#### Neddy Bate

**Valued Senior Member**

I think he used the term "equivalence" strictly to mean that all measurements would be the same in the two cases. For example, the lengths and widths of the two groups of yard sticks (those along the circumference, and those along the diameter) would be the same in both scenarios. I think he definitely considered the two scenarios to be different: in one case, you have acceleration with no gravitational field, and in the other case, you have a gravitational field, and no acceleration.

This sounds like the opposite of what Q-reeus just said. He said the rotating and gravitational cases produce different measurements. I don't know which is correct, but it seems like Einstein had the opportunity to say the measurements would be the same in both cases, but I don't recall him doing so specifically.

I also believe that he saw the value in the equivalence principle to be in what special relativity can tell us about general relativity, not the reverse.

Yes, that is what I think as well. SR is built up from its own postulates, in flat spacetime, and should therefore stand on its own in flat spacetime. GR is built up from SR, for the cases where there is not flat spacetime. GR should therefore reduce to SR in the case of flat spacetime.