Test of Einstein equivalence principle near the Galactic center
  supermassive black hole
  During its orbit around the four million solar mass black hole Sagittarius A*
the star S2 experiences significant changes in gravitational potential. We use
this change of potential to test one part of the Einstein equivalence
principle: the local position invariance (LPI). We study the dependency of
different atomic transitions on the gravitational potential to give an upper
limit on violations of the LPI. This is done by separately measuring the
redshift from hydrogen and helium absorption lines in the stellar spectrum
during its closest approach to the black hole. For this measurement we use
radial velocity data from 2015 to 2018 and combine it with the gravitational
potential at the position of S2, which is calculated from the precisely known
orbit of S2 around the black hole. This results in a limit on a violation of
the LPI of $|\beta_{He}-\beta_{H}| = (2.4 \pm 5.1) \cdot 10^{-2}$. The
variation in potential that we probe with this measurement is six magnitudes
larger than possible for measurements on Earth, and a factor ten larger than in
experiments using white dwarfs. We are therefore testing the LPI in a regime
where it has not been tested before.
