508
E. Rabinovici
These connected regions of valleys trace out very elaborate geography and
describe different possible solutions of string theory. All known stable ones share in
common the property of describing a supersymmetric theory. In such theories each
bosonic particle has a fermionic particle companion. It then turned out that there
actually are several different types of string theories, each having its own infinite set
of degenerate ground states. What made the string theories different were details,
most of which are rather technical, which seemed to affect the particle content and
gauge symmetries of the low energy physics. There was a stage, in the eighties of
the last century, at which some researchers imagined they were on the verge of being
able to perform reliable perturbative calculations of a realistic theory containing all
the aspects of the standard model. That turned out not to be the case. However, in
the process, the geography of many interesting ground states was surveyed. Next, a
large amount of circumstantial evidence started to accumulate hinting that all these
theories are after all not that distinct and may actually be mapped into each other
by a web of surprising dualities. This led once again to the conjecture that there is
but one string theory, this time, the theory that was supposed to encompass all string
theories and bind them, was given a special name, M theory. Even in this framework
all string theories brought as dowries their infinite moduli space. M theory still had
an infinite number of connected different degenerate supersymmetric ground states,
they were elaborated and greatly enriched by what are called brane configurations.
Branes are a special type of solitons that appear in string theory. They come in
various forms and span different dimensions. What is common to several of them,
is that open strings may end on them.
Thus what was thought to be a theory of only closed strings contains open strings
excitations in each of its sectors that contain branes. Moreover, every allowed brane
configuration leads to its own low energy physics. In fact there are suggestions that
our four dimensional experience results from us living on one or several branes
all embedded in a larger brane configuration placed in a higher dimensional spacetime. How does that fit with the expectations to have the One and Only string theory?
Well, in this framework there is one theory but an infinite amount of possible ground
states. Are we owed only once such state? There are those who hope that the vacua
degeneracy will be lifted by some yet to be discovered mechanism leaving the one
unique ground state. Others are loosing their patience and claiming that the NOH
principle takes over at this stage, the theory will retain its large number of ground
states.
Moreover a crucial ingredient of string theory at present is super symmetry
which was discussed obove, a beautiful symmetry not yet detected in nature, and
even if detected, it seems at this stage to play only a role of a broken symmetry.
It is not easy to obtain even remotely realistic models of string theory in which
supersymmetry is broken (that is the symmetry between fermions and bosons is
broken), a nearly vanishing cosmological constant and light particles whose mass
values are amendable to a calculation within a reliable approximation. Candidates
for such ground states have been suggested recently, these particular candidates
consist of very many isolated solutions living off the shore of the supersymmetric
connected valleys. Many of these solutions actually have different values for the
E. Rabinovici
These connected regions of valleys trace out very elaborate geography and
describe different possible solutions of string theory. All known stable ones share in
common the property of describing a supersymmetric theory. In such theories each
bosonic particle has a fermionic particle companion. It then turned out that there
actually are several different types of string theories, each having its own infinite set
of degenerate ground states. What made the string theories different were details,
most of which are rather technical, which seemed to affect the particle content and
gauge symmetries of the low energy physics. There was a stage, in the eighties of
the last century, at which some researchers imagined they were on the verge of being
able to perform reliable perturbative calculations of a realistic theory containing all
the aspects of the standard model. That turned out not to be the case. However, in
the process, the geography of many interesting ground states was surveyed. Next, a
large amount of circumstantial evidence started to accumulate hinting that all these
theories are after all not that distinct and may actually be mapped into each other
by a web of surprising dualities. This led once again to the conjecture that there is
but one string theory, this time, the theory that was supposed to encompass all string
theories and bind them, was given a special name, M theory. Even in this framework
all string theories brought as dowries their infinite moduli space. M theory still had
an infinite number of connected different degenerate supersymmetric ground states,
they were elaborated and greatly enriched by what are called brane configurations.
Branes are a special type of solitons that appear in string theory. They come in
various forms and span different dimensions. What is common to several of them,
is that open strings may end on them.
Thus what was thought to be a theory of only closed strings contains open strings
excitations in each of its sectors that contain branes. Moreover, every allowed brane
configuration leads to its own low energy physics. In fact there are suggestions that
our four dimensional experience results from us living on one or several branes
all embedded in a larger brane configuration placed in a higher dimensional spacetime. How does that fit with the expectations to have the One and Only string theory?
Well, in this framework there is one theory but an infinite amount of possible ground
states. Are we owed only once such state? There are those who hope that the vacua
degeneracy will be lifted by some yet to be discovered mechanism leaving the one
unique ground state. Others are loosing their patience and claiming that the NOH
principle takes over at this stage, the theory will retain its large number of ground
states.
Moreover a crucial ingredient of string theory at present is super symmetry
which was discussed obove, a beautiful symmetry not yet detected in nature, and
even if detected, it seems at this stage to play only a role of a broken symmetry.
It is not easy to obtain even remotely realistic models of string theory in which
supersymmetry is broken (that is the symmetry between fermions and bosons is
broken), a nearly vanishing cosmological constant and light particles whose mass
values are amendable to a calculation within a reliable approximation. Candidates
for such ground states have been suggested recently, these particular candidates
consist of very many isolated solutions living off the shore of the supersymmetric
connected valleys. Many of these solutions actually have different values for the