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Unformatted text preview: DOES THE INERTIA OF A BODY DEPEND UPON ITS ENERGY-CONTENT? By A. EINSTEIN September 27, 1905 The results of the previous investigation lead to a very interesting conclusion, which is here to be deduced. I based that investigation on the Maxwell-Hertz equations for empty space, together with the Maxwellian expression for the electromagnetic energy of space, and in addition the principle that: The laws by which the states of physical systems alter are independent of the alternative, to which of two systems of coordinates, in uniform motion of parallel translation relatively to each other, these alterations of state are referred (principle of relativity). With these principles * as my basis I deduced inter alia the following result ( 8): Let a system of plane waves of light, referred to the system of co-ordinates ( x, y, z ), possess the energy l ; let the direction of the ray (the wave-normal) make an angle with the axis of x of the system. If we introduce a new system of co-ordinates ( , , ) moving in uniform parallel translation with respect to the system ( x, y, z ), and having its origin of co-ordinates in motion along the axis of x with the velocity v , then this quantity of lightmeasured in the system ( , , )possesses the energy l * = l 1- v c cos p 1- v 2 /c 2 where c denotes the velocity of light. We shall make use of this result in what follows....
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