Star orbiting massive black hole lends support to Einstein’s theory

Star orbiting massive black hole lends support to Einstein’s theory

Scientists have tested Einstein's general relativity's predictions in weak gravitational fields, such as those on Earth and in the solar system, but the predictions of general relativity may crumble in stronger gravitational fields such as those of supermassive black holes.

An global team of astronomers co-led by UCLA professor Andrea Ghez published the results of the study in Science.

The outcomes provide a better check on general relativity and making it the most detailed study ever conducted on supermassive black holes and Einstein's theory of general relativity.

"We wondered how gravity behaves near a supermassive black hole, and whether Einstein's theory tells the whole story", Ghez said in a statement from her university. "We can absolutely rule out Newton's law of gravity".

The team led by Ghez could now not only gain the most accurate moving image of the star on the black hole in three dimensions, but also discovered with him the gravitational red shift, a characteristic feature of the General Theory of Relativity.

Einstein's theory foresees the wavelength of electromagnetic radiation including light lengthening as it escapes the pull of gravity exerted by a massive celestial body like a black hole.

The general theory of relativity says that what we perceive as gravity arises from the curvature of space and time. The researchers' analysis involved detecting an effect known as "gravitational redshift" in the light emitted by a star closely orbiting the supermassive black hole, as the star was at its closest point to the blackhole in its 16-year orbit.

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Ghez and colleagues tracked the complete orbit of the star in 3D using the Keck Observatory, Gemini Observatory and Subaru Telescope in Hawaii, and combined the data with other measurements made over the past 24 years. The star got as close as 120 astronomical units (AU) from the black hole - an AU is the average distance between Earth and the sun, about 93 million miles (150 million kilometers) - traveling as fast as 2.7% the speed of light.

Scientists are excited to again put relativity to the test by astrometrically measuring the precession of the orbit of SO-2 - that is, gradual rotation of the orbital plane predicted by general relativity.

The team's results are consistent with general relativity and substantially favor the theory over Newtonian gravity, which can not account for the observed redshift.

Ghez's research team was able to see the co-mingling of space and time near the supermassive black hole.

This research, which was completed in the Fall of previous year, published Thursday, July 25, 2019, in the journal Science. "Through their rigorous efforts, Ghez and her collaborators have produced a high-significance validation of Einstein's idea about strong gravity". "It's one of four fundamental forces and the one we have tested the least", she said. The team took spectrograph observations of all three events, studying the changes in the star's light as it zoomed around the black hole. Most of the stars Ghez studies have orbits of much longer than a human lifespan. The gravity this generates is huge, creating the most extreme environment in the Milky Way. "It's easy to be overconfident and there are many ways to misinterpret the data, many ways that small errors can accumulate into significant mistakes, which is why we did not rush our analysis".

A figure showing the challenges the Ghez team had in processing decades of image data and spectroscopy input to follow the star S0-2.

The UC team followed the star SO-2, which is far enough from the event horizon to still be visible.

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