Warp wrote:
>   It's not even a question of "definition" of "force". According to GR
> gravity is *not* a force because it doesn't cause an acceleration, and
> a free-falling object is *not* accelerating.

I agree with everything you say.

Given that, some definitions of "acceleration" include "a change in 
velocity". Clearly, gravity changes an object's velocity. That's the 
sort of thing I meant. If you define gravity *not* to be a "force", then 
you have to define "force" differently than Newton did (obviously), at 
which point I don't know what the definition would be.

Interestingly enough, I understand that if you actually do the math, the 
curvature of space by mass is surprisingly (to me) tiny. For example, 
the circumference of the entire sun is only about a kilometer different 
than you'd expect from multiplying the radius by Pi. (I don't know how 
to do that math, but I read it in a text by someone you could safely 
assume did.) It seems kind of odd that such a small change in the 
curvature of space would lead to such a relatively large effect.

Oh, there it is. Wow, that was a bit of searching for the right google 
terms.

"""
There is a simple formula for a spherical mass M - the change in the 
radius (delta r) = MG/3c^2 For the earth this turn out to be about 1.5 
millimeteres
"""

Also,
"""
If you consider the universe as a whole - it has the correct mass for a 
black hole having a size equal to the Hubble radius.
"""
altho I can't say I know that's true.

<http://www.physicsforums.com/archive/index.php/t-7152.html>

Cool stuff. I guess light doesn't really bend very much going past the 
sun, but it still seems like the earth has a pretty large gravitational 
effect for something that's less than 2mm wrong. :-)

-- 
   Darren New / San Diego, CA, USA (PST)
     It's not feature creep if you put it
     at the end and adjust the release date.

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