tyro question

wolram

in quantum gravity theories, can a quanta of gravity exist in isolation
or would it need other interacting quanta to perpetuate?

marcus

this is a good question in that it
could produce a shower of divergent opinion,
almost a "full spectrum" of views

I think in asking it you probably want to elicit a wide spectrum.
I shall give one very focused view

I dont think of gravity as a substance that you can have quanta of.
I think of gravity as revealing geometry
the shape of space

I think that the correct name for "quantum gravity" is really
Quantum Geometry. the quantum theory of how space is shaped
(because the shape determines how matter flows around, how
planets orbit etc)

I think that one should ask not about quanta of gravity but about
the quanta of geometry
and these are things like quanta of area
and quanta of volume

distance and curvature might come in discrete amounts too
anyway it is geometrical entities that should out to be quantized
in a quantum theory of geometry

I think that a "graviton"
is a fictional entity that is convenient in analyzing the behavior of the
gravitational field in certain restricted (approximately flat) situations.
it is a ripple in the fabric of geometry
but in highly curved dynamic situations analyzing in terms of "gravitons"
wouldnt work very well---they dont really exist it is just handy sometimes
to pretend they do.

The real quanta of geometry are quanta of area and volume and suchlike geometrical things, and if you know the volumes of enough regions and the areas of enough surfaces you have a handle on the geometry of space.

In quantum gravity one of the landmark results is that areas and volumes of things are in fact quantized and are somewhat like the energy levels of an atom. That's what I look for from a quantum theory of spacetime geometry,
that is, a quantized version of GR.

other people doubtless have different perspectives on this

wolram

i will be quite happy now, if i can relate how an inspiraling body
gives of geometry, or how mass is converted to geometry, thanks
for your patience.

marcus

help!

right as usual, very hard-to-understand stuff here
how does mass effect the curvature of space?
when two neutron stars spiral in towards each other
they radiate energy (carried away by ripples in the gravitational field)
and the system loses mass
as it loses energy
how can this possibly be?

there are equations to describe what happens
but I have no inkling of the mechanisms that make those equations work
and why they fit

maybe someone else can add insight here

wolram

if i considered that the destruction of mass initiated a shock wave that perturbed the quanta of geometry akin to the pixels on an oscilloscope, and the continuum of quanta transfered this shock energy to new born mass perterbances of geometry would this be a conservation of mass, geometry?
or am i thinking utter ***P

marcus

there is a group of people who use a solid state model (condensed matter model: analogous to crystal lattice model) to describe gravity

there was one of them here a couple of months ago, I will try to find a link

I imagine these people think like that (maybe similar to how you are thinking)
and they relate gravity to how shocks and defects propagate thru a crystal lattice
someday, if they havent already done it, they might make computer-generated animated films of these "solid-state" gravity ideas

it's way out of my ken
all I can do is try to remember who that guy was that came to PF
and try to dig up some keywords for a search

IIRC the guy was unhappy. from his point of view so much attention is given to string and LQG that there is not enough attention left over for
"condensed matter" models of gravity

one thing you can say for those people is they have some different ways of visualizing gravity

--------------
incredibly, wolram, the lady and I went scubadiving today
it had been several years since I had a tank on my back
speaking of gravity, it is a very different experience in neutral
buoyancy

the difference between up and down becomes more subtle

one rolls over or drifts heels over head rather easily but it doesnt make
much difference

drawing a deep breath (of air from the tank) makes you more buoyant and you start to rise, letting out a breath (as a cloud of bubbles) makes you less buoyant and you stop rising and slowly settle back down

I admit this is irrelevant, but it was a nice way to spend the afternoon
--------------
you are venturing ahead of the crowd, if you want to speculate what makes gravity work
talk about pushing to the ragged edge!
What I see going on today is they're mainly just trying to get a satisfactory quantum description of GR----able to match the usual GR effects---no easy
matter in itself

I'd guess they wont try to imagine actual mechanisms until after GR is quantized.

progress on gravity is like tunneling through a mountain of solid rock
a few inches a day, and they forgot to bring the dynamite

wolram

a wonderful thing diving under water, its a different world, i could never
do it, fly light aircraft yes, i intend to look up condensed matter theory
and see if i can find a gem of wisdom, happy S,Dving....