In 2019, the scientific collaboration of the Event Horizon Telescope should have finished analyzing the 27 petabytes of data relating to the two supermassive black holes at the center of the Milky Way, and M87. Once the process is complete, we should get, for the first time, the image of the event horizon of a black hole. But how can the EHT achieve such a feat?
Black holes are fairly easy objects to detect, once you know what clues to look for. It might seem counter-intuitive because they don't emit light, but they have three surefire signatures that let us know they're there.
Black holes generate enormous gravity — a distortion/curvature of space — in a very small volume. If it is possible to observe the gravitational effects of a bulky and compact mass, then we can deduce the existence of a black hole and potentially measure its mass.
Black holes strongly affect the environment around them. Any material nearby will not only experience intense tidal forces, but also acceleration and heating responsible for emitting electromagnetic radiation within the accretion disk.
When this radiation is detected, it is possible to analyze it to reconstruct the source object, which is always a black hole. Finally, black holes can spiral and merge, causing them to emit detectable gravitational waves for a brief period.
The Event Horizon Telescope , however, aims to go further than any of these detection methods. Instead of taking steps to infer the properties of a black hole indirectly, it gets to the heart of the matter and considers imaging the event horizon of a black hole directly.
The method to do this is simple and straightforward, but it has not been technologically possible until very recently. The reason for this is a combination of two important factors that normally go together in astronomy:resolution and light gathering.
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Because black holes are very compact objects, extremely high resolution must be achieved. But because we are not looking for light itself, but the absence of light, we must collect large amounts of light with extreme precision to determine where the shadow of the event horizon really is.
Conventionally, a telescope with better resolution and a telescope with better light-gathering power should constitute the same telescope. The resolution of the telescope is defined among other things by the range of wavelengths of light that can be captured by its antenna. Therefore, larger telescopes have higher resolution. Similarly, the amount of light collected is determined by the area of the telescope. Knowing that all photons hitting the telescope will be collected, the larger the area of the telescope, the greater the amount of light captured.
The main limiting factor of technology has always been its resolving power. The size (radius) of a black hole appears to be proportional to its mass, but inversely proportional to its distance from us. To see the largest black hole from our perspective — Sagittarius A*, the one at the center of the Milky Way — requires a telescope the size of planet Earth.
Of course, the construction of such an instrument is not an option. But there is another solution:build a network of telescopes. With a large number of telescopes, it is possible to sum the light-gathering power of the individual telescopes. But the resolution, if properly calibrated, can see objects as fine as the spacing between the most distant telescopes.
In other words, light gathering is really limited by the size of the telescope. But the resolution, if the technique of long baseline interferometry (or its cousin, very long baseline interferometry) is used, can be significantly improved by using an array of telescopes with a large space between them.
This video shows how the Event Horizon Telescope works :
This translates to a resolution of just 15 microarcseconds (μas), which is the size of a fly that would appear to us here on Earth while located on the Moon. There are black holes in the Universe whose angular size is greater than 15 μas. More precisely, there are two:Sagittarius A*, at the center of the Milky Way, and the black hole located at the center of the M87 galaxy.
The black hole at the center of M87 is located about 50-60 million light-years away, but is over 6 billion solar masses, making it over 1000 times larger than the giant black hole in our galaxy.
The Event Horizon Telescope works by using this huge array of radio telescopes and observing these black holes simultaneously, allowing us to reconstruct a very high resolution image of anything we look at, as long as enough light is collected. P>
The key to the operation of the Event Horizon Telescope is therefore to gather enough light to see the shadow cast by the black hole's event horizon, while correctly imaging the light coming from around and behind it.
Black holes accelerate matter, and the acceleration of charged particles creates both magnetic fields and — if charged particles accelerate in the presence of magnetic fields — the emission of radiation.
The safest thing to do is to look in the radio part of the spectrum, which is the area with the least energy. All matter-accelerating black holes are believed to emit radio waves, which we have detected from supermassive black holes at the center of the Milky Way and M87. The difference is that, at these new higher resolutions, we should be able to spot the "vacuum" where the event horizon lies.
