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In his Nobel lecture (https://www.nobelprize.org/prizes/physics/1965/feynman/lecture/) Richard Feynman states that by varying the Schwarzschild-Tetrode-Fokker direct interparticle action
$$A=-\sum_i m_i\int\big(\mathbf{\dot X_i}\cdot\mathbf{\dot X_i}\big)^{1/2}d\alpha_i+\frac{1}{2}\sum_{i\ne j}e_ie_j\iint\delta(I_{ij}^2)\ \mathbf{\dot X_i}\cdot\mathbf{\dot X_j}\ d\alpha_i\ d\alpha_j\tag{1}$$
where
$$I_{ij}^2=\big[\mathbf{X_i}(\alpha_i)-\mathbf{X_j}(\alpha_j)\big]\cdot\big[\mathbf{X_i}(\alpha_i)-\mathbf{X_j}(\alpha_j)\big]$$
one can reproduce classical electrodynamics without using the concept of the electromagnetic field.
In their paper Classical Electrodynamics in Terms of Direct Interparticle Action (https://cds.cern.ch/record/1062647/files/RevModPhys.21.425.pdf) John Wheeler and Richard Feynman showed in the section Action and Reaction on pages 429-430 that the energy-momentum transferred by retarded forces from particle ##i## to particle ##j## along a null worldline connecting them is equal and opposite to the energy-momentum transferred by advanced forces from particle ##j## back to particle ##i## along the same null worldline. Therefore they had discovered a Lorentz covariant generalization of Newton's principle of action and reaction.
Following the Feynman Lectures vol.1 ch. 28 section 28-2 (https://www.feynmanlectures.caltech.edu/I_28.html) I imagine two stationary particles with charges ##e_1## and ##e_2## separated by a large distance ##r## so that only the radiative electromagnetic forces, which decay as ##1/r##, are relevant.
Let us suppose that I apply a contact force to particle ##1## at time ##t## that gives it an acceleration ##\mathbf{a}(t)## perpendicular to the line joining the two particles.
The retarded electromagnetic force received by particle ##2## at time ##t+r/c##, having been emitted by particle ##1## at time ##t##, is given by
$$\mathbf{F^{21}}(t+r/c)=\frac{-e_1 e_2}{4\pi\epsilon_0 c^2r}\mathbf{a}(t)\tag{2}$$
For simplicity let us suppose that the mass of particle ##2## is very large so that it hardly accelerates at all and therefore does not produce a retarded force back on particle ##1##.
However if the direct interparticle action ##(1)## is a correct description of Nature then there should be an advanced reaction force back on particle ##1## at time ##t##, emitted by particle ##2## at time ##t+r/c##, given by
$$\mathbf{F^{12}}(t)=\frac{e_1 e_2}{4\pi\epsilon_0 c^2r}\mathbf{a}(t)\tag{2}$$
As this back reaction force is proportional to the acceleration then it will manifest itself as an apparent change in the inertia of particle ##1##, ##\Delta m_1##, given by
$$\Delta m_1=\frac{e_1 e_2}{4\pi\epsilon_0 c^2r}\tag{3}$$
This apparent change in inertia of particle ##1## should be detectable. Has any such effect been measured?
$$A=-\sum_i m_i\int\big(\mathbf{\dot X_i}\cdot\mathbf{\dot X_i}\big)^{1/2}d\alpha_i+\frac{1}{2}\sum_{i\ne j}e_ie_j\iint\delta(I_{ij}^2)\ \mathbf{\dot X_i}\cdot\mathbf{\dot X_j}\ d\alpha_i\ d\alpha_j\tag{1}$$
where
$$I_{ij}^2=\big[\mathbf{X_i}(\alpha_i)-\mathbf{X_j}(\alpha_j)\big]\cdot\big[\mathbf{X_i}(\alpha_i)-\mathbf{X_j}(\alpha_j)\big]$$
one can reproduce classical electrodynamics without using the concept of the electromagnetic field.
In their paper Classical Electrodynamics in Terms of Direct Interparticle Action (https://cds.cern.ch/record/1062647/files/RevModPhys.21.425.pdf) John Wheeler and Richard Feynman showed in the section Action and Reaction on pages 429-430 that the energy-momentum transferred by retarded forces from particle ##i## to particle ##j## along a null worldline connecting them is equal and opposite to the energy-momentum transferred by advanced forces from particle ##j## back to particle ##i## along the same null worldline. Therefore they had discovered a Lorentz covariant generalization of Newton's principle of action and reaction.
Following the Feynman Lectures vol.1 ch. 28 section 28-2 (https://www.feynmanlectures.caltech.edu/I_28.html) I imagine two stationary particles with charges ##e_1## and ##e_2## separated by a large distance ##r## so that only the radiative electromagnetic forces, which decay as ##1/r##, are relevant.
Let us suppose that I apply a contact force to particle ##1## at time ##t## that gives it an acceleration ##\mathbf{a}(t)## perpendicular to the line joining the two particles.
The retarded electromagnetic force received by particle ##2## at time ##t+r/c##, having been emitted by particle ##1## at time ##t##, is given by
$$\mathbf{F^{21}}(t+r/c)=\frac{-e_1 e_2}{4\pi\epsilon_0 c^2r}\mathbf{a}(t)\tag{2}$$
For simplicity let us suppose that the mass of particle ##2## is very large so that it hardly accelerates at all and therefore does not produce a retarded force back on particle ##1##.
However if the direct interparticle action ##(1)## is a correct description of Nature then there should be an advanced reaction force back on particle ##1## at time ##t##, emitted by particle ##2## at time ##t+r/c##, given by
$$\mathbf{F^{12}}(t)=\frac{e_1 e_2}{4\pi\epsilon_0 c^2r}\mathbf{a}(t)\tag{2}$$
As this back reaction force is proportional to the acceleration then it will manifest itself as an apparent change in the inertia of particle ##1##, ##\Delta m_1##, given by
$$\Delta m_1=\frac{e_1 e_2}{4\pi\epsilon_0 c^2r}\tag{3}$$
This apparent change in inertia of particle ##1## should be detectable. Has any such effect been measured?