Archive for the ‘Seein' the light’ Category

Loop quantum cosmology: a brief overview

Wednesday, December 3rd, 2008

quantum-loop-cosmology

Abhay Ashtekar, a physicist at the Pennsylvania State University is one of the founders of loop quantum cosmology and also a part-time populariser of science.

Today, he uses both of these attributes to produce a fascinating overview of loop quantum cosmology that non-specialists will find enlightening.

A recommended read.

Ref: arxiv.org/abs/0812.0177: Loop Quantum Cosmology: An Overview

Two new SETI searches see first light

Friday, November 21st, 2008

seti-spots-crab-pulsar

The Search for Extraterrestrial Intelligence is picking up steam. The folks over at the Berkeley SETI group now have 7 separate searches underway at infrared, visible and radio wavelengths.

Today, Andrew Siemiona and pals outline the two newest programs which have recently seen first light and are hunting for pulses just a few hundred nanosceonds long. By contrast, most searches up till now have looked only for pulses a few seconds long.

The first, a project called Fly’s Eye at the Allen Telescope Array in northern California, can watch huge areas of the sky up to 100 degrees square and spot pulses as short as 0.625 ms

The second is called Astropulse at the Arecibo Observatory in Puerto Rico and will be 30 times more sensitive than any search gone before.

The early results from these searches are being processed on the SETI@Home network which the authors claim is the second most powerful supercomputer on the planet.

Nevertheless, it looks as if they have long hard slog ahead of them: ET hasn’t revealed herself just yet.

Ref: arxiv.org/abs/0811.3046: New SETI Sky Surveys for Radio Pulses

And here is the sunspot forecast…

Thursday, November 13th, 2008

Astronomers have been monitoring sunspot numbers since 1700 and using them as an indicator of solar cycles since 1913. Today we know that peaks in sunspot numbers have an important influence on the Earth, increasing the amount of drag on satellites and contributing to telecoms and and power outages. Accurate forecasts of sunspot activity could help mitigate against these effects.

Ali Kilcik at Akdeniz University in Turkey and an international group of buddies have used a variety of techniques to analyse the data going back to the 1700s and use it to make predictions about the forthcoming maximum solar cycle 24, which we entered earlier this year.

Is prior performance a good guide to the future? It certainly hasn’t been in the past. Predicting the number of sunspots at the solar max has been notorioulsy unreliable.

That hasn’t stopped Kilcik and co sticking their necks out. They say solar cycle 24 will peak in December 2012 with 89 sunspots, a relatively small number considering that cycle 23 peaked with 170 or so in 2000.

Guess we’ll have to wait and see.

Ref: arxiv.org/abs/0811.1708: Nonlinear Prediction of Solar Cycle 24

Why PAMELA may not have found dark matter

Thursday, October 30th, 2008

pamela.jpg

This is the one we’ve been waiting for. For months, the astrophysical world has been abuzz with rumors that the orbiting observatory PAMELA has found evidence of dark matter.

Various people have speculated on the nature of this dark matter but the PAMELA team has been cautious, refusing to release the data until they are happy with it. (Although that hasn’t stopped data being smuggled out of private presentations using digital cameras to capture slides).

Now the wait is over. The PAMELA team has put its data on the arXiv and the evidence looks interesting but far from conclusive
Here’s the deal: PAMELA has seen more positrons above a certain energy (10GeV) than can be explained by known physics. This excess seems to match what dark matter particles would produce if they were annihilating each other at the center of the galaxy. That’s what has got everybody excited

But there’s a fly in the ointment in the form of another explanation: positrons of this kind of energy can also be generated by nearby pulsars.

So PAMELA isn’t the smoking gun for dark matter that everybody hoped. At least not yet.

For that, we’ll need some way to distinguish between the positron signature of dark matter annihilation and the positron signature of pulsars.

That means a whole lot more data and some refreshing new ideas. You can be sure that more than a few  astrobods are onto the case.

Ref: arxiv.org/abs/0810.4995: Observation of an Anomalous Positron Abundance in the Cosmic Radiation

On the origin of Saturn’s rings

Thursday, October 9th, 2008

s-rings.jpg

One of the outstanding mysteries of our Solar System is how Saturn’s rings formed.

We know they rings are made of water ice with very few contaminants. We know they are different to the rings around Jupiter, Neptune and Uranus which are much smaller and probably the result of the surface erosion of nearby moonlets.

But Saturn’s spectacular rings are different. They are far more massive, probably several times the mass of the Saturnian moon Mimas. So how did they get there?

There are three main theories, says Julien Salmon from the Université Paris Diderot in France and a couple of mates.

The first is that the rings are leftovers from the primordial cloud and never formed into a moon around Saturn. That seems unlikely say the researchers, because the rings have a different chemical composition to other Saturnian satellites which must have formed from the same cloud.

The next idea is that the rings formed when a comet collided with and destroyed an ancient Saturnian moon.

The final theory is that the rings formed when Saturn’s gravity captured one or more comets and tidal forces broke the comets apart.

These last two are much more difficult to tease apart because we know that about 4 billion years ago, the solar system was filled with comets which bombarded the planets and their moons. This period, known as the Late Heavy Bombardment, could have caused either scenario.

But a detailed analysis by Salmon and co cause them to lean towards the theory that a comet must have collided with an existing moon. Here’s why: if passing comets could be captured and torn apart by tidal forces, then all four gas giants ought to have Saturn-like rings. And Saturn’s ring system ought to be the smallest of the lot because of the planet’s low density and mass compared to Jupiter and its distance from the main body of comets compared to Uranus and Neptune.

So Saturn’s rings must have been formed by a collision between a comet and moon, say Salmon and buddies. And it turns out that only  Saturn (and possibly Jupiter) could have had a moon at the relatively close distance that the rings have formed. (At that distance, moons around the other gas giants would not have been stable because of tidal forces.)

So that settles it: Saturn’s rings formed about 4 billions years ago when  a number of comets smashed apart one of its moons.

Well, not quite. There are still a number of important outstanding details. For instance,  moons and comets are known to contain relatively high fractions of silicates. And yet the rings contain very little silicates. Nobody has adequately explained where these silicates have gone.

And then there is the annoying evidence that the rings may be much younger than 4 billion years old because we can see some of them darkening at a rate which cannot have been going on for too long without turning the rings black.

Salmon and co say that on balance, the late heavy bombardment is your best bet if you wnat to plump for a mechanism that created the rings.

But there’s no need to be hasty– there’s more mileage in this mystery yet.

Ref: arxiv.org/abs/0809.5073: Did Saturn’s Rings Form During The Late Heavy Bombardment?

The amazing powers of silicon carbide

Monday, September 8th, 2008

silicon-carbide.jpg

Silicon carbide is one of those workhorse materials that can do almost anything. Because it has a high melting point, it is used in high performance brake discs, as a the matrix for particulate filters in engines, and because it is a semiconductor in high temperature and high voltage applications.  Come to think of it, silicon carbide is also used as an abrasive in sand paper.

Today you can add terahertz generator to the list. Jared Straight and pals from Cornell University bombarded silicon carbide with infrared light pulses and found that it in response, it emitted coherent radiation with a frequency of between 1 and 6 Thz.

That’s handy because the current best way to make terahertz radiation in this way is to bombard galium arsenide or zinc tellurium with infrared light. But both of these material are relatively fragile.

Silicon carbide, on the other hand, is unusually tough and so could lead to terahertz devices that are much more robust.

Ref: arxiv.org/abs/0809.0756: Emission of Terahertz Radiation from SiC

Why aluminum should replace cesium as the standard of time

Monday, August 25th, 2008

micromagic-clock

The second is defined as 9,192,631,770 vibrations of a cesium atom and measured in a device known as a fountain clock. These work by cooling a tiny cloud of cesium atoms to a temperature close to zero, tossing it up in the air and zapping it with microwaves as it falls.

Then you watch the cloud to see if it fluoresces. This fluorescence is maximised when the microwave frequency matches a hyperfine transition between two electronic states in the atoms, at exactly 9,192,631,770 Hz.

Various labs around the world use this method to run clocks with an accuracy of around 0.1 nanoseconds per day. That’s impressive but not perfect. Fountain clocks have one drawback: the clouds of cesium tend to disperse quickly and that limits how accurately you can take data.

Now there’s a new kid on the block which looks as if it’s going to be better at keeping time.

Today some chaps from the the University of Nevada in Reno and the University of New South Wales in Sydney outline a new clock that relies on an effect called the Stark shift in which a spectral line is split by an electric field (this is the electric analogue of the Zeeman effect in which spectral lines are split with a magnetic field).

This is a complex phenomenon but the key thing is that the same electric field can influence the split in different ways. In fact, a couple of groups have recently discovered that in certain circumstances these can cancel out each other at specific “magic” frequencies of an electric field. When that happens, the line splitting vanishes.

This should be pretty straightforward to measure. The electric field is supplied by trapping the atoms in a standing electromagnetic wave, otherwise known as a standard optical lattice. Then change the laser frequency while looking at the atomic spectra. When the line splitting vanishes, you’ve hit the magic frequency.

The big advantage of this method is that you can trap millions of atoms easily in an optical lattice and that should make such a clock much more robust than a fountain, while achieving at least the same kind of accuracy.

So what kind of atom should we choose to sit at the heart of these “micromagic clocks”? The Ozzie-American group says that, contrary to previous reports, cesium does not have a magic frequency and so can’t be used in this technique. Aluminum, on the other hand, should be perfect.

The second is dead, long live the second.

Ref: arxiv.org/abs/0808.2821: Micromagic Clock: Microwave Clock Based on Atoms in an Engineered Optical Lattice

The physics of skin vision

Tuesday, August 12th, 2008

skin-vision.jpg

Most animals use optical systems to form images but a substantial number rely on optics-less cutaneous vision or skin vision. And while computer scientists have spent a good deal of time and effort trying to reproduce the former, how many will even have heard of skin vision?

So a systematic investigation of this kind of imaging is long overdue, argue Leonid Yaroslavsky and mates from Tel Aviv University in Israel on the arXiv today.

What exactly are we talking about here?  Yaroslavsky and pals give a list of examples from the natural world including:

The ability of some plants to orient their leaves or flowers
towards the sun

Cutaneous photoreception in reptiles

“Pit organs” of vipers located near their normal eyes – these organs are sensitive
to infra-red radiation and do not contain any optics

They even mention one or two anecdotal examples in humans to which I might add the common sense of knowing where the sun is with your eyes closed, by feeling its heat alone.

Skin vision has a number of advantages over conventional optics. Since it requires no lenses skin vision can be adapted to virtually any type of radiation at any wavelength. It can work on almost any surface and its resolution is determined by number of sensors and not by diffraction limits.

Equally, there are significant disadvantages, chief among them being that a lens creates an image using no processing power at all whereas skin vision needs significant post-processing to produce an image.

In fact, the trade off between lenses and optics-less vision systems seems to be between simplicity of design and and computational complexity. As Yaroslavskyputs it:

What a lens does in parallel and at the speed of light, optics-less vision must replace by computations in neural machinery, which is slower and requires high energy (food) consumption.

But where does the crossover occur that makes one type of vision better than another? This looks to be a field that is too important and potentially useful to be overlooked  by biologists and computer scientists alike.

Ref: arxiv.org/abs/0808.1225: Optics-less Smart Sensors and a Possible Mechanism of Cutaneous Vision in Nature

The Casimir conundrum

Thursday, July 31st, 2008

When it comes to the Casimir force, physicists are in an embarrassing position.

“Weak intermolecular forces have a truly pervasive impact, from biology to chemistry, from physics to engineering. It may therefore come as a surprise to know that there still exist, in this well established field, unresolved problems of a fundamental character. This is indeed the case with respect to the problem of determining the Van der Waals-Casimir interaction between two metallic bodies at finite temperature. As of now, people simply don’t know how to compute it, and the numerous recent literature on this subject provides contradictory recipes, which give widely different predictions for its magnitude”

So writes Giuseppe Bimonte at the Istituto Nazionale di Fisica Nucleare in Naples, Italy. That doesn’t sound good but Bimonte has a way out of the conundrum.

His proposal is to measure the change in Casimir pressure between two superconducting plates as their temperature is raised through their critical value so that they no longer superconduct.

Bimonte claims that the results should unambiguously distinguish between the various competing theories.

Get to it.

Ref: arxiv.org/abs/0807.2950: The Casimir Effect in a Superconducting Cavity: a New Tool to Resolve an Old Controversy

The magnetic magic of liquid mirrors

Thursday, July 17th, 2008

 Liquid mirror

Liquid mirror telescopes are amazing contraptions. They start life as a puddle of mercury in a bowl. Set the whole thing spinning and the mercury spreads out in a thin film up the sides of the bowl.

The result is a fabulously cheap mirror that can be used for a variety of astronomical surveys. If we ever put a telescope on the moon, many astronomers have suggested that it should be one of this type.

It won’t have escaped your attention that liquid mirrors have important limitations. First, they can only point straight up. One or two people  have played with fluids that have a higher viscosity than mercury and so can be tilted a few degrees this way or that but with limited success. And second, they cannot be made adaptive to correct for blurring introduced by the Earth’s atmosphere.

But that may change thanks to some interesting work being done by Denis Brousseau at Université Laval in Quebec et amis.  Their machine controls the shape of the surface of a liquid mirror using a magnetic field. Mercury cannot be used, however, because it is too dense and changing its shape requires impractically powerful fields.

Instead the team have used a suspension of ferromagnetic nanoparticles in oil. A thin highly reflectivity layer of silver particles can then be spread across the surface of the ferrofluid to create a mirror.

Brousseau and co use an array of tiny coils behind the liquid to create a field that deforms the fluid surface as required.  Their tests show this can be done fast and furiously enough to cope with the usual array of optical aberrations that the atmosphere throws up.

However, it may also be possible to use this technique to tilt liquid mirrors further than ever before.  Ferrofluids can easily be made much more viscous than mercury and so combat the deforming pull of gravity. But they can also be deformed in a way that opposes gravity during each rotation of the supporting bowl. That could make them much more tiltable than mercury mirrors.

Of course, such a mirror would be mechanically more complex than the spinning bowls we have today and correspondingly more expensive. And sending one to the moon seems an unnecessary extravagance given the absence of an atmosphere there.

But here on Earth they could be made much more useful.  It’s a combination of new-found utility and value for money that many astronomy projects on a budget will find irresistible.

Ref: arxiv.org/abs/0807.2397: Wavefront Correction with a Ferrofluid Deformable Mirror: Experimental Results and Recent Developments