RADIOACTIVE materials decay at a predictable rate—so predictable, in fact, that scientists widely use them to date artefacts and geological objects. That, at least, is the received wisdom, which Jere Jenkins and Ephraim Fischbach, from Purdue University in Indiana, think may need revising. In 2006 Dr Jenkins noticed that the decay rate of the radioactive isotope manganese-54 dipped 39 hours before a solar flare came crashing into Earth's protective magnetic field. Now it seems that the sun might affect other types of decay, too.

As the researchers report in Astroparticle Physics, the decay rate of chlorine-36 increases as Earth approaches the sun. The difference is tiny: the rate fluctuates by less than 1% between the aphelion and perihelion, the points on Earth's orbit when it is farthest and closest to the sun, respectively. But it is discernible and persistent. As-yet-unpublished data for manganese-54 suggest that isotope follows a similar pattern. If confirmed, the insight might, among other things form the basis of a system for forecasting dangerous cosmic storms.

Solar flares, in which charged particles are ejected from the sun, can damage satellites and ground-based electronic infrastructure. In 2005 an unseasonal solar storm knocked out a number of Global Positioning System (GPS) birds, some of them for good. It also forced airliners to be redirected from Arctic routes, where Earth's magnetic field provides least cover from the nefarious effects a hail of such particles can have on the people's, and machines', health. And that was a mere breeze compared with the solar storm of 1859, thought to have been many times more devastating on the basis of the disruption it caused to the nascent telegraph service. These days, another Carrington Event, as the 19th-century episode is known, risks crippling a planet increasingly reliant on all sorts of electronic gubbins.

A number of advance-warning systems, enabling countermeasures such as temporary shutdown of vulnerable electronics, are in the works. But reliable forecasts are scarce. This is because solar storms are not yet well understood. Paradoxically, Dr Jenkins and Dr Fischbach think this might change with the help of neutrinos, the ethereal particles which pervade the universe but rarely interact with anything—and themselves a cause of much head-scratching among physicists.

Neutrinos are a byproduct of the nuclear fusion which powers the sun. Earth's elliptical orbit means that the flux of solar neutrinos which stream through it varies during the year. The changes in chlorine-36 and manganese-54 decay rates observed by the Purdue team, including the dip prior to the flare in 2006, mirror the changes in neutrino flux detected by other experiments. Unlike their tiny radioactive sample, though, those existing neutrino detectors are vast (to shorten the odds that the elusive particles deign to react with at least one atom inside it) and often sit deep underground (to shield the detectors from other particles which leave neutrino-like traces; only neutrinos, thanks to their signature unwillingness to react, are able to penetrate ). As a result, any system based on such detectors would be hard to scale up.

If Dr Fischbach and Dr Jenkins are right about neutrinos affecting radioactive decay, it would herald a new era in neutrino physics, not just space-weather forecasting. That is still a prodigious if. For a start, like many things neutrino-related, the mechanism through which the particles might affect decay rates remains a mystery. On the rare occasions that they do interact, neutrinos do so via the weak nuclear force, which is also responsible for the sorts of radioactivity present in chlorine-36 and manganese-54. Physicists critical of the work point out that in the Purdue team's proposal the strength of the force, which can be calculated from the observed changes in decay rates, is much larger than established particle theory would have it.

Such discrepancies might be explained if a neutrino somehow amplifies the decay rates. In the conventional view, most neutrinos pass through matter without so much as a shudder. Those that do interact tend to do so only once; the likelihood of a single neutrino scattering off one atom and then another in short order is infinitesimal. However, rather controversially, Dr Fischbach thinks that the large number of neutrinos that seem not to be interacting may in fact be doing so, just that the effects of these interactions in stable matter are too small to see. In an unstable radioactive sample, he speculates, they might come to light, because decay rates are known to be extremely sensitive to the energy released in the process. As a result, if solar neutrinos transferred a mere millionth of their energy to a decaying nucleus, that might have a big effect on the rate at which it breaks up.

Whatever the mechanism, the correlation between radioactive decay rates and neutrino flux looks striking, and has been observed in a number of samples in different laboratories. Wary neutrino physicists warn that it could all yet prove to be an artefact of the way the experiments were conducted. That was the case in 2011, when their colleagues in Italy clocked neutrinos travelling faster than light, only to discover that the result, at odds with Einstein's cherished theory of relativity, was down to a loose cable.

Even if this time all cables were taut, many hurdles remain. Dr Fischbach admits that while whatever process generated the flare in 2006 also caused a dip in neutrino flux, and a corresponding drop in radioactive decay rates, other processes seem to have the opposite effect. For example, a storm in 2008 was preceded by a spike in manganese-54 decay rates.He suspects that what is loosely termed a "solar storm" may in fact be a number of distinct processes whose common feature is that they affect neutrino production in one way or another. That is a far cry from a reliable space-weather forecast. But it has not stopped the university from applying for a patent on a decay-based neutrino detector technology, just in case.

Source: economist.com