# Confusion with the equivalence principle and inertial frames

This is an explanation of the equivalence principle. A. says accelerating frames can simulate gravity. Then, I am confused. Are the frames presented in A. inertial frames? Compared to B. I have the impression that the frames in A. are not inertial frames. Then, does it mean that any observer standing on earth is not an inertial observer? But this is contrary to my common sense... Also, the Einstein's postulate says that the speed of light is same in all inertial frames. Then, the speed of light measured from frames in A are not equal to $c$?

• There are some significant issues with the principle of equivalence Keith. Einstein said the special theory of relativity is “nowhere precisely realized in the real world”. It’s only valid “in the infinitesimal”. Your room has to be an infinitesimal room for the principle of equivalence to be exactly valid. See what John Synge said on pages ix and x in his 1960 preface to relativity: the general theory. Commented Apr 9, 2018 at 17:09
• Then, does it mean that any observer standing on earth is not an inertial observer? But this is contrary to my common sense... Common sense is not a good guide to what is correct in physics.
– user4552
Commented Apr 9, 2018 at 20:35

Are the frames presented in A. inertial frames?

No, the frames are accelerated reference frames - an accelerometer at rest in either frame reads non-zero acceleration - and not inertial reference frames.

Then, does it mean that any observer standing on earth is not an inertial observer?

Yes, an accelerometer attached to an observer at rest on the surface of the Earth reads non-zero acceleration and so the observer's reference frame is an accelerated reference frame and not an inertial reference frame.

Then, the speed of light measured from frames in A are not equal to c?

You might find the answers here useful: Does the speed of light vary in non-inertial frames?

An inertial reference frame is a reference frame where an object not subjected to a force is at rest or moves with uniform velocity. The Einstein equivalence principle in its basic formulation states that the gravitational force as experienced locally is not distinguishable from the force experienced in a non-inertial (accelerated) reference frame.
Locally the speed of light is $c$. Locally means measured by an inertial reference frame instantaneously at rest with the accelerated frame.