Magnetism is intimately related to special relativity. Maxwell’s equations are invariant under a Lorentz transformation; the electromagnetic wave equation gives the speed of light c. Many have explained magnetic force as a consequence of relativistic length contraction, for example Richard Feynman in page 13-8 of his ≪The Feynman Lectures on Physics, Volume II≫ and Steve Adams in page 266 in his ≪Relativity: An Introduction to Spacetime Physic≫. If magnetic force is really created by relativistic length contraction, we should be able to derive the expression for magnetic force from the length contraction formula. And indeed we can, as I will show below.
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