M-Theory Progress Sparks
New Interest in String
by Simon Mitton
The Physics Top Ten captures the spirit of a great
revival of interest in
branes and string theory, following
a lengthy absence of the subject from the ratings. Hot
Papers #3, #4, and #6 demonstrate a surge of activity in
M2-brane theory, which has sparked a flurry of
What’s string theory all about? The short answer is
that it combines general relativity and quantum mechanics
in a quest to find a quantum theory of gravity. The theory
is geometrical: strings are one-dimensional objects which
we perceive as electrons and quarks. Higher-dimensional
objects are branes, a word derived from membrane. From the
point of view of being accessible to curious
non-specialists, the theory is heavy duty in terms of its
mathematical formulation and its technical language, which
is a barrier to a wider understanding of the theory.
String theory is multifaceted in the sense that its
mathematical landscapes are richly varied, and only a small
class of its constructs appear to connect to reality.
Historically, theoretical physics has been strongly driven
by the quest for unification and simplification, although
at the new frontiers it may not seem like that.
Maxwell’s electromagnetic theory combined electricity
and magnetism. In that spirit, string theorists search for
the Theory of Everything, while their critics dismiss the
efforts as a theory of nothing, on the grounds that it has
never made a testable prediction.
In 1995 string theorists grazing at a smorgasbord of tasty
options identified 11-dimensional M theory as the bedrock
on which to anchor their constructs. The interest stems
from the fact that M theory has the maximum possible amount
of supersymmetry in three dimensions. But there’s
more: M-theory has duality, meaning that quantities thought
to be separate are in fact linked through the mathematics.
Duality is a powerful concept in theoretical physics.
M-branes are mysterious objects, and little is known apart
from the special case of a single M-brane. That’s
quite a contrast with D-branes, where a description in
terms of open strings has driven great progress in string
theory and gauge theory.
In 1997 Juan Maldacena, a co-author of Hot Paper #3, made a
conceptual breakthrough with the conjecture that a string
theory defined in one space is equivalent to a quantum
field theory without gravity defined on the boundary of the
space. This duality set in motion many new lines of
research in quantum gravity. M-theory has a known
gravitational dual, which is the first step towards
understanding M2-branes at singularities (such as
Although the dynamics of a single M-brane are well
understood, very little is known about the interactions of
multiple M2-branes. That problem is addressed in Hot Paper
#4, in which
Jonathan Bagger and Neil Lambert construct a
supersymmetric field theory in three dimensions that is
consistent with all the symmetries expected of a multiple
M2-brane theory. This paper immediately attracted the
attention of other string theorists who have carried out
computations using the algebraic framework developed by
Bagger and Lambert. [View a Research Front Map titled:
Paper #4 motivated the research of Ofer Aharony and
colleagues that is reported in Hot Paper #3, which examines
M2-branes in flat space. For Science Watch from
Reuters, Professor Aharony offered the following
comment on the technical aspects of the paper: "Our work
led to many generalizations of M2-branes in other
backgrounds. Integrable structures have been found that may
eventually lead to a solution of the theory of M-branes."
The highly supersymmetric three-dimensional conformal field
theories examined in the paper are interesting for various
In Hot Paper #6, Bagger and Lambert offer some comments on
various physical aspects of the multiple M2-brane set-up
proposed in #4. They have tested the model further, and
conclude that it meets all expectations for M-theory. In
terms of what the model may lead to, they state "the most
pressing open issue is obtaining an infinite class of three
algebras that can represent an arbitrary number of
Elsewhere in the Physics Top Ten, the papers on
observational cosmology (#1, #2, #5, and #10) continue to
attract a large following. Due to the mandatory two-year
"retirement" age for Hot Papers, this is the last time we
shall see #2, which has been continuously in the Top Ten
for virtually all two years of its eligibility, registering
1,608 citations. The new #1, in fact, replaces #2 as the
key reference on the cosmological interpretation of WMAP
Dr. Simon Mitton is a Fellow of St. Edmund’s
College, Cambridge, U.K.
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