Information, information processing and gravity

I discuss fundamental limits placed on information and information processing by gravity. Such limits arise because both information and its processing require energy, while gravitational collapse (formation of a horizon or black hole) restricts the amount of energy allowed in a finite region. Specifically, I use a criterion for gravitational collapse called the hoop conjecture. Once the hoop conjecture is assumed a number of results can be obtained directly: the existence of a fundamental uncertainty in spatial distance of order the Planck length, bounds on information (entropy) in a finite region, and a bound on the rate of information processing in a finite region. In the final section I discuss some cosmological issues related to the total amount of information in the universe, and note that almost all detailed aspects of the late universe are determined by the randomness of quantum outcomes. This paper is based on a talk presented at a 2007 Bellairs Research Institute (McGill University) workshop on black holes and quantum information.Comment: 7 pages, 5 figures, revte

Synchrotron Emission as the Source of GRB Spectra, Part II: Observations

We test the models of synchrotron emission presented in Part I of this series (Lloyd & Petrosian, these proceedings) against the distributions and evolution of GRB spectral parameters (particularly the low energy index, $\alpha$). With knowledge of the $E_{p}$ distribution and the correlation between $\alpha$ and $E_{p}$ presented in Part I, we show how to derive the expected distribution of $\alpha$ from fits to optically thin synchrotron spectra, and compare this with the observed distribution. We show that there is no difficulty explaining bursts below the ``line of death'', $\alpha < -2/3$, and that these bursts indicate that the spectrum of accelerated electrons must flatten or decline at low energies. Bursts with low energy spectral indices that fall above this limit are explained by the synchrotron self-absorption frequency entering the lower end of the BATSE window. Finally, we discuss a variety of spectral evolution behavior seen in GRBs and explain this behavior in the context of synchrotron emission from internal shocks.Comment: To appear in the proceedings of the 5th Huntsville Symposium on Gamma Ray Burst

The school food environment and student body mass index and food consumption: 2004 to 2007 national data

PURPOSE: This study identifies trends in the availability of various food choices in United States' middle and high schools from 2004 to 2007, and examines the potential associations between such food availability and students' self-reported eating habits and body mass index (BMI)-related outcomes. METHODS: Data are based on nationally representative samples of 78,442 students in 684 secondary schools surveyed from 2004 to 2007 as part of the Youth, Education, and Society (YES) study and the Monitoring the Future (MTF) study. In the YES study, school administrators and food service managers completed self-administered questionnaires on their school's food environment. In the MTF study, students in the same schools completed self-administered questionnaires, providing data used to construct BMI and food consumption measures. RESULTS: Overall, there was a decrease in the availability of regular-sugar/fat food items in both middle and high schools, and some indication of an increase in high school availability of reduced-fat food items through school lunch or a la carte. Some minimal evidence was found for relationships between the school food environment and student BMI-related outcomes and food consumption measures. CONCLUSIONS: United States secondary schools are making progress in the types of foods offered to students, with food items of lower nutritional value becoming less prevalent in recent years. Continued monitoring of food environment trends may help clarify whether and how such factors relate to youth health outcomes.The Youth, Education, and Society (YES) project (part of a larger research initiative, entitled Bridging the Gap: Research Informing Policy and Practice for Healthy Youth Behavior) is funded by the Robert Wood Johnson Foundation. The Monitoring the Future study is funded by the National Institute on Drug Abuse (DA01411). The views expressed in this article are those of the authors and do not necessarily reflect the views of the funders. (Robert Wood Johnson Foundation; DA01411 - National Institute on Drug Abuse

Quantised orbital angular momentum transfer and magnetic dichroism in the interaction of electron vortices with matter

Following the very recent experimental realisation of electron vortices, we consider their interaction with matter, in particular the transfer of orbital angular momentum in the context of electron energy loss spectroscopy, and the recently observed dichroism in thin film magnetised iron samples. We show here that orbital angular momentum exchange does indeed occur between electron vortices and the internal electronic-type motion, as well as center of mass motion of atoms in the electric dipole approximation. This contrasts with the case of optical vortices where such transfer only occurs in transitions involving multipoles higher than the dipole. The physical basis of the observed dichroism is explained

Ground-layer wavefront reconstruction from multiple natural guide stars

Observational tests of ground layer wavefront recovery have been made in open loop using a constellation of four natural guide stars at the 1.55 m Kuiper telescope in Arizona. Such tests explore the effectiveness of wide-field seeing improvement by correction of low-lying atmospheric turbulence with ground-layer adaptive optics (GLAO). The wavefronts from the four stars were measured simultaneously on a Shack-Hartmann wavefront sensor (WFS). The WFS placed a 5 x 5 array of square subapertures across the pupil of the telescope, allowing for wavefront reconstruction up to the fifth radial Zernike order. We find that the wavefront aberration in each star can be roughly halved by subtracting the average of the wavefronts from the other three stars. Wavefront correction on this basis leads to a reduction in width of the seeing-limited stellar image by up to a factor of 3, with image sharpening effective from the visible to near infrared wavelengths over a field of at least 2 arc minutes. We conclude that GLAO correction will be a valuable tool that can increase resolution and spectrographic throughput across a broad range of seeing-limited observations.Comment: 25 pages, 8 figures, to be published in Astrophys.