Lubos Motl
Oct11-06, 02:07 PM
Hep-th papers on Tuesday
************************
http://motls.blogspot.com/2006/05/hep-th-papers-on-tuesday.html
Let me summarize each of the 24 hep-th papers that appeared last night.
Freed, Moore, and Segal argue that the electric and magnetic fluxes on a
spacetime manifold can't be measured simultaneously because of a version
of the uncertainty principle. That should not be unexpected. Only the
electric potential or the magnetic potential may be chosen as a
fundamental degree of freedom which makes their fluxes well-defined for a
given configuration: the fluxes of the dual field are then undetermined.
Moreover, the authors argue that the uncertainty principle only applies to
torsion classes. One of the implications is that the K-theory class of the
Ramond-Ramond fields can't be physically measured. There also exists a
less mathematically formal version of that paper even though I am a bit
confused which of the two is addressed to the physicists. The insights
have special consequences for self-dual form fields; they're connected
with Pontrjagin classes.
Ben Craps offers the written version of his CERN lectures about
big-bang-like singularities in string theory. He reviews the role of the
winding modes, the Milne orbifold, attempts to use the AdS/CFT
correspondence, and the matrix big bang.
Dasgupta, Grisaru, Gwyn, Katz, Knauf, and Tatar re-derive the metric
describing certain wrapped fivebranes (based on functions among which
"cot" is the most complex one). Also, they write down certain new
non-Kähler geometries, and offer you a table that relates the non-Kähler
deformations of certain backgrounds to other effects related by dualities,
something that Anke Knauf has shown us some time ago.
Washington Taylor has written a mostly non-technical review of string
field theory for a book. While it does not annoy the reader with too many
formulae, it contains many references, including the references on newest
developments, and may be viewed as a state-of-the-art summary of the
subject.
Donagi, Reinbacher, and Yau take one of the simplest Calabi-Yau manifolds,
namely the quintic hypersurface, define heterotic string theory on them,
but consider very general configurations of the gauge field (the bundle).
Using similar diagrams that were important for the recent heterotic
standard model papers, they derive things as complicated as the Yukawa
couplings of these backgrounds.
Bambah, Mahajan, and Mukku propose the non-Abelian version of an unusual
kind of unification that one of the authors proposed in the Abelian case.
What unification do we mean? It is a unification of Maxwell's theory with
fluid dynamics, speculated to be relevant for the quark-gluon plasma.
Well, I personally see no unification in that paper. What I see is some
theory that contains the Yang-Mills field (previously Maxwell) together
with an additional scalar field and a vector field, and they are coupled
in some strange way. The Republican Party and the Democratic Party exist
in the same country and interact, but that still does not make them
unified. As you can see, it seems that the entertainment value of this
paper exceeds most others (but not all of them, as you will see below).
Damiano Anselmi offers a proof (?) that general relativity is a
renormalizable theory, if defined as "acausal gravity". The first
sentences of the body of the paper indicate that the acausality is far
from being the worst thing in this proposal: the first sentence argues
that "the necessity to quantize gravity is a debatable issue". Because it
seems likely that these initial sentences play an important role and
because I personally consider a non-quantized gravity within the quantum
world to be an obvious inconsistency, I can't recommend you to read the
full paper.
Svrek and Witten study the axion decay constant "F" in various scenarios
in string theory, including M-theory on G2 manifolds, intersecting
braneworlds, heterotic M-theory, and others. Recall that the axions are
useful to explain the strong CP-problem i.e. the smallness of the QCD
theta-angle that couples to the instanton number. Also, the axions could
be relevant to describe various potentially exciting recent experiments.
However, astrophysical bounds tell you that the axions that couple to
visible matter should not be too heavy. They should be lighter than the
GUT scale or so. The authors find that this is naturally achieved only if
the visible matter lives on shrunk cycles.
Buividovich and Kuvshinov investigate gauge theories - such as QCD - using
the method of the random walk. Define the open Wilson line from a given
point in spacetime to the point "(t,0,0,0)". You will get a point in the
gauge group manifold, and you may ask what is the probability distribution
to obtain a certain particular element of the group. The heat kernel
equation on the group manifold becomes relevant.
W.F. Kao starts by saying that recently there is a growing interest in the
Kantowski-Sachs (KS) universe - a term that I've never heard of - based on
some purely higher-derivative gravity (?). While the phrase "pure
higher-derivative gravity" may sound insulting, it does not seem to play
much role in the paper. On the other hand, the KS universe seems to be
nothing else than the FRW universe with an extra dependence on the
coordinate "theta", making it anisotropic. The growing interest in this
geometry is proved by three papers from 1999, 2003, 2004. This KS Universe
is supposed to be relevant for inflation. Because I don't know where to
put this paper in my understanding of the world, I can't tell you more.
Laamara, Drissi, and Saidi want to relate topological string theory on the
conifold with the fractional quantum hall effect (FQHE). Well, there are
already quite many things that are claimed to be equivalent to topological
string theory on the conifold. The links between FQHE and fluids of
D-branes (with their characteristic non-commutativity) were investigated
by Susskind, Hellerman, Polychronakos, and others years ago, but the
conifold seems to be a novelty introduced by the present authors.
Justin David and Ashoke Sen prove the formula for the generating function
for the degeneracies of dyons of the CHL string - the kind of formulae
that Davide Gaiotto knows very well. They map the system to the D1-D5
system and use, among other tools, various properties of the modular
functions.
Delduc and Ivanov explain the origin of some dualities in supersymmetric
quantum mechanics. They're dualities between theories with the same number
of fermions and different numbers of bosons. The equivalence is shown by
gauging additional superfields.
Gherghetta and Giedt realize the Randall-Sundrum models in terms of type
IIB string theory on "AdS5 times T11" where "T11" is the base of the
conifold, equipped with probe D7-branes. The tip is governed by the
Klebanov-Strassler theory; the following region by the Klebanov-Witten
regime; and the rest is connected to a compact Calabi-Yau manifold. The
additional D7-branes provide you with bulk gauge fields - something very
popular among the RS phenomenologists. They also claim to have some purely
stringy natural predictions for physics of these models but I have not
found what they are.
R.P. Malik studies QED using the BRST formalism written in terms of
superfields. Not only that I have never heard of the "horizontality
condition", but the BRST formalism itself seems like an overkill to me if
it is applied to QED, especially in this BRST superfield formalism.
Lee and Yee construct new 7-manifolds "X" that can be multiplied by "AdS4"
to give you new solutions of M-theory. These seven-dimensional manifolds -
in fact, "tri-Sasakian" manifolds - are obtained from
twelve-real-dimensional hyperKähler manifolds. Four dimensions are lost by
a method I have not understood, and the last fifth dimension disappears by
quotienting by a U(1). They also say something about the dual CFT3 theory,
for example things that follow from their calculated volume of the
seven-manifold.
Neznamov only offers a PDF file with a few pages that only seem to contain
some trivial - but not necessarily correct - manipulation with the Dirac
gamma matrices. His following PDF paper is a bit longer but even more
entertaining. He argues that the Standard Model fermions may be massive
even without Yukawa couplings by generalizing - more precisely, by
screwing - the covariant derivative. Neznamov's covariant derivative has
the standard time-component, but the spatial component has an extra term
"inverse nabla times squared mass". Lorentz invariance, unitarity, gauge
invariance, mathematics, and common sense must be sacrificed for more
noble goals.
Hortacsu and Taskin provide more than the PDF file. They argue that in
some model, a composite spin 1 particle must interact although the spin
1/2 partons don't interact. I am not sure what they exactly claim and why
but it sounds like an attempt to violate the Weinberg-Witten theorem -
although the latter is not even cited.
Janos Polonyi wants to study the emergence of the classical limit in QED.
Well, yes, I don't quite follow what is the open question that is being
answered. At any rate, to answer "this" question, the author defines some
kind of generalized density matrix, to make the quantum theory look more
classical. Because I don't believe that anything from the quantum
character of quantum field theory has to be sacrificed to understand the
classical limit properly, I doubt that one can learn something new and
true from that paper.
Edward Shuryak discusses AdS/QCD. He claims that he was the first
discoverer of the statement that confinement in the field theory should be
described as an explicitly added potential in the AdS bulk that depends on
the holographic dimension and increases as you go away from the UV. This
picture, recently promoted by Karch et al., is certainly a natural and
qualitatively correct dual parameterization of the same effect except that
I don't know how this follows from string theory, which makes it hard to
imagine that a new precise understanding of physics of confinement.
Berenstein and Cotta look at "emergent geometry" within the AdS/CFT
context. It is an orbifold of a previous paper by David Berenstein.
David's conceptually intriguing goal is to construct the bulk geometry
from certain partons - particles that live on the moduli space of
D3-branes in flat space - that can be identified within the gauge theory.
These partons effectively repel each other which makes them arrange
themselves into the right geometry, assuming that you believe the
statistical mechanical kind of reasoning. It is a sort of cute heuristic
description of the origin of the compact part of the geometry. It would be
even more interesting if one could derive something about the moduli space
of possible geometries from this starting point.
Mintz, Farina, Maia Neto, and Rodrigues look at particle production
(controlled by the Bogoliubov coefficients) of bosons in a 1+1-dimensional
theory with a boundary where the boundary conditions interpolate between
the Neumann ones and the Dirichlet ones.
__________________________________________________ ____________________________
E-mail: lumo@matfyz.cz fax: +1-617/496-0110 Web: http://lumo.matfyz.cz/
eFax: +1-801/454-1858 work: +1-617/384-9488 home: +1-617/868-4487 (call)
Webs: http://schwinger.harvard.edu/~motl/ http://motls.blogspot.com/
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
************************
http://motls.blogspot.com/2006/05/hep-th-papers-on-tuesday.html
Let me summarize each of the 24 hep-th papers that appeared last night.
Freed, Moore, and Segal argue that the electric and magnetic fluxes on a
spacetime manifold can't be measured simultaneously because of a version
of the uncertainty principle. That should not be unexpected. Only the
electric potential or the magnetic potential may be chosen as a
fundamental degree of freedom which makes their fluxes well-defined for a
given configuration: the fluxes of the dual field are then undetermined.
Moreover, the authors argue that the uncertainty principle only applies to
torsion classes. One of the implications is that the K-theory class of the
Ramond-Ramond fields can't be physically measured. There also exists a
less mathematically formal version of that paper even though I am a bit
confused which of the two is addressed to the physicists. The insights
have special consequences for self-dual form fields; they're connected
with Pontrjagin classes.
Ben Craps offers the written version of his CERN lectures about
big-bang-like singularities in string theory. He reviews the role of the
winding modes, the Milne orbifold, attempts to use the AdS/CFT
correspondence, and the matrix big bang.
Dasgupta, Grisaru, Gwyn, Katz, Knauf, and Tatar re-derive the metric
describing certain wrapped fivebranes (based on functions among which
"cot" is the most complex one). Also, they write down certain new
non-Kähler geometries, and offer you a table that relates the non-Kähler
deformations of certain backgrounds to other effects related by dualities,
something that Anke Knauf has shown us some time ago.
Washington Taylor has written a mostly non-technical review of string
field theory for a book. While it does not annoy the reader with too many
formulae, it contains many references, including the references on newest
developments, and may be viewed as a state-of-the-art summary of the
subject.
Donagi, Reinbacher, and Yau take one of the simplest Calabi-Yau manifolds,
namely the quintic hypersurface, define heterotic string theory on them,
but consider very general configurations of the gauge field (the bundle).
Using similar diagrams that were important for the recent heterotic
standard model papers, they derive things as complicated as the Yukawa
couplings of these backgrounds.
Bambah, Mahajan, and Mukku propose the non-Abelian version of an unusual
kind of unification that one of the authors proposed in the Abelian case.
What unification do we mean? It is a unification of Maxwell's theory with
fluid dynamics, speculated to be relevant for the quark-gluon plasma.
Well, I personally see no unification in that paper. What I see is some
theory that contains the Yang-Mills field (previously Maxwell) together
with an additional scalar field and a vector field, and they are coupled
in some strange way. The Republican Party and the Democratic Party exist
in the same country and interact, but that still does not make them
unified. As you can see, it seems that the entertainment value of this
paper exceeds most others (but not all of them, as you will see below).
Damiano Anselmi offers a proof (?) that general relativity is a
renormalizable theory, if defined as "acausal gravity". The first
sentences of the body of the paper indicate that the acausality is far
from being the worst thing in this proposal: the first sentence argues
that "the necessity to quantize gravity is a debatable issue". Because it
seems likely that these initial sentences play an important role and
because I personally consider a non-quantized gravity within the quantum
world to be an obvious inconsistency, I can't recommend you to read the
full paper.
Svrek and Witten study the axion decay constant "F" in various scenarios
in string theory, including M-theory on G2 manifolds, intersecting
braneworlds, heterotic M-theory, and others. Recall that the axions are
useful to explain the strong CP-problem i.e. the smallness of the QCD
theta-angle that couples to the instanton number. Also, the axions could
be relevant to describe various potentially exciting recent experiments.
However, astrophysical bounds tell you that the axions that couple to
visible matter should not be too heavy. They should be lighter than the
GUT scale or so. The authors find that this is naturally achieved only if
the visible matter lives on shrunk cycles.
Buividovich and Kuvshinov investigate gauge theories - such as QCD - using
the method of the random walk. Define the open Wilson line from a given
point in spacetime to the point "(t,0,0,0)". You will get a point in the
gauge group manifold, and you may ask what is the probability distribution
to obtain a certain particular element of the group. The heat kernel
equation on the group manifold becomes relevant.
W.F. Kao starts by saying that recently there is a growing interest in the
Kantowski-Sachs (KS) universe - a term that I've never heard of - based on
some purely higher-derivative gravity (?). While the phrase "pure
higher-derivative gravity" may sound insulting, it does not seem to play
much role in the paper. On the other hand, the KS universe seems to be
nothing else than the FRW universe with an extra dependence on the
coordinate "theta", making it anisotropic. The growing interest in this
geometry is proved by three papers from 1999, 2003, 2004. This KS Universe
is supposed to be relevant for inflation. Because I don't know where to
put this paper in my understanding of the world, I can't tell you more.
Laamara, Drissi, and Saidi want to relate topological string theory on the
conifold with the fractional quantum hall effect (FQHE). Well, there are
already quite many things that are claimed to be equivalent to topological
string theory on the conifold. The links between FQHE and fluids of
D-branes (with their characteristic non-commutativity) were investigated
by Susskind, Hellerman, Polychronakos, and others years ago, but the
conifold seems to be a novelty introduced by the present authors.
Justin David and Ashoke Sen prove the formula for the generating function
for the degeneracies of dyons of the CHL string - the kind of formulae
that Davide Gaiotto knows very well. They map the system to the D1-D5
system and use, among other tools, various properties of the modular
functions.
Delduc and Ivanov explain the origin of some dualities in supersymmetric
quantum mechanics. They're dualities between theories with the same number
of fermions and different numbers of bosons. The equivalence is shown by
gauging additional superfields.
Gherghetta and Giedt realize the Randall-Sundrum models in terms of type
IIB string theory on "AdS5 times T11" where "T11" is the base of the
conifold, equipped with probe D7-branes. The tip is governed by the
Klebanov-Strassler theory; the following region by the Klebanov-Witten
regime; and the rest is connected to a compact Calabi-Yau manifold. The
additional D7-branes provide you with bulk gauge fields - something very
popular among the RS phenomenologists. They also claim to have some purely
stringy natural predictions for physics of these models but I have not
found what they are.
R.P. Malik studies QED using the BRST formalism written in terms of
superfields. Not only that I have never heard of the "horizontality
condition", but the BRST formalism itself seems like an overkill to me if
it is applied to QED, especially in this BRST superfield formalism.
Lee and Yee construct new 7-manifolds "X" that can be multiplied by "AdS4"
to give you new solutions of M-theory. These seven-dimensional manifolds -
in fact, "tri-Sasakian" manifolds - are obtained from
twelve-real-dimensional hyperKähler manifolds. Four dimensions are lost by
a method I have not understood, and the last fifth dimension disappears by
quotienting by a U(1). They also say something about the dual CFT3 theory,
for example things that follow from their calculated volume of the
seven-manifold.
Neznamov only offers a PDF file with a few pages that only seem to contain
some trivial - but not necessarily correct - manipulation with the Dirac
gamma matrices. His following PDF paper is a bit longer but even more
entertaining. He argues that the Standard Model fermions may be massive
even without Yukawa couplings by generalizing - more precisely, by
screwing - the covariant derivative. Neznamov's covariant derivative has
the standard time-component, but the spatial component has an extra term
"inverse nabla times squared mass". Lorentz invariance, unitarity, gauge
invariance, mathematics, and common sense must be sacrificed for more
noble goals.
Hortacsu and Taskin provide more than the PDF file. They argue that in
some model, a composite spin 1 particle must interact although the spin
1/2 partons don't interact. I am not sure what they exactly claim and why
but it sounds like an attempt to violate the Weinberg-Witten theorem -
although the latter is not even cited.
Janos Polonyi wants to study the emergence of the classical limit in QED.
Well, yes, I don't quite follow what is the open question that is being
answered. At any rate, to answer "this" question, the author defines some
kind of generalized density matrix, to make the quantum theory look more
classical. Because I don't believe that anything from the quantum
character of quantum field theory has to be sacrificed to understand the
classical limit properly, I doubt that one can learn something new and
true from that paper.
Edward Shuryak discusses AdS/QCD. He claims that he was the first
discoverer of the statement that confinement in the field theory should be
described as an explicitly added potential in the AdS bulk that depends on
the holographic dimension and increases as you go away from the UV. This
picture, recently promoted by Karch et al., is certainly a natural and
qualitatively correct dual parameterization of the same effect except that
I don't know how this follows from string theory, which makes it hard to
imagine that a new precise understanding of physics of confinement.
Berenstein and Cotta look at "emergent geometry" within the AdS/CFT
context. It is an orbifold of a previous paper by David Berenstein.
David's conceptually intriguing goal is to construct the bulk geometry
from certain partons - particles that live on the moduli space of
D3-branes in flat space - that can be identified within the gauge theory.
These partons effectively repel each other which makes them arrange
themselves into the right geometry, assuming that you believe the
statistical mechanical kind of reasoning. It is a sort of cute heuristic
description of the origin of the compact part of the geometry. It would be
even more interesting if one could derive something about the moduli space
of possible geometries from this starting point.
Mintz, Farina, Maia Neto, and Rodrigues look at particle production
(controlled by the Bogoliubov coefficients) of bosons in a 1+1-dimensional
theory with a boundary where the boundary conditions interpolate between
the Neumann ones and the Dirichlet ones.
__________________________________________________ ____________________________
E-mail: lumo@matfyz.cz fax: +1-617/496-0110 Web: http://lumo.matfyz.cz/
eFax: +1-801/454-1858 work: +1-617/384-9488 home: +1-617/868-4487 (call)
Webs: http://schwinger.harvard.edu/~motl/ http://motls.blogspot.com/
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^