(a) what we call the curvature of 4D spacetime can be regarded as the result of embedding it in an x4-dependent 5D manifold; (b) the sign of the 4D curvature depends on the signature of the 5D metric, which here is +(−−− )± and admits the twotime option; (c) the magnitude of the 4D curvature depends strongly on the scalar field or the size of the extra dimension (g44 =Φ2), so while it may be justifiable to neglect this in astrophysics(where the 4D curvature is small) it can be crucial in cosmology and particle physics. The 4D energy–momentum tensor that follows from (3)–(5) is