Reference: Einstein’s 1920 Book
Section II (Part 1)
The System of Co-ordinates
Please see Section II at the link above.
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Summary
In reality, the body of reference is the rigid body with the point with which the position of an object or an event coincides. A length is correct only when it is measured directly with the use of a rigid rod of standard length.
Distance to locations that are far from the body of reference can be determined by means of optical observations according to the rules and methods laid down by Euclidean geometry, and taking into account the properties of the propagation of light.
We may apply the Cartesian system of co-ordinates to describe a position in space. Such a system shall consists of three rigid plane surfaces perpendicular to each other and rigidly attached to the body of reference. Perpendiculars can be dropped from that position to those three plane surfaces to describe it by means of three coordinates.
The measurements are generally made by indirect means as described above. However, the physical meaning of specifications of position must always be sought.
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Comments
“Every description of events in space involves the use of a rigid body to which such events have to be referred. The resulting relationship takes for granted that the laws of Euclidean geometry hold for ‘distances’, the ‘distance’ being represented physically by means of the convention of two marks on a rigid body.” ~Einstein
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