Einstein's General Relativity Illustrated by a Single Star

July 25, 2019

By observing a single star orbiting around the supermassive black hole at the center of our Milky Way Galaxy, a team of astronomers have tested Einstein’s one hundred-year-old theory of General Relativity in an unprecedented new regime. Unlike Sir Isaac Newton’s law of gravity, which states that matter pulls on other matter across empty space, General Relativity theory proposes that massive objects like supermassive black holes distort both space and time, which in turn affects the motions of objects around them. The team’s result: “Einstein’s right, at least for now,” said Andrea Ghez at the University of California Los Angeles (UCLA), a co-lead author of the research.

Gemini Observatory played an integral part in this research using the Gemini North telescope on Maunakea in Hawai‘i with the Near Infrared Integral Field Spectrometer (NIFS). Using NIFS the team acquired critical spectroscopic observations of the star, known as S0-2, as it orbited the supermassive black hole Sagittarius A* (pronounced A star), which has a mass four million times that of our Sun.

"One important aspect of the Gemini data obtained is that it provided critical information on the speed of S0-2 during its close approach to the black hole,” said Tuan Do, Associate Research Scientist at UCLA and lead author of the paper published in the July 25 report of the journal Science.

S0-2 moves around the black hole at a blistering 16 million miles per hour at its closest approach. Even at that speed, it takes the star 16 years to complete one orbit around Sagittarius A*. According to General Relativity theory, as the star makes its closest approach, it not only must travel faster but light leaving the star have to work extra hard to escape the black hole’s powerful gravitational field.

“Making a measurement of such fundamental importance has required years of patient observing, enabled by state-of-the-art technology,” said Richard Green, director of the National Science Foundation’s division of astronomical sciences. For more than two decades, the division has supported Ghez, along with several of the technical elements critical to the research team's discovery. “Through their rigorous effort to define in detail the three-dimensional orbit of that star around the supermassive black hole at the center of our galaxy, Ghez and her collaborators have produced a high significance validation of Einstein’s idea about strong gravity.”

“We’re learning how gravity works,” Ghez said. “It’s one of four fundamental forces and the one we have tested the least. 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.”

The observations show how well S0-2’s motion follows the predictions of General Relativity, even relatively close to the event horizon of a black hole — the boundary around a black hole beyond which no light or other radiation can escape.

Astronomers have tracked S0-2 for more than two decades as it moves along its 16-year orbital path, paying special attention to when the star is traveling its fastest, and the points at which it is closest and furthest from Sagittarius A*. Detailed observations of S0-2 carried out at multiple observatories on Hawai‘i’s Maunakea show how a truly massive object affects the motion and the fingerprints of atoms and molecules of an orbiting star. The star’s motion reached several percent of the speed of light, the universal speed limit imposed by Einstein’s theory.

"Keck, Gemini, and Subaru all contributed to the measurements, and it's very special to us that these observatories are on Maunakea," said Do. "The velocity of the star was changing quickly every night! So having all three observatories participate was essential."

In addition to providing a greater quantity of data during this highly time-sensitive period, using multiple telescopes allowed for a careful cross-checking of results. "The instruments on the various telescopes are quite different, so comparing their measurements and seeing that they agree gives us great confidence that the results are accurate," explained Do.