8.3.5 Competing theories
One of the reasons that there has been so much interest in testing general relativity as accurately as possible is that there are some competing theories to Einstein's. These theories generally have the same starting point, but differ in their details. The result of the tests of these competing theories is that the experimental foundations of Einstein's theory are now much stronger than they were when the theory was initially studied.
If we set r1 = 2GM/c2 (the Schwarzschild radius), we find that A2 is infinite, even if r2 is only slightly greater than r1. This means that no electromagnetic energy can escape from within the Schwarzschild radius. We call an object that is contained within its Schwarzschild radius a black hole.
Example 8.2 Schwarzschild radius
Find the Schwarzschild radius for an object of one solar mass.
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