Re: Newton's 3rd law? was Re: inertial forces (definition)

[Cliff Parker <cparker@EMPOWERING.COM>]

David Bowman wrote:

> Is it OK if I butt in here?

Yes of course!  Thank you for your concise and clear explanation.  Please
entertain a few follow-up questions.

>   Such a field propagates changes no
> faster than c.  This means that Newton's third law can't hold for any two
> spatially separated objects interacting via the field because they can't
> keep in instantaneous communication with each other so they can exert
> their respective forces on each other in a way that can be precisely
> synchronized as exactly "equal & opposiite".

OK, lets see if I am thinking correctly.  Are you saying that the fact that
forces work at a distance means that during the time the initiating force (from
body one)  travels to a second body the interaction of the fields through this
transmittance process changes the initial force and the reacting force so that
they do not exactly match?  Does this process also indicate that the action
reaction forces do not occur instantaneously but rather "dance" with each other
over a period of time?

> Of course, this does not
> mean that Newton's third law is not an excellent *approximation* in many
> important instances.  In the case of gravitation it is an even better
> approximation than it is for electromagnetism because gravitational
> interactions tend to have the effect of their actual retarded interaction
> cancel out in the near field to a higher order in 1/c^2 than they do for
> electromagnetic forces.

I do not understand the significance of 1/c^2 in your statement.  It would seem
that the "dance" of interacting forces would tend to in large part cancel each
other out.  Why would it work out this way more for gravitational force than for
electromagnetic or other forces?

> General relativity *is* a field theory.  The field mediating the
> interaction between sources is the metric of spacetime, and in GR
> anything with energy, momentum, and/or stress counts as a source of
> gravitation.

Yes that is correct.  I had not thought calling space-time a field before.  Now
what is this about stress counting as a source of gravitation?  That is a new
idea for me.