We consider a Lorentzian metric in R R ⁿ. We show that, if we know the lengths of the space-time geodesics starting at (0, y, ) when t=0, then we can recover the metric at y. We prove the rigidity of Lorentzian metrics. We also prove a variant of the rigidity property for the case of null-geodesics: if two metrics are close and if the corresponding null-geodesics have equal Euclidian lengths, then the metrics are equal.
G. Eskin (Sun,) studied this question.
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