Test of the Gravitational redshift with RadioAstron
Quantum mechanics (QM) and general relativity (GR) are the two pillars of our understanding of the nature of the universe from the subatomic world to cosmology. However they are incompatible. Large efforts are being made to find a unifying theory. From the experimental point of view it is of prime importance to test whether predictions can be confirmed or whether deviations from predictions can be found that would perhaps, as in many cases before in the history of science, lead to new insights and perhaps to a more fundamental theoretical understanding of the physical world. We want to contribute to such tests. QM has been tested with high precision. For instance the fine structure constant was determined with a relative standard error of 3.7x10^-10 (Hanneke et al. 2008). In contrast, GR has been tested with orders less accuracy. In 1916 Einstein suggested three tests of GR, later called the three classical tests: the perihelion precession of Mercury, the bending of light and the gravitational redshift (Einstein 1916). Of these the latter is most accurately measured. The gravitational redshift was measured with NASA’s Gravity Probe A (GP-A) mission with a relative 1-σ upper limit of 1.4x10-4 (Vessot et al. 1980).

Although the gravitational redshift test is one of the three classical tests of GR, it is actually a more fundamental test, namely a test of the Einstein Equivalence Principle (EEP), which is the foundation of metric gravitation theories including GR. A test of EEP is in effect a test of the curvature of spacetime in the gravitational field of a massive body (Will, 2014). According to EEP an electromagnetic wave propagating in a region of space where the gravitational potential is not constant, experiences a gravitational frequency shift Δf_grav, proportional to the gravitational potential difference ΔU between the measurement points and the frequency f of the wave. Any violation of Eq. in an experiment with two identical frequency standards can be parameterized by a violation parameter ε. We want to extend our previous engagement in GP-B to a direct test of spacetime with space-VLBI using RadioAstron. Our first results indicate that we measured the gravitational redshift with an accuracy of about 3%.