- #1
Sardano
- 31
- 0
I ask this question because some time ago I was attending some course in LQG, and I was very surprised of the lack of knowledge of the teacher in anything that was not completely related to his investigation. Even things that he knew, he only knew it in the particular context of his work and couldn't relate it to the same concept in more general context. I am talking about things that every ST physicist have used, like, fermion representations, arbitrary dimensional vielbiens, perturbative techniques, fiber bundles etc. It was also evident that all his education was coming from the same book: the thiemman QG book. Something that seems to be the usual among LQG people. For example, he used the triad (of course) which is nothing but the "three dimensional vielbien" of a riemannian metric. However, he didnt know anything about this, he just knew the specific definitio given in the LQG context (I guess from the Thiemman book, of course) that is based on the SU(2) group and his killing form. But he didnt know that this construction can be used in arbitrary dimensions, that is a non-coordinate change of basis, that this construction allow to introduce fermions in curved spacetime. In his own words: "I don't know what is a fermion, I have never seen one": he didnt know about spinorial Lorentz representations and anything related to fermions!. He was talking about holonomies in the LQG context, but he didnt know what the holonomy group of connection in a fibre bundle was!. These are just some examples of much more facts that have made me run into the conclusion of the question. It was obvious that he knew NOTHING about QFT, even the QFT's are the more tested theories (much more than GR, which only has a few tests). And I am pretty sure that he knew nothing about QFT because when we were talking about ST and LQG he commented:
"Well, is true that ST has a perturbative description of gravity that is finite order by order...but no one has showed that the whole sum is not divergent!"
I was astonished, and replied:
"The thing is that the sum IS divergent...otherwise everything would be wrong..."
And he said:
"What?, Why?"
My reply:
"Non perturbative physics...but this is something that happens already in QED..."
On the other hand, in ST we have to work with all this things in a mixed complicated set up, so we have to learn how to work with fermions, but not only in four dimensions but in arbitrary dimensions, we have to learn about huge gauge groups also in several dimensions, we have to learn also about gravity in several dimensions and we have to learn not only about pointlike actions but only about extenden objects and their gravitational an gauge interactions. And then we have to put everything together!. I think the background for ST is much wider than the background for LQG. I am not saying that is more difficult or something like that, but I think if you specialize on LQG, then you are not prepared to do antything else in Theoretical Physics. I got to say, I was going to start as PhD student in LQG when i attended this course, but the formation of the teachers and the stuff the were working in (not because it was LQG, but because other reasons) made me change mind. They seemed to be locked in his particular GR research and the have forgotten about all the others branchs and tools of theoretical physics.
"Well, is true that ST has a perturbative description of gravity that is finite order by order...but no one has showed that the whole sum is not divergent!"
I was astonished, and replied:
"The thing is that the sum IS divergent...otherwise everything would be wrong..."
And he said:
"What?, Why?"
My reply:
"Non perturbative physics...but this is something that happens already in QED..."
On the other hand, in ST we have to work with all this things in a mixed complicated set up, so we have to learn how to work with fermions, but not only in four dimensions but in arbitrary dimensions, we have to learn about huge gauge groups also in several dimensions, we have to learn also about gravity in several dimensions and we have to learn not only about pointlike actions but only about extenden objects and their gravitational an gauge interactions. And then we have to put everything together!. I think the background for ST is much wider than the background for LQG. I am not saying that is more difficult or something like that, but I think if you specialize on LQG, then you are not prepared to do antything else in Theoretical Physics. I got to say, I was going to start as PhD student in LQG when i attended this course, but the formation of the teachers and the stuff the were working in (not because it was LQG, but because other reasons) made me change mind. They seemed to be locked in his particular GR research and the have forgotten about all the others branchs and tools of theoretical physics.