Re: centrifugal force

[Leigh Palmer <palmer@sfu.ca>]

> Come now fellas. This argument seems to be getting a little harsh.

I hadn't detected that.

> By "agent" he means another body as a source of the force. For
> "centrifugal" force there is no such body, unless you want to believe
> that the rotation of the stars around the Earth causes the effect.

It is generally acknowledged that the "source" of the gravitational
force is the gravitational field at the point in question. If you
believe otherwise then you adopt an action at a distance model.
Such models are cognitively troubling, and there is evidence that
they are unphysical. I have previously cited the ptroblem that
arises if one considers the source of the gravitational force which
acts on the Earth to be the Sun, without the intermediary agency of
the gravitational field. A consequence of this is that the light
from the Sun comes from a different direction than that in which
the gravitational force acts.

> Why are we making a mountain out of this? The definition: "An inertial
> frame is one in which the law of inertia holds" is circular! How do you
> know if the law of inertia holds? You have to decide whether a body is
> force free. How do you do that? Well if it doesn't accelerate? But then
> you are assuming that you are in an inertial frame!

If there is nothing in contact with the body then you are in an
inertial frame; you've got it! Why do you consider that to be
circular?

> It was clear many decades ago that there is no absolute way to decide
> whether you are in an inertial frame.

To whom was this clear? Please cite the enlightened source.
I have just explained exactly how to decide whether or not you
are in an inertial frame.

The only thing that is clear is that there is no *unique*
inertial frame without external reference. Of course the
comoving frame of the Hubble flow is a unique frame, but one
cannot detect the Hubble flow locally, within a closed
laboratory.

> [portion deleted]

> It is from the theoretic point of view arbitrary to decide whether these
> "inertial" forces are real or not, but is useful to do so in order to
> maintain that there is something definite about the idea of inertial
> frames.

Here we part company. If you decide that *any* accelerations
of particles are produced by forces that are, somehow, not real,
then you will have to give me an operational way to decide on
that reality. I maintain that it is not useful to make what you
acknowledge are arbitrary distinctions when no physical
distinctions exist.

> SOme of my research colleagues (relativists mostly) still do not
> agree on whether one can speak in absolute terms of accelerated frames.
> There is a subtle distinction between centrifugal and Coriolis effects
> on the one hand and gravitation on the other,  . . . . but that's enough
> for now.

Ask your colleagues again if they understand what inertial
frames are. All relativists I know do, and they can be
defined locally without difficulty.

Leigh