129 research outputs found
The reality conditions for the new canonical variables of General Relativity
We examine the constraints and the reality conditions that have to be imposed
in the canonical theory of 4--d gravity formulated in terms of Ashtekar
variables. We find that the polynomial reality conditions are consistent with
the constraints, and make the theory equivalent to Einstein's, as long as the
inverse metric is not degenerate; when it is degenerate, reality conditions
cannot be consistently imposed in general, and the theory describes complex
general relativity.Comment: 11
Spatial control of extreme ultraviolet light with opto-optical phase modulation
Extreme-ultraviolet (XUV) light is notoriously difficult to control due to
its strong interaction cross-section with media. We demonstrate a method to
overcome this problem by using Opto-Optical Modulation guided by a geometrical
model to shape XUV light. A bell-shaped infrared light pulse is shown to
imprint a trace of its intensity profile onto the XUV light in the far-field,
such that a change in the intensity profile of the infrared pulse leads to a
change in the shape of the far-field XUV light. The geometrical model assists
the user in predicting the effect of a specific intensity profile of the
infrared pulse, thus enabling a deterministic process
Macroscopic effects in noncollinear high-order harmonic generation.
We study two-color high-order harmonic generation using an intense driving field and its weak second harmonic, crossed under a small angle in the focus. Employing sum- and difference-frequency generation processes, such a noncollinear scheme can be used to measure and control macroscopic phase matching effects by utilizing a geometrical phase mismatch component, which depends on the noncollinear angle. We further show how spatial phase effects in the generation volume are mapped out into the far field allowing a direct analogy with temporal carrier envelope effects in attosecond pulse generation
Reality conditions for Ashtekar gravity from Lorentz-covariant formulation
We show the equivalence of the Lorentz-covariant canonical formulation
considered for the Immirzi parameter to the selfdual Ashtekar
gravity. We also propose to deal with the reality conditions in terms of Dirac
brackets derived from the covariant formulation and defined on an extended
phase space which involves, besides the selfdual variables, also their
anti-selfdual counterparts.Comment: 14 page
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Digital Epidemiology
Mobile, social, real-time: the ongoing revolution in the way people communicate has given rise to a new kind of epidemiology. Digital data sources, when harnessed appropriately, can provide local and timely information about disease and health dynamics in populations around the world. The rapid, unprecedented increase in the availability of relevant data from various digital sources creates considerable technical and computational challenges
New constraints for canonical general relativity
Ashtekar's canonical theory of classical complex Euclidean GR (no Lorentzian
reality conditions) is found to be invariant under the full algebra of
infinitesimal 4-diffeomorphisms, but non-invariant under some finite proper
4-diffeos when the densitized dreibein, \tilE^a_i, is degenerate. The
breakdown of 4-diffeo invariance appears to be due to the inability of the
Ashtekar Hamiltonian to generate births and deaths of \tilE flux loops
(leaving open the possibility that a new `causality condition' forbidding the
birth of flux loops might justify the non-invariance of the theory).
A fully 4-diffeo invariant canonical theory in Ashtekar's variables, derived
from Plebanski's action, is found to have constraints that are stronger than
Ashtekar's for rank\tilE < 2. The corresponding Hamiltonian generates births
and deaths of \tilE flux loops.
It is argued that this implies a finite amplitude for births and deaths of
loops in the physical states of quantum GR in the loop representation, thus
modifying this (partly defined) theory substantially.
Some of the new constraints are second class, leading to difficulties in
quantization in the connection representation. This problem might be overcome
in a very nice way by transforming to the classical loop variables, or the
`Faraday line' variables of Newman and Rovelli, and then solving the offending
constraints.
Note that, though motivated by quantum considerations, the present paper is
classical in substance.Comment: Version to appear in Nuclear Physics B. Discussion of 4-diffeo
invariance, Dirac brackets improved. Proof of theorem connecting self-dual
2-forms and orthonormal tetrads replaced. Latex 57 pages, 7 uuencoded
postscript figures. Uses macro psfig.tex available from this archive (and
appended to this posting for your convenience). After latexing use dvips -
not - dvi2ps to get postscript file
The Standard Model with gravity couplings
In this paper, we examine the coupling of matter fields to gravity within the
framework of the Standard Model of particle physics. The coupling is described
in terms of Weyl fermions of a definite chirality, and employs only
(anti)self-dual or left-handed spin connection fields. It is known from the
work of Ashtekar and others that such fields can furnish a complete description
of gravity without matter. We show that conditions ensuring the cancellation of
perturbative chiral gauge anomalies are not disturbed. We also explore a global
anomaly associated with the theory, and argue that its removal requires that
the number of fundamental fermions in the theory must be multiples of 16. In
addition, we investigate the behavior of the theory under discrete
transformations P, C and T; and discuss possible violations of these discrete
symmetries, including CPT, in the presence of instantons and the
Adler-Bell-Jackiw anomaly.Comment: Extended, and replaced with LaTex file. 25 Page
Localized qubits in curved spacetimes
We provide a systematic and self-contained exposition of the subject of
localized qubits in curved spacetimes. This research was motivated by a simple
experimental question: if we move a spatially localized qubit, initially in a
state |\psi_1>, along some spacetime path \Gamma from a spacetime point x_1 to
another point x_2, what will the final quantum state |\psi_2> be at point x_2?
This paper addresses this question for two physical realizations of the qubit:
spin of a massive fermion and polarization of a photon. Our starting point is
the Dirac and Maxwell equations that describe respectively the one-particle
states of localized massive fermions and photons. In the WKB limit we show how
one can isolate a two-dimensional quantum state which evolves unitarily along
\Gamma. The quantum states for these two realizations are represented by a
left-handed 2-spinor in the case of massive fermions and a four-component
complex polarization vector in the case of photons. In addition we show how to
obtain from this WKB approach a fully general relativistic description of
gravitationally induced phases. We use this formalism to describe the
gravitational shift in the COW 1975 experiment. In the non-relativistic weak
field limit our result reduces to the standard formula in the original paper.
We provide a concrete physical model for a Stern-Gerlach measurement of spin
and obtain a unique spin operator which can be determined given the orientation
and velocity of the Stern-Gerlach device and velocity of the massive fermion.
Finally, we consider multipartite states and generalize the formalism to
incorporate basic elements from quantum information theory such as quantum
entanglement, quantum teleportation, and identical particles. The resulting
formalism provides a basis for exploring precision quantum measurements of the
gravitational field using techniques from quantum information theory.Comment: 53 pages, 7 figures; v2: published version with further corrections.
v3: some references and equation typesetting fixe
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Mitigation of Moral Hazard and Adverse Selection in Venture Capital Financing: The Influence of the Country’s Institutional Setting
A venture capitalist (VC) needs to trade off benefits and costs when attempting to mitigate agency problems in their investor-investee relationship. We argue that signals of ventures complement the VC’s capacity to screen and conduct a due diligence during the pre-investment phase, but its attractiveness may diminish in institutional settings supporting greater transparency. Similarly, whereas a VC may opt for contractual covenants to curb potential opportunism by ventures in the post-investment phase, this may only be effective in settings supportive of shareholder rights enforcement. Using an international sample of VC contracts, our study finds broad support for these conjectures. It delineates theoretical and practical implications for how investors can best deploy their capital in different institutional settings whilst nurturing their relationships with entrepreneurs
Clonal expansion and epigenetic reprogramming following deletion or amplification of mutant
IDH1 mutation is the earliest genetic alteration in low-grade gliomas (LGGs), but its role in tumor recurrence is unclear. Mutant IDH1 drives overproduction of the oncometabolite d-2-hydroxyglutarate (2HG) and a CpG island (CGI) hypermethylation phenotype (G-CIMP). To investigate the role of mutant IDH1 at recurrence, we performed a longitudinal analysis of 50 IDH1 mutant LGGs. We discovered six cases with copy number alterations (CNAs) at the IDH1 locus at recurrence. Deletion or amplification of IDH1 was followed by clonal expansion and recurrence at a higher grade. Successful cultures derived from IDH1 mutant, but not IDH1 wild type, gliomas systematically deleted IDH1 in vitro and in vivo, further suggestive of selection against the heterozygous mutant state as tumors progress. Tumors and cultures with IDH1 CNA had decreased 2HG, maintenance of G-CIMP, and DNA methylation reprogramming outside CGI. Thus, while IDH1 mutation initiates gliomagenesis, in some patients mutant IDH1 and 2HG are not required for later clonal expansions
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