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Title: How the quark-gluon plasma becomes a fluid so quickly
Plain: Fluid models describe heavy-ion collisions starting about 3e-24 seconds after impact, which seems too early for the matter to have settled down. How the system starts behaving as a fluid so soon is not fully understood.
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.
Settled by: QCD effective kinetic theory simulations with realistic transverse geometry, compared with photon and dilepton data sensitive to the early pressure anisotropy.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: partially-resolved
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.
Posed since: (empty)