Summary of session C1: experimental gravitation

The fact that gravity is a metric theory follows from the Einstein equivalence principle. This principle consists of (i) the universality of free fall, (ii) the universality of the gravitational redshift and (iii) the local validity of Lorentz invariance. Many experiments searching for deviations from standard general relativity test the various aspects of the Einstein equivalence principle. Here we report on experiments covering the whole Einstein equivalence principle. Until now all experiments have been in agreement with the Einstein equivalence principle. As a consequence, gravity has to be described by a metric theory. Any metric theory of gravity leads to effects such as perihelion shift, deflection of light, gravitational redshift, gravitational time delay, Lense–Thirring effect, Schiff effect, etc. A particular theory of that sort is Einstein’s general relativity. For weak gravitational fields which are asymptotically flat any deviation from Einstein’s general relativity can be parametrized by a few constants, the PPN parameters. Many astrophysical observations and space experiments are devoted to a better measurement of the effects and, thus, of the PPN parameters. It is clear that gravity is best tested for intermediate ranges, that is, for distances between 1 m and several astronomical units. It is highly interesting to push forward our domain of experience and to strengthen the experimental foundation of gravity also beyond these scales. This point is underlined by the fact that many quantum gravity and unification-inspired theories suggest deviation from the standard laws of gravity at very small or very large scales. In this session summary we briefly outline the status and report on the talks presented in session C1 about experimental gravitation. PACS numbers: 04.20.−q, 04.80.Cc, 04.90.+e, 95.40.+s 1. The importance and basis of relativistic gravity The meaning and importance of relativity and gravity can be summarized in the statement that it is the physics of space and time. Since all physical phenomena have to happen within 0264-9381/08/114023+09$30.00 © 2008 IOP Publishing Ltd Printed in the UK 1 Class. Quantum Grav. 25 (2008) 114023 C Lämmerzahl space and time relativistic gravity is at the very basis of all physics. Space and time have to be measured and explored by clocks and moving objects. Correspondingly, at the end all tests of relativistic gravity are based on clocks and on the observation of the motion of particles. This is also mirrored by the basic principle underlying general relativity, the Einstein equivalence principle (EEP). This principle consists of three parts: the principle of the universality of free fall (UFF), also called the weak equivalence principle, the principle of the universality of the gravitational redshift (UGR), also called local position invariance, and the local validity of Lorentz invariance (LLI). The validity of the EEP implies that gravity has to be described by a pseudo-Riemannian metric. If one of these principles fails to hold, then there will be more gravitational field than just a spacetime metric. UFF states that two different pointlike neutral particles move along the same spacetime trajectory when released at the same spacetime point with the same initial velocity. This is a pure orbit comparison test. UGR states that the rates of two different clocks (which are not allowed to be based on the interaction with an external gravitational field like a pendulum) do not depend on the position of these clocks. Finally, LLI is based on the comparison of clocks having different orientations and velocities. Therefore, these merely orbit and clock comparison tests imply that gravity is metric. One particular metric theory is Einstein’s general relativity (GR). If one of these tests fails then GR will no longer hold. Any metric theory of gravity predicts effects which are not present in the Newtonian theory and which are related to the rates of clocks and trajectories. These effects are light bending, perihelion shift, gravitational redshift, a gravitational time delay and the Lense– Thirring effect. The Schiff effect, the precession of gyroscopes in a gravitomagnetic field is an effect of the motion of an extended particle which can be derived from the motion of point particles with restricted degrees of freedom. A further predicted effect is gravitational waves whose detection is also related to the motion of test particles, the mirrors and light rays. Einstein’s theory of relativity is then characterized by a certain magnitude of these effects. Accordingly, there are two possibilities for deviations from GR: (i) within the class of metrical theories but with different magnitudes of effects. Examples of that are all theories which are related to non-standard PPN parameters, see e.g. a list given in [1]. (ii) non-metric theories of gravity. In this case there are additional fields, in general tensor fields of any rank, related to gravitational phenomena. Examples of these are theories with torsion, with a non-metricity or with a Finslerian metric. In the following we first describe these tests in more detail and point to the corresponding talks held in the session C1. 2. Tests of the universality of free fall UFF applies to neutral point-like particles only. The corresponding tests are described in terms of the acceleration of these particles in the reference frame of the gravitating body: the Eötvös factor compares the normalized accelerations of two bodies η = a1−a2 1 2 (a1+a2) in the same gravitational field. In the frame of Newtons theory this can be expressed as η = μ1−μ2 1 2 (μ1+μ2) with μ = mg/mi where mg is the gravitational and mi the inertial mass, respectively. Though there are no point particles it is possible experimentally to manufacture macroscopic bodies such that their higher gravitational multipoles either are very small or very well under control. This is used in the various tests of the UFF. There are hints from quantum gravity inspired scenarios that the UFF might be violated below the 10−13 level [2, 3]. Also from cosmology with a dynamical vacuum energy (quintessence) one can derive a violation of the UFF at the 10−14 level [4]. The validity