Can black holes affect our galaxy?
"Black holes have not been taken seriously for a long time"
This year's Nobel Prize in Physics honored the researchers Andrea Ghez, Reinhard Genzel and Roger Penrose for their discoveries on black holes. But while black holes are now the focus of numerous research groups and experiments, the mysterious celestial objects were dismissed by many scientists as a theoretical gimmick until a few decades ago. In an interview with Welt der Physik, the theoretical physicist Jutta Kunz from the University of Oldenburg reports what findings this gradually changed and what role the contributions of the Nobel Prize winners played.
World of physics: Black holes are probably one of the most puzzling objects in our universe. How did researchers get the idea that there must be black holes in the first place?
Jutta Kunz: As early as the 18th century, researchers John Michell and Pierre-Simon Laplace thought about what it would mean if stars were much denser than our sun, for example. According to your imagination, the gravity of these stars could then become so great that other objects could only escape the strong attraction of these stars at extremely high speeds. If a star were compact enough, even the speed of light would no longer be sufficient. The researchers called these hypothetical objects, from which we cannot see light, dark stars. But at the time it was still unclear whether light is even influenced by the force of gravity of an object. To describe this, a new theory was needed - general relativity.
What has changed with general relativity?
When Albert Einstein published his general theory of relativity in 1915, he revolutionized our understanding of space and time. Because he no longer describes space and time as a rigid framework in which physical processes take place. In his theory, Einstein combines space and time into a common structure, the so-called space-time. This spacetime is warped by matter. The matter in turn moves through space-time and is influenced by the curvature in its movement. Light also follows the geometry of space-time and is thus deflected by the force of gravity from celestial bodies such as the sun. This complex interplay between space, time, matter and light is described by Einstein's equations.
And these equations also describe black holes?
Exactly. The possibility that black holes exist is already in the equations of general relativity. But the equations are very complex. For the first time, the astronomer Karl Schwarzschild discovered the possibility of black holes in the equations when attempting to apply the general theory of relativity to our solar system: To do this, he calculated how much our sun bends space-time and how this curvature the orbit of the planets and the course of Light affects. Indeed, during a solar eclipse in 1919, astronomers could see starlight being deflected by the sun's gravity.
But where does a black hole appear in Schwarzschild's calculations?
In order to find a solution for the complex equations, Schwarzschild greatly simplified our solar system: In his model he assumed that the entire mass of the sun is concentrated in one point and calculated how much this point-like mass bends space-time in its surroundings. In reality, however, the sun extends over a radius of almost 700,000 kilometers - and what the equations showed for a smaller radius did not matter for the solar system. In this inner area, however, Schwarzschild's calculations revealed strange behavior that the scientists were initially unable to explain. Today we know: This inner area of the solution describes a black hole.
What does this enigmatic inner area look like?
Simulation of the event horizon of a black hole
The closer you get to the compact mass, the more space-time curves. At some point this curvature is so strong that even light is not fast enough to be able to escape. This boundary is called the event horizon and separates the inner area from its surroundings. No light and therefore no information can get out from there. In the middle of the center, the curvature of spacetime becomes infinitely great. But at that time the scientists did not yet know what this so-called singularity and the event horizon were supposed to mean. Many doubted that this theoretical solution was even relevant for our physical reality. It was impossible to imagine an object that was compact enough to produce such a large curvature.
What changed this attitude?
The decisive factor was a new finding from the physicist Subrahmanyan Chandrasekhar, who studied the evolution of stars. He was particularly interested in the possible final stages of stars: as long as a star has enough fuel, it generates a thermal pressure outwards that counteracts the attractive force of gravity - the star is stable. If this fuel runs out, a star like our sun will eventually turn into a white dwarf. However, Chandrasekhar found that stars significantly heavier than our Sun can collapse under their own gravity. For the first time, a physical process was conceivable in which an extremely compact object could be created. Theoretical physicists Robert Oppenheimer and Hartland Snyder later investigated gravitational collapse in more detail, and Roger Penrose, one of this year's Nobel Prize winners in physics, also did research in this area.
What did Roger Penrose receive the Nobel Prize for?
The calculations by Chandrasekhar as well as by Oppenheimer and Snyder were based on many simplifications. Critics suspected that the gravitational collapse of stars is possible in theory, but would never take place in our complex physical reality. However, Penrose proved the opposite in the 1960s: Using sophisticated mathematical methods, he showed that a gravitational collapse of stars to black holes is possible even without the major simplifications. Soon afterwards the designation of the enigmatic objects as black holes established itself.
And when did astronomers first observe such a black hole?
Rosette orbit of the star S2
In the early 1970s, the astronomers Tom Bolton, Louise Webster and Paul Murdin finally discovered the first black hole called Cygnus X-1 with about fifteen times the mass of the sun in our Milky Way. Today researchers assume that there are several hundred million of these so-called stellar black holes in our galaxy alone. In addition to the stellar black holes, which are formed during the gravitational collapse of stars, there are also much heavier, so-called supermassive black holes, but we still know little about their formation. There is also a supermassive black hole in the center of our galaxy, as researchers working with Andrea Ghez and Reinhard Genzel were able to show in the 1990s: For years, researchers tracked the position of stars near our galactic center. They observed the elliptical orbits of the stars, which can only be explained by an extremely massive, invisible object in the center of the Milky Way, which binds the stars to itself through its enormous gravity.
And for this discovery, the researchers now received half of this year's Nobel Prize in Physics?
Exactly. The simultaneous award of theory and experimental observation also shows how important this interaction was in research on black holes. Time and again, the two areas have mutually enriched each other and stimulated new insights. Personally, I was also particularly pleased that Andrea Ghez is now another woman among the Nobel Prize winners for physics. In this way, young women are shown that they too can decisively advance research in physics.
It has been some time since these discoveries. What does our understanding of black holes look like today?
First image of a black hole
We can now describe the area around black holes much better. Many exciting processes happen there, because black holes attract matter from their environment, which makes them grow. The matter shines brightly and can be observed, as was impressively shown by the recordings of the Event Horizon Telescope in 2019. Thanks to our enormous advances in data processing, this environment can now be modeled very precisely with large computers and it agrees wonderfully with the observations. However, we still do not know what happens inside black holes, where the curvature of space-time becomes infinite according to general relativity. To understand this area, we arguably need a new, more comprehensive theory - a theory of quantum gravity.
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