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Re: is free-fall an inertial frame?

At 01:20 AM 8/18/99 -0400, Stefan Jeglinski wrote:


let me try to quote a
notion that keeps revolving in my head and drives me crazy: "when I'm
in free-fall, I can't feel that it is gravity causing it; to see that
gravity causes the free-fall, I have to stand outside the earth, in
an inertial frame. But free-fall is an inertial frame to begin with;
why then do I need to stand outside in another inertial frame to see
gravity as a "known" force?" I don't seem to have this conflict with
any other force. I seem to be able to identify "pushes" and "pulls"
and friction and drag (etc) and electrostatic force as known forces,
and coriolis and centrifugal forces (etc) as pseudo forces due to
accelerated or rotating frames-of-reference. But gravity itself...

Two answers:

1) The equivalence principle applies to gravity, not to any other field.
For other fields, you can have a small piece of apparatus, half of which
responds to the field while the other half does not.

2) To measure gravity, no *small* apparatus will ever suffice. Think of
the proverbial bug on a beach ball. For small circles, the bug sees no
excess radius. The bug must pace off rather large circles in order to see
the intrinsic curvature. The absolute excess goes like the cube of the
radius; the percentage excess goes like the square.

Therefore, let me argue that when you measure gravity by standing on the
earth, the earth plays *two* roles:
a) it is the source of the gravitational field, and
b) it is large and firm.

Suppose you were in space, five earth-radii away from the earth. No small
apparatus would give you a good measurement of gravity. But if you had a
shell of firm scaffolding five earth-radii in radius around the earth, the
scaffolding would allow you to probe the distant parts of the field and get
a measurement. Note that the scaffolding itself is not a significant
source of gravity.

======

I suspect this doesn't fully answer your question, but maybe it'll help some.