NUT 4-momenta are forever

The asymptotic structure of the gravitational field at null infinity is re-examined by allowing certain potentials to develop "wire singularities", keeping the physical fields smooth. This relaxation of the regularity conditions leads to the introduction of the Newman-Unti-Tamburino (NUT) 4-momentum which is the "magnetic" or the "dual" counterpart of the Bondi-Sachs 4-momentum. It is shown that, unlike the Bondi-Sachs 4-momentum, the NUT 4-vector is absolutely conserved even in the presence of gravitational radiation. Thus, while the gravitational field resembles the nonabelian Yang-Mills fields in its "electric" properties, it is analogous to the abelian Maxwell field in its "magnetic" properties. It is pointed out that gravitational fields with nonvanishing NUT 4-momenta may have a substantial role in quantum gravity even though they are not physically significant in classical general relativity.