What Determines the Nature of Gravity? A Phenomenological Approach

Abstract

The gravitational field can only be explored through the motion of test objects. To achieve this one first has to set up the correct equations of motion. Initially these equations are based on Newton's laws. Corresponding experiments that support Newton's laws are described. Furthermore, the basic characteristics of the motion of test objects in gravitational fields are described. This leads to the notion of Einstein's Equivalence Principle which has as consequence a metric theory of gravity. One particular metric theory is General Relativity based on Einstein's field equations with its particular predictions for effects like periastron advance, light deflection, etc. An overview over the experimental confirmation of General Relativity, in particular those presented at this workshop, is given. This workshop summary ends with open problems. We also describe some of the strategies for the experimental search for a quantum gravity theory.

Gravitational anomalies in the solar system?

“…As in the case of the PA and, to a lesser extent so far, the FA, also such an alleged anomaly motivated many researches to find viable explanations in terms of various either conventional or unconventional physical mechanisms. 159,206,[282][283][284][285][286][287][288][289][290][291][292][293][294][295][296] The various attempts 154,[279][280][281] to investigate it have been performed so far within the framework of the International Astronomical Union (IAU) 1976 System of Astronomical Constants, available on the Internet at http://www.iau.org/static/resolutions/ IAU1976_French.pdf, which, in turn, is reminiscent of pre-relativistic eras when, among other things, only relative distances (expressed in astronomical units) in the Solar system could be determined since absolute ones could not be estimated with high accuracy. More precisely, the IAU 1976 system prescribed to measure the masses in units of Solar masses, the durations in days D (made of 86400 seconds), and the distances in astronomical units of length.…”

Quantum gravity effects in geodesic motion and predictions of equivalence principle violation

“…where the above expression is linearized in the last step. Hence for two clocks A and B [3], with (β m ) A = (βm 2 A )/2, . .…”

“…An alternative manifestation of EP, especially convenient in low energy (Newtonian) domain, was suggested by Bondi [2], where the idea of active and passive masses of a particle was introduced and their inequality, (if observed experimentally), would signal EP violation. Most of the terrestrial experiments [3][4][5] showing validity of EP are based on the latter scenario. However, the Newtonian potential approach [2] is derived as a low energy limit of the basic geodesic motion so, quite obviously, any violation in EP in Newtonian physics has to be present as a deviation in the geodesic motion (which is more fundamental).…”

“…However, the Newtonian potential approach [2] is derived as a low energy limit of the basic geodesic motion so, quite obviously, any violation in EP in Newtonian physics has to be present as a deviation in the geodesic motion (which is more fundamental). In this paper we use this top-down approach and show that Quantum Gravity (QG) effects induce a deviation in the geodesic motion that can have low energy experimental consequences related to EP violation as in [3][4][5].…”

An Empirical Explanation of the Anomalous Increases in the Astronomical Unit and the Lunar Eccentricity

“…Such phenomena attracted the attention of various scientists dealing with them in different contexts [9,10,11,12,13,14,15,16,17,18,19,20,21]. Thus, several more or less sound attempts to find, or to rule out, possible explanations [2,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,1,41,42,43] for both the anomalies were proposed so far, both in terms of standard known gravitational physical phenomena and of long-range modified models of gravity.…”