Piero Scaruffi(Copyright © 2013 Piero Scaruffi | Legal restrictions )
These are excerpts and elaborations from my book "The Nature of Consciousness"
The Principle of Equivalence Revisited
Einstein enunciated the principle of equivalence like this: no local experiment can detect the presence of a gravitational field. We cannot determine whether we are being accelerated in the absence of gravitational fields or we are at rest in the middle of a gravitational field. He knew that this statement is true only if one considers small regions of space and over brief intervals of time. Einstein concluded that gravitation is not really a force: it is a side effect of a different phenomenon.
Free fall (rather than stillness and rather than uniform speed) is the natural state of motion. After all, all objects fall the same way into a gravitational field. An object in free-fall towards a bigger body is actuality moving by inertia. The reason it looks like it is accelerating is that its time scale stretches at an accelerated rate as the object approaches the bigger body. In reality, the falling object is not accelerating at all. There is no force at work. It is just the time scale that changes. The change is in the geometry spacetime, not in the forces operating on the object.
The principle of equivalence basically states that there is no difference between an object in free-fall in a gravitational field and an object accelerating far away from any gravitational field.
This principle of equivalence is obviously false, if nothing else because of the tidal effects of a gravitational field that do not exist for an accelerating object far away from gravitational fields. But it also false simply because there is no such place without gravitational fields: it is just a matter of how small a field we are willing to measure. The key word is "local": no local experiment can detect the presence of a gravitational field; but the definition of "local" seems arbitrary, and, in general, nothing is truly "local". Einstein's principle is true only in an infinitely small region of spacetime. (In fact, it is debatable whether Einstein's very General Relativity complies with the principle of equivalence that inspired it).
However the outcome of the principle of equivalence was the equivalence between gravitation and acceleration.
When viewed from the broader canvas of spacetime, a change in velocity is but a change in orientation, and an acceleration is equivalent to a rotation in spacetime.
The principle of equivalence was just the thought experiment that helped realize how gravitation is not really a force like electromagnetism but has to do with spacetime itself.