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Star's Black Hole Encounter Puts Einstein's Theory of Gravity To the Test (sciencemag.org)
An anonymous reader quotes a report from Science Magazine: For more than 20 years, a team of astronomers has tracked a single star whipping around the supermassive black hole at the center of our galaxy at up to 25 million kilometers per hour, or 3% of the speed of light. Now, the team says the close encounter has put Albert Einstein's theory of gravity to its most rigorous test yet for massive objects, with the light from the star stretched in a way not prescribed by Newtonian gravity. In a study announced today, the team says it has detected a distinctive indicator of Einstein's general theory of relativity called "gravitational redshift," in which the star's light loses energy because of the black hole's intense gravity. The star, called S2, is unremarkable apart from a highly elliptical orbit that takes it within 20 billion kilometers, or 17 light-hours, of the Milky Way's central black hole -- closer than any other known star. A team led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, has been tracking S2 since the 1990s, first with the European Southern Observatory's (ESO's) 3.6-meter New Technology Telescope in Chile's Atacama Desert and later with ESO's Very Large Telescope (VLT), made up of four 8-meter instruments. Ghez's team at UCLA also began to observe the star around the same time with the twin 10-meter Keck telescopes in Hawaii. In a paper published today in Astronomy & Astrophysics, Genzel's group reports seeing the combined action of the relativistic effects, with the black hole's gravity redshifting S2's radial velocity by 200 kilometers per second, a small fraction of its overall speed. The results match closely with the predictions of relativity and are inconsistent with Newtonian gravity.
Not long ago, it was impossible to see stars at this distance. Now adaptive optics have improved the resolution so much that they are able to track stars at the center of the galaxy. This is through all the intervening dust and closer stars obstructing the view. I have trouble getting a good image of Saturn due to atmospheric turbulence, and these guys are imaging the center of the galaxy. Well done.
That Newtonian gravity is imprecise is already well known, milions of people who use GPS, Glonass or Galileo already use the general relativity calculations in practice. It is far more interesting to know how the results compare to alternative theories of gravity, like for example Verlinde's: https://en.wikipedia.org/wiki/...
Figure 3 is showing the residual velocity, not the total velocity of the star. They have to subtract the red or blue shift caused by the star moving toward or away from us before they measure the red shift due to the light climbing out of the gravity well.