7,455 research outputs found

### String and M-theory: answering the critics

Using as a springboard a three-way debate between theoretical physicist Lee
Smolin, philosopher of science Nancy Cartwright and myself, I address in
layman's terms the issues of why we need a unified theory of the fundamental
interactions and why, in my opinion, string and M-theory currently offer the
best hope. The focus will be on responding more generally to the various
criticisms. I also describe the diverse application of string/M-theory
techniques to other branches of physics and mathematics which render the whole
enterprise worthwhile whether or not "a theory of everything" is forthcoming.Comment: Update on EPSRC. (Contribution to the Special Issue of Foundations of
Physics: "Forty Years Of String Theory: Reflecting On the Foundations",
edited by Gerard 't Hooft, Erik Verlinde, Dennis Dieks and Sebastian de Haro.
22 pages latex

### How fundamental are fundamental constants?

I argue that the laws of physics should be independent of one's choice of
units or measuring apparatus. This is the case if they are framed in terms of
dimensionless numbers such as the fine structure constant, alpha. For example,
the Standard Model of particle physics has 19 such dimensionless parameters
whose values all observers can agree on, irrespective of what clock, rulers,
scales... they use to measure them. Dimensional constants, on the other hand,
such as h, c, G, e, k..., are merely human constructs whose number and values
differ from one choice of units to the next. In this sense only dimensionless
constants are "fundamental". Similarly, the possible time variation of
dimensionless fundamental "constants" of nature is operationally well-defined
and a legitimate subject of physical enquiry. By contrast, the time variation
of dimensional constants such as c or G on which a good many (in my opinion,
confusing) papers have been written, is a unit-dependent phenomenon on which
different observers might disagree depending on their apparatus. All these
confusions disappear if one asks only unit-independent questions.
We provide a selection of opposing opinions in the literature and respond
accordingly.Comment: Note added. 30 pages latex. 7 figures. arXiv admin note: text overlap
with arXiv:hep-th/0208093 (unpublished

### State of the Unification Address

After reviewing how M-theory subsumes string theory, I report on some new and
interesting developments, focusing on the ``brane-world'': circumventing no-go
theorems for supersymmetric brane-worlds, complementarity of the Maldacena and
Randall-Sundrum pictures; self-tuning of the cosmological constant. I conclude
with the top ten unsolved problems.Comment: 16 pages, Latex. Plenary talk delivered at The Division of Particles
and Fields Meeting of The American Physical Society, August 9-12 2000, Ohio
State University. Minor corrections and references adde

### M-theory on manifolds of G2 holonomy: the first twenty years

In 1981, covariantly constant spinors were introduced into Kaluza-Klein
theory as a way of counting the number of supersymmetries surviving
compactification. These are related to the holonomy group of the compactifying
manifold. The first non-trivial example was provided in 1982 by D=11
supergravity on the squashed S7, whose G2 holonomy yields N=1 in D=4. In 1983,
another example was provided by D=11 supergravity on K3, whose SU(2) holonomy
yields half the maximum supersymmetry. In 2002, G2 and K3 manifolds continue to
feature prominently in the full D=11 M-theory and its dualities. In particular,
singular G2 compactifications can yield chiral (N=1,D=4) models with realistic
gauge groups. The notion of generalized holonomy is also discussed.Comment: Notes added on n, the number of allowed M-theory supersymmetries.
Asymmetric orbifold compactifications of Type II strings from D=10 to D=2
permit n=0,1,2,3,4,5,6,8,9,10,12,16,17,18,20,24,3

### The world in eleven dimensions: a tribute to Oskar Klein

Current attempts to find a unified theory that would reconcile Einstein's
General Relativity and Quantum Mechanics, and explain all known physical
phenomena, invoke the Kaluza-Klein idea of extra spacetime dimensions. The best
candidate is M-theory, which lives in eleven dimensions, the maximum allowed by
supersymmetry of the elementary particles. We give a non-technical account.
An Appendix provides an updated version of Edwin A. Abbott's 1884 satire {\it
Flatland: A Romance of Many Dimensions}. Entitled {\it Flatland, Modulo 8}, it
describes the adventures of a superstring theorist, A. Square, who inhabits a
ten-dimensional world and is initially reluctant to accept the existence of an
eleventh dimension.Comment: Oskar Klein Professorship Inaugural Lecture, University of Michigan,
16 March 2001. 38 pages, Latex, 15 color figure

### M-Theory (the Theory Formerly Known as Strings)

Superunification underwent a major paradigm shift in 1984 when
eleven-dimensional supergravity was knocked off its pedestal by ten-dimensional
superstrings. This last year has witnessed a new shift of equal proportions:
perturbative ten-dimensional superstrings have in their turn been superseded by
a new non-perturbative theory called {\it $M$-theory}, which describes
supermembranes and superfivebranes, which subsumes all five consistent string
theories and whose low energy limit is, ironically, eleven-dimensional
supergravity. In particular, six-dimensional string/string duality follows from
membrane/fivebrane duality by compactifying $M$-theory on $S^1/Z_2 \times K3$
(heterotic/heterotic duality) or $S^1 \times K3$ (Type $IIA$/heterotic duality)
or $S^1/Z_2 \times T^4$ (heterotic/Type $IIA$ duality) or $S^1 \times T^4$
(Type $IIA$/Type $IIA$ duality).Comment: Version to appear in I.J.M.P.A. References added; typographical
errors corrected; 25 pages Late

### Electric/magnetic duality and its stringy origins

We review electric/magnetic duality in $N=4$ (and certain $N=2$) globally
supersymmetric gauge theories and show how this duality, which relates strong
to weak coupling, follows as a consequence of a string/string duality. Black
holes, eleven dimensions and supermembranes also have a part to play in the big
picture.Comment: A few minor improvements; 23 pages LaTe

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