What would be the response of a string theorist...

[Tony]

... to justifying the use of the flat background D dimensional manifold
for string theory and including gravitation at the same time?

I tend to see the lack of justification for the ad hoc flat background
as one of the strongest LQG arguments against the string theory. In
comparison. string theory appears completely ad hoc. No philosophy of
space time, no concepts of space time, just a flat Lorentzian theory
that, like Kaluza-Klein, reaps all of its rewards only and only from
working in more dimensions than are physically observable.

Apparently, it is only its mathematical richness that makes is worth
investigating.

Am I just poorly informed?

Tony

[alfansome]

I think you should consider the fact that many very smart people have
decided to work on string theory for many years. This does not in and
of itself make it a good theory but clearly there is more to it than
you describe. I would suggest you read Brian Greene's "The Elegant
Universe" to get a more comprehensive view of how and why people are
exploring this theory.

Al

[Tonko Juricic]

I have found one of, perhaps, most famous responses to the issue of
background independence.

In his critique of Rovelli's book on Quantum Gravity that can be found at
http://schwinger.harvard.edu/~motl/rovelli.html
Motl writes:

'A background-independent formulation of a quantum theory of gravity -
such as string theory - is not a physical necessity but rather one of
the aesthetic dreams that would allow us to see the whole physical
theory (the same physical theory) in a more illuminating and general
light.'

IMO Motl is effectively reintroducing Newtonian absolute space and time in
physics (albeit with the difference that today it has 11 dimensions) and
saying that we don't need to be bothered by that because it is just
aesthetics. It just so happens that physics is well defined on some absolute
background manifold which we can factor out on account that it is merely an
aesthetic convenience with no physical content.

The fact that Motl really prefers the existence of absolute space is best
revealed in his following comment:

'Moreover, the physicists will only be excited once the spacetime -
more precisely all the different existing backgrounds - emerge from a
space-less starting point.'

IOW, since the space is absolute why would anybody sane want to get rid of
it?