dipole selection rule removal of apparent inconsistency there is now an apparent inconsistency in the article as to why no spectrum is observed in molecules with no dipole the intro says that a permanent non zero electric dipole moment is required to have a spectrum but the spherical top section says that spherical tops have no spectrum because there is no change in dipole moment with rotational quantum number a reader may wonder why change in dipole is not mentioned in the intro i understand that this inconsistency is only apparent because the statement in the intro refers to a molecular frame of reference in which a dipole is constant at least in each quantum state whereas the statement about spherical tops describes the rotation of the dipole vector with the molecule as a change in the lab frame of reference but this is not mentioned in the article and i think it would be unnecessarily complicated to explain properly instead i will change the language in the spherical top section to conform to the intro