to identify an inertial frame of reference just use the wep as for the eep i would just say that the underlying laws of physics are the same in all inertial frames of reference those laws permit tidal effects to appear in non uniform gravitational fields apr utc i hold the science of physics to be the following the endeavour to account for the whole corpus of observations in a coherent mathematical framework it all begins with observations and observations should always be the final word whenever a principle can be stated in a form that is not committed to any specific theory it should be stated in that uncommitted form the equivalence principle is prime example of that the harvard tower experiment indicates that there is no clash between quantumphysics and gr as far as measureable aspects of physics are concerned at present the formal frameworks of the two theories cannot be reconciled but the measurements agree the principle of relativity of special relativity can be formulated in a theory committed form but einsteins choice of principle is best in all frames of reference that are not measurably accelerated measurement of the velocity of electromagnetic radiation will yield the same value talk apr utc