despite rather many interruptions I want to continue pursuing this thread
here is the link I promised to Enrique Alvarez "Loops versus Strings"
the article is interesting and credible for several reasons
he is actively engaged in string research currently
(around 16 string/brane articles since mid-1999 show in arxiv, you can check in arxiv to see how much they have been cited by other authors---compare e.g. Kaku zero articles since the one in mid-1999 and that one has been cited only once, according to the arxiv citation-bot)
the article is recent----mid 2003
the article was an invited talk at the mid-2003 HEP
conference at Portoroz called "What comes beyond the standard model?"----that is, other HEP theorists want to hear what Alvarez has to say
the guy is senior and broadly knowledgeable----so capable of doing an intelligent overview and making a useful string vs loop comparison
CAVEAT: his viewpoint (and that of his HEP audience) is very
UN-general relativity, his perspective is HEP/QFT/string. So you dont get a relativist perspective. I cannot vouch for his article
beyond mentioning the circumstantial evidence that he seems to be senior, currently active in research, and respected.
Mentat, the situation in quantum gravity is currently exciting, the picture is changing, the split between General Relativity and the relativists on one hand and the (far mor numerous, almost mob) of string/braners on the other is fascinating, or so I find. It points to a deep division in the foundations of physics concerning the nature of space and time (this is what "background-independence" is about---and the problem that string-thinking doesnt have it). So these things are IMHO very worth trying to follow, even enlightening, maybe historical in some sense.
I dont know of any other 2003 article of the "Loops versus Strings" overview/comparison type from the string side! This
Alvarez one is all I could find! So I offer it to you or anyone who wants to follow the action, to look over.
But on the other hand there are quite a few recent overviews of developments in quantum gravity from the loopers or, to put it more clearly, the GR people involved in quantizing GR.
Smolin has a 2003 paper called "How far are we from a theory of quantum gravity?" which has been extensively cited, to mention just one, and Rovelli even has this book "Quantum Gravity" in current draft form at his site.
I will get some links for these things, and also check the arxiv bot to get citation numbers for some current Ashtekar, Smolin or Rovelli work---it is an interesting quantitative measure of how important or useful a research paper has been---I've mostly been judging by the citations I see in other papers I happen to read, a partially subjective assessment which I believe is also important to make. Bear in mind that QGR is a small field and the numbers are small, but there seems to be a shift underway (in science a small active minority can sometimes bring about change)
Oh BTW you mentioned the fact that, while GR appeared in 1915, Einstein spent much of his time in later life working on a unified theory of other stuff besides gravity. The essence of the 1915 GR is its background-independence---you dont specify a static spacetime geometry ahead of time but let the geometry be dynamic and variable. I DON'T KNOW if Einstein's later efforts at unification retained this essential, and new, view of space and time! He may have given up and gone back to some fixed background approach like "special" relativity. Today's particle/field theories (QFT, Standard Model, whatever) are fixed-background.
My GUESS would be that Einstein would have kept to the dynamic GR spacetime and tried to bring other forces besides gravity into the picture which as we know is very hard and would help explain why he met with frustration. In effect that line of effort is only just beginning to get results and make testable predictions! So what you decide, on a verbal level, to call a continuation of Einstein's project is kind of subjective and depends on how detailed a picture you have of how
he was trying to construct a unified model.