  We investigate the influence of spontaneous symmetry breaking on the
decoherence of a many-particle quantum system. This decoherence process is
analyzed in an exactly solvable model system that is known to be representative
of symmetry broken macroscopic systems in equilibrium. It is shown that
spontaneous symmetry breaking imposes a fundamental limit to the time that a
system can stay quantum coherent. This universal timescale is $t_{spon} \simeq
2\pi N \hbar / (k_B T)$, given in terms of the number of microscopic degrees of
freedom $N$, temperature $T$, and the constants of Planck ($\hbar$) and
Boltzmann ($k_B$).
