Constructing Time Machines
Department of Physics
University of Wales, Swansea
Swansea, SA2 8PP, U.K.
The existence of time machines understood as spacetime constructions exhibiting physically realized closed timelike curves (CTCs), would raise fundamental problems with causality and challenge our current understanding of classical and quantum theories of gravity. In this paper, we investigate three proposals for time machines which share some common features: cosmic strings in relative motion, where the conical spacetime appears to allow CTCs; colliding gravitational shock waves, which in Aichelburg-Sexl coordinates imply discontinuous geodesics; and the superluminal propagation of light in gravitational radiation metrics in a modied electrodynamics featuring violations of the strong equivalence principle. While we show that ultimately none of these constructions create a working time machine, their study illustrates the subtle levels at which causal self-consistency imposes itself, and we consider what intuition can be drawn from these examples for future theories.
SWAT 02/348 October 2002
Invited review article prepared for Int. J. Mod. Phys. A
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