The basic idea of this chapter is that we get the correct equations of nature, by minimizing something. What could this something be? One thing is for sure: The object mustn’t change under Lorentz transformations, because otherwise we get different laws of nature for different frames of reference. In mathematical terms this means the object we are searching for must be a scalar, which is an object transforming according to the (0, 0) representation of the Lorentz group. Together with restricting to the simplest possible choice this will be enough to derive the correct equations of nature. Nature likes it simple.
KeywordsOrbital Angular Momentum Product Rule Lagrangian Density Lagrangian Formalism Lorentz Group
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