Towards the fast scrambling conjecture

Many proposed quantum mechanical models of black holes include highly nonlocal interactions. The time required for thermalization to occur in such models should reflect the relaxation times associated with classical black holes in general relativity. Moreover, the time required for a particularly strong form of thermalization to occur, sometimes known as scrambling, determines the time scale on which black holes should start to release information. It has been conjectured that black holes scramble in a time logarithmic in their entropy, and that no system in nature can scramble faster. In this article, we address the conjecture from two directions. First, we exhibit two examples of systems that do indeed scramble in logarithmic time: Brownian quantum circuits and the antiferromagnetic Ising model on a sparse random graph. Unfortunately, both fail to be truly ideal fast scramblers for reasons we discuss. Second, we use Lieb-Robinson techniques to prove a logarithmic lower bound on the scrambling time of systems with finite norm terms in their Hamiltonian. The bound holds in spite of any nonlocal structure in the Hamiltonian, which might permit every degree of freedom to interact directly with every other one.Comment: 34 pages. v2: typo correcte

Manejo da hipertensão arterial na isquemia cerebral aguda Management of arterial hypertension in patients with acute ischemic stroke

OBJETIVO: Avaliar o nível de conhecimento dos médicos, através de sua conduta, em paciente com quadro de hipertensão arterial na fase aguda da isquemia cerebral. Também comentamos as principais condutas nesta fase, com ênfase na tensão arterial (TA). MÉTODO: Foram entrevistados 120 médicos da clínica médica e da cirurgia geral, em dez dos maiores Hospitais de Belo Horizonte, em 1997. Todos responderam a um questionário contendo um caso clínico de paciente hipertenso leve, admitido com quadro de isquemia cerebral e tensão arterial de 186x110 mmHg. Os profissionais deveriam optar por reduzir, aumentar ou manter a TA. RESULTADOS: Dos entrevistados, 38 (31,7%) responderam que reduziriam os níveis tensionais, 82 (68,3%) optaram pela manutenção e nenhum aumentaria (p<0,05). Estes índices foram iguais nas duas especialidades. Já em relação ao tempo de formado, observou-se que aqueles com mais de 10 anos de graduação, apresentaram maior tendência a redução da TA (p<0,05), quando comparado aos mais jovens. CONCLUSÃO: A redução da TA prejudica a perfusão cerebral, podendo aumentar a área isquêmica ("zona de penumbra"). Concluimos que, apesar de estar a maioria dos médicos ciente da conduta recomendada no manejo da hipertensão arterial na fase aguda dos quadros isquêmios cerebrais 31,7% ainda é uma taxa elevada de médicos despreparados para o atendimento emergencial de isquemia cerebral, considerando os prejuízos trazidos ao paciente em decorrência da conduta inadequada. Talvez o desenvolvimento de unidades especializadas no atendimento desses pacientes ("stroke units") possa melhorar tais índices.<br>PURPOSE: We aimed with study to assess the current clinical practice about the management of high blood pressure in patients in the acute phase of ischemic stroke. We also comment some topics of ischemic stroke treatment. METHODS: A case report of a patient admitted 8 hours after onset of ischemic stroke and with blood pressure of 186x110 mmHg was presented to 120 surgeons and clinician. They were asked to decide the best therapeutic option: to increase, decrease or maintenance blood pressure. RESULTS: Thirty-eight physicians (31,7%) considered decreasing blood pressure the best therapeutics, 82 (68,3%) considered maintenance and none decided to increase it (p < 0.05). There was no difference between the two specialties conduct. The physicians, with more than 10 years of graduation, had a tendency to decrease the blood pressure (p <0.05). CONCLUSION: The maintenance of blood pressure may present a sufficient blood support to compensate brain flow. A high percentage of the physicians (31,7%) do not know about the current concepts of therapeutics considering hypertension in acute ischemic stroke. The development on special units to treat these patients ("stroke units") may eventually decrease the morbimortality rates of ischemic stroke