Archive for September, 2008

Flyby anomalies explained by special relativity

Thursday, September 18th, 2008


On 23 January 1998, when NASA’s Near spacecraft swung past Earth on a routine flyby towards more interesting lands, a curious thing happened to its speed. It jumped by 13 mm/s.

This wasn’t the first time such an effect had been seen. Engineers saw similar jumps in speed during the Earth flybys of Galileo (in 1990 and 1992), Cassini (in 1999), Messenger (in 2005) and
Rosetta (also in 2005).

Various exotic explanations have been put forward but today it looks as if the explanation is far more prosaic.  Jean Paul Mbelek from CEA-Saclay near Paris, France, says special relativity explains all.
The speed of the spacecraft is measured by the Doppler shift in radio signals from the craft. That makes the speed  easy to calculate.

But Mbelek’s argument is that the relative motion of the spacecraft and the Earth (which is spinning) have not been properly accounted for. And when they are factored in, using special relativity, the flyby anomalies disappear.


Ref: Special Relativity May Account for the Spacecraft Flyby

The incredible climbing droplets

Wednesday, September 17th, 2008


Here’s a curious finding from the University of Bristol in the UK.

Place a droplet onto an inclined plexiglass sheet and shake it up and down. I know what you’re thinking: even without the shaking the drop should dribble down the plate due to gravity unless it is pinned in place by surface tension. Vertical shaking should loosen the drop’s “grip” on the surface and so enhance the speed at which it dribbles. Right?

Actually, exactly the opposite, say Philippe Brunet and pals.  The drop starts to climb up the plate.

Brunet and friends think the shaking introduces a non-linear effect in the frictional force between the drip and plate because of the way the surface area of the drop in contact with the sheet changes with time. So there is less resistance to its upwards motion than its downward progress.

Interesting, no? And useful too, they say.  Engineers need to know how to move droplets around inside microfluidic devices and this could give them an extra tool to play with.

Ref: Vibration-induced climbing of drops

Nanotube springboard is world’s most sensitive weighing scales

Tuesday, September 16th, 2008


Vibrating springboards have long been the darlings of nanomechanics wanting to measure the mass of small things.

Their thinking goes like this: a springboard vibrates at a specific resonant frequency that depends on its stiffness and mass. So you can work out the mass of anything that becomes stuck to the springboard by measuring any change in its resonant frequency.

Various groups have used this idea to detect all kinds of organic and inorganic molecules using springboards carved out of silicon.

But improving the sensitivity even further means reducing the mass of these springboards. The question is how.

The answer is provided today by Alex Zettl and his team at the University of California, Berkeley, who have created a springboard out of a single carbon nanotube. And their machine is one helluvan elegant device.

For starters, they exploit nanotubes’ unusual ability to act as radio transmitters to determine how fast it is vibrating. They zap the nanotube with radio waves and listen out for the radio signals it emits in return. This signal tells them how fast the nanotube is vibrating.

And because nanotubes are four orders of magnitude lighter than silicon cantilevers, they are four orders of magnitude more sensitive to mass.

All that adds up to device that is able to measure the mass of individual gold atoms as they settle on to its surface. “The sensitivity of our device is 0.40 Au atoms/√Hz. This is the lowest mass noise ever recorded for a nanomechanical resonator,” say Zettl and buddies.

And if that doesn’t impress you, how about this: their measurements were made at room temperature rather than in cryogenic conditions.

The team points out that their device works as a unique kind of mass spectrometer: it is compact, does not require powerful magnets and can easily be built into a chip. Expect to see more of them.

Ref: An Atomic-Resolution Nanomechanical Mass Sensor

First printed graphene circuits

Monday, September 15th, 2008


The world of solid state electronics is in awe of graphene. This single layer of carbon chickenwire has the potential to revolutionise electronics (and much else) because it has enviable electronic, mechanical and thermal properties that no other material can match.

The news today is that Ellen Williams and buddies at the University of Maryland at College Park have worked out how to use graphene in printed circuits. The technique is conceptually straightforward: simply stamp the graphene onto a plastic circuit board in the required shape and hope it sticks. Then stamp other components on top of it. And voila! A printed graphene circuit.

Williams and her team have wasted no time demonstrating the technique, by printing a field effect graphene transistor onto a plastic substrate. “This represents the ultimate extension of printing technology to a single atomic layer,” they say.

Engineers are going to be fascinated by the properties of these transistors, not least because they are almost perfectly transparent at optical frequencies.  Anybody think why that might be useful?

Of course, there is much work to be done, not least in making graphene itself. At the moment, Williams (and almost everyone else) can only get graphene by chipping away at a block of graphite and hunting for graphene in the debris.  That’s a technique even Fred Flintstones would recognise. Any advance is going to be hugely welcomed.

Ref: Printed Graphene Circuits

Holes ‘n’ spaces (part 1)

Saturday, September 13th, 2008

The best of the rest from the physics preprint server this week:

Matter-wave Cavity Gravimeter

Fractality Feature in Oil Price Fluctuations

Digital Control of Force Microscope Cantilevers Using a Field Programmable Gate Array

Modeling of Plasma-Assisted Conversion of Liquid Ethanol into Hydrogen Enriched Syngas in the Nonequilibrium Electric Discharge Plasma-Liquid System

Traversable Wormholes from Surgically Modified Schwarzschild Spacetimes

How supermassive black holes help galaxies evolve

Friday, September 12th, 2008


It’s easy to imagine that our understanding of the way galaxies form and evolve is more or less complete. After all, we’ve been fitting missing pieces into the jigsaw at an alarming rate in recent years with all this data from WMAP etc about the structure of the early universe, a better understanding of the distribution of dark matter and the vast computer simulations that show how galaxies should appear out of this maelstrom.

But there are one or two hairs in this astrophysical ointment. For example, our models of galaxy formation indicate that certain types of galaxies should become surrounded by huge clouds of gas in which stars ought to be forming. But observations show that there are far fewer of these types of galaxies than the models predict.

Today, Timothy Heckman of Johns Hopkins University in Baltimore discusses the idea that supermassive black holes at the center of these galaxies might explain the difference. The thinking is that black holes generate and spread enough energy to the outer reaches of the galaxy to regulate star formation in a way that fits with observations. We’ve certainly seen good evidence of supermassive black holes in various galaxies, including our own.

But what makes Heckman’s discussion highly provocative is the suggestions that a symbiotic relationship exists between galaxies and supermassive black holes, that they need each other to form. So supermassive black holes are as important in galactic evolution as gas, dust and gravity. What an idea!

What that means is that far from being a done deal, galaxy formation is set to become one of the hottest topics in astronomy as data from the next generation of space telescopes comes flooding in.

PS: Heckman has a great name for the study of gas-star-black hole cosmic ecosystems. He calls it gastrophysics. Like it!

Ref: The Co-Evolution of Galaxies and Black Holes: Current Status and Future Prospects

Supernova over south pole caused Ordovician mass extinction

Thursday, September 11th, 2008


About 444 million years ago, more than half of all marine invertebrates were wiped out at the end of the Ordovician era in the third worst mass extinction in history.

A couple of years ago, Brian Thomas at the University of Kansas pointed out that this holocaust could have been caused by a nearby supernova zapping the Earth with gamma rays. A 10 second burst of gamma rays, they said, would have done for about half Earth’s ozone layer, leaving life here more or less unprotected from the Sun’s harmful UV rays for 10 years or more.

Organisms living deep beneath the waves would, of course, have been protected from UV rays anyway but those nearer the surface would have been wiped out within that time. What makes Thomas’ idea interesting is that the geological record seems to indicate that species living nearer the surface were hardest hit in the Ordovician extinction.

Today, Thomas and a pal give us an update based on more detailed simulations. They say the geological data is consistent with a gamma ray burst somewhere over the South Pole.

And this also allows them to predict that any large land mass well above the equator would have been shielded from the burst and so the geological record there ought to be different. Thomas suggests that northern China would be as good a place as any to start looking.

Better get digging.

Ref: Late Ordovician Geographic Patterns of Extinction Compared with Simulations of Astrophysical Ionizing Radiation Damage

The LHC: let the lead fly

Wednesday, September 10th, 2008


There’s no doubt that protons will be the stars of the show when the LHC switches on this morning. But in all the fuss it’s easy to forget that the machine is designed to carry other particles too.

So Paolo Giubellino at the National Institute of Nuclear Physics in Turin, Italy, outlines what to expect next year when the machine starts smashing together nuclei of lead. The idea is to reproduce the conditions that existed in the first three minutes after the Big Bang.

Giubellino is unusually optimistic about the results. He says that every new generation of heavy ion collider as immediately produced  revolutionary  data and, in a strange application of logic, says we should therefore expect the same from the LHC

The first results obtainable with Heavy-Ion beams at the LHC will qualify it as a discovery machine, capable to provide fundamental new insight to our knowledge of high-density QCD matter

Let’s hope he’s not tempting fate.

Ref: Heavy Ion Physics at the LHC

The puzzle of knotted proteins

Tuesday, September 9th, 2008


At one time, molecular biologists swore blind that proteins would never become knotted, at least not in the natural course of things.

But in recent years, they’ve been forced to eat their words as  one protein after another has been shown to have a knotted structure.

The question is why; what purpose do knots serve in protein structures?

We know how they form: in exactly the same way that string becomes knotted in your pocket, by the random motion of the protein.

But it turns out that knots do not occur as frequently as you’d expect if proteins became knotted at random. In fact most proteins never become knotted. It “has remained largely unclear why nature steers some proteins to form a complicated knotted structure, while most are discouraged,” say Thomas Bornschlögl from the Technische Universität München in Germany and colleagues.

One possibility is that knots make proteins more mechanically stable. To test this idea, Bornschlögl and pals set out to measure how much force is needed to untangle a knotted protein called apo phytochrome using an atomic force microscope as a pair of tweezers.

It turns out that it unfolds with only 47 picoNewtons of force, making it less stable than many proteins that aren’t knotted. Nature can’t have chosen that structure for its superstability then, reason Bornschlögland and co.

Instead they think that the knot prevents the protein domains from slipping relative to each other when the structure absorbs light energy. The knot helps turn this energy into free vibrations which dissipate as heat.

That’s an interesting idea but by no means a slam dunk. So we’re back to square one: why are some proteins knotted and what advantage does this confer on them?

Ref: Tightening the knot in phytochrome by single molecule atomic force microscopy

The amazing powers of silicon carbide

Monday, September 8th, 2008


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: Emission of Terahertz Radiation from SiC