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It is shown that if the universal cover of a manifold M is an open manifold, then two
different fibres of the spherical cotangent bundle ST*M cannot be connected by
a non-negative Legendrian isotopy. This result is applied to the study of
causality in globally hyperbolic spacetimes. It is also used to strengthen a result
of Eliashberg, Kim and Polterovich on the existence of a partial order on
Cont0(ST*M).
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