Swartzchild discovered that any mass compressed into a small enough space creates what we today call...

• A. A boundary
• B. A white dwarf
• C. A black hole
• D. A pulsar

Answer: (C)

The main idea is how gravity in general relativity can trap light if a mass is squeezed small enough. Schwarzschild showed that for a non-rotating mass there’s a specific radius, the Schwarzschild radius, inside which all of the mass would be hidden from outside observers because not even light can escape. When matter is confined within that radius, the spacetime geometry forms an event horizon, and the object becomes a black hole. That’s why the best choice is black hole.

Why the other ideas don’t fit: a boundary by itself isn’t a physical object with this threshold behavior; a white dwarf is a stable stellar remnant supported by electron degeneracy pressure and does not arise simply from crushing matter to a small size, at densities far below the Schwarzschild threshold; a pulsar is a rapidly spinning neutron star that emits beams due to strong magnetic fields, not the result of compressing all mass into a region where light cannot escape. The key point is that crossing the Schwarzschild radius leads to a region from which nothing can escape, which defines a black hole.