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Re: operationally inertial frames



Hi --

At 06:15 PM 10/12/99 -0800, John Mallinckrodt wrote:
1>
1>For me, it is because there is no satisfactory operational way to
1>determine whether or not you are in a so-called "inertial frame."

2> I feel
2>liberated by the modern viewpoint which holds that *all* frames are
2>equivalent and that any inertial force is a gravitational force.

And at 07:51 AM 10/13/99 -0500, Joel Rauber seemed to agree with passage
<1>, saying:
3>
3>I should've picked up on this aspect; as I've repeatedly preached this in
3>several other contexts on this list. Mea Culpa


Passage <2> expresses the corret physics. I cannot understand passage <1>;
it seems totally inconsistent with the correct physics.



------ begin parenthetical remark ------
First, a reminder: the literature contains two inconsistent definitions of
"inertial frame".
a) The high-school level literature typically deals with what might be
called "Newtonian inertial frames", such as frames attached to the earth's
surface.
b) The modern physics literature (in particular the general relativity
literature) typically reserves the term "inertial frame" to refer to
"freely falling frames".

In forums such as this list, where there are people with wildly divergent
backgrounds and interests, it is best to be ultra-explicit. I will use the
term "Newtonian frame" for item (a) and "freely-falling frame" for item (b).
------ end parenthetical remark ------



The operational procedure for determining whether you are in an inertial
frame is to do an experiment to detect non-inertial effects. If you get a
null result, you are in an inertial frame, within your chosen level of
accuracy.

Example 1 (Newtonian): it is easy to show that the earth's surface is not
quite a Newtonian frame. Just set up a Foucault pendulum and watch it for
a while. OTOH the Foucault effect and other nonidealities are small, so
for most purposes it is an appropriate approximation to consider the
earth's surface to be a Newtonian frame.

Example 2 (GR): it is operationally quite possible to determine if you are
in a freely-falling frame, to very high accuracy. Take a corner-cube,
shield it from stray electromagnetic fields etc., and drop it. Track its
progress with a laser interferometer. This is not just a
Gedanken-Experiment; it's been done, e.g. by James Faller at JILA.


______________________________________________________________
copyright (C) 1999 John S. Denker jsd@monmouth.com