Gerry Quinn
Jan4-09, 06:00 AM
[[Mod. note -- I mistakenly deleted this article's References: headers
when saving it from my inbox into my s.p.r articles-to-be-moderated queue.
I apologise to the author and other readers for this, and for the ensuing
loss of threading in many newsreaders.
-- jt]]
In article <ivr7l.10266$D32.4985@flpi146.ffdc.sbc.com>, tjroberts137
@sbcglobal.net says...
> Gerry Quinn wrote:
> > [about spin-2 gravitons on flat background spacetime]
> > my conclusion would be that
> > (assuming that gravity is explained based on a non-fundamental graviton
> > field) non-trivial topologies would be unphysical.
>
> I would state this quite differently: Assuming that gravity is explained
> based on a non-fundamental graviton field, it is an experimental issue
> to determine the underlying manifold, including its curvature and
> topology.
Agreed... but have you any reason at all to suppose it is anything but
approximately flat on the scale of the observable universe, and of some
pretty simple topology? In that case any small-scale non-trivial
topology - or any topology at all that differs from the background -
would be unphysical. [Unless spacetime can somehow have multiple
topologies at once and still be stable and consistent.]
> Current attempts to formulate such theories use Minkowski
> spacetime for simplicity, but there's no reason to expect that to hold
> in the real world.
But there's no reason either for any background curvature corresponding
to gravity. The whole point of the graviton theory is to explain
gravity as just another field, like electromagnetism. Sure, the
background might in principle be curved or have an exotic topology for
some other reason, but any such curvature would be unrelated to any
curvature or topology predicted by GR.
> Many/most current attempts to explore quantum gravity are abandoning the
> whole notion of manifolds, so this may well be moot.
They can do what they like at the Planck scale and beyond, but at lower
energy scales it is obvious that spacetime is best described in terms of
manifolds (effective manifolds, if you prefer). It is consistency
between effective manifolds of this kind that is relevant to this
thread.
- Gerry Quinn
when saving it from my inbox into my s.p.r articles-to-be-moderated queue.
I apologise to the author and other readers for this, and for the ensuing
loss of threading in many newsreaders.
-- jt]]
In article <ivr7l.10266$D32.4985@flpi146.ffdc.sbc.com>, tjroberts137
@sbcglobal.net says...
> Gerry Quinn wrote:
> > [about spin-2 gravitons on flat background spacetime]
> > my conclusion would be that
> > (assuming that gravity is explained based on a non-fundamental graviton
> > field) non-trivial topologies would be unphysical.
>
> I would state this quite differently: Assuming that gravity is explained
> based on a non-fundamental graviton field, it is an experimental issue
> to determine the underlying manifold, including its curvature and
> topology.
Agreed... but have you any reason at all to suppose it is anything but
approximately flat on the scale of the observable universe, and of some
pretty simple topology? In that case any small-scale non-trivial
topology - or any topology at all that differs from the background -
would be unphysical. [Unless spacetime can somehow have multiple
topologies at once and still be stable and consistent.]
> Current attempts to formulate such theories use Minkowski
> spacetime for simplicity, but there's no reason to expect that to hold
> in the real world.
But there's no reason either for any background curvature corresponding
to gravity. The whole point of the graviton theory is to explain
gravity as just another field, like electromagnetism. Sure, the
background might in principle be curved or have an exotic topology for
some other reason, but any such curvature would be unrelated to any
curvature or topology predicted by GR.
> Many/most current attempts to explore quantum gravity are abandoning the
> whole notion of manifolds, so this may well be moot.
They can do what they like at the Planck scale and beyond, but at lower
energy scales it is obvious that spacetime is best described in terms of
manifolds (effective manifolds, if you prefer). It is consistency
between effective manifolds of this kind that is relevant to this
thread.
- Gerry Quinn