Astronomy: picture with a hole (

Center of the Milky Way in infrared light. The blue spots are X-ray sources near the black hole Sgr A *.

Photo: X-ray: NASA / CXC / SAO

They are cunningly thick beyond the event horizon, illuminating the eyes of astronomers and astronomy enthusiasts while remaining in the dark by themselves: black holes. It was only the second time that an international team of astronomers managed to take a picture of such a massive monster. But what do we actually see in the image of “our” black hole sitting in the center of the Milky Way like a black cat with its eyes closed at night?

Looking towards the center of the Milky Way, gas and dust are quickly blocking our view in visible light. In terms of infrared and radio radiation, we have much more vision and can look deep into the central regions of our Milky Way. It has long been known that there must be an extremely massive and dense object in the center. For example, stars can be observed orbiting an object in eccentric orbits at incredible speeds – the fastest stars reach around 24,000 km / s, which is eight percent the speed of light. The published photo is the first visible evidence that “the object at the center of the galaxy is in fact a black hole,” says Anton Zensus, director of the Max Planck and founder of the EHT Supervisory Board.

Despite its enormous mass of 4.5 million solar masses, “Sagittarius A *” or “Sag A *” is only half the distance between the Sun and Earth in diameter. About 26,000 light years away, it is just a tiny speck in the sky. The black hole in galaxy M87, the photo of which was released in 2019, is much heavier and larger, weighing 6.5 billion times the mass of the Sun – but it’s also about 2,000 times farther from Earth, which is as much as two black holes. a similar sized hole in the sky.

Like a beer mat on the moon

How can one study in detail an object whose apparent size – astronomers call it angular extent – is as “large” as a beer mat on the moon?

In the case of recordings, black holes were not observed in the range of visible light, but in the range of radio waves – here they radiate particularly strongly and during their long journey to Earth, these waves are weakly weakened by gas or dust. Terrestrial radio telescopes, which usually look like huge satellite dishes, take advantage of the fact that the Earth’s atmosphere is transparent to a wide range of radio waves, including the 1.3-mm waves from new observations. Like visible light, radio waves are part of the electromagnetic spectrum and are collected, bundled and converted into an electrical signal in a radio telescope.

Waves can be described by their frequency, the number of oscillations per second, and the amplitude and strength of deflection. So you have a more or less rapid succession of wave crests and wave valleys. Like all waves, radio waves can overlap and “interfere” with a characteristic interference pattern. If the wave crest meets the wave crest (and the bottom wave meets the bottom wave), this results in a strong signal (constructive interference) – but if the bottom wave and the wave crest meet, the waves are canceled completely with the same amplitude (destructive interference) . However, there are also situations that lie in between: If the crest of the wave is almost exactly in contact with the valley, the signal is attenuated by a very specific amount.

The radio telescope receives radio waves from the black hole over the entire surface of its dome, reflects them, and combines the radiation at the focus for interference. If the telescope is pointing directly at the black hole, a strong signal is being picked up; However, if the radio telescope looks a little beyond the black hole, the signal is more or less attenuated. When a radio telescope scans the sky piece by piece, you get a “radio picture” of the sky.

Ideally, to get the sharpest possible image of an object, you only want to capture the signal when the telescope is pointing directly at the source. One possibility would be to observe radio waves of shorter wavelengths: destructive interference occurs faster, and the object’s boundaries and structures would be clearer. However, since the permeability of the Earth’s atmosphere drops sharply in this wavelength range, a second option should be used: increasing the diameter of the telescope. And here it gets a little impractical: to get the desired resolution, the satellite dish would need to be roughly the diameter of the Earth. But there is a way out: instead of a giant canopy, several radio telescopes can be connected to each other. The “Event Horizon Telescope” is a global network of radio telescopes which, using so-called long-base interferometry, enables the registration of radio signals from black holes with the desired accuracy.

To this end, the signals of the individual telescopes are recorded and mathematically linked together with their precise location data and precise time measurements (in the femtosecond range). The amounts of data that need to be recorded and merged here are in the petabyte range (typical computer hard drives contain one-thousandth of that). Such amounts of data overwhelm even the fastest internet connections. Data is thus transported to its destination by means of hard drives and “on foot” – often referred to with a grain of salt “sneakernet” or “sneaker network”.

Equipped in this way with the data, the next steps can then be taken in a sporty way. Each pair of telescopes provides an interference pattern whose light-dark sequence correlates with the distance between the telescopes: if they are close together, the interference pattern is slightly wider; if they are far apart, the light-dark sequence is narrower. If all the patterns of the pair of telescopes are now combined using extremely complex algorithms, the result is a polluting image: the black hole has finally been photographed! But the question is: what do we actually see in the recording?

If a black hole were completely isolated, completely free of surrounding matter – it would live up to its name and we would not be able to see anything, even with the most sophisticated tricks. A black hole, a really heavy ball whose radius (the so-called Schwarzschild ray) corresponds to an “event horizon” from which nothing can escape: matter certainly cannot, and not even a photon can escape from there.

But a black hole is never so lonely: in the center of the galaxy, fast-moving stars point to a mass giant. In addition, it can still be surrounded by huge amounts of matter that collapses into the black hole as a spinning disk. The gas and dust of this accretion disk heat up to several million degrees and reach speeds that can reach even a considerable fraction of the speed of light. And it is the radiation produced in this hot roundabout that ultimately reveals the black hole.

It changes every minute

The material of the accretion disk causes the black hole to grow increasingly massive over time – although the “appetite” can vary from object to object. “Say A *” is one of the quieter candidates, while the black hole in M87 is known for its uninterrupted, high material hunger. This was also evident when two recordings were made: large and constantly fed, the M87 seemed calmer and more stable in the datasets, which made imaging a bit “simpler” than with “Sag A *” where changes in appearance sometimes they can even occur every minute.

But no matter if we are hungry or satiated: in both cases, the huge mass of the black hole leads to a phenomenon with which on Earth we only deal with a lot of imagination and as an analogy: if we sit on a comfortable and very worn-out couch, we immerse ourselves in deep, and the padding curves toward our, well, lower backs. Cookie crumbs, a sleepy cat and a pilot do not want to “turn”, but inevitably follow the curved surface of the upholstery and land on our laps. In the case of a black hole, “space-time” is curved – and even if light follows a straight path, space-time is curved and thus dictates the path of light.

Now consider the situation where the black hole accretion disk is oriented exactly perpendicular to our line of sight – we are looking straight down at the hot bud of matter. Any light that falls on the event horizon is absorbed by the black hole – and we see that we can’t see anything here: the shadow of the hole. But there is also a curvature: even light that is slightly beyond the event horizon is bent towards the hole in such a way that it also disappears: the shadow we perceive is ultimately larger than the actual black hole.

But now it’s more likely that we are looking at the accretion disk from some angle – perhaps even at the very edge of the disk. In this case, the radiation from the accretion disk behind the black hole is deflected by the curvature of space-time in such a way that it can still reach our telescopes: the light from the top of the accretion disk “bends” upwards and the light from the bottom of the accretion disk downwards. The image we see is somewhat reminiscent of a “double Jupiter” with glowing rings. We look both from the side of the rings and from above – except that Jupiter is our black hole and the rings are a feeding accretion disk.

Even if there is a photo of “Sag A *” now – there are still some questions that the black hole has yet to reveal: “The exciting question is what properties this black hole has, such as whether it rotates, is it charged or electrically neutral,” he says Dominik Schwarz, who studies radio astronomy at the University of Bielefeld and was not involved in the research himself.

If you’re a little dizzy: no problem, the accretion disc does the same! As the matter in the disk spirals towards the black hole, the radiation moves towards us on one side and away from us on the other side. This is similar to the acoustic Doppler effect, whereby the siren of a police car becomes terrifying when you approach it and a bit deaf as you go away. In the case of a black hole, there is a so-called a relativistic or doppler ray in which relativistic effects change the apparent brightness of the emitted matter moving at a speed close to the speed of light. One side of the black hole image appears lighter and the other side is darker.

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