Re: 4

From: Bill Hobba (bhobba_at_rubbish.net.au)
Date: 02/15/05 

Date: Tue, 15 Feb 2005 00:13:12 GMT

"Mike" <eleatis@yahoo.gr> wrote in message
news:1108412967.036928.219010@g14g2000cwa.googlegroups.com...
>
> Sam Wormley wrote:
> > St. M wrote:
> > > An accelerating spaceship gives the
> > > astronauts the sensation of weight. Can
> > > we say that this accelerating motion
> > > simulates gravity or is it real gravity
> > > that is being produced?
> > >
> >
> > No experiment can tell the difference in a closed environment...
>
> I guess you are pretty sure you know all the kind of experiments that
> can be performed.
>
> That is locally, not "closed". This terminology you are using is not
> correct. Accelerated motion and gravitational free-fall are
> indistinguishable locally, that is in a infinitesimally flat region of
> curved spacetime. Coordinate based experiments, such as laser ranging
> for instance, can differentiate between accelerated motion and
> gravitational free-fall unambiguisly.

?????????? - I would like to see the detials of those experiments. The
problem is examined in detail in the first chapter of Ohanian and Ruffini -
Gravitation and Space-time. It is possible to unambiguously tell
acceleration from gravity by the existence of tidal forces. Ohanian and
Ruffini describe an experiment using water drops that will always tell if
they are present - even in principle locally. The limiting factor is
quantum effects from which they conclude in principle it is possible - even
locally - to distinguish gravity from acceleration. But not all (including
myself) necessarily agree. It is possible to have a configuration of masses
such that in a finite region gravity is constant. So for a closed finite
region it is possible that you can not tell the difference. Caveat I am not
so sure if a GR analysis of those special configurations will not show it is
in fact not constant ie space-time is curved - I have never seen an analysis
of such. Others may like to comment. But the problem is GR is concerned
about more than such special configurations - it is a theory applicable to
any mass/energy distribution.

> Same can be achieved using
> inertial sensors such as test masses attached to spings.

Again I would like to see the detail of those experiments.

>
> That is, the EP in GR is a postulate.
>

It certainly is - but a postulate with strong experimental support.

> It is a principle that cannot be verified experimentally

It can and has been. Indeed Uncle Al is conducting an experiment to test it
as we speak.

> but the theory can be corroborated
> experimentally by measuring its predictions, i.e. the Universallity of
> Free Fall (UFF).

That is the EEP is different words.

>
> When people say that the EP has been confirmed experimentally they
> really mean its predicted effects have been demonstrated to hold to a
> high accuracy, in the order of some parts in a trillion or so and
> increasing.

The same is true of any test of any physical law - we test the experimental
consequences of the assumption.

>
> It is important though to understand that this experimental
> corroboration does not imply the EP is a law of nature, such assertion
> would be a logical fallacy. It simply corroborates the theory using the
> EP postulate in its axiomatic basis.
>

The EEP is a law of nature on exactly the same footing as any law - it is
true until experimentally shown otherwise. And even then may still be true
in a limited domain of applicability.

>
> One could further assert that all experiments that claim to confirm the
> weak EP version of GR actually confirm the EP version of Newton
> (equivalence of inertial and gravitational mass) while only corroborate
> GR. What about that?

As usual you are very confused about the scientific process. Of course
saying such will simply illicit your usual incoherent diatribe.

Bill

>
> Mike
>

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