Einstein’s theory of general relativity underpins our whole understanding of the large-scale universe. It also has limitations, and researchers have tested it time and time again over the last 100 years. The latest test builds on previous work using an incredible laboratory, a star orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way.
Star S0-2 orbits Sagittarius A* every 16 years, getting as close as 17 billion kilometers (11 billion miles) from it. The orbit is stable, but the intense gravitational field is still enough to produce effects such as gravitational redshift. The light of the star is slightly redder because photons have to escape the gravitational field.
S0-2's last close passage happened in 2018, and one year ago researchers published data on the gravitational redshift of the star. This new research, published in Science, combines that data with observations made since 1995. Their result is consistent with general relativity and it categorically excludes a Newtonian model of gravity.
"Einstein's right, at least for now," co-lead author Professor Andrea Ghez, from UCLA, said in a statement. "We can absolutely rule out Newton's law of gravity. Our observations are consistent with Einstein's theory of general relativity. However, his theory is definitely showing vulnerability. It cannot fully explain gravity inside a black hole, and at some point we will need to move beyond Einstein's theory to a more comprehensive theory of gravity that explains what a black hole is."
Tracking this star for such a long time has given researchers precious insights into how the gravitational dynamics around a black hole play out. During the crucial close passage of May 2018, Ghez’s team took measurements of the star every four nights.
"What's so special about S0-2 is we have its complete orbit in three dimensions," explained Ghez. "That's what gives us the entry ticket into the tests of general relativity. We asked how gravity behaves near a supermassive black hole and whether Einstein's theory is telling us the full story. Seeing stars go through their complete orbit provides the first opportunity to test fundamental physics using the motions of these stars."
S0-2 is not the only star to orbit close to Sagittarius A* and it's not the closest either. In 2012, Ghez and her team discovered S0-102, which orbits the black hole in 11.5 years. It will experience its closest pass at some point next year, moving at over 1 percent of the speed of light.