|
Black holes are the densest objects in space. Their gravity is so strong that nothing, not even light, can escape from within a region around them known as the "event horizon." When a ball is thrown upward from Earth's surface, it slows down and its course is eventually reversed by Earth's gravity. Unlike objects with mass, photons of light cannot slow down. But a beam of light aimed upward from near a massive body loses energy by shifting toward the redder (low energy) part of the electromagnetic spectrum. Within the event horizon of a black hole, photons of light lose so much energy fighting the intense gravity that they cannot escape at all, and so the object looks completely black. If a flashlight were dropped into a black hole its would light get redder and redder until eventually it would fade out entirely.