# Can light penetrate a black hole

## Physics compact, basic knowledge 8, textbook

92 Macrocosm 23 Example of the discovery of black holes How can you see an object from which no light can penetrate? Black holes can be detected by three effects: 1. Curvature of light - gravitational lens: In 1979 two closely spaced quasars were found whose spectroscopic properties were very similar. It eventually turned out to be a single object whose light is split into two images by the gravitational field of a galaxy lying between the quasar and our Milky Way. Black holes would have to show a similar property, but the probability that light from a distant object will just get past a black hole on its way to us is very small. 2. Radiation of energy from matter falling into the black hole: If matter falls into a black hole, it reaches speeds just below the speed of light. If one compares the kinetic energy of such a falling object, according to theoretical calculations it makes up about 40% of the rest energy. If a quantity of gas orbits a black hole, a gas particle can only reach deeper orbits with a loss of energy. This occurs through friction, so that gravitational energy is converted into thermal energy. Up to 400% of the rest energy of the incident mass can be radiated in this way. If a black hole is a partner in a binary star system, a considerable amount of matter can migrate from the partner star to the black hole compared to interstellar gas capture. 3. Evaporation through e - -e + pair formation: Stephen Hawking has drawn attention to an effect which shows that black holes do not have to exist permanently: Finds an electron-positron pair formation near the Schwarzschild radius instead, one of the two newly formed particles could be drawn into the black hole. The mass of the black hole would have to decrease by the mass of the free particle until the black hole disappears after a lot of such processes after about 10 67 years. Fig. 92.1 The “quartz wills” (A, B) could be caused by a gravitational lens. Orbit of the neutron star or black hole normal star Neutron star or black hole Fig. 92.2 Black holes reveal themselves through interactions with the “neighborhood”. Black holes If the star's mass is greater than 2.5 solar masses, the collapse of a star under its own gravity can no longer be stopped by the quantum pressure of the neutrons. The radius of the star is getting smaller and smaller. Let us consider the escape speed v (r): v r r G M 2 2 \$ \$ = ^ h v (r)… escape speed G… gravitational constant M… mass of the star r… distance from the center With decreasing r, v (r) increases. According to chap. 20 we know that v cannot exceed the value c 0. With decreasing r a limit R s is reached, for which the following applies: cvr RGM 2 s 0 2 2 \$ \$ = = ^ h Objects whose distance r to the center is less than R s cannot even move at the speed of light of the gravitation of the Escape from the star. In particular, light from the areas within a sphere with a radius R s cannot reach the outside either. This limit radius R s is called the Schwarzschild radius. A1 Calculate the Schwarzschild radius R s for the following objects: a) people, b) trucks (estimate ≈ 50 t), c) house (1000 t), d) moon, e) earth, f) sun, g) white dwarf and h) neutron star! What can be concluded from the results? Note: The ratio R s: R determines the effects of general relativity. R s… Schwarzschild radius of an object R… radius of the object A2 Give the order of magnitude of the ratio R s: R for the objects from A1! For which objects will the results of general relativity be essential? A3 Read in Chapter 20.5.5 about particle-antiparticle pair formation! For testing purposes only - property of the publisher öbv