The key word here is 'if'.

There is no quantum theory of gravity, that looks even close to passing all relevant tests, so speculating about what role any 'graviton' might (or might not) play in any such successful theory is rather a waste of time.

Perhaps we could get some clearer statements about the boundary of what we know today, re what any such quantum theory of gravity would contain? Once we've done that, I think we'll have gone as far as we can within the scope of the Q&A section.

I wonder

Light and matter and other stuff can't escape a black hole. Why? - because of gravity. So the jailer is gravity. Is the graviton bound by gravity? If it was, then the graviton couldn't escape the singularity - and therefore no gravitational field and no black hole. You must conclude that the graviton can't be bound by gravity.

I like the m-theory view on gravity - gravitons are not bound to the 11-brane that the universe is on (unlike light). Gravity is weak because they spend most of the time elsewhere. This would mean that the graviton could be a 2-spin baryon travelling at c, but not necessarily parallel to the three spatial dimensions of the universe

Russ,

Let's make sure we're talking about the same paper:

http://arxiv.org/PS_cache/gr-qc/pdf/9909/9909087.pdf

That is the one I posted I believe. I don't read anything in there about GR not being precise enough. Can you point out the exact place where you read this in there? If I recall, you mentioned something about this in the thread where I posted it. What Carlip was saying is that Van Flandern's (rather wacky) idea that gravitational forces propagate much faster than light, but gravitational radiation propagates at c, is "imprecise". It would require the two to be treated differently.

If gravity didn't "miss" in the manner GR says it does, then you'd need "something else" to explain where the radiation comes from when systems do radiate. And that would have to agree with GR as well.

That's what he's talking about, and that may be confusing.

Russ, "radiation" has a precise mathematical meaning. Propagation delay can be seen as the cause of it, which I find as a most enlightening way to look at it, but not all propagation delay effects are radiation.

In EM terms, a radiation field is one "that carries energy (and momentum)away to infinity", put in very rough terms. That has a rigorous mathematical definition, which you can find in EM texts (I forget exactly how it goes, but has to do with integrating the Poynting vector over a closed surface and letting that surface go to infinity. With a radiation field, that integral remains finite and constant, meaning energy is being carried away to infinity).

The field equations of GR are much more complex and therefore "richer" that plain ol' Maxwellian EM, but a radiation field/metric has similiar properties.

Mercury's precession has nothing to do with propagation delay. The GR calculation for that is based on the absolutely static Schwarzschild metric. Any slight perturbations due to propagation delay aren't in that metric, and would be a much more complex mess involving the Sun's actual "jerk" as it wobbles around the solar system barycenter. And I imagine other small effects, like the Sun's non-spherical moments, plus frame dragging and all that would be much stronger than any "radiative" effects.

The solar system is technically producing gravitational radiation, but it's so small, so vanishingly small, that is neglible over the lifetime of the solar system. The sun will be a white dwarf, and all sorts of other perturbations will have long since made much larger changes than any due to radiation.


-Richard

Yes, this is the same paper.

And thanks for taking the time to respond to this. I really Appreciate your's and Tensor's very clear and precise posts!

I see, and that is part of it.

Yes, and I have no particular problems with any of the baryonic matter producing energies (well maybe a couple, but they don't apply here) and how they are treated.

Ah, once again clear and precise, and I see now.

When I asked the question about the 'gravity time dilation' of the sun's gravity well in relation to Mercury, it was more or less a side question to the one asked in the OP, so I was just thinking that that might apply once gravitational radiation was brought up again.

I intensely dislike posting questions like my next one, because I know that certain people will come to the conclusion, that if he doesn't even know that, then...
But, here goes. Is the solar system gravitational radiation you are talking about here, the same as the GEM (gravito-electromagnetism) you have talked about so much??? I was thinking that they were different, but after reading this, I got the impression that they might be the same.

And finally, I see this a little differently.

It is not really a matter of gravity NOT missing...I'm sure it does...it is more that 'gravitational radiation' is mimicking something else that has the same basic affect, and once that something else is found, it will agree with GR.

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

Very wise words: let's not forget...:

1) Even though there are rough similarities between gravity and EM (there both 'attracting' forces*, they both extend beyond the atomic/molecular structures from which they "emanate", they both don't need a medium such as air (needed by sound waves for instance)...), there are also very big differences between them: magnetic forces only apply to metals, whereas gravity affects literally everything with a mass, and the 'reach' of magnetic fields is much smaller then that of gravity... And most importantly, gravity only comes into play when considering 'big enough' objects, whereas even a small piece of metal can be turned into a magnet with a bit of electric current.

2) How it all started: 1 objection formulated by Einstein against the Newtonian point of view was the answer to the question "What would happen if the Sun suddenly disappeared?". According to Newtonian physics, all objects in the solar system would be immidiately and at the same time propelled outward by lack of a binding force to keep them inside the same 'space' that we define as being the solar system. According to Einstein, however, the inner planets would react earlier to this event then the outer planets, introducing something like a 'propagation speed' of gravity. I wonder if people consider the fact that the sudden disappearance of the Sun may indeed cause 'ripples' in the space-time fabric like suggested by Einstein, however, this then does not necessarily imply a mediating particle: all you need is space itself, unless you propose a 'granular' quality for space, which will be hard to sell since any granularity in itself already implies space (I think)...

Just my opinion, but QM and Relativity are already hugging 3 of the 4 basic forces: I say, at least leave something for the 'mathematically impaired'...

* According to me, this is a very though cookie for the "push" theories: I can't imagine a distant object pushing a metal object towards an electro-magnet and keep doing so in accordance with me switching the thing on/off...

Well, something's wacky then. Light doesn't escape a black hole, but certainly the effect of the black hole's mass does.

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Everyone is entitled to his own opinion, but not his own facts.

Yeah, that's the wacky thing about it: as I see it, the effect of the mass is what we call gravity, no? (Yes, I know, Newtonian point of view... guilty as charged).

As Nereid said at the top of this page, nobody's really sure what's going on with gravitational interaction, how its effect is "communicated" to (1) the affected body OR (2) the region of space (which then affects the bodies in that space), whichever way you look at it. Sure, we can measure and predict the effect extremely accurately, but exactly how it works is unknown. Like you say, though, mass is obviously intimately related.

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Everyone is entitled to his own opinion, but not his own facts.

I know, but the fact that nobody exactly knows what's going on, doesn't mean we can't make determined decisions about what's not going on: here's the OP again:

According to me (fwiw), gravity can't be travelling in a gravity well. It can (perhaps) travel through space, causing a gravity well, but can't be said to travel inside or through a (pre-existing?) gravity well. Things can't cause themselfs (no religious debate intended)...

It seems wacky, but it's not. Consider a sphere collapsing into a black hole. In our far away frame, it takes infinite time for the sphere to cross its own event horizon. So what do we see? We see a bunch of mass in a spherical shell at a small radius. That makes the same gravity externally as if it were all concentrated at a point.

The space-time outside the event horizon is "shaped" by the mass as it falls in, not when it's past the horizon.

Oh, and for Layman, actually gravity itself does make gravity -- that's one of the non-linear effects of the Einstein Field Equations.

-Richard

Russ,

No, it's very different from the radiation the GEM approximation would predict. GEM is Maxwell like, and would predict *dipole* gravitational radiation just like EM (well, it would be slightly different, as GEM's g and B_g do not transform exactly the same as E and B -- the difference is electric charge is invariant between frames, while relativistic mass is not.

Applying GEM to the solar system would have gravity "missing" way too much, and the orbits would decay. It wouldn't be as bad as Van Flandern adding a propagation delay to pure inverse square Newtonian gravity and coming up with the 2*10^10c speed, but the solar system would still decay rapidly on astronomical time scales. GEM also would not predict the precession of Mercury, because the non-linear aspects of GR that produce that are dropped in the GEM linearization.

So what good is GEM? Not much for any practical work, but it does let you see how gravity has magnetic aspects, and how it works similiar to EM, with "gravitational induction" (ie changing gravitomagnetic field makes a non-conservative rotational g field) and all that. Using GEM, one can calculate gravitomagnetostatic fields, like the LT precession and that kind of thing based on straightfoward EM analogies, so I suppose that would be a limited usefulness.

But when it comes to radiation and strong fields, GEM is way off, as bad as plain Newton.

-Richard

Right, that's it, I'm outta here... or maybe out there?

Sorry, just feel a bit 'queezy' thinking about gravitons attracting gravitons by means of 'gravity'?

I'm starting to think it wasn't Newton who got hit on the head by an apple, but Einstein... and pretty hard too.

Quote:
Originally Posted by Cougar
Well, something's wacky then. Light doesn't escape a black hole, but certainly the effect of the black hole's mass does.

And here is something else to consider.

Since galaxies are permeated by DM, DM has to be continually being pulled into the massive gravity well of the Massive Black Hole, and since the DM is not interacting with the Baryonic Matter, once the DM crosses the event horizon, it cannot escape either. So (and I just realized this) the DM continually being pulled in, is what is forming the Black Hole structure.

Now that is Dynamical!

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

Thank You!

It is a relief to know that they are different, and that my original thinking on this was okay.

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

[According to me (fwiw), gravity can't be travelling in a gravity well]

[It can (perhaps) travel through space,]

LayMan, if gravity can be travelling in space and Einstein says that 'ponderable matter' warps space/time, then what do you think the gravity 'well' is made of?

What has not yet been figured out though, is that all of space is 'gravity', (otherwise, what is getting warped?) but not the gravity of the body travelling through it.

Which is one of the reasons I have the same problem as you do with...

Quote:
Originally Posted by publius
Oh, and for Layman, actually gravity itself does make gravity -- that's one of the non-linear effects of the Einstein Field Equations.

-Richard

It is NOT Einsteins fault!!! GR is correct, but It's just that it hasn't been figured out correctly yet.

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

The graviton has yet to be found. Until now, our only really solid theory for gravity is Einstein's GR, i.e. geometrical warping of space and not a force.

Aside from virtual paticles causing a black hole to leak, nothing escapes a black hole. GR is consistent with this, since gravity is considered warping of space, and there is no carrier force particle involved. The black hole's effect is felt by the warping of space, not because of a gravitational force.

Well, I'm not sure anymore myself... That's what you get from hanging around on this forum too long. Should have grabbed the rotating Moon and made a dash for it...