In the early 1970s, NASA made history with the launch of the Pioneer 10 and 11 space probes, which became the first spacecraft to obtain detailed images of Jupiter from a short distance away, in addition to achieving the milestone of passing through the Asteroid belt. Today, Pioneer 10 is more than 8 billion miles away from Earth, and its last message was received by NASA on January 23, 2003.

“NASA engineers report that Pioneer 10’s radioisotope power source has decayed,” the agency’s website reported, “and it may not have enough power to send additional transmissions to Earth. NASA’s Deep Space Network (DSN) did not detect a signal during a contact attempt on 7 February 2003. The previous three contacts, including the 23 January signal, were very faint, with no telemetry received. The last time a Pioneer 10 contact returned telemetry data was 27 April 2002.”

Amidst observations of the Pioneer 10 and 11 craft, at least one anomalous aspect has been noted by the agency: that each of the distant probes are moving away from the Sun at a rate that is slightly slower than calculations indicate they should be. This oddity led to an investigation by the Jet Propulsion Lab in Pasadena, California, which was able to determine “no conventional explanation” for what was causing this apparent discrepancy.

However, this anomalous force, while having an unknown cause, appears to be similar to another seemingly unrelated discrepancy of nature. Specifically, the force affecting each of the Pioneer probes seems roughly equivalent to anomalous measurements with gravimeters that occur only during solar eclipses.

Though this connection–if indeed there is one–may appear innocuous at first glance, its implications may even point to a flaw in Einstein’s theory of General Relativity.

The story of these “anomalous measurements” is an interesting one, which involves an award-winning French economist who, decades ago, often spent his spare time conducting physics experiments as a hobby.

In 1954, Maurice Allais, who eventually was awarded the Nobel prize for economics in 1988, had been engaged in an experiment with pendulums where, over the course of a 30-day period, he observed and recorded their movements. Incidentally, during the 30-day period that Allais chose to make his observations, a solar eclipse was also going to occur; hence, while observing the pendulum as the moon began to pass before the sun, Allais noticed that the movement of the pendulum appeared to quicken slightly, moving faster than it normally should. Several decades later, this odd little perturbation of physics remains unexplained, and is known today as the Allais effect.

Granted, for Allais’ discovery to qualify as being a legitimate phenomenon, it would have to have been repeatable… and although the strange effect has been observed under independent testing elsewhere over the years, the results of those tests occasionally vary, leading the more skeptically minded to doubt whether there is any real phenomenon afoot in the first place.

In 2004, The Economist reported on a study by Chris Duif with the Delft University of Technology in the Netherlands, who was convinced after further research of his own that the phenomenon known as the Allais effect is not only real, but that it could still qualify as being unexplained (although Duif, it should be mentioned, was also the first to draw comparisons between the Allais phenomenon and slower than calculated movements of the Pioneer spacecraft).

Among the factors that convinced Duif of the legitimacy of the phenomenon had been the fact that, in testing for potential falsehood of the observation, a number of factors could seemingly be ruled out as the underlying cause for an apparent change, when in fact no such gravitation deviations are actually taking place. These include the idea that large crowds gathering to view an eclipse within the vicinity of where it can be observed geographically may lead to minor “seismic disturbance”; Duit contested this based on experiments carried out in remote corners of China and Belgium that yielded the same results even in the absence of large crowds (of equal interest had been that, according to Duit’s research, one of the inconclusive tests of the Allais effect had, in fact, occurred in an unusually crowded population center).

Other explanations which Duit felt could confidently be ruled out included the proposed notion that cooler air present during an eclipse might cause minor variances in the way gravity could influence the movement of certain equipment used in tests seeking to replicate the Allais effect.

At present, there are two ways one might reconcile with the Allais effect, which rely on either the notion that the anomaly, though perhaps legitimate, can be explained in some way that remains in keeping with General Relativity, or perhaps as a result of something other than the direct influence of solar eclipses (the results of numerous tests have varied slightly, after all). Alternatively, there is always the possibility that there is something else wrong with this picture, which in the most extreme sense could have to do with problems associated with Einstein’s theories, rather than Allais’ observations.

Those who doubt General Relativity are, of course, in the distinct minority. Of this, quoting Allais himself, one might also say that, “Too many theorists have a tendency to ignore facts that contradict their convictions.” There may yet be something to learn from future study of this peculiar anomaly, which Allais first began to toy with in his spare time decades ago; it is one that is still very much a matter of debate, and hence, if studied properly, perhaps an opportunity for new future discoveries as well.