The idea is to peer right at a black hole's thin edge, known as the event horizon, beyond which gravity is so strong that nothing crossing it can get back out.
The Event Horizon Telescope (EHT) is a virtual dish, nearly the size of Earth, and the amount of information it will get from its observations will surpass any previous data gathering: the EHT is predicted to generate 2 petabytes of data each night.
The method of combining several telescopes in this way is known as Very Long Baseline Interferometry (VLBI), which has never before been done on such a huge scale.
Scientists hope to take the first-ever images of the event horizon and try to determine the black hole’s mass and spin, as well as put Einstein’s general theory of relativity to one of the most rigorous tests yet.
"The Event Horizon Telescope will allow us to test Einstein's theory of gravity, which predicts a particular shape and size for the silhouette of known black holes," says Harvard university astronomer Avi Loeb. "A deviation from the expected image could signal new physics. We know that Einstein's theory breaks down at the singularity of a black hole because it does not incorporate quantum mechanics."
Astronomers do not expect this breakdown to result in a modification of the black hole image, Loeb adds, but it could happen, "we might be surprised," he says.
“These are the observations that will help us to sort through all the wild theories about black holes. And there are many wild theories,” says astronomy research professor Gopal Narayanan at the University of Massachusetts Amherst. “With data from this project, we will understand things about black holes that we have never understood before.”
A grapefruit on the moon
Narayanan adds that creating the huge virtual telescope has been a technological and logistical challenge. The EHT started taking data in 2006, and has been adding more and more telescopes to its network over the years.
Even now, when everything is set up for the first image of Sagittarius A*, there are no guarantees that observations will go smoothly; for once, each and every instrument will need extremely clear skies. That’s why collaborating telescopes have been able to make their observations only for two weeks every year.
The instruments joining together are the Large Millimeter Telescope (LMT), a project of UMass Amherst and Mexico’s Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE), and a number of telescopes in Hawaii, Arizona, at the South Pole, in Chile and in Spain.
For telescopes, size matters – and the further away something is, the bigger the telescope should be to image it.
Our backyard’s central black hole is 4 million times more massive than the Sun, but it is 26,000 light years away from us. So imaging it requires a resolution more than 1,000 times better than that of the Hubble Space Telescope – it is “like trying to image a grapefruit on the surface of the moon," says Narayanan.
Researchers operating the eight telescopes will account for the Earth’s rotation. Since every instrument will be aimed at the black hole, over the course of many hours their sampled curves, combined, will resemble the observational effect of one gigantic, Earth-sized telescope.
"The gas in the vicinity of the black hole at the centre of the Milky Way - as well as in the galaxy M87 - does not blur the silhouette of the black on the luminous 'wallpaper' of matter falling into it for a wavelength below 1 millimetre," says Loeb. "A telescope as big as the Earth can resolve the image of the black hole shadow at a millimetre wavelength."
The EHT will also study the physics of accretion - the process by which a black hole’s gravity pulls in nearby matter. It will try to shed light on the behaviour of large plasma jets launched from the central black holes of most galaxies.