Dam Thanh Son from the University of Washington, US, and his colleagues used string theory to model a 10-dimensional black hole as a liquid. String theory tries to explain fundamental properties of the universe by predicting that seven dimensions exist on top of the known three spatial dimensions. While the concept is currently unproven as a cosmological model, the tools of string theory can sometimes provide answers to real quantum problems.

That means that while the “black holes” modelled by Son are not astrophysical black holes, but mathematical objects that exist in string theory, their findings may have relevance to the real world.

The fluid has two properties that relate to the black hole’s surface area: viscosity, which describes how thick the liquid is, and entropy density, which is a measure of the internal disorder. Son’s team found that the ratio of these two properties is a constant which can be expressed as a mixture of fundamental constants from the quantum world.

Super-cooled atoms

They suggest this constant acts as a universal lower limit for the ratio of the viscosity to entropy in real fluids. This backs an argument based on Heisenberg’s famed uncertainty principle suggesting that such a limit should exist.

“That is what we hypothesise. We couldn’t prove that it’s the case, but we couldn’t find anything that is less viscous,” says Son. For example, the value of this ratio for water is 400 times greater than for black hole fluid. Even liquid helium is nine times more viscous.

Fluids that could approach the limit include super-cooled clouds of atoms, or the plasmas created in particle colliders, suggests Son.

Physicists have already drawn comparisons between the fireball produced at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York, US, and string-theory black holes. “I’ve started taking it seriously,” says Peter Steinberg from Brookhaven National Laboratory, who works with one of the teams collecting data at the RHIC.

Although RHIC have not yet measured the viscosity of their fireball, this would allow an experimental test of Son’s prediction. “The final word will come from the experimentalists,” Son says.

Journal reference: Physical Review Letters (vol 94, p111601)

Wrinkles and the leathery feel of old skin are thought to result from changes in the dermis, the deepest layer of skin. This becomes fibrous, making the skin less elastic. But Igor Sokolov’s team at Clarkson University in Potsdam, New York, US, has shown that changes may also be taking place in the epithelial cells above the dermis. “What we have discovered is that the epithelial cells themselves become more rigid with age,” says team member Craig Woodworth.

When the researchers measured rigidity by poking human epithelial cells grown in a dish with the tip of an atomic force microscope, they found that after many generations of cell division, cells were two to 10 times as stiff as “younger” cells. They think the increased rigidity is due to the cells’ cytoskeleton – the internal scaffolding of protein fibres – becoming more dense.

The team expect to find the same changes, which appear in cells over weeks or months, in skin samples from elderly people. And some existing drugs might inhibit the stiffening of the cytoskeleton and perhaps slow the ageing process. The team is already testing skin creams containing low doses of these compounds on mice, and expects results in a few months.

Little is known about ageing mechanisms in epithelial cells even though this is the layer that can be targeted with creams, says Marie-Madeleine Cals-Grierson, an expert in ageing at cosmetics company L’Oréal in Paris, France. “The cellular ageing mechanism described in the article seems quite new,” she says. “We are always interested in the results of clinical trials involving anti-ageing compounds.”

Journal reference: Physics and Medicine in Biology (vol 50, p 81)

Although planet hunters have bagged almost 150 extrasolar planets since the first one was spotted 10 years ago, until now they have only inferred the planets’ presence by measuring the wobble in the host star’s orbit or the dimming of the starlight as the planet passes in front of it. No one had yet seen the light from a planet directly.

Earlier this year, Glenn Schneider of the University of Arizona in Tucson, US, and his colleagues directly saw an object that was hailed by many as an extrasolar planet. But Schneider’s object, visible in infrared light, is five times as massive as Jupiter and is in orbit around a small, failed star known as a brown dwarf.

Schneider himself refuses to use the “P-word”, preferring instead to call the thing a “planetary-mass object”. “Everyone was calling this a planet in the press,” says Schneider. “But this wouldn’t form as planets do.”

“Hot Jupiters”

By contrast, Deming’s team and another led by David Charbonneau at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, have each studied a planet of the class called “hot Jupiters”, which orbit Sun-like stars. These gas giants probably evolved in a manner similar to those in our solar system.

The orbits of both planets, as observed from Earth, take them behind their host stars. The teams took advantage of this to tease out the planets’ radiation from infrared images captured by NASA’s Spitzer space telescope.

First they measured the radiation from each star-planet system when the planet was to one side of its star as viewed from Earth. Then they measured the radiation when the planet was behind the star. The difference between the two readings showed the radiation coming from the planet.

“From the first moment we saw the data, we were ecstatic,” says Charbonneau. “We could see the signal of the planet passing behind the sun and coming back again.”

Surface temperatures

Based on these measurements, the teams estimated the temperature of the planets. Both gas giants have surface temperatures of at least 700°C.

“It is clearly a direct measurement of the flux from the planet,” says Didier Queloz from the Geneva Observatory in Sauverny, Switzerland, who was behind the discovery of the first extrasolar planet 10 years ago. “It is very impressive.”

Deming’s planet, HD 209458b, has a mass two-thirds that of Jupiter and orbits a sun-like star in the constellation of Pegasus about 140 light years from Earth (Nature, DOI: 10.1038/nature03507).

Charbonneau studied a more distant planet called TrES-1, which has a similar mass and zips around its star in just three days. He reports these calculations in a forthcoming issue of The Astrophysical Journal.

The rich are getting richer while the poor remain poor. If you doubt it, ponder these numbers from the US, a country widely considered meritocratic, where talent and hard work are thought to be enough to propel anyone through the ranks of the rich. In 1979, the top 1% of the US population earned, on average, 33.1 times as much as the lowest 20%. In 2000, this multiplier had grown to 88.5. If inequality is growing in the US, what does this mean for other countries?

Almost certainly more of the same, if you believe physicists who are using new models based on simple physical laws to understand the distribution of wealth. Their studies indicate that inequality in market economies may be very hard to get rid of.

Economists will join physicists to discuss these issues next week in Kolkata, India, at the first ever conference on the “econophysics” of wealth distribution. “We are interested in understanding whether there is some kind of social injustice behind this skewed distribution,” says Sudhakar Yarlagadda of the Saha Institute of Nuclear Physics (SINP) in Kolkata.

It is well known that wealth is shared out unfairly. “People on the whole have normally distributed attributes, talents and motivations, yet we finish up with wealth distributions that are much more unequal than that,” says Robin Marris, emeritus professor of economics at Birkbeck, University of London.

Pareto’s law

In 1897, a Paris-born engineer named Vilfredo Pareto showed that the distribution of wealth in Europe followed a simple power-law pattern, which essentially meant that the extremely rich hogged most of a nation’s wealth (żěè¶ĚĘÓƵ print edition, 19 August 2000). Economists later realised that this law applied to just the very rich, and not necessarily to how wealth was distributed among the rest.

Now it seems that while the rich have Pareto’s law to thank, the vast majority of people are governed by a completely different law. Physicist Victor Yakovenko of the University of Maryland in College Park, US, and his colleagues analysed income data from the US Internal Revenue Service from 1983 to 2001.

They found that while the income distribution among the super-wealthy – about 3% of the population – does follow Pareto’s law, incomes for the remaining 97% fitted a different curve – one that also describes the spread of energies of atoms in a gas (see graphic).

Gas analogy

In the gas model, people exchange money in random interactions, much as atoms exchange energy when they collide. While economists’ models traditionally regard humans as rational beings who always make intelligent decisions, econophysicists argue that in large systems the behaviour of each individual is influenced by so many factors that the net result is random, so it makes sense to treat people like atoms in a gas.

The analogy also holds because money is like energy, in that it has to be conserved. “It’s like a fluid that flows in interactions, it’s not created or destroyed, only redistributed,” says Yakovenko.

Yakovenko also found that the total income of those in the poorer part of the distribution did not change significantly with time after accounting for inflation. But incomes for those in the Pareto curve shot up nearly five times from 1983 to 2000, before declining with the US stock market crash of 2001.

Class jumping

This, along with research data from other countries, suggests that there are two economic classes. In one, the rich grow richer while in the other the poor stay poor. Yakovenko explains this by going back to the analogy of atoms in a gas.

The atoms assume an exponential distribution of energy when they are in thermal equilibrium, and pushing the gas away from this state takes a lot of energy and it could prove similarly difficult to shift an economy to a different state. Randomness in the model does, however, mean that individuals can jump from one class to another.

“It suggests that any kind of policy will be very inefficient,” says Yakovenko. It would be very difficult to impose a policy to redistribute wealth “short of getting Stalin”, says Yakovenko, who will talk in Kolkata next week.

Saving plans

A more sophisticated model developed by Bikas Chakrabarti of the SINP and his colleagues paints a slightly less bleak picture for the poor. His team adjusted the gas model to allow people to save various proportions of their money.

This model predicts both the wealth classes that Yakovenko found. It also suggests that if you save more you are more likely to end up rich, although there are no guarantees. Changing people’s saving habits could be an effective way of making the wealth distribution fairer, rather than enforcing taxes, says Chakrabarti, who is one of the Kolkata conference organisers.

Macroeconomist Makoto Nirei at Utah State University in Logan, US, whose own work will be presented at the conference, is supportive of the physicists’ work but he has reservations about how they model the exchange of money. “The model seems to me not like an economic exchange process, but more like a burglar process. People randomly meet and one just beats up the other and takes their money.”

Other economists warn it is too early to use such models to inform policies. “The models are too abstract,” says Thomas Lux, an economist at the University of Kiel in Germany. But J. Doyne Farmer, a physicist from the Santa Fe Institute in New Mexico, US, points out that these models have their place: “Many economic theories don’t even come close to producing the wealth distribution we see, and if you can’t produce that you’re dead in the water.”

While the bungle does not undermine the main scientific conclusions drawn from the data collected by the rovers, it is an embarrassing slip-up for a space agency that once lost a Mars spacecraft because engineers mixed up metric and imperial units.

The rovers made near-perfect touchdowns on the surface of Mars in January 2004, and their mission has been considered an unqualified success. Spirit and Opportunity provided the first irrefutable evidence that there was once liquid water on the surface of the Red Planet and are still roaming long after their scheduled 90-day mission.

But something was worrying Ralf Gellert of the Max Planck Institute for Chemistry in Mainz, Germany. Gellert runs an instrument on the rovers called the alpha-particle X-ray spectrometer (APXS), which analyses the chemical composition of rocks. Opportunity had found higher concentrations of certain elements in the soil at its Meridiani Planum landing site than Spirit had at the Gusev Crater, but on a windswept Mars the concentrations should have evened out all over the planet.

Gellert published the data last year (Science, vol 306, p 1746). “No one really asked about the differences,” he recalls. “I thought about it very, very heavily, but I couldn’t come up with a solution.”

“Now the reason for this is obvious,” Gellert told (żěè¶ĚĘÓƵ. “We found proof that we unintentionally swapped both instruments.” Spirit is carrying the spectrometer destined for Opportunity, while Opportunity is in possession of Spirit’s.

Creeping errors

Although their designs are identical, each instrument is unique because of quirks in the materials they are made from. So before the rovers were launched, each instrument was calibrated using known rock samples. The measurements from each rover are then processed using the calibration files, but because of the mix-up, researchers were using the wrong ones. As a result, small errors have crept into the APXS results, affecting measurements of sodium, magnesium and aluminium abundance.

Fortunately, now that the goof-up has been spotted, it is easily fixed by reanalysing the raw data with the right calibration. Corrected values for the first year’s data will be available soon, says Steve Squyres, the chief scientist for the rovers.

The corrections are very small because the instruments are so similar, he says. “None of our substantive scientific conclusions are affected.” Gellert is relieved: “This turns out to be a lucky case where the data makes more sense now.”

Squyres is “not embarrassed at all” about the slip-up with the rovers. “It was an easy mistake to make,” he says. “It happened during some very busy and stressful times.” He also says it is not fair to compare it to past mishaps because the spacecraft suffered no damage. “There isn’t going to be an investigation. We know when it happened,” he says. “There was a point when both of them were sitting on the same bench, and that has to have been it.”