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How the system starts behaving as a fluid so soon is not fully understood.","posed_since":"","precise":"Determine how and when the energy-momentum tensor of matter formed in $A+A$ collisions reaches viscous-hydrodynamic form (hydrodynamization) while the pressure anisotropy $P_L/P_T$ is still far from 1, at proper times $\\tau$ about $0.5\\ \\text{to}\\ 1\\ \\mathrm{fm}/c$. Answer: a QCD-based mechanism (attractor solutions, weak-coupling kinetic theory, or strong-coupling holography) with a predicted hydrodynamization time as a function of $\\eta/s$ and energy density, tested against data.","problem_ref":null,"references":"","settled_by":"QCD effective kinetic theory simulations with realistic transverse geometry, compared with photon and dilepton data sensitive to the early pressure anisotropy.","status_note":"Hydrodynamic attractor solutions found since 2015 (Heller and Spalinski) explain early hydrodynamic behavior in simplified expansions; the full three-dimensional QCD case remained open as of 2026.","title":"How the quark-gluon plasma becomes a fluid so 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