Einstein's theory of relativity has passed the most rigorous test to date, this time withstanding the super-strong gravitational field of a supermassive black hole.

This is the latest in a series of demonstrations of Einstein's 103-year-old theory of general relativity applied in the most extreme circumstances.

Previously, scientists have proven that Einstein's explanation of how the universe works stands in the face of highly dense neutron stars and mysterious particles called ghost neutrinos.

They also found an entire galaxy that bends space itself and, in 2015, discovered gravitational waves, ripples in the fabric of space-time.

Relativity Theory Can Withstand A Black Hole

An international team of scientists studying Sagittarius A*, the supermassive black hole at the center of the Milky Way, has found that, even in the vicinity of the strongest gravitational field in the galaxy, Einstein's relativity applies.

The team specifically examined S2, a star that circles Sagittarius A*, and measured its velocity and orbit as it came to its closest point to the black hole.

They found that S2 behaved exactly as Einstein predicted. At such a close distance to an object with a much stronger gravitational field, the star exhibited a phenomenon called gravitational redshifting.

The study, which is published in the Astronomy & Astrophysics journal, is the culmination of more than a quarter of a century of observation by a team of experts led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics at Garching, Germany.

Genzel says this is the second time the team observed S2 making its closest pass to Sagittarius A*. However, they were able to make observations at unprecedented resolutions with the development of new instruments.

"We have been preparing intensely for this event over several years, as we wanted to make the most of this unique opportunity to observe general relativistic effects," says Genzel.

What Is Gravitational Redshifting?

S2 is one of three stars orbiting the supermassive black hole at the center of the Milky Way, which is about 26,000 light-years from Earth.

In May, the researchers observed S2 passing dangerously close to Sagittarius A* at around 12 billion miles away from the black hole. This is roughly four times the distance from the Sun to Neptune.

At this point, the star is so close to the black hole that its proximity speeds it up to 15 million miles per hour, which is nearly 3 percent the speed of light.

According to Einstein, the black hole's gravitational field is so strong that it stretches out the light of the star to longer wavelengths, pulling it toward the red end of the electromagnetic spectrum. This phenomenon is called gravitational redshifting.

New Equipment Used To Study Black Hole

Observing the region of Sagittarius A* has proven to be challenging for experts. The center of the galaxy is clouded by a thick veil of dust, which makes observations of visible light almost impossible.

Using highly advanced instruments in the Very Large Telescope at the European Southern Observatory in Chile, Genzel's team were able to see through the dusty veil and take infrared and near-infrared measurements of S2.

The researchers used the SINFONI near-infrared spectrograph to determine the star's velocity in conjunction with the GRAVITY interferometer to map out S2's positions as it moved around the black hole.

Frank Eisenhauer, principal investigator for both instruments, says the team first made measurements of S2 two years ago.

"During the close passage, we could even detect the faint glow around the black hole on most of the images, which allowed us to precisely follow the star on its orbit, ultimately leading to the detection of the gravitational redshift in the spectrum of S2," he says.

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