The proton and neutron are in a scheme of categories being at once fermions, hadrons and baryons. The proton carries a positive net charge and the neutron carries a zero net charge; the proton's mass is only about 0.13% less than the neutron's. Thus, they can be viewed as two states of the same nucleon, and together form an isospin doublet (). In isospin space, neutrons can be transformed into protons via SU(2) symmetries, and vice versa. These nucleons are acted upon equally by the strong interaction, which is invariant under rotation in isospin space. According to the Noether theorem, isospin is conserved with respect to the strong interaction.</math>