Stephen Hawking has decided to pull out of the fifth Israeli Presidential Conference: Facing Tomorrow 2013, which istaking place in June. The world-famous British cosmologist and science communicator was due to deliver a keynote speech at the conference in Jerusalem, which boasts other presenters including Tony Blair, Bill Clinton and Mikhail Gorbachev. But it appears that Hawking has made a late U-turn. That is according to a statement published by the British Committee for Universities for Palestine – an organization of UK-based academics, set up in response to the Palestinian call for an academic boycott of Israel.
We understand that Professor Stephen Hawking has declined his invitation to attend the Israeli Presidential Conference Facing Tomorrow 2013, due to take place in Jerusalem on 18–20 June. This is his independent decision to respect the boycott, based upon his knowledge of Palestine, and on the unanimous advice of his own academic contacts there.
Cosmic rays interacting with water droplets within thunderclouds could play an important role in initiating lightning strikes. That is the claim of researchers in Russia, who have studied the radio signals emitted during thousands of lightning strikes. The work could provide new insights into how and why lightning occurs in the first place.
Although most people have witnessed a flash of lightning during a thunderstorm at some point in their lives, scientists still do not completely understand what triggers the discharge in the first place. Lightning has been studied for hundreds of years, yet while many possibilities for observation are available – there are about 40 to 50 lightning strikes per second across the globe – predicting the onset of a strike is difficult.
There are three basic types of lightning: lightning that occurs within a single cloud; lightning that occurs between two clouds; and lightning that occurs between a cloud and the Earth’s surface. In a typical cloud-to-ground lightning strike, scientists know that an electrically-conducting plasma channel forms between the cloud and the ground, which allows the discharge to occur. However, the factors that cause the initial charging of the cloud and its subsequent discharge are not clearly understood.
Cosmic ray kick-off
Now, Aleksandr Gurevich of the Lebedev Physical Institute in Moscow and Anatoly Karashtin of the Radiophysical Research Institute in Nizhny Novgorod have suggested a new model that includes two crucial factors that could help explain the process: the behaviour of water or ice particles inside clouds, dubbed “hydrometeors”; and showers of ionized electrons that might be created by cosmic rays.
The theory that cosmic rays may cause the ionized showers that initiate lightning was first put forward by Gurevich more than 20 years ago. Known as “runaway breakdown”, Gurevich suggested that the ionized particles create free electrons within thunderclouds that are then accelerated to extremely high energies by electric fields within the clouds. These electrons collide with other atoms in the air to cause an “avalanche” of high-energy particles within the cloud – and this provides the seed for the onset of lightning. While the theory was widely discussed, Gurevich was not able to find proof that cosmic rays do indeed trigger the avalanche.
In a bid to gather more evidence, Gurevich and Karashtin have now done a new analysis using a radio interferometer of radio pulses emitted at the onset of 3800 lightning strikes across Russia and Kazakhstan. A long series of these short yet strong pulses is emitted just before lightning strikes and, according the researchers, the pulse data match Gurevich’s model of electrical breakdown.
Pulses of information
The researchers also point out that the amplitude of a pulse is proportional to the number of secondary electrons, and so also to the energy of the initial cosmic ray that generates the shower. But when they calculated the cosmic-ray energy, Gurevich and Karashtin found it to be about 1017 eV – a surprising figure as cosmic rays of this energy are too rare to explain what was measured.
To explain why such high energies were observed, the researchers suggest that the hydrometeors they used become electrically polarized as the strong electric field inside the cloud builds up and that a further “micro-discharge” occurs at the hydrometeor as the field reaches its threshold, thereby effectively amplifying the cosmic-ray-initiated breakdown. When this is taken into consideration, then much more common cosmic-ray particles with energies of about 1012–1013 eV are sufficient to initialize a discharge.
Physicist and lightning expert Joseph Dwyer of the Florida Institute of Technology, who was not involved in the current research, says that the new model is “an interesting idea, but much more work is still needed, for example experiments to measure radio pulses and air showers at the same time”, which is something that Dwyer and his colleagues are currently working on themselves.
Gurevich and Karashtin say that their observations show that the radio emissions are generated by the specific discharges in thunderclouds, which are different from the conventional electric discharges expected and that the “runaway breakdown” plays a significant role too. Further observations will be necessary to finally crack the mystery of the atmospheric crackle.
In less than 100 seconds, science blogger Andrew Jaffe shares his thoughts on what scientists can gain from sharing their insights and experiences with the wider scientific community.
After discovering the Higgs boson last year, researchers at the Large Hadron Collider are now trawling through the data as the collider undergoes an 18-month shutdown for repairs and upgrades. The goal is to discover hints of physics beyond the Standard Model of particle physics – but tantalizing glimpses of new physics have been harder to spot than many physicists had expected.
Three years, eight-quadrillion particle collisions and the discovery of the most infamous particle of them all: the Higgs boson. With such achievements under their belt, you might think that physicists working on the Large Hadron Collider (LHC) at CERN, near Geneva, would be taking a well-earned break. But the current shutdown – a two-year period of repairs and upgrades that began in February – is affording them no holiday. “This is actually the most intense period we’ve ever had,” says Joseph Incandela, spokesperson for the LHC’s CMS experiment. “The schedule is so tight there’s almost no contingency to prepare for the next run. It’s a bit insane.”
Since starting up in October 2008 – and following a major malfunction a few weeks afterwards that took a year to fix – the LHC has rarely been out of the headlines. The focus of news has been on the search for the Higgs boson, the last piece in the Standard Model puzzle of elementary particles and forces. Strong hints of the Higgs’s existence came last July, when CERN announced the discovery of a new particle, the mass and production rate of which appeared close to Higgs parameters in Standard Model predictions. By March this year, other measured properties, such as zero spin and positive parity, had led the laboratory to claim that the new particle was a Higgs almost without any doubt.
Triumph and disappointment
But while the public has largely taken the discovery of the Higgs boson as mission accomplished for the €3.8bn collider, many particle physicists have been shaking their heads in disappointment. Since it started collecting data, the LHC has exposed few – if any – traces of physics beyond the Standard Model, a framework that is now some 40 years old. There has been no solid evidence for dark matter, supersymmetry, miniature black holes, extra dimensions or any of the other exotic phenomena that theorists excitedly talked about prior to the machine’s switch-on. If there is new physics still waiting to be found, the question is: where? And will it turn up in the current shutdown period from an analysis of existing data or in the next, higher energy run?
Those waiting for new physics can take comfort in the fact that the LHC has achieved far more than the discovery of the Higgs over its three-year operation. A year before the Higgs’s detection, for instance, the ATLAS experiment found another new boson: the so-called Chi-b(3P) quark-antiquark pair. That was followed by the discovery last year of a new excited Xi(b) baryon by CMS. Although not elementary particles like the Higgs is thought to be, Chi-b(3P) and Xi(b) have helped tie up some of the Standard Model’s loose ends by confirming the nature of the strong force, which binds quarks together.
Precision particle physics
Perhaps more important than these particle discoveries, however, have been the LHC’s precise measurements of existing Standard Model phenomena. Some of these are quantities that cannot be accurately predicted, such as the high-energy structure of the photon that is being studied by the ALICE experiment. But other measurements can put the latest theories to the test. These include the energy distribution of particle jets (which are produced when quarks collide), and the production rate of pairs of heavyweight elementary particles such as W and Z bosons (which carry the weak force, responsible for radioactive decay) and top quarks. “Those calculations have been taken now to a higher degree of precision,” says Incandela. “We have a very good match between our data and our simulations, which tells you that our calculations are very good.”
Will the next big news at CERN be about supersymmetry? Physicists in CERN’s main auditorium listen as the latest data from the Large Hadron Collider are presented on 4 July 2012. (Courtesy: CERN)
Testing the Standard Model in this way is not merely an excuse for self-congratulation; it allows theorists to figure out which of their more speculative hypotheses are worth pursuing. In 2008, for instance, the CDF and D0 experiments at the Tevatron collider at Fermilab in the US accumulated evidence for an unexpected asymmetry in the production of top- and antitop-quark pairs, such that more of the top quarks seemed to fly in the direction of the collider’s proton beam than ought to, given Standard Model predictions. Theorists rushed to explain the effect, invoking extra dimensions, supersymmetry and other new physics.
The problem was that the top quark was so heavy – more than 180 times as massive as the proton – that the Tevatron could not generate it in sufficient quantities to give reliable statistics. Conversely, the LHC, which collides protons at record-breaking energies of 7 TeV, has been able to generate millions of top-quark pairs. Although CERN’s collider has not been able to shed light directly on the Tevatron’s measured asymmetry, it has managed to show, via measurements of a related top-quark asymmetry at ATLAS, that most of those theories proposed to explain it must be wrong (arXiv:1203.4211).
Where are the sparticles?
Besides the discovery of the Higgs, then, one of the main achievements of the LHC to date has been in ruling out new-physics theories, or at least restricting the elbow room, or “parameter space”, in which they can operate. Top among all of these theories was always supersymmetry, the idea that every known elementary particle has one or more heavier partners, known as sparticles. Supersymmetry potentially offers a solution to the “hierarchy problem” – why the weak force is 1032 times stronger than gravity – and presents candidates for dark matter, the mysterious substance thought to make up 26.8% of the universe’s total mass–energy content. According to particle theorist Ben Allanach at the University of Cambridge in the UK, data taken at the LHC have excluded roughly half of supersymmetry’s parameter space.
If supersymmetry doesn’t crop up, I’ll then be getting pretty depressed
Ben Allanach, University of Cambridge
Much of the data from the LHC’s first run has not yet been analysed – the CMS collaboration, for instance, still has to comb through 40% of the 4.7 billion events it recorded last year. Allanach thinks there is a chance that hints of new physics, such as supersymmetry, will crop up in the data analysed during the shutdown period, but he thinks that any big discoveries will have to wait until 2015 when the accelerator restarts at the higher collision energy of 13 TeV. “My hopes are pinned on the next run,” he says. “The energy jump now is going to make the big difference. And if supersymmetry is the correct theory of nature, I would be expecting to see a big signal within the first month. If it doesn’t crop up, I’ll then be getting pretty depressed.”
Blind scanning
Others are not so optimistic. Last year, the Latvian theoretical physicist Mikhail Shifman posted an essay on the arXiv preprint server claiming that supersymmetry had failed its basic experimental tests, and that theorists should “stop blindly scanning the parameter space and start thinking and developing new ideas” (arXiv:1211.0004v1). But despite growing pessimism, there may already be signs of supersymmetry in current data. It may even arise out of the recently discovered Higgs boson. Its 125 GeV/c2 mass, combined with its production and decay rate, might fit well with Standard Model predictions, but it might also point to the lightest of several Higgs particles predicted to exist in the simplest versions of supersymmetry.
Null results are hard to sell to newspapers, but they are really important to scientific progress
Bill Murray, ATLAS
Bill Murray, deputy physics co-ordinator of the ATLAS collaboration – which, together with CMS, made the Higgs discovery – says that a supersymmetric Higgs would require a supersymmetric partner for the top quark, the “stop”, to be found at masses below 1000 GeV/c2. “That is a region we’re testing enthusiastically,” he says. But he stresses that he is open to the possibility of finding no evidence for supersymmetry. “Proving [supersymmetry] wrong would be as important as proving it right,” he says. “Null results are hard to sell to newspapers, but they are really important to scientific progress.”
A composite Higgs?
Many physicists have latched onto the Higgs discovery, hoping to find out whether it really does fit Standard Model predictions or whether, over the next few years, they will find hints of a more exotic nature. Even if the Higgs is not supersymmetric, there is the possibility that it is not elementary but a composite of smaller particles, or that its existence stretches over higher dimensions.
Theorist John Ellis of King’s College London is doubtful whether these more exotic possibilities will be correct, given, he says, that the Higgs’s properties are already known to agree with Standard Model predictions to within some 10% on average. But he thinks the next two years of shutdown could offer news on supersymmetry, as the rest of the first run’s data is analysed. “Experiments have looked under the most obvious lampposts for supersymmetric signatures,” he says. “Now, they’ve got two years during which all the graduate students can fan out and look under the many other possible lampposts. If we’re lucky, something might be lurking underneath one.”
Hard at work at the LHC (Courtesy: CERN/Samuel Morier-Genoud)
By Hamish Johnston
Recently I had the pleasure of speaking to CERN’s Steve Myers who is supervising the herculean task of upgrading the superconducting magnets that guide protons around the Large Hadron Collider (LHC).
Scientists in California have proposed a new type of gravitational-wave detector that is immune to laser noise – a problem that adds to the expense of current detector designs. The researchers believe that their proposal – a modified form of an atom interferometer – would be cheaper and easier to implement in space than current laser interferometers.
Gravitational waves are tiny perturbations in the curvature of space–time that arise from accelerating masses – according to Einstein’s general theory of relativity. The first hint that the waves exist was spotted in 1974 as a gradual decrease of the orbital period of the pulsar PSR B1913+16, which circles a neutron star. However, no-one has directly detected a gravitational wave. Such a discovery would provide confirmation of general relativity and also open a new field of gravitational-wave astronomy, in which distant objects could be studied by the waves they emit.
Huge and costly
The conventional way to try to detect gravitational waves involves a long-baseline laser interferometer. A passing gravitational wave should cause the pathlengths of the two beams to change slightly, causing a shift in the interference fringes when the beams are recombined. None of these detectors have yet succeeded in detecting a gravitational wave; so to increase sensitivity, astronomers need to put detectors in space. Constructing a traditional L-shaped interferometer in outer space would require three satellites, which poses severe technological and financial challenges. The proposed Laser Interferometer Space Antenna (LISA) project, originally scheduled for launch in 2015, has been revised because of its high cost.
A single-baseline interferometer, which measures the change in length of a single path by interfering the emitted and reflected waves in a mirror cavity, would require just two satellites. But in this set-up it would be difficult to distinguish changes in pathlength from random fluctuations in the frequency of the laser – a phenomenon called phase noise.
Atom interferometers were proposed in the late 1980s and first built in the early 1990s by physicists including Mark Kasevich and Steven Chu at Stanford University. Instead of measuring the difference in phase between two beams of light, an atom interferometer measures the change in the phase of a matter wave made of atoms in a superposition of quantum states. An atom interferometer can be created by repeatedly exciting and de-exciting one half of the wavefunction using a laser while holding the other half in the ground state. The wavelength of an atom shortens when the atom is in its excited state, creating a phase shift between the two halves of the wavefunction that depends on how long the first half has spent in the excited state.
Each atom cloud is like a stopwatch
Mark Kasevich, Stanford University
In this latest work, Kasevich and colleagues, led by theoretical physicist Peter Graham at Stanford University, propose placing two atom interferometers a long distance apart and using the same pulsed lasers – one originating at one interferometer, one at the other – to excite and de-excite the atoms in both interferometers. The time each atom spends in the excited state depends on the travel time of the laser pulses between the two atom interferometers. “Each atom cloud is like a stopwatch,” explains Kasevich. “When the laser pulse comes from one direction, it starts the clock. When it comes from the other direction, it stops it.”
Putting atoms to work
If the length of the baseline between the interferometers is constant, the atoms at both interferometers will accumulate the same phase shift. But if one interferometer accelerates relative to the other, the time between excitation and de-excitation of half the wavefunction will differ at the two locations and the atoms will accumulate a different relative phase shift. The same laser pulses excite and de-excite the atoms in both interferometers, so the laser-phase noise affects both atoms in the same way and does not affect the difference between the phase shifts detected at the two interferometers. “The light is just acting as the gate to turn off and on the clock,” says Kasevich. “The atom is doing all the hard work.”
Gravitational-wave expert B S Sathyaprakash of Cardiff University is cautiously optimistic. “The scheme is obviously very exciting,” he says. “But I think the big question is what kind of technology is required in space to run this thing for three to five years? I’m not saying anything negative or positive, but I would like to see numbers.” In an attempt to provide these, the Stanford team is currently planning to build a prototype in the laboratory to ascertain whether or not there are any unforeseen technical challenges with the proposal.
I think it’s safe to say that Peter Woit was never going to like Gordon Kane’s latest book about string theory. Woit, who is at Columbia University, is a prolific anti-string-theory blogger and author of Not Even Wrong: The Failure of String Theory and the Continuing Challenge to Unify the Laws of Physics, whereas Kane is a leading string theorist who is based at the University of Michigan.
Kane’s latest tome is called Supersymmetry and Beyond: From the Higgs Boson to the New Physics and it will be published later this month by Basic Books. On his blog – also called Not Even Wrong – Woit compares the new book with Kane’s previous effort Supersymmetry: Unveiling The Ultimate Laws Of Nature, which was published in 2000.
Woit makes the controversial claim that about 75% of Supersymmetry and Beyond is a simply a rehash of the 2000 book. To make his point, Woit focuses on several examples of how Kane has updated the text to paper over the fact that little experimental evidence for supersymmetry has been found over the past 13 years.
They are the bane of car owners worldwide: star-shaped cracks that spread out from a tiny chip until they cover the entire windscreen. So, to find out more about how these common but little-understood cracks grow, a group of physicists in France has carried out a series of simple experiments using brittle plates of both plastic and glass. The researchers found a scaling law that describes the cracking process in terms of several fundamental parameters – a finding that could have a wide range of applications from forensics to planetary science.
The impact experiments were done by Nicolas Vandenberghe, Romain Vermorel and Emmanuel Villermaux from Aix-Marseille University, who used an airgun to fire steel and ceramic projectiles at plates of either glass or poly(methyl methacrylate) – better known as PMMA, Plexiglas or Perspex – that were no more than 3 mm thick. The resulting cracking process was monitored using a high-speed camera for about half a second, which is the time it takes the initial shock wave to reach the edges of the plates. After the wave reaches the plate edge it reflects back to the centre and complicates the cracking process.
Counting cracks
Each individual experiment involved firing a projectile at a speed of between 5 m s–1 and 100 m s–1 at a plate and counting the number of cracks that emanate from the point of impact. Vandenberghe and colleagues found that, for both materials, the number of cracks increased in proportion to the square-root of a dimensionless impact velocity that is based on the material properties of the glass or the PMMA. This relationship was true for glass that was just 0.15 mm thick as well as PMMA plates varying in thickness from 0.5–3.0 mm.
The existence of a universal law, which was successfully able to describe all the data collected in the study, was based on calculations that express the number of radial cracks in terms of the velocity of the projectile and the thickness of the plate. Also considered were the material’s speed of sound, its Young’s modulus (stiffness) and fracture energy – the latter being the minimum energy required for a crack to form in a material.
The team also examined what happens when the projectiles are fired so fast that they travel at speeds beyond the point where just simple star-shaped cracks are seen. The researchers found that, as the velocity increases, circular cracks surrounding the point of impact also appear. For a given PMMA plate thickness, the radius of the first circular crack was found not to vary with projectile speed, but the velocity at which the first such crack appeared obeyed a scaling law that involved the thickness of the plate. The team was also able to calculate this relationship from the material properties of the plates.
Archaeology and meteorites
Vandenberghe told physicsworld.com that the main goal of the study was to get a better understanding of the fragmentation that occurs when an object strikes a thin material. This is crucial in forensics and archaeology, where shattered glass and other materials can provide important clues about past events. Further afield, it could also help planetary scientists gain better insights into the composition of planetary and lunar surfaces by studying how these materials crack when struck by meteorites.
In particular, Jupiter’s moon Europa is covered in cracks – leading scientists to believe that its surface is a thin layer of ice floating on a liquid sea. So far, Vandenberghe and colleagues have not had much success relating Europa’s cracks to the material properties of its surface. However, the researchers are now planning to do experiments on the cracking of plates floating on liquid to see if they can gain further insights.
