Hello all. From Laurent – Introduction to Spacetime. Referring to the Hafele and Keating experiment. ---Let us neglect gravitation and picture the earth as a merry-go-round all alone in empty space. The rim of the merry-go-round corresponds to the equator. Let us further assume that two space ships are flying along the rim in opposite directios with such a speed that they circle once relative to the merry-go-round while it rotates one turn. This has the effect that centrifugal forces are felt on board one of the ships but not on board the other one.----- Why!!!!! Obviously some fundamental law or principle is involved. Matheinste.