Recall that the pull of gravity depends on both the mass that is pulling you and your distance from the center of gravity of that mass. Now imagine that we begin to compress the Sun, forcing it to shrink in diameter. The escape velocity from the surface of the Sun is higher yet-618 kilometers per second. Only those objects launched with a speed greater than this escape velocity can get away from Earth. In fact, any object-rocket, ball, astronomy book-that is thrown into the air with a velocity less than 11 kilometers per second will soon fall back to Earth’s surface. A rocket must be launched from the surface of Earth at a very high speed if it is to escape the pull of Earth’s gravity. We want to know what speeds are required to escape from the gravitational pull of different objects. We will examine this situation from two perspectives: first from a pre-Einstein point of view, and then with the aid of general relativity. We saw that if the core’s mass is greater than about 3 M Sun, theory says that nothing can stop the core from collapsing forever. Let’s now apply what we have learned about gravity and spacetime curvature to the issue we started with: the collapsing core in a very massive star. Recognize why the concept of a singularity-with its infinite density and zero volume-presents major challenges to our understanding of matter.Use the concept of warped spacetime near a black hole to track what happens to any object that might fall into a black hole.Discuss why the popular notion of black holes as great sucking monsters that can ingest material at great distances from them is erroneous.Explain the event horizon surrounding a black hole.By the end of this section, you will be able to:
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