Accoding to Einstein’s 1915 theory of general relativity, what we perceive as the force of gravity arises from the curvature of space and time. The scientist proposed that objects such as the sun and the Earth change this geometry. Einstein’s theory is the best description of how gravity works. Now, in the most comprehensive test of general relativity near the monstrous black hole at the center of our galaxy, the Galactic Center group (GCG) reports that Einstein’s theory of general relativity holds up.
The GCG team is one of only two groups in the world to monitor a star known as S0-2 make a complete orbit in three dimensions around the supermassive black hole at the center of the Milky Way. The full orbit takes 16 years, and the black hole’s mass is about four million times that of the sun.
This is the most detailed study ever conducted into the supermassive black hole and Einstein’s theory of general relativity. Our measurements absolutely rule out Newton's law of gravity and are consistent with the theory of general relativity.
Using the Keck 10m Telescopes on Mauna Kea, we have been measuring the highly eccentric 16-year orbit of for more than 20 years, since 1995. This star reached its closest approach - within 120 AU of the SMBH - in May 2018, corresponding to about 1000 times the black hole's event horizon (or Schwarzschild radius), with a velocity reaching 2.7% of the speed of light.
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 S0-2 approaches near the SMBH, the light it emitts 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. During the pericenter passage in 2018, 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.
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 was an approximately 3% effect.
Because several different parameters of the orbit must be accurately measured, in order to accurately determine the relativistic signals, observations were done not just at the closest approach but also at "turning points" in the star's orbit.
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 have undergone three turning points: two for the velocity (max & min) and one for the positional data
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