Black Holes
Black holes are well known from science fiction and yet they are very real. They are not made of matter and yet they have mass. That is why, until recently, they could not be observed directly, but only through the effect of their gravity on their surroundings: they bend space and time and have an almost irresistible attraction. The idea behind these exotic objects is over 230 years old but it took some time before they were finally discovered - with the significant involvement of researchers from the Max Planck Society.
What is a black hole?
The mass of a star, concentrated into a single point. A point of infinite density, a singularity. A sphere around this point, from which a spaceship could only escape the central gravitational pull at the speed of light. In other words, the event horizon, where nothing actually happens, except that even light is trapped behind it. Otherwise, it is quite uneventful at the event horizon; you could fly through it and not hit anything. All these definitions are correct; they describe a black hole. Its properties are curious and yet conceivably simple. It is completely described by its mass and angular momentum, and in a very general definition also by a charge. Fun fact: If you were to squeeze the Earth to the size of a cherry, none of the known forces could stop it from collapsing any more. The Earth would become a black hole.
For those who want to know more: black holes warp space-time. It is as if they were tearing deep troughs in an imaginary net in which space and time are interwoven. As part of the general theory of relativity, Einstein developed equations, the so-called Einstein field equations, that describe this space-time and its curvature. Black holes are mathematical solutions to these equations, as are wormholes. However, the latter have not yet been observed or proven, and thus remain science fiction for the time being.
What kind of black holes are there?
There are black holes with a wide range of masses. Some were formed directly after the Big Bang and are only as large as an atom, yet as heavy as a skyscraper. Others are formed when heavy stars, much heavier than our sun, collapse at the end of their lives. However, the largest gravitational traps reside in the centers of galaxies and reach billions of times the mass of the sun. How they can become so heavy is still an open research question. Black holes can quickly gain mass when smaller specimens merge with each other. In the centers of active galaxies, a constant influx of matter also ensures that they continue to grow at a rate of about one solar mass per year. Over the 13.8 billion years of the universe's development to date, a lot has accumulated in many places.
How do scientists detect black holes?
Black holes are characterized above all by their eternal darkness and remain undetected in photographs of the night sky. They interact with their surroundings only through the laws of gravity. When two black holes orbit each other, they send waves through space-time, which were only measured a few years ago with the significant participation of the Max Planck Society. The data from the gravitational wave detectors testify to a multitude of black holes that would otherwise have remained undetected.
Until now, astronomers have only been able to detect black holes when matter orbits them and heats up or when matter falls onto them. In both cases, electromagnetic radiation, i.e. light, is produced, which sensitive telescopes can measure. In this way, only a handful of small black holes in our Milky Way have been found, which are the result of the death of massive stars. However, there could be many more black holes in the Milky Way that have so far completely eluded detection. Primordial black holes also fall into this category. They do not pose any danger, as they are apparently located at a sufficient distance from Earth and, if they exist, are no heavier than a moon or a planet. They could be so numerous that some researchers are even trying to use them to explain dark matter.
And how was the first photo of a black hole taken?
In 2001, Andrea Ghez of the University of California and Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics observed how stars orbited an invisible point in the center of the Milky Way like planets orbit a star. For this discovery of a supermassive compact object at the center of our home galaxy, both received the Nobel Prize in Physics in 2020.
The first direct evidence, the first image of one of these cosmic gravity traps, was only obtained in 2019 by an international team with the participation of the Max Planck Society. A simple camera on a telescope would never have achieved the necessary resolution. The solution to the problem is interferometry. Here, researchers operated eight ground-based radio telescopes from around the world as a single virtual telescope the size of the Earth. The Event Horizon Telescope was thus able to take a picture of the black hole at the center of the galaxy M87, which is 55 million light years from Earth. More precisely, a picture of glowing gas that surrounds the black hole like a ring. From Earth, the ring appears to be only as large as a concert spotlight on the moon.
A little later, in 2022, astronomers finally took a picture of the black hole at the center of our own galaxy, the Milky Way. Due to the large amount of gas between the center and the Earth, the data analysis was more difficult here than in the case of M87. Even though it is much closer than the gravity trap at the center of M87, at "only" 27,000 light years, the ring appeared to be similarly large for the telescopes in the sky. This is because it is more than a thousand times smaller and much lighter.
