Quantum Geons and Noncommutative Spacetimes

Physical considerations strongly indicate that spacetime at Planck scales is noncommutative. A popular model for such a spacetime is the Moyal plane. The Poincar\`e group algebra acts on it with a Drinfel'd-twisted coproduct. But the latter is not appropriate for more complicated spacetimes such as those containing the Friedman-Sorkin (topological) geons. They have rich diffeomorphism groups and in particular mapping class groups, so that the statistics groups for N identical geons is strikingly different from the permutation group $S_N$. We generalise the Drinfel'd twist to (essentially) generic groups including to finite and discrete ones and use it to modify the commutative spacetime algebras of geons as well to noncommutative algebras. The latter support twisted actions of diffeos of geon spacetimes and associated twisted statistics. The notion of covariant fields for geons is formulated and their twisted versions are constructed from their untwisted versions. Non-associative spacetime algebras arise naturally in our analysis. Physical consequences, such as the violation of Pauli principle, seem to be the outcomes of such nonassociativity. The richness of the statistics groups of identical geons comes from the nontrivial fundamental groups of their spatial slices. As discussed long ago, extended objects like rings and D-branes also have similar rich fundamental groups. This work is recalled and its relevance to the present quantum geon context is pointed out.Comment: 41 page

Observational study of organisational responses of 17 US hospitals over the first year of the COVID-19 pandemic

Objectives The COVID-19 pandemic has required significant modifications of hospital care. The objective of this study was to examine the operational approaches taken by US hospitals over time in response to the COVID-19 pandemic. Design, setting and participants This was a prospective observational study of 17 geographically diverse US hospitals from February 2020 to February 2021. Outcomes and analysis We identified 42 potential pandemic-related strategies and obtained week-to-week data about their use. We calculated descriptive statistics for use of each strategy and plotted percent uptake and weeks used. We assessed the relationship between strategy use and hospital type, geographic region and phase of the pandemic using generalised estimating equations (GEEs), adjusting for weekly county case counts. Results We found heterogeneity in strategy uptake over time, some of which was associated with geographic region and phase of pandemic. We identified a body of strategies that were both commonly used and sustained over time, for example, limiting staff in COVID-19 rooms and increasing telehealth capacity, as well as those that were rarely used and/or not sustained, for example, increasing hospital bed capacity. Conclusions Hospital strategies during the COVID-19 pandemic varied in resource intensity, uptake and duration of use. Such information may be valuable to health systems during the ongoing pandemic and future ones

Dysfunctional oligodendrocyte progenitor cell (OPC) populations may inhibit repopulation of OPC depleted tissue

We have attempted to extend a previously described rat model of focal oligodendrocyte progenitor cell (OPC) depletion, using 40 Gy X-irradiation (Chari and Blakemore [2002] Glia 37:307–313), to the adult mouse spinal cord, to examine the ability of OPCs present in adjacent normal areas to colonise areas of progenitor depletion. In contrast to rat, OPCs in the mouse spinal cord appeared to be a comparatively radiation-resistant population, as 30–35% of OPCs survived in X-irradiated tissue (whereas <1% of OPCs survive in X-irradiated rat spinal cord). The numbers of surviving OPCs remained constant with time indicating that this population was incapable of regenerating itself in response to OPC loss. Additionally, these OPCs did not contribute to remyelination of axons when demyelinating lesions were placed in X-irradiated tissue, suggesting that the surviving cells are functionally impaired. Importantly, the length of the OPC-depleted area did not diminish with time, as would be expected if progressive repopulation of OPC-depleted areas by OPCs from normal areas was occurring. Our findings therefore raise the possibility that the presence of a residual dysfunctional OPC population may inhibit colonisation of such areas by normal OPCs. © 2003 Wiley-Liss, Inc

Modelling large areas of demyelination in the rat reveals the potential and possible limitations of transplanted glial cells for remyelination in the CNS

Transplantation of myelin-forming glial cells may provide a means of achieving remyelination in situations in which endogenous remyelination fails. For this type of cell therapy to be successful, cells will have to migrate long distances in normal tissue and within areas of demyelination. In this study, 40 Gy of X-irradiation was used to deplete tissue of endogenous oligodendrocyte progenitors (OPCs). By transplanting neonatal OPCs into OPC-depleted tissue, we were able to examine the speed with which neonatal OPCs repopulate OPC-depleted tissue. Using antibodies to NG-2 proteoglycan and in situ hybridisation to detect platelet-derived growth factor alpha-receptor Rα (PDGFRα) mRNA to visualise OPCs, we were able to show that neonatal OPCs repopulate OPC-depleted normal tissue 3–5 times more rapidly than endogenous OPCs. Transplanted neonatal OPCs restore OPC densities to near-normal values and when demyelinating lesions were made in tissue into which transplanted OPCs had been incorporated 1 month previously, we were able to show that the transplanted cells retain a robust ability to remyelinate axons after their integration into host tissue. In order to model the situation that would exist in a large OPC-depleted area of demyelination such as may occur in humans; we depleted tissue of its endogenous OPC population and placed focal demyelinating lesions at a distance (≤1 cm) from a source of neonatal OPCs. In this situation, cells would have to repopulate depleted tissue in order to reach the area of demyelination. As the repopulation process would take time, this model allowed us to examine the consequences of delaying the interaction between OPCs and demyelinated axons on remyelination. Using this approach, we have obtained data that suggest that delaying the time of the interaction between OPCs and demyelinated axons restricts the expression of the remyelinating potential of transplanted OPCs. GLIA 38:155–168, 2002. © 2002 Wiley-Liss, Inc

Key-evolution schemes resilient to space-bounded leakage

Much recent work in cryptography attempts to build secure schemes in the presence of side-channel leakage or leakage caused by malicious software, like computer viruses. In this setting, the adversary may obtain some additional information (beyond the control of the scheme designer) about the internal secret state of a cryptographic scheme. Here, we consider key-evolution schemes that allow a user to evolve a secret-key K1 via a deterministic function f, to get updated keys K2 = f(K1), K3 = f(K2),.... Such a scheme is leakage-resilient if an adversary that can leak on the first i steps of the evolution process does not get any useful information about any future keys. For such schemes, one must assume some restriction on the complexity of the leakage to prevent pre-computation attacks, where the leakage on a key Ki simply pre-computes a future key Ki+t and leaks even a single bit on it. We notice that much of the prior work on this problem, and the restrictions made therein, can be divided into two types. Theoretical work offers rigor and provable security, but at the cost of having to make strong restrictions on the type of leakage and designing complicated schemes to make standard reduction-based proof techniques go through (an example of such an assumption is that only the data actually used in computation can leak to the adversary). On the other hand, practical work focuses on simple and efficient schemes, often at th

A leakage-resilient mode of operation

Abstract. A weak pseudorandom function (wPRF) is a pseudorandom functions with a relaxed security requirement, where one only requires the output to be pseudorandom when queried on random (and not adversarially chosen) inputs. We show that unlike standard PRFs, wPRFs are secure against memory attacks, that is they remain secure even if a bounded amount of information about the secret key is leaked to the adversary. As an application of this result we propose a simple mode of operation which – when instantiated with any wPRF – gives a leakage-resilient stream-cipher. Such a cipher is secure against any side-channel attack, as long as the amount of information leaked per round is bounded, but overall can be arbitrary large. This construction is simpler than the only previous one (Dziembowski-Pietrzak FOCS’08) as it only uses a single primitive (a wPRF) in a straight forward manner.