General Darwin Interaction Energy

Remember that all terms whose time dependence has not explicitly been noted are evaluated at time fi. Eq. (3.238) finally allows us to write the interaction energy without any reference to the retardation time fi2 and to assign all terms a unique order in 1 /c. 

This is the interaction energy of two moving particles derived by Darwin [56]. Darwin, however, started his derivation of the interaction energy of two moving particles from an expression for the scalar and vector potentials of a single 

241) already contains a prefactor of 1/c, and therefore the vector potential is easily obtained to the same order as 

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