Physics
Black Holes
Quick fact
The largest black holes, called supermassive black holes, can have masses billions of times that of our Sun and sit at the centers of most galaxies.
Why this is interesting
You know how if you throw a ball upwards, it comes back down? Imagine a place where even light cannot escape—where gravity is so strong it bends space and time. What lies inside that darkness?
Read the full explanation
Understanding Black Holes
A black hole is a region of spacetime with gravity so intense that nothing—not even light—can escape. They form when a massive star (more than about 20 times the Sun's mass) runs out of nuclear fuel and its core collapses under its own gravity. If the core's mass exceeds roughly three solar masses, it collapses into a black hole. The outer boundary is the event horizon—once anything crosses it, it is forever hidden. At the center is a singularity, a point of infinite density where our laws of physics break down. In a simple analogy, imagine gravity as a well in a sheet of rubber: a black hole is an infinitely deep well whose sides are so steep that even light (moving at top speed) cannot climb out.
A deeper explanation
The mechanism behind black holes is described by Einstein's general relativity: mass and energy warp spacetime. When a large amount of mass is compressed into a tiny volume, the curvature becomes extreme. The event horizon corresponds to the distance from the singularity where the escape velocity equals the speed of light; this distance is the Schwarzschild radius. Inside, all paths lead inward, and time and space swap roles. Black holes are not cosmic vacuum cleaners—they only exert strong gravity at close range. They matter because they are the ultimate laboratories for testing general relativity and quantum gravity. Observations of stars orbiting the supermassive black hole at our galaxy's center and the detection of gravitational waves from merging black holes confirm these predictions and continue to reshape our understanding of the universe.