On April 10, 2019, the results of direct observation of a black hole obtained using the event horizon telescope were presented for the first time. Well, at least, the results are as close to direct observation as possible, in particular, to observe the shadow, cast by the so-called photon sphere of the black hole.
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The resulting image is extremely important. Because it is accurate enough, it has specific data that can be used to confirm or refute the theory.
In the picture you can see something more than it seems to the normal eye.
In astrophysics, the event horizon is the boundary beyond which events cannot affect the observer.
The image provides an accurate estimate of the apparent diameter of the photon sphere. This value, consistent with some estimates of the mass of the black hole M87, is a confirmation of Einstein’s theory of gravity in a regime (near the event horizon) which was previously inaccessible to us.
The object in the above photo is called the Lagoon Nebula. It is located about 5,000 light years from us. In fact, the picture is not detailed at all. The lagoon nebula is ridiculously huge. A tiny cloud thee is bigger than our entire solar system.
To see more details, you need a large lens antenna.
Now, back to the black hole M87. It is located at a distance of 55 million light years, more than 10,000 times further than the Lagoon Nebula.
Such a photograph requires the largest telescope that could be built. So, the event horizon telescope project turned the entire Earth into a telescope.
If you install two telescopes on opposite sides of the planet and make them function as one, the result is a lens the size of a planet.
They all work in unison, observing the same place, recording everything. Then incredibly powerful programs are needed to organize data. Accordingly, we need real professionals who write such programs.
In addition, it was not a direct shot of the M87 black hole. There are other stars and galaxies on the way. In a sense, one can “see through them.” A massive object has a so-called gravitational lens. Because of it, the light bends, creating distorted images.
The telescope can be directed to the lens and record distorted data. Then another very powerful program, written by another group of outstanding professionals, smooths the data, thus allowing you to see what is hidden behind the galaxy.
Probably, to get to the black hole, such actions had to be repeated more than once.
Here’s what you had to do to photograph the black hole of the M87 galaxy, in addition to a telescope the size of a planet.