Milky Way’s Central Black Hole Confirms General Relativity Yet Again

Credit: ESO (public domain)

You have to be pretty smart for your name to become synonymous with intelligence. So, it’s not exactly shocking that many of Albert Einstein’s theories have been supported by observable evidence. Chalk up another posthumous win for Einstein — a new study of the black hole at the center of our galaxy confirms the theory of general relativity (again). 

In 1915, Einstein published his theory of general relativity, which made numerous predictions about the nature of the universe. Relativity caused scientists to rethink a great many things, including the nature of Mercury’s orbit around the sun. A month after Einstein published his work, Karl Schwarzschild completed the first solutions of Einstein’s field equations to describe an orbital rotation now known as a Schwarzschild precession. Mercury has a mild precession due to the mass of the sun, and something orbiting a black hole should have a much more extreme version. Scientists now have good evidence that is the case. 

At the center of our galaxy lurks a supermassive black hole known as Sagittarius A* (pronounced Sagittarius A star). A star known as S2 orbits Sagittarius A* every 16 years, but the nature of its orbit is a real mind-bender. For decades, astronomers have used the elliptical orbit of S2, which is 15 times larger than the sun, to test various elements of general relativity. For example, confirming gravitational red shifts as predicted by the theory. It turns out that orbit doesn’t remain static — it rotates around the black hole in a rosette shape (think spirograph) indicative of a Schwarzschild precession. 

The team analyzed S2 using the SINFONI and NACO adaptive optics instruments on the European Southern Observatory’s Very Large Telescope (VLT). In 2017, they added the beam-combiner instrument known as GRAVITY to the mix, which helped refine the observations and show a clear Schwarzschild precession. The distinctive orbital shifts seen in S2 are a result of the incredible mass possessed by Sagittarius A*. Essentially, the black hole’s intense gravity warps space-time and causes S2’s orbit to shift. 

General relativity has been confirmed time and time again, so no one expected this to be the nail in its coffin. That said, general relativity has its limits. Problems like quantum gravity and dark energy appear to exist outside the model of general relativity. We’ll need new physics to fully explain the universe, but Einstein nailed many predictions more than a century ago.

Top image credit: ESO (public domain)

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