Re: Centrifugal force

[Leigh Palmer <palmer@sfu.ca>]

I had a really bad crash on my home Mac, and I had to give a talk
at a meeting on Friday, so I'm just catching up. After much hard
work I discovered which of the 147 items in my Extensions folder
was preventing the machine from booting from its internal hard
disk, and everything now seems OK. (Anyone out there know what a
TrueDocDisplayer.dlm file is?)

This minor calamity gave the messages time to accumulate and
prevented this rash fellow from stirring the pot before tasting
all the ingredients. Thus I am finally ready to reply.

Donald said:

> On Fri, 1 May 1998, Leigh Palmer wrote:
>
> > Thanks are due only to Donald Simanek; I just jazzed it up a bit.
>
> I really hate to say this, but so far as I can remember, I had nothing to
> do with this thought experiment or this thread. Someone else deserves the
> credit. When too many levels of quotes are nested, one can easily
> attribute something to the wrong person. Anyway, I like the experiment,
> too.

My error; I hope I didn't upset the true author too much. John?
Was it you I slighted? I'm sorry; please come back! The important
thing is the physics, of course, but I wanted to make it clear
that I was not the clever fellow who thought up this wrinkle.

Hugh wrote:

> Leigh,
>  I agree with everything you wrote above. I was trying to say that
> detecting the convergence (not divergence) of the earth's gravitational
> field by dropping two objects simultaneously and trying to determine
> whether they would move closer together as they fell could just as well
> have been done with the rocket resting on the launch pad. Although
> detection of the convergence of the earth's gravitational field might be
> beyond the capability of the instrumentation, one would expect to detect
> it if sufficiently sensitive instruments were available. But there would
> be no reason to expect the convergence to have any possibility of being
> detected once the earth were whisked away. I think Chuck is concerned
> that the rocket ship might be too large (being of "finite size")for the
> gravitational field to be considered locally uniform in the cases where
> the earth is in close proximity. (I referred to a positive detection of
> the convergence of the earth's gravitational field as a "tidal effect"
> in my previous messages).

As I pointed out in another message it is quite simple to produce a
gravitational field which either converges or diverges, or else is
homogeneous to any hypothetical degree one specifies, over a volume
as large as one wishes, within reason. Since we were talking about
local measurements the question of field gradient did not arise.
The point of Einstein's principle of equivalence is that within the
region over which the field can be considered to be uniform, there
is *no observable difference* between phenomena which might be
attributed to a gravitational force originating outside the frame
and phenomena attributable to an inertial force due to a uniform
acceleration of the frame.

A difference, to be a difference, must make a difference. (I've
also forgotten the attribution for this sage observation.)

Leigh