element that quantum mechanics is correct and that general relativity must be wrong.”
It’s one of the most memorable moments of my career – and not in a good way. I was giving a talk to a room packed full of eminent astrophysicists, but there had been a bit of a childcare crisis, so child number two was sitting grumpily on the front row. I was in full flow, proudly leading up to my new result, when an all-too-familiar voice cut through the air. “She doesn’t know what she’s talking about!”
As time has passed, I have slowly recovered from this mortifying experience, comforted in the realization that for a four year old, my son was being quite astute. You see, I specialize in observing the dark side of our universe – a kind of shadow realm that we can’t see or touch, but which extends throughout the whole universe and even permeates our everyday world.
What we do know about this invisible stuff is that it appears to make up over 95% of our universe and comes in two forms. Dark matter is a special type of matter that, unlike normal matter, cannot interact via the electromagnetic force – the one that light uses to travel by. Dark energy, meanwhile, is a mysterious source of energy that is causing the rate at which our ever-expanding universe grows to get faster and faster each and every day. Numerous independent observations point to the existence of both entities via the effects they have on the matter that we can see.
As physicists, we have begun to quantify this realm, and we have thought long and hard about what it is made of. But in the grand scheme of things, we are still pretty clueless. And so, you see, to some extent my son was right. To prove him wrong – to truly know the nature of the dark side – would involve solving some of the biggest challenges facing science today.
Well, I do like a good challenge! And so, using a powerful astronomical technique called gravitational lensing, I am working on several projects that are beginning to expose the mysterious dark side of the universe.
Cosmic raindrops
If you are unfamiliar with gravitational lensing – and even if you’re not you may like this analogy – take a look out of your nearest window and ask yourself: how do you know the glass is there? Perhaps there are some little imperfections, or some raindrops that distort your view? If you see raindrops, the reason they are apparent is that these transparent globules bend the light travelling from an object beyond the window – from a tree, say – to your eye. But because you know how the scene outside should look in the absence of distortions, you infer the existence of the raindrops.
To apply the same thinking to gravitational lensing, simply swap the trees for galaxies billions of light-years away. As for the raindrops, replace these tiny objects with huge transparent clumps of dark matter, sitting between the distant galaxies and you. The physical reason why the light bends is, however, different: while raindrops simply refract light, clumps of dark matter bend the very fabric of space–time, and the path that the light is travelling along gets bent with it (see figure 1).
1 Bending the light of distant galaxies
(Courtesy: NASA/ESA)
The gravitational field of a massive object extends far into space, warping space–time. The path of any light rays passing close to that object (and thus through its gravitational field) will become bent. The light is then refocused somewhere else. The more massive the object, the stronger its gravitational field and hence the greater the deflection of the light.
Anything with mass warps space–time to some extent, according to Einstein’s general theory of relativity, but we only perceive the phenomenon when the mass, and hence the distortion, is very big – and dark matter has a very big mass, making up 83% of all matter in the universe. In fact, we can now image these distant, distorted galaxies using extremely powerful telescopes. And by combining the measured distortion in these images with the equations of general relativity, we can directly weigh all of the matter lying in-between us and the galaxies, irrespective of whether it is luminous or dark.
The dark question
How can we be so sure, though, that the majority of the matter we detect through this lensing technique is an unknown and mysterious dark substance? One reason is that from studies of the Sun and other stars, we know roughly how much mass in each galaxy is locked up in the stars, and we find that there simply is not enough luminous stellar mass in the galaxies to account for the lensing effects that we measure. Indeed, the same conclusion is drawn from the observation that stars orbit their galaxies faster than the mass that we can see should allow them to.
But could dark matter be non-luminous objects that we cannot see with telescopes, such as the failed faint brown dwarf stars that never summoned up the temperatures required to switch on nuclear fusion in their core, or a multitude of primordial black holes that have been lurking since the birth of the universe? Again, lensing can help us answer this question. If these entities, known as massive compact halo objects (MaCHOs) existed in the quantities required to account for the missing mass, they would regularly pass between us and distant stars. In doing so, their mass would warp space–time, focusing more of a certain star’s light towards us such that for a passing moment, we would see that star brighten, and then dim again. Although these “micro-lensing” observations are not easy – requiring the dedicated monitoring of stars over many years – the relatively few events that several teams have found have essentially ruled out MaCHOs as a major source of dark matter. Excitingly, this dark-matter search instead led to the discovery of many exoplanets orbiting distant stars.
The arguments so far cannot quite rule out the possibility that the extra mass is down to something mundane that we already know about – namely gas. In massive galaxy clusters – collections of hundreds of galaxies – there is indeed a significant amount of gas between the galaxies, which we can see because it emits X-ray light. However, in rare events in which two galaxy clusters collide, such as the Bullet Cluster, we can cleanly distinguish the gas from the galaxies and the dark matter, proving that they are separate entities (see box).
A shot in the dark
Dark discovery Galaxy cluster 1E 0657-56, better known as the Bullet Cluster. The visible-light image (left) has been overlaid with X-ray observations of hot gas (pink) and the total mass distribution inferred by gravitational lensing (blue). The artist’s representation (right) shows two galaxy clusters colliding in four stages (a–d) to form the Bullet Cluster. The interacting gas (pink) contains most of the normal matter, implying that the majority of the mass in the system must consist of weakly interacting dark matter (blue). (Courtesy: NASA/CXC/ESO/Clowe and Markevitz; NASA/CXC/M Weiss)
The power of weakness
In the most powerful examples of gravitational lensing, it is plain to see that very strong lensing is occurring. Galaxy clusters, such as Abell 2218 (see lead image), are the strongest lenses we know and provide us with the most striking images of this gravitational physics in action. The giant arcs that encircle these clusters show the highly distorted light emitted by distant galaxies situated almost directly behind the cluster.
But in our goal to map dark matter, we would be left with a very patchy map if galaxy clusters – which are fairly scarce – were our only indicators of mass. Thankfully, every galaxy tells us something, even if it is only mildly distorted, showing that a relatively small amount of mass lies between us and that galaxy. In fact, most distant galaxies are only “weakly” lensed and it is these galaxies that we mostly rely on to make dark-matter maps. Still, even with these weakly lensed galaxies, how do you know if an elliptical galaxy, say, looks that shape because it actually is that shape, or because its light has been gravitationally lensed?
Help to answer this question comes from the other galaxies in the neighbourhood. Imagine light travelling towards us from two nearby galaxies. As it journeys across the universe, the light will pass by the same structures of dark matter, and hence experience the same gravitational distortion. So when we look at those two galaxies in the sky they will appear to be weakly aligned, with the level of alignment increasing with the amount of dark matter they have passed. If there were no dark matter in a particular patch of the sky, the average galaxy shape would just be a circle, assuming that galaxies are randomly oriented in the universe. With dark matter, however, we find the average galaxy shape is an ellipse. The stronger the average galaxy ellipticity is in the patch, the more dark matter there is in that region of the universe. This induced ellipticity is a faint signature that dark matter writes across the cosmos to tell us exactly where it is and how much of it there is.
Simulated… The Millennium Simulation Project calculated the evolution of structure in our universe using 10 billion particles. Shown here is the dark matter when our universe was 4.7 Gyr old. (Courtesy: Millennium Simulation Project)
The biggest map of dark matter to date, made using this weak lensing technique, was the very result that caused my son’s abrupt outburst. Over five years from 2003 to 2008, the absolute best weather at the Canada–France–Hawaii Telescope in Hawaii was reserved to map dark matter. By analysing the weak alignment of the images of over 10 million galaxies, whose light was emitted when the universe was only six billion years old, we had our first direct glimpse at dark matter on the largest of scales. The survey revealed a cosmic web of dark matter in each of the four directions we looked. Our map exposed massive clumps of matter, wispy filamentary structures joining them together, and expansive voids between them spanning millions of light-years (2013 MNRAS 433 3373).
Even before these observations were complete, theorists had simulated dark-matter maps by taking our best theories about the nature of dark matter and using supercomputers to build a dark universe, allowing it to grow and evolve. These computer studies, such as the Millennium Simulation in 2005, had provided us with a glimpse of the invisible dark side, predicting the giant cosmic web we saw in our observations. In fact, theorists had also predicted another feature we saw in the maps: the visible universe largely overlaps with the dark universe, because the dark-matter web has dictated when and where the visible universe should form.
…and for real The Canada–France–Hawaii Telescope Lensing Survey showed that dark matter is distributed as a network of dense (light) and empty (dark) regions. (Courtesy: Van Waerbeke, Heymans and CFHTLenS collaboration)
As for dark energy, it too has an important role to play in the cosmic web of dark matter. In a universe without dark energy, the clumps of dark matter would be even more dense than they are now due to the attractive forces of gravity causing the densest regions to accrete neighbouring structures of matter. But dark energy – the mysterious source of energy that is causing the post-Big Bang expansion of our universe to accelerate – slows this process down. Together, the two dark entities play out a cosmic battle of epic proportions. While the gravity of dark matter slowly pulls structures together, dark energy causes the dark-matter structures to get further and further apart, making it harder for them to grow.
Looking further away in our universe is the same as looking back in time – with gravitational lensing surveys so far having mapped objects as far away as when the universe was only six billion years old. This technique has therefore let us map dark matter in different epochs in the history of the universe. So by studying the evolution of the dark-matter web we have been able to measure how dark energy has affected the growth of those structures, and we are slowly learning about what this mysterious dark energy could be.
Technical challenges
Gravitational lensing has been heralded as the most powerful technique for studying the dark universe, but it is also the most technologically challenging. The typical distortion induced by dark matter, as a galaxy’s light travels through the universe, is only enough to alter the ellipticity of that galaxy by less than 1%. But in the last few moments before that light is captured on Earth, the atmosphere, telescope and detector can together change the ellipticity of the galaxy by 10% or more. So to isolate the alignment signature that dark matter imprints, we need to model all the distortions introduced by technology and the atmosphere to very high precision and then invert these terrestrial effects to accurately recover the cosmological signal. Just to up the ante, the terrestrial effects change every second as the wind and ground temperature alter the density of the air in different layers of the atmosphere, and the telescope slowly moves to track the rotation of the Earth.
Gravitational lensing has been heralded as the most powerful technique for studying the dark universe, but it is also the most technologically challenging
Furthermore, this lensing effect is so weak that to detect it we need to analyse the images of hundreds of millions of galaxies, which involves rapidly processing petabytes of data. For the past decade, however, astronomers have been setting “big data” challenges to crowd-source the best minds to solve this monumental computational task. In 2011, for example, the Kaggle “Mapping Dark Matter” challenge saw 700 non-astronomers competing for a prized tour around NASA’s Jet Propulsion Laboratory. Their submissions fuelled a new range of machine-learning ideas for the astronomers to put into practice.
One final astrophysical challenge persists, which the astute reader will have already recognized. How valid is our assumption that galaxies are randomly oriented throughout the universe before their light is lensed? We know that the way galaxies form and evolve depends on their local environment, and hence two galaxies in the same district of the universe may well have a natural-born alignment with each other. However, we have measured this effect by looking at the alignment of galaxies in tight-knit communities, in contrast to the alignment of galaxies widely dispersed throughout the universe. What we found was that the average natural alignment between galaxies is roughly 100 times smaller than the observed alignment that dark matter induces. This is small, but not negligible and we do take it into account (2013 MNRAS432 2433).
Current and future missions
Three lensing teams are currently competing to be the first to reveal the next major leap in our understanding of the dark universe. Researchers from Europe (with their Kilo-Degree Survey) and from Japan (with their Hyper-Suprime Cam survey) are imaging 1500 square degrees of the cosmos – nearly 5% of our sky and 10 times as much sky as our current best lensing survey. Astronomers in the US, with the Dark Energy Survey, will eventually cover three times that area. All three surveys will conclude their observations over the next few years.
Lensing teams are competing to be the first to reveal the next major leap in our understanding of the dark universe
There is great interest in whether these surveys will uncover the same “tension” that we see between current lensing observations of the invisible dark universe and the Planck satellite’s observations of the cosmic microwave background. Gravitational-lensing surveys are very sensitive to how dark matter clumps, but the Planck data imply a much clumpier universe than the lensing surveys are currently seeing. This lack of agreement could mean a flaw in one or both of the methods. If it persists, as the methods and data quality improve, it has been speculated that this could be evidence for the existence of a new type of neutrino called the sterile neutrino. (See “What’s the matter?” Physics World July 2014, pp30–31.)
Over the next decade, three major new international projects will work in tandem in the final stages of our quest to understand the dark side. The Euclid satellite will be launched above the atmosphere, providing Hubble-Space-Telescope-quality imaging across the whole sky. Getting above the atmosphere gives us a much clearer view of the universe, and the keen vision of Euclid will be extremely sensitive to the weak dark-matter distortions that we are trying to detect. Euclid will also measure the spectra – and hence redshift and distance information – of millions of galaxies with which to chart the expansion of the universe.
Meanwhile, the Large Synoptic Survey Telescope will image the whole southern sky every three nights and provide deep multicolour imaging with which to measure distances to the galaxies without spectra. Not only will this allow us to chart the evolution of dark-matter structures, but this telescope will also be able to detect killer rocks in our solar system that may one day obliterate planet Earth!
Finally, the Square Kilometre Array will provide high-resolution imaging in the radio part of the electromagnetic spectrum, with precision redshift and polarization observations that will allow us to untangle the lensing alignment signature of dark matter from naturally arising alignments. In combination, these surveys will be able to use gravitational lensing to map dark matter and dark energy over the last 10 billion years of the history of the universe, testing gravity on the largest of scales in space and time.
With 35 years still left before I retire, my hope is that I will be able to see these projects through to their conclusion and truly know the nature of the dark side. So one day I will finally be able to tell my son that I really do know what I’m talking about!
Observed change in Tatooine surface temperature 100BBY – 10ABY. (Courtesy: David Ng)
What’s it like to have a nuclear bomb dropped on you? Okay, I know the question is a bit heavy for this light-hearted column but I was really inspired by this piece about Shinji Mikamo who was less than a mile from the epicentre of the Hiroshima bomb. He was 19 at the time and not surprisingly the bomb changed the course of his life in many ways. What I found most amazing is that Mikamo managed to survive an explosion so intense that it blasted off the glass and hands of his father’s pocket watch, but not before imprinting the time of the blast on the watch’s melted face. The article is called “When time stood still” and it appears on the BBC website.
First there was Schrödinger’s cat, now an international team of physicists has come up with a new animal-related paradox involving “quantum pigeons”.
For nearly a century students have struggled to understand the many counter-intuitive implications of quantum physics. Perhaps the most famous paradox is Schrödinger’s cat, whereby a cat being both dead and alive at the same time illustrates the fact that a particle can exist simultaneously in two quantum states.
Now, Jeff Tollaksen of Chapman University in California and colleagues in Israel, Italy and the UK have proposed an equally bizarre scenario dubbed the “quantum-pigeonhole effect”. The paradox begins with the observation that when you put three pigeons in two pigeonholes, there will always be at least two pigeons in the same hole. But according to the team’s quantum analysis, it is possible for none of the pigeons to share a hole.
“It’s one of those things that seem to be impossible,” says Tollaksen. But it is a direct consequence of quantum mechanics and, he adds, “It really has immense implications.”
Nondeterministic measurements
Classical physics is deterministic. This means that measuring the initial state of a system will, in principle, tell you everything you need to determine the final state. But in 1964 Yakir Aharonov of Chapman University and Tel Aviv University helped discover that in quantum mechanics, you can choose initial and final states that are entirely independent, Tollaksen says.
Now Aharonov has teamed up with Tollaksen and colleagues to use this and other concepts of quantum mechanics to postulate the quantum-pigeonhole effect. They reckon that the effect will arise when an observer makes a sequence of measurements while trying to fit three particles in two boxes. First, you make an initial, “pre-selection” measurement of the locations of the particles. Next, you can perform an intermediate measurement to see whether two particles share a box. Finally, you make a final, “post-selection” measurement of the locations. You can make the pre-selection and post-selection measurements such that they are completely independent. In the intermediate step, you can make what’s called a weak measurement to look at all three particles simultaneously. And when you do, it turns out that no two particles share a box.
Spooky and profound
The implications of these results, Tollaksen says, complement the well-known Einstein–Podolsky–Rosen (EPR) paradox. In this scenario, two particles that start in the same place can become intimately correlated, a relationship called entanglement. Measuring the state of the first particle seems to influence the state of the second one, even if they are subsequently separated by distances so great that it would be impossible to explain the influence using classical physics. This unsettling conclusion led Einstein to call entanglement “spooky action at a distance”.
“EPR is one of the most profound discoveries in science,” Tollaksen says. “But that’s only half the story.” The quantum-pigeonhole principle creates a somewhat opposite situation, he explains. Three particles can begin separated with no connections or correlations at all. You bring them together and force them to interact by squeezing them in two boxes. During this intermediate stage, they are more strongly correlated than classically possible. But in the final stage, they are not correlated at all.
The implications of the EPR paradox are important and shape our understanding of information and the fundamental physics of matter. Although it is too early to predict every implication, he believes that the quantum-pigeonhole principle could prove to be just as influential – if not more so. “This is at least as equally profound, if not more profound,” he says. It implies a new concept of correlation that is surprising.
Electronic pigeons
To verify their conclusions, Tollaksen and colleagues propose an experiment in which three electrons travel through an interferometer. This is essentially a beam splitter that creates two separate paths for the electrons, which then meet again.
Because there are only two possible paths, you would expect at least two electrons to share a path. If so, then the two will be close together and interact: their identical electric charges will repel each other, slightly deflecting their trajectories. Then physicists will be able to detect these deflections when all three electrons reunite after the paths converge. But, Tollaksen says, because their calculations show that no two of the three electrons will actually follow the same path, no deflections will be observed.
Physicists have not done these experiments yet, but Tollaksen is confident in their results. “I’m sure it will be confirmed experimentally very soon,” he says.
The new results seem “fascinating,” says Leonard Susskind of Stanford University. “I would guess that the new effect is a serious step in understanding quantum correlations.”
The research is described on the arXiv preprint server.
I chatted with Carroll when he was in the UK speaking at the recent Cheltenham Science Festival and, in the podcast, you can find out about his favourite science-fiction films and why he thinks it’s important to get the science in such films right. Carroll also reveals who he thinks he’s most like in TV’s The Big Bang Theory.
Carroll, who’s a cosmologist at the California Institute of Technology, has worked on films such as Thor, Avengers Assemble and TRON: Legacy. In this podcast, he talks to Physics World about what makes a good science-fiction film and why getting accurate science doesn’t mean ruining a story line. Carroll also discusses his role on TV’s The Big Bang Theory and reveals which character on the show he thinks he resembles most.
CERN’s 27-km Large Hadron Collider (LHC) is gradually restarting after being shut down for 16 months following a major maintenance and upgrade programme. CERN scientists hope that the upgrade – costing SwFr 150m (€124m) – will now boost the energy of the collider to the full design energy of 13 TeV.
The LHC is at one end of a chain of proton accelerators that support a large number of diverse experiments. Some of these experiments have also undergone significant upgrades. Others are new and have goals ranging from finding physics beyond the Standard Model to developing electronic components that are resistant to radiation damage.
The LHC has been out of action since February 2013 when it was turned off following a successful three-year run operating at 7 TeV. Although the collider was not running at its full design energy of 13 TeV, this was enough in July 2012 to enable scientists to announce the detection of the elusive Higgs boson particle, which had first been theorized in 1964 and led to François Englert and Peter Higgs sharing the 2013 Nobel Prize for Physics.
Methodical restart
Having upgraded the LHC, which involved consolidating 10,000 superconducting magnet interconnections, CERN is now methodically restarting the accelerator chain step by step and conducting tests before the planned resumption of full operations next year. The first stage in a proton’s journey to the LHC is the Proton Synchrotron (PS), which was fired up in mid-June. Several experiments that use protons from the PS are already taking data, including AIDA, which is a test bed for new particle-detector technologies. The long-running ISOLDE radioactive ion-beam facility on the PS is expected to resume operations by the end of July. Meanwhile, two new irradiation facilities – IRRAD and the CHARM – are nearing completion and should be ready in September.
Protons from the PS are also fired into a block of metal to create high-energy antiprotons, which are then slowed by the Antiproton Decelerator (AD). The AD is now in the process of being powered up and should be fully operational by 19 August. The AD supplies antiprotons to five experiments. Stefan Ulmer who works on two of them – BASE and ASACUSA – told physicsworld.com that the commissioning process at the AD is proceeding as planned. BASE is a new experiment that aims to measure the magnetic moment of the antiproton and should start gathering data in early September.
Extremely precise measurements
Early last month, CERN also began to power up the Super Proton Synchrotron (SPS), which accelerates protons from the PS and feeds them to the LHC. The physics programme at the SPS will begin again in October and will include the new NA62 experiment. NA62 is looking for new physics beyond the Standard Model of particle physics by trying to make an extremely precise measurement of the probability that a positively charged kaon will decay to a positively charged pion plus a neutrino/antineutrino pair.
From early 2015 the beam will be back at the LHC, and in spring 2015 the physics programme will restart at the LHC’s four experiments.
“The machine is coming out of a long sleep after undergoing an important surgical operation,” says Frédérick Bordry, CERN’s director for accelerators and technology. CERN’s main objective for next year is to run the LHC at 13 TeV – a level that it is hoped will enable deeper studies of the Higgs boson and the hunt for supersymmetric particles.
There is much more about CERN’s NA62 experiment in this video:
