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Home » Page 1060

Could quantum ‘clocks’ tread two different paths to general relativity?

A new way of probing the intersection between quantum mechanics and Einstein’s general theory of relativity using interferometry has been devised by physicists in Israel. The researchers have developed a “self-interfering clock” that comprises two atomic spin states put into a quantum superposition. The researchers hope that their proof-of-principle experiment will provide new insights into the study of time, the interplay between quantum mechanics and relativity, and in particular the role that gravity could play in destroying the coherence of a quantum system.

Different ticks?

Quantum mechanics and general relativity are both well-established and well-tested theories. Despite this, the two are not always in agreement. The concept of time, for example, is treated differently by both: while quantum theory states that time is global and all clocks “tick” uniformly, general relativity dictates that time is influenced by gravitational fields, and so clocks tick at different rates in different places. The latter having been verified experimentally using clocks at different heights above the Earth.

Another inherent property of quantum mechanics is “superposition”, wherein a quantum particle such as an electron is considered to simultaneously be in all possible “states” (or spatial positions) until a measurement is made and the wavefunction collapses. This is more commonly known as the Schrödinger’s cat paradox.

Using an interferometer (the simplest being the double-slit experiment), researchers can make photons or electrons take two paths simultaneously. As long as the observer does not know which of the two paths is taken, when the paths are rejoined at a detector, an interference pattern – which is the hallmark of superposition – is seen, and the particles are thereby in spatial superpositions. On the other hand, if the observer is able to tell which path was taken, the interference pattern will disappear because there was no superposition. Such “which path” information can be revealed using a tag referred to as a “which path” witness. For example, a polarization filter placed in one path would allow the observer to distinguish light that had taken that path.

Which way?

So, what would happen if a “quantum clock” is simultaneously sent along two paths of an interferometer? General relativity says that time can “tick” at different rates along each path, and therefore time itself could be a “which path” witness.

This is precisely the question that Ron Folman and colleagues at the Ben-Gurion University of the Negev aimed to answer in their latest research. Thanks to the discovery of Bose–Einstein condensates (BEC) and the idea of using ultracold atoms as “clocks”, it is possible to send such a clock through an interferometer. “What we have shown in our proof-of-principle experiment is that time itself can also be a ‘which path’ witness,” says Folman. To do this, the researchers used ultra-cold rubidium atoms at nano-kelvin temperatures in a new Stern–Gerlach type of interferometer that they developed and demonstrated two years ago. Here, a strong magnetic field from an atomic chip interacts with the spin of the atoms, “and if the atom is in a superposition of two spin states, then it will evolve into a superposition of two momentum states that form (after some time) a spatial superposition”, explains Folman. In their latest experiment, the researchers do not actually send their clock down an interferometer – instead, two copies of the clock (wavepackets), are separated in space, thereby forming the two interferometer paths.

“We turn the atom into an atomic clock by manipulating its internal degrees of freedom (spin states),” says Folman, further explaining that as their clock is not sensitive enough to feel the different ticking rates caused by gravity, “we induce an artificial difference in the ticking rate by exposing the two paths to different magnetic fields that make the two clock wavepackets tick at different rates”. When the team actually induced its time lag – the wavepackets were put in easily differentiable orthogonal states – it found that the interference pattern disappeared, thereby proving that time may serve as a “which path” witness, according to the researchers.

Images of the interference patterns seen via the self-interfering clock
Strange pattern? Clock interference (click for full view)

Folman says that as the team was able to show revivals of the interference pattern, it is not clear yet if this may be called decoherence and there is a debate among theoreticians about the role general relativity may be playing. “Our proof-of-principle experiment opens the road to investigate this interplay,” he says. Indeed, recent theoretical work done by Časlav Brukner of the University of Vienna and colleagues looked into this, and suggested sending a cold-atom clock through an interferometer to test the boundary between the quantum and classical worlds, and Folman’s simulated interferometer clock is a first step in that direction.

Brukner tells physicsworld.com that the new work beautifully simulates what he and colleagues theoretically predicted about what a single “clock” – a time-evolving internal degree of freedom of a particle – undergoes when put in a superposition of regions of space–time with different ticking rates. “The time as shown by the clock is not well defined, and gets entangled with its position,” Brukner explains. He adds that “this implies that by ‘reading-out time’ from the clock, one could reveal the “which path” information, and consequently, one has a loss of coherence of the clock’s centre-of-motion degree of freedom”. This latest work “succeeded to demonstrate the very exact effect that we expect in a future experiment with a natural lag due to time dilation”, says Brukner.

Folman also points out that their device is a new type of interferometer that produces signals not seen before. “For example, people have become accustomed to the fact that when you join a split BEC, you always get an interference pattern in each repetition of the experiment. This new interferometer completely destroys the interference pattern of a BEC in every single repetition, when the two clock wavepackets have orthogonal time readings,” he says.

Higher sensitivities

Chad Orzel, a physicist at Union College in the US who was not involved in the work, says it is a clever idea, although he remains sceptical that this sort of mechanism could have anything to do with the quantum-to-classical transition because of the very small time lag that is induced. “In terms of implications for other experiments or tests of quantum gravity and the like, I think it will be a massive challenge to do anything with this. They’re using an artificial phase shift of order π between their clocks, and that much of a shift would be hard to realize with gravitational shifts near the Earth,” he says. Orzel adds that even if the technical challenges involved in developing a more sensitive clock (using say strontium) were surpassed, it would still be very difficult to show that it is indeed gravity degrading the contrast of interferometer fringes.

Folman acknowledges that the challenge now facing his team is to reach a sensitivity that would “allow us to directly observe the effect of general relativity on the interferometer. For this to happen, the distance between the two paths must be enlarged (so that the difference in ticking rate is larger) and the clock needs to be made much more accurate. It remains to be seen how quickly this can be achieved.” In addition to testing the overlap between relativity and quantum mechanics, the team hopes its work will help us to “learn more about time itself”.

The research is published in Science.

Browsing the Milky Way at the IAU General Assembly in Honolulu

 

Artist's impression showing the Milky Way over Hawaii
Kai‘aleleiaka: artist’s impression showing the Milky Way over Hawaii. (Courtesy: IAU)

By Hamish Johnston

Earlier this week the triennial XXIX General Assembly of the International Astronomical Union (IAU) kicked off in Honolulu, Hawaii. Founded in 1919, the IUA has about 10,000 members based in 96 countries worldwide. About 3500 astronomers are attending this year’s meeting, which runs until 14 August and is hosted by the American Astronomical Society.

A long-standing tradition of the congress is the production of a daily newspaper for delegates and 2015 is the first year that an electronic version is available to the general public. You can catch up with all the daily news by downloading a copy of Kai‘aleleiaka, which is pronounced “kah EE ah lay-lay-ee AH kah” and means “the Milky Way” in Hawaiian.

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Re-examining the decision to bomb Hiroshima

Not forgotten: The Hiroshima Peace Memorial is dedicated to the 140,000 people killed in the city. (Courtesty: iStockphoto/Colin13362)

By Hamish Johnston

Today marks the 70th anniversary of the bombing of Hiroshima – the first time that a nuclear weapon was used in war. Many argue that the bombing of Hiroshima, and three days later Nagasaki, was a necessary evil that saved hundreds of thousands of lives by ending the war and avoiding an allied invasion of Japan.

Over on The Nuclear Secrecy Blog, the science historian Alex Wellerstein asks “Were there alternatives to the atomic bombings?”. Wellerstein argues that the choice facing the US in 1945 was not as simple as whether to bomb or to invade. He points out that some physicists working on the Manhattan Project – which built the bombs – argued for a “technical demonstration” of the weapons.

In June 1945 the Nobel laureate James Franck and some colleagues wrote a report that argued that the bomb should first be demonstrated to the world by detonating it over a barren island. Wellerstein surmised that “If the Japanese still refused to surrender, then the further use of the weapon, and its further responsibility, could be considered by an informed world community”. Another idea being circulated at the time was a detonation high over Tokyo Bay that would be visible from the Imperial Palace but would result in far fewer casualties than at Hiroshima, where about 140,000 people were killed.

On the other hand, Wellerstein points out that Robert Oppenheimer and three Nobel laureates wrote a report that concluded “we can propose no technical demonstration likely to bring an end to the war; we see no acceptable alternative to direct military use”. This report was written for a US government committee, which decided to use the weapon against a “dual target” of military and civilian use.

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Buckyballs give copper a magnetic attraction

Thin layers of two non-magnetic metals – copper and manganese – become magnets when they are in contact with buckminsterfullerene molecules. This discovery has been made by physicists in the UK, US and Switzerland, and could lead to new types of practical electronic devices and even quantum computers.

Ferromagnets – such as familiar fridge magnets – are materials that have permanent magnetic moments. There are only three metals that are ferromagnetic at room temperature – iron, nickel and cobalt – and this is explained in terms of the “Stoner criterion”, which was first derived in 1938 at the University of Leeds by Edmund Stoner.

Stoner knew that magnetism in metals is a property of the conduction electrons. These electrons are subject to the exchange interaction that allows them to reduce their energy by aligning their spin magnetic moments in the same direction – thus creating a ferromagnetic metal. However, having spins that point in the same direction increases the overall kinetic energy of the electrons. Stoner realized that ferromagnetism will only occur when the reduction in energy caused by exchange is greater than the gain in kinetic energy. Quantitatively, he showed that this occurs when the product of the electron density of states (DOS) – the number of energy states available to the electrons – and the strength of the exchange interaction (denoted by U) is greater than one.

Giving U a boost

U is called the Stoner criterion, and it is greater than one for iron, nickel and cobalt but not for their neighbours in the periodic table – manganese and copper. Now, an international team including Fatma Al Ma’Mari and Tim Moorsom of the University of Leeds in the UK has found a way to boost the DOS and exchange interaction in copper and manganese so that they are ferromagnetic at room temperature.

The team made its samples by depositing several alternating layers of C60 and copper (or manganese) onto a substrate. The copper layers were about 2.5 nm thick and the C60 layers about 15 nm thick. C60 is used because it has a large electron affinity, which means that each molecule will take up to three conduction electrons from the copper. This is expected to increase both the DOS and the strength of the exchange interaction in copper.

The team then measured the magnetization of the layered samples and found them to be ferromagnetic materials. The researchers also looked at samples in which the copper and C60 layers were separated by layers of aluminium and found no evidence of magnetism, which suggests that ferromagnetism occurs at the interfaces between the copper and C60. This was backed up by experiments using muons, which are depth-sensitive and showed that the ferromagnetism occurs in the copper near to the C60 interface. The team also found room-temperature ferromagnetism in C60/manganese layers, but with a weaker magnetization.

Critical field

Surprisingly, when the researchers calculated U for their copper samples, they found it to be less than one. In other words, the samples should not have been ferromagnetic according to the Stoner criterion. However, further theoretical investigations suggest that the samples should become ferromagnet when exposed to a relatively small magnetic field – something that would have happened during the preparation of the samples. This suggests that other non-magnetic metals could be made ferromagnetic by boosting U but not necessarily all the way to one.

Although further work is needed to increase the strength of the copper and manganese magnets, the research could result in the development of new types of tiny magnetic components. These could find use in spintronic devices, which use the spin of the electron to store and process information, or even in quantum computers in which electron spins are used as quantum bits of information.

The research is described in Nature.

Giant Magellan Telescope Organization president resigns

Ed Moses, the head of the body constructing the $1bn Giant Magellan Telescope (GMT), has announced he has left the organization, citing personal reasons. Moses, who was president of the Giant Magellan Telescope Organization (GMTO), has departed after only 10 months in the role “to deal with family matters that require his attention”, according to a GMTO statement. The news of Moses’ departure comes just weeks after Wendy Freedman, chair of the organization’s board, stepped down in early July.

The GMT is scheduled to be fully operational in Chile’s Atacama Desert by 2024, when it will become the world’s largest optical telescope. Moses joined the GMTO as its first president last September, after a stint running the National Ignition Facility at the Lawrence Livermore National Laboratory. In June the project received a major boost when the GMT’s 11 international partners committed more than $500m to start construction of the telescope.

Rapid growth

Colleagues of Moses have praised his achievements. “[He] brought his deep experience and has left us stronger,” says Patrick McCarthy, former executive vice president of GMTO, who is now interim president. That view is backed up by Taft Armandroff, director of the McDonald Observatory, who is the new board chair of the GMTO. He pays tribute to Moses’ recruitment programme, with the project office growing from 30 to 90 people in the space of a year. “We now have a strong technical and corporate staff dedicated to GMT,” he says. “Their experience from past projects makes this team ideally suited to establish the GMT as one of the most powerful telescopes in the world.”

‘Comprehensive search’

Freedman became GMTO’s first board chair in 2003, and has now stepped down “to do more science”. Indeed, she joined the University of Chicago last year, and is now principal investigator on a project to measure the Hubble constant to higher accuracy. “The GMTO has been fortunate to have had her guidance for so long,” says Armandroff, who adds that the board will now conduct a “comprehensive search” for a new president.

Meanwhile, construction of the $1.4bn Thirty Meter Telescope (TMT) on Hawaii’s Mauna Kea has yet to begin, following protests that have blocked building work. TMT board member Michael Bolte from the University of California, Santa Cruz, says in an official statement that a restart date for construction has yet to be determined. “In the construction timetable, the delay is small, and the time has been well spent in better understanding the concerns about the project,” adds Bolte.

Plan for supersized entanglement is unveiled by physicist

An experiment that could lead to the quantum-mechanical entanglement of everyday objects in the form of two 100 g mirrors has been proposed by Roman Schnabel of the University of Hamburg and the Max Planck Institute for Gravitational Physics in Germany. If successful, the mirrors would be by far the largest objects ever to be entangled, and the experiment would confirm that quantum physics applies to large and heavy objects, not just tiny particles. It could also test a prediction made in 2010 about how the mutual gravitational attraction of the mirrors affects their entanglement.

Entanglement is a purely quantum-mechanical phenomenon that allows two particles, such as photons or electrons, to have a much closer relationship than is predicted by classical physics. The concept of entanglement was introduced in the 1930s when physicists were debating the seemingly bizarre implications of quantum mechanics as identified by the Einstein–Podolsky–Rosen (EPR) paradox. EPR points out that if two particles are entangled and separated by some distance, then a measurement made on one particle seems to instantaneously affect the outcome of a measurement made on the other particle. Since no communications can travel between the particles faster than the speed of light, EPR surmises that “hidden variables” unknown to the experimenter have caused the effect. In 1964 John Bell came up with a way of disproving the existence of hidden variables – by a violation of Bell’s inequality – and subsequent experiments have confirmed that entanglement cannot be explained by hidden variables.

Since then, entanglement has become a fascinating phenomenon for physicists to study and has also found practical application in quantum-encryption systems. Indeed, over the past decade or so, physicists have been successful at entangling ever-larger objects, including micron-scale mechanical resonators. Now, Schnabel has come up with a way to entangle two mirrors that are enormous in comparison with previously entangled objects.

Swapping entanglement

The mirrors are entangled via photon radiation pressure and a process called entanglement swapping. This is done by placing two mirrors into a Michelson-type interferometer (see figure). Two beams of light are sent into the interferometer so that each mirror is struck on both sides by light. As light travels through the system, it is reflected from the surfaces of the mirrors. If the mirrors are free to oscillate, then momentum can be transferred between the mirrors and the light. The motion of the mirrors will also affect the phase of the light that is reflected from them. In this way, the light in the interferometer and the motion of the mirrors becomes entangled.

This entanglement is then “swapped” to become an entanglement between the two mirrors. This is done by measuring the interference of the two light beams as they exit the interferometer. Crucially, this measurement provides information about the nature of the entanglement but does not destroy it because the measurement does not provide any information about an individual mirror. The Michelson-type interferometer is ideal for this because it can be set up to measure the relative difference between the positions of the mirrors and the relative difference between the momenta of the mirrors – but not the individual positions and momenta of each mirror.

Once entanglement is achieved, the next step is to verify that the motions of the mirrors are indeed entangled. This involves switching off the light to allow the system to evolve for a few milliseconds before further measurements are made. Then the set-up is modified by removing one beamsplitter, which allows the experimenter to measure the position and momenta of each mirror individually.

Repeated measurements

In a practical experiment, the mirrors would be entangled, allowed to evolve for a few microseconds and then have their positions and momenta measured. This would be repeated over and over again, and the presence of entanglement would be signalled by correlations between the measured properties of the mirrors that are greater than those allowed by classical physics.

Schnabel and colleagues have already started building the experiment in the lab, but Schnabel says that there are several important practical challenges that must be overcome. The most significant challenge will be how to cool the mirrors to a temperature of about 4 K and how to keep them isolated from their surroundings so they do not absorb heat energy, which would affect their motion and destroy entanglement.

If they can realize the experiment, Schnabel and colleagues will prove that not only small particles can show quantum behaviour, but also massive objects. If successful, they will also be able to test a prediction made in 2010 by Haixing Miao – then at the University of Western Australia – and colleagues. This group calculated that the mutual gravitational energy of the mirrors would destroy the entanglement on the microsecond timescale, which is something that Schnabel’s experiment should be able to see.

Earlier this year, Schnabel and colleagues demonstrated a new way of cooling a mirror using light in a Michelson interferometer (see “Physicists reveal new way of cooling large objects with light”).

The experimental proposal is described in Physical Review A.

Between the lines

Hot topics in domestic science

If you wanted to heat your house simply by inviting some nice warm people into it, rather than by turning on your central heating, how many of your fellow humans would you need to entice over your threshold? The answer will depend on a number of factors, including the size of your house and how toasty you like it, but in his book Atoms Under the Floorboards, author Chris Woodford reckons that, for a small terraced house, somewhere between 70 and 140 warm bodies ought to do the trick. The resulting mental image of a house stuffed to the eaves with wriggling radiator-people is just one of the many delights in this fun and accessible book, which is written for a general audience and loosely focused on the science of objects found in and around the home. This broad remit gives Woodford licence to explore a host of topics, from the physics of bicycle and car movement to the chemical make-up of the vapours given off by old plastics as they decay. (Note: taking big sniffs near old plastic dolls is best avoided unless you like the vomitous aroma of urea formaldehyde.) The marvels of modern technology also get their due, and Woodford’s discussion of communication devices is particularly interesting. If telephones were restricted to transmitting speech at the speed of sound, he observes, a single 30-sentence conversation between people in New York and Los Angeles would take around four days and nights. Woodford’s eclectic background includes a stint as an advertising copywriter, and he has a good ear for language, as when he describes shaking electrons up and down “like a jar of reluctant ketchup” to produce radio waves. The best part of the book, though, is the sense of imagination it brings to everyday phenomena.

  • 2015 Bloomsbury Sigma £16.99hb 336pp

See you on Mars

The prospect of sending humans to Mars has tantalized space aficionados for decades. Indeed, among dedicated enthusiasts, a crewed mission to the Red Planet has long since become a matter of when, rather than if. For the veteran science journalist and publisher Stephen Petranek, however, both “if” and “when” are a bit passé. In How We’ll Live on Mars, Petranek zooms straight past these quotidian questions to focus on the future – a future that will, he believes, contain Mars‑dwelling humans by 2027 and a viable colony of 50 000 settlers before the end of this century. The book is based on a talk that Petranek gave at the 2015 TED conference, and while a film of this talk is not yet available to the general public, he seems to have used the longer format to flesh out his ideas and address some audience criticisms. A keen advocate of private space exploration (particularly Elon Musk’s firm SpaceX), Petranek gives bullish answers to questions about how much a Mars craft will cost and how reliable it will be. He does, however, concede that protecting astronauts from radiation is “still a big bugaboo” and that keeping their bodies from falling apart on a long space mission remains “a significant challenge”. He also acknowledges that the history of exploration is not solely one of noble pioneers and daring adventures, noting that while “exploration may be connected to human survival…it has also led to colonization of lands already occupied, the devastation of cultures, and the plundering of resources”. Will Mars settlers end up repeating those past mistakes? Petranek isn’t sure, but at least he’s thinking about it, and his willingness to engage with tough questions adds heft to this slender but fascinating book.

  • 2015 Simon & Schuster £7.99/$16.99hb 96pp

Small to large

The smallest SI-accepted prefix is yocto, denoting 10–24 of something. The largest, yotta, corresponds to 1024. In most fields of human endeavour, numbers outside the yocto-to-yotta sandpit rarely get a look-in – but physics is, of course, an exception. (Greetings, Planck’s constant, you lovely little lump of almost-nothing.) In Physics in 100 Numbers, science writer Colin Stuart ably demonstrates the discipline’s scale. The book begins with the smallest meaningful number in physics (the Planck time, 5.39 × 10–44 s) and, in a series of short, illustrated essays, races through 98 others before pitching up at the largest (1 × 10500, the number of possible configurations in the string theory landscape). It’s an impressive journey, but non‑specialist readers who begin at the beginning may find it frustrating, as the book’s structure makes it hard to comprehend just how small the first few numbers in the book really are. The classic film Powers of Ten avoided this problem by starting with familiar numbers and then zooming out and then in again; readers of Physics in 100 Numbers, in contrast, get plonked directly into the Planck scale, and are given only the sketchiest information about why these tiny distances and time intervals are important (and none at all about how their numeric values were arrived at). That’s a pity, because, within the constraints of the book’s somewhat gimmicky format (the same publishers have also produced a book called Chemistry in 100 Numbers; one suspects that biology and mathematics are also in the pipeline), there are plenty of good explanations in Stuart’s text.

  • 2015 Apple Press £12.99hb 176pp

Faraday explodes in court, NIST is entangled in dance, and Oliver Sacks’ periodic table

 

By Hamish Johnston and Michael Banks

You may remember back in 2013 when researchers at the National Institute of Standards and Technology (NIST) in the US entangled the motion of a tiny mechanical drum with a microwave field for the first time ever. Not content with that feat, NIST physicist Ray Simmonds, who was involved in the work, has now made a dance about it (but no song, yet). Teaming up with choreographer Sam Mitchell, the duo has created a modern dance piece entitled Dunamis Novem (“The chance happening of nine things”). Featuring four dancers, their movements are based on nine quantized energy levels of a harmonic oscillator – like the microscopic drum in the NIST work. For each level, Mitchell created corresponding dance actions, while Simmonds created a random-number generator – to add some “quantum randomness” – for the sequence of levels that the dancers perform at. If the dancers happen to touch each other, their actions become synchronized, which can then only be broken by a beam of light – demonstrating that a measurement collapses the entanglement.

NIST has published a Q&A with Mitchell and Simmonds with links to videos of the dance and the animations of the corresponding energy levels of the harmonic oscillator. A video of the first half of Dunamis Novem is shown above and a video of the entire dance is also available.

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Brown dwarf’s powerful auroras shine bright

The first aurora ever seen in an object beyond our solar system has been discovered by an international team of researchers. The team was studying a brown dwarf 20 light-years away, using both radio and optical telescopes, when the researchers picked up signals that suggest that the object hosts the most powerful aurora seen to date. Indeed, the aurora is hundreds of thousands of times more powerful than any detected in the solar system. This discovery also reveals a major difference between the magnetic activity of more-massive stars and that of brown dwarfs and planets.

Auroras are natural light displays in the sky that form when charged particles from cosmic rays or the solar wind enter a planet’s magnetosphere – the region where charged particles are affected by a planet’s magnetic field. Once in the magnetosphere, the particles – mainly electrons and protons – are accelerated along the magnetic-field lines to the planet’s poles. There they collide with gas atoms in the atmosphere and produce the bright light emissions that we see as auroras. They occur on all of the magnetized planets in the solar system, and the gas giant Jupiter has the most powerful auroras.

In the middle

Brown dwarfs are “sub-stellar” objects with masses that lie between the heaviest gas giants and the lightest stars, with an upper limit of about 75 to 80 Jupiter masses. They are too small to sustain the hydrogen-fusion reactions that power main-sequence stars like the Sun. However, they are too big to be thought of as planets. Indeed, they contain characteristics of both, and are often referred to as “failed stars”.

More than a decade ago, astronomers began picking up radio waves emitted by brown dwarfs. It was initially thought that the waves were produced in much the same way as signals from other stars – in an extremely hot upper atmosphere or “corona” that is heated via magnetic activity occurring in the lower atmosphere. But brown dwarfs do not generate large flares and charged-particle emissions in the way that main-sequence stars like the Sun do, so the radio emissions remained something of a mystery.

In 2006, while a postgraduate, Gregg Hallinan – who is now assistant professor of astronomy at California Institute of Technology – discovered that brown dwarfs actually pulse at radio frequencies. “We see a similar pulsing phenomenon from planets in our solar system,” says Hallinan, “and that radio emission is actually down to auroras.” Hallinan wondered if the radio emissions coming from brown dwarfs might be caused by auroras too.

Hallinan – together with Stuart Littlefair from the University of Sheffield in the UK and colleagues worldwide – made extensive observations of a sub-stellar object, called LSR J1835+3259, that is most likely a brown dwarf. To study the object at radio wavelengths, the team used the Very Large Array (VLA), while optical observations were made using the Hale Telescope on Palomar Mountain and the Keck telescopes in Hawaii.

Peculiar pulses

The researchers’ radio observations showed a bright pulse of radio waves that appeared as the brown dwarf rotated every 2.84 hours – the team was able to observe nearly three full rotations over the course of a single night. The Hale Telescope revealed that the brown dwarf varied optically on the same period as the radio pulses. Focusing on one of the spectral lines associated with the H-alpha emission line of hydrogen, the researchers saw that the object’s brightness varied periodically. Lastly, the Keck telescopes were used to make a precise measurement of the brown dwarf’s brightness over time – a difficult task considering the objects’ dimness compared with the Sun.

With all of the observations combined, the team was able to establish that the hydrogen emission is a signature of auroras near the dwarf’s surface. “As the electrons spiral down toward the atmosphere, they produce radio emissions, and then when they hit the atmosphere, they excite hydrogen in a process that occurs on Earth and other planets, albeit tens of thousands of times more intense,” explains Hallinan. According to the researchers, their findings suggest that brown dwarfs are not like small stars in terms of their magnetic activity; instead, they are much more like giant planets with hugely powerful auroras.

Secret source?

A mystery remains, however. There are no stars close enough to the brown dwarf, so what is the source of the charged particles? The researchers believe that some other source, such as an orbiting planet moving through the dwarf’s magnetosphere, could generate a current and produce the auroras. “But until we map the aurora accurately, we won’t be able to say where it’s coming from,” says Hallinan. Another possibility may be an interaction between the brown dwarf’s magnetic field and orbiting moons – something that drives some of Jupiter’s auroras, thanks to interactions with its moon Io.

Hallinan also points out that further studies of brown dwarfs could also prove useful for understanding exoplanets. “For the coolest brown dwarfs we’ve discovered, their atmosphere is pretty similar to what we would expect for many exoplanets, and you can actually look at a brown dwarf and study its atmosphere without having a star nearby that’s a factor of a million times brighter obscuring your observations,” says Hallinan, who also wants to study the magnetic fields of exoplanets. Whether or not a planet has a magnetic field plays an important role in the object’s potential to host life.

The research is published in Nature.

The August 2015 issue of Physics World is now out

Mention the two words “science policy” and most physicists’ eyes will probably glaze over. Most of us dream of discovering a new planet or finding the Higgs boson – not poring over budget spreadsheets, championing science to politicians or commenting on legislation.

But science policy is vital in today’s world, which depends hugely on scientific research and in the cover feature of the August issue of Physics World, which is now out, Len Fisher and John Tesh offer 12 practical tips for scientists who want their ideas incorporated into science policy. You’ll be intrigued by what the two authors have to say.

Elsewhere in the issue, as my colleague Tushna Commissariat explains in the video above, there’s a great feature based on an interview with the French physicist Hélène Langevin-Joliot – the granddaughter of Marie Curie. In the article, Langevin-Joliot explains what’s known as the “Curie complex” and gives her own tips for scientific success. Langevin-Joliot didn’t suffer from the complex herself, but she acknowledges that it is a big problem for others and, these days, spends her time actively promoting careers for women in science.

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