8 research outputs found
Curvature conditions for the occurrence of a class of spacetime singularities
It has previously been shown [W. Rudnicki, Phys. Lett. A 224, 45 (1996)] that
a generic gravitational collapse cannot result in a naked singularity
accompanied by closed timelike curves. An important role in this result plays
the so-called inextendibility condition, which is required to hold for certain
incomplete null geodesics. In this paper, a theorem is proved that establishes
some relations between the inextendibility condition and the rate of growth of
the Ricci curvature along incomplete null geodesics. This theorem shows that
the inextendibility condition may hold for a much more general class of
singularities than only those of the strong curvature type. It is also argued
that some earlier cosmic censorship results obtained for strong curvature
singularities can be extended to singularities corresponding to the
inextendibility condition.Comment: RevTeX, 6 pages, no figures. To be published in J. Math. Phy
On the strength of the Kerr singularity and cosmic censorship
It has been suggested by Israel that the Kerr singularity cannot be strong in
the sense of Tipler, for it tends to cause repulsive effects. We show here
that, contrary to that suggestion, nearly all null geodesics reaching this
singularity do in fact terminate in Tipler's strong curvature singularity.
Implications of this result are discussed in the context of an earlier cosmic
censorship theorem which constraints the occurrence of Kerr-like naked
singularities in generic collapse situations.Comment: RevTeX, 6 pages, no figures, to appear in Phys. Lett.
Generalized Strong Curvature Singularities and Cosmic Censorship
A new definition of a strong curvature singularity is proposed. This
definition is motivated by the definitions given by Tipler and Krolak, but is
significantly different and more general. All causal geodesics terminating at
these new singularities, which we call generalized strong curvature
singularities, are classified into three possible types; the classification is
based on certain relations between the curvature strength of the singularities
and the causal structure in their neighborhood. A cosmic censorship theorem is
formulated and proved which shows that only one class of generalized strong
curvature singularities, corresponding to a single type of geodesics according
to our classification, can be naked. Implications of this result for the cosmic
censorship hypothesis are indicated.Comment: LaTeX, 11 pages, no figures, to appear in Mod. Phys. Lett.
Salient signatures of entanglement in the surrounding environment
We develop a model in which presence of entanglement in a quantum system can
be confirmed through coarse observations of the environment surrounding the
system. This counter-intuitive effect becomes possible when interaction between
the system and its environment is proportional to an observable being an
entanglement witness. While presenting intuitive examples we show that: i) a
cloud of an ideal gas, when subject to a linear potential coupled with the
entanglement witness, accelerates in the direction dictated by the sign of the
witness; ii) when the environment is a radiation field, the direction of
dielectric polarization depends on the presence of entanglement; iii)
quadratures of electromagnetic field in a cavity coupled with two qubits (or a
four-level atom) are displaced in the same manner
GENERALIZED STRONG CURVATURE SINGULARITIES AND WEAK COSMIC CENSORSHIP IN COSMOLOGICAL SPACETIMES
Diffusion-weighted magnetic resonance imaging of 103 patients with rectal adenocarcinoma identifies the apparent diffusion coefficient as an imaging marker for tumor invasion and regional lymph node involvement
BACKGROUND: This retrospective study included 103 patients diagnosed with rectal adenocarcinoma at a single center in Poland who underwent preoperative diffusion-weighted magnetic resonance imaging (DWI) and aimed to determine whether the apparent diffusion coefficient (ADC) was an imaging marker for tumor invasion and regional lymph node involvement. MATERIAL/METHODS: We analyzed primary staging magnetic resonance examinations of the rectum of 103 consecutive patients with histologically proven non-mucinous adenocarcinoma who underwent surgical treatment. In 85 patients, surgery was preceded by long-course chemoradiotherapy (n=18) or short-course radiotherapy (n=67). The following DWI parameters were measured: ADC mean, minimum, maximum, and standard deviation in the region of interest (ADC SD-in-ROI). Values were compared between subgroups based on histological parameters from the report: tumor stage, lymph node stage, differentiation grade, the presence of extranodal tumor deposits, angioinvasion, and perineural invasion. Statistical analysis was performed using the Mann-Whitney U test and the unilateral t test. RESULTS: ADC mean values were lower for cases in which postoperative histopathological examination lymph node invasion (P=0.04) and tumor deposits were found (P=0.04). Minimal ADC value was higher in cases in which tumor deposits were not found (P=0.009). ADC SD-in-ROI values were lower in cases in which lymph nodes invasion was confirmed (P=0.014). There were no statistically significant differences for other parameters. CONCLUSIONS: The ADC values in pre-treatment DWI in patients with rectal adenocarcinoma were correlated with tumor invasion and regional lymph node metastases. Therefore, ADC values from the pre-treatment MRI may help plan adjuvant therapy in patients with rectal adenocarcinoma