Astrophysicists in the UK have created the largest ever map of voids and superclusters in the universe, which they say helps solve a long-standing cosmological mystery. The team, based at the University of Portsmouth, have mapped the positions of cosmic voids – large empty spaces that contain relatively few galaxies – and superclusters – huge regions with many more galaxies than normal. Lead author Seshadri Nadathur says their new technique allowed them to “make a very precise measurement of the effect that these structures have on photons from the cosmic microwave background (CMB)” as they pass through the structures. According to Nadathur, the photons are affected by the “stretching effect of dark energy”, which causes tiny changes in the temperature of CMB light depending on where it came from. Indeed, photons travelling through voids should appear slightly colder than normal and those navigating through superclusters should be hotter. This effect – known as the integrated Sachs–Wolfe (ISW) effect – has been previously studied, but early maps of the supercluster and void structure seemed to suggest that the effect was five times greater than the predicted value. The Portsmouth team’s new dataset – which used nearly a million galaxies from the Sloan Digital Sky Survey – is much larger. They also created a new statistical technique to be able to measure the ISW effect on the CMB data, as the effect was negligible. This allowed the team to make a very precise measurement of the ISW effect, and they found that the new result agreed extremely well with predictions using Einstein’s theory of general relativity. The work is published in The Astrophysical Journal Letters.
Plasma pressure record set for fusion
Plasma pressure: inside the Alcator C-Mod tokamak at MIT. (Courtesy: MIT)
A new world record for the highest plasma pressure created within a tokamak fusion reactor has been set by Earl Marmar and colleagues working on the Alcator C-Mod facility at the Massachusetts Institute of Technology (MIT). Achieving a high plasma pressure is crucial to the development of practical fusion reactors because the amount of power output by a reactor scales as the square of the plasma pressure. The MIT team achieved a plasma pressure of 2.05 atm within a space of 1 m3 – smashing the previous record of 1.77 atm, which was set by Alcator C-Mod in 2005. The plasma temperature reached more than 35 million Kelvin, which is twice as hot as the centre of the Sun. The plasma produced 300 trillion fusion reactions per second, but much more work must be done before fusion reactors become a viable source of clean energy.
Physicists predict rogue ocean waves
Sisters and rogues: numerical simulations of how waves can interfere in the ocean to produce rogues. The top left shows a normal sea state and the top right a rogue hole. Bottom left is a rogue wave and bottom right is a rogue wave group, also known as the ‘three sisters’. (Courtesy MBI)
Mariners have long known that certain parts of the sea are prone to extremely high rogue waves, which are more than twice the size of the surrounding waves. Now, physicists in Germany have made some progress towards predicting when these rare and dangerous wave will occur. Günter Steinmeyer at the Max Born Institute in Berlin and colleagues from the Leibniz University in Hannover and the Technical University in Dortmund have developed a new way of measuring the number of waves that interfere at specific locations in the ocean, and have shown that it can be used to alert mariners when sea conditions are right for the emergence of rogue waves. Measuring the “phase space dimension” metric can be done on board a ship, so it could provide an early warning of dangerous seas. In the future, the metric could be combined with meteorological data to provide forecasts of rogue conditions.
You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on a new frequency comb sensor.
It’s not often that a police officer greets you when you enter a physics laboratory. But then the Center for Neutron Research (CNR) at the National Institute of Standards and Technology (NIST) in the US is no ordinary lab. Nestled amid the green lawns of NIST’s Gaithersburg campus just north of Washington, DC, it houses a 20 MW nuclear reactor – powered by highly enriched uranium – that first went critical in 1967. For added safety, the centre is surrounded by large boulders to prevent unauthorized vehicles from driving up too close.
Once you are past security though, the CNR looks and feels like any ordinary neutron-physics facility. Neutrons from the reactor are used by more than 2000 researchers from university, business and government for almost 200 days each year. Seven of the 28 experimental stations are diffractometers and spectrometers that use “thermal” neutrons that have been slowed by the reactor’s heavy water to energies of 15–20 meV. Researchers scatter these neutrons off materials to reveal their properties, to develop radiation detectors and to maintain dosimetry standards.
The other 21 instruments at the CNR are different. Located in a separate experimental hall, they sit at the end of guide tubes some tens of metres long. The neutrons that enter these tubes from the reactor are “cold”, having been slowed to energies of 5 meV or less after passing through a liquid hydrogen moderator. The inside walls of the guide tubes are coated with nickel, which has the highest total external reflection of any element. Cold neutrons that graze the walls at shallow angles therefore do not get absorbed but instead bounce off the inner surfaces, like stones skipping over a lake. Researchers use the cold neutrons for studying everything from polymers and proteins to magnets and non-Newtonian fluids.
One fascinating new project using cold neutrons at NIST involves building what the lab claims is the world’s most advanced “neutron microscope”. Like a medical X-ray of the human body, the intensity of the neutron “shadow” of an object provides information about its internal make-up. Water, for example, blocks neutrons whereas lead, aluminium and other dense materials let them through. So by comparing, say, the neutron image of a fuel cell containing water with images of the same object as it dries, researchers could study how water diffuses as the cell’s electrodes degrade. Other applications might include imaging lithium batteries, studying fluid flow in rocks or even analysing works of art.
Beyond the pinhole
NIST is not the only lab creating a neutron microscope, but its approach has certain advantages. Researchers at the Paul Scherrer Institute in Switzerland, for example, are building a neutron microscope by creating a detector with very high resolution. But to reach the resolution, incoming neutrons have to be sent through a tiny pinhole aperture to form a collimated beam that shines on a sample. The detector records the transmitted neutrons, which can then be analysed to yield information on the properties of the sample.
The problem is that in creating a collimated beam, few neutrons pass through the aperture in a given period, meaning that it can take a long time to image a sample. To take images faster or to improve the spatial resolution, your options are to either improve the detector, place the sample up close to the detector or use very thin samples. “But if you’re forced to have thin samples, you lose all the advantages of neutrons over X-rays,” says Dan Hussey, who leads NIST’s neutron-microscope project. “Even with better detectors, in a 1 μm pixel with a typical flux of 106 cm–2 s–1, there’s only one neutron every 100 seconds.”
In focus Consisting of mirrors nested together like an onion, a Wolter lens (top right) is the key part of NIST’s planned neutron microscope. Neutrons that hit the lens at a shallow angle bounce off and can be focused onto a detector (blue lines), while neutrons that hit at a steep angle are lost or get stopped by an aperture block (dashed blue lines). (Courtesy: Sean Kelley/NIST PML)
The neutron microscope at NIST instead does away with pinholes and uses a neutron lens. That might seem an odd approach given that neutrons have no charge and interact weakly with matter, meaning they can’t easily be focused into beams. NIST, however, is using a technique first proposed by German physicist Hans Wolter in 1952 to focus X-rays, which – like neutrons – are also hard to shepherd with conventional optics. “Wolter lenses” have been successfully used on various space-based X-ray missions, including NASA’s Chandra telescope and the European Space Agency’s XMM-Newton telescope.
In the NIST neutron microscope, neutrons from the lab’s reactor shine directly on a sample before passing through a Wolter lens and onto a detector. The barrel-shaped lens, which is roughly 20 cm long and 13 cm in diameter, consists of 10 nested parabolic mirrors each made of a 0.5 mm layer of highly polished nickel (see figure, right). Neutrons that hit a nickel cone at a shallow enough angle bounce off the inside and can be focused onto the sample. The neutrons then pass to a detector containing a 20 μm thick layer of gadolinium oxysulphide. Light given off when a neutron hits this material is recorded by a CMOS camera and fed to a computer for analysis.
Our target is to increase the beam intensity by a factor of 100 and achieve a spatial resolution of 1 μm
Dan Hussey
“The Wolter lenses mean that many more neutrons strike the sample than if there was a pinhole because you no longer need to collimate the beam,” says Hussey, who is developing the lenses with teams led by Mikhail Gubarev from NASA’s Marshall Space Flight Centre and Boris Khaykovich at the Massachusetts Institute of Technology. “Our target is to increase the beam intensity by a factor of 100 and achieve a spatial resolution of 1 μm.” Currently NIST’s best effort using conventional pinhole optics is just 15 μm.
Forging ahead
The 10 mirrors, which range in radius from 55 to 68 mm, are being made at NASA’s Marshall lab by electroplating nickel onto a specially shaped and polished aluminium block, and then peeling the metal off after it’s been cooled in an ice bath. NASA is still fine-tuning the manufacture process, which is not easy. Gravity makes the foils sag by about 80 nm and so they have to be carefully adjusted in the Wolter lens to avoid losing resolution. “However, the beauty of the method is that the mirrors can be replicated from substrates so if any other neutron lab had similar needs they could use the same substrates to create their own neutron microscope,” says Hussey.
The other advantage of using Wolter lenses is that the sample can be placed a long way from the detector – 7 m in the case of NIST – allowing researchers to obtain magnified images of the sample, with Hussey hoping for magnifications of at least ×10. With a pinhole camera, in contrast, the sample has to be stationed right up close to the detector to maximize neutron flux, meaning it is hardly magnified at all. When the Wolter lens is complete in 2018, Hussey says that rather than taking 20 minutes to capture an image – as with pinhole optics – it will take just 20 seconds or less.
NIST’s plan is for the neutron microscope to become a fully functional user facility in 2018. The lab also wants to improve the system to allow neutron phase imaging, which uses variations in the phase of the signal – not its attenuation – to determine the details of a material’s structure. As Hussey and colleagues put it in a paper demonstrating the principle of the Wolter lens for neutron microscopy in 2013 (Appl. Phys. Lett.102 183508), their work could allow “game-changing improvements in the neutron-imaging technique”.
Sounds of Earth: An original golden record. (Courtesy: NASA)
By Hamish Johnston
An online initiative to reissue Carl Sagan’s golden record, which was attached to NASA’s Voyager 1 and 2 craft, has so far raised a whopping $1.1m, smashing its $198,000 goal. The campaign was created in September by David Pescovitz, editor and managing partner at the technology news site Boing Boing, after teaming up with Timothy Daly from Amoeba Music in the US, who was the original producer of the record, as well as US graphic designer Lawrence Azerrad. The original LP, which was created in 1977, contains sounds of the Earth along with recorded greetings and a mix of music, and has been unobtainable for decades, having been available only on CD-ROM in the early 1990s. Now that the cash has been raised, the golden record will be released next year as an LP to mark the 40th anniversary of the Voyager launches. So how much will it set you back? It’s yours for only $98, what a bargain.
“Movies” of electrons as they move across a semiconductor junction have been made by researchers at the Okinawa Institute of Science and Technology in Japan using a new imaging technique. Combining photoemission electron microscopy with femtosecond laser pump-probe methods, the technique tracks the motion of electrons on timescales shorter than 1 ps. The researchers say it could provide a better understanding of how semiconductor devices work and lead to more efficient solar cells.
Solar cells – along with diodes, transistors and other semiconductor devices – rely on the flow of electrons across “heterojunctions” between two different types of semiconductor. While this motion is crucial to just about every modern technology, it is not easy to image in real time. The problem is that pulsed laser techniques – which can measure the energy of electrons on very short time scales – do not offer high enough spatial resolution to track the electrons. On the other hand, electron microscopy techniques offer high spatial resolution, but cannot keep up with the fast-moving electrons.
Best of both
Now Keshav Dani, Michael Man and colleagues have unveiled a new technique that combines high spatial resolution with high temporal resolution. Their method uses the well-established “pump-probe” technique involving laser pulses that are only 200 fs in length. It involves first firing a relatively intense probe pulse at a sample in the form of an indium selenide/gallium arsenide heterojunction that functions as a solar cell. The probe pulse plays the role of sunlight, putting electrons into excited energy states from which electrical energy can be extracted.
About 1 ps after the pump pulse, an image of the excited electrons is taken by firing a less intense probe pulse at the heterojunction. This ejects some electrons from the semiconductor, carrying with them information about the excited electrons in the semiconductor. These ejected electrons are captured and studied using spectroscopic photoemission electron microscopy, which creates an image of the electrons in the semiconductor.
Variable delay
By varying the delay between the pump and probe pulses, the team acquired a series of images of the excited electrons with a spatial resolution of about 250 nm and a temporal resolution of less than 1 ps.
This is a new door to understanding the motion of electrons in semiconductor materials
Keshav Dani, Okinawa Institute of Science and Technology
“We have made a video of a very fundamental process: for the first time we are not imagining what is happening inside a solar cell, we are actually seeing it,” explains Dani. “This is a new door to understanding the motion of electrons in semiconductor materials”.
When creating a solar cell, it is important that the electron excited states endure for long enough for energy to be extracted from the semiconductor. The images revealed that the lifetimes of excited electrons varied throughout the sample, with thinner regions of the semiconductor sample having faster decay rates. The images also suggested that electrons get trapped in thicker regions of the sample – something that could affect the performance of solar cells.
A tiny carbon nanotube has been inflated and deflated much like a balloon by making small changes to the voltage applied along its length. The work was done by Hamid Reza Barzegar at Umeå University in Sweden, and Alex Zettl at the University of California, Berkeley and could someday be used to create tiny machines and actuators for a wide range of applications. Carbon nanotubes have walls that can be as thin as just one atom. They can be made defect free, which the researchers say could make nanotube-based actuators resistant to mechanical wear and fatigue. Indeed, they we able to inflate and deflate their balloon many times without causing any apparent damage to the nanotube. The research is described in Nano Letters.
Quantum coherence lasts forever, almost
Physicists based in Brazil, Italy, Germany and the UK have shown it is possible to defeat that great enemy of quantum technology: decoherence. The ability of quantum systems to exist in superposition states holds the promise, among other things, of computers that are exponentially faster than today’s classical devices. However, these states are usually destroyed by environmental interference after just a fraction of a second. Physicists can get around this problem by adding additional controls to a system, but doing so uses up huge resources. Quantum error-correction schemes, for example, require multiplying the number of quantum bits (qubits) many times over. In the new work, Gerardo Adesso of the University of Nottingham and colleagues have found that they can delay the onset of decoherence in certain composite systems that do not lose energy to the environment by at least a second – and in principle, they say, indefinitely – without resorting to error correction or other artificial schemes. The trick, they reveal, is to prepare states such that they are observed using a quantum basis that orthogonal to that of the noise source. The researchers observed this “everlasting quantum coherence” in spin systems containing two and four qubits made using nuclear magnetic resonance at room temperature, and they say that such states in future might be used to carry out low-noise magnetometry. Team member Rosario Lo Franco of the University of Palermo adds that plants might exploit a similar scheme in order to maintain the very long coherence times observed when they harvest light. The research will be reported in Physical Review Letters and a preprint is available on arXiv.
Universal census tots up to two trillion galaxies
Across the universe: among other data, scientists used the galaxies visible in the Great Observatories Origins Deep Survey (GOODS) to recalculate the total number of galaxies in the observable universe. The image was taken by the NASA/ESA Hubble Space Telescope and covers a portion of the southern field of GOODS. This is a large galaxy census, a deep-sky study by several observatories to trace the formation and evolution of galaxies. (Courtesy: NASA, ESA/Hubble)
The most accurate cosmic census ever has found that there are many more galaxies in the observable universe as was previously thought. An international team of astronomers, led by Christopher Conselice from the University of Nottingham, UK, used the latest data and images from the NASA/ESA Hubble Space Telescope to estimate that the visible universe contains around two trillion galaxies – some 20 times more than the previously estimated count of 100-200 billion galaxies. “It boggles the mind that over 90% of the galaxies in the universe have yet to be studied. Who knows what interesting properties we will find when we observe these galaxies with the next generation of telescopes,” says Conselice. The team converted Hubble images into 3D images to make accurate measurements of the number of galaxies at different epochs through the evolution of the universe, looking 13 billion years into the past. They found that galaxies are not evenly distributed throughout the universe’s history and that there were 10 times more galaxies per unit volume (albeit relatively small and faint galaxies) when the universe was only a few billion years old. The team also developed new mathematical models that allowed them to infer the existence of galaxies that are too faint or far away to be seen by today’s telescopes. The work is described in the Astrophysical Journal (ApJ830 83)
You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on how electron microscopy has been combined with femtosecond spectroscopy.
The laser altimeter for Europe’s first mission to Mercury has been fitted to the Mercury Planetary Orbiter (MPO). Developed by a European team led by the University of Bern in Switzerland, the BepiColombo Laser Altimeter (BELA) is part of the European Space Agency’s BepiColombo mission to Mercury, which is due to launch in April 2018. It’s the first laser altimeter for inter-planetary flight to be built in Europe.
Coupled system
The instrument will measure the topography of Mercury from aboard the MPO, which is one of two spacecraft that will orbit the planet as part of the BepiColombo mission. The MPO is being built by ESA, while the Mercury Magnetospheric Orbiter (MMO) is being built by the Japan Aerospace Exploration Agency (JAXA).
The aim of the BepiColombo mission – Europe’s first to Mercury – is to provide information on the composition, geophysics, atmosphere, magnetosphere and history of Mercury. The two spacecraft will travel to Mercury as part of a coupled system. When they reach Mercury in 2024, the MMO will study the planet’s magnetosphere via an orbit such that it will be 590 km from Mercury’s surface at its closest approach, while the MPO will follow a closer orbit at 480 km and survey the planet’s surface and internal composition.
BELA is one of 11 instruments that will be carried on the MPO. The laser altimeter uses the direct-detection approach. A high-power laser, which was developed by Cassidian Optroniks, a subsidiary of Airbus Defence and Space, based in Germany, will emit 50 mJ pulses with a wavelength of 1064 nm at a frequency of 10 Hz. These will be reflected back from the surface of Mercury and received around 5 ms later by the receiver telescope (RTL). The image is then refocused onto silicon-based avalanche photodiodes – these are based on the photodiodes used on the laser altimeters on NASA’s Mars Global Surveyor spacecraft and MESSENGER spacecraft, which orbited Mercury between 2011 and 2015. The signal from the photodiode is then analysed by an electronics module developed by Swiss technology company RUAG, to determine the time of flight (and therefore range and altitude), the integrated pulse intensity, and its width.
Mirror mirror
The RTL is a two-mirror telescope that was designed and developed by RUAG. Located inside the spacecraft, it needs to cope with temperature ranging from –20 °C to +45 °C without deforming, but also needs to be lightweight. “We decided to build the telescope entirely from beryllium to provide thermal compensation,” says Nicolas Thomas, BELA co-principal investigator, based at the University of Bern. “The 20 cm-diameter system weighs only 600 grams.” The optical surface was produce by diamond-machining copper deposited on the beryllium.
Due to Mercury’s proximity to the Sun, BELA will have to deal with intense heat and sunlight. The RTL is protected by a “baffle unit” that reflects 90% of sunlight striking it. “If we had had a traditional black baffle, not only would it have reached 450 °C, but we would have injected more than 300 W of heat into the spacecraft,” explains Thomas. “The outer (ceramic) ring still reaches 200 °C worst case but the 30 W we now dump to the spacecraft can be handled.” The laser is also protected by a similar but smaller unit developed by the German Aerospace Center, DLR, in Berlin.
Feel the heat
To deal with sunlight reflected from Mercury, both the optics system, which transfers light from the telescope to the photodiodes, and the laser are fitted with interference filters. The filter for the optics system isolates light at a laser wavelength of 1064 nm, while the filter for the laser prevents potentially dangerous light levels reaching the system.
“BELA will contribute a lot to understanding Mercury,” says Thomas, who adds that “Einstein’s studies of the motion of Mercury have been so important to the theory of general relativity. It is nice to think that with this instrument, the University of Bern, where he used to work, can play a leading role in studying this particular planet in detail.”
The $180m Five-hundred-meter Aperture Spherical radio Telescope (FAST) – the world’s largest single-aperture radio receiver – has joined the Breakthrough Listen programme, which launched in July 2015 to look for intelligent life beyond Earth. FAST was completed in September and is located in a natural depression in Guizhou province in southern China. The telescope consists of 4450 reflecting panels with a collection area that is more than twice as big in size as its nearest rival – the 300 m Arecibo telescope in Puerto Rico. FAST will now join the Green Bank Telescope in the US and the Parkes Observatory in Australia in hunting for alien signals, with the three observatories exchanging observing plans, search methods and data. “‘Are we alone?’ is a question that unites us as a planet,” says Yuri Milner, founder of the Breakthrough Initiatives, “And the quest to answer it should take place at a planetary level too. With this agreement, we are now searching for cosmic companions with three of the world’s biggest telescopes across three continents.”
Warm dense matter simulation sheds light on fusion
Warm and dense: simulation of electron density in warm dense matter. (Courtesy: Travis Sjostrom)
A new computer simulation of warm dense matter that could improve laser plasma fusion has been unveiled by physicists in Germany, the US and the UK. The simulations allowed Matthew Foulkes and colleagues at Imperial College London, Christian Albrechts University Kiel and Los Alamos National Laboratory to determine the phase diagram of warm dense matter – which exists in the temperature range between condensed matter and plasma (1000–100,000 K) and is characterized by hot electrons that move around within tightly packed atoms. It is an important step in the process of laser fusion, whereby intense lasers compress and heat a solid target driving the atomic nuclei together until they fuse and release large amounts of energy. As the electrons in the target are heated by the lasers, the target transforms into warm dense matter for just a few microseconds. This fleeting phase can be crucial to achieving fusion because it affects how the nuclei will be further compressed. If this compression is uneven, then significant amounts of fusion will not occur. Understanding the behaviour of the electrons during the warm dense matter phase could help physicists to improve the compression process. The simulations are described in Physical Review Letters and they could also shed light on warm dense matter in astronomy, including the behaviour of Jupiter’s core and the atmosphere of white-dwarf stars.
Theorist trio bag APS prize for particle accelerators
The 2017 Robert R Wilson Prize for Achievement in the Physics of Particle Accelerators has been awarded to theoretical physicists Sekazi Mtingwa of the Massachusetts Institute of Technology, James Bjorken from SLAC National Accelerator Laboratory and Anton Piwinski at DESY. The trio were given the prize “for the detailed, theoretical description of intrabeam scattering, which has empowered major discoveries in a broad range of disciplines by a wide variety of accelerators, including hadron colliders, damping rings/linear colliders, and low emittance synchrotron light sources”. The annual prize is awarded by the American Physical Society and recognizes outstanding achievement in the field of particle accelerators. The $7,500 award will be split between the recipients. According to Fermilab, Mtingwa “is the first African-American scientist to receive a prize from the American Physical Society”. Mtingwa, who now sits on the committee for the African Light Source, penned a Forum article for Physics World earlier this year – “A shining light for African science” – that called for physicists to get behind African plans to build the continent’s first ever synchrotron light source.
You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on the first laser altimeter for inter-planetary flight.
In sync. (Courtesy: Ted Kinsman/Science Photo Library)
What goes up must come down. But does everything come back down at the same time? Galileo said yes. Newton said yes. Einstein said yes. Still, many physicists today secretly believe the answer might be no.
That belief might seem strange. Countless experiments over the years have concluded that two objects dropped from a height will – regardless of their composition – fall to the ground at precisely the same moment, provided they do not suffer disparities in air resistance. Schoolchildren are routinely taught about this “universality of free fall”, often with reference to the famous 1971 video of the US astronaut David Scott standing on the Moon and demonstrating that, in the absence of any air, even a feather and a hammer fall in unison. If the universality is not clear from everyday experience, it is at least implied by Newton’s laws of motion and gravitation, which combine to suggest that the acceleration of a body due to gravity is proportional only to the mass of the planetary object it is being attracted to, not to its own mass. The conclusion would appear irrefutable.
Yet, some violation of the universality of free fall could come in very useful. One of the greatest obstacles to progress in physics is the gaping chasm between the classical world of Einstein’s general theory of relativity, our current best theory of gravity, and the fuzzy, largely microscopic world of quantum mechanics, which accurately describes the other three known forces of nature: electromagnetism; and the strong and weak nuclear forces. A bridge between the two worlds – a quantum theory of gravity – is the neatest theoretical solution, but it has been elusive. Some candidate theories would seem to entail additional forces that, at very fine timescales, create an imbalance in the pull of gravity for different objects. Indeed, the observation of a tiny and hitherto imperceptible difference in acceleration for two falling objects could be the first evidence that general relativity is flawed, ushering in a new paradigm in modern physics.
Before the turn of this century, the best tests of gravitational free fall could find no deviation in the acceleration of two masses to within one part in 10 trillion. But a new host of lab- and space-based experiments promises up to a 10,000-fold increase in this precision, potentially offering the first chance of testing quantum gravity theories. What is more, some experimentalists are presenting new ways to approach tests of free fall – for example, by employing purely quantum systems, or antimatter. The question “Does everything fall back to Earth at the same speed?” may soon have an answer far more accurate than ever before.
An old story
To Galileo, the answer was certainly obvious. Even as a young medical student at the University of Pisa in Italy, in the late 16th century, he argued that all bodies must fall to the ground at the same speed, because otherwise, in a shower of hailstones, large stones would reach the ground before small stones – assuming they all start their fall at the same altitude. This post-Aristotelian logic was famously tested in the (almost certainly apocryphal) story of Galileo dropping two different weights from the top of the Leaning Tower of Pisa.
This retroreflector (left) was put on the Moon by astronauts on the Apollo 11 mission. Astronomers all over the world have reflected laser light off the reflectors to measure precisely the Earth–Moon distance. NASA also uses laser-ranging to track the Lunar Reconnaissance Orbiter 28 times per second (right). (Courtesy: left: NASA Apollo Archive; right: Tom Zagwodzki/Goddard Space Flight Center)
But it was only the better part of a century later with the Newtonian revolution that a mathematical basis for the universality of free fall was established. Combine Newton’s second law of motion (the force on an object is equal to its mass multiplied by its acceleration in the direction of the force) and his law of universal gravitation (gravitational attraction is directly proportional to the product of two gravitating masses and inversely proportional to the square of the distance between them), and you naively find that the acceleration of a gravitating object is proportional to the mass of the object it is being attracted to, not to its own mass. Naively, that is, because the combination of these laws implicitly assumes an equivalence between two types of mass. On the one hand there is inertial mass, which we feel in situations describable by the second law of motion – turning a corner in a car, say – and on the other hand there is gravitational mass, which we feel all the time, being attracted to the surface of the Earth.
This implicit assumption of an equivalence between inertial and gravitational mass was pointed out by the German physicist Heinrich Hertz in the late 19th century. “[The properties] must be thought of as being completely independent of each other,” he wrote, “but in our experience, and only in our experience, appear to be exactly equal. This correspondence must mean much more than being just a miracle”: there must be “a deeper explanation”.
In 1915, within his general theory of relativity, Einstein established what has since become the agreed explanation: space–time. Like Galileo and Newton before him, Einstein accepted that objects travel in a straight line unless a force drives them otherwise. Unlike his predecessors, however, he established that this straight line exists on the fabric of 4D space–time, which is warped by mass. In the vicinity of very massive objects such as our planet, there is a pronounced depression in space–time. Roughly speaking, that means that a straight line in our everyday, 3D Euclidean geometry is bent inwards, towards the centre of the Earth, in much the same way that a straight flight path from London to New York appears on a 2D map to be an arc.
The Micro-Satellite à traînée Compensée pour l’Observation du Principe d’Equivalence, or Microscope, contains two test cylinders (one of which is pictured above right) of different masses that are orbiting Earth in free fall, with their acceleration being measured precisely. (Courtesy: CNES)
In this description, we feel gravity pulling us towards Earth simply because the ground under our feet – or the chair under our bottom – is diverting us from this straight line in distorted space–time. Likewise, we experience inertia when we are pushed from our existing straight line by another force – the grip of car tyres, the blast of a rocket or the sobering contact of an easy-to-miss lamppost.
In short, Einstein showed that inertia and gravity are locally two sides of the same coin: that they are the same is embodied in his “equivalence principle”.
Today, physicists describe three versions of the equivalence principle, of which one is the universality of free fall, or the so-called weak equivalence principle. Another version, known for historical reasons as the Einstein equivalence principle, states that this weak equivalence principle holds whenever and wherever an experiment is carried out. The third version, the so-called strong equivalence principle, states that the Einstein equivalence principle holds even if a mass is large and has substantial internal gravitational interactions.
In general, the equivalence principle can be seen to form the basis for general relativity, which is defined as a “metric” theory – that is, one in which matter behaves according to functions of distance on the fabric of space–time. Quantum mechanics is not a metric theory, and it is therefore widely assumed that any future theory bridging quantum mechanics with general relativity will have to ditch one or more aspects of the equivalence principle. “If you’re looking for a unified theory, you may have to abandon this concept of a metric space–time theory,” says experimental physicist Sven Herrmann of ZARM at the University of Bremen, Germany. “That would mean the equivalence principle was violated.”
Weak principle
One well-established method to test the strong equivalence principle is lunar laser-ranging, which takes advantage of retroreflectors – devices like cat’s eyes that reflect light back to its source – placed on the Moon during the US and Soviet Moon landings. The best results using this technique were reported in the mid-1970s by physicists Irwin Shapiro and Charles Counselman at the Massachusetts Institute of Technology in Cambridge, US, together with Robert King of the US Air Force Cambridge Research Laboratories. They analysed nearly 1400 measurements of the time required for laser light to go from a telescope on Earth to a retroreflector on the Moon and back, and found that the Earth and Moon must be “falling” towards the Sun with exactly the same acceleration, give or take one part in a trillion (1976 Phys. Rev. Lett.36 555). That precision is expected to be bettered soon with new data taken as part of the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) in New Mexico, US.
At the Bremen Drop Tower in Germany, experiments are performed under a microgravity comparable to one millionth of the Earth’s gravitational force (10–6 g). (Courtesy: ZARM, University of Bremen)
Indeed, it has been bettered already by another tool: the torsion pendulum, which consists of two different masses suspended on a wire. If the Sun’s gravity pulls on the masses differently due to a violation of the universality of free fall, there should be a twist of the pendulum. In 1999 Eric Adelberger and others in the Eöt-Wash group at the University of Washington in Seattle, US, found no such twist to a precision of one part in 10 trillion (1013), setting the current record for tests of the weak equivalence principle (Phys. Rev. Lett.83 3585).
In more than 15 years since this result, the Eöt-Wash group has struggled to improve its experimental precision, though it still hopes to do so by an order of magnitude by cooling its apparatus to near absolute zero. But many experimentalists believe it will prove too difficult to surpass it here on Earth. “It seems to me that a significant improvement in accuracy will only take place in space-based experiments in the future,” says Herrmann. “It’s hard for me to see that a 10–15 experiment will be done on the ground. But, with some clever new ideas, maybe!”
As it happens, this year brings the first possibility of a 10–15 precision from a space-based experiment, namely Microscope, or the Micro-Satellite à traînée Compensée pour l’Observation du Principe d’Equivalence (the drag-compensated micro-satellite for the observation of the equivalence principle), which was launched in April. Developed by the French National Centre for Space Studies (CNES), Microscope contains two cylindrical test masses – one made of the light metal titanium and the other of an alloy of the two heavy metals platinum and rhodium. The cylinders sit concentrically (to have coincident centres of mass) on separate accelerometers, undisturbed by terrestrial sources of noise, such as seismic perturbations.
Being in orbit, the satellite and its contents are in free fall towards Earth, and if general relativity is correct, the two test masses should remain motionless relative to each other. If there is a violation of the weak equivalence principle, however, one of the test masses will experience slightly more free-fall acceleration than the other, and this will be recorded by its accelerometer. Prior to launch, scientists from the French national aerospace centre ONERA, who are responsible for interpreting the Microscope data, claimed that the first results could come in as little as a few months, but so far none have been announced. The CNES is not the only institution hoping to exploit the calmer benefits of space, however. For several years physicists Robert Reasenberg and James Phillips, formerly of the Harvard–Smithsonian Center for Astrophysics in Massachusetts, US, have been developing an experiment designed to fly on a sounding rocket that briefly enters space in free fall. Unlike Microscope, SR-POEM (Sounding-Rocket based Principle Of Equivalence Measurement) has a pair of test masses that are interleaved with each other in order to keep their centres of mass coincident; one of the masses is made of solid aluminium, while the other contains hollow lead inserts. Moreover, SR-POEM has a markedly different system to determine acceleration. Each of the masses forms one end of a triplet of optical cavities, and if either mass moves, its cavities lengthen or shorten. Using a “laser gauge” based on a technique known as Pound–Drever–Hall locking, Reasenberg and Phillips lock the frequency of a separate laser to each cavity, so that if the cavity lengths change due to the acceleration of the test mass, so do the laser frequencies – and frequency can be measured to very high precision.
The physicists plan to measure the acceleration of the test masses towards Earth, with the apparatus pointed up and then – rotating the entire payload – with the apparatus pointed down. “We move the Earth to the other side of the experiment, then do it again,” jokes Reasenberg, who is now based at the University of California, San Diego, in the US. In this way, any spurious accelerations should perfectly cancel out, yielding a signal only if one of the test masses experiences more gravity than the other (2012 Class. Quantum Grav.29 184013). “If it’s not zero, either you’ve got a systematic error, or you’ve discovered a violation of the equivalence principle,” Reasenberg says.
SR-POEM should have an accuracy 100 times greater than Microscope, due to the isolation of the test masses and the acuity of the laser gauges. Reasenberg also reckons it could be done at 10% of the cost, and in as little time as half an hour. But currently there is no funding to realize the experiment as, according to Reasenberg, NASA funds are devoted to the forthcoming James Webb Space Telescope, which is scheduled for launch in late 2018. “I don’t think there’s a good chance of getting funding until the James Webb is launched,” he says.
What goes up, keeps going up?
The good news for Reasenberg and Phillips is that their highly precise system for distance measurement has found its way onto an altogether different test of the equivalence principle – one that investigates antimatter. Believe general relativity, and antimatter should fall to the Earth exactly like matter; but some alternative gravity theories predict it should do the opposite, and “fall” upwards. This kind of antigravity effect could explain why we don’t observe equal quantities of matter and antimatter in our local universe, even though the two types are predicted to have been generated in equal quantities after the Big Bang. Simply put, the two types of matter might have repelled each other, driving all the antimatter into distant regions of the universe. “Maybe clusters of galaxies, or even portions of the universe as big as we can see, are made of antimatter,” says Reasenberg.
The antimatter “free fall” test to which Reasenberg and Phillips are contributing is being led by Daniel Kaplan at the Illinois Institute of Technology in Chicago, US. It involves sending a beam of muonium – a hydrogen-like atom, consisting of an electron orbiting an antimuon – into an atom interferometer, horizontally. The muonium beam is split at a diffraction grating, with each new beam taking a slightly different path before being diffracted again at a second diffraction grating, generating an interference pattern at a third diffraction grating. From the position of this pattern, which can be measured with the help of the third grating, it is possible to work out whether the muonium atoms fall down or up (EPJ Web Conf.95 05008).
One difficulty with this type of experiment comes down to the lifetime of muonium: with a half life of just a couple of microseconds, the window for observing any up or down movement is very small, even if large numbers of particles are employed. The challenge, therefore, is incredibly high precision in the interferometer’s alignment to observe tiny changes in the position of the interference pattern. Kaplan’s group plans to use Reasenberg and Phillips’s laser gauge to ensure that the motion of the third grating is known to within 10 picometres (10–11 m) or so, and calibrate zero acceleration through use of an X-ray beam that has roughly the same wavelength as muonium. “If you found antimatter was falling up, that would be a major discovery,” says Reasenberg. “It changes physics. It changes cosmology.”
The benefit of investigating antimatter is that it provides an alternative way to tackle the equivalence principle, one for which no precise precedent has yet been set. That is also the case with PRIMUS, the Präzisionsinterferometrie mit Materiewellen unter Schwerelosigkeit (precision interferometry with matter waves in zero gravity), which is being funded by the German space agency DLR and is being performed at the 146 m-high drop tower at the University of Bremen by Herrmann as well as Dennis Schlippert and others at the University of Hannover in Germany. It also involves sending atom-waves into an interferometer, but these atoms are in a Bose–Einstein condensate (BEC): they are in the same, lowest quantum state, and behave as a single quantum entity. The speculation is that such a decidedly quantum system could, like antimatter, behave differently when it comes to the universality of free fall. “It might be more sensitive to violation,” says Herrmann, “because it is closer to the boundary between quantum theory and gravity.”
As a matter-wave in an interferometer, a BEC has a phase, although this phase can be altered with exposure to a laser of the right wavelength. In PRIMUS, a BEC is allowed to free fall in the drop tower before being hit by such a laser pulse, which simultaneously splits the quantum entity into two paths (like a diffraction grating) and changes its phase, with the new phase dependent on when and where the laser struck. Farther down the drop tower the BEC is struck by another laser, which recombines the two halves to create an interference pattern. By examining the periodicity of this pattern, it is possible to work out the initial phase change, where it took place, and therefore the acceleration on the BEC due to gravity.
Last year, as a precursor to PRIMUS, Schlippert and colleagues performed a similar free-fall test for two BECs, one made of potassium and one of rubidium, to a precision of one in 10–7 (Phys. Rev. Lett. 112 203002). No violation yet, but the experiments are breaking ground for a different type of test of the equivalence principle. Indeed, Herrmann says that a future version of the PRIMUS experiment may one day find its way into space.
And what if a violation is found? That could well be the first evidence for a theory of quantum gravity – perhaps string theory, in which all fundamental particles are in fact twisted loops that, when expanded, spread into 10 or more dimensions. There are currently no firm predictions of where experimentalists should expect violations of the equivalence principle according to string theory, but some theorists expect them to become visible at precisions greater than 10–13.
Even then, there is the natural caution with which one interprets a single experimental result, particularly when it affects a theory that has proved unshakeable for 100 years. “You don’t topple a major piece of physics like general relativity based on one experiment,” says Reasenberg. “There’s always the possibility that there’s some systematic error nobody thought of, and there’s no physics in it at all.”
Last May the Huffington Post ran a piece entitled “Why physics experiments at the subatomic level may cause ‘unknown unknowns’ to destroy the world”. It was written by Peter Reynosa, a poet and painter from San Francisco, who’s also the author of a dystopian novel about the dangers of rationalism. As an enthusiastic newcomer to the old debate over whether the Large Hadron Collider (LHC) will destroy the universe, Reynosa reckoned the LHC is a monster that is “nightmaring itself into our world”. Though vague on specifics, Reynosa claimed the collider is staging “very dangerous experiments that may cause the world’s destruction”.
To support his claim, Reynosa cited the Bush Administration defence secretary Donald Rumsfeld’s famous phrase concerning “unknown unknowns”, which he originally used in 2002 to defend the invasion of Iraq. The LHC is designed to investigate matter at its fundamental levels. Although we may not think this poses any dangers, there could be, Reynosa claimed, things we don’t know that may cause its operation to trigger “catastrophic repercussions” that will create an “unthinkable horror”.
Bush league
Frightening! But arguments using “unknown unknowns” were dishonest when Rumsfeld first made them to defend the Iraq invasion, and are equally dishonest made against the LHC. Iraq, after all, had nothing to do with the terrorist attack of 11 September 2001, but in its aftermath the Bush administration prepared to invade the country anyway. As a shameless pretext, it cited the possibility Iraq might give terrorists weapons of mass destruction.
At a news conference on 12 February 2002, a reporter asked Rumsfeld what evidence he had. Rumsfeld replied evasively. “There are,” he said, “things we know we don’t know.” Then he added: “But there are also unknown unknowns – the ones we don’t know we don’t know.” Those, he implied, were sufficient to justify invasion. As it turned out, there was no evidence of weapons of mass destruction, and an inspection team that hunted for them came up empty-handed.
Rumsfeld’s use of “unknown unknowns” to justify the decision to invade Iraq is like justifying pulling out a gun and shooting a suspect who shows no sign of ill intent by saying “You never know!” It was a smokescreen to rationalize the already-taken decision. The appeal to “unknown unknowns” uses naked fear-mongering to try to turn a lack of evidence for an action, such as invading Iraq or shutting down the LHC, into a positive reason for proceeding as if there were evidence.
We might call this a Chicken Little argument, as it’s like the children’s fable in which Chicken Little, after getting hit on the head with an acorn, tells the other animals to run because “the sky is falling”. But this new version is a “could-have” Chicken Little argument, for the chicken is claiming the sky is falling because she “could have” been hit by an acorn. The “unknown unknowns” argument is a fallacy.
Two kinds of probabilities
Theoretical physicists who’ve looked into whether heavy-ion collisions can produce “strangelets” or black holes that will consume other forms of matter have concluded that the probability of such events is non-zero. So isn’t there at least minuscule evidence that the LHC is a threat? No, because there are two kinds of probabilities. One – established probability – is based on sound principles and empirical data of frequencies of actual occurrences. This is the probability of coin tosses, winning the lottery, human mortality, the molecular behaviour of gases and so on. We’re sure of the principles and data, and no magical thinking is involved. Established probabilities are reliable guides to the world.
The other kind of probability is fictive or subjective. This is when you assume certain principles and initial conditions and use them to estimate possible outcomes. You don’t know whether all the principles are sound, and you have no data on outcomes from relevant similar experiments. You have no reason to suppose the probabilities reflect the real world, and it would be folly to use them as a guide to action.
We can show this by turning the argument on Reynosa’s own profession. I’m sure someone can concoct a theory of insanity that yields a non-zero probability that poetry and painting can drive individuals to commit mass murder. If there were no empirical data to back up this finding, wouldn’t it be insane and unjust to use it as a reason to ban poets and painters from practising?
The safe working of our scientific, technological and medical infrastructure is highly vulnerable to dishonest unknown unknown arguments. If you use that argument, then it’s easy to claim that cell phones may cause cancer, vaccines autism, and genetically modified organisms disease, and so they should be banned. After all, you never know!
The problem is that those who advance such arguments often have no real interest in public safety but only in self-advancement or special pleading. Politicians and activists exploit such arguments to counter policies they do not want to support. Others use such arguments to sell books or get attention. Media sources lap up such incendiary messages. In my May 2007 column, I called people who seek to advance themselves by sowing unwarranted suspicion “social Iagos”. Iago, the villain in Shakespeare’s Othello, used a handkerchief to cause his boss to doubt his wife; today’s scoundrels use “unknown unknowns”.
The critical point
Arguments that use “unknown unknowns” to promote a particular course of action are harmful in several ways. They can lead to different kinds of harmful and destructive courses of action, such as shutting down accelerators or stopping medications. They can be used to create distrust of legitimate and valuable institutions, such as review committees. Finally, they are harmful in seeking to curtail investigating the dynamics of the world, which is far safer than not investigating it at all.
There are good reasons to get excited by the €2bn European Spallation Source (ESS), which is currently under construction in Lund, Sweden. When the machine’s user programme finally starts in 2023, if all goes to plan, it will be the world’s most intense particle accelerator, generating up to 100 times more neutrons than any of today’s sources. Like a giant microscope, it will allow unprecedented studies into various fields – particularly the science of the everyday, such as plastics, pharmaceuticals, biological matter and nanotechnology. The ESS is a fitting tribute to Europe’s neutron research community, which is estimated to be by far the world’s largest, comprising some 6000 scientists and engineers.
Against this starry-eyed picture, however, a recent report published by the Neutron Landscape Group (NLG) makes for sobering reading, by pointing out that many sources are set to close within a decade. By the mid-2030s, according to the NLG, the best-case scenario is a 30% drop in neutron instrument time, while the worst-case scenario is a 60% reduction. In decades to come the ESS may be a transformative neutron source, but there may not be much of a neutron community left to use it. “The renewal of intermediate neutron sources becomes necessary to maintain a ‘critical mass’ for the neutron user community, otherwise new powerful sources such as the ESS become almost useless,” says Jacques Ollivier, a physicist at the Institut Laue-Langevin (ILL) in Grenoble, France. The ILL itself could close in 2023 unless its partners agree otherwise.
The renewal of intermediate neutron sources becomes necessary to maintain a ‘critical mass’ for the neutron user community, otherwise new powerful sources such as the ESS become almost useless
Jacques Ollivier, Institut Laue-Langevin
Fortunately, there may be ways to plug this neutron deficit. Physicists at the Forschungszentrum Jülich, in collaboration with those at the Laboratoire Léon Brillouin (LLB) in Saclay, France, are exploring the possibility of cheap, scalable neutron sources that could be installed at universities or at national facilities. The technology has already been put into practice outside Europe, for example at Indiana University in the US and at the Japan Collaboration on Accelerator-driven Neutron Sources – a nationwide network of researchers working on neutron-beam technologies and moderators. But the researchers at Jülich and the LLB want to push the technology to its limit, boosting the power output by at least a factor of 100.
Tailored solution
There are currently two main types of neutron source: spallation and reactor. In spallation, charged particles such as protons are accelerated into a heavy-metal target, kicking neutrons from the target’s nuclei. This process, which is employed at various facilities including the ISIS Neutron and Muon Source in Oxfordshire, UK, and which will also be employed at the ESS, is the most efficient, generating some 20 neutrons per incident proton. The second type of neutron source, the fission reactor, involves a single neutron splitting a uranium atom to generate three neutrons, which then split more uranium atoms, provoking a chain reaction. The ILL and the LLB are both reactor neutron sources.
For sociopolitical reasons, new research reactors are deemed unlikely to be built in Europe. But the chances of building further spallation sources in addition to the ESS are slim too – partly because big accelerators themselves are pricey, but also because the intense radiation requires huge amounts of shielding. The ESS’s inner monolith alone will comprise some 2000 tonnes of steel. “[Spallation] is a very efficient process, but it comes with a huge price tag,” says Thomas Brückel, director of the Jülich Centre for Neutron Science.
Brückel’s answer is a compact accelerator source, which accelerates protons or deuterons (bound protons and neutrons) to MeV rather than – in the case of the ESS – GeV energies. Such modest energies mean that the target must be made of a light metal, most likely beryllium. That in turn means a drop in efficiency, with an average of 10 incident protons or deuterons required to generate a single neutron. But the accelerator should be much cheaper and should produce far less unwanted radiation.
There are other benefits too. With less risk of radiation damage, the moderators (which reduce the energy of the emitted neutrons to a usable meV range) and the neutron optics (which guide the neutrons to the instruments) can be placed much closer to the target. That means several moderators can be squeezed around the target, each tailored to the needs of a specific instrument. In bigger spallation sources, where moderators have to be placed metres away from the target, the moderators have to be compromised to suit the needs of several instruments. “If you go to a shoe shop to buy some shoes, you do not buy size 50 so that they fit any feet – you buy shoes that fit your feet,” explains Brückel.
Consider this benefit, says Brückel, and suddenly this inefficient process becomes competitive. It is not better than current medium-flux sources, but he adds that it will be possible to build sources with dedicated beams for instruments “with a price tag that is much lower than what you have nowadays” – as little as €20m, about 100 times less than a new research reactor. “The beauty is that you can downscale it,” he notes.
Indeed, Brückel believes that the compact source could fit on one side of a football pitch, making it a viable prospect for universities as well as national facilities like ISIS. “At university, everybody has an electron microscope, everybody has an X-ray tube,” he says. “But normally university students can’t learn how to work with neutrons and professors can’t do challenging neutron experiments; they have to go to a large-scale facility. And this step, if you don’t have the experience, is huge.” The compact source could be just the ticket to foster the broader neutron community in the run up to the ESS and beyond.
To develop the technology, Brückel and his colleagues are working closely with Alain Menelle and others at the LLB. One of the greatest challenges, Menelle explains, is constructing the targets to produce neutrons effectively. “There are lots of [potential] solutions, [but] we can’t explore them all,” he says. There has already been interest from other institutions, he adds, including ESS-Bilbao in Spain, which is providing in-kind contributions to the ESS in Lund, and the Paul Scherrer Institut near Zürich, Switzerland. Once up and running, the compact neutron source ought to be able to perform neutron imaging, small-angle neutron scattering, powder and single-crystal diffraction, and perhaps other novel techniques; its limitation will be in the use of relatively large samples and an inability to reveal atomic dynamics (in contrast to the ESS).
Menelle’s group is hoping to have a low-flux demonstrator source ready for 2020, paving the way for the initial research version five years later, dubbed the Source compacte de neutrons s’Appuyant sur la technologie des accélérateurs (SONATE). Brückel’s group at the LLB is less ambitious on timescale, hoping to have its prototype ready for 2024 and the research version, the High Brilliance neutron Source (HBS), for 2030 or later. Neither SONATE nor the HBS will therefore plug the imminent deficit created by the closing medium-flux sources, but they could ease the loss – if it happens – of the ILL in the early 2020s.
Whether or not the ILL closes will come down to politics. Next year it will undergo an evaluation by its three major partners – France, the UK and Germany – and the 11 smaller national partners. Already it has undergone some €27m of safety improvements to protect the reactor from floods and other Fukushima-style disasters, but more funds will be needed to prolong its lifetime – cash that few partnered countries want to commit. Yet its loss would have a dramatic impact on the neutron community, making the new breed of sources like the HBS and SONATE all the more necessary.
