Star S2 completes an orbit around the black hole at the Milky Way's center every 16 years, reaching speeds of 7650 kilometers per second. Its orbit is a stretched ellipse, and at its closest approach, the star comes within a distance 120 times the Earth-Sun separation. Astronomers have watched this dance for two decades, pinpointing the star's position with enough precision to spot deviations of just a few tens of kilometers.
These measurements act as a rigorous exam for gravity. Standard relativity predicts that space and time are the same in all directions, like a flat field with no hidden paths. But some extensions of the theory allow this flatness to break down in the strong field of a black hole: a preferred direction emerges, like an invisible groove that slightly alters the star's orbit. The team searched for such a groove by analyzing S2 data with a sophisticated statistical method, akin to testing thousands of possible motion scenarios.
The result: no groove found. Any possible distortion is less than one ten-thousandth of the characteristic scale of the black hole's gravitational field. Spacetime near the black hole remains as smooth as a well-stretched canvas, and the star continues its predictable dance.
At its closest approach to the black hole, star S2 moves at about 2.5% of the speed of light — roughly 27 million kilometers per hour.
In the novel 'The Three-Body Problem,' a civilization uses spacetime warping for travel, but our universe, judging by S2, doesn't allow such tricks.