My research interest has been on a wide variety of fundamental problems in theoretical high energy physics. At present time, my main effort is devoted to the study of superstring theory, as a prime candidate for the unified theory of nature, and the deeply related topics of quantum gravity and the
so-called AdS/CFT.
With the experimental observation of the Higgs particle, the "Standard
Model" has been established as a remarkably successsful effective
theory of particle physics up to the distance of 10^(-17)cm. However it
is well-recognized that there are a number of important phenomena which
cannot be explained by the Standard Model. Among them, a crucial shortcoming
is the fact that the Standard Model is incapable of providing any fundamental
understanding of the oldest-known and perhaps the most important force
of Nature, namely the gravitational force. In particular, at and beyond
the Planck scale, the gravity becomes strong and the quantum effects becomes
crucial, and the deep understanding of the nature of quantum gravity is
one of the most important issues of the 21st century.
For this task, the superstring theory, which contains gravity in a very
natural way, is certainly the most promising candidate theory. However,
so far, what the string theory can compute are rather scarec, such as the
scattering amplitudes of particles including the graviton in a flat background,
the entropy of rather special black holes which can be described as D-branes,
and not much more. Therefore it is rather difficult to obtain a picture
of what is quantum gravity from string theory at the moment.
An epoch-making conjecture called AdS/CFT, proposed in 1997, which has
its roots in string theory and its extension called M-theory, has the possibility
of offering a possibility of capturing the essence of quantum gravity from
a rather different perspective. It claims that string theories in the background
of the Anti-de-Sitter(AdS) space are "equivalent" or "dual"
to certain class of conformal field theories (CFT), which are defined on
the boundary of the AdS space, hence in a spacetime which is one-dimension
lower. For this reason this correspondence is called holographic. Although
this correspondence relies heavily on the fact that the specific isometry
of the AdS is the same as the global symmetry of the CFT and hence applies
to rather special situations, it is possible that the true understanding
of this correspondence, which has not yet been obtained, would lead to
a pictaure which will be applicable to quantum gravity in more general
spacetime. In particular it may give an important hint for the non-perturbative
understanding of the quantum black holes, which is an object of fundamental
importance in unravelling the strongly coupled regime of quantum gravity.
In fact the study of the quantum properties of the black holes and the
fields around them in the more general setting than the AdS/CFT would be
of great importance in the understanding of the superstring theory and
the D-branes that it contains. In any case, we are at the stage where we
need a rather drastically new view of the quantum gravity in order to make
an essentia lprogress. I would like to concentrate my effort in finding
such a new view to understand quantum gravity.
Explicitly, the topics of my present research include