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The causal boundary of string propagation -- defined as the hypersurface in loop space bordering the timelike(spacelike) domains in which two successive measurements of the string field do(do not) interfere with one another -- is argued to be 0=\∫ d\σ\(\δ X(\σ)\)² = \∑\=-\∞^\∞ \δ x^\μ-\\δ x\μ \. Some possible consequences are discussed.
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Emil J. Martinec (1993) studied this question.
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