DOES THE INERTIA OF A BODY DEPEND UPON ITS ENERGY-CONTENT?

DOES THE INERTIA OF A BODY DEPEND UPON ITS ENERGY-CONTENT?

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 The fact that the energy withdrawn from the body becomes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 L/9 × 1020, the energy
being
measured in ergs, and the mass in grammes.

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.

If the theory corresponds to the facts, radiation conveys inertia
between the
emitting and absorbing bodies.

Very truly yours,

Albert Einstein