The Mystery of Distance
Even more vexing was the fact that astronomers could not tell how far away the GRBs were. They came and went so fast that astronomers never had time to get a good fix on their location on the sky, by looking for some “host” object of known distance. What makes this problem so important is a simple link between an object’s distance and its brightness.
The energy from a GRB can only be measured based on how much light is intercepted at the Earth (this is called the flux of light). The intrinsic energy output of the GRB can’t be measured at its source, since we are not there to make that measurement. Luckily the properties of light are well known. Just as Newton described the force of gravity as obeying an “inverse square law”, growing increasing weaker as distance increases, a similar effect works with light. As a light a pulse travels away from its source, radiating outward in an ever-increasing sphere, the light intercepted at any point on the surface of that sphere drops with the square of distance from the source. Like the headlights on a car, sources of light appear dimmer the more distant they are. The only way to know how intrinsically powerful the headlights are, is to know how far away they are.
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