![]() If the Sun could then shrink just a little more, even that one remaining light beam would no longer be able to escape. All others would fall back onto the star (Figure 24.14). As gravity increases (as in the collapsing Sun of our thought experiment), the curvature gets larger and larger.Įventually, if the Sun could shrink down to a diameter of about 6 kilometers, only light beams sent out perpendicular to the surface would escape. what is gravity according to theory of general relativity? General relativity tells us that gravity is really a curvature of spacetime. black hole An object in space whose gravity is so strong not even light can escape. ![]() In the last picture, the astronaut is just outside the sphere we will call the event horizon and is stretched and squeezed by the strong gravity. Note that the size of the astronaut has been exaggerated. Eventually the mass collapses into so small a sphere that the escape velocity exceeds the speed of light and nothing can get away. As the same mass falls into a smaller sphere, the gravity at its surface goes up, making it harder for anything to escape from the stellar surface. formation of a black hole At left, an imaginary astronaut floats near the surface of a massive star-core about to collapse. An object with such large escape velocity emits no light, and anything that falls into it can never return. If the speed you need to get away is faster than the fastest possible speed in the universe, then nothing, not even light, is able to escape. Ultimately, as the Sun shrinks, the escape velocity near the surface would exceed the speed of light. Suppose we continue to compress the Sun to a smaller and smaller diameter. When the shrinking Sun reaches the diameter of a neutron star (about 20 kilometers), the velocity required to escape its gravitational pull will be about half the speed of light. compressing the sun If the Sun is compressed, its mass will remain the same, but the distance between a point on the Sun's surface and the center will get smaller Thus, as we compress the star, the pull of gravity for an object on the shrinking surface will get stronger and stronger escape velocity The velocity an object must reach to fly beyond a planet's or moon's gravitational pull. The distortion of spacetime is greater near the surfaces of these compact, massive objects than near the surface of the Sun. The paperweight in our analogy might be a white dwarf or a neutron star. Something with a mass like our Sun's was necessary to detect the effect Einstein was describing A white dwarf, with its stronger surface gravity, produces more distortion just above its surface than does a red giant with the same mass How large does a mass have to be before we can measure a change in the path followed by light? Einstein first proposed his theory, no distortion had been detected at the surface of Earth (so Earth might have played the role of the grain of sand in our analogy). Stars produce measurable distortions in spacetime. The amount of distortion in spacetime depends on the mass of material that is involved and on how concentrated and compact it is. As American physicist John Wheeler summarized it: "Matter tells spacetime how to curve spacetime tells matter how to move." When something else-a beam of light, an electron, or the starship Enterprise-enters such a region of distorted spacetime, its path will be different from what it would have been in the absence of the matter. This curving of spacetime is identified with gravity. ![]() Spacetime the inseparable, four-dimensional combination of space and time how does spacetime work? The gist of Einstein's general theory is that the presence of matter curves or warps the fabric of spacetime.
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