FOCUS_Extreme Space Traps in Space-Time Black holes are a permanent fixture in science fiction literature. In reality, there is hardly a more extreme location in the universe. These mass monsters swallow everything that ventures too close to them: light, gas, dust and even entire stars. It sounds quite simple, but the nature of black holes is complex. Maria Rodriguez, Minerva Group Leader at the Max Planck Institute for Gravitational Physics in Golm, wants to solve some of the puzzles these exotic cosmic bodies present. TEXT FELICITAS MOKLER or a long time it seemed that with 62 solar masses. Only a few months work of Newtonian gravitational theo- they existed only in the minds later, the researchers registered gravi- ry and the corpuscular theory of light. of theoretical physicists – black tational waves of source GW151226 At the beginning of the 20th cen- holes. But since at least a year from a similar system. These events tury, Albert Einstein revolutionized our ago now, we finally have clari- gave the space-time structure a violent understanding of gravitation – and thus ty: theyF really do exist. On September shake – and provided impressive con- also of matter, space and time – with his 14, 2015, gravitational waves caused firmation of the existence of the mass general theory of relativity. His idea re- space-time to oscillate to a measurable monsters. (MAXPLANCKRESEARCH 2/2016, placed the classical concept of gravity extent in the American LIGO detectors page 78 ff.) as the direct interaction between two for the first time. masses that move through rigid, Euclid- The detection of the undulation of EINSTEIN’S THEORY REPLACES ian space. Instead, it held that three-di- space-time predicted 100 years earlier THE CLASSICAL CONCEPT mensional space and time are united to by Albert Einstein (1879 to 1955) is si- form a four-dimensional space-time multaneously deemed to be experimen- Naturalists John Mitchell (1724 to 1793) and are themselves now deformable tal proof of the physical existence of and Pierre Simon de Laplace (1749 to and dynamic. A mass such as the Sun black holes. The first measured signal 1827) were already speculating in the bends space-time in its vicinity – the GW150914 originated from two black late 18th century about dark stars or larger and, in particular, the denser a holes with 36 and 29 solar masses, re- dark astrophysical bodies whose gravi- mass, the stronger the effect. spectively, that used to orbit one anoth- ty was so strong that light couldn’t es- The motion of a second, smaller 2.5 er in a kind of binary star system until cape them. The ideas of the two re- mass, such as a planet, then follows this they finally merged into one object searchers were still within the frame- curvature. If the local curvature chang- Under the spell of gravity: Black holes have been keeping astrophysicists busy for more than a century. None has yet been observed directly, but the discovery of gravitational waves at least shows that these bizarre structures really do exist. Graphic: Alain r/CC-BY-SA 26 MaxPlanckResearch 1 | 17 Graphic: Alain r/CC-BY-SA 2.5 FOCUS_Extreme Space es because a mass accelerates across this the black hole,” says the Minerva Group (1873 to 1916) in the metric that bears space, this change propagates at the Leader at the Max Planck Institute for his name. The so-called event horizon speed of light as a gravitational wave in Gravitational Physics in Golm. that shields the interior of the black the space-time structure. There is no escape for either matter hole from the outside world is defined Einstein used Riemannian geome- or light. This is what makes it so diffi- by the Schwarzschild radius, which de- try as the mathematical basis for dy- cult for astronomers to observe these pends on the mass. “Its shape is a per- namic space-time. Black holes emerge objects. The explanation is simple: in fect sphere for conventional black holes, from the field equations, which de- order to leave a conventional celestial which don’t rotate and have no electric scribe the curvature of space brought body such as Earth, objects require a charge,” says Rodriguez. The researcher about by matter and, conversely, the minimum kinetic energy that depends investigates the boundary surfaces of effect of curvature on matter, as a nat- on the body’s mass. The velocity that black holes with different properties. ural solution for matter concentrated rockets require, for example, is called But black holes with no angular at one point. The gravity there is in- the escape velocity. For a black hole, momentum are probably very rare in finitely large, and space-time has an this velocity corresponds to the speed nature. This is because these objects infinitely large curvature: mathemati- of light. are formed, for instance, when partic- cally, it is a singularity. ularly massive stars have exhausted “Physically, however, such infinities GRAVITY EVENTUALLY GAINS the supply of fusionable material in don’t tell us anything,” says Maria Ro- THE UPPER HAND their interior toward the end of their driguez. “What is relevant is the separa- life. The temperature there then drops tion from the point mass below which How space-time behaves under these and thermal and radiation pressure are gravity becomes so strong that nothing conditions was first described in 1916 no longer sufficient to counteract the can escape from it and can only fall into by astrophysicist Karl Schwarzschild star’s gravitation. The stellar interior implodes and the once free electrons are pushed into the positively charged atomic nuclei. Event horizon In such a supernova, the neutrinos re- leased in this process transport a large Ring singularity portion of the energy away and hurl the outer stellar shell into space. The matter in the core, in contrast, con- tracts more and more under its own gravity. If the original mass of the star was more than eight solar masses, the ultimate result is a black hole. Since stars usually rotate about their own axis and the angular momentum is con- served, they pass it on to the black hole. Astronomers are still puzzled by the birth of the particularly massive black Ergosphere Inner horizon holes – with a few million to a billion solar masses – that are presumed to lie at the centers of galaxies. They may pos- sibly have formed from the first gener- Clear structures: The physics might be complex, but the explanation of the structure is simple. ation of extremely heavy stars that, The graphic shows a rotating black hole with a ring singularity in the center and the inner having become a black hole, rapidly ac- horizon. The event horizon is something like the surface of the black hole; when something disappears behind it, it is truly out of this world. The ergosphere marks the region in which creted more and more mass and thus every object is forced to co-rotate. more and more angular momentum. Graphic: designergold, based on original material from the MPI for Gravitational Physics 28 MaxPlanckResearch 1 | 17 Engine at the center: Centaurus A is around 15 million light-years away, making it the nearest radio galaxy, but it also emits intense X-ray and gamma radiation. The jet structure visible in this image is apparently caused by the activity of the central black hole with around 55 million solar masses. The hypothesis of so-called primordial black hole is called its ergosphere and high-energy jets in around 20 active black holes, according to which these is ellipsoid in shape. Matter from out- galaxies or quasars. These objects prob- cosmic heavyweights formed shortly side that gets caught up in it doesn’t ably have an extremely massive, rapid- after the Big Bang together with the necessarily plunge directly into the ly rotating black hole at their center,” first galaxies, is also being discussed. As- space-time trap, but rather initially says the Max Planck researcher. tronomers also speculate about the ex- co-rotates with it. Nevertheless, it is un- To be able to correctly interpret istence of medium-weight black holes, able to escape from it. these phenomena, the researchers are with 100 to one million solar masses, “Unless the matter is electrically attempting to compute the behavior that could be present at the centers of charged,” says Rodriguez, because the of rotating black holes and the physi- large globular clusters. co-rotating charged matter causes a cal processes in their immediate vicin- magnetosphere to form. Some of the ity according to the laws of the gener- PARTICLES ARE EJECTED charged particles are deflected along al theory of relativity. Analytically, they the magnetic field lines. Depending on have initially succeeded in doing this When a black hole rotates, its event the sign of the charge and the orienta- only for very slowly rotating black horizon deforms slightly. Furthermore, tion of the field lines, the particles then holes, but Maria Rodriguez is interest- the space-time of its immediate vicini- fall more rapidly into the black hole – ed in the very opposite: objects that ty and the matter located there rotate or are ejected along the rotational axis. rotate about their own axis at almost Graphic: designergold, based on original material from the MPI for Gravitational Physics Photo: ESO/WFI (optical); MPIfR/ESO/APEX/A.Weiss et al. (submillimeter); NASA/CXC/ CfA/R.Kraft et al. (X-ray) with it. This sphere of influence of the “Astronomers have since observed very the speed of light. > 1 | 17 MaxPlanckResearch 29 Surface of the event horizon Angular momentum Advanced mathematics: During her time as a postdoc at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI), Maria Rodriguez computed possible surface configurations of event horizons of black holes in more than three spatial dimensions.