Snapping Black Holes, This is How the EHT Supertelescope Works


Capable of seeing golf balls on the moon, the Event Horizon Telescope (EHT) is a network of radio dishes designed to detect the beam of light as matter disappears into the mouth of a black hole. The EHT is one of a number of extraordinary astronomical endeavors that in recent years have helped broaden our view of the universe.

On Thursday (13/5), scientists released the first images of the supermassive black hole at the center of our Milky Way galaxy. This giant galaxy is known as Sagittarius A*. So what is EHT and how does it work?

What is EHT?

The EHT is a unique worldwide network of antennas that together form a virtual telescope as wide as the Earth itself of about 10,000 kilometers (6,200 miles). The radio dish network was trained towards our galaxy, the Milky Way, and launched in 2015 involving 80 different astronomical institutions.

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In 2019, EHT revealed the first images of a black hole named M87* in a galaxy far from our own.

How can he see black holes?

On Thursday, an international team of astronomers gave us a glimpse of the supermassive black hole at the center of the Milky Way. Dubbed Sagittarius A*, the gravitational and light-sucking monster some 26,000 light-years from Earth has the same mass as four million Suns.

Observing a black hole is, by definition, impossible, because no light can escape from it. However EHT avoids this problem.

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The EHT captures the flashes of light that are produced when matter–planets, debris, anything that comes too close–is sucked into the black hole’s outer boundary called the event horizon. “We were able to detect the silhouette of a black hole against a glowing background of gas and dust,” Frederic Geth of the Franco-German Institute of Milimetric Radio-astronomy told AFP.

British cosmologist Stephen Hawking once compared the event horizon to crossing Niagara Falls with a canoe. If you’re at the top of a waterfall, it’s still possible to escape if you paddle hard enough. Once you cross the line, however, there is no turning back.

How to find out Sagittarius A*?

The cloud of matter swirling around the black hole is only visible using a very precise radio frequency band called millimeter wave and using only a radiotelescope — like a TV satellite dish but much larger. It needs to be very large to detect weak radio signals emitted by objects at great distances from Earth.

However, none of the radiotelescopes with current technology have a sufficiently high resolution. So astronomers use interferometry, which connects a pair of trained radio antennas on the same object in the sky to create a virtual telescope called an interferometer. It can see fine details, such as the camera zoom lens.

The EHT project goes even further, using radiotelescopes at eight observatories around the world–from America to Europe, Greenland to Antarctica–to create new, much more powerful telescopes. This technique is known as very long baseline interferometry (VLBI).

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As the Earth rotates, different telescopes pick up slightly different waves of light emitted by the matter around the black hole. These patterns can eventually be combined to form a more complete picture. The signal received at each antenna must be matched wave by wave, even if the dishes are half a world apart. So each location is equipped with an atomic clock.

Why is that important?

EHT’s success in detecting M87* and now Sagittarius A* provides double evidence of supermassive black holes. This is a huge leap forward in consolidating concepts about the way the cosmos is structured.

Einstein’s theory of general relativity has so far been unable to explain what happens in black holes on an infinitely small scale. A black hole is the “most extreme, chaotic and turbulent” environment ever, German astrophysicist Heino Falcke told AFP. But thanks to EHT, this aspect of fundamental theory can now be tested. (OL-14)


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