Why medical students choose not to carry out an intercalated BSc : a questionnaire study.

Peer reviewedPublisher PD

The tail effect in gravitational radiation-reaction: time non-locality and renormalization group evolution

We use the effective field theory (EFT) framework to calculate the tail effect in gravitational radiation reaction, which enters at 4PN order in the dynamics of a binary system. The computation entails a subtle interplay between the near (or potential) and far (or radiation) zones. In particular, we find that the tail contribution to the effective action is non-local in time, and features both a dissipative and a `conservative' term. The latter includes a logarithmic ultraviolet (UV) divergence, which we show cancels against an infrared (IR) singularity found in the (conservative) near zone. The origin of this behavior in the long-distance EFT is due to the point-particle limit -shrinking the binary to a point- which transforms a would-be infrared singularity into an ultraviolet divergence. This is a common occurrence in an EFT approach, which furthermore allows us to use renormalization group (RG) techniques to resum the resulting logarithmic contributions. We then derive the RG evolution for the binding potential and total mass/energy, and find agreement with the results obtained imposing the conservation of the (pseudo) stress-energy tensor in the radiation theory. While the calculation of the leading tail contribution to the effective action involves only one diagram, five are needed for the one-point function. This suggests logarithmic corrections may be easier to incorporate in this fashion. We conclude with a few remarks on the nature of these IR/UV singularities, the (lack of) ambiguities recently discussed in the literature, and the completeness of the analytic Post-Newtonian framework.Comment: 24 pages. 3 figures. v2: Extended discussion on the nature of IR/UV singularities. Published versio

Antioxidants that protect mitochondria reduce interleukin-6 and oxidative stress, improve mitochondrial function, and reduce biochemical markers of organ dysfunction in a rat model of acute sepsis

Funding This study was funded by the Medical Research Council (Grant number G0800149). Research material from this study is not available. Acknowledgement We are very grateful to Dr Robin A.J. Smith, Department of Chemistry, University of Otago, Dunedin, New Zealand, for the generous gifts of MitoE and MitoQ, without which this work would not have been possible.Peer reviewedPublisher PD

Quantum reference frames for general symmetry groups

A fully relational quantum theory necessarily requires an account of changes of quantum reference frames, where quantum reference frames are quantum systems relative to which other systems are described. By introducing a relational formalism which identifies coordinate systems with elements of a symmetry group $G$, we define a general operator for reversibly changing between quantum reference frames associated to a group $G$. This generalises the known operator for translations and boosts to arbitrary finite and locally compact groups, including non-Abelian groups. We show under which conditions one can uniquely assign coordinate choices to physical systems (to form reference frames) and how to reversibly transform between them, providing transformations between coordinate systems which are `in a superposition' of other coordinate systems. We obtain the change of quantum reference frame from the principles of relational physics and of coherent change of reference frame. We prove a theorem stating that the change of quantum reference frame consistent with these principles is unitary if and only if the reference systems carry the left and right regular representations of $G$. We also define irreversible changes of reference frame for classical and quantum systems in the case where the symmetry group $G$ is a semi-direct product $G = N \rtimes P$ or a direct product $G = N \times P$, providing multiple examples of both reversible and irreversible changes of quantum reference system along the way. Finally, we apply the relational formalism and changes of reference frame developed in this work to the Wigner's friend scenario, finding similar conclusions to those in relational quantum mechanics using an explicit change of reference frame as opposed to indirect reasoning using measurement operators

Gravitational self-force in the ultra-relativistic limit: the "large-N" expansion

We study the gravitational self-force using the effective field theory formalism. We show that in the ultra-relativistic limit γ → ∞, with γ the boost factor, many simplifications arise. Drawing parallels with the large N limit in quantum field theory, we introduce the parameter 1/N ≡ 1/γ^2 and show that the effective action admits a well defined expansion in powers of λ ≡ N ϵ at each order in 1/N , where ϵ ≡ E_m/M and E_m = γm is the (kinetic) energy of the small mass. Moreover, we show that diagrams with nonlinear bulk interactions first enter at O (λ^2/N^2) and only diagrams with nonlinearities in the worldline couplings, which are significantly easier to compute, survive in the large N /ultra-relativistic limit. Finally, we derive the self-force to O (λ^4/N) and provide expressions for some conservative quantities for circular orbits

Fast and accurate prediction of numerical relativity waveforms from binary black hole coalescences using surrogate models

Simulating a binary black hole (BBH) coalescence by solving Einstein's equations is computationally expensive, requiring days to months of supercomputing time. Using reduced order modeling techniques, we construct an accurate surrogate model, which is evaluated in a millisecond to a second, for numerical relativity (NR) waveforms from non-spinning BBH coalescences with mass ratios in $[1, 10]$ and durations corresponding to about $15$ orbits before merger. We assess the model's uncertainty and show that our modeling strategy predicts NR waveforms {\em not} used for the surrogate's training with errors nearly as small as the numerical error of the NR code. Our model includes all spherical-harmonic ${}_{-2}Y_{\ell m}$ waveform modes resolved by the NR code up to $\ell=8.$ We compare our surrogate model to Effective One Body waveforms from $50$-$300 M_\odot$ for advanced LIGO detectors and find that the surrogate is always more faithful (by at least an order of magnitude in most cases).Comment: Updated to published version, which includes a section comparing the surrogate and effective-one-body models. The surrogate is publicly available for download at http://www.black-holes.org/surrogates/ . 6 pages, 6 figure

A Surrogate Model of Gravitational Waveforms from Numerical Relativity Simulations of Precessing Binary Black Hole Mergers

We present the first surrogate model for gravitational waveforms from the coalescence of precessing binary black holes. We call this surrogate model NRSur4d2s. Our methodology significantly extends recently introduced reduced-order and surrogate modeling techniques, and is capable of directly modeling numerical relativity waveforms without introducing phenomenological assumptions or approximations to general relativity. Motivated by GW150914, LIGO's first detection of gravitational waves from merging black holes, the model is built from a set of $276$ numerical relativity (NR) simulations with mass ratios $q \leq 2$, dimensionless spin magnitudes up to $0.8$, and the restriction that the initial spin of the smaller black hole lies along the axis of orbital angular momentum. It produces waveforms which begin $\sim 30$ gravitational wave cycles before merger and continue through ringdown, and which contain the effects of precession as well as all $\ell \in \{2, 3\}$ spin-weighted spherical-harmonic modes. We perform cross-validation studies to compare the model to NR waveforms \emph{not} used to build the model, and find a better agreement within the parameter range of the model than other, state-of-the-art precessing waveform models, with typical mismatches of $10^{-3}$. We also construct a frequency domain surrogate model (called NRSur4d2s_FDROM) which can be evaluated in $50\, \mathrm{ms}$ and is suitable for performing parameter estimation studies on gravitational wave detections similar to GW150914.Comment: 34 pages, 26 figure

Pharmacological activation of endogenous protective pathways against oxidative stress under conditions of sepsis

Funding The study was funded entirely by institutional funds.Peer reviewedPostprin

Melatonin as a potential therapy for sepsis : a phase I dose escalation study and an ex vivo whole blood model under conditions of sepsis

This article is protected by copyright. All rights reserved. This study was funded by the Chief Scientist Office, NHS Scotland. We would like to thank all the volunteers who gave up their time and blood to take part in the study and the data monitoring committee and staff of the intensive care unit for their support. In addition, thanks to Dr Malachy Columb for performing Page's trend test for us and to Annette Fearnley at Nu-Pharm Ltd for her advice.Peer reviewedPublisher PD

Reliability of lithium dilution cardiac output in anaesthetized sheep

Background Cardiac output (CO) measurement with lithium dilution (COLD) has not been fully validated in sheep using precise ultrasonic flow probe technology (COUFP). Sheep generate important cardiovascular research models and the use of COLD has become more popular in experimental settings. Methods Ultrasonic transit-time perivascular flow probes were surgically implanted on the pulmonary artery of 13 sheep. Paired COLD readings were taken at six time points, before and after implantation of a left ventricular assist device (LVAD) and compared with COUFP recorded just after lithium injection. Results The mean COLD was 5.7 litre min−1 (range 3.8-9.6 litre min−1) and mean COUFP 5.9 litre min−1 (range 4.0-9.2 litre min−1). The bias (standard deviation) was 0.3 (1.0) litre min−1 [5.1 (16.9)%] and limits of agreement (LOA) were −1.7 to 2.3 litre min−1 (−28.8 to 39.0%) with a percentage error (PE) of 34.4%. Data to assess trending [rate (95% confidence intervals)] included a 78 (62-93)% concordance rate in the four-quadrant plot (n=27). In the half moon polar plot (n=19), the mean polar angle was +5°, the radial LOA were −49 to +35° and 68 (47-89)% of data points fell within 22.5° of the mean polar angle. Both tests indicated moderate to poor trending ability. Conclusion COLD is not precise when evaluated against COUFP in sheep based on the statistical criteria set, but the results are comparable with previously published animal studie