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We have Begun the Third and Final Phase (July-Sep) of the

First Direct Test of Einstein's Theory of General Relativity Near a

Supermassive Black Hole

After two decades of monitoring the orbital motions of stars in the Galaxy's central gravitational well, the Galactic Center Group stands on the precipice of carrying out measurements that provide the unique opportunity to test the General Relativity description of gravity, the least tested of the four fundamental forces forces of nature, in an unexplored regime. Measurements of the short-period orbits around the Galactic Center's Supermassive Black Hole (SMBH) probe the structure of space-time on a mass scale that is 400,000 times larger than any other existing test.

This is now possible because we are measuring the highly eccentric 16-year orbit of a star, S0-2, which reached its closest approach--within 100 AU of the SMBH--in May 2018. This corresponds to about 1000 times the black hole's event horizon (or Schwarzschild radius). During the pericenter passage, the difference between S0-2's orbit described by Newtonian or Einsteinian gravity was significantly larger than the measurement uncertainty, offering a key new test of Einstein's Theory of Generay Relativity in an unexplored regime.

Einstein's General Relativity predicts that spacetime is curved around any massive object. The curvature caused by gravity is stronger nearer to a more compact object. As a closely orbiting star, such as S0-2, plunges near the SMBH, the light it emits must propagate through the curved space-time to reach us; it needs to "climb out" of the graviational potential well and will lose energy. So as the light travels through the curved space, it shows a gravitational redshift (or click here) that is independent of the Doppler effect. As S0-2 has reached its closest approach to the Galactic Center supermassive black hole during the summer of 2018, our group is now measuring the effect of the spacetime curvature around the SMBH on its orbit.

The measurement of the relativistic redshift of S0-2 relies on measuring its orbit accurately enough that the difference between a Keplerian model and a relativistic model will be distinguished at high significance. The gravitational redshift signal from S0-2 at its closest approach is expected to be about 1 part in 200, readily measurable. Because several different parameters of the orbit must be accurately measured, it is not enough to determine the redshift only at closest approach. By using sophisticated simulations, we have determined that the “turning points” in the star's orbit are the most sensitive times to measure the relativistic signal. In 2018, S0-2 will undergo three turning points: two for the velocity (max & min) and one for the positional data (minimum separation). So far, our group has successfully measured two of these turning points while the measurements related to the last one are currently on-going. Exciting results are coming...

Observing on Keck II telescope remotely from UCLA the closest approach of S0-2 around the Galactic Center Supermassive Black Hole on May 19th, 2018! Credit: Tuan Do

S0-2's orbit (exaggerated!) as it nears the SMBH. The curvature of spacetime causes the light emitted by S0-2 to redshift as it climbs out of the gravational potential well. Credit: Aurelien Hees

Credit: A. Hees and T. Do

For further reading:

• Hees et al. 2017 Testing General Relativity with Stellar Orbits around the Supermassive Black Hole in Our Galactic Center

• Ghez et al. 2008 The Galactic Center: A Laboratory for Fundamental Astrophysics and Galactic Nuclei

Einstein's Ultimate Test: Star S0-2 To Encounter Milky Way's Supermassive Black Hole