921 research outputs found
Arrow of time in a recollapsing quantum universe
We show that the Wheeler-DeWitt equation with a consistent boundary condition
is only compatible with an arrow of time that formally reverses in a
recollapsing universe. Consistency of these opposite arrows is facilitated by
quantum effects in the region of the classical turning point. Since
gravitational time dilation diverges at horizons, collapsing matter must then
start re-expanding ``anticausally" (controlled by the reversed arrow) before
horizons or singularities can form. We also discuss the meaning of the
time-asymmetric expression used in the definition of ``consistent histories".
We finally emphasize that there is no mass inflation nor any information loss
paradox in this scenario.Comment: Many conceptual clarifications include
Unitarity of Quantum Theory and Closed Time-Like Curves
Interacting quantum fields on spacetimes containing regions of closed
timelike curves (CTCs) are subject to a non-unitary evolution . Recently, a
prescription has been proposed, which restores unitarity of the evolution by
modifying the inner product on the final Hilbert space. We give a rigorous
description of this proposal and note an operational problem which arises when
one considers the composition of two or more non-unitary evolutions. We propose
an alternative method by which unitarity of the evolution may be regained, by
extending to a unitary evolution on a larger (possibly indefinite) inner
product space. The proposal removes the ambiguity noted by Jacobson in
assigning expectation values to observables localised in regions spacelike
separated from the CTC region. We comment on the physical significance of the
possible indefiniteness of the inner product introduced in our proposal.Comment: 13 pages, LaTeX. Final revised paper to be published in Phys Rev D.
Some changes are made to expand our discussion of Anderson's Proposal for
restoring unitarit
Perturbative two- and three-loop coefficients from large beta Monte Carlo
Perturbative coefficients for Wilson loops and the static quark self-energy
are extracted from Monte Carlo simulations at large beta on finite volumes,
where all the lattice momenta are large. The Monte Carlo results are in
excellent agreement with perturbation theory through second order. New results
for third order coefficients are reported. Twisted boundary conditions are used
to eliminate zero modes and to suppress Z_3 tunneling.Comment: 6 pages, 5 figures. Contributions of Howard Trottier and Paul
Mackenzie to Lattice '9
Scaling of the B and D meson spectrum in lattice QCD
We give results for the and the meson spectrum using NRQCD on the
lattice in the quenched approximation. The masses of radially and orbitally
excited states are calculated as well as -wave hyperfine and -wave fine
structure. Radially excited -states are observed for the first time. Radial
and orbital excitation energies match well to experiment, as does the
strange-non-strange -wave splitting. We compare the light and heavy quark
mass dependence of various splittings to experiment. Our -results cover a
range in lattice spacings of more than a factor of two. Our -results are
from a single lattice spacing and we compare them to numbers in the literature
from finer lattices using other methods. We see no significant dependence of
physical results on the lattice spacing.
PACS: 11.15.Ha 12.38.Gc 14.40.Lb 14.40.NdComment: 78 pages, 29 tables, 30 figures Revised version. Minor corrections to
spelling and wordin
Hybrid configuration content of heavy S-wave mesons
We use the non-relativistic expansion of QCD (NRQCD) on the lattice to study
the lowest hybrid configuration contribution to the ground state of heavy
S-wave mesons. Using lowest-order lattice NRQCD to create the heavy-quark
propagators, we form a basis of ``unperturbed'' S-wave and hybrid states. We
then apply the lowest-order coupling of the quark spin and chromomagnetic field
at an intermediate time slice to create ``mixed'' correlators between the
S-wave and hybrid states. From the resulting amplitudes, we extract the
off-diagonal element of our two-state Hamiltonian. Diagonalizing this
Hamiltonian gives us the admixture of hybrid configuration within the meson
ground state. The present effort represents a continuation of previous work:
the analysis has been extended to include lattices of varying spacings, source
operators having better overlap with the ground states, and the pseudoscalar
(along with the vector) channel. Results are presented for bottomonium
(, ) using three different sets of quenched lattices. We
also show results for charmonium (, ) from one lattice set,
although we note that the non-relativistic approximation is not expected to be
very good in this case.Comment: 9 pages, 7 figures, version to appear in Phys Rev
One-Loop Matching of the Heavy-Light A_0 and V_0 Currents with NRQCD Heavy and Improved Naive Light Quarks
One-loop matching of heavy-light currents is carried out for a highly
improved lattice action, including the effects of dimension 4 O(1/M) and O(a)
operators. We use the NRQCD action for heavy quarks, the Asqtad improved naive
action for light quarks, and the Symanzik improved glue action. As part of the
matching procedure we also present results for the NRQCD self energy and for
massless Asqtad quark wavefunction renormalization with improved glue.Comment: 25 pages, 3 eps-figure
Adjoint "quarks" on coarse anisotropic lattices: Implications for string breaking in full QCD
A detailed study is made of four dimensional SU(2) gauge theory with static
adjoint ``quarks'' in the context of string breaking. A tadpole-improved action
is used to do simulations on lattices with coarse spatial spacings ,
allowing the static potential to be probed at large separations at a
dramatically reduced computational cost. Highly anisotropic lattices are used,
with fine temporal spacings , in order to assess the behavior of the
time-dependent effective potentials. The lattice spacings are determined from
the potentials for quarks in the fundamental representation. Simulations of the
Wilson loop in the adjoint representation are done, and the energies of
magnetic and electric ``gluelumps'' (adjoint quark-gluon bound states) are
calculated, which set the energy scale for string breaking. Correlators of
gauge-fixed static quark propagators, without a connecting string of spatial
links, are analyzed. Correlation functions of gluelump pairs are also
considered; similar correlators have recently been proposed for observing
string breaking in full QCD and other models. A thorough discussion of the
relevance of Wilson loops over other operators for studies of string breaking
is presented, using the simulation results presented here to support a number
of new arguments.Comment: 22 pages, 14 figure
Physics in the Real Universe: Time and Spacetime
The Block Universe idea, representing spacetime as a fixed whole, suggests
the flow of time is an illusion: the entire universe just is, with no special
meaning attached to the present time. This view is however based on
time-reversible microphysical laws and does not represent macro-physical
behaviour and the development of emergent complex systems, including life,
which do indeed exist in the real universe. When these are taken into account,
the unchanging block universe view of spacetime is best replaced by an evolving
block universe which extends as time evolves, with the potential of the future
continually becoming the certainty of the past. However this time evolution is
not related to any preferred surfaces in spacetime; rather it is associated
with the evolution of proper time along families of world linesComment: 28 pages, including 9 Figures. Major revision in response to referee
comment
Perturbation theory vs. simulation for tadpole improvement factors in pure gauge theories
We calculate the mean link in Landau gauge for Wilson and improved SU(3)
anisotropic gauge actions, using two loop perturbation theory and Monte Carlo
simulation employing an accelerated Langevin algorithm. Twisted boundary
conditions are employed, with a twist in all four lattice directions
considerably improving the (Fourier accelerated) convergence to an improved
lattice Landau gauge. Two loop perturbation theory is seen to predict the mean
link extremely well even into the region of commonly simulated gauge couplings
and so can be used remove the need for numerical tuning of self-consistent
tadpole improvement factors. A three loop perturbative coefficient is inferred
from the simulations and is found to be small. We show that finite size effects
are small and argue likewise for (lattice) Gribov copies and double Dirac
sheets.Comment: 13 pages of revtex
Leptonic decay constants f_Ds and f_D in three flavor lattice QCD
We determine the leptonic decay constants in three flavor unquenched lattice
QCD. We use O(a^2)-improved staggered light quarks and O(a)-improved charm
quarks in the Fermilab heavy quark formalism. Our preliminary results, based
upon an analysis at a single lattice spacing, are f_Ds = 263(+5-9)(+/-24) MeV
and f_D = 225(+11-13)(+/-21) MeV. In each case, the first reported error is
statistical while the is the combined systematic uncertainty.Comment: Talk presented at Lattice2004(heavy), Fermilab, June 21-26, 2004. 3
pages, 2 figure
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