 My name is Reinhard Gänsel. I'm an astrophysicist, an experimental astrophysicist. I work at a Max Planck Institute, Max Planck Institute for Extraterrestrial Physics. And I'm interested in my team and I'm interested in studying massive black holes and the formation and evolution of the universe. So first of all, the center of our galaxy is a testing ground for gravity. How did you come to this idea? Well, it's a longer story. I mean, the general activity, as you know, was invented, so to speak, as an improvement over Newton's theory by Albert Einstein about 100 years ago. Then it took 50 years until the quasars were discovered. The quasars are very bright but very, very distant phenomena, which we now know are, with great likelihood, very, very massive black holes between billions to billions of solar masses, which attract creed material and are very bright. And this idea that you would have these supermassive black holes is one of the things which the theory of Einstein predicts. But the question is, is it true? And so the quasars are much too far away at this point to make detailed measurements and test whether they are really truly black holes. So that's where the galactic center comes in. The galactic center is 24,000 light-years away. That's really very close. And so over these last three decades, we've been able to make measurements with ever-better precision, mostly of stars orbiting the very center, showing that there is indeed a mass, a compact mass there, about 4 million solar masses. And the evidence has gotten better and better and better that this is indeed an Einsteinian, if you like, or a Kerr, a Kerr black hole. The fact that Einstein's general relativity, relativity theory, predicted, is the Schwarzschild precession. What is it? Can you explain this to me? Well, if you envision the solar system and what Newton and Kepler knew about it, then you have the planets orbiting the Sun on elliptical or circular orbits. And these ellipses, if there's nothing else in the way of a planet around the Sun, is a figure which is always the same. Okay, it stays the same in the orbit and the planet orbits around the Sun for all times, not so in general relativity. And there are several effects here. One is just due to the fact that the space time, which the Sun generates, makes the orbit of the planet process, move forward, rotate, if you like, a little bit. Forward over time. And the second thing, if the planet, if the Sun has an angular momentum, so it rotates, then there's a second effect, which is also a precession. So it's the biggest effect of these two is the precession due to the mass. So in the case of the Sun, the precession of a planet was actually detected before general relativity was known. That's not very well known that the Mercury, the innermost planet, was looked at in the 19th century by astronomers, mainly to look for disturbances on its orbit by other planets. And this is where Uranus was discovered, actually. And many people at the time thought there might be a planet inside of Mercury called Vulcan. And they were sort of looking for an effect which could not be explained by the other planets. And indeed they found some, but not Vulcan. They had, in fact, detected 30, 40 years before Einstein the Schwarzschild precession. And so when Einstein then wrote it down, everyone said, hey, okay, there it is, okay. So if we already knew about Mercury, why do we need to look at the black hole in the middle of our own galaxy? Yes, so why are we testing and testing general relativity? Well, because that's the nature of physics. Theories and the famous theory of Einstein is no different here. Our only transitory truth, if you like. In fact, we believe there is good reason that the Einstein's theory must be wrong. On small scales, for instance, very small scales where you would have quantum effects and quantum atomic effects and so forth. Plus, there are other theories which are similar to what not the same as general relativity, which predict slightly different things. So we must, as scientists, that's our, that's the scientific method. Keep on testing in different parts of parameter space whether a theory is correct in order to either, you know, find it is correct or it is not correct. So that's what we're doing here. We're looking in part of a parameter space in mass, which has not been looked at. I mean, the laboratory experiments or the solar system tests on very small scales, very small masses. Then LIGO looked at and tested general relativity with so-called stellar black holes, a few tens of solar masses. Here we're looking at an object which has millions of solar masses. So that's very different. And so that's one way, that's one way we want to test the theory. There's a second motivation we have. And that is to actually show that the object which is in the center of the Milky Way is a massive black hole. It plausibly is so far, but it might not. It might be a double object. It might be a triple object. Who knows? And so by making the measurements we are measuring or have been making, we are basically firming up the evidence ever more that this is indeed a supermassive black hole. Now the third, if you like, is the biology of black holes in their environment because black holes are not lonely objects because of their gravity. They are attracting other objects. One would expect, in fact, to be a massive black hole in the center of a galaxy to be tightly surrounded by a cluster of stars and maybe stellar black holes, or maybe so-called intermediate mass black holes. So these would be objects of, say, a few hundred to a few thousand solar masses. They have not been seen, but they might exist. Our method, in fact, has been able to test that and to our surprise, if you like, they have not seen anything. The object in the galactic center is pretty lonely. In its vicinity there is the star which we have been using, but so far we have not seen anything inside. Your team has worked on this project for almost 30 years or around 30 years, which is a really long time. What has happened in those last three decades? Which are the technological advancements that have happened since? Yes, indeed. I can see I'm getting older and older here. I started this when I was in my 20s. Over this time, over this period of time, we have made measurement improvements of factors between a few thousand to a million in resolution, in sensitivity, etc. The most recent step we took is to replace a single telescope, which one usually uses to make images, by four telescopes which we combine, that's called interferometry. We take four eight-meter-class telescopes of the European Southern Observatory, the very large telescope, in Chile and combine it optically to an equivalent 130-meter telescope. The bigger the telescope, the better the resolution. With this interferometer called gravity, we can measure basically a few centimeters on the moon, if you like. That gives us the precision to really nail the motion of a star around the black hole so well that we can see these effects of general relativity. With gravity you could show in 2018 another effect of the general relativity. What is that and why do we need to prove it all over and over again? Well, the effect we are seeing now is basically true for objects with a mass. Now, general relativity in contrast to Newton also says that gravity affects the motion of massless things like light. So if you have a flashlight or a laser and you're close to a black hole and you shine that to us at a large distance, then the laser light has to climb out of the massive gravity around the black hole and it gets less energy lessens, and so we see it redshifted. So that's one of the effects. The effect we are seeing now is a second effect, but there are many more. The next one we would like to see is the effect of the spin. So if a black hole is rotating, then it takes a space-time around with it and a star which would be moving there or gas which would be moving there would see that and start to wobble, and if you can see that you can measure the spin of the black hole. Now these are very subtle effects, very difficult to measure. So you have to slowly learn to climb the ladder of 7000 meter mountains, then you go to 8000 meter mountains, and then finally you hopefully end up at Mount Everest. It's an amazing project. Congratulations on it. What's next? What's the next project that you and your team are working on? Well, we are in the process of improving the gravity instruments still more, maybe as much as a factor of 100. We'll see. With that we want to look at distant black holes. So far we've been looking at the closest massive black hole, but quasars are very distant from us. With gravity we can see the motion of gas and resolve the sizes of these regions and thereby measure them the mass precisely. So if you can do this for many quasars, many distant objects, then you can solve perhaps the riddle how massive black holes played a role in the evolution of galaxies. We now know that basically every galaxy has at its center a massive black hole of different masses. We would like to understand that in detail. There is, if you like, a symbiosis between these black holes and galaxies, and we need to understand that in order to understand the evolution of the universe. So what we see here is the elliptical orbit of the star we've been looking at. It moves around the black hole which is located in this still on the lower right. We see this black spot there. It does so in 16 years. For most of this orbital motion, Newton's theory is just fine. It's only when the star enters within a year the region around the black hole here where it comes as close as about two or three times the orbital radius of Neptune when generativity plays a role. And then what happens is that this ellipse which is stationary in Newton's theory all of a sudden starts to shift. It precesses forward by a small amount, but evermore. And basically we have been able to precisely measure this procession and shown that it is identical to what generativity would predict. As I said, this has several aspects. One is the testing of the theory. And the other one is that if there were a deviation from the massive black hole, if there were two black holes this wouldn't look like that. If there were a big black hole and an intermediate mass black hole, it wouldn't look like that. And so by seeing the effect so pure you can say a lot about the environment of the big black hole. Perfect. It's super, super interesting. Thank you very much for the time you've given me. And good luck to you and a lot of success for the next project. Okay, thank you all.