Re: [Phys-L] GR and Gravitons

[John Denker <jsd@av8n.com>]

On 11/18/2015 03:57 PM, Jeffrey Schnick wrote:
> I have a question about general relativity and the hypothetical 
> graviton.  It is my understanding that the presence of a massive 
> object alters the geometry of spacetime thus causing the observed 
> motion of free particles in the vicinity of the massive object.

So far so good.  If you want to know what the curvature
looks like, to a decent approximation, see:
  https://www.av8n.com/physics/geodesics.htm#fig-darts

> The particles aren’t experiencing a force but simply following the 
> contours of spacetime.

Yes.

> The question is, why would anyone think that there would be an
> exchange particle governing the (non-existent) interaction between
> the massive object and any one of the particles.

This is essentially analogous to the rant I recently posted about 
trying to explain the electromagnetic radiation field in terms of
the static Coulomb field.  There is no reasonable way to do it.
HOWEVER there are other (non-Coulomb) terms in the field equation
that explain radiation just fine.

I recently expanded my discussion of this point, including a long 
list of reasons why the "usual" diagram is bogus:
  https://www.av8n.com/physics/lienard-wiechert.htm#sec-bogus

> The answer I have come across is that general relativity is a field
> theory and fields can be quantized—the graviton is the quantum of the
> field. 

That's a true observation, but we agree it's not much of an
explanation.

>  Then the question becomes, what is the field?   Is it
> spacetime itself?

There's no harm in thinking the field is spacetime itself.

Beyond that, the question has no answer and needs no answer.  The 
field is an abstraction.  The same goes for electromagnetism:  it is
pointless to ask what the field "is".  To me, it's an abstraction.
It's vastly more important to understand what the field /does/ than
to fuss over what the field "is".  We know how it behaves, and that's
really all we need to know.

The same goes for many other things in physics, including energy.
It is vastly more useful to understand how energy /behaves/ than to
fuss over a pithy dictionary-style definition of what energy "is".

==========

Also:  Relativistic causality guarantees that there has to be "some"
sort of wavelike disturbance.  Again this is analogous to the story
used to construct the infamous I'itoi diagrams such as:
  https://www.av8n.com/physics/lienard-wiechert.htm#fig-iitoi-half-bogus
A source particle is initially at rest, then it moves for a while,
and then it stops.   Outside the light-cone of the start-event you 
have the plain old static Coulomb-style field.  Inside the light-
cone of the stop event you have the static Coulomb-style field, 
just shifted a bit.  In between, there's a mess of radiation fields. 
It's a bunch of work to figure out exactly what goes on in the messy
region, but you know it cannot possibly be Coulomb-like.  And you 
know that the mess propagates with the speed of light.  So you might
as well call it a wave.