Resolving the observer reference class problem in cosmology

The assumption that we are typical observers plays a core role in attempts to make multiverse theories empirically testable. A widely shared worry about this assumption is that it suffers from systematic ambiguity concerning the reference class of observers with respect to which typicality is assumed. As a way out, Srednicki and Hartle recommend that we empirically test typicality with respect to different candidate reference classes in analogy to how we test physical theories. Unfortunately, as this paper argues, this idea fails because typicality is not the kind of assumption that can be subjected to empirical tests. As an alternative, a background information constraint on observer reference class choice is suggested according to which the observer reference class should be chosen such that it includes precisely those observers who one could possibly be, given one's assumed background information.Comment: 15 pages, no figure

Gauge symmetry breaking in gauge theories---In search of clarification

The paper investigates the spontaneous breaking of gauge symmetries in gauge theories from a philosophical angle, taking into account the fact that the notion of a spontaneously broken local gauge symmetry, though widely employed in textbook expositions of the Higgs mechanism, is not supported by our leading theoretical frameworks of gauge quantum theories. In the context of lattice gauge theory, the statement that local gauge symmetry cannot be spontaneously broken can even be made rigorous in the form of Elitzur's theorem. Nevertheless, gauge symmetry breaking does occur in gauge quantum field theories in the form of the breaking of remnant subgroups of the original local gauge group under which the theories typically remain invariant after gauge fixing. The paper discusses the relation between these instances of symmetry breaking and phase transitions and draws some more general conclusions for the philosophical interpretation of gauge symmetries and their breaking.Comment: 30 pages, penultimate version, forthcoming with minor changes in "European Journal for Philosophy of Science

Biological Motion induced mu suppression is reduced in Early Psychosis (EP) patients with active negative symptoms and Autism Spectrum Disorders (ASD).

There is evidence of genetic and neural system overlap in Autism Spectrum Disorder (ASD) and Early Psychosis (EP). Five datasets were pooled to compare mu suppression index (MSI), a proxy of mirror neuron activity, in EP, high functioning ASD, and healthy subjects (HS). ASDs and EPs with "active" negative symptoms showed significant differences in mu suppression, in response to Biological Motion/point-light display animation, compared to HS. Preliminary findings suggest that similar neural network deficits in ASD and EP could be driven by the expression of negative symptoms in the latter group of patients. These findings may aid future studies on EP and ASD and facilitate the formulation of new hypotheses regarding their pathophysiology

Trading anonymity and order anticipation

Does it matter to market quality if broker identities are revealed after a trade and only to the two traders involved? We find that implementing full anonymity dramatically improves liquidity and reduces trader execution costs. To explain this, we compare theories based on asymmetric information to an order anticipation mechanism, where identity signals trader size, allowing strategic agents to predict the future order flow of large traders. Evidence supports the anticipation hypothesis: liquidity improves most in stocks where trading is heavily concentrated among a few brokers and in stocks susceptible to temporary price pressure. Also, only traders having large market shares benefit from anonymity

Order-to-trade ratios and market liquidity

We study the impact on market liquidity of the introduction of a penalty for high order-to-trade ratios (OTRs), implemented by the Italian Stock Exchange to curtail high-frequency quote submission. We find that the fee is associated with a collapse in the quoted depth of the stocks that make up the bulk of trading in Italian equities and with an increase in price impacts of trading across the treated stocks. Spreads do not change, however. Stocks from a pan-European control sample show no such liquidity changes. Thus, the Italian OTR fee had the effect of making Italian stocks markets more shallow and less resilient. Large stocks are more severely affected than midcaps. We also find evidence of a limited decrease in turnover. Consolidated liquidity, constructed by aggregating across all electronic trading venues for these stocks, decreases just like that on the main exchange. Thus, liquidity was not simply diverted from the main exchange, it was reduced in aggregate

The use of computer-assisted orthopedic surgery for total knee replacement in daily practice: a survey among ESSKA/SGO-SSO members

Computer-assisted orthopedic surgery (CAOS) for total knee arthroplasty is an emerging surgical tool, yet little is known about how it is being used in everyday orthopedic centers. We sought to better understand physicians' current practices and beliefs on this topic through performing a Web-based survey. Between December 2006 and January 2007, a 24-question survey was emailed to 3,330 members of the European Society of Sports Traumatology Knee Surgery and Arthroscopy (ESSKA) and the Swiss Orthopedic Society (SGO-SSO), with 389 (11.7%) agreeing to participate. Of this group, 202 (51.9%) reported that their center was equipped with a navigation system, which was an image-free based system for most (83.2%) and was primarily used for total knee arthroplasty (61.4%). In terms of the proportion of use, 50.5% of respondents used their navigation system in less than 25% of cases, 16.3% in 25-50% of cases, 7.4% in 51-75% of cases, and 25.7% in more than 75% of cases. The potential for improving the alignment of prosthesis was the most strongly cited reason for using a navigation system, while the potential for increasing operation times and the risk of infections were the most strongly cited reasons for not using a navigation system. Approximately half of respondents surveyed believed navigation systems were a real innovation contributing to the improvement of total knee implantation. However, heavy usage of computer-assisted navigation (≥51% of cases) was observed in only 33.1% of respondents, with only a quarter using it at rates that could be considered frequent (>75% of cases). Forty-eight percent of respondents said they will use a navigation system in more cases and 39.1% that their usage will stay the same. These findings indicate that CAOS is being used only moderately in current practices, though respondents generally had a positive opinion of its potential benefits. Physicians may be awaiting more data before adopting the use of these systems, though survey responses also suggest a projected increase in their use in the coming year