Partial creation in de Sitter spacetime is studied in a complex-time WKB approximation. It is found that the particle production parallels the description of particle creation by a collapsing spherical body. The appropriate vacuum for an inflationary early universe is the Hartle--Hawking vacuum; this is also a general feature of expanding spacetimes. The effect of particle production back on the metric is treated in a way that differs from the standard semiclassical approach.

The first observation of dc-electric-field-induced optical rectification is reported. In this process a dc polarization is produced in a medium (in this case nitrobenzene) by the simultaneous presence of dc and optical electric fields. The relation between this process and the Kerr effect is found to be consistent with that predicted by permutation symmetry. A bolometerlike response also seen in these experiments is discussed.

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We show that the Dirac equation in Robertson--Walker (RW) spacetime can be written in a two-dimensional equivalent form that parallels the motion of an electron in a time-varying electric field. Using the effect of particle production, a RW spacetime that describes a universe which contracts and re-expands almost symmetrically and becomes asymptotically static is simulated. The back-reaction problem is considered only through the quantum behaviour of particle production without any reference to Einstein field equations.

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We present a nonperturbative method of calculating the pair production amplitude in a time-varying electric field. The method is a generalization of the complex multiple reflection technique of Knoll and Schaeffer, not in space but in time. We compare the results obtained by this method with standard results.

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