From: leao@BUPHY0.BU.EDU (Joao Leao)
Newsgroups: alt.sci.physics.new-theories
Subject: Re: Gravitons
Date: 9 Apr 92 16:36:35 GMT

In article <1992Apr03.121450.2463518@locus.com>, richard@locus.com (Richard M. Mathews) writes:
|> leao@BUPHY0.BU.EDU (Joao Leao) writes:
|> >It may worth to point out that gravitons do not carry the whole of the
|> >gravitational interaction but only the linear part! They cannot be responsible
|> >for the effects of curvature or tidal forces which are very non-linear
|> >effects of gravitation.
|> 
|> I know very little about quantum theories of gravity, but it is my
|> understanding that the above is false.  I believe that the above is
|> true only if you do not allow gravitons to be affected by gravity.
|> When you do allow that, you get exactly full blown GR.  The "resulting
|> theory eradicates original flat geometry from all equations, showing it
|> to be unobservable" (quoted from Misner-Thorne-Wheeler, Box 18.1).  Or
|> to quote a passage from MTW section 18.1:
|> 
|> 	Just as one can "descend" from general relativity to
|> 	linearized theory by linearizing about flat spacetime.
|> 	so one can "bootstrap" one's way back up from linearized
|> 	theory to general relativity by imposing consistency
|> 	between the linearized field equations and the equations
|> 	of motion, or, equivalently, by asking about: (1) the
|> 	stress-energy carried by the linearized gravitational
|> 	field h_mu_nu; (2) the influence of this stress energy
|> 	acting as a source for corrections h(1)_mu_nu to the
|> 	field; (3) the stress-energy carried by the corrections
|> 	h(1)_mu_nu; (4) the influence of this stress energy
|> 	acting as a source for corrections h(2)_mu_nu to the
|> 	corrections h(1)_mu_nu; (5) the stress-energy carried
|> 	by the corrections to the corrections; and so on.
|> 
|> And while I am only somewhat certain that gravitons can account for all
|> of GR, I am 100% certain that even the linear theory of gravity (whether
|> geometric or graviton based) can produce curvature, tides, perihelion
|> shift, bending of light, gravitational redshift, etc.  See Box 7.1 or
|> Chapter 18 of MTW.
|> Richard M. Mathews			D efend
|> Internet:   richard@locus.com		 E stonian-Latvian-Lithuanian
|> UUCP:	    ...!uunet!lcc!richard	  I ndependence
|> MIL/BITNET: richard%lcc@UUNET.UU.NET

My point does not conflict with anything you or MTW say but maybe I 
should elaborate a bit. Gravitons are NOT "creatures" of Classical GR
which is what the whole of MTW is about (except for fragments of the
last 4 or 5 chapters). As far as Classical GR is concerned all the 
above "observable" effects can be "bootstrapped" from the linearized 
theory only by adding a battery of additional constraints to the equations of motion which "interpret" dynamically the effects of curvature. This type of
artifice does not work for the Quantized version of linearized GR because
one cannot build but a very low-energy pastische of GR (though soft-graviton
theorems allow you to calculate the perihelion of Mercury pretty convincingly)
a genuine perturbation theory cannot really be built for gravitons because
beyond first order even the linear theory is unrenormalizable. So what is
called sometimes pompously "The Quantum Theory of Gravitation" (1st order
soft graviton scattering theory) is very far from Quantum GR and though it
can reproduce the best know emprical predictions of GR it can't hardly
deal with space-time Curvature or the principle of General Covariance.

There have been some other heroic approaches to QGD (Quantm GeometroDynamics)
which address these problems but they as a rule put asside the whole
graviton business and try to Quantize the Non-Linear Theory from
scratch...but that is a whole other can of worms...

							Joao Pedro Leao (Artificial Iconoclast and Director of Computer Resources
Artificial Physics Lab * Boston University - Physics Dept. Boston MA 02215)
 	leao@buphy.bu.edu | leao@buphyc.bitnet | BUPHYC::LEAO 
"Well I am sitting here in Tahiti/ I am laying in the sun and sipping a...
...chartreuse tropical drink!/ and I say: I know those Bermuda shorts!..."