GRAVITY Collaboration, a group of researchers at various world renowned institutes recently tested a part of the Einstein Equivalence Principle, the local position invariance(LPI) near a supermassive black hole. Their work focused on the dependency of various atomic transitions on gravitational potential so as to find upper limit on violations of the principle.
Einstein’s equivalence principle in the theory of general relativity implies that gravitational force experienced by an observer while standing on a huge body like Earth is same as the pseudo force experienced in an accelerated or non-inertial frame of reference. Although this is the best explanation of gravity right now, there are still many questions not answered. Hence, putting this principle to test is crucial as it could open up new observations and expand our present knowledge.
Equivalence principle of Einstein has three principles. The local position invariance mentions that the non-gravitational measurements must be independent of location in space time(the gravitational potential).
Galaxies have a supermassive black hole which is present at the center of the galaxy. For Milky Way, the mass of its supermassive black hole, Sagittarius A*, is 4 million times more than that of the sun. Hence it is the ideal place to experiment on the principles of general relativity as it creates the strongest gravitational field.
The star S2, a very bright star in Milky Way’s inner region makes a very close encounter with the galatic supermassive black hole at 16 light years. So the GRAVITY team decided to use this to test Einstein’s equivalence principle as it keeps moving in and out of the gravitational field of the black hole. When the star is closest to the black hole, there is a gravitational redshift in the star’s light. This is due to the intense gravity on the surface of the star which results in decreasing vibrations of light waves, thus they appear redder than usual from Earth.
Scientists used hydrogen and helium atoms to test the local position invariance principle. According to it, the gravitational redshift in a star is only dependent on the gravitational potential and not on any other factors.
The change in frequency of light from the atoms was measured as they moved through a varying potential. This was done with the help of the line-of-sight velocity of S2 using the spectral lines of both atoms. The change in the frequency did not violate the LPI. Violation of LPI means different vibration of light from helium and hydrogen atoms.
After this successful testing, scientists want to keep testing various aspects of the general relativity theory in different conditions and improve our understanding of the universe.