You can't see a black hole. At least not yet. They form after the collapse of massive stars, and their gravitational waves have been sensed by observatories in Louisiana and Washington state. But astronomers are anxious to know what they look like.
Researchers from the University of Arizona are part of the international team hoping to take the first image of the black hole that's in the center of the Milky Way galaxy and another black hole 53 million light years away, in the M87 galaxy.
What scientists are aiming at is called the "event horizon," the edge of a black hole. The aptly named Event Horizon Telescope — a group of 11 arrays at nine locations around the globe that operate as one Earth-sized telescope — may produce that first image.
"What we really expect to see is practically a shadow, a silhouette of the black hole that is cast around the emission of the hot plasma that is going into the black hole," said UA astronomy professor Dimitrios Psaltis, the lead scientist for the telescope project. The university's submillimeter telescope on Mount Graham east of Tucson is one of the instruments aimed at the black holes.
All of the telescopes point at the same location at the same time. The data from all are combined by supercomputers to construct the images, he said.
"In order for us to be able to combine all of the telescopes, as if it was a single telescope the size of the Earth, then not only do we have to do the observations simultaneously but we need to be able to time-tag, to tell exactly the time that each and every one of the waves of light come on every telescope at extremely high accuracy," Psaltis said.
The full Event Horizon Telescope experiment array made its first observations in April 2017.
One key telescope is at the South Pole, in a dry polar desert where there's no water in the atmosphere to absorb the light the astronomers are trying to see. Outfitting that telescope to look at the black hole greatly expanded the experiment's capabilities, said Dan Marrone, another UA astronomer who worked on those upgrades at the South Pole telescope.
"The Event Horizon Telescope gets better the bigger you make it, the further apart the antennas are, and the more antennas there are. And if you put a telescope at the very bottom of the Earth, obviously that's making the telescope just about as big as you can make it. And so a few years ago I started working on building an instrument to add on to the South Pole telescope so that we could include it in the Event Horizon Telescope array," Marrone said.
The downside of the South Pole telescope is that it took until December for the two crates of discs containing 800 terabytes of data from April to be flown back to the United States. Marrone said it would have taken 15 years to transmit that much data via the internet.
All of the data from the various telescopes was shipped to the supercomputer sites due to the volume.
Psaltis said the team has an idea of what the telescopes observed by creating simulations, but there is scientific value in surprises.
"Physics thrives when we find things that we do not understand, and physics thrives when we have experiments that contradict our expectations. If we find something in that silhouette of the black hole that is against our expectations then that I think is the most fascinating promise of that experiment" — Dimitrios Psaltis
UA astronomer Feryal Ozel explained how a discovery could shake up a theory that's been in place for over a century.
"While we love Einstein's theory of general relativity, we know that it has to break down at some point because it's incompatible with how we are describing the microscopic world using quantum mechanics," she said.
Psaltis said the team hopes to release the first images this fall.
Meanwhile, the researchers will head back to the telescopes for more observing this April. The observing happens for a week, once a year.