Bone has to satisfy two nearly contradictory requirements – it must be stiff without being brittle. To avoid splintering, a stressed bone needs to dissipate energy, like a shock absorber. And nobody is sure how rigid bone manages to absorb the energy.

James Thompson and his team from the University of California, Santa Barbara, think the energy gets used up ripping apart tiny Velcro-like connections between tangled collagen molecules in the bone. When these sacrificial bonds break they soak up the energy that might otherwise crack the bone.

Delicate operation

To test their idea, the researchers used a delicate device called an atomic force microscope to repeatedly stretch individual collagen fibres attached to a glass slide, as well as similar ones sticking out of a sample of rat bone. The fibres soaked up more energy in each cycle of stretch and relaxation if there was more time between consecutive pulls.

This suggests that when the fibres are stretched, bonds between the coiled collagen molecules break until the molecules are more or less straight. As the fibre contracts again, the bonds need time to reform. If the bonds were simply stretching rather than breaking the fibres should soak up the same amount of energy no matter how long the interval between pulls.

The idea of sacrificial bonds is plausible, says John Currey, a biologist at the University of York, but he’s not convinced yet. He says it’s a nice result, but points out that the Santa Barbara researchers don’t know for certain that the fibres sticking out of the bone are in fact collagen.

“They’ve got to find out what it is they’re pulling on,” he says, “and that may be difficult.”

More at: Nature (vol 414, p 773)

Guo-meng Zhao and Yong Sheng Wang of the University of Houston in Texas found subtle signs of superconductivity. It wasn’t zero resistance, but it’s the closest anyone’s got so far. “I think all the experimental results are consistent with superconductivity,” Zhao says. “But I cannot rule out other explanations.”

At the moment no superconductor will work above about 130 kelvin (-143°C). But if a material could carry current with no resistance at room temperature, no energy would be lost as heat, meaning faster, lower-power electronics. And electricity could be carried long distances with 100 per cent efficiency.

Nanotube bundles

Zhao and Wang studied the effects of magnetic fields on hollow fibres of carbon known as “multiwall carbon nanotubes”. Each nanotube is typically a millionth of a metre long, several billionths of a metre in diameter and with walls a few atoms thick. The nanotubes cling together in oblong bundles about a millimetre in length.

The researchers did not see zero resistance in their bundles. They think this is because the connections between the tiny tubes never become superconducting. But they did see more subtle signs of superconductivity within the tubes themselves.

For example, when the researchers put a magnetic field across a bundle at temperatures up to 400 kelvin (127°C), the bundle generated its own weak, opposing magnetic field. Such a reaction can be a sign of superconductivity.

And when the team cooled the bundles from even higher temperatures then turned the external field off, they stayed magnetised. A current running around within the tubes could generate this lingering field if there wasn’t any resistance to make it fade away.

While each effect could have a more prosaic explanation, they varied in similar ways as the temperature of the bundles changed. The correlation suggests superconductivity was responsible, Zhao and Wang argue in a paper to be published in Philosophical Magazine B.

Dominating effect

However, their argument doesn’t convince Paul Grant, a physicist with the Electric Power Research Institute in Palo Alto, California. “Generally, superconductivity is such a dominating effect that when it occurs it just shouts out at you,” Grant says. “It doesn’t appear in these indirect ways.”

Superconductivity theories do not forbid the phenomenon at very high temperatures, says Sasha Alexandrov, a theoretical physicist at Loughborough University in the UK.

A material becomes superconducting when its electrons pair up. Normally such negatively charged particles would repel each other, but in a positively charged crystal structure, vibrations called phonons help them get together. In carbon nanotubes, the frequency of these vibrations is very high, which, in theory at least, means superconductivity at higher temperatures.

“The results on the magnetic response are very intriguing, and favour the explanation they present,” Alexandrov says. “It’s certainly possible,” agrees David Caplin, head of the Centre for High Temperature Superconductivity at Imperial College, London.

To decide whether or not the nanotubes really are superconductors, you need to measure the resistance through a single tube, Alexandrov says. “To be convinced, I’d like to see zero resistance.”

When a type of neutrino called a “muon neutrino” blasts through an atomic nucleus it can either remain a neutrino or change into a muon, a heavier cousin of the electron. Using the theory known as the standard model, particle physicists can predict precisely how often the neutrino will remain a neutrino.

However, the prediction is not quite right, say physicists working with the NuTeV experiment at Fermilab, outside Chicago. They studied more than three million collisions between high-energy muon neutrinos and iron nuclei. The neutrinos stayed neutrinos about one per cent less often than predicted, says Sam Zeller, a team member from Northwestern University in Evanston, Illinois.

“One percent may not sound like a lot, but this sort of measurement is so precise that one percent is a big deal,” she says.

Extra weak force

The neutrinos interact with the quarks in a nucleus through the weak nuclear force, and the discrepancy might be evidence for a second weak force that interferes with the well-studied one.

The extra force would help explain other tantalizing oddities observed in the behavior of atoms, says Jens Erler, a theoretical physicist at the University of Pennsylvania in Philadelphia.

But the extra weak force might prove hard to reconcile with the results of other experiments at particle colliders, says Gordon Kane, a theorist at the University of Michigan in Ann Arbor. And it would not fit into the leading theories proposed to unify the four known forces, Kane says. “I’m very skeptical of this being a clue to new physics,” he says.

The neutrino result is not part of any “darling theory” concedes NuTeV team member Kevin McFarland of the University of Rochester in New York. However, theorists do not always anticipate important results, he says: “Sometimes it happens the other way around, sometimes the experiments lead the theory.”

The supermassive black hole lies in the heart of a galaxy 120 million light years away, where it spins in the middle of a gigantic disk of hot matter and energy. This “accretion disc” drags on the black hole, causing it to slow and lose energy, say researchers working with the orbiting XMM-Newton X-ray observatory.

The lost energy heats the inner edge of the accretion disk so that it produces telltale X-rays, report Jörn Wilms of University of TĂŒbingen, Germany, and colleagues in a forthcoming paper.

Black holes that consume accretion disks are thought to power astronomical phenomena ranging from quasars to gamma ray bursts, says Christopher Reynolds, a team member and astrophysicist at the University of Maryland in College Park. “This new powerful energy source might be relevant in a whole host of objects,” he says.

Uncertain model

The result is exciting, but it depends on uncertain mathematical models of accretion disks, says Chris Done, an astrophysicist at the University of Durham in the UK. “I don’t think they have shown that from the data their conclusions are absolutely necessary,” she says.

The X-rays in question come from iron in the accretion disk. Usually such X-rays all have nearly the same energy, but the X-rays from the black hole have a broader range of energies. That implies they are coming from the very inner edge of the disk, where matter swirls at a range of speeds, causing the energies of the X-rays to spread.

However, the disc also produces other X-rays and the mathematical models are needed to account for this background.

Angels’ wings

If the black hole is losing energy, it confirms a prediction made more than 25 years ago by astronomers Roger Blandford and Roman Znajek.

They suggested that an accretion disc could generate a magnetic field that would latch on to a spinning black hole as it twists space and time around it. This “friction” would slow the black hole down, heating the disc.

But the idea remains speculative because researchers still do not understand precisely how the disc produces a field, Done says: “When most astrophysicists start invoking magnetic fields, they might as well say ‘when angels flap their wings’.”