Re: the basis of relativity


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

.

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