One interesting thing about black holes is that, well, we’ve never actually seen one. In fact, our only direct evidence that they exist came from the discovery of gravitational waves last year, in the form of two merging black holes.
But astronomers are now preparing to produce an image of a black hole, Sagittarius A* at the center of our Milky Way, thanks to an international collaboration called the Event Horizon Telescope (EHT). It will use nine radio telescopes around the world to create a virtual telescope with a much wider area, known as interferometry.
To make this work, though, requires a bit of trickery, and that’s where a team from the Massachusetts Institute of Technology (MIT) comes in. They’ve developed an algorithm (described in a paper) that uses other images of space to create a mosaic of the combined observational data. It’s called CHIRP (Continuous High-resolution Image Reconstruction using Patch priors), and the result is that it should let us directly image a black hole.
“Suppose you want a high-resolution video of a baseball,” Yoav Schechner, a professor of electrical engineering at Israel’s Technion who was not involved in the paper, told MIT News. “The nature of ballistic trajectory is prior knowledge about a ball's trajectory. In essence, the prior knowledge constrains the sought unknowns. Hence, the exact state of the ball in space-time can be well determined using sparsely captured data.”
One of the reasons we need such a large telescope is that black holes are very far away, and also very small. For example, Sagittarius A* is just 17 times the Sun’s diameter but 25,000 light-years away.
“[Taking a picture of the black hole in the center of the Milky Way galaxy is] equivalent to taking an image of a grapefruit on the Moon, but with a radio telescope,” said MIT graduate Katie Bouman, who led the algorithm’s development, to MIT News. “To image something this small means that we would need a telescope with a 10,000-kilometer diameter, which is not practical, because the diameter of the Earth is not even 13,000 kilometers.”
Using telescopes around the world will let astronomers in the EHT project achieve such a resolution. And the new CHIRP algorithm will be essential in piecing together all the observations to make this ambitious endeavor possible.
There’s no firm date in 2017 for when we can expect the first image yet, but there’s every reason to be excited for when it does get up and running.
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