We compute the energy tensor and the energy-momentum tensor for electrodynamics coupled to the current of a charged scalar field and for electrodynamics coupled tothe current of a Dirac spinor field, without using the equations of motion.

Asim Barut once,en passant, asked the question “For what transitions of the hydrogen atom do the spectral lines coincide”? It is a pleasure for us to give the answer in this paper.

Starting from the study of the physical world, we develop the concept of a dual pair, Comparing dual pairs is known as duality. We show that duality is a basic mechanism for our intellectual curiosity.