The idea of black holes dates back to 1783, when Cambridge scientist John Michell realized that a fairly massive object in a small enough space could attract even light without letting it escape. More than a century later, Karl Schwarzschild found an exact solution to 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 the region from which light cannot escape, and the mass of the black hole.
For 103 years after Schwarzschild's prediction, it could not be verified. And only on April 10, 2019, scientists uncovered the first ever photograph of the event horizon. Einstein's theory worked again, as it always did.
Although we already knew quite a lot about black holes, even before the first snapshot of the event horizon, it changed and clarified a lot. We had a lot of questions that now have answers.
On April 10, 2019, the Event Horizon Telescope collaboration presented the first successful snapshot of the black hole event horizon. This black hole is located in Messier 87: 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 it takes 23 quadrillion black holes of the same size to cover the entire sky.
At 55 million light-years away, the black hole's estimated mass is 6.5 billion times that of the Sun. Physically, this corresponds to a size greater than the size of Pluto's orbit around the Sun. If there were no black hole, it would take light about a day to pass through the diameter of the event horizon. And only because:
- the event horizon telescope has enough resolution to see this black hole
- black hole emits radio waves strongly
- very little radio waves in the background to interfere with the signal
we were able to get this first shot. From which we have now learned ten profound lessons.
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We learned what a black hole looks like. What's next?
This is really a black hole, as predicted by general relativity. If you've ever seen an article titled “theorist boldly claims that black holes don't exist” or “this new theory of gravity could turn Einstein around,” you guess physicists have no problem coming up with alternative theories. Even though general relativity has passed all the tests we put it to, physicists have no shortage of extensions, replacements, or possible alternatives.
And observing a black hole rules out a huge number of them. We now know that this is a black hole, not a wormhole. We know that the event horizon exists and that this is not a naked singularity. We know that the event horizon is not a solid surface, since the falling matter must give off an infrared signature. And all these observations are consistent with general relativity.
However, this observation says nothing about dark matter, the most modified theories of gravity, quantum gravity, or what lies behind the event horizon. These ideas are beyond the scope of the EHT's observations.
The gravitational dynamics of stars gives good estimates for the masses of a black hole; gas observation - no. Before the first image of a black hole, we had several different ways to measure the masses of black holes.
We could either use measurements of stars - like the individual orbits of stars near a black hole in our own galaxy, or absorption lines of stars in M87 - that gave us gravitational mass, or emissions from gas that moves around the central black hole.
For both our galaxy and M87, these two estimates were very different: gravitational estimates were 50-90% higher than gaseous ones. For M87, gas measurements showed that the black hole had 3.5 billion suns, and 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 black hole masses, but the gas conclusions are shifting towards lower values. This is a great opportunity to revisit our astrophysical assumptions about orbital gas.
It should be a spinning black hole, and its axis of rotation points away from the Earth. Through observations of the event horizon, radio emissions around it, a large-scale jet, and extended radio emissions measured by other observatories, the EHT has determined that it is a Kerr black hole (rotating), not a Schwarzschild black hole (not rotating).
There is 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 itself and the matter outside it, and then develop them to understand what is happening. When you look for possible signals that might emerge, 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 points from the Earth at about 17 degrees.
We were finally able to determine that there is material around the black hole, corresponding to accretion disks and streams. We already knew that M87 had a jet - from optical observations - and that it also emitted in the radio and X-ray ranges. This kind of radiation cannot be obtained only from stars or photons: you need matter, 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 took an incredible amount of modeling work. By tweaking all the possible parameters of all possible models, you will learn that these observations not only require accretion streams to explain radio results, but also necessarily predict non-radio wave results - like X-rays. 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 should be heating up, as evidenced by the spectrum of M87's magnetic emissions, in accordance with relativistic accelerating electrons in a magnetic field.
The visible ring demonstrates the force of gravity and gravitational lensing around the central black hole; and again general relativity was tested. This ring in the radio range does not correspond to the event horizon itself and does not correspond to the ring of rotating particles. And it is also not the most stable circular orbit of a black hole. No, this ring arises from a sphere of gravitationally lensed photons whose paths are bent by the gravity of the black hole on their way to our eyes.
This light bends into a larger sphere than one would expect if gravity were not as strong. As the Event Horizon Telescope Collaboration writes:
"We found that more than 50% of the total flux in arcseconds passes near the horizon and that this radiation is sharply suppressed when it hits this region, by a factor of 10, which is direct evidence of the predicted shadow of a black hole."
Einstein's general theory of relativity proved to be correct once again.
Black holes are dynamic phenomena, their radiation changes over time. With a mass of 6.5 billion suns, it will take light about a day to traverse the black hole's event horizon. This roughly sets the time frame in which we can expect to see changes and fluctuations in the emission observed by the EHT.
Even observations that lasted for several days allowed us to confirm that the structure of the emitted radiation changes over time, as predicted. The 2017 data contains 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 properties of radiation around a black hole in M87 do change over time.
EHT will in the future reveal the physical origin of black hole bursts. We have seen, in both X-ray and radio bands, that a black hole at the center of our own Milky Way is emitting short bursts of radiation. Although the very first image of a black hole presented showed a supermassive object in M87, a black hole in our galaxy - Sagittarius A * - will be just as large, only changing faster.
Compared to 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 no longer be observed during the day, but within even one minute. The features of the black hole will change rapidly, and when an outbreak occurs, we can reveal its nature.
How are the flares related to the temperature and luminosity of the radio picture we saw? Is there magnetic reconnection, as in our Sun's coronal mass ejections? Is anything bursting in the accretion streams? Sagittarius A * flashes daily, so we will be able to associate all the necessary signals with these events. If our models and observations are as good as they were for M87, we may be able to determine what drives these events and perhaps even know what is falling into the black hole, creating them.
Polarization data will emerge that will reveal whether black holes have their own magnetic field. While we were all definitely happy to see the first snapshot of a black hole's event horizon, it's important to understand that a completely unique picture will soon emerge: the polarization of light emanating from a black hole. Because of the electromagnetic nature of light, its interaction with the magnetic field will imprint a particular polarization signature on it, allowing us to reconstruct the black hole's magnetic field, 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 accretion streams, or pass through the event horizon, forming a kind of "anchor" in the black hole. There is a connection between these magnetic fields, accretion and black hole growth, and jets. Without these fields, matter in accretion flows could not lose angular momentum and fall into the event horizon.
Polarization data, thanks to the power of polarimetric imaging, will tell us about this. We already have the data: it remains to perform a full analysis.
The Event Horizon Telescope enhancement will reveal the presence of other black holes near galactic centers. When a planet revolves around the Sun, it 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 affects the sun. Likewise, when an object orbits a black hole, it also exerts gravitational pressure on the black hole. In the presence of a whole set of masses near the centers of galaxies - and, in theory, many invisible black holes so far - the central black hole should literally tremble in its place, being pulled apart by the Brownian motion of the surrounding bodies.
The trick to making this measurement today is that you need a reference point to calibrate your position relative to the location of the black hole. The technique for such a measurement assumes that you look at the calibrator, then at the source, again at the calibrator, again at the source, and so on. At the same time, you need to move your gaze very quickly. Unfortunately, the atmosphere changes very rapidly, and a lot can change in 1 second, so you simply won't have time to compare two objects. In any case, not with modern technology.
But technology in this area is developing incredibly fast. The tools used on the EHT are awaiting updates and may be able to reach the required speed by the mid-2020s. This puzzle could be solved by the end of the next decade, thanks to improved instrumentation.
Finally, Event Horizon Telescope will eventually see hundreds of black holes. To disassemble a black hole, the resolution of the telescope array needs to be better (i.e. high resolution) than the size of the object you are looking for. Currently, the EHT can only make out three known black holes in the Universe with a sufficiently large diameter: Sagittarius A *, the center of M87, the center of the galaxy NGC 1277.
But we can increase the power of the Event Horizon Telescope eye to Earth's size if we launch the telescopes into orbit. In theory, this is already technically achievable. The increase in the number of telescopes increases the number and frequency of observations, as well as the resolution.
By making the necessary improvements, instead of 2-3 galaxies, we will be able to find hundreds of black holes or even more. The future of black hole photo albums looks bright.
The Event Horizon Telescope project was expensive, but it paid off. Today we live in the era of black hole astronomy and have finally been able to observe them with our own eyes. This is just the beginning.
Ilya Khel