> [...] Two test particles with the same initial position and velocity, but different electric charges, can behave quite differently in the same electric field.

> [...] Two test particles with the same initial position and velocity, but different electric charges, can behave quite differently in the same electric field. > One possible response is to say “oh, maybe our notion of force should really be something like F = mj, where j is the jerk, i.e., the third derivative of position”. > I’ve never worked it out in detail, but wouldn’t be surprised if such an approach can be made to work. It might not be exactly what he's looking for, but in Kaluza-Klein theory, charge is related to the velocity of the particle in an unseen 5th dimension. Through some miracle, if you work out general relativity with one dimension extra, the resulting theory bears a striking resemblance to general relativity in the usual four dimensions plus electromagnetism. If you then assume the 5th dimension is extremely small, which would explain why we can't see it, you get quantization of charge for free. https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory