Gravitational wave scintillation by a stellar cluster

The diffraction effects on gravitational waves propagating through a stellar cluster are analyzed in the relevant approximation of Fresnel diffraction limit. We find that a gravitational wave scintillation effect - similar to the radio source scintillation effect - comes out naturally, implying that the gravitational wave intensity changes in a characteristic way as the observer moves.Comment: 9 pages, in press in IJMP

Asymptotic Behaviour of the Proper Length and Volume of the Schwarzschild Singularity

Though popular presentations give the Schwarzschild singularity as a point it is known that it is spacelike and not timelike. Thus it has a "length" and is not a "point". In fact, its length must necessarily be infinite. It has been proved that the proper length of the Qadir-Wheeler suture model goes to infinity [1], while its proper volume shrinks to zero, and the asymptotic behaviour of the length and volume have been calculated. That model consists of two Friedmann sections connected by a Schwarzschild "suture". The question arises whether a similar analysis could provide the asymptotic behaviour of the Schwarzschild black hole near the singularity. It is proved here that, unlike the behaviour for the suture model, for the Schwarzschild essential singularity $\Delta s$ $\thicksim$ $K^{1/3}\ln K$ and $V\thicksim$ $K^{-1}\ln K$, where $K$ is the mean extrinsic curvature, or the York time.Comment: 13 pages, 1 figur

Weyl collineations that are not curvature collineations

Though the Weyl tensor is a linear combination of the curvature tensor, Ricci tensor and Ricci scalar, it does not have all and only the Lie symmetries of these tensors since it is possible, in principle, that "asymmetries cancel". Here we investigate if, when and how the symmetries can be different. It is found that we can obtain a metric with a finite dimensional Lie algebra of Weyl symmetries that properly contains the Lie algebra of curvature symmetries. There is no example found for the converse requirement. It is speculated that there may be a fundamental reason for this lack of "duality".Comment: 9 page

Cultural influences on perception of disability and disabled people: A comparison of opinions from students in the United Kingdom (UK) Pakistan (PAK) about a generic wheelchair using a semantic differential scale

Assistive Technology (AT) product use occurs within a socio-cultural setting. The growth internationally in the AT product market suggests that designers need to be aware of the influences that diverse cultures may have on the societal perception of an AT product through its semantic attributes. The study aimed to evaluate the visual interaction with an AT product by young adults from Pakistan, a collectivist society, and the United Kingdom (UK), an individualist society. A paper-based questionnaire survey was carried out with 281 first-year undergraduate students from the UK and Pakistan to evaluate their perception towards the visual representation of a generic conventional wheelchair image. A semantics differential (SD) scale method was used involving a seven-point bipolar SD scale incorporating sixteen pairs of adjectives defining functional, meaning, and usability attributes of the product. The mean (M) and standard deviation (sd) values were obtained for each pair of adjectives and compared between both groups by employing appropriate parametric tests. The results show that having a diverse cultural background did not appear to have overtly influenced the meanings ascribed to the generic manual wheelchair, which was unexpected. The University ‘Internationalist’ environment may have influenced the results. Some minor but critical differences were found for some pairs of adjectives (bulky-compact, heavy-light), having p-value less than 0.05 (p<0.05) that related to previous experience of wheelchairs and/or their use. Further studies are planned to investigate and validate outcomes with other student and non-student groups

Stabilization of self-mode-locked quantum dash lasers by symmetric dual-loop optical feedback

We report experimental studies of the influence of symmetric dual-loop optical feedback on the RF linewidth and timing jitter of self-mode-locked two-section quantum dash lasers emitting at 1550 nm. Various feedback schemes were investigated and optimum levels determined for narrowest RF linewidth and low timing jitter, for single-loop and symmetric dual-loop feedback. Two symmetric dual-loop configurations, with balanced and unbalanced feedback ratios, were studied. We demonstrate that unbalanced symmetric dual loop feedback, with the inner cavity resonant and fine delay tuning of the outer loop, gives the narrowest RF linewidth and reduced timing jitter over a wide range of delay, unlike single and balanced symmetric dual-loop configurations. This configuration with feedback lengths of 80 and 140 m narrows the RF linewidth by ∼ 4–67x and ∼ 10–100x, respectively, across the widest delay range, compared to free-running. For symmetric dual-loop feedback, the influence of different power split ratios through the feedback loops was determined. Our results show that symmetric dual-loop feedback is markedly more effective than single-loop feedback in reducing RF linewidth and timing jitter, and is much less sensitive to delay phase, making this technique ideal for applications where robustness and alignment tolerance are essential

Malicious entities are in vain : preserving privacy in publish and subscribe systems

Publish and subscribe (pub/sub) system is a decoupled communication paradigm that allows routing of publications. Through a set of dedicated third party servers, referred to as brokers, publications are disseminated without establishing any link between publishers and subscribers. However, the involvement of these brokers raises security and privacy issues as they can harvest sensitive data about subscribers. Furthermore, a malicious broker may collude with malicious subscribers and/or publishers to infer subscribers’ interests. Our solution is such that subscribers’ interests are not revealed to curious brokers and published data can only be accessed by the authorised subscribers. Moreover, the proposed protocol is secure against the collusion attacks between malicious brokers, publishers, and subscribers

Cross-cultural influences on the semantics ascribed to assistive technology product and its envisaged user

Culture is an important variable when considering the communication of meaning through an artefact. A literature review has highlighted distinct differences in the cognitive processing that delivers perception between individuals from individualist and collectivist societies. The projected growth in Assistive Technology (AT) online marketing suggests industrial designers need to be more aware of the influence that diverse cultures may have on consumer’s perception of an AT product attributes. Artefact semantic language is the vehicle to deliver design intent during an online user-product visual interaction. Little is published about how cultural differences in cognition relate to semantic preferences of AT product attributes and their users. This study aims to evaluate visual interaction of an AT product and its perceived user by individuals from culturally distinct countries; United Kingdom (individualist) and Pakistan (collectivist). A survey was conducted with first-year undergraduate students (N=281) from both countries, to evaluate their perception of a conventional attendant wheelchair. A Semantics Differential (SD) scale was employed having sixteen pairs of adjectives defining functional, meaning, and usability attributes of the product. The mean, standard deviation values were acquired for each pair of adjective and compared between both groups by performing appropriate statistical tests. In results, diverse cultures did not appear to have overtly influenced the meanings ascribed to the product, which was unexpected. Following statistical analysis minor but critical differences were found for some pairs of adjectives (bulky-compact, heavy-light), with p-value of less than 0.05 indicating the differences. Studies are planned to further investigate outcomes and validate results

Collusion defender : preserving subscribers’ privacy in publish and subscribe systems

The Publish and Subscribe (pub/sub) system is an established paradigm to disseminate the data from publishers to subscribers in a loosely coupled manner using a network of dedicated brokers. However, sensitive data could be exposed to malicious entities if brokers get compromised or hacked; or even worse, if brokers themselves are curious to learn about the data. A viable mechanism to protect sensitive publications and subscriptions is to encrypt the data before it is disseminated through the brokers. State-of-the-art approaches allow brokers to perform encrypted matching without revealing publications and subscriptions. However, if malicious brokers collude with malicious subscribers or publishers, they can learn the interests of innocent subscribers, even when the interests are encrypted. In this article, we present a pub/sub system that ensures confidentiality of publications and subscriptions in the presence of untrusted brokers. Furthermore, our solution resists collusion attacks between untrusted brokers and malicious subscribers (or publishers). Finally, we have implemented a prototype of our solution to show its feasibility and efficiency. Index Terms: Collusion Resistance, Secure Pub/sub, Subscribers’ Privacy, Publications’ Confidentialit