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VLT optics provide key to Relativity breakthrough

26 Jul 2018

Adaptive optics and interferometry instruments help confirm effects of space-time around supermassive black hole at center of Milky Way.

An effect predicted by Einstein’s Theory of General Relativity has for the first time been observed around a supermassive black hole, thanks in large part to cutting-edge optics and photonics technology.

Astronomers using the Very Large Telescope (VLT) facility at the European Southern Observatory (ESO) in Chile used adaptive optics and interferometric techniques to follow a star (known as “S2”) orbiting close to the center of our own Milky Way galaxy. The latest results are the culmination of some 26 years of observations, using increasingly advanced techniques.

Thanks to the sharpness of the images collected and precision instrumentation able to monitor subtle wavelength changes, the team was able to confirm the gravitational effects of the supermassive black hole on light from the orbiting star.

Odele Straub, a member of the multi-national team at the Paris Observatory, said during an ESO press conference to announce the finding that understanding gravity was the key to understanding the universe – and that the Milky Way’s galactic center was effectively providing a laboratory to test Einstein’s predictions.

“When the star is close to the supermassive black hole, the light [emitted] has to ‘fight’ against the gravity of both the star and the black hole,” she explained, adding that because the speed of that starlight cannot change, its wavelength is forced to stretch and undergo a red-shift.

Because it is so close to the galactic center – it is orbiting the supermassive black hole every 16 years - the star in question is itself moving at colossal speeds, up to around 3 per cent of the speed of light. But just to observe S2’s precise position and the red shift induced by space-time has required the development of cutting-edge photonics technology.

Explaining how that technology works, Françoise Delplancke – who heads up ESO’s VLT Interferometer team – likened the required accuracy and precision to that of trying to watch a football match taking place on the Moon from Earth, and being able to measure the ball’s position to within 6 centimeters.

Key instrumentation
Among the instruments that have provided that level of accuracy and precision are GRAVITY, which combines infrared light collected by four 8 meter-diameter telescopes at the VLT site. That light is then transmitted along four single-mode optical fibers and recombined to deliver ultra-precise positional data.

Another key piece of equipment is the SINFONI spectrograph, which is used in combination with adaptive optics to reveal the slight gravitational red-shift of the orbiting star’s light.

Team leader Reinhard Genzel from the Max-Planck Institute for Extraterrestrial Physics said: “This is the second time that we have observed the close passage of S2 around the black hole in our galactic center.

“But this time, because of much improved instrumentation, we were able to observe the star with unprecedented resolution. 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.”

Genzel’s colleague Frank Eisenhauer, principal investigator on the GRAVITY and SINFONI instruments, added: “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 red-shift in the spectrum of S2.”

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