But how can we talk about the "surface" of a black hole?
The German astronomer Karl Schwarzschild (1873-1916) first calculated what is now called the Schwarzschild radius of a black hole. This value is the radius at which escape velocity would equal the speed of light (and, therefore, the radius within which escape is impossible) for a star of a given mass. For the earth, the Schwarzschild radius is the size of a garbanzo bean, about 0.4 inches, or 1 centimeter. For a neutron star at 3 solar masses the Schwarzschild radius is 5.58 miles (9 km). So this radius doesn't define a literal "surface" so much as a characteristic property of a black hole.
Remember that the collapse of a black hole is in some sense infinite. Our three-or-more solar-mass stellar core will not stop shrinking just because it has reached the Schwarzschild radius. It keeps collapsing. Once it is smaller than the Schwarzschild radius, however, it will effectively disappear. Its electromagnetic radiation (and the information that it carries) is thereafter unable to escape. We spoke earlier about electromagnetic radiation carrying energy and information. Since we cannot get radiation from within the Schwarzschild radius, we cannot get any information from there, either. Events that occur within that radius are hidden from our view. For this reason, the Schwarzschild radius is also called the event horizon. We cannot see past this ultimate horizon.
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