Inquire about purchasing the book | Table of Contents | Annotated Bibliography | Class on Nature of Mind
These are excerpts and elaborations from my book "The Nature of Consciousness"
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The Principle of
Equivalence Revisited Einsteinenunciated 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.
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