>> After all, we know from Einstein that there is no measurable difference between gravitational acceleration and inertial acceleration (like, say, bumping a se

>> After all, we know from Einstein that there is no measurable difference between gravitational acceleration and inertial acceleration (like, say, bumping a sensor). I think it's better to say that we know it from Newton's laws of motion and gravity - they are older, much easier to understand, and can still explain the principles of the technique.