Einstein is a renowned physicist best known for his theory of relativity and E-MC2, but even though his reputation largely precedes him, some researchers have taken up the task of testing his theories. The GRAVITY Collaboration, a team of researchers at several renowned institutes, has recently tested part of the Einstein Equivalence Principle, namely the local position invariance or LPI, near the galactic center supermassive black hole. Their study investigated the dependency of different atomic transitions on the gravitational potential in order to give an upper limit on LPI violations.
The equivalence principle states that the gravitational force experienced in any small region of space-time is the same as the pseudo-force experienced by an observer in an accelerated frame of reference. Testing this principle is of key importance, as it could lead to interesting observations and broaden our current understanding of gravity.
"Einstein's equivalence principle consists of three main principles," Habibi explained. "LPI, states that non-gravitational measurements should be independent of the location in space-time where they are carried out. The main part of our study focuses on testing the LPI principle."
Star S2, one of the brightest stars in the Milky Way's innermost region, has its closest encounter with the galactic center supermassive black hole at a distance of 16.3 light hours. S2 moves in and out of the black hole's gravitational field, hence the GRAVITY collaboration team decided to use it to test part of Einstein's equivalence principle.
To test Einstein's LPI principle, the researchers used two different types of atoms in S2's stellar atmosphere: hydrogen and helium atoms. The LPI principle states that the gravitational redshift is seen in a star that is flying in and out of a strong gravitational field only depends on the gravitational potential and does not rely on other parameters, such as the internal structure of the atom.
"The past year was exceptionally successful for the GRAVITY collaboration," Widmann said. "For the first time, we observed relativistic effects in the orbit of a star around a supermassive black hole and used this star to test the Equivalence Principle. We also observed material orbiting very close to the black hole, another observation which would have been impossible without GRAVITY. However, this is more of a start than an end for us."
With the ideal season for galactic center observation just around the corner, the researchers at GRAVITY collaboration will continue to point their telescopes to S2 and the galactic center supermassive black hole. In their future observations, the researchers also hope that they will see more flare activity around the black hole, as this would enable further studies aimed at broadening their understanding of the Milky Way's galactic center black hole and black holes in general.
