Archive for May, 2008

Can dark matter explain the flyby anomalies?

Wednesday, May 21st, 2008

Flyby anomaly

The flyby anomalies, you may remember, are a set of fascinating data indicating that spacecraft flying past Earth undergo a strange, step-like change in their acceleration.

The Galileo, Near, Cassini and Rosetta spacecraft all seem to have been hit by this weird phenomenon and while that’s not a large number of data points, it is an impressive proportion of the few spacecraft that have flown past Earth on their way to other parts of the solar system.

Nobody knows what causes this effect but there are a growing number of fascinating ideas. For example, I’ve blogged about a Casimir force-like change in inertia. And today, Stephen Adler at the Institute for Advanced Study in Princeton considers the possibility that these spacecraft are banging into lumps of dark matter as they swing past the planet.

In an impressive analysis, Adler doesn’t rule out an interaction with dark matter but he does impose some severe limits on how this process might occur. The problem is that we’ve witnessed both increases and decreases in the acceleration of these spacecraft so any dark matter model would have to allow for this.

Adler says that to fit the flyby data, the dark matter near Earth would have to be much denser than in the rest of the solar system and many orders of magnitude more dense than expected in our galaxy. It would have to be confined to a Saturn-like ring around the Earth. And it would have to consist of at least two types of dark matter.

Of course that’s possible but dark matter would have to be wildly more complex than most scientists are willing to accept at the moment. So the phenomenon remains a puzzle.

Since I last blogged about this, a major peer-reviewed study from the Jet Propulsion Laboratory has examined the data and pronounced the effect real. Which means that this is becoming one of the great outstanding challenges in modern physics. Expect to hear a lot more about it.

Ref: arxiv.org/abs/0805.2895: Can the Flyby Anomaly be Attributed to Earth-bound Dark Matter?

Why ET will phone using neutrinos not photons

Tuesday, May 20th, 2008

Neutrino communication

The search for extraterrestrial intelligence assumes that ET will be communicating using photons. But despite decades of listening out, we’ve heard nothing.

But today, John Learned from the University of Hawaii and pals say forget photons. We should be looking for evidence of ET using neutrinos.

The reason is that any civilisation advanced enough to colonise the galaxy would need a reliable way to communicate over intragalactic distances and photons simply don’t pass muster. There is a huge amount of noise in the electromagnetic spectrum, photons are easily scattered and would almost certainly be absorbed if they had to travel from one side of the galaxy to the other.

By contrast, the neutrino spectrum is relatively noise free and neutrinos intereact so weakly with matter that a signal could travel unhindered from one side of the galaxy to the other.

They propose testing the idea by generating a neutrino signal using a particle acclerator to genreate Z nought particles which decay into neutrinos of a relatively easily detectable energy. They would encode information in the time structure of the beam, like Morse code.

What’s more, Learned and co say that the kind of neutrino signals that ET might be expected to beam should be detectable by the generation of neutrino detectors now under construction.

Ref: arxiv.org/abs/0805.2429: Galactic Neutrino Communication

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

In case ya missed ‘em…

Sunday, May 18th, 2008

This week’s sure things from the arXiv blog:

Speech therapy revolutionised by hi-tech dentures

World’s oldest social network reconstructed from medieval land records

Phobos could form Saturn-like ring around Mars

How ESA plans to search for other Earth’s

How orbiting electrons can lengthen nuclear half-life

Hafnium ‘n’ hafnotium

Saturday, May 17th, 2008

The best of the rest from the physics arXiv this week:

A Limiting Rule for the Variability of Coding Sequence Length in Microbial Genomes

High-Sensitivity Diamond Magnetometer with Nanoscale Resolution

Parity-Violating Macroscopic Force between Chiral Molecules and Source Mass

Statistics of Multiphoton Events in Spontaneous Parametric Down-Conversion

Neutrinos from active black holes, sources of ultra high energy cosmic rays

Evidence of Phonon-assisted Tunnelling in Electrical Conduction through DNA Molecules

Pedestrian Flow at Bottlenecks

Does Avian Magnetoreception Rely on Both Magnetite and Maghemite?

How orbiting electrons can lengthen nuclear half-life

Friday, May 16th, 2008

Fission

Nuclear fission is the process in which a nucleus decays into two fragments. For large nucleii, this process is a complicated one in which the nucleus undergoes several stages of deformation before tearing itself apart.

In recent years, physicists have predicted that fission ought to be affected by the presence of electrons in orbit about the nucleus. That’s because any change in the shape of the nucleus naturally affects the electrons which tend to absorb energy making fission less likely. And the more electrons there are, the more energy they absorb. But the effect has never been observed because ordinary, naturally ocurring elements simply don’t have enough electrons to make this effect significant.

Today, Vlad Dzuba and Vic Flambuam at the University of New South Wales in Australia have calculated the strength of this effect for superheavy elements which would have more electrons. They say that although the effect is tiny for naturally ocurring nuclei with fewer than 100 or so protons, it would be hugely significant for these larger nuclei. In fact, they calculate that an atom with 160 protons would have double the expected half life because of this effect.

That could have significant implications for how much of this stuff we’re likely to find because elements that decay quickly tend to be rarer) . Last week, arxivblog reported on the potential discovery of element 122 (with 122 protons). Perhaps the groups looking for superheavies should be setting their sights much, much higher.

Ref: arxiv.org/abs/0805.1961: The Effect of Atomic Electrons on Nuclear Fission

How ESA plans to search for other Earth’s

Thursday, May 15th, 2008

Darwin

We’re getting close to the day when we’ll spot an Earth-like planet orbiting another star. Astronomers have already seen a number of superEarth candidates–rocky planets in the habitable zone that are many times larger than Earth. They’ve even begun to analyse the atmosphere of these places and got some idea of what it might be like on their surfaces. Earth-sized planets won’t be far away now.

But if we are to spot the signs of life on these bodies, what should we look for? The European Space Agency has been giving this some serious thought for a mission called Darwin currently pencilled in for launch in 2015. It’s goal is to look for signs of life on Earth-like planets.

Today, the team behind the mission explain some of the reasoning behind their design for the spacecraft. To look for life, they’ve had to make some important assumptions about the form it might take. For example, they’re plumping for carbon-based life forms that rely on water as a solvent. Fair enough but their assumptions go a lot further:

“We assume that extraterrestrial life is similar to life on Earth in its use of the same input and output gases, that it exists out of thermodynamic equilibrium, and that it has analogs to microorganisms on Earth.”

That’s getting pretty specific but they say their hand is forced by the fact that they’ve never seen any other type of life and so can’t possibly know what else to look for.

So Darwin will look for carbon dioxide, ozone and of course water in the atmospheres of these planets as well as methane and ammonia.

Finding those in the right abundances will be good evidence that something interesting is happening on these planets although finding any other gases that are out of geochemical equilibrium will also be an eye-opener.

The trouble is that finding these signatures will by no means be a slam dunk in favour of life.

In one of the classic scientific papers of the 20the century, Carl Sagan and colleagues published their analysis of the data from the Galileo spacecraft’s 1990 flyby of Earth. The spacecraft saw all those gases and more. Their conclusion? “Our results are consistent with the hypothesis that widespread biological activity exists…on Earth”.

Not quite conclusive and that’s from a distance of a thousand kilometres. So it’s hard to imagine that data from Darwin could provide conclusive evidence of life at a distance measured in dozens or hundreds of lightyears. But I guess that’s nothing a good PR team couldn’t solve.

Ref: arxiv.org/abs/0805.1873: Darwin – A Mission to Detect, and Search for Life on, Extrasolar Planets

Phobos could form Saturn-like ring around Mars

Wednesday, May 14th, 2008

Mars

The martian moon Phobos is spiralling towards Mars at a rate of 20 cm a year

The question is when will it hit. On the arxiv today, calculates that we have about 11 million years before it smashes in to the Red Planet. That’s plenty of time to visit although he also points out that Phobos may break up long before that because of tidal forces. If that’s going to happen it’ll probably occur about 7 million years from now.

But it’s not all bad news. Sharma says if tidal forces get the better of Phobos, it should form a Saturn-like ring around Mars. Now that would be worth seeing.

Ref: arxiv.org/abs/0805.1454: Theoretical Formulation of the Phobos, moon of
Mars, rate of altitudinal loss.

World’s oldest social network reconstructed from medieval land records

Tuesday, May 13th, 2008

Medieval network

The network of links between peasants who farmed a region of small region of south west France called Lot between 1260 and 1340 have been reconstructed by Nathalie Villa from the Universite de Perpignan in France et amis.

The team took their data from agricultural records that have been preserved from that time. This is a valuable dataset because it records the date, the type of transaction and the peasants involved.

Villa and co used this to recreate the network of links that existed between individuals and families in th 13th and 14th centures in this part of France. They then drew up a self organising map of the network (see above).

But the best is surely to come. What Vilal hasn’t yet done is analyse the network’s properties. Does this medieval network differ in any important ways from the kind of networks we see between individuals in the 21st century? If so, what explains the differences and if not what are the invariants that link our world with 13th century France. The team promises an analysis in the near future.

In the meantime, it’s worth reflecting on the significance of this work. These kinds of networks could provide anthropolgists with an exciting new way to study historical societies.

And while this may be the world’s oldest social network (if anyone knows of an older network, let us know), it’s unlikely to remain so for long. Excellent records survive of transactions in ancient Rome, from the earlier Greek empire and even from the Egyptian civilizations that built the pyramids some 4000 years ago.

If Villa work turns up any useful insights into the nature of medieval society in France, you can be sure that anthroplogists will rush to repeat the method usnig data from from even older societies.

All that’s left is to christen the new science of the study ancient social networks Any suggestions?

Ref: arxiv.org/abs/0805.1374: Mining a Medieval Social Network by Kernel SOM and Related Methods

Speech therapy revolutionised by hi-tech dentures

Monday, May 12th, 2008

Dentures

Dentures rarely find their place at the cutting edge of science (some say unfairly) but today is an exception.

Christophe Jeannin at the Institut de la Communication Parlée in Grenoble, France, and a few pals have developed a set of hi-tech dentures that contain a number of tiny pressures sensors that record the position of the tongue and the force it applies during speech.

That’s important because nobody has ever accurately measured that behaviour of the tongue during speech which is a serious impediment to the development of computer models of humans speech, which in turn makes it harder to perfect robot speech and various kinds of computerised speech therapy.

Jeannin and friends place to change that and have even recruited 20 denture wearers to try out the device (they each had their own personalised set of pressure sensitive dentures rather than having to take turns on the same set).

Just one problem however: are the speech patterns of denture wearers really the same as those with a full set of clappers? Who knows?

But if they can work that out, researchers will have a good data set to chew on which they can use to determine the mechanics of tongue-tooth interaction during speech for the first time.

Ref: http://arxiv.org/abs/0805.0744: Tongue Pressure Recordings during Speech using Complete Denture