Archive for the ‘Seein’ the light’ Category

The weather on HD 189733b

Tuesday, July 15th, 2008

HD 189733B

Our old friend HD 189733b is in the news again this week. As a Jupiter-sized gaseous planet orbiting a yellow dwarf in the constellation of Vulpecula, HD 189733b has become one of the best studied exoplanets.

The reason is that it’s relatively big  and close to its sun, which shines through the atmosphere as the planet transits.

This phenomenon has allowed astronomers to measure many properties of HD 189733b’s atmosphere. For example, they have found that this ball of gas is rich in water and methane and that to our eyes the planet would appear a rich dark blue, a bit like Uranus.

Last year, the Spitzer Space Telescope produced a heat map of the planet showing global temperature differences. Unsurprisingly, HD 189733b is warmer at the equator than at the poles.

Astronomers also think the atmosphere is filled with particles of the condensates of iron, silicates and aluminium oxide. And today, Sujan Sengupta from the Indian Institute of Astrophysics in Bangalore, India, adds another nugget of information. It seems that these particles must be distributed in a thin layer of cloud in the upper atmosphere.

So if you’re wondering what the weather is like on HD 189733b, the answer is cloudy.

What’s emerging is the most amazing picture of a planet orbiting another sun, something that only a few years ago would have been deemed impossible.

To put this in perspective, our understanding and knowledge of HD 189733b is comparable to, and in some ways better than, our pre-Voyager knowledge of Neptune and Uranus in the 1970s.

That’s truly astounding. There can’t be many more persuasive examples of the fact that we’re currently living through a golden age of astronomy.

Ref: arxiv.org/abs/0807.1794: Cloudy Atmosphere of the Extra-solar Planet HD189733b : A Possible Explanation of the Detected B-band Polarization

Dark energy and the bitterest pill

Monday, July 14th, 2008

 Copernican principle

It’s hard to get your head around dark energy, this universe-accelerating stuff that is supposed to fill the cosmos. Dark energy was invented to explain measurements that seem to show that the most distant supernovas all appear to be accelerating away from us. The thinking is that something must be pushing them away and that stuff is dark energy.

But for many astrophysicists, dark energy is a difficult pill to swallow. It requires the universe to be fine tuned in a previously unexpected, and frankly, unimaginable way.

So astronomers have begun a systematic investigation of all the assumptions on which the notion of dark energy depends. Nothing is sacrosanct in this hunt–these guys are tearing up the floorboards in the search for an alternative hypothesis. And that means revisiting some of our most fundamental assumptions.

One of these is the Copernican principle, that the universe is more or less the same wherever you happen to be. Principles don’t come much more fundamental than this but the evidence in its favour, at least on the scale that dark energy seems to behave, is pretty thin.

In fact, a number of theorists have calculated that the supernova data can be explained without the need for dark energy if our local environment were emptier than the universe as a whole. But to make this idea work, the earth must be sitting in the middle of a void that is roughly the size of the observable universe and that’s not compatible with the Copernican principle, not by a long shot.

Now Timothy Clifton  and pals at the University of Oxford in the UK have worked our how to tell whether such a void exists or not. They say that the next round of highly accurate measurements of nearby supernova should be able to tell us whether we’re in a void or not. So we shouldn’t have long to wait.

Either way, astronomers will find it hard to settle that troubling sensation in the pit of their stomachs. The truth is that when it comes to swallowing uncomfortable ideas, dark energy may turn out to be a sugar-coated doughnut compared to a rejection of the Copernican principle.

Ref: arxiv.org/abs/0807.1443: Living in a Void: Testing the Copernican Principle with Distant Supernovae

Simple mod turns diode into photon counter

Tuesday, July 8th, 2008

Avalanche photodetection

Counting photons is a tricky business. They’re slippery beasts that arrive silently, often and in packs, in ways that are almost impossible to count.

One of the most widely used of devices that can spot the arrival of a single photon is the avalanche photodiode. These cheap and easy to use devices rely on the ability of diodes to allow the flow of electrons when the voltage across them is in one direction but prevent that flow when the bias is reversed. But  if the reverse bias is increased beyond a specific threshold then a breakdown occurs and a reverse current suddenly starts to flow.

Choose the right material for your photodiodes and this breakdown can be triggered by a the arrival of a single photon  smashing into an electron which goes on to hit other electrons causing a chain reaction. The result is an avalanche of current that signals the arrival of your photon.

Avalanche photodiodes are widely use to detect single photons but have an important limitation: they cannot distinguish between the arrival of a single photon and the arrival of two or more photon’s simultaneously.

But that is set to change. Today, our old friend Andrew Shields, at Toshiba’s research labs in Cambridge UK, explains how to soup up a bog-standard avalanche photodiode so that it can count photons as they arrive. That’s like turning a Fiat 500 into a Ferrari.

He says that the trick is to measure the characteristics of the avalanche current in the very first instants that it forms. At this early stage, say Shields and friends, the avalanche current  is proportional to the number photons that have struck.

Simple really but with enormous potential. The ability to count photons is one of the key enabling technologies for optical quantum computing. A number of schemes are known in which it is necessary to count the arrival of 0,1 or 2 photons at specific detectors.

Various people, including Shields himself, have  come up with complex, cooled devices that can count photons. But this is a major step forward. Avalanche photodiodes are cheap, widely available and easy to use. With such a cheap detector now available (as well as decent photon guns), we could see dramatic progress in this field in the coming months.

If you haven’t quite seen the significance of this, imagine overclocking your calculator and matching the performance of a workstation. Or polishing up the 3 inch reflector in your attic and outclassing Hubble with your images.

Impressive stuff.

Ref: arxiv.org/abs/0807.0330: An Avalanche-Photodiode-Based Photon-Number-Resolving Detector

Why black holes could be antimatter factories

Tuesday, June 24th, 2008

Black hole

Here’s an interesting chain of thought…

Imagine a black hole sucking in protons and electrons. With their higher mass,  protons are likely to be preferentially sucked, giving the black hole a positive charge. (That’s not so unusual in space: a similar mechanism can give planets a charge because electrons escape their gravity more easily.)

But black holes also create such strong electrostatic fields at the horizon that positrons and electrons simply appear out of the vacuum.

In those circumstances, it’ll look as if the protons being sucked into the black hole are being converted into positrons.

So these kinds of black holes will look and behave like antimatter factories, say Cosimo Bambi from Wayne State University in Detroit and pals.

How might we we spot these exotic objects? Bambi and friends say a sure signature would be an excess of positrons in cosmic rays  with an energy between 1 and 100 MeV coming from a black hole.

Anybody seen any of these?

Ref: arxiv.org/abs/0806.3440: Black Holes as Antimatter Factories

The embarrassing lightness of photons

Wednesday, June 18th, 2008

Photon force

Here’s a conundrum for you. What is the momentum of light in a transparent dielectric medium?

If the answer doesn’t trip off your tongue, that might be because nobody else knows either. Amazingly, there are two lines of thought:
In 1908, the German mathematician Hermann Minkowski guessed that the momentum was equal to nE/c (where n is the refractive index, and E and c are the energy and speed of light in a vacuum).

A year later,  his contemporary Max Abraham suggested that the momentum is equal to E/nc.

A century since then and we’re none the wiser. An embarrassing state of affairs for theoretical physics, wouldn’t you agree?

Today Weilong She and pals from Sun Yat-Sen University in Guangzhou, China, announce that they have the answer. And they got it by measuring the recoil on the end face of a nanometre-sized fibre exerted by outgoing light. (This isn’t the well known pressure caused by specular reflection but something  a little more subtle.)

The experiment is impressive because it is designed in such a way that if Minkowski were correct the fibre should be pushed in one direction and in the other if Abraham were correct.

So the result is the first to unambiguously favour one theorist over the other.

And the winner is…drum roll…Abraham.

It’s about time.
Ref:  arxiv.org/abs/0806.2442: Observation of a Push Force on the End Face of a nm Fiber Taper Exerted by Outgoing Light

Supernova echoes give first glimpse of ancient explosions

Monday, June 2nd, 2008

Supernova echoes

Back in 2005, Armin Rest from Harvard and a few mates, spotted the echo of a supernova in the Large Magellanic Cloud. The explosion had kicked off some 900 years ago but what Rest and co were seeing was its reflection from cold dark dust in the cloud. Since then, the team has even measured the spectrum of the echo to determine that it was Type 1a supernova

Impressive work but Team Rest have not been idle. They have been busy hunting for other echos and today it looks as if they’ve come up trumps. Instead of looking for echos from the Large Magellanic Cloud, the team looked at the Milky Way (a harder task because it takes up a much larger portion of the sky) and found numerous clusters of echos from two recent supernova: Tycho (SN1572) observed by the Danish astronomer Tycho Brahe in 1572 and Cassiopeia A, which went more or less unobserved when it exploded in 1667.

This is important for astronomers because it gives them access to a a kind of exotic wayback machine. All of a sudden, they can study the physics of these supernovae at the moment they exploded and compare that with the properties of the remnants today.What’s more, by looking for echoes of different supernovae that have reflected off the same dustcloud, they have a way to directly measure the distance between them. They conclude that Cassiopeia A is (probably) 1900 light years further away from us than Tycho.

So it provides a new way to measure distance too.

The team has now begun a program to look for the echoes from five other supernova that have occured in the last 2000 years or so. Expect to hear more about that in the near future

Ref: arxiv.org/abs/0805.4607: Scattered-light Echoes from the Historical Galactic Supernovae Cassiopeia A and Tycho (SN 1572)

How to turn a narrow slit into a large window

Thursday, May 29th, 2008

Narrow slit

How do you turn a narrow slit into a large window? Fill it with a metamaterial that captures and transmits as much light as the bigger window. At least, that’s what Xiaohe Zhang and colleagues at Shanghai Jiao Tong University in China tell us.

Metamaterials are substances constructed in a way that gives them exotic bulk properties that aren’t otherwise found in nature, such as the ability to manipulate electromagnetic radiation in unheard of ways. Much of the publicity about metamaterials has revolved around their potential ability to form invisibility cloaks that can hide an object from view. But less well known are a menagery of designs that do other strange things such as rotate the appearance of a cloaked object.

Now Xiaohe Zhang and pals have weighed in with yet another design: a material “that can transmit the information outside a domain through a small slit, with the transmittance identical to the one of a big window”. In other word, they’ve designed a small window with the same transparency as a larger one, albeit one that works in the microwave region of the spetrum

But why on Earth would you want one of these? It’s one of those things that has a useful smell about it but the team don’t mention any applications their paper so I’m kinda stumped.

Ref: arxiv.org/abs/0805.3039: Transformation Media that Turn a Narrow Slit into a Large Window

The mystery of the Plutonic color scheme

Monday, May 26th, 2008

Pluto

Pluto’s three satellites, Hydra, Nix and Charon, are all a similar shade of grey. In fact, Nix and Hydra have exactly the same colour to within our ability to measure it. Pluto, on the other hand, is a beautiful shade of red. How come?

The current thinking is that Charon, Hydra and Nix are a similar colour because they were all formed in the giant impact that created this satellite system.

But today, Alan Stern, former head of NASA’s Planetary Science’s division and principal investigator for the New Horizons mission to Pluto, puts forward an alternative hypothesis.

His idea is that the impact of debris from the Kuiper belt on these bodies could send enough surface material into orbit to coat the satellites nearby. Interesting idea.

Stern calculates that the ejecta velocities on Pluto and Charon would be too low to escape. However, the ejecta from Nix and Hydra could easily escape in enough quantity to cover one another to a depth of tens of metres and to cover Pluto and Charon to a depth of tens of centimetres.

The weather on Pluto generates regular frosts which cover this up as qucikly as it was laid down but no such mechanism operates on the other satellites.

So that might explain the differences and similarities in the Plutonic color scheme. Stern also predicts that if he is right, the colours and albedos of Nix, Hydra and Charon should change slowly as more material is ejected and deposited. So by keeping a sharp eye on them, he can gain further evidence for his theory.

What’s more, he says that this mechanism may be common in the solar sytem wherever small binary systems are found, such as in the asteroid and Kuiper belts. And where this happens, these bodies should have similar colours too.

All we have to do now is to look out for the flurry of papers pointing to evidence that he’s right.
Ref: arxiv.org/abs/0805.3482: Ejecta Exchange, Color Evolution in the Pluto System, and Implications for KBOs and Asteroids with Satellites

The trouble with optical invisibility cloaks

Monday, May 19th, 2008

Nanoring

You could be forgiven for thinking that invisibility cloaks are a few R&D dollars away from hitting the high streets. Not so.

While it’s true that a number of high profile cloaks have been built, the best of these work only in the radio and microwave regions of the spectrum and then only in at a single frequency and in two dimensions . So unless you are a flatlander viewing the world through microwave eyes, these cloaks are not much use.

The one claim for an optical invisibility cloak works only inside a strange, exotic material made from gold nanorods and even then over a distance of a only few nanometres.

What an invisibility cloak has to do is steer light around an internal cavity in way that makes it appear to have passed straight through. That’s possible, in theory, if you can design a material in which its permeability and permittivity (the way it interacts with an electromagnetic wave) can be tailored throughout its structure.

That can be done relatively easily at microwave frequencies. The materials in question are extraordinary honeycombs of repeating patterns of split ring resonators and wires. The pattern has to be about the same size as the wavelength of the microwaves– a few centimetres or so.

So why not just make everything smaller to match the wavelength of visible light? The first reason is that we’re talking about a material with a feature size measured in nanometres and that is just beyond what’s possible today. The second is that optical frequencies tend to match the resonant frequency of electrons within these materials. What that means in plain English is that the materials absorb light rather than transmit it (which is why the one demonstration so far has worked only over a distance of a few nanometres before the light was absorbed).

So what to do? One idea is to make the cloaks out of lasing materials which constantly replace the light as it is absorbed. But a better one is to create a material that doesn’t absorb light in the first place. There’s no way to get rid of the electronic resonance that is responsible for absorbing light so the trick is to design a structure in which the resonance can be made to cancel itself out or help to propel the light through the material.

So physicists are desperately examining the properties of various nanostructures to see whether they might have the properties that fit the bill.

Today, Andrea Alu and Nadar Enghet at the University of Pennsylvania in Philadelphia, publish an analysis of just such a metamaterial made of nanoparticles arranged in a ring, as shown above. They say that this material gives “cleaner magnetic dipole response”.

Unfortunately, they are less clear over whether they’ve hit the jackpot with regards optical invisibility. In fact they can’t be sure whether this will have the required properties at optical frequencies or not.

The trouble is that it’s not possible to know the bulk properties of a material made from a particular nanostructure without some heavyweight calculating. And choosing which structure to investigate is little more than guesswork at the moment. As Alu and Enghet show with this work.

Back to the drawingboard, I’d say. Looks as if they’ll need to kiss a few more frogs before they find their prince.

Ref: arxiv.org/abs/0805.2329: Dynamical Theory of Artificial Optical Magnetism Produced by Rings of Plasmonic Nanoparticles

A new class of photon gun

Monday, March 31st, 2008

Photon gun

Photons are easy to produce, at least en masse. But making them one at a time in a controlled fashion is much harder. Until recently the only trick physicists had for this was to reduce the brightness of a beam until it contained only one photon at a time, on average.

Of course, the “on average” clause allows for any manner of multiphoton sin: the photons may be produced in pairs or bunches or not at all. That’s a serious problem for techniques such as quantum cryptography because any extra photons are equivalent to a leak of information that an eavesdropper can use to crack the code.

Enter the photon gun brigade, a growing band of physicists attempting to build devices that can fire identical, single photons on demand. The single photon part of this problem has been cracked by a number of groups who have built guns that work by inducing atoms, ions, molecules or quantum dots to fluoresce in a cavity.

What has turned out to be more difficult, but just as important, is making each of these single photons identical. What tends to happen is that the cavites vibrate in various ways and this subtly distorts the wavelength of the photons they produce. The result is that each photon is very slightly different, which spoils their ability to interfere and become entangled with each other.

Here’s the breakthrough: Andrew Shields, at Toshiba’s research labs in Cambridge, UK, plus a few pals from a nearby university, say they can create the photons using a quantum dot that they zap with two precisely timed voltage pulses. The first pulse injects charge carriers into the diode while the second suddenly shifts the quantum dot’s emission characteristics so that it can emit light. When that happens, the dot emits a photon of a specific energy.

But the key is to use such short voltage pulses that the photon doesn’t have time to be influenced by its surroundings. And that means the quantum dot always emits photons of precisely the same wavelength.

So it looks as if Shields and co have a neat new trick up their sleeves, albeit one that has to be performed at 4K. But it’s also an entirely new mechanism for producing photons of this kind so improvements are gonna be forthcoming. How long before we see a commercial version?

Ref: arxiv.org/abs/0803.3700: Indistinguishable Photons from a Diode