Unstable Physical World

A very interesting paper just appeared on the arxiv.org preprint server. It presents the work of two different groups measuring the radiactive decay rates of two specific isotopes, Silicon-32 and Radium-226, who found some strange and intriguing results indeed.

Scientists believe we live in a universe where physical "laws" apply the same everywhere. It shouldn't matter whether we are on the earth, the moon, or the depths of outer space, the equations and theories should be the same. Radioactive decay is governed by the so-called "weak force," which is in principle a nuclear-scale force that shouldn't really care about the anything beyond the nucleus itself. So it is natural to expect that Silicon and Radium should decay radioactively at the same rate wherever we find it.

But that's not what was found by Jenkins et al. in their analysis. Taking data from two different laboratories, at Brookhaven National Laboratories in New York State and another in Germany, the authors found the decay rate goes up and down with an annual cycle. See this blog entry for a graph. The variations are less than a tenth of a percent over the course of a year for both radioisotopes.

The authors speculate that the effect is dependent on the distance between the sun and earth, which also has an annual periodic modulation as the earth moves in its orbit between perihelion and aphelion. Of course, this would violate our precepts about the physical laws governing radioactive decay. They further speculate that this may be due to a changes in the amount of solar neutrinos that reach the earth, which might somehow modulate the nuclear decay rates. Or, perhaps it is a variation in the "fine structure constant" with distance from the sun (the fine structure constant dictates the strength of the weak force).

However, if you look closely at the graph, you will see that the radioactive decay rates and the distance variations do not match up quite that well. In fact, the measurements lag the distance template by a few months. I can't imagine what force law could describe such an oddity, but it is definitely not a simple distance relationship.

An experimenter needs to be worried about subtle biases. Both of the experiments were done in the northern hemisphere. Perhaps there is some kind of earth-bound seasonal effect? For example, there could be seasonal temperature variations that affect the sensors. Or, it could be something even more subtle, like annual changes in the cosmic ray flux which affect detector dead-time. An interesting test would be to use a southern hemisphere lab where any seasonal biases would be reversed. There have been some other spectacular results which have been later retracted due to failure to account for equipment (mal)function. A famous example of this was the discovery of a 2000 Hz optical pulsar in Supernova 1987a, which later turned out to be electrical interference from another piece of equipment. These people are not dumb, it's just that the physical world can be more complicated than the ways we can control.

It's worth noting that this paper has apparently not yet been refereed. It's quite possible that during the refereeing process, a lot of these points will be addressed, or the conclusions of the paper might change. Until then, it's still a quite fascinating result!