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Abstract
We consider the time evolution of nonequilibrium quantum scalar fields in the O(N)
model, using the next-to-leading order 1/N expansion of the 2PI effective action.
A comparison with exact numerical simulations in 1+1 dimensions in the classical limit
shows that the 1/N expansion gives quantitatively precise results already for moderate
values of N. For sufficiently high initial occupation numbers the time evolution of
quantum fields is shown to be accurately described by classical physics. Eventually
the correspondence breaks down due to the difference between classical and quantum
thermal equilibrium.