Archive for the ‘Seein' the light’ Category

How superconducting sheets could reflect gravitational waves

Friday, March 6th, 2009

gravitational-wave-mirror

Gravitational waves are the elusive distortions in spacetime created by the universe’s most violent events–collisions between black holes, stars exploding and even the big bang itself.

Nobody has bagged a confirmed sighting of these waves but that may change thanks to an intriguing idea from Raymond Chiao and pals at the University of California, Merced. They propose the existence of a new kind of mirror that reflects gravitational waves and may even convert them into electromagnetic waves.

First some background. Theoretical physicists have long noticed that in certain circumstances, Einstein’s equations of general relativity, which predict the existence of gravitatonal waves, bear a remarkable similarity to Maxwell’s equations that describe the behaviour of electromagnetic radiation. That’s an important clue for understanding how gravitational waves  behave, says Chiao.

He points to the specific case in which a thin superconducting film reflects em waves. If that works for em waves, then the mathematics indicates that it must also work for gravitational waves.

Here’s the thinking. A gravitational wave stretches and squeezes space as it moves through the universe. Any object in its way will appear to be squashed  and stretched in the same way, the particles within this object will move with the distorted space in a specific trajectory (called geodesic motion).

The new idea comes from considering what happens to a superconducting sheet when a gravitational wave passes by. The Cooper pairs within the sheet are quantum objects governed by the uncertainty principle and so cannot have specific trajectory: they are entirely delocalised. On the other hand, the ions that make up the crystal structure of the superconductor are not delocalised and so can move along a geodesic trajectory when a gravitational wave passes.

This is the basis on which a gravitational wave can interact with a superconducting sheet. “Quantum delocalization causes the Cooper pairs of a superconductor to undergo non-geodesic motion relative to the geodesic motion of its ionic lattice,” says Chiao and buddies.

They speculate that this difference in motion causes the sheet to absorb energy from the  gravitational wave and then re-radiate it as gravitational wave travelling in the opposite direction–in other words specular reflection.

That’s an extraordinary claim which needs some further investigation, not least because there’s a fair amount of disagreement over the GR-Maxwell link in the first place.

Nevertheless, Chiao and co go even further by ending their paper with this:

This implies that two charged, levitated superconducing spheres in static mechanical equilibrium, such that their Coulombic repulsion balances their Newtonian attraction, should be an efficient transducer for converting EM waves into GR waves and vice versa. A Hertz-like experiment in which a transmitter and receiver of GR microwaves are constructed using two such transducers should therefore be practical to perform.

So a pair of levitating, superconducting spheres would act as an antenna for gravitational waves and convert them into electromagnetic waves.

Why wait for LIGO? What’s the betting that superconducting spheres can make the detection first?

Ref: arxiv.org/abs/0903.0661: Do Mirrors for Gravitational Waves Exist?

First evidence of a supernova in an ice core

Monday, February 23rd, 2009

antarctic-supernova

There hasn’t been a decent supernova in our part of the universe in living memory but astronomers in the 11th century were a little more fortunate. In 1006 AD, they witnessed what is still thought to be the brightest supernova ever seen on Earth (SN 1006) and just 48 years later saw the birth of the Crab Nebula (SN 1054).

Our knowledge of these events come from numerous written accounts, mainly by Chinese and Arabic astronomers (and of course from the observations we can make today of the resultant nebulae).

Now we can go one better. A team of Japanese scientists has found the first evidence of supernovae in an ice core.

The gamma rays from nearby supernova ought to have a significant impact on our atmosphere, in particular by producing an excess of nitrogen oxide. This ought to have left its mark in the Earth’s ice history, so the team went looking for it in Antarctica.

The researchers took an ice core measuring 122 metres from Dome Fuji station, an inland site in Antarctica. At a depth of about 50 metres, corresponding to the 11th century, they found three nitrogen oxide spikes, two of which were 48 years apart and easily identifiable as belonging to SN 1006 and SN 1054. The cause of the third spike is not yet known.

That’s impressive result and their ice core was obligingly revealing about other major events in the Earth’s past. The team saw a 10 year variation in the background levels of nitrogen oxide, almost certainly caused by the 11-year solar cycle (an effect that has been seen before in ice cores). They also saw a number of sulphate spikes from known volcanic eruptions such as Taupo, New Zealand, in 180 AD and El Chichon, Mexico, in 1260 AD.

The team speculate that the mysterious third spike may have been caused by another supernova, visible only from the southern hemisphere or hidden behind a cloud.

That would make the 11th century a truly bounteous time for supernovae. Of course, statistically, there ought to be a supernova every 50 years or so in a galaxy the size of the Milky Way, which means that the Antarctic ice is due another shower of nitrogen oxide any day now.

Ref:  arxiv.org/abs/0902.3446: An Antarctic Ice Core Recording both Supernovae and Solar Cycles

The puzzle of planet formation

Wednesday, February 18th, 2009

planet-formation

“The formation of planets is one of the major unsolved problems in modern astrophysics.” That’s how Rafael Millan-Gabet at Caltech and John Monnier from the University of Michigan begin their account of how our understanding of planet formation is about to undergo a revolution.

Driving this change will be a new generation of telelscopes and techniques capable of measuring and in some cases imaging planet formation in action.

It’s worth pointing out the poverty of our current understanding. At the heart of the problem is the fascinating question: why are all the planets different?

The ones in our solar system ought to have formed out of the same stuff at more or less the same time and yet no two are alike. And now the extrasolar planets seem to be demonstrating a similar variety.

The  trouble is that astronomers have only the vaguest understanding of what goes on inside  the circumstellar discs where planets are supposed to form. They have little idea of the circumstances in which accretion dominates over gravitational instability, whether “dead zones” exist in circumstellar discs where planets cannot form or what mechanisms are at work in transporting angular momentum within early solar systems.  They don’t even know when planets form.

The new measurements that will be possible in the coming years should hep to answer at least of these puzzles. And that makes this an exciting field to be in. Watch this space for developments

Ref: arxiv.org/abs/0902.2576: How and When do Planets Form? The Inner Regions of Planet Forming Disks at High Spatial and Spectral Resolution

The telescope that Antarctica broke

Tuesday, February 17th, 2009

coronagraph

First light from any instrument is always exciting but particularly so when exotic locations and exciting goals are involved.  The CORONA experiment offers both.

CORONA is a stellar coronagraph designed to spot extrasolar planets orbiting other stars. It is based at Dome C, some 10,000 feet above sea level in in Antarctica, a location that boasts some of the best seeing on the planet.

The goal of the project is to test the feasibility of operating such a device in the harsh conditions that exist in the Antarctic, where temperatures can drop to -80 degrees C in winter.

The result: at -65 degrees C the telescope showed some strong aberrations in test images of Sirius, probably the result of thermal distortions.

Geraldine Guerri from the Université de Nice Sophia-Antipolis in France and buddies say: “The coronagraph could not be operated in these conditions, and thus, no nighttime images are presently available. CORONA has been sent back to France to be modified.”

That must have been disappointing, although their paper puts a brave face on things. Their plan is to eventually build a much more sophisticated coronagraph with adaptive optics.

That won’t be easy and given their experience so far, it looks highly, perhaps overly, ambitious.

Ref: arxiv.org/abs/0902.2326: First Light from the Dome C (Antarctica) of a Phase Knife Stellar Coronagraph

Watch this space for the chemistry of dust

Wednesday, February 11th, 2009

optical-doughnut

It’s not often that chemists get new tools with which to investigate the building blocks of the world around us, so  a paper on the arXiv today gives them good reason to pop a few corks.

Vladlen Shvedov at the Australian National University in Canberra and a few mates have today unveiled a way of confining and steering aerosol particles in a beam of light.

That hasn’t been possible until now because aerosols are slippery blighters. They absorb huge amounts of light that increases their temperature making them hard to hang on to.

That makes the conventional  way of handling small particles using optical tweezers more or less useless. Optical tweezers rely on radiation pressure to push their charges around. But this is dwarfed by thermal forces and so only works for particles that are relatively cool.

Thermal heating makes particles misbehave because  nonuniform heating makes one side of a particle hotter than the other, causing gas molecules on either side to bounce off with different velecities. This generates a force known as photophoresis.

What Shvedov and his cobbers have done is exploit the photophoretic force to trap partices using  two light beams in the shape of doughnuts.

The result is a device that can trap aerosol particles up to 10 micrometres across  and steer them along trajectories several millimetres long at a rate of around a centmetre per second.

All of a sudden that makes possible a whole host of experiments that were previosuly impossible: developing ecologically clean and safe nanotechnologies, modeleling the chemical proceses at work in the atmosphere and best of all (IMO) simulating interstellar dust .

If you see any chemists with smiles on their faces, you’ll know why. In the meantime look out for some fascinating insights into the chemistry of dust.

Ref:  arxiv.org/abs/0902.1205: Optical Guiding of Absorbing Nanoclusters in Air

Challenging the nature of black holes

Wednesday, February 4th, 2009

black-hole-formation

The nature of black holes has puzzled physicists for decades. But while the debate has fizzled in recent years, some new thinking is about to set it alight again.

Black holes are fundamentally a product of general relativity, which allows for a gravitational collapse so violent that no other force can oppose it. When that happens, the collapse continues until the density of matter becomes infinite and gravity becomes so strong so as to prevent even light from escaping. This generates an “event horizon”, a volume of space around the black in hole inside whihc events cannot affect an outside observer.

But perhaps there’s more to it than that, suggest Matt Visser at Victoria University of Wellington in New Zealand and pals who ask whether quantum processes can have an affect on the collapse of a star.

It’s fair to say that the consensus among astrophysicists is that quantum physics can be safely ignored when considering the collapse of a star. As Visser and co put it: “There is a widespread feeling in the general relativity community that semiclassical quantum back-reaction effects are always small, and never enough to significantly alter the classical picture of collapse to a black hole.”

But Visser and pals beg to differ. The standard thinking is that if an event horizon forms, then quantum field theory is well behaved there. But that makes the assumption that am event horizon will form.

Visser and co say this may not be a valid assumption and go on to show how the vacuum energy might stifle the formation of an event horizon.

What does this mean for black holes? What Visser and co end up with is something very similar to a black hole but without an event horizon–a black hole mimic, they say.

Debating the nature of black holes is a well trodden path. But what’s interesting is that numerous avenues of thought–from loop quantum gravity to abstract studies of the nature of horizons–are now hinting at something more subtle and interesting about the nature of star collapse.

Black holes might never be the same again.

Ref: arxiv.org/abs/0902.0346: Small, dark, and heavy: But is it a black hole?

Trick of the light boosts atom interferometer sensitivity

Thursday, January 15th, 2009

atom-interferometer

While preparing for the job of US Secretary of Energy in the incoming Obama administration (and being  director of one the top labs in the US and Nobel Prize winner to boot), Steven Chu has somehow found time to post the results of his latest experiment on the arXiv. And it’s an impressive piece of work too.

Chu, who is director of the Lawrence Berkeley National Laboratory, and his colleagues have built an atom interferometer with a sensitivity that is dramatically higher than previous models. To prove its worth, they’ve measured the fine structure constant to an accuracy of 3.4 parts per billion, which is within an order of magnitude of the best measurements.

But the real benefit of the new device is that, among other things, it will allow a new generation of tests of the equivalence principle. That is, it will test whether  the m in F=ma and the m’s in F = Gm1.m2/r^2 refer to the same thing.

In physics-speak, the question is whether gravitational and inertial mass are the same. It’s something we always assume but have never proven and there are a number of ongoing programs to study the question.

Here’s how Chu’s work will change the game…

(more…)

Why Saturn’s rings are so sharp

Monday, January 5th, 2009

outer-b-ring

Here’s a conundrum for you: why do Saturn’s rings have such sharp edges?

It’s question that has puzzled planetary scientists for many years. Various ideas have been put forward but none adequately explain the structure we see today.

To understand just  how sharp the edges are consider this: pictures from Cassini show that the density of particles at the edge of the outer B ring, for example, drops by an order of magnitude over a distance of only 10 metres or so.

That’s extraordinary given that the ring is 25580 kilometres wide.

(more…)

The strange right hand of the universe

Monday, December 22nd, 2008

galactic-handedness

Is the Universe right handed? If Michael Longo at the University of Michigan in Ann Arbor is to be believed, the answer is yes; and the evidence comes from the right or left-handed shape of spiral galaxies.

Astronomers have images of many thousands of spiral galaxies. But classifying them as left or right handed is tricky for a computer program. So a project called the Galaxy Zoo asks humans to analyse galaxies and mark them as either left or right handed.

It’s fair to say the results have caused some controversy. The Universe, according to this data, seems to prefer right-handed galaxies and by some distance.

Various critics have said this is a result of the way the tests are set up and that we humans have an in-built bias for right-handed spirals. But this has been challenged by other data. Longo has trawled through the lot and and has settled on the conclusion that there is some kind of universal preference for right handedness, an Axis of Evil along which galaxies tend to spin.

What to make of this claim? Longo says that various studies have shown all kinds of biases, some such as the cold spot in the cosmic background radiation, more convincing than others.

But it always pays to tread carefully in areas like this. A number of claimed biases have disappeared after a more thorough analysis of the data.

A fascinating idea it may be, but we need to treat claims of a universal handedness with caution.

Ref: http://arxiv.org/abs/0812.3437: Does the Universe Have a Handedness?

Astronomers find hottest and fastest exoplanet

Friday, December 19th, 2008

wasp-12b-data

As astronomers discover greater numbers of planets orbiting other stars, they are able to revise their theorie sof planet formation accordingly. Exotic planets are particularly prized because they push the boundaries of theoretical understanding to its limits.

Today,  Leslie Hebb  at the University of St Andrews in Scotland and colleagues announce the discovery of one of the most exotic planets yet seen. WASP-12b orbits a star about 3 times as bright as the sun. Its orbit has a radius about 1.8 times that of Jupiter  but it is only about a quarter as dense.

But get this. WASP-12b has an orbital period of only  1.09 days, the shortest ever seen, and a surface temperature of 2516 K because it is the most highly radiated planet ever discovered. That’s too records for one planet.

How these kinds of planets can form, nobody is certain. But one thing’s for sure: WASP-12b must be on one helluva roller coaster ride.

Ref: arxiv.org/abs/0812.3240: WASP-12b: the Hottest Transiting Extra-Solar Planet Yet Discovered