Abstract
The following four statements for indefinitemetrics of Lorentz signature do not possess an analogousstatement in the definite Euclidean signature case: (1)All curvature invariants of a gravitational wave vanish, in spite of the fact that itrepresents a nonflat spacetime. (2) The eigennullframecomponents of the curvature tensor (the Cartan“scalars”) do not represent curvaturescalars. (3) The Euclidean topology in the Minkowskispacetime doesnot possessa basis composedofLorentzinvariant neighborhoods. (4) There are pointsin the de Sitter spacetime which cannot be joined toeach other by any geodesic. We show that these fourstatements all follow from the noncompactness of theLorentz group. We conclude that the popular (and oftenuseful) imaginary-coordinate rotation from Euclidean to Lorentzian signature (called Wick rotation) isnot an isomorphism.
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Schmidt, HJ. Consequences of the Noncompactness of the Lorentz group. International Journal of Theoretical Physics 37, 691–696 (1998). https://doi.org/10.1023/A:1026660211608
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DOI: https://doi.org/10.1023/A:1026660211608