Archive for June, 2008

How to build a quantum internet

Monday, June 30th, 2008

Quantum internet

You could be forgiven for thinking that a quantum version of the internet is a couple of-afternoons-in-the-lab away from being plumbed into your living room. In reality, there are significant engineering challenges to overcome, says Jeff Kimble from the California Institute of Technology in Pasadena and one of the leading thinkers on the links between the quantum and information sciences.

If you want to know about some of the bigger hurdles that physicists face in wanting to build a quantum internet, you could do worse than look at his account of this field on the arXiv today.

Some of the challenges are particularly daunting such as the unambiguous creation and verification of entanglement.

But, ever the optimist,  he concludes:

“I have every confidence that extending entanglement across quantum networks will create wonderful scientific opportunities for the exploration of physical systems that have not heretofore existed in the natural world”.

Ref: arxiv.org/abs/0806.4195: The Quantum Internet

In case ya missed ‘em…

Sunday, June 29th, 2008

The cocktails from the physics arxivblog this week:

The latest social network: binge drinking

Why black holes could be antimatter factories

Surfing solves puzzle of water snail locomotion

Solar system filled with dark matter, say astronomers

The popcorn experiment and spooky action-at-a-distance

Pop ‘n’ rock

Saturday, June 28th, 2008

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

Predicting Future Duration from resent Age: Revisiting a Critical Assessment of Gott’s Rule

Nanonewton Force Generation and Detection Based on a Sensitive Torsion Pendulum

Shannon Meets Carnot: Mutual Information Via Thermodynamics

Factors Influencing the Earth’s Magnetic Field Evolution

Consequences of Increased Longevity for Wealth, Fertility, and Population Growth

The popcorn experiment and spooky action-at-a-distance

Friday, June 27th, 2008

Macroscopic EPR experiment

In 1964, John Bell became fascinated by the EPR paradox, an idea that Einstein had dreamt up to highlight what he saw as a major flaw in quantum mechanics.

The paradox (called EPR after Einstein and his mates Boris Podolsky and Nathan Rosen) is a thought experiment involving two particles that share the same quantum state. The particles become separated. Then a measurement is made on one particle which immediately determines the state of the other, regardless of the distance between them. This, said Einstein, violates special relativity and is in an act of “spooky action-at-a-distance”.

For thirty years or so, physicists ignored this paradox, all that is, except Bell, a physicist at CERN, the European particle physics laboratory near Geneva.

Bell developed a set of inequalities that could be tested against experiment. If violated, Bell’s inequalities would prove that quantum mechanics and relativity really were at odds.

At first everbody ignored Bell’s ideas but in 1984, a French team succeeded in showing that quantum mechanics did violate the inequalities. Today Bell’s inequalities are routinely violated in quantum laboratories all over the world, leaving little doubt over the issue.

Except for Joy Christian at the University of Oxford, who says that Bell’s inequalities ought to be violated on a macroscopic scale as well as the quantum level.

His assertion is based on an argument about the topology of space. In particular, he relies on a bizarre property of space that, like the EPR paradox,  physicists have tended to  ignore. It is this: turn an object through 360 degrees and it returns to its starting position, right? Actually, no. Not if you’re dealing with fermions such as protons and electrons which have a 720 degree symmetry. To get back to the start, you actually need to rotate them through two full turns.

Christian’s argument is that Bell’s inequalities take no account of this property, which he likens to taking an image apart pixel by pixel but without numbering them and then trying to put them back together again. He says this is the reason why Bell’s inequalities are violated, because they do not take account of the toplogy of space, not because of any spooky action-at-a-distance (although this doesn’t rule that out).

He suggests a somewhat tricky experiment that could be done on the macroscopic scale which would also violate Bell’s inequalities as strongly as on the microscopic scale. It involves measuring how balls pop apart when they’re heated,  like popcorn (this is not a joke, see the paper for full details).

So is Christian implying that there’s nothing strange about the quantum world that isn’t also strange about the macroscopic world? And that perhaps Einstein was onto something after all?

Obviously, we need to take a closer look at this “macroscopic world” everybody is talking about.

Ref: arxiv.org/abs/0806.3078: Can Bell’s Prescription for Physical Reality Be Considered Complete?
His says that a macroscopic test of Bell’s inequalities and today he explains why.

Solar system filled with dark matter, say astronomers

Thursday, June 26th, 2008

Solar dark matter

As the evidence for dark matter builds, astronomers have begun modelling how it ought to be distributed around the cosmos.  They’ve shown how it must be distributed on the largest scale to make clusters of galaxies form in the way we see, various other simulations show that it forms a kind of halo around galaxies such as the Milky Way.

But what of smaller scales? Today, Ethan Siegel and his student Xiaoying Xu at the University of Arizona produce the first model showing how much dark matter there is in the Solar System.

They say that throughout its 4.5 billion year history, the Sun will have been sweeping up the dark stuff as it has moved through the Milky Way. Siegel estimates that in this time it will have gathered some 8 x10^19 kilograms of dark matter, or about 300 times the background levels in the Milky Way.

That’s a lot of mass and Siegel points out that that much dark matter should have profound implications for the various teams searching for it. It means there ought to be more dark matter than anybody expected although this will have a smaller velocity relative to Earth because it should be moving through the galaxy with the Sun. Keep ‘em peeled, guys.

It’s also easy to think that this much dark matter  might have a bearing on the Pioneer anomaly, the unexplained acceleration towards the Sun of our most distant space probes. But no, says Siegel.

This amount of dark matter is much several orders of magnitude smaller than  than the mass of Pluto or any of the larger moons in the solar system. So there’s not nearly enough to explain the observed accelerations, he concludes.

So that’s that then.

Ref: arxiv.org/abs/0806.3767: Dark Matter in the Solar System

Surfing solves puzzle of water snail locomotion

Wednesday, June 25th, 2008

Snail surfing

Snails move using a mechanism called adhesive locomotion. Through muscular contraction and expansion of their foot, they transmit a force to the ground through a thin layer of mucus which is adhesive at low strains but otherwise flows like a liquid.

But what of water snails that move upsidedown along the underside of a liquid surface? Water snails seem to move their foot in the same undulating way as their terrestrial cousins but adhesive locomotion can’t answer for their albeit small, velocity because there’s nothing to stick to.

Today, Sungyon Lee, an engineer at  the Massachusetts Institute of Technology in Cambridge  and a few pals put forward their own suggestion. Their idea is that the undulating motion of the foot deforms the surface of the water. And this generates a pressure that causes the mucus, which is sandwiched between the foot and water surface, to flow.

In other words, water snails surf on waves of their own making.

Neat idea but Lee and company have more work to do to make their argument water tight.

First, it isn’t clear whether their model can account for the kinds of speeds water snails actually achieve (whatever these are).

And second, the model assumes that water snail mucus is newtonian. That’s probably wrong. Terrestrial snail mucus is non-newtonian and that is crucial for locomotion.

I’d be willing to bet a bowl of steaming escargot a la poulette that water snail mucus also turns out to be non-newtonian and that this is crucial for amplifying whatever forces snails use to surf.

Ref: arxiv.org/abs/0806.3651: Crawling Beneath the Free Surface: Water Snail Locomotion

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 latest social network: binge drinking

Monday, June 23rd, 2008

Binge drinking

Binge drinking is “the rapid consumption of large amounts of alcohol, especially by young people, leading to serious anti-social and criminal behavior in urban centres,” say Paul Ormerod, an economist at Volterra Consulting in London, also linked to the University of Durham.

Binge drinking is a growing problem in city centers in the UK, with some 1.5 million binge drinking “events” across the country each week. Ormerod says:

“Vomiting, collapsing in the street, shouting and chanting loudly, intimidating passers-by and fighting are now regular night-time features of many British towns and cities. A particularly disturbing aspect is the huge rise in drunken and antisocial behaviour amongst young females.”

You know who you are. But why has this behavior spread so rapidly?

Ormerod and his mate Greg Wiltshire point to an interesting study of patterns of health conducted in the US, in which people were found to be much more likely to become obese if they had an outsize  friend or to stop smoking if their spouse had also stopped. Perhaps a similar kind of pattern occurs in binge drinkers, Ormerod reasoned.

To find out, he commissioned a survey of Britain’s youth, 504 18-24 year-olds, in which he asked them about their drinking patterns and those of their friends, family and colleagues.

The results are striking.  It turns out that:

  • 30 per cent of binge drinkers think that ‘all’ or ‘almost all’ of their work colleagues binge drink, compared to only 8 per cent of non-binge drinkers.
  • 54 per cent of binge drinkers think that “all” or “almost all” of their friends are binge drinkers, compared to 15 per cent of non-binge drinkers for whom “all” or “almost all” friends are binge drinkers.
  • only 3 per cent of binge drinkers have “no” or “hardly any” friends that binge drink compared to 19 per cent of non-binge drinkers.

Clearly the behaviour of people you know is linked in a significant way to your drinking habits.

Ormerod then used the data to determine that the kind of network that best describes the way these people are linked is a small world:  a network in which people are linked by a small number of steps in the manner of six degrees of separation.

And if that’s the case, binge drinking is a fashion that must be spreading like like tie-dyed T-shirts and nose studs.

That’s interesting because the fact that this is a small world network could have important implications for how to tackle the problem.

One line of thought has focused on individuals who are linked to a large number of others. These so-called hubs can have a disproportionate effect on the spread of behaviours and so targeting them would be one way to halt the spread of binge drinking.

For example, it may be that the behaviour of film stars or other widely known figures can have a disproportionate effect on the behaviour of others.

But that doesn’t seem to be the case with binge drinking. Hubs only exist in scale free networks in which the distance between people varies as a power law (so the network looks the same regardless of the resolution at which you view it).

If Ormerod’s conclusion is confirmed (he needs more data to nail it) and the pattern of binge drinking in the UK really do reflect a small world network, then this kind of targeting is unlikely to work.

Ref: arxiv.org/abs/0806.3176: ‘Binge’ Drinking in the UK: a Social Network Phenomenon

In case ya missed ‘em…

Sunday, June 22nd, 2008

The treasures from this week’s physics arXiv blog:

Extraterrestrial nucleobases found in meteorite

How to bury an ion (and find it again later)

The embarrassing lightness of photons

Why gamma ray bursts are not standard candles

First X-ray diffraction image of a single virus

X-rays ‘n’ gamma rays

Saturday, June 21st, 2008

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

How Do Schrodinger Cats Die?

On a possible quantum contribution to the red shift

Galactic and Extragalactic Distance Scales: The Variable Star Project

Optical Metamagnetism and Negative Index Metamaterials

Cluster structure of functional networks estimated from high-resolution EEG data

Thermodynamics of natural images

Measuring Value in Healthcare