Researchers have devised a new and extreme test for general relativity. They are using the supermassive black hole at the center of the Milky Way and the stars that orbit it to see if Einstein’s theory of gravity is still valid under these extreme conditions.
The study, which is published in Physical Review Letters, describes how 19 years’ worth of observations of two stars, S0-2 and S0-38, was used to look for a deviation from the theory. The team supposed the existence of a fifth force to explain why we can’t include gravity in our particle physics models.
"This is really exciting," co-author Andrea Ghez, director of the UCLA Galactic Center Group, said in a statement. "It's taken us 20 years to get here, but now our work on studying stars at the center of our galaxy is opening up a new method of looking at how gravity works."
No deviation was found in the current data and general relativity continues to pass every test we have thrown its way. However, the test has not been used in the most extreme case yet. In summer 2018, star S0-2 will be at its closest point from Sagittarius A*, the Milky Way’s supermassive black hole.
"By watching the stars move over 20 years using very precise measurements taken from Keck Observatory data, you can see and put constraints on how gravity works," Ghez stated. "If gravitation is driven by something other than Einstein's theory of General Relativity, you'll see small variations in the orbital paths of the stars."
Nothing in science is unquestionable, and while relativity has an unbeaten record, it still deserves scrutiny. On top of that, there’s the fact that gravity doesn’t play well with the other three fundamental forces (electromagnetism, strong nuclear, and weak nuclear), which suggests that there’s a lot more we don’t know about the true nature of gravity.
"It's exciting that we can do this because we can ask a very fundamental question – how does gravity work?" continued Ghez. "Einstein's theory describes it beautifully well, but there's lots of evidence showing the theory has holes. The mere existence of supermassive black holes tells us that our current theories of how the universe works are inadequate to explain what a black hole is."
The test could tell us something new about gravity or once again confirm how well it is described by general relativity. Either way, we’ll have a better understanding of how the universe works.