When a star passes close to a supermassive black hole, it enters a region where space is more severely curved, and hence the light emitted from it has a greater potential well to climb out of. The loss of energy results in a gravitational redshift, independent of and superimposed atop any doppler (velocity) redshifts we’d observe.(NICOLE R. FULLER / NSF)

General Relativity Rules: Einstein Victorious In Unprecedented Gravitational Redshift Test

A star orbiting past our galaxy’s supermassive black hole offered a chance to test relativity as never before.

The supermassive black hole at the center of the Milky Way is the most extreme astrophysical object located within a million light-years of Earth. With an estimated four million solar masses, it’s the largest black hole in our galaxy and second largest, behind Andromeda’s, in the entire Local Group. If your goal is to probe Einstein’s theory of General Relativity more stringently than ever before, the environment around this black hole is the best testing ground provided by nature.

Since 1995, a team of astronomers led by Andrea Ghez at UCLA has been studying the orbits of stars near the galactic center. As time has progressed, their observational tools and techniques have improved. In 2018, the closest orbiting star to our supermassive black hole, S0–2, made its closest approach, reaching 2.7% the speed of light. In a tremendous new result, Einstein’s theory has been confirmed as never before. Here’s how.