Re: [Phys-l] definition of gravity

[John Denker <jsd@av8n.com>]

On 11/08/2011 09:29 AM, Edmiston, Mike wrote:

> I'm way out of my comfort zone here, so I might be asking a dumb
> question.  What if it turns out that gravitons exist, and the force
> we commonly associate with Newton's Law of Gravity (Gm1m2/r2) is
> mediated by gravitons.  Does a strict adherence to "equivalence" mean
> that gravitons would also necessarily be involved in the observed
> forces John Mallinckrodt is speaking of in his two questions
> (below)?
...
 
> So what is the current thinking about gravitons and how they fit into
> the equivalence principle?

The short answer is that gravitons do exist, in the sense that
there is theoretical evidence that they must exist, observational
evidence that they do exist, and no good reason to doubt that
they exist.

The ordinary wavelike gravitons can be thought of as "second order"
creatures that cause tidal stress when they interact with matter.
As such, the equivalence principle has nothing to say about them.
They peacefully coexist with the equivalence principle.

Just as the DC Coulomb field can be thought of in terms of the
exchange of /virtual/ photons, the DC gravitational field can be 
thought of in terms of /virtual/ gravitons/ ... but these are 
rather strange creatures and your intuition about waves may not 
be a reliable guide in this case.

> If gravitons are involved in F= Gm1m2/r2, but not involved in F=ma,
> then it seems we would have a problem saying that the
> rocket-generated force is a "gravitational force."

I say they are involved in both, so there is no problem.

Indeed, if they were not involved in both, you would have a
big problem explaining higher-order effects, such as arise
when the rocket begins or ends its acceleration.

To repeat:  When the rocket is undergoing steady acceleration
(not near the beginning or end of the acceleration), you are
talking about DC virtual gravitons, which do not radiate.
That is to say, you are always in the /near field/ limit, not
in the far field (radiation) limit.  Also there are some 
funny issues with the polarization.  All this is consistent 
with what we know about Coulomb's law and virtual photons.