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In an old 2002 google thread sci.physics.research which is moderated:

http://groups.google.com/group/sci.physics.research/browse_thread/thread/7237fb8a564e0656/534ebd7afdfd3015?lnk=gst&q=Einstein+Field+Equations+and+Flat+Space+Time#

Steve Carlip seemed to agree when he said:

"There's a bit more to it. You also need, at least, a massless spin two interaction that couples universally. While this doesn't involve general covariance in an obvious way, a massless spin two field has a gauge invariance that's ``as big'' as diffeomorphism invariance (i.e., that's parametrized by a vector field), and the universality of the coupling rules out any noninvariant ``background.''

Steve Carlip"

Bill Hobba who is now a member of Physicsforums wrote this at sci.physics. (need comment how true it is).

Someone asked (in 2002) at sci.physics: "But in string theory, spacetime still has curvature."

Bill Hobba replied all the following:

"No it doesn't. It emerges as a limit - but the underlying geometry of space-time - if it has one - is not known."

"As Steve Carlip once explained, it is experimentally impossible to tell a theory formulated in flat space-time that makes rulers and clocks behave as if it was curved from a curved one, so the question is basically meaningless at our current level of knowledge."

"Up to about the plank scale the assumption it is flat is fine, with gravitons making it behave like it had curvature or actually giving it curvature (we can't determine which) works quite well. "

True? If yes, how much is it supported in String Theory? If not, why?