Quantum mechanics has an iron rule: information cannot be destroyed. Every process must be reversible in principle. If you burn a book, the information is scrambled into smoke, ash, and heat, but it still exists and could theoretically be reconstructed. Black holes seem to violate this by permanently erasing whatever falls in.

When a black hole evaporates through Hawking radiation, the radiation appears to be perfectly thermal — random, featureless heat with no encoded information about what fell in.