Page:The Meaning of Relativity - Albert Einstein (1922).djvu/65

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SPECIAL RELATIVITY
53

where we have written [1]

(48)

The physical meaning of equation (47) becomes evident if in place of this equation we write, using a new notation,

(47a)

or, on eliminating the imaginary,

(47b)

When expressed in the latter form, we see that the first three equations state the principle of momentum; are the Maxwell stresses in the electromagnetic field, and is the vector momentum per unit volume of the field. The last of equations (47b) expresses the energy principle; \mathbf{s} is the vector flow of energy, and the energy per unit volume of the field. In fact, we get from (48) by introducing the well-known expressions for the components of the field intensity from electrodynamics,

  1. To be summed for the indices and .