> there is an elevator that works with electromagnetic force and not with=
> gravity.
>
> How does it differ from Einstein's elevator when it is restrained and
> when it descends in free fall?
>
> [[Mod. note -- *IF* the ratio electromagnetic_force / mass is the
> same for the elevator and the test mass ("robot"), then this is indeed
> exactly analogous to an Einstein elevator. If this ratio differs, then
> the weight force won't be zero when the constraint is removed (i.e.,
> when the elevator is free-falling).
>
> For gravity, Einstein's (weak) principle of equivalence says that the
> corresponding ratio gravitational_force / mass is indeed the same for
> *all* objects, so Einstein's elevator automatically gives zero weight
> when its constraint is removed (i.e., when the elevator is free-falling=
).
> -- jt]]
The analogy I was referring to is the following.
Einstein says: "In the elevator stopped at the floor there is the force
of gravity that pushes the man against the floor. I prove you that it
is not a real force. And the proof is this: if the cable breaks and the
elevator goes into free fall, in this accelerated reference the force
of before is no longer there and, therefore, it was a fictitious
force".
In the case of my animation, in the elevator stopped at the floor there
is the electromagnetic force that pushes the robot against the floor. I
prove to you that it is not a real force. And the proof is this: if the
cable breaks and the elevator goes into free fall, in this accelerated
reference the force from before is no longer there and, therefore, it
was a fictitious force.
Is this statement the same as Einstein's?
[[Mod. note -- Einstein's statement implicitly assumed the weak
equivalence principle, i.e., it is based on the observation that in
a uniform gravitational field, *all* objects free-fall at the same
rate (and thus there is no relative motion between the free-falling
elevator and the free-falling objects in the elevator). It's this
*universality* of free-fall that's the key precondition for Einstien's
argument.
*IF* it were the case that all objects had the same ratio
electromagnetic_force / mass , then the electromagnetic-elevator
situation would be exactly analogous to the Einstein-gravitational-
-elevator situation.
But, in reality different objects have very different
electromagnetic_force / mass ratios (e.g., consider objects made of
copper vs objects made of wood vs objects made of iron). This means
that in a free-falling electromagnetic elevator there may still be
(a lot of) relative motion between different objects, i.e., "free fall"
in an electromagnetic field is *not* universal.
-- jt]]