6,411 research outputs found
Revisiting Event Horizon Finders
Event horizons are the defining physical features of black hole spacetimes,
and are of considerable interest in studying black hole dynamics. Here, we
reconsider three techniques to localise event horizons in numerical spacetimes:
integrating geodesics, integrating a surface, and integrating a level-set of
surfaces over a volume. We implement the first two techniques and find that
straightforward integration of geodesics backward in time to be most robust. We
find that the exponential rate of approach of a null surface towards the event
horizon of a spinning black hole equals the surface gravity of the black hole.
In head-on mergers we are able to track quasi-normal ringing of the merged
black hole through seven oscillations, covering a dynamic range of about 10^5.
Both at late times (when the final black hole has settled down) and at early
times (before the merger), the apparent horizon is found to be an excellent
approximation of the event horizon. In the head-on binary black hole merger,
only {\em some} of the future null generators of the horizon are found to start
from past null infinity; the others approach the event horizons of the
individual black holes at times far before merger.Comment: 30 pages, 15 figures, revision
Simple and Effective Type Check Removal through Lazy Basic Block Versioning
Dynamically typed programming languages such as JavaScript and Python defer
type checking to run time. In order to maximize performance, dynamic language
VM implementations must attempt to eliminate redundant dynamic type checks.
However, type inference analyses are often costly and involve tradeoffs between
compilation time and resulting precision. This has lead to the creation of
increasingly complex multi-tiered VM architectures.
This paper introduces lazy basic block versioning, a simple JIT compilation
technique which effectively removes redundant type checks from critical code
paths. This novel approach lazily generates type-specialized versions of basic
blocks on-the-fly while propagating context-dependent type information. This
does not require the use of costly program analyses, is not restricted by the
precision limitations of traditional type analyses and avoids the
implementation complexity of speculative optimization techniques.
We have implemented intraprocedural lazy basic block versioning in a
JavaScript JIT compiler. This approach is compared with a classical flow-based
type analysis. Lazy basic block versioning performs as well or better on all
benchmarks. On average, 71% of type tests are eliminated, yielding speedups of
up to 50%. We also show that our implementation generates more efficient
machine code than TraceMonkey, a tracing JIT compiler for JavaScript, on
several benchmarks. The combination of implementation simplicity, low
algorithmic complexity and good run time performance makes basic block
versioning attractive for baseline JIT compilers
Bounds on 4D Conformal and Superconformal Field Theories
We derive general bounds on operator dimensions, central charges, and OPE
coefficients in 4D conformal and N=1 superconformal field theories. In any CFT
containing a scalar primary phi of dimension d we show that crossing symmetry
of implies a completely general lower bound on the central
charge c >= f_c(d). Similarly, in CFTs containing a complex scalar charged
under global symmetries, we bound a combination of symmetry current two-point
function coefficients tau^{IJ} and flavor charges. We extend these bounds to
N=1 superconformal theories by deriving the superconformal block expansions for
four-point functions of a chiral superfield Phi and its conjugate. In this case
we derive bounds on the OPE coefficients of scalar operators appearing in the
Phi x Phi* OPE, and show that there is an upper bound on the dimension of Phi*
Phi when dim(Phi) is close to 1. We also present even more stringent bounds on
c and tau^{IJ}. In supersymmetric gauge theories believed to flow to
superconformal fixed points one can use anomaly matching to explicitly check
whether these bounds are satisfied.Comment: 47 pages, 9 figures; V2: small corrections and clarification
Horizon Entropy
Although the laws of thermodynamics are well established for black hole
horizons, much less has been said in the literature to support the extension of
these laws to more general settings such as an asymptotic de Sitter horizon or
a Rindler horizon (the event horizon of an asymptotic uniformly accelerated
observer). In the present paper we review the results that have been previously
established and argue that the laws of black hole thermodynamics, as well as
their underlying statistical mechanical content, extend quite generally to what
we call here "causal horizons". The root of this generalization is the local
notion of horizon entropy density.Comment: 21 pages, one figure, to appear in a special issue of Foundations of
Physics in honor of Jacob Bekenstei
Tethered Monte Carlo: computing the effective potential without critical slowing down
We present Tethered Monte Carlo, a simple, general purpose method of
computing the effective potential of the order parameter (Helmholtz free
energy). This formalism is based on a new statistical ensemble, closely related
to the micromagnetic one, but with an extended configuration space (through
Creutz-like demons). Canonical averages for arbitrary values of the external
magnetic field are computed without additional simulations. The method is put
to work in the two dimensional Ising model, where the existence of exact
results enables us to perform high precision checks. A rather peculiar feature
of our implementation, which employs a local Metropolis algorithm, is the total
absence, within errors, of critical slowing down for magnetic observables.
Indeed, high accuracy results are presented for lattices as large as L=1024.Comment: 32 pages, 8 eps figures. Corrected Eq. (36), which is wrong in the
published pape
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