Hi all! I am reading a book on Classical Electrodynamics (Hehl and Obukhov, Foundations of Classical Electrodynamics, Birkhauser, 2003). In this manual I found the following statement: ============ Consider a 4-dimensional differentiable manifold which is: -connected (every 2 points are connected by continuous curve) -Hausdorff (http://en.wikipedia.org/wiki/Hausdorff_space#Definitions) -orientable (http://en.wikipedia.org/wiki/Orientability#Orientation_of_differential_manifolds) -paracompact (manifold covered by finite number of coordinate charts) This manifold always has a foliation by 3-dimensional hypersurfaces (each hypersurface is a hypersurface of constant "time"). ============ Does anyone know another reference which confirms this statement? In other words, do you know why a connected Hausdorff orientable paracompact manifold always has such foliation? Any help is massively appreciated! Cheers!