Lichtgedanken 04

S C HW E R P U N K T 43 04 | LICHT GEDANKEN of massive stars, which implode due to their own gravitational force. If light particles reach the sphere of influ- ence—the so-called event horizon—of a black hole, they are diverted along curved space and swallowed; just as water disappears down a drain in a vortex. When light has traversed the event horizon—in other words, has practically fallen into the black hole—it can no longer leave. What happens to the light in the black hole? Gies: We can only speculate based on our knowledge of mathematics and Einstein's theory of relativity: just as with the Big Bang, a singularity exists inside a black hole in which all matter appears to fall. These point-like sin- gularities are, however, incompatible with quantum mechanics. That is why we are looking for a complete theory of quantum gravity, which can explain the resolution of such singularities. For, in contrast to what Einstein's the- ory purports, we now know that black holes also emit radiation. So, a black hole is not actually black? Ammon: Exactly. To put it simply, black holes are not black; they are more a grey colour. In a ground-breaking work, physicist Stephen Hawking, who re- cently passed away, showed that black holes emit light and material particles through a quantum effect—Hawking radiation. Having said that, even in the case of large black holes, the radiation is in the nanokelvin range and its ex- istence can, therefore, not currently be proven using the technology we have today. How can black holes be observed? Gies : When we look into the universe, we see the light emitted by the celestial bodies. Because black holes don’t emit anything that we have yet been able to measure, we cannot see them directly. But we can see their effects; for exam- ple, how other stars move around black holes. Or we can see gas clouds and other forms of matter, which fall into black holes and, in doing so, produce gigantic X-ray flashes. Can black holes capture infinite amounts of matter and light? Ammon: The theory does not predict a maximum mass for black holes. In re- ality, we see black holes with masses ranging from a few to several million solar masses. The black hole in the cen- tre of our galaxy is around four million solar masses in weight, for example. Will black holes eventually swallow all light? Gies: No, that won’t happen. But at some point, our universe will become completely dark. But this won’t be due to the growing number of black holes; rather because the universe is expand- ing at an ever-increasing rate. The number of stars and galaxies, whose light has yet to find us, will decline over the long term. Thus, it will be the expansion of the universe that will eventually switch off the light. When will this happen? Ammon: We can’t answer that yet. The accelerating expansion of the universe is attributed to dark energy, which makes up around 70 percent of the en- ergy of the universe. By way of com- parison: conventional matter and light only make up around four percent of the energy content of the universe. De- spite extensive research, we still know almost nothing about dark energy and dark matter. In order to be able to precisely predict the expansion of the universe, we must first come to under- stand dark energy. Image left: The centre of the Milky Way. It is the loca- tion of the central black hole »Sagittarius A*«, which weighs around four million solar masses. © NRAO/AUI/NSF Image right: In order to explain what happens to light and matter in a black hole, Junior Prof. Dr Martin Ammon (left) and Prof. Dr Holger Gies are attempting to combine Einstein’s general theory of relativity with quantum theory.