**A telescope as big as planet Earth –Will we soon be looking directly at a black hole?**

Of the many bizarre things that theoretical physics has confronted us over the last 100 years, „black holes“ are probably among the most peculiar. After Albert Einstein had formulated the basic equations of his general theory of relativity in November 1915, it took less than a year for the physicists to understand the dramatic consequences for the nature of our world they entail. Thus already in 2016the German astronomer Karl Schwarzschild succeeded inspecifying a space-time metric, which corresponds to a solution of the Einstein equations for the gravitational field of a point-shaped mass at point zero (r = 0). It describes such a large curvature of space-time that not even light can escape. Specifically, the space-time structure of such an object describes what physicists call a “singularity”: here common physical quantities are no longer defined as finite entities, which means nothing else than at this point space and time cease to exist in their usual form. However, the necessary mass density for such a phenomenon is so great (i.e. the so-called ‘event horizon’ at which it occurs so small) that neither Einstein nor Schwarzschild knew what to do with such a solution(for the mass of the Earth the event horizon corresponds of a sphere with a radius of 9 mm).

Thus astronomers regarded such structures as a long-winded curiosity within a theory that was already quite bizarre by itself. It was only decades later that they realized that such high mass densities can indeedexist in the universe for larger event horizons. Thus only in 1967the physicist John Wheeler popularized the term „black hole“ for such a structure, when he looked for a better word than „a gravitationally completely collapsed object“. The first candidate for a real black hole was „Cygnus X-1“ in the constellation Swan, discovered in 1971.

Today the astrophysicists understand the evolutionary dynamics of black holes quite well (although not yet in every detail): With all bodies the gravitational force leads to a compression. The more massive the body, the stronger the compression. Normally, this is stopped by internal forces – thermodynamic pressure, repulsion between the atoms or nucleons, or the Fermi pressure (electrons or nucleons can never be in the same quantum state), which ultimately leads to a balance between gravitation and the opposing forces. Beyond a critical mass however the opposing forces are no longer sufficient to compensate for the gravitational force. Then a complete gravitational collapse begins: the mass shrinks into a decreasingly smaller volume (howeverfrom the viewpoint of an external observer it never reaches zero, since the ever-increasing gravitation locally distorts the space and the course of time more and more. This means that the collapsing gets slower and slower, and the volume never converges to a single point; plus quantum processes whose nature we do not yet knowand which are beyond general relativity begin to play their part).

How about looking directly at a black hole (more precisely, the area immediately outside its event horizon)? Since black holes are very small on the cosmic scale and very distant from the earth, this is extremely difficult. The challenges are of elementary optical nature: the resolution of a telescope depends (besides on the wavelength of the light) on its lens diameter, the so-called “aperture”. Larger telescopes make it possible to dissolve smaller structures. Looking for example at the black hole „Sagittarius A*“ (short “Sgr A*”) in the heart of our Milky Way, the telescope (for those wavelengths that make it to the surface of the Earth) would have to be as large as the Earth itself, as seen from the Earth, the size of Sgr A* iscomparable with that of a saucer on the surface of the moon. But can we constructa telescope the size of the Earth? This is actually possible: Simply connect several observatories into one virtual telescope which then possesses an effective diameter as large as the distance between the individual observation points. If these are distributed all over the planet, we getto a telescope of the necessary size for hunting black holes.

This is exactly the aim of the project „Event Horizon Telescope (EHT)“ which went live in April 2017. However, the project requires the analysis of an immense amount of data only to get a few blurred pictures consisting of only a bunch of pixels from the surroundings of theblack holeSgr A*. But even these could put Einstein’s general relativity theory to an entirely new test. After all, events so close to such an extreme cosmic structure have never been directly observed. The structure of space-time near a black hole also makes it possible to observe its entire event horizon, i.e.we can also look behind the black hole. This is because the gravitational distortions of the space-time are so strong that light beams can literally go around the black hole (the movie „Interstellar“ from 2014 also illustrated that effect). First results from EHT are only expected in 2018-.

Now one might think: Why all this effort to measure a theory that has so far passed every test of measurement up to the last detail? Here one should know that the theoretical description of black holes entails some fundamental differences between the two theories that form the foundation of modern physics: general relativity and quantum theory. So far these had coexistedwith each otherundisturbed, quantum theory describing the atomic world of the microcosm, the general theory of relativity the macrocosm of galaxies and the universe as a whole. Bringing both together in a single theory with a scope over all scales proved with increasing efforts to be theoretically impossible (from the perspective of quantum theory, the general theory of relativity is still a “classic theory”‘, which requires no quantum leaps or probability waves, while from the perspective of general relativity, quantum theory remains an “background independent theory”, i.e.it knows no influence of matter on the structure of space and time). Black holes now fall within the scope of general relativity as well as quantum theory, and therefore represent a simultaneous test case for both. Such have astrophysicists not yet been able to examine with comparable precision. This leads us,next to the exciting possibility of looking directly at such a bizarre object, to potentially gainingcompletely new insights about the nature of our universe.