“It’s mind-boggling to actually witness material orbiting a massive black hole at 30% of the speed of light,” marveled Oliver Pfuhl, a scientist at the Max Planck Institute for Extraterrestrial Physics.
“Funhouse-mirrored space-time” of Sgr*A
In 2018, Astronomers found something orbiting the innermost possible orbit of the supermassive black hole at the Milky Way’s center. Their measurements suggest that this stuff — perhaps blobs of plasma — is spinning not far from the innermost orbit allowed by the laws of physics, reports Joshua Sokol in Quanta. If so, he wrote, this affords astronomers their closest look yet at the funhouse-mirrored space-time that surrounds a black hole. And in time, additional observations will indicate whether those known laws of physics truly describe what’s going on at the edge of where space-time breaks down.
“A new, totally unknown environment”
“They have clearly seen something moving,” said Shep Doeleman, an astronomer at the Harvard-Smithsonian Center for Astrophysics. “What it is, is not exactly clear.”
The European Southern Observatory (ESO) exquisitely sensitive GRAVITY instrument has added further evidence to the long-standing assumption that a supermassive black hole lurks in the center of the Milky Way. New observations show clumps of gas swirling around at about 30% of the speed of light on a circular orbit just outside its event horizon — the first time material has been observed orbiting close to the point of no return, and the most detailed observations yet of material orbiting this close to a black hole.
For astrophysicists, this glimpse at plasma is interesting in and of itself. “We have a totally new environment, which is totally unknown,” said Nico Hamaus, a cosmologist at Ludwig Maximilian University in Munich, who also developed the early hot spot theory.
Cosmic Lab –Oddities of General Relativity revealed in their full glory
“The central black hole accretes matter in its vicinity, which settles in a disk. Differential rotation leads to friction in the disk and thereby creates hot and charged particles, a plasma. The motion of charges then creates a magnetic field which gets wound up by the disk’s rotation,” writes Hamaus in an email to The Daily Galaxy. “At some point,” he continues, “the magnetic field becomes so strong that it instantaneously accelerates particles in some region of the disk, a so-called “magnetic reconnection event” occurs. This eruption heats up those particles and appears as a bright “hot spot”, which continues to rotate with the accretion disk. Similar phenomena occur on the surface of our own sun, but unlike the latter, SgrA* provides a laboratory where the oddities of General Relativity reveal themselves in their full glory.”
Nothing, not even light, has ever returned from there
“Hot spots are used as guinea pigs for experiments in this cosmic lab,” Hamaus told The Daily Galaxy. “While orbiting the black hole,” he explains, “the light they emit is warped by the strongly curved spacetime and itself may circle the black hole multiple times until it reaches us, the observer. This makes the hot spot appear at multiple locations at the same time and distorts its blob-like shape into arcs. Eventually it goes down the “rabbit hole” towards the center, beyond the event horizon of SgrA*. Nothing, not even light, has ever returned from there…”
Image credit top of page: ESO, material orbiting a black hole