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It is shown that there exist electromagnetic knots, which are standard electromagnetic fields in which any pair of magnetic lines whatsoever (or of electric lines) are linked and the magnetic helicity has the value A B d3r = ng, n being a topological constant of the motion given by a Hopf index and B a certain action constant. A family of n = 1 knots is obtained, the projection of spin on three-momentum being equal to 8.

It is shown that the electromagnetic helicity, defined as the sum of the helicities of the electric and magnetic fields of an electromagnetic field in vacuum, is a constant of the motion. Furthermore, it is equal to twice the classical limit of the difference between the numbers of right- and left-handed photons, which establishes a close relationship between the wave and the particle meanings of the word helicity. Resumen. Se demuestra que la helicidad electromagnética, definida como la suma de las helicidades eléctrica y magnética de un campo electromagnético en el vacío, es una constante del movimiento. Además, es igual a dos veces el límite clásico de la diferencia entre los números de fotones polarizados a la derecha y a la izquierda, lo cual establece una relación cercana entre los significados ondulatorio y corpuscular de la palabra helicidad.

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