860 research outputs found
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Bondi mass with a cosmological constant
The mass loss of an isolated gravitating system due to energy carried away by
gravitational waves with a cosmological constant was recently
worked out, using the Newman-Penrose-Unti approach. In that same article, an
expression for the Bondi mass of the isolated system, , for the
case was proposed. The stipulated mass would ensure
that in the absence of any incoming gravitational radiation from elsewhere, the
emitted gravitational waves must carry away a positive-definite energy. That
suggested quantity however, introduced a -correction term to the Bondi
mass (where is the usual Bondi mass for asymptotically flat
spacetimes) which would involve not just information on the state of the system
at that moment, but ostensibly also its past history. In this paper, we derive
the identical mass-loss equation using an integral formula on a hypersurface
formulated by Frauendiener based on the Nester-Witten identity, and argue that
one may adopt a generalisation of the Bondi mass with
\emph{without any correction}, viz. for any .
Furthermore with , we show that for \emph{purely quadrupole
gravitational waves} given off by the isolated system (i.e. when the "Bondi
news" comprises only the components of the "spherical
harmonics with spin-weight 2"), the energy carried away is \emph{manifestly
positive-definite} for the case. For a general having
higher multipole moments, this perspicuous property in the case
still holds if those contributions are weak --- more precisely, if they
satisfy any of the inequalities given in this paper.Comment: 29 pages, accepted for publication by Physical Review
Professional Ethics in the Construction Industry
The results are provided of a small, but reprersentative, questionnaire survey of typical project managers, architects and building contractors concerning their views and experiences on a range of ethical issues surrounding construction industry activities. Most (90%) subscribed to a professional Code of Ethics and many (45%) had an Ethical Code of Conduct in their employing organisations, with the majority (84%) considering good ethical practice to be an important organisational goal. 93% of the respondents agreed that "Business Ethics" should be driven or governed by "Personal Ethics", with 84% of respondents stating that a balance of both the requirements of the client and the impact on the public should be maintained. No respondents were aware of any cases of employers attempting to force their employees to initiate, or participate in, unethical conduct. Despite this, all the respondents had witnessed or experienced some degree of unethical conduct, in the form of unfair conduct (81%), negligence (67%), conflict of interest (48%), collusive tendering (44%), fraud (35%), confidentiality and propriety breach (32%), bribery (26%) and violation of environmental ethics (20%)
No-boarding buses: Synchronisation for efficiency
We investigate a no-boarding policy in a system of buses serving bus
stops in a loop, which is an entrainment mechanism to keep buses synchronised
in a reasonably staggered configuration. Buses always allow alighting, but
would disallow boarding if certain criteria are met. For an analytically
tractable theory, buses move with the same natural speed (applicable to
programmable self-driving buses), where the average waiting time experienced by
passengers waiting at the bus stop for a bus to arrive can be calculated. The
analytical results show that a no-boarding policy can dramatically reduce the
average waiting time, as compared to the usual situation without the
no-boarding policy. Subsequently, we carry out simulations to verify these
theoretical analyses, also extending the simulations to typical human-driven
buses with different natural speeds based on real data. Finally, a simple
general adaptive algorithm is implemented to dynamically determine when to
implement no-boarding in a simulation for a real university shuttle bus
service.Comment: 49 pages, 9 figures. Video available here:
https://www.youtube.com/watch?v=SBNqvTr1Aj
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