Do the Bosenova

Desktop astrophysics could be the next event on the horizon thanks to the synthesis of a new form of matter known as a Bose-Einstein condensate.

In 1924, Indian physicist Satyendra Nath Bose came up with an idea about the behaviour of photons. He reckoned they might exist in different energy states and he used his idea to derive Planck’s Law of Radiation without resorting to classical physics. Einstein extended Bose’s work to atoms, particles of matter and predicted that if a gas were cooled enough the atoms would condense into their lowest possible energy state. This weird form of matter, the Bose-Einstein condensate, was first created in a laboratory in 1995 and earned Eric Cornell (of NIST), Carl Wieman (University of Colorado) and Wolfgang Ketterle (MIT) the 2001 Nobel Prize in Physics.

Burst movie

Burst movie

The quantum mechanical blob that is a BEC is not peculiar to the physics laboratory though. Astrophysicists believe that BECs could be at the core of one of the strangest entities in the universe – the neutron star. Ketterle cautions, however, that, while the two have some aspects in common, and it is nice to explore these analogies, I doubt that BEC work will contribute in a major way to advances in astrophysics.

Neutron stars are, as their name suggests, composed of neutrons and form under the enormous pressure as a star collapses. A thimbleful of neutron star would weigh a billion tonnes and the whole thing would sit neatly on Manhattan Island, New York. With a mass about 1.5 million times that of the sun, these iron-encrusted stars spin at hundreds of revolutions per second. The brittle, iron-rich crust of the neutron star hides an interior that is even more mysterious than their enormous density. Inside, they are fluid. In their swirling depths is a vast sea of neutrons – the debris from atoms crushed by a supernova explosion – and within those depths rage quantum storms.

Crab Nebula

Crab Nebula

You might think such an object would be impossible to handle, but physicists would like to take a closer look at such stellar species as neutron stars and their cousins, the white dwarfs and black holes. They all represent extreme forms of matter that could betray the secrets of universe. The discovery of how to make a BEC in the laboratory provided a technical tip on handling the types of extreme matter that exist in a neutron star.

Magnetar

Magnetar

BECs are 100,000 times less dense than air, and are colder than interstellar space. In contrast, neutron stars have a density of up to 100 million tonnes per cubic centimetre, and their internal temperature is 100 times that of the centre of the sun. The link between the two, however, is in that they are both superfluids – they are liquids that flow with zero friction and zero viscosity.

Swiss cheese!

Swiss cheese!

Setting a superfluid in motion is difficult. Try to rotate a container of supercold liquid helium-4, for instance, and the lack of friction between it and the inside wall of the container means it doesn’t budge. But, superfluids can swirl according to Ketterle. And the concept that unifies neutron stars and BECs is that they share this characteristic of superfluids. Ketterle and his colleagues at MIT began experimenting with swirling BECs. At the time, they did not have neutron stars in mind. BECs are a new form of matter, and we wanted to learn more about them, he explains, By rotating BECs, we force them to reveal their properties.

Quantum swirls

Quantum swirls

To set a BEC swirling Ketterle’s team shone a rotating laser beam on it while holding it in place with strong magnets. The experiment is like stroking a ping-pong ball with a feather until it starts spinning, muses Ketterle. The surprising thing was that suddenly, a regular array of whirlpools appeared in the BEC. It was a breathtaking experience when we saw those vortices, recalls Ketterle. Researchers had seen such whirlpools before (in liquid helium and in BECs) but never so many at once. This array of superfluid whirlpools was exactly the kind of storm system astronomers predicted would swirl beneath the iron crust of a neutron star.

Lighthouse? Neutron star

Lighthouse? Neutron star

Evidence for the swirling depths of neutrons stars is based on the fact that some neutron stars are pulsars – the emit a powerful beam of radiation as they spin – like a cosmic lighthouse. The pulses are very regular but occasionally there is a glitch and a pulse might come slightly too early or too late and it is these glitches that are thought to be due to superfluid vortices hammering into the inside of the neutron star’s crust.

Ketterle adds that attractions between atoms in a BEC could parallel the collapse of a neutron star so emulating the distant and massive in the laboratory too. The explosive collapse of a BEC, dubbed a Bosenova (pronounced bose-a-nova) by Wieman releases only a tiny quantity of energy, just enough to raise the temperature of the BEC by 200 billionths of a degree. Supernovae release many times the energy.

Further reading

Nature, 416, 211-218 (2002)
DOI: 10.1038/416211a

Satyendra Nath Bose
http://www.calcuttaweb.com/people/snbose.shtml

Wolfgang Ketterle
http://www.rle.mit.edu/rleonline/People/WolfgangKetterle.html

2001 Nobel Prize in Physics
http://nobelprize.org/nobel_prizes/physics/laureates/2001/

Neutron star
http://www.astro.umd.edu/~miller/nstar.html

Suggested searches

Neutron stars
Bose Einstein condensation
Superfluids

Flowing hot and cold

It is a relatively easy to control the direction of an electric current – use a rectifier – but heat usually flows from hot to cold. Now, Italian and French researchers have devised a rectifier for heat in a computer simulation. They reckon a heat rectifier might be made from DNA or another biological molecule.

Giulio Casati, Michael Peyrard and Marcello Terraneo of Insubria University in Como, Italy, have simulated the vibrations of atoms in a crystal lattice and reckon that given the right arrangement of atoms a device could be built that carried heat energy in one direction only. They looked at one-dimensional solids whose vibrational frequency depends on the size of the vibration. A material such as a long strand of DNA in the solid state might behave in this way for instance.

Thermal transfer Renaissance style (Georges de La Tour, particular of Saint Joseph, Paris, Musée du Louvre)

Thermal transfer Renaissance style (Georges de La Tour, particular of Saint Joseph, Paris, Musée du Louvre)

In such an anharmonic material, energy is transferred less efficiently from one end to the other than in a harmonic material where the vibrational frequency (temperature) is independent of the amplitude of the vibration.

At its simplest level, thermodynamics describes atoms in a solid as vibrating with heat energy flow as though they are connected to each other by springs. Casati believes that computational tools can provide a deeper insight and as such allow interesting and potentially surprising properties, such as allowing heat to travel in only one direction.

Giulio Casati

Giulio Casati

A thermal rectifier might consist of an anharmonic chain molecule tethered by a stiff molecular spring at one end and a loose spring at the other. When the spring is at a higher temperature, the chain molecule will vibrate more slowly because energy is not carried through the chain so effectively. Imagine the effect of plucking a guitar string on to which small pieces of children’s play clay have been stuck. The sound produced would be muffled because of the inefficiency of the string’s vibration. Suppose such a thermal rectifier material as we describe is constructed and you put some object inside this material, then if what is inside is cold then it remains cold; if instead it is warm it will cool down. The heat can flow only in one direction, Casati explained to Spotlight.

Computer simulated rectifier

Computer simulated rectifier

Such a thermal rectifier might have applications in electronics – as a cooling device for computer chips and in biotechnology as a heat flow controller for microscale chemical reactors. As to applications, I feel it is too early to say… much work needs to be done, confesses Casati, but what is important is that we now know that possibility exists.

Further reading

Phys. Rev. Lett. 88, 094302
http://link.aps.org/abstract/PRL/v88/e094302/

DOI: 10.1103/PhysRevLett.88.094302

Michael Peyrard
http://perso.ens-lyon.fr/michel.peyrard/

Suggested searches

Thermal Rectifiers
Thermodynamics

Planetary shortlist

There is a bright disc of interplanetary dust particles that surrounds our solar system, starting beyond the orbit of Saturn and stretching beyond the outer reaches, according to European astronomers. The discovery could lead to a speedier way of determining whether distant stars have their own planets too.

Markus Landgraf of the European Space Agency and colleagues have obtained the first direct evidence of a bright dust disc surrounding our Solar System and they reckon it should help illuminate the search for planets orbiting other stars in our galaxy. The finding will help mission planners short list stellar candidates for observation with ESA’s future planet-search missions, Eddington and Darwin.

Markus Landgraf

Markus Landgraf

Planetary systems are thought to condense from a cloud of gas and dust. Planets form near the central star, where the material is densest. However, at great distances from the star, the gas and dust is sparse and can coalesce only into a vast band of small, icy bodies. A dust ring would therefore seem indicative of a mature star with a planetary system. In our Solar System, the dust particles that reach out beyond the orbit of Neptune form the Edgeworth-Kuiper belt. The dust is continually lost to deeper space though and according to Landgraf there has to be something to replenish it for the disk to be maintained. Indeed, 50 tonnes of dust have to be produced every second.

If you have a dust disc around a star that’s not particularly young, then it’s extremely interesting because the dust has to come from somewhere. The only explanation is that the star has planets, comets, asteroids or other bodies that collide and generate the dust, explains ESA’s Malcolm Fridlund.

Traces of the disc surrounding our Solar System are visible in this image taken by COBE. The blue band curving across this image is created by the dust disc surrounding our Solar System (Copyright Michael Hauser (Space Telescope Science Institute)

Traces of the disc surrounding our Solar System are visible in this image taken by COBE. The blue band curving across this image is created by the dust disc surrounding our Solar System (Copyright Michael Hauser (Space Telescope Science Institute)

To prove the existence of the dust disk, Landgraf and colleagues sieved vast quantities of NASA’s Pioneer 10 and 11 data from the 1970s and early 1980s. These spacecraft found dust particles of unknown origin beyond Saturn’s orbit. It was initially suggested that the dust might come from comets but these objects only discard dust when they are close to the sun, they are frozen solid beyond Saturn. They compared the size of the dust particles from beyond Saturn with those measured by ESA’s Ulysses spacecraft, which has orbited the poles of the Sun for more than a decade.

Artist’s impression of Pioneer’s Jovian fly-by

Artist’s impression of Pioneer’s Jovian fly-by

The interstellar grains detected by Ulysses are typically ten to a hundred times smaller than the smallest grain that could be detected by Pioneer. Thus, the Pioneer grains have to be made somewhere within our Solar System.

The only plausible origin is collisions between the small, icy objects in the Edgeworth-Kuiper belt. Since these are the remnants of planet formation, the team believe that planetary systems around other stars will also produce constantly replenishing dust rings.

Brightly shining discs around the stars Vega and Epsilon Eridani have already been observed and the ESA results hint that these discs are evidence of their planets. If we see a similar dust ring around a main sequence star (a mature star, like the Sun), we’ll know it must have asteroids or comets. If we see gaps in the dust ring, it will probably have planets which are trapping the dust grains in resonant orbits, or ejecting them from the Solar System during close encounters, explains Landgraf.

The results appear in the May 2002 issue of The Astronomical Journal

Further reading

Suggested searches

Interplanetary Dust Particles
Extrasolar Planets