10 scientific facts that we have learned from the first photo of the black hole

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10 scientific facts that we have learned from the first photo of the black hole

The idea of ​​black holes dates back to 1783, when the Cambridge scientist John Michelle realized that a sufficiently massive object in a sufficiently small space can attract even light, preventing it from escaping. After more than a century, Karl Schwarzschild found the exact solution for Einstein's general theory of relativity, which predicted the same result: a black hole. Both Michell and Schwarzschild predicted a clear connection between the event horizon, or the radius of a region from which light cannot escape, and the mass of a black hole.

For 103 years after Schwarzschild's predictions, they could not verify it. And only on April 10, 2019, scientists uncovered the first ever photograph of the horizon of events. Einstein's theory worked again, as always.

Although we already knew quite a lot about black holes, even before the first snapshot of the event horizon appeared, it changed and clarified a lot. We had a lot of questions that now have answers.

By the way, here are 10 facts about black holes that everyone should know.

On April 10, 2019, the Event Horizon Telescope collaboration presented the first successful shot of the black hole event horizon. This black hole is in the Messier 87 galaxy: the largest and most massive galaxy in our local supercluster of galaxies. The angular diameter of the event horizon was 42 micro-arc-seconds. This means that in order to cover the whole sky, you need 23 quadrillion black holes of the same size.

At a distance of 55 million light years, the estimated mass of this black hole is 6.5 billion times the solar one. Physically, this corresponds to a size greater than the size of the Pluto orbit around the Sun. If there were no black hole, the light would need about a day to go through the diameter of the event horizon. And just because:

the event horizon telescope has enough resolution to see this black hole
black hole strongly radiates radio waves
very few radio wave emissions in the background to interfere with the signal

we were able to make this first shot. From which we have now learned ten deep lessons.

We learned what a black hole looks like. What's next?

This is a true black hole, as predicted by GR. If you have ever seen an article with a title like “the theorist boldly asserts that black holes do not exist” or “this new theory of gravity can turn Einstein around”, you can guess that physicists have no problem with inventing alternative theories. Despite the fact that GR has passed all the tests with which we subjected it, physicists have no shortage of extensions, replacements or possible alternatives.

And the observation of a black hole eliminates a huge number of them. Now we know that this is a black hole, not a wormhole. We know that the event horizon exists and that it is not a bare singularity. We know that the event horizon is not a hard surface, as the falling substance must produce an infrared signature. And all these observations correspond to the general theory of relativity.

However, this observation says nothing about dark matter, the most modified theories of gravity, quantum gravity, or what lies behind the horizon of events. These ideas are beyond the scope of EHT observations.

The gravitational dynamics of stars gives good estimates for the masses of a black hole; gas observation – no. Before the first black hole image, we had several different ways to measure the mass of black holes.

We could either use measurements of stars — such as the individual orbits of the stars near the black hole in our own galaxy or the absorption lines of stars in M87 — that gave us gravitational mass, or emissions from gas that moves around the central black hole.

Both for our galaxy and for M87, these two estimates were very different: gravitational estimates were 50–90% more than gas. For M87, gas measurements showed that the black hole had a mass of 3.5 billion suns, while gravitational measurements were closer to 6.2–6.6 billion. But the EHT results showed that the black hole has 6.5 billion solar masses, which means , gravitational dynamics is an excellent indicator of the mass of black holes, but the conclusions on gas shift towards lower values. This is a great opportunity to review our astrophysical assumptions about orbital gas.

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It must be a rotating black hole, and its axis of rotation points away from the Earth. Through observations of the event horizon, radio emission around it, a large-scale jet and extended radio emission measured by other observatories, the EHT determined that it was a Kerr black hole (rotating) and not Schwarzschild (not rotating).

Not a single simple feature of a black hole that we could study to determine this nature. Instead, we have to build models of the black hole and the matter outside it, and then develop them to understand what is happening. When you look for possible signals that may appear, you get the opportunity to limit them so that they are consistent with your results. This black hole should rotate, and the axis of rotation indicates approximately 17 degrees from Earth.

We were able to finally determine that around the black hole there is a substance corresponding to the accretion disks and streams. We already knew that the M87 had a jet – from optical observations – and that it also emitted in the radio wave and X-rays. This kind of radiation can not be obtained only from stars or photons: you need a substance, as well as electrons. Only by accelerating electrons in a magnetic field can we get the characteristic radio emission that we saw: synchrotron radiation.

And it also required an incredible amount of modeling work. Twisting various parameters of all possible models, you will learn that these observations not only require accretionary fluxes to explain radio results, but also necessarily predict non-radio-wave results, such as X-ray radiation. The most important observations were made not only by the EHT, but also by other observatories such as the Chandra X-ray telescope. Accretion fluxes must be heated, as evidenced by the M87 magnetic radiation spectrum, in accordance with relativistic accelerated electrons in a magnetic field.

The visible ring shows gravity and lensing around a central black hole; and again the GRT was tested. This ring in the radio band does not correspond to the event horizon itself and does not correspond to the ring of rotating particles. And this is also not the most stable circular orbit of a black hole. No, this ring arises from the sphere of gravitationally lensed photons, the paths of which are bent by the gravity of a black hole on the way to our eyes.

This light bends into a larger sphere than one would expect if gravity was not so strong. As he writes in Event Horizon Telescope Collaboration:

"We found out that more than 50% of the total flow in arc-seconds passes near the horizon and that this radiation is sharply suppressed when it enters this area by 10 times, which is direct evidence of the predicted shadow of a black hole."

Einstein's general theory of relativity once again proved to be true.

Black holes are dynamic phenomena, their radiation changes with time. With a mass of 6.5 billion suns, the light will take about a day to overcome the horizon of events of the black hole. This roughly sets the time frame within which we can expect to see changes and fluctuations of the radiation observed by the EHT.

Even observations that lasted several days allowed us to confirm that the structure of the emitted radiation changes with time, as predicted. The data for 2017 contain four nights of observations. Even looking at these four images you can visually see that the first two have similar features and the last two also, however, there are significant differences between the first and the last. In other words, the radiation properties around the black hole in M87 do change over time.

EHT in the future will reveal the physical origin of black hole flares. We saw, both in the X-ray and in the radio range, that a black hole in the center of our own Milky Way emits short-term flashes of radiation. Although the very first black hole image shown showed a supermassive object in M87, the black hole in our galaxy – Sagittarius A * – will be just as big, only it will change faster.

Compared with the mass of M87 – 6.5 billion solar masses – the mass of Sagittarius A * will be only 4 million solar masses: 0.06% of the first. This means that fluctuations will be observed not only during the day, but within even one minute. The features of a black hole will change quickly, and when a flash occurs, we will be able to reveal its nature.

How do flashes relate to the temperature and luminosity of the radio pattern we saw? Does a magnetic reconnection occur, as in the ejections of the coronal mass of our Sun? Is anything breaking in accretion streams? Sagittarius A * flashes daily, so we can associate all the necessary signals with these events. If our models and observations are as good as they turned out to be for the M87, we can determine what drives these events and maybe even find out what falls into a black hole, creating them.

Polarization data will appear, which will reveal whether black holes have their own magnetic field. Although we all were definitely happy to see the first shot of the black hole event horizon, it is important to understand that a completely unique picture will soon appear: the polarization of light emanating from a black hole. Due to the electromagnetic nature of light, its interaction with the magnetic field will imprint a special polarization signature on it, allowing us to reconstruct the magnetic field of a black hole, as well as how it changes over time.

We know that matter outside the event horizon, being essentially moving charged particles (like electrons), generates its own magnetic field. Models indicate that field lines can either remain in accretionary flows, or pass through the event horizon, forming a kind of “anchor” in a black hole. There is a connection between these magnetic fields, accretion and the growth of a black hole, as well as jets. Without these fields, matter in accretion flows could not lose its angular momentum and fall into the event horizon.

Polarization data, due to the power of polarimetric visualization, will tell us about it. We already have the data: it remains to perform a complete analysis.

An enhancement to the Event Horizon Telescope will show the presence of other black holes near the galactic centers. When the planet rotates around the Sun, this is not only due to the fact that the Sun has a gravitational effect on the planet. There is always an equal and opposite reaction: the planet has an effect on the sun. Similarly, when an object spins around a black hole, it also exerts a gravitational pressure on the black hole. In the presence of a whole set of masses near the centers of galaxies — and, in theory, a multitude of invisible yet black holes — the central black hole should literally tremble in its place, being pulled away by the Brownian motion of the surrounding bodies.

The difficulty with this measurement today is that you need a reference point to calibrate your position relative to the location of a black hole. The technique for such a measurement implies that you look at the calibrator, then at the source, again at the calibrator, again at the source, and so on. In this case, you need to move the eye very quickly. Unfortunately, the atmosphere is changing very rapidly, and in 1 second a lot can change, so you just do not have time to compare the two objects. In any case, not with modern technology.

But technologies in this area are developing incredibly fast. The tools that are used at the EHT are awaiting updates and may be able to achieve the required speed by the mid-2020s. This puzzle can be solved by the end of the next decade, and all thanks to the improvement of the toolkit.

Finally, Event Horizon Telescope will eventually see hundreds of black holes. To disassemble a black hole, it is necessary that the resolving power of the telescope array be better (that is, with high resolution) than the size of the object you are looking for. At present, the EHT can only make out three known black holes in the Universe with a sufficiently large diameter: Sagittarius A *, center M87, center of the galaxy NGC 1277.

But we can increase the power of the eye of the Event Horizon Telescope to the size of the Earth if we launch telescopes into orbit. In theory, this is already technically achievable. An increase in the number of telescopes increases the number and frequency of observations, and at the same time, the resolution.

By making the necessary improvements, instead of 2-3 galaxies, we can find hundreds of black holes or even more. The future of black hole photo albums seems bright.

The event horizon telescope project was expensive, but it paid off. Today we live in the era of astronomy of black holes and finally were able to observe them with our own eyes. This is just the beginning. Subscribe to our Telegraph channel to receive all the news from this invisible front.

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