第13章万有引力

The Schwarzschild radius is also called the event horizon of a black hole. We noted that both space and time are stretched near massive objects, such as black holes. Figure 13.31 illustrates that effect on space. The distortion caused by our Sun is actually quite small, and the diagram is exaggerated for clarity. Consider the neutron star, described in Example 13.15. Although the distortion of space-time at the surface of a neutron star is very high, the radius is still larger than its Schwarzschild radius. Objects could still escape from its surface.

However, if a neutron star gains additional mass, it would eventually collapse, shrinking beyond the Schwarzschild radius. Once that happens, the entire mass would be pulled, inevitably, to a singularity. In the diagram, space is stretched to infinity. Time is also stretched to infinity. As objects fall toward the event horizon, we see them approaching ever more slowly, but never reaching the event horizon. As outside observers, we never see objects pass through the event horizon—effectively, time is stretched to a stop.

The first validated neutron star was discovered by Jocelyn Bell Burnell in 1967. Burnell observed a regular radio pulse from an distant object, a phenomenon not previously observed. After further analysis and the discovery of a second pulsar (as the objects would later be called), the researchers and scientific community recognized that the pulses were caused by rapidly rotating neutron stars. At about the same time, a number of other astronomers, including Iosof Shklovsky, observed radio sources that would be proven to be neutron stars. These discoveries renewed interest in black holes. Evidence for black holes is based upon several types of observations, such as radiation analysis of X-ray binaries, gravitational lensing of the light from distant galaxies, and the motion of visible objects around invisible partners. We will focus on these later observations as they relate to what we have learned in this chapter. Although light cannot escape from a black hole for us to see, we can nevertheless see the gravitational effect of the black hole on surrounding masses.

The closest, and perhaps most dramatic, evidence for a black hole is at the center of our Milky Way galaxy. The UCLA Galactic Group, using data obtained by the W. M. Keck telescopes, has determined the orbits of several stars near the center of our galaxy. Some of that data is shown in Figure 13.32. The orbits of two stars are highlighted. From measurements of the periods and sizes of their orbits, it is estimated that they are orbiting a mass of approximately 4 million solar masses. Note that the mass must reside in the region created by the intersection of the ellipses of the stars. The region in which that mass must reside would fit inside the orbit of Mercury—yet nothing is seen there in the visible spectrum. In 2020, UCLA astrophysicist Andrea Ghez and Plank Institute's (Germany) Reinhardt Genzel shared the Nobel prize for the discovery of the supermassive object. They shared the prize with Richard Penrose, whose mathematical models proved that black holes can actually form and align with Einstein's theory. Since their work, the world has actually seen a black hole, thanks to the Event Horizon Telescope. The global network of observatories combined massive amounts of data in order to produce remarkable images of two black holes: first at the center of galaxy Messier 87, and second at the center of the Milky Way, Saggitarious A.

The physics of stellar creation and evolution is well established. The ultimate source of energy that makes stars shine is the self-gravitational energy that triggers fusion. The general behavior is that the more massive a star, the brighter it shines and the shorter it lives. The logical inference is that a mass that is 4 million times the mass of our Sun, confined to a very small region, and that cannot be seen, has no viable interpretation other than a black hole. Extragalactic observations strongly suggest that black holes are common at the center of galaxies.