A Source Book in Astronomy, 1900–1950

Author: Albert Einstein  | Date: 1905

Albert Einstein "Does the Inertia of a Body Depend upon its Energy Content?" Harlow Shapley Annalen der Physik 18 639–641 1905


By Albert Einstein

The results of my recently published electrodynamic 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 change do not depend on which of two systems of co-ordinates, in uniform motion of parallel translation relatively to each other, these alterations of state are referred to (principle of relativity).

With these principles1 as my basis I deduced inter alia the following result:—

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 ø with the axis of x of the system. if we introduce a new system of co-ordinates (ξ, η, ζ) moving in uniform parallel translation with respect to the system (x, y, z), and its origin of co-ordinates moving along the axis of x with the velocity v, then this quantity of light measured in the system (ξ, η, ζ)—possesses the energy

where c denotes the velocity of light. We shall make use of this result in what follows.

Let there be a stationary body in the system (x, y, z), and let its energy—referred to the system (x, y, z)—be E0. Let the energy of the body relative to the system (ξ, η, ζ), moving as above with the velocity v, be H0.

Let this body send out, in a direction making an angle ø with the axis of x, plane waves of light, of energy ½L (measured relatively to (x, y, z)), and simultaneously an equal quantity of light in the opposite direction. Meanwhile the body remains at rest with respect to the system (x, y, z). The principle of energy must apply to this process, and in fact (by the principle of relativity) with respect to both systems of co-ordinates. If we call the energy of the body after the emission of light E1 or H1 respectively, measured relatively to the system (x, y, z) or (ξ, η, ζ) respectively, then by employing the relation given above we obtain

By subtraction we obtain from these equations

The two differences of the form H — E occurring in this expression nave simple physical meanings. H and E are energy values of the same body referred to two systems of co-ordinates which are in motion relatively to each other, the body being at rest in one of the two systems (namely, (x, y, z)). Thus it is clear that the difference H — E can differ from the kinetic energy K of the body, with respect to the other system (ξ, η, ζ), only by an additive constant C, which depends on the choice of the arbitrary additive constants of the energies H and E. Thus we may place

since C does not change during the emission of light. So we have

The kinetic energy of the body with respect to (ξ, η, ζ) diminishes as a result of the emission of light, and the amount of diminution is independent of the properties of the body. Moreover, the difference K0K1, like the kinetic energy of the electron, depends on the velocity.

Fig. 1. Einstein’s original statement of the mass-energy equivalence.

Neglecting magnitudes of fourth and higher orders we may place

From this equation it directly follows that:—

If a body gives off the energy L in the form of radiation, its mass diminishes by L/c2. The fact that the energy withdrawn from the body becomes energy of radiation evidently makes no difference so that we are led to the more general conclusion that:

The mass of a body is a measure of its energy-content; if the energy changes by L, the mass changes in the same sense by

the energy being measured in ergs, and the mass in grams.

It is not impossible that with bodies whose energy-content is variable to a high degree (e.g. with radium salts) the theory may be successfully put to the test.

1 The principle of the constancy of the velocity of light is of course contained in Maxwell’s equations.


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Chicago: Albert Einstein, "The E=Mc2 Equation," A Source Book in Astronomy, 1900–1950, trans. Harlow Shapley in A Source Book in Astronomy, 1900–1950, ed. Harlow Shapley (Cambridge: Harvard University Press, 1960), 344–346. Original Sources, accessed June 14, 2024, http://www.originalsources.com/Document.aspx?DocID=P4RKQ97S2V2K795.

MLA: Einstein, Albert. "The E=Mc2 Equation." A Source Book in Astronomy, 1900–1950, translted by Harlow Shapley, Vol. 18, in A Source Book in Astronomy, 1900–1950, edited by Harlow Shapley, Cambridge, Harvard University Press, 1960, pp. 344–346. Original Sources. 14 Jun. 2024. http://www.originalsources.com/Document.aspx?DocID=P4RKQ97S2V2K795.

Harvard: Einstein, A, 'The E=Mc2 Equation' in A Source Book in Astronomy, 1900–1950, trans. . cited in 1960, A Source Book in Astronomy, 1900–1950, ed. , Harvard University Press, Cambridge, pp.344–346. Original Sources, retrieved 14 June 2024, from http://www.originalsources.com/Document.aspx?DocID=P4RKQ97S2V2K795.