The main surprise found in 1962 was that at the future null infinity, " u -translations" of the retarded time u to u + α α ( θ θ , φ φ ) at any given direction ( θ θ , φ φ ) are asymptotic symmetry transformations, which were named supertranslations. As α α ( θ θ , φ φ ) can be expanded as an infinite series of spherical harmonics, it was shown that the first four terms (ℓ = 0, 1) reproduce the four ordinary spacetime translations, which form a subgroup of the supertranslations. In other words, supertranslations are direction-dependent time translations on the boundary of asymptotically flat spacetimes and include the ordinary spacetime translations. Loosely speaking, "neighboring" points on the future null infinity with slightly different ( θ θ , φ φ ) coordinates are actually very "far apart" in space such that they are not causally connected. Light rays from one point can't reach another, clocks can not be synchronized, and thus an arbitrary time offset α α ( θ θ , φ φ ) can be added to the clocks in each ( θ θ , φ φ ) direction, viz., a supertranslation. In fact, for any point on the future null infinity with a given ( θ θ , φ φ ) , the only other points on the future null infinity that it is causally connected to are points with the same ( θ θ , φ φ ) coordinates with different u coordinates.