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Never before have we felt so small

The Cassini spacecraft has captured Saturn’s rings, the Earth and our Moon in the same frame. (Courtesy: NASA/JPL-Caltech/Space Science Institute)

By Madeleine Fowler, who is doing a work experience placement at Physics World

It is hard to believe while standing among 7 billion other people on this huge and diverse planet we call home that it is not the centre of the universe in the same way that it is the centre of our lives. From the point of view of an ordinary person such as myself, the stars and the other planets seem almost to rotate around us as we go about our everyday lives. But as we all know, this is not the case. In this photo of the Earth taken on 19 July by the Cassini Interplanetary Spacecraft, approximately 900 million miles away, the Earth and the Moon occupy less than one pixel of the photograph. So perhaps we are not quite as important as we thought. Not a big fish in a small pond, but a very small fish in an infinite pond.

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Square Kilometre Array precursor begins search of early universe

The most powerful low-frequency radio telescope in the southern hemisphere – the A$51m (£30m) Murchison Widefield Array (MWA) – has started its search for signals from the first billion years of the universe’s life, the last unexplored epoch. Located at the Murchison Radio-Astronomy Observatory in a remote, radio-quiet region of Western Australia, the MWA is the first of three Square Kilometre Array (SKA) precursor telescopes to be operational.

The MWA consists of 128 aperture arrays – each comprising 16 antennas – spread out across an area of seven square kilometres. It detects signals between 80 and 300 MHz and has an unparalleled field of view of thousands of square degrees that will enable it to rapidly image the entire southern sky. Developed by an international consortium of 13 institutions in four countries, the MWA’s launch is the culmination of nine years of preparation.

Hot topics in astrophysics

With no moving parts, the MWA can be controlled remotely and data that are gathered by the telescope will be transmitted 800 km via a dedicated optical-fibre data link for processing in Perth. In one of nine inaugural research programmes, the MWA will search for the radio signature of hydrogen emitted during the first billion years of the universe’s life. Known as the era of reionization, the first stars and galaxies formed during this time, but it remains largely unexplored. “This is one of the hottest topics in global astrophysics,” says Steven Tingay, director of the MWA and an astronomer at Curtin University in Perth.

In another project, Martin Bell, astronomer at the University of Sydney, is leading a survey that will use monthly snapshots to observe the dynamic behaviour of supermassive black holes in the southern sky and look out for new, previously undetected classes of objects. “We’ve never done anything as wide or as frequent as this. In some sense, we don’t really know what we’re going to find,” says Bell. The first results from the new telescope are expected by the end of the year.

Sharing data

The vast quantities of data produced by the MVA are being processed at the Pawsey Centre for supercomputing in Perth. According to Andreas Wicenec of the nearby University of Western Australia, 400 Mbyte/s of data are currently being processed by the centre. “The technical challenge isn’t just in saving the observations but how you then distribute them to astronomers from the MWA team in far-flung places,” says Wicenec.

Sharing is currently done via two links – one with MWA colleagues at the Massachusetts Institute of Technology in the US and the other with team members at the Victoria University of Wellington in New Zealand. A link to MWA collaborators in India is planned for the future. The Pawsey Centre will also provide computing facilities for another SKA precursor, the Australian Square Kilometre Array Pathfinder (ASKAP).

Physicists call for €5bn Neutrino Factory

An international group of physicists has called for the construction of the Neutrino Factory as the next high-intensity neutrino facility in Europe. Taking four years to prepare, the report was written by the EUROnu collaboration, which consists of 15 institutions. The report backs the Neutrino Factory – which is estimated to cost between €4.6bn and €6.5bn – rather than two less-expensive options: the €1.6bn Super-Beam experiment and the €2.3bn Beta Beam facility.

The Neutrino Factory will involve producing neutrinos by firing a high-power proton beam at a target to make pions, which are then captured and allowed to decay to muons. The muons are then accelerated and injected into a storage ring where they decay into neutrinos, which are sent some 2000 km to the Magnetized Iron Neutrino detector, which would be made from 100,000 tonnes of iron.

Long-distance oscillations

One possible scenario for the Neutrino Factory is to have the accelerator based at CERN with a detector in Finland, the UK or even the US. The primary aim of this experiment – and other existing and planned experiments that send neutrino beams over long distances – is to study neutrino oscillation. This is the process by which neutrinos of one flavour (muon neutrinos at the Neutrino Factory) can with time change into neutrinos of a different flavour. Making more precise measurements of neutrino oscillations could help solve several important mysteries of physics, including why there is much more matter than antimatter in the universe.

The EUROnu report concludes that the 10 GeV Neutrino Factory “clearly has the best physics performance” over the two other designs, adding that the better performance level “offsets the additional cost”. The report recommends construction “as soon as possible” and if funding can be found, then the facility could be operational between 2025 and 2030.

Enormous potential

Kenneth Long, an experimental particle physicist at Imperial College London and a member of EUROnu, says that the scientific impact of the Neutrino Factory is “potentially enormous” and has the capacity to solve some of the biggest challenges in physics, such as the nature of dark matter. However, he warns that the ultimate fate of the facility will lie in the hands of funding agencies. “The scale of investment would require an international agreement,” he says.

The Super-Beam experiment would have focused a beam of pions and then measured how they decay into neutrinos as they travel to a detector. Beta Beam was designed to produce beams of neutrinos, possibly at CERN, and then send them towards the proposed MEMPHYS water-Cerenkov detector that could be based at the Fréjus Underground Laboratory near the Swiss–Italian border. But Alain Blondel, a neutrino physicist at the University of Geneva who is not involved in the EUROnu collaboration, says the Super-Beam and Beta Beam have “serious difficulties or shortcomings”, while “the Neutrino Factory is a much more powerful, long-term facility”.

Long explains the fundamentals of neutrino physics in the video “Why do neutrinos change flavour?”.

Learning to adapt: an interview with Joshua Miele

The podcast is a follow-up to an interview with Joshua Miele published earlier this year as part of a series of articles about people who studied physics but went on to work in other fields. In it, you will hear Miele describe some more personal aspects of his work at California’s Smith-Kettlewell Eye Research Institute, as well as specific devices such as an audio-enhanced Braille periodic table and the events that inspired him to pursue a career in adaptive-devices research.

Listen to the podcast now to learn more about Miele and his work.

T2K discovery puts neutrino oscillation beyond doubt

The SuperKamiokande detector lies 1 km underground in the Mozumi mine in the city of Hida. (Courtesy: Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo)

By Hamish Johnston

Physicists working on the Tokai to Kamiokande (T2K) experiment have confirmed what many have suspected for nearly three decades – over time, a neutrino of one flavour will change into a neutrino of another flavour in a process called neutrino oscillation.

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Attractive force arises from black-body radiation, say physicists

Black-body radiation can give rise to a net attractive force between tiny objects. That is the claim made by physicists at the University of Innsbruck in Austria, who have calculated the strength of this new force between a speck of dust and a hydrogen atom. The team believes that in some situations the force could be more significant than gravity – which means that its presence could have important effects on the behaviour of clouds of gas and dust in space.

Pulling things with electromagnetic radiation – the so-called tractor beam – has long been a mainstay of science fiction. While physicists have enjoyed some luck creating specialized systems to achieve it, any system must overcome a fundamental challenge: a particle absorbing a photon is pushed, not pulled.

Incident radiation can affect an atom in two ways. If the photon has precisely the right energy, then it can promote an electron into an excited atomic state. As the atom absorbs the photon, it also absorbs its momentum. This pushes the atom away from the light source, causing radiation pressure. More subtly, the electric field of the light wave causes its energy levels to shift slightly – a process called the Stark effect. While some excited states are raised in energy, the ground state is generally lowered.

Attracted to radiation

Unless the atom has recently been excited by a photon, then the electrons will be in the ground state; so reducing its energy reduces the total energy of the atom. More intense radiation creates a stronger electromagnetic field and a larger Stark shift, so the natural tendency of atoms to minimize their energy creates an attractive force towards the source of the radiation. This force is used by optical tweezers to trap atoms at a laser focus.

Theoretical physicist Helmut Ritsch of the University of Innsbruck explains that the current work arose from a speculative discussion with his wife, biomedical physicist Monika-Ritsch Marte, who studies optical tweezers at the Medical University of Innsbruck. They pondered whether or not an attractive optical potential could be created using broadband light. “Most people would first say no,” he explains. The pair teamed up with PhD student Matthias Sonnleitner to study the case of black-body radiation – the most broadband light imaginable.

Stark contributions

The black-body radiation emitted by an object contains a continuous spectrum of frequencies, so the photon energies required to excite atoms will be present, and these photons create a repulsive force. However, the energies of most common atomic transitions, at least in the lighter elements that make up most of the universe, correspond to photon frequencies in the visible or ultraviolet region of the electromagnetic spectrum. Radiating black bodies with temperatures below about 6000 K – the temperature of the surface of the Sun – emit the vast majority of their radiation as infrared photons. Because these photons have an energy below that needed to excite the electronic transitions they are not absorbed and do not cause radiation pressure. They do, however, contribute to the attractive force created by the Stark effect. In most physically realistic cases, therefore, this attractive “black-body optical force” is greater than the radiation pressure.

The force decays rapidly with distance and therefore the researchers believe that it will be challenging to measure in the laboratory. Under specific astrophysical conditions, however, it may play a key role. It is likely to be most significant for objects that, while below the temperatures of thousands of Kelvin necessary for radiation pressure to become significant, are hot enough to radiate appreciably. Moreover, in particles that are very light, it could be more significant than gravity. For example, modelling of an interplanetary dust cloud of micrometre-sized particles at 100 K suggests that the black-body potential at its surface is more than 100 million times the potential caused by gravity.

Astrophysical feedback

Helmut Ritsch now hopes to explore the detailed implications of the model in various scenarios. “We have certainly got a lot of feedback from the astrophysics community,” he says. “They have suggested a few scenarios that we should look at.” He says, for example, that immediately after the first hydrogen formed in the early universe, there would still have been plenty of radiation free, so radiation-mediated binding between hydrogen could perhaps have altered the evolution of density fluctuations.

Miles Padgett, an optical physicist at the University of Glasgow, is enthusiastic. “I think it is lovely,” he says. “It is a new mechanism completely different from all the others that have been previously discussed in the optical-trapping community.” He believes that, under high vacuum and on a small scale, it may be possible to detect the force directly in the laboratory.

Theoretical atomic physicist Andrei Derevianko of the University of Nevada says that, in principle, the attractive effect of electromagnetic radiation had previously been known in theory and used in practice in specific cases but that spelling out the full implications may have a significant impact. “This whole subject of atom–light interactions is quite exotic,” he says, “It is not like standard science. You really have to come in and have somebody say that this could have astrophysical implications for the other community to become aware of it. Work like this builds bridges.”

The research is described in Physical Review Letters.

Watch the Physics World Hangout about the physics of cancer

By James Dacey

A little earlier today we hosted a Google+ Hangout about the July issue of Physics World – a special issue about an emerging new research field called the “physics of cancer”. In case you were unable to join us for the live event (or would like to enjoy it all over again), you can watch it again via this YouTube recording.

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GERDA puts new limit on neutrinoless double beta decay

The GERDA experiment at Gran Sasso (Courtesy: INFN) — The GERDA experiment at Gran Sasso. (Courtesy: INFN)

By Hamish Johnston

This stylish chap is looking for an incredibly rare nuclear process called neutrinoless double beta decay. The picture was taken deep under a mountain at Italy’s Gran Sasso National Laboratory, which is about 160 km north-west of Rome. He is standing in a cavern containing the GERDA experiment, which has been searching for the rare decay since 2011.

GERDA hasn’t actually detected a decay event, but the collaboration claims to have measured the best value yet of the lower limit on its half-life in germanium-76. They researchers say that it’s about 2.1 × 10²⁵ years – or 21 yottayears!

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‘Snow line’ of neighbouring star comes into view

An artist's illustration of the snow line around TW Hydrae — Ring of ice: an artist’s illustration of the snow line around TW Hydrae showing water-covered ice grains in the inner disc (4.5–30 AU, blue) and CO ice-covered grains in the outer disc (>30 AU, green). The transition from blue to green marks the CO snow line. (Courtesy: Bill Saxton and Alexandra Angelich, NRAO/AUI/NSF)

The carbon monoxide (CO) “snow line”, or the distance from a star beyond which CO can freeze, has been directly imaged for the first time by an international team of researchers. The snow line was observed in a protoplanetary disc around the star TW Hydrae and was imaged using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. Knowing the locations of such snow lines could help astronomers better understand how planetary systems form and what the planets are composed of, according to the team.

Just as snow lines on Earth are typically seen at high elevations where falling temperatures turn atmospheric moisture to snow, stellar CO snow lines are thought to form in the far, cold reaches of protoplanetary discs that orbit young stars. Depending on the distance from the star, however, other more exotic molecules can freeze and turn to snow.

Freezing point

In addition to CO, other hydrogen compounds such as water, ammonia and methane also condense into solid ice grains at the snow line at a temperature of about 150 K. Water ice freezes first, followed by the other abundant gases that form a sort of frost on the dust grains that will ultimately be the building blocks of planets and comets in that system. The frost line for our solar system is at around 5 AU.

In the new study, Chunhua Qi, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachuttes, along with Karin Oberg, of Harvard University in the US, and colleagues used data from ALMA to study the protoplanetary disc that surrounds TW Hydrae, a young star 175 light-years from Earth. TW Hydrae was choosen because the researchers believe that its disc is comparable to the nebula from which our solar system arose.

“ALMA has given us the first real picture of a snow line around a young star, which is extremely exciting because of what it tells us about the very early period in the history of our solar system,” says Qi. He goes on to say that it is now possible to “see previously hidden details about the frozen outer reaches of another solar system, one that has much in common with our own when it was less than 10 million years old”.

ALMA image showing CO snow region around TW Hydrae — Emerald hues: this ALMA image (green) shows the region where CO snow has formed around the star TW Hydrae (centre). The blue circle represents where the orbit of Neptune would be when comparing it to the size of our solar system. (Courtesy: Karin Oberg, Harvard University/University of Virginia)

Lifting the veil

But directly imaging a snow line is no easy task – to date, they have only been detected via their spectral signatures and so their precise location and extent could not be nailed down. The difficulty arises because the snow lines form almost exclusively within the narrow central plane of a protoplanetary disc. Above and below the plane, stellar radiation warms the gases and so they cannot freeze. However, the insulating effect of the concentrated dust and gas in the central plane of the disc allows temperatures to drop sufficiently for CO and other gases to cool and freeze, according to the team. And it is this radiation that veils the snow line, preventing astronomers from peering in.

To get round this problem, the researchers looked for a reactive ion – diazenylium (N₂H+) – which only appears when CO freezes. That is because diazenylium is destroyed in the presence of CO gas and so would only appear in detectable amounts where the CO had frozen. Diazenylium shines brightly in the millimetre portion of the electromagnetic spectrum, which can be detected by radio telescope such as ALMA. According to the researchers, ALMA’s sensitivity and resolution allowed them to trace the presence and distribution of diazenylium around TW Hydrae, finding a defined boundary at about 30 AU from the parent star.

Diazenylium marks the spot

“Using this technique, we were able to create, in effect, a photonegative of the CO snow in the disc surrounding TW Hydrae,” says Oberg. “With this, we could see the CO snow line precisely where theory predicts it should be – the inner rim of the diazenylium ring.”

Current theories suggest that snow lines help dust grains overcome their normal tendency to collide and self-destruct by giving the grains a stickier outer coating. They also increase the number of solids available and may dramatically speed up the planet-formation process. There are multiple snow lines – such as a water snow line and the CO snow line – each thought to link to the formation of specific kinds of planets. In our solar system, the snow line is thought to separate the terrestrial planets from the jovian planets.

Oberg also points out that the CO snow line is particularly interesting because CO ice is needed to form methanol, which is considered a basic building block that forms more complex organic molecules that are essential for life. “Imaging of snow-line locations in large samples of discs will directly trace how planet formation varies between different systems. The snow line regulates the bulk compositions of planetesimals and planets formed in such systems,” says Qi. The researchers hope that future observations will reveal other snow lines and provide additional insights into the formation and evolution of planets.

The research is published in Science.

Explaining CERN, the Higgs and the LHC

By Matin Durrani

[brightcove videoID=phw.live/2013-07-18-balloon-vox/1 playerID=106573614001 height=268 width=390]

How well would you do if someone asked you to explain the Higgs boson or the Large Hadron Collider (LHC) at CERN?

If you’re a physicist, you’ll probably find it hard enough. But if you’ve never done any physics in your life, things must surely be trickier still, more so if a film crew from Physics World has shoved a camera up your nose.

These two short videos show the results of a straw poll of randomly selected visitors at last summer’s Bristol International Balloon Fiesta when we asked them to describe the Higgs boson and the LHC.

The reason we were at the fiesta is that we were making a separate film about a project by Bristol University physicist Dave Cussans where school students were measuring cosmic rays during a hot-air balloon flight – it being the centenary of Victor Hess’s discovery of these rays in a balloon flight in central Europe.

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Never before have we felt so small