Its mentioned in the Speed of Gravity response that gravity can and has been considered a particle of sorts as light is. How can this be, it could not have mass or energy because it would slowly deplete an object that radiated it. This would disqualify it for particle consideration, has it been called this for lack of a better word. Also, you seem to be infering that the speed of gravity cant be measured because we would need an instantaneous account of both objects positions. Why would this be nessassary when we can calculate the distance each object if from the earth and make a compensatory adjustment e.g. if both objects radiate light(star of sorts) and are very far from each other but so massive that they are captured in each others orbit and one goes through some sort of astronomical change. Could we not observe the light from both, make a compensatory calculation becuase of propagation distances and determine how fast mass change effects gravitational pull.

Anything which radiates real gravitons is, in fact, losing energy. As for an object merely sitting there attracting other objects, it’s not emitting real gravitons, but virtual ones. And virtual particles, counterintuitive though it may be, are allowed to have zero or even negative energy, so the books all balance. At least, so goes the argument by analogy with electromagnetic forces, which are mediated by virtual photons. We don’t really have a quantum theory of gravity yet, so we can’t make such statements definitively, but it seems very likely that there’s at least a superficial resemblence between gravity and electromagnetism (at least to the point of gravitons existing).

And the bit about simultaneous measurement wasn’t meant to imply that we can’t measure the speed of gravity: We can, by approximately the method you describe. It was just meant to show that the speed of gravity can’t be infinite, which would mean that one object reacts at the same time as the other changes.

Sure we do:

Yes, it’s still incomplete and experimentally unproven thus far, but it’s the most promising thing going. The fact that gravity *has* to be a part of superstring theory (it emerges naturally from the equations) and is not something we have to try to incorporate into it is one of the most beautiful parts of it.

From here.

Well, string theory certainly looks promising, and it may well be that some direct decendant of it may give us the answers we’re looking for. But string theory still leaves a lot to be desired. There are still too many unknowns for it to be a useful theory, and I’ll hold off on saying that we have a quantum theory of gravity until it can make some testable predictions. To be more accurate, one should probably consider “string theory” to be a collection of related theories, and we don’t yet know which one (if any) is accurate.

From my perspective as a layman, it seems like string theory is a pretty wild shot in the dark. You say that it’s promising, and that a quantum theory of gravity just falls out of it, but it seems to me that we stumbled upon something which has this mathematical property, therefore we persued that path. Then we found out it didn’t work because of specific reasons. Then someone else figured that if you add six extra dimensions, you overcome those objections. Now the math is too hairy to figure whether there might be other objections, so the theorists and mathematicians are out there working away on it.

To my mind, it’s just not grounded enough in reality to have much hope in. I’m currently reading the book *The Elegant Universe*, by the way, and although Brian Greene is trying really hard to sell me on the idea that string theory is the bees knees, I’m not buying it so far.

When you finish Greene’s elegant book, I hope you understand that nobody just added six extra dimensions for a lark. The idea of using extra dimensions to incorporate forces has been around since the days of Kaluza and Klein 75 years ago.

And as a layman myself, I understand that my perspective is totally irrelevant and meaningless to the process.