later on they say “Currently, the value of m_e has an uncer-tainty of 3.0 parts in 10^10, while the uncertainty in the Rydberg constant is 1.9 part in 10^12. These uncertainties are several orders of magnitude larger than the present realizations of the unit of time of the current SI system (few parts in 10^16) and even further away from the capabilities of optical frequency standards (10^−18 or better). Consequently, Option 3 is not practical in the current state of science and technology.”