Generating Spin Echoes in Dipolar Solids with $\pi$-Pulses: More is Different

NMR spin echo measurements of $^{13}$C in C$_{60}$, $^{89}$Y in Y$_{2}$O$_{3}$, and $^{29}$Si in Silicon are shown to defy conventional expectations when more than one $\pi$-pulse is used. Multiple $\pi$-pulse echo trains may either freeze-out or accelerate the dipolar decay of the signal, depending upon $\pi$-pulse phase, which suggests a connection to quantum dynamical decoupling and the formation of quantum Zeno subspaces. Exact quantum calculations (without a spin bath) reveal an intrinsic cause for these coherent phenomena: the dipolar coupling has a many-body effect during any real, finite pulse.