High energy physics has obtained spectacular successes during
this century, culminating with the (far from linear)
establishment of quantum field theory as the general form of
dynamics and with the comprehensive success of the
Standard Model. Thanks to this success, now a few decades old,
physics is in a position in which it has very rarely been: There
are no experimental results that clearly challenge, or clearly
escape, the present fundamental theory of the world. The theory
we have encompasses virtually everything  except gravitational
phenomena. From the point of view of a particle physicist,
gravity is then simply the last and weakest of the interactions.
It is natural to try to understand its quantum properties using
the strategy that has been so successful for the rest of
microphysics, or variants of this strategy. The search for a
conventional quantum field theory capable of embracing gravity
has spanned several decades and, through an adventurous sequence
of twists, moments of excitement and disappointments, has lead to
string theory. The foundations of string theory are not yet well
understood; and it is not yet entirely clear how a supersymmetric
theory in 10 or 11 dimensions can be concretely used for deriving
comprehensive univocal predictions about our world.
But string theory may claim extremely remarkable theoretical
successes and is today the leading and most widely investigated
candidate theory of quantum gravity.
In string theory, gravity is just one of the excitations of a
string (or other extended object) living over some background
metric space. The existence of such background metric space, over
which the theory is defined, is needed for the formulation and
for the interpretation of the theory. This is the case not only
in perturbative string theory, but, to my understanding, in the
recent attempts at a nonperturbative definition of the theory,
such as
M
theory. Thus, for a physicist with a high energy background, the
problem of quantum gravity is now reduced to an aspect of the
problem of understanding: What is the mysterious nonperturbative
theory that has perturbative string theory as its perturbation
expansion? And how does one extract information on Planck scale
physics from it?

Loop Quantum Gravity
Carlo Rovelli
http://www.livingreviews.org/lrr19981
© MaxPlanckGesellschaft. ISSN 14338351
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