Re: the basis of relativity


"Baugh" <baconbaugh@xxxxxxxxxxx> wrote in message
news:UP3le.784$fe2.313@xxxxxxxxxxx
> Let me clearify further. My analogy with regard to electrostatic
> potential was aimed at the point of the relativity being broken by
> fixing an aspect of the theory by convention. This was in response
> to your claim that the theory contradicted itself in its practice.
>
> With regard to the theory of gravity as I described it. The
> point is that the equivalence principle states that you
> can either treat gravity as a "real" force or as a pseudo-force
> or as a hybrad of the two. You can't distinguish between
> a "real" gravitational force and a pseudo-force due to curvature.
> (curving time coordinates is equivalent to accelerating the frame.)
>
> It is not completely correct to say gravity is "just geometry" rather
> one should say gravity is indistinguishable from geometry.
> It is a subtile but possibly important distinction.
>
> Take some solution to Einstein's equations, then perturb the geometry
> but at the same time "add by hand" an additional field of forces in
> such a way that the combination predicts particles
> will follow the original paths.

Baugh I must admit I find your responses fascinating, a bit different, and
insightful. Often I read them and think I am not so sure about that - then
I think a bit more and say he has a point. But with regard to the above
even after thinking about it a bit I am not so sure. I am thinking of
Kretchmans objections to the principle of general covariance - he showed it
had no physical basis - any law can be put in covariant form Which is why
we need to add in the requirement of invariance - namely all the absolute
terms appearing in the equations remain unchanged (Ohanian - Gravitation and
Space-Time - page 374). Doing what you suggest would seem to violate this
requirement - or am I missing something?

Thanks
Bill

>You have the same theory with
> slightly changed metaphysical interpertation. Since it is redundant
> it is just as well to only work with purely geometric form.
> But it is by no means an affirmation of metaphysical facts.
>
> You can look at the perturbative analysis of gravity waves as an example
> of a hybrid description of both geometric and dynamic components to the
> gravitational field. You can also look at it as simply "all geometry"
> but treated perturbatively which is the usual "interpretation".
> The point is that neither "interpretation" is a true interpretation.
> The true interpretation is that test particles will go "that-a-way"
> in the presence of matter distributions as predicted by the theory.
>
>
>
>
> Neil G wrote:
> > Baugh wrote:
> >
> >>Neil G wrote:
> >>
> >>>relativity is based on the equivalent principle,
> >>>then the developed relativity shows that the equivalent
> >>>principle is wrong
> >>
> >>You misunderstand. The equivalence principle states that
> >>given a dynamic gravitational field ("real force") in
> >>a given geometry you can alter the geometry and alter the
> >>dynamic force to yield an equivalent predictive theory.
> >>
> >
> >
> > are the "real forces" considered "dynamic gravitational fields"?
> >
> > I thought that there was a huge difference between gravitational and a
> > Newtonian force
> >
> >
> >>Given this then you can *by convention* choose a geometry
> >>in which the dynamic force goes away and in that choice of
> >>geometry the gravitational force is just geodesic evolution.
> >>
> >
> >
> > "geodesic evolution" means no Newtonian forces?
> >
> >
> >>It is similar to saying you can set "zero electrostatic potential"
> >>to be at any point you like. When doing problems you set the ground
> >>of your device to be at zero volts. That is another "relativity
> >>principle" namely that voltage is relative and thus it is only
> >>meaningful to speak of voltage differences.
> >>
> >
> >
> > thanks, but I still can't see the connection between the two forces
> >
> >
> >>Setting the ground to be zero volts does not mean the original
> >>relativity is wrong, it rather relies implicitly on the relativity
> >>principle being right. Otherwise you'd have to worry about whether
> >>the ground is "really at zero volts".
> >>
> >
> >
> > you sounds convincing, but I still can't understand
> >
> >
> >>These choices of convention are loosly refered to as "gauge
> >
> > conditions".
> >
> >>There is a deep connection between "equivalence principles" and gauge
> >
> >
> >>theories. One is effectively considering a whole class of equivalent
> >>models with an explicit group of equivalence transformations (the
> >
> > gauge
> >
> >>group). One then insists that physical phenomena which one may
> >
> > predict
> >
> >>be independent of the choice of model (choice of gauge).
> >>
> >
> >
> > I think I begin to understand, thanks
> >
> >
> >>This is how Einstein formulated his field equations.
> >>
> >>Note however that in the case of gravitation the purely geometric
> >>formulation leads some to take geometry too seriously as a physical
> >>quality instead of a feature of the formal language. Hence attempts
> >>to quantize gravitation by "quantizing geometry". This I believe
> >>to be the major flaw of quantum string and 'brane' models
> >
> > inaccurately
> >
> >>refered to as "theories".
> >>
> >>--
> >>Regards,
> >>James Baugh
> >
> >
> > I understand now, so the "forces" are actually the same type of forces
> > depending on one's point of view, thanks
> >
>
>
> --
> Regards,
> James Baugh


.

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