Quantum Gravity: General relativity describes gravity on cosmic scales; quantum mechanics governs the subatomic world. But they are mathematically incompatible—especially in extreme environments like black holes or the Big Bang singularity. A theory of quantum gravity is needed to unify them. Leading candidates include string theory (vibrating strings in extra dimensions) and loop quantum gravity (quantized spacetime). Both remain untested experimentally due to the Planck scale’s inaccessibility. Quantum gravity might explain black hole information loss, the universe’s birth, or whether spacetime is fundamental or emergent. Success would complete physics’ foundational framework, potentially revealing a multiverse or new symmetries. Until then, the deepest workings of reality remain split between two brilliant but conflicting descriptions.
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