String Theory
The role of solitonic extended objects known as
branes
has been pivotal in obtaining detailed knowledge of strongly coupled
dynamics
in string theory. In an effort to uncover non-perturbative,
non-supersymmetric
contributions to the string spectrum, I got involved in dicussions with
Roberto
Emparan (Bilbao/Cern) which led to the
construction
, together with one of my graduate students, Auttakit
Chattaraputi (Durham) , of new
four-dimensional
solutions of General Relativity (called composite diholes) which can be
embedded in 10 or 11-dimensional supergravities, interpreted as systems
of intersecting Dirichlet branes (D-branes) and intesecting
anti-D-branes,
generically called `non BPS states' (hep-th/9911007)
Dirichlet branes can also be studied as objects
described
by the conformal field theory of strings that end on them, and this
picture
has amazing consequences in our understanding of the emergence of all
ten-dimensional
string theories from a 26-dimensional parent bosonic string
theory.
Together with a long-term collaborator, Francois
Englert (Brussels) , and Laurent
Houart (Brussels) , we extend to
the open string sector of the bosonic string the universal truncation
procedure which allows
one to encode all ten-dimensional closed fermionic string theories in
the 26-dimensional bosonic string theory compactified, in the left
and/or right sector, on the Lie algebra lattice of E8 x SO(16). The
study of the open string sector revealed that crucial features of the
fermionic strings can be
explained from purely bosonic considerations. First of all, through a
brane fusion mechanism , the imposition of a tadpole condition in the
bosonic string explains the cancellation of anomalies in Type I theory
by the Chan-Paton group SO(32) and yields the Chan-Paton groups of the
Type O theories. In addition, the tensions of the fermionic
space-filling D-branes are computed using properties of the bosonic
string only. These results follow from specific properties
of the E8 x SO(16) lattice, singling it out of all possible toroidal
compactifications. They also point towards a dynamical origin of the
truncation, but such dynamics cannot be handled in the context of the
perturbative approach given so far. We are now thinking of a
non-perturbative approach to the bosonic string
(hep-th/0106235)
. Work in this area continues with the extra
active
participation of Auttakit Chattaraputi
(Durham).