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42
Relativity: The Special and General Theory
If now, as we find from experience, the acceleration is to be independent of the nature and the
condition of the body and always the same for a given gravitational field, then the ratio of the
gravitational to the inertial mass must likewise be the same for all bodies. By a suitable choice of
units we can thus make this ratio equal to unity. We then have the following law: The
gravitational mass of a body is equal to its inertial maw.
It is true that this important law had hitherto been recorded in mechanics, but it had not been
interpreted. A satisfactory interpretation can be obtained only if we recognise the following fact :
The same quality of a body manifests itself according to circumstances as " inertia " or as " weight "
(lit. " heaviness '). In the following section we shall show to what extent this is actually the case,
and how this question is connected with the general postulate of relativity.
Next: The Equality of Inertial and Gravitational Mass as an argument for the General Postule of
Relativity
Relativity: The Special and General Theory