Spin echoes and Loschmidt's paradox

Isolated systems of many particles often appear to behave irreversibly. though they are in principle dynamically reversible. In some cases this reversibility can be made evident in 'echo phenomena. in which the system is literally restored to a dynamical statc which existed in the past. Whether such echoes are in principle possible for all isolated systems is not known. However. they do not require the absence of interactions among the particles. Thus the inhomogeneaus and 'magic sandwich' echoes are aspects of the same phcnomenon.