The minima described above for thermal diffusivity and kinematic viscosity are analogous to Bohr's treatment of the atom. Just as minimal viscosity and thermal diffusivity "turns out to be quantum, " Niels Bohr developed an atomic model that assumes an electron bound to an atom in its ground state could be situated at some distance away from the atomic nucleus, but no closer. That distance became known as the "Bohr radius" 𝑎 0 and its modern formula-definition is 𝑎 0 ≡ ℏ (𝛼𝑚 e 𝑐) ⁄ . In today's SI definition, ℏ and c are defined exactly and 𝛼 and 𝑚 e are known with relative uncertainties of 150 parts per trillion and 310 parts per trillion, respectively .