Can we understand gravity?
Observation of the Solar System has always been the basis for our understanding of gravitation: The first major success of Newton’s gravitational law was to explain Kepler’s empirical rules for the motions of the planets. Einstein’s theory of general relativity was triggered by the tiny anomalous precession of Mercury, that could not be explained by Newton’s law. General relativity’s first independent test was the deflection of light from distant stars in the gravitational field of the Sun during a Solar eclipse. Hence for 300 years the Solar System has been our natural laboratory of gravitational physics. In the past decades attention has moved towards studying the gravity round distant stars leading for instance to the indirect discovery of gravitational waves in the binary pulsar system PSR 1913+16 and increasingly convincing evidence for a black hole event horizon in microquasars.
Nevertheless the Solar System might not yet have revealed all its gravitational secrets. NASA’s Pioneer 10 and 11 deep-space probes showed an unexplained constant offset of their tracking signals. An intense investigation, which took several years, did not manage to find an explanation of this anomaly. It cannot be ruled out that the anomaly is due to a new physical effect. From the data delivered by the Pioneer probes it is not possible to decide if the anomaly is caused by a deviation of the spacecraft from its nominal trajectory or by an increased blueshift of the radio signal from the spacecraft. Although both of these physical effects would be truly revolutionary, there are several reasons to remain sceptical about interpreting the Pioneer anomaly as new physics: Firstly the orbit of Neptune shows no perturbation whereas the Pioneer probes showed the anomaly already at the same distance from the Sun as the Neptune orbit. Secondly the observed anomaly is constant (within the precision of the data). In physical terms this would indicated a considerable violation of the weak equivalence principle ("all bodies fall equally fast"). Furthermore it would indicate an instability in the gravitational forces since the anomaly would act over arbitrarily large distances). Up to now there is no consistent theory for the laws of gravity that could account for these peculiarities.
Despite the serious doubts about the origin of the Pioneer anomaly some concrete investigations need to be carried out by ESA already at such an early stage of our knowledge. Most importantly the impact of the anomaly for other missions has to be assessed. In particular the performance of the LISA gravitational-wave observatory could be influenced if the Pioneer anomaly were caused by a universal blueshift of electromagnetic waves. The ACT will be assessing the consequences of a possible anomaly for the LISA mission in the coming month. On the other hand one would not want to miss the opportunity of an experimental test of the Pioneer anomaly if it would be possible onboard one of the upcoming deep space missions of ESA’s Cosmic vision 2015-2025 programme. To this end the ACT is currently investigating which requirements a satellite and its trajectory need to fulfil to enable a verification of the Pioneer anomaly. For first results, on the case of a Pluto orbiter mission, read more here.
Still controversial is also the possible influence of the cosmic expansion on the dynamics of the Solar System. Although the cosmic expansion does not seem to have a connection to the Pioneer anomaly – The cosmic expansion should lead to a weakening of the gravitational attraction and not to an enhancement! – it could nevertheless be measurable in the Solar System. An Ariadna study will investigate this issue in depth in the first half of the coming year.
For another area of gravitational physics in scales accessible to direct experiments it is commonly agreed that we have no idea how to describe the gravitational laws: the interaction of a quantum system with gravity. In this realm we might even expect strong modifications of the gravitational laws and not just the tiny ones we have indications for from the Pioneer anomaly. This realm has been associated with new technologies like the possible generation of gravitational waves for telecommunications. The current state of the field was reviewed by the University of Cologne in the framework of the Ariadna programme.
Obviously, there are still important gaps in our understanding of gravity even within our neighbourhood. The question is do these gaps hide surprises?