Re: Apparent weight

["A. R. Marlow" <marlow@loyno.edu>]

On Thu, 19 Feb 1998, Tim Folkerts wrote:

> ... 
> How do other forces, like E&M fit in to GR?  

E&M and nuclear forces fit into GR as real forces that cause deviations
from inertial freefall motion. 

> ...Imagine the earth having a 
> net charge, and a person having enough charge that electrostatics 
> exactly counterbalance gravity (and ignore rotation).  Is the "inertial 
> frame" still the one accelerating downward at -9.8 m/s due to curved 
> space time, or is it the one with no acceleration.  
According to GR the inertial frame is still the freefall frame and the
electrostatic force is causing a deviation from that inertial motion.

> If you choose the first, it would seem you need to talk about THREE 
> fundamental forces, and a "fundamental curver of spacetime."

Yes,  GR recognizes E&M and the nuclear forces as fundamental forces, and
the presence of mass-energy-momentum is the fundamental "curver of
spacetime."

> If you choose the second, then all four force are on an even status: 
> either as "forces" or as "curvers of spacetime."  

This was a dream Einstein was never able to accomplish, but it has been
accomplished in the modern gauqe theory of electromagnetic and nuclear
forces, but at the cost of working in additional dimensions over and above
the four dimensions of spacetime.

> If we want to interpret gravity as "curving spacetime" instead of as a 
> "force", that would be fine.   Both give the same motion, which 
> is the ultimate test.  It is just a question of a MODEL not a 
> question of the PHYSICS.  

But both do not give the same motion, and so it *is* a question of
physics. The geometric model provided by general relativity predicts, for
example, extra precessional motion in the orbits of planets that is not
predicted by Newtonian gravitation.  The extra motion has been
experimentally verified, as has the bending of light by gravitation (also
not predicted correctly be Newtonian gravitation). 

> But then why not choose a model so that the 
> other three forces "curve spacetime" as well?   
>
> Or put another way, why is the "intertial frame" the one where only 
> GRAVITY is acting?  Why not the one where only E&M is acting, or only 
> the strong force is acting?

The basic reason why this cannot be done in only four dimensions is that 
electromagnetic and nuclear forces accelerate differing amounts of their
respective charges in differing ways.  Only gravitation accelerates its
differently "charged" objects (masses) in exactly the same way, so only
gravitation gets "geometrized" in four dimensions.

> To me, putting all four forces on the same level is simpler and more 
> appealing. Of course, experiment, not "appeal" is the final arbitrator. 
> Can experiment point to a fundamental difference between gravity and 
> other forces that justifies giving it this special status?

To me too, which is why I have been motivated to look into gauge theories
of fundamental forces.  They are beautiful and very intriguing.  As
regards the final question, yes, there seems to be  a fundamental
difference between gravity and the other forces as outlined above. 

A. R. Marlow                          E-MAIL:  marlow@loyno.edu
Department of Physics, Box 124        PHONE:   (504) 865 3647  (Office)  
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