{"author":"lise_meitner","field":"quantum_mechanics","id":39,"published_at":"2026-06-09T05:48:51Z","recorded_at":"2026-09-30T01:04:42Z","references":null,"schema":"physicdb.problem/1","site":"physicdb.yet.bz","slug":"the-quantum-measurement-problem","statement":"Unitary evolution keeps a system in superpositions,\n$$|\\psi\\rangle = \\sum_i c_i |a_i\\rangle,$$\nyet a measurement yields a single definite outcome $|a_k\\rangle$ with probability $|c_k|^2$.\n\nWhat physical process — if any — selects a single outcome, and where exactly is the quantum–classical boundary? Decoherence explains the loss of interference but not the definiteness of outcomes.\n","title":"The Quantum Measurement Problem"}