GRAVITY THEORY SEMINARS 2002
Abstracts:
Feb. 8, Fri., 1:30pm, Room 4208
David Mattingly, UMD
``Astrophysical Constraints on Lorentz Breaking
Dispersion Relations''
Current astrophysical observations can strongly
constrain Lorentz breaking dispersion relations, even when the deviation
from Lorentz symmetry is suppressed by the Planck energy. We discuss these
constraints as well as new phenomena that may occur when Lorentz invariance
is broken.
Feb. 8, Fri., 2:00pm, Room 4208
Jim van Meter, UC Davis
``Post Minkowskian approach to binary black
hole dynamics''
Mar. 4, Mon. 1:00pm, Room 4102
Akikazu Hashimoto, IAS Princeton
``Observables of String Field Theory''
Mar. 22, Fri. 1:00pm, Room 4208
BeiLok Hu, UMD
``A Kinetic Theory Approach to Quantum Gravity''
We describe a kinetic theory
approach to quantum gravity  by which we mean a theory of the microscopic
structure of spacetime, not necessarily equivalent to a theory obtained
by quantizing general relativity. A figurative conception of this program
is like building a ladder with two knotted poles: quantum matter field
on the right and spacetime on the left. Each rung connecting the corresponding
knots represent a distinct level of structure. The lowest rung is hydrodynamics
and general relativity; the next rung is semiclassical gravity, with the
expectation value of quantum fields acting as source in the semiclassical
Einstein equation. We recall how ideas from the statistical mechanics of
interacting quantum fields helped us identify the existence of noise in
the matter field and its effect on metric fluctuations, leading to the
establishment of the third rung: stochastic gravity, described by the EinsteinLangevin
equation. Our pathway from stochastic to quantum gravity is via the correlation
hierarchy of noise and induced metric fluctuations.
Three essential tasks beckon:
 Deduce the correlations of metric fluctuations
from correlation noise in the matter field;
 Reconstruction of quantum coherence this is
the reverse of decoherence  from these correlation functions
 Use the BoltzmannLangevin equations to identify
distinct collective variables depicting recognizable metastable structures
in the kinetic and hydrodynamic regimes of quantum matter fields and how
they demand of their corresponding spacetime counterparts.
This will give us a hierarchy of generalized stochastic
equations  call them the BoltzmannEinstein hierarchy of quantum gravity
 for each level of spacetime structure, from the the macroscopic (general
relativity) through the mesoscopic (stochastic gravity) to the microscopic
(quantum gravity).
Apr. 5, Fri. 1:30pm, Room 4208
Stefano Liberati, UMD
``Towards the observation of Hawking radiation
in BoseEinstein condensates''
Acoustic analogues of black holes
(dumb holes) are generated when a supersonic fluid flow forms a trapped
region from which sound cannot escape. The surface of no return, the acoustic
horizon, is qualitatively very similar to the event horizon of a black
hole in general relativity. In particular Hawking radiation (a thermal bath
of phonons with temperature proportional to the ``surface gravity'') is
expected to occur. In this talk I shall consider a supersonic flow of a
BoseEinstein condensate. First I shall discuss how an effective geometry
emerges for the propagation of phonons in these system. After I shall expose
the opportunities and the pitfalls on the way to obtain an experimental
confirmation of the analog Hawking effect.
Apr. 9, Tue. 12:30pm, Room
4208
Niall O'Murchadha, University Collge, Cork, Ireland
``Relativity without relativity: emergent time
and the classical fields''
Jacobi introduced a parameter
time action, called the Jacobi Principle, of the form sqrt{E V
}sqrt{T} in particle mechanics, where E is a constant, the
total energy, V is the potential energy, and T the kinetic
energy. He showed that it reproduced all of the standard equations but with
an emergent time. I will consider an action quite like the Jacobi Principle
but I will take the square roots of the individual kinetic energies of each
particle and then sum. This is still reparametrisation invariant. This new
action has a large set of constraints which are not mirrored by any symmetry
of the action, and thus is not, in general, selfconsistent. What it does
do is act as a filter. If the particles do not interact everything works beautifully.
The `missing' symmetries appear in the solution because each particle trajectory
turns out to be independently reparametrisable. The action has only a global
time reparametrisation but the solutions have local time reparametrisation.
This local square root action can be easily generalised to field theories
and one can use it to pick out each of the standard fields of modern classical
physics. If we choose the space of threemetrics as the configuration space
we get general relativity, if we add a threevector we get the Maxwell field,
if we have several interacting vector fields, they must satisfy the YangMills
equations. In each case we go from a low symmetry action to a high symmetry
solution just by demanding that the action is selfconsistent.
Apr. 26, Fri.. 1:30pm, Room 4208
Matt Visser, Washington University, St. Louis.
``The quantum physics of chronology protection''
Classical general relativity
(Einstein gravity) has many ``solutions'' that appear to be ``time machines''.
This makes most physicists very nervous. There are several ways of fixing
things up. In particular, Stephen Hawking has formulated the ``chronology
protection conjecture'' wherein quantum physics steps in to save the day
(and so ``makes the universe safe for historians'').
I will assume the audience is familiar with Quantum
Field Theory, and knows some basic General relativity.
May 10, Fri.. 1:30pm, Room 4208
Esteban Calzetta, Buenos Aires University.
``Thermalization
in LargeN quantum field theory''
We shall discuss whether the next to leading order
LargeN approximation correctly describes the thermalization process in
quantum field theory. We shall consider a O(N) invariant scalar field theory
with unbroken symmetry. We shall show that, in this approximation, the
only translation invariant solutions to the Schwinger  Dyson equations
are thermal. Together with the familiar KadanoffBaym approach to quantum
kinetic theory, this shows that this approximation predicts thermalization,
at least for certain initial conditions. This analytic result provides support
for recent claims in the literature based on numerical evidence.
However, there is a glitch. If we adhere strictly to the next to leading
order approximation, then the truncated theory, unlike the exact theory,
allows thermalization with non vanishing chemical potential. To obtain the
correct behavior, we must admit, within the perturbative SchwingerDyson
equations, corrections to all orders in the density of states. But these
are of the same order of magnitude than other terms previously discarded
because of the Large N approximation.
We conclude that LargeN theory is a nice theoretical framework to study
these issues, but probably not a realistic depiction of the phenomenon.
June 21, Fri.. 1:30pm, Room 4208
Amit Ghosh, Penn State University.
``Black hole entropy from semiclassical methods
and 2nd law''
Symmetry based approaches to the black hole entropy
are attractive because they are very general and do not depend on details
of the quantization method. However we point out that, of the two available
approaches, one faces conceptual problems (also emphasized by others), while
the second contains certain technical flaws. We correct these errors and,
within the new, improved scheme, calculate the entropy of 3dimensional black
holes. We find that, while the new symmetry vector fields are welldefined
on the "stretched horizon", and lead to welldefined Hamiltonians satisfying
the expected Lie algebra, they fail to admit a welldefined limit to the
horizon. This suggests that, although the formal calculation can be carried
out at the classical level, its real, conceptual origin probably lies in
the quantum theory. We then discuss some open issues related to the 2nd law
of black hole mechanics  this is an ongoing project.
Oct 4, Fri.. 1:30pm, Room 4208
Eric Poisson, University of Guelph.
``Radiation reaction in strong gravitational fields''
In this talk I will explore the fundamental physics
of radiation reaction (the response of an orbiting body to the radiative
loss of energy and angular momentum) in strong gravitational fields. After
some motivational remarks I will introduce the concept of a gravitational
selfforce acting on a massive object moving in curved spacetime. I will
then pull back and describe the physics of radiation reaction in the simpler
context of electrodynamics, both in flat and curved spacetime. I will then
return to the gravitational selfforce and describe recent work (by me and
many others) to compute it and use it in calculations of gravitational waves
emitted by compact objects in orbit around massive black holes, one of the
most promising type of signals for detection by the Laser Interferometer
Space Antenna.
Nov. 1, Fri.. 1:30pm, Room 4208
Albert Roura, University of Maryland.
``EinsteinLangevin equation as the
large N limit for quantum metric fluctuations''
The socalled EinsteinLangevin equation has been
proposed in order to extend the applications of the semiclassical theory
of gravity to those situations in which the fluctuations of the stress tensor
operator for the quantum matter fields are important. After briefly reviewing
some aspects of nonequilibrium quantum field thoery, it will be explained
how a stochastic description can be introduced to gain information on quantum
properties of simple open systems. This will then be extended to the gravitational
case by considering N quantum matter fields interacting with the metric perturbations
around a given background spacetime. In particular, it will be shown that
the correlation functions obtained using the EinsteinLangevin equation are
equivalent to the leading order contribution in the large N limit to the
correlation functions that would follow from a purely quantum treatment.
Nov 8, Fri.. 1:30pm, Room 420
8
Jong Hyuk Yoon, Konkuk University,
Korea / U. of Chicago
``Quasilocal dynamics of black holes''
A set of exact quasilocal conservation equations
is derived from the Einstein's equations using the firstorder (2,2)formalism
of general relativity of 4dimensional spacetime. These equations are interpreted
as quasilocal energy, linear momentum, and angular momentum conservation
equations. In the asymptotic region of asymptotically flat spacetimes, it
is shown that these quasilocal conservation equations reduce to the conservation
equations of Bondi energy, linear momentum, and angular momentum, respectively.
When restricted to the event horizon of a generic dynamical black hole,
the quasilocal conservation equations coincide with the conservation equations
studied by Thorne et al. All of these quasilocal quantities are expressed
as invariant twosurface integrals, and geometrical interpretations in terms
of the area of a given twosurface and a pair of null vector fields
orthogonal to that surface are given. Finally we apply these quasilocal
conservation equations to study the dynamics of a generic black hole, and
obtain the {\it dynamical} version of the first law of black hole mechanics,
which states that the change in the internal quasilocal energy of a generic
black hole is given by the positivedefinite heat influx into the black
hole minus the work done by the black hole itself in deforming and rotating
its horizon.
Nov 15, Fri.. 1:30pm, Room
4208
Enric Verdaguer, University of Barcelona
``Cosmological Structure Formation in Stochastic Gravity''
Stochastic gravity describes the interaction of the gravitational field
with quantum matter fields beyond the semiclassical limit. The EinsteinLangevin
equations are introduced as a consistent set of equations for a first order
correction to semiclassical gravity. The solutions of these equations
allow the computation of the symmetric twopoint quantum correlations of
the metric fluctuations induced by the quantum matter fields over the semiclassical
background. This theory is particularly useful in the cosmological structure
formation problem. A simple inflationary cosmological model driven by an
inflaton field is described. It is shown that the twopoint correlation functions
for the gravitational fluctuations induced by the inflaton fluctuations lead
to an almost scale invariant spectrum at large scales. We also comment on
the application of the theory to the structure formation problem in an inflationary
model not driven by an inflaton field such as the trace anomaly induced inflation
model (Starobinsky inflation). The EinstenLangevin equations for this problem
have been computed.
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Last updated:June 13, 2002