Reference: Einstein’s 1920 Book
Section XIX (Part 2)
The Gravitational Field
Please see Section XIX at the link above.
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Summary
Current science regards action at a distance as a process impossible without the intervention of some intermediary medium. With the aid of this conception electromagnetic phenomena can be theoretically represented much more satisfactorily than without it, and this applies particularly to the transmission of electromagnetic waves.
We may apply this conception also to the gravitational phenomena. We may regard the earth to produce a field in its immediate neighborhood directly. The intensity and direction of the field at points farther removed from the earth are determined by the law which governs the properties in space of the gravitational fields themselves.
The gravitational field exhibits a most remarkable property. Bodies which are moving under the sole influence of a gravitational field receive an acceleration, which does not in the least depend either on the material or on the physical state of the body.
Inertial mass relates to an object’s resistance to acceleration, while gravitational mass relates to its interaction with gravitational fields. Inertial mass is measured by applying a known force and measuring acceleration (F/a), while gravitational mass is typically measured by comparing gravitational forces using a balance scale. These two masses are found to be equal.
The same quality of a body manifests itself according to circumstances as “inertia” or as “weight” (lit. “heaviness”).
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Final Comments
The gravitational phenomenon is directly related to the property of inertia.
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