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Textbooks on classical electrodynamics define two
different kinds of
the electromagnetic energy-momentum tensor called canonical and
symmetric, respectively. The canonical energy-momentum tensor is
derived from the Lagrangian density of electromagnetic fields. This
quantity has several drawbacks which are corrected by a
mathematical trick that yields the physically acceptable symmetric
energy momentum tensor. This state of affairs casts doubt on the
usefulness of the variational principle, which plays a key role
in the present structure of theoretical physics.
The following paper proves that in the case of radiation fields,
the variational principle yields the correct symmetric energy-momentum
tensor. As explains above, this result removes a certain doubt
concerning the physical meaning of the variational principle. For details,
click here
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