Re: Apparent weight

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

On Wed, 18 Feb 1998 Raacc@aol.com wrote:

> ...
> Now I am confused.  In GR, is the gravitational force fictional or not?
> ...

In GR gravitation is very real:  it is the effect of living in and moving
through curved spacetime.  The real geometric curvature of spacetime
cannot be transformed away by any coordinate transformations, say to
accelerated reference frames, so it is about as real as you can get.  In 
regions of strong curvature, the tidal compression effects can be
devastatingly strong and destructive.  To study gravitational effects in
the vicinity of earth, isolate yourself by putting yurself into freefall
so that only gravitation is acting on you --  launch yourself off a diving
board or better get into a much bigger orbit in the space shuttle.  Then
any accelerations you measure will be due only to gravitation  --  they
are very real effects:  the effects of inertial motion in curved
spacetime.  They are small in the vicinity of earth, however, and they are
certainly swamped when you try to measure them when earth is pushing on
you with a big electrochemical force.  The effect of the push of earth on
us is to keep us safely away from studying the effects of inertial motion
in spacetime, just as the push of centrifuge walls on our backs safely
keeps us in the centrifuge and prevents us from studying the effects of
Newtonian inertial motion in flat spacetime --  the rectilinear constant
speed motion we would go into if the centrifuge wall suddenly vanished.
In both cases real forces prevent us from going into inertial motion in
spacetime.  It's just that inertial motion in Einstein's curved spacetime
is much richer and more interesting than inertial motion in Newton's flat
space + time-as-parameter.  We can't blame Newton for doing the best he
could to build an instantaneous action-at-a-distance force model for
gravitation and consequently not discovering Einstein's much more
satisfying inertial frames in curved spacetime,  and we can still teach it
as the best model available until we want to introduce and study curved
spacetime.  

Thus in Newton's model gravitational force is a real force, with a real
source and a real third law counterpart acting instantaneously elsewhere
even over immense distances.  Einstein's model makes such a force
unnecessary and uncovers a different class of inertial frames.  In neither
model should fictitious forces not called for by the model be introduced.
In the context of Einstein's model (which most physicists believe is an
improvement, supported by experiment), introduction of Newton's
action-at-a-distance force would be a fiction.  If and when our students
become strong enough to do without it, they generally positively enjoy
Einstein's beautiful model.  I feel strongly that Newton himself would
have embraced it immediately with glee if it had been explained to him.

A. R. Marlow                          E-MAIL:
marlow@loyno.edu
Department of Physics, Box 124        PHONE:   (504) 865 3647  (Office)  
Loyola University                                    865 2245  (Home)
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