The Black Hole Information Paradox exists at the catastrophic intersection of Albert Einstein’s General Theory of Relativity (GR) and the laws of quantum mechanics (QM). GR predicts the black hole, defined by its event horizon (the "point of no return") and governed by the "no-hair theorem" which suggests complex information is rendered permanently inaccessible to the outside universe. Conversely, QM is built upon the mathematical principle of unitarity, which dictates that the complete quantum state (information) of a closed system can never be truly created or destroyed; it can only be transformed. Violation of unitarity is considered a collapse of physics, implying a breakdown of the conservation of energy.
The paradox was catalyzed by Stephen Hawking's 1974 discovery that black holes must emit Hawking radiation and therefore evaporate completely over vast timescales. His calculations suggested this radiation was purely thermal, meaning it was a random, high-entropy "mixed state". The core problem—the "pure-to-mixed" problem—is that a complex, ordered object (a "pure state") falling in and subsequently evaporating into only featureless, thermal radiation fundamentally violates the law of unitarity.
The conflict developed into a four-decade intellectual debate, known as "The Black Hole War". The paradox was sharpened by the work of Don Page, who showed that the conflict between Hawking's calculation and unitarity occurs when the black hole is still enormous (at the Page Time). This led to the 2012 AMPS Firewall paradox, which demonstrated that assuming unitarity requires sacrificing the "smooth" event horizon mandated by Einstein's Equivalence Principle, due to a violation of the "monogamy of entanglement".
The overwhelming modern consensus is that information is conserved, asserting that unitarity is victorious. This resolution emerged from breakthroughs like the Holographic Principle (which posits that information describing a 3D volume is encoded on its 2D boundary) and the 2019 successful calculation of the Page Curve using semiclassical gravity. This calculation required introducing new spacetime configurations called "replica wormholes" and their associated "islands"—regions inside the black hole that are mathematically considered part of the distant radiation system. The result not only reproduces the required entropy curve but also resolves the firewall paradox by saving the Equivalence Principle. The resolution suggests a profound synthesis, where the geometry of spacetime (GR) is- fundamentally built from the non-local threads of quantum entanglement (QM).