Reference: Einstein’s 1920 Book
Section XVIII (Part 2)
Special and General Principle of Relativity
Please see Section XVIII at the link above.
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Summary
To detect or describe motion we may choose any body as our reference. But, according to the principle of relativity, the general laws of nature must have exactly the same form regardless of the body we choose as our reference.
So far, we have chosen only those bodies as our reference, which are limited to having uniform and non-rotary motion. This provides us only with some special cases of the laws of nature. To obtain the general laws of nature that apply to all cases, we should be able to choose bodies of reference regardless of their state of motion.
As long as the body of reference is moving uniformly, its own motion does not influence the facts of the motion being observed. However, when the body of reference moves non-uniformly, then the facts of the motion being observed also change, and the same laws as before cannot be expected to hold.
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Final Comments
The non-uniform motion of the body of reference brings in additional factors into play that must be taken into account to determine the form of the laws of nature. Such forms would be more general.
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