== A model that breaks down == 

 :There is nothing wrong with an observer on a rotataing disk describing his experiences as being under the influence of a centrifugal force.   

 I think the following example is interesting: an observer inside a rotating wheel-shaped space-station. As long as the observer is in physical contact with something inside the space-station, strong enough contact to keep the observer in co-acceleration with the wheel, then what he measures is consistent with being in a pervasive force-field. 

 But when the observer is free-floating inside the rotating space station the force-field model breaks down. The observer can be floating inside the space-station with a velocity of, say, 1 centimeter per second with respect to the center of gravity of the space-station. He will not accelerate away from the center of gravity of the space-station, that velcity will remain constant. (Of course, when he finally does come in contact with the perimeter it will be hazardous, for the perimeter is moving fast in a perpendicular direction.)  

 The 'floating observer' thought experiment illustrates the fact that in the case of rotational motion there is no pervasive alteration of the rate of time, no field. The perimeter is moving faster than the hub and that corresponds to a slower rate of time for objects co-moving with the rotating structure.  

 I am primarily interested in the physical difference between gravitation on one hand, and acceleration and rotation on the other hand. Gravitation is a pervasive alteration of space-time geometry itself, spherically symmetrical around a celestial body. But when a structure is rotating in flat space-time (pulling G's), then the space in and around that rotating structure is still plain old flat space-time.  

  
 I suspect that Einstein was thinking about propagation of electromagnetic radiation in formulating the equivalence principle. Einstein's first quantative prediction was about frequency shift of light, and I think that is not a coincidence. I think the equivalence principle is particularly striking when thinking about propagation and detection of photons.  |