## Archive for March, 2009

### Important changes to the Physics arXiv Blog

Sunday, March 15th, 2009

From Monday 13 March, the Physics arXiv Blog will appear exclusively on technologyreview.com

This is an exciting move for the blog because it will allow me to concentrate on reading and filtering the fantastic ideas on the arXiv while leaving the increasingly onerous task of administering a popular website to the talented tech guys at TR.

If all goes smoothly, your RSS feeds and email subscriptions will be transferred seemlessly to our new hosts. And if it doesn’t go smoothly, let us know. We’ll be working hard to iron out any teething problems as soon as we can.

The new URL is:  arxivblog.technologyreview.com

I hope you’ll join me.

Kentucky

### Chops ‘n’ changes

Saturday, March 14th, 2009

### The secret of world class putting

Friday, March 13th, 2009

Watch professional golfers putt and you’ll eventually notice three common features about their style,  says Robert Grober, an expert on the physics of golf at the Yale University.

First, the putter head always moves at a constant speed when it hits the ball. Second, the length of time the putting stroke takes has little impact on the speed of the ball (and therefore the length of the putt). And finally, a professional golfer’s backswing takes about twice as long as the  downswing.

Grober has used these observations to construct a mathematical model of a putting swing and to explore other properties of such a system.

It turns out that the model that best accounts for this behaviour is a simple pendulum driven at twice its resonant frequency.

That explains a number of other observations about professional golfers, says Grober. For example, a common putting tip is that longer backswings equate to longer putts. This model has exactly this characteristic: the length of the backswing is proportional to the speed of the club at impact.

It is also relatively straightforward to get a sense of the tempo of the required putt by swinging the club back and forth in resonance, like a pendulum. The duration of the actual stroke is exactly half the length of the putter cycle (i.e. from the address position moving backward, to the address position moving forward). “In fact, one often observes golfers instinctively doing this before they hit a putt,” says Grober.

So now the secret is out. Make a careful note for next time you’re out on the links.

Ref: arxiv.org/abs/0903.1762: Resonance in Putting

### How to narrow the search for ET

Wednesday, March 11th, 2009

The search for extraterrestrial intelligence  needs all the help it can get. Depending on who you listen to, the chances of us spotting an intelligent technological society vary from an almost certainty to practically zero.

The trouble is the sheer size of the search. The Milky Way contains around 10^10 sun-like stars, any one of which may have a planet whose citizens are at this very moment pointing their beady eyes  or antennae in our direction.

But if we want to peer back, in which direction should we look?

Shmuel Nussinov at Tel Aviv University in Israel makes a thoroughly sensible suggestion of narrowing the search: why not look only towards stars that have a reasonable chance of having seen Earth?

We know of several ways to detect planets aroudn other stars but only one that might reveal an Earth-like body and that is to look for changes in brightness that are the signature of a transiting planet.

Earth passes in front of the sun for 13 hours once a year, dimming it by 77 parts per million. Venus transits for 11 hours every 7 months with even less dimming. Mars gives  three-fold weaker eclipse every 1.9 years and Mercury dimming is ten times weaker than Earth’s but occurs four times a year.

Only stars within a narrow angle of the ecliptic will be able to detect these transits. And so only civilisations on planets around these stars could possibly be aware of Earth might be broadcasting our way.

Common sense really.

Ref: arxiv.org/abs/0903.1628: Some Comments on Possible Preferred Directions for the SETI Search

### Visible light metamaterials on the cheap

Tuesday, March 10th, 2009

Only a couple of years, more than a few physicists doubted that it would ever be possible to build decent metamaterials with a negative refractive index for visible light.

Metamaterials have bulk properties that depend on the structure of their components rather than the bulk properties of the materials from which they are made. The thinking is that they can make light do all kinds of things that are no possible in naturally occurring stuff such as bending light backwards and imparting it with a reverse Doppler shift.

Metamaterials that bend microwaves backwards are straightforward to make: it’s just a question of arranging components, such as conducting wires and split rings, in a periodic 3D array on a centimetre scale.

It’s easy to think that similar structures would work for visible light were they shrunk to the nanometre scale. But, as many physicists have pointed out, the electrical properties of conducting metals do not scale with wavelength in quite the same way. Instead of transmitting light, many of these designs would be opaque to visible light.

Some people said it may never be possible to make efficient negative refraction index metamaterials for visible light. Others, who were a little more optimistic, were vindicated  last August, Xiang Zhang at the University of California, Berkeley, revealed that a periodic array of parallel silver nanowires embedded in aluminium oxide worked perfectly well as metamaterial with negative refractive index for visible light.

Now Akhlesh Lakhtakia  at Pennsylvania State University and pals have worked out how to make sheets of this stuff using a vapour deposition technique that is common in the optical industry.

So in a couple of years, we’ve gone from having little prospect of a negative refractive index material for visible light to a way of making sheets of it at extremely low cost.

That’ll make negative refractive index materials available to almost anybody who wants to play with them. Expect to see some ingenious applications in the coming months.

Ref: arxiv.org/abs/0903.1177: Vapor-deposited thin Films with Negative refractive Index in the Visible Regime

### The fundamental patterns of traffic flow

Monday, March 9th, 2009

Take up the study of earthquakes, volcanoes or stock markets and the goal, whether voiced or not, is to find a way to predict future “events” in your field. In that sense, these guys have something in common with scientists who study traffic jams.

The difference is that traffic experts might one day reach their goal. The complexity of traffic flow, while  awe inspiring, may well be fundamentally different  to the complexity of stock markets and earthquake events.

At least that’s how Dirk Helbing at the Institute for Transport & Economics at the Technical University of Dresden in Germany, and his buddies see it.

Helbing says that one long standing dream of traffic experts is to identify the fundamental patterns of traffic congestion from which all other flows can be derived,  a kind of periodic table of traffic flows.

Now he thinks he has found it: a set of fundamental patterns of traffic flow that when identified on a particular road, can be used to create a phase diagram of future traffic states.

The phase diagrams can then be used to make forecasts about the way in which the flow might evolve.

That’ll be handy. But only if it’s then possible to do something to prevent the congestion. And that may be the trickiest problem of all.

Ref: arxiv.org/abs/0903.0929: Theoretical vs. Empirical Classification and Prediction of Congested Traffic States

### Sheets ‘n’ pillows

Saturday, March 7th, 2009

### How superconducting sheets could reflect gravitational waves

Friday, March 6th, 2009

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?

### Centimetre scale models could compute Casimir forces

Thursday, March 5th, 2009

The Casimir force is notoriously difficult to measure. So tricky is it, that the first accurate measurements weren’t made until 1997 and even today only a handful of labs around the world of capable of taking its measure.

Of course there are various ways of modelling what goes on theoretically but even the most powerful simulations these struggle to cope with simple shapes let alone complex geometries. Consequently, our knowledge of the Casimir  force and how to exploit it is poor.

Now John Joannopoulos and pals at MIT are suggesting a rather entertaining third way: to calculate Casimir forces using scale models that work like analogue computers.

What the team has noticed is a mathematical analogy between the Casimir force acting on microscopic bodies in a vacuum and the electromagnetic behaviour of macroscopic bodies floating in a conducting fluid.

So imagine you want to know what Casimir forces will act on a particular geometry. The idea is to build a centimetre scale metal model of this set up and place it in a conducting liquid such as saline. Then bombard it with microwaves and see what happens.

The result should give an accurate representation of the Casimir forces that would act on the microscopic scale.

The group explains:

Such a centimeter-scale model is not a Casimir “simulator,” in that one is not measuring forces, but rather a quantity that is mathematical related to the micron-scale Casimir force. In this sense, it is a kind of analog computer.

There’s no reason why those kinds of tests can’t be done now.  And that should give researchers a way of testing machines designed to reliably exploit the Casimir force for the first time.

Ref: arxiv.org/abs/0903.0599: Ingredients of a Casimir Analog Computer

### Were gravitational waves first detected in 1987?

Wednesday, March 4th, 2009

In 1987, Joe Weber, a physicist  at the University of  Maryland, claimed to have detected gravitational waves at exactly the same moment that other astronomers witnessed the famous supernova of that year, SN1987A.

His equipment consisted of several massive aluminium bars that were designed to vibrate in a unique way when a large enough gravitational wave passed by.

His claims were ignored largely because other physicists calculated that gravitational waves ought to be several orders of magnitude too weak to be picked up by this kind of gear. (And he’d made several similar claims throughout the 60s and 70s that others had failed to repeat.)

But Weber’s claims may have to be re-examined, says Asghar Qadir, a physicist at  the National University of Sciences and Technology in Rawalpindi, Pakistan. He points out that predicting the strength of a gravitational wave is by no means easy and until recently, only first order effects have been considered.

He and colleagues have now worked out that in certain circumstances, second order effects can enhance the  waves. But this only happens when there is a certain kind of assymetry in the event that created the waves.

But get this: the assymetry can enhance the waves by a factor of 10^4.

He also  points out that SN1987A is aspherical in exactly the way that might create this enhancement. So if SN1987A generated gravitational waves, Weber would have been perfectly able to detect them.

Qadir concludes: “The claim of Weber to have observed gravitational waves from [SN1987A] needs to be re-assessed”.

By all accounts, Weber was a careful experimenter who got something of a rough deal for his efforts (the most comprehensive telling of the tale is in a book called Gravity’s Shadow by Harry Collins) .

Weber died in 2000 but it wouldn’t do any harm to re-examine his work in the light of this development.

Ref: arxiv.org/abs/0903.0252: Gravitational Wave Sources May Be “Closer” Than We Think