And it's what Grant told me, it just took me a while for me to understand.
The answer seems to be, that you have some state/process, and it can be described as an infinite sum of of ones involving virtual particles. After all, particles aren't what's really fundamental, so you *can* describe it as such an infinite sum, but you don't have to. But they're not just an approximation scheme -- the infinite sum is a valid description of what's actually going on, but of course you can cut it off to get a decent approximation. (And that's the method of Feynman diagrams.)
Now apparently there's some new computational method in QFT known as the "unitarity method", which is supposed to be more powerful than Feynman diagrams, even yielding exact mathematical answers? Since unitarity is such a fundamental principle it's pretty surprising that there'd be a recent new method where unitarity of all things is the key working principle, but I assume in reality there's much more to it than this. I don't suppose anybody knows anything more about it? Is its scope more limited than Feynman diagrams or something? I would expect there would be some restriction like that.
I don't suppose there's any chance such a method can get around the problems that occur when you try to do QFT with gravitons in it (and no arbitrary energy cap)? I assume those are real problems that don't go away just by changing the computational method? (Even if you did get a sensible result out of it, there's no way the result could be compatible with GR, is there?)
-Harry
The answer seems to be, that you have some state/process, and it can be described as an infinite sum of of ones involving virtual particles. After all, particles aren't what's really fundamental, so you *can* describe it as such an infinite sum, but you don't have to. But they're not just an approximation scheme -- the infinite sum is a valid description of what's actually going on, but of course you can cut it off to get a decent approximation. (And that's the method of Feynman diagrams.)
Now apparently there's some new computational method in QFT known as the "unitarity method", which is supposed to be more powerful than Feynman diagrams, even yielding exact mathematical answers? Since unitarity is such a fundamental principle it's pretty surprising that there'd be a recent new method where unitarity of all things is the key working principle, but I assume in reality there's much more to it than this. I don't suppose anybody knows anything more about it? Is its scope more limited than Feynman diagrams or something? I would expect there would be some restriction like that.
I don't suppose there's any chance such a method can get around the problems that occur when you try to do QFT with gravitons in it (and no arbitrary energy cap)? I assume those are real problems that don't go away just by changing the computational method? (Even if you did get a sensible result out of it, there's no way the result could be compatible with GR, is there?)
-Harry
