The holographic principle

There is strong evidence that the area of any surface limits the information content of adjacent spacetime regions, at 10^(69) bits per square meter. We review the developments that have led to the recognition of this entropy bound, placing special emphasis on the quantum properties of black holes. The construction of light-sheets, which associate relevant spacetime regions to any given surface, is discussed in detail. We explain how the bound is tested and demonstrate its validity in a wide range of examples. A universal relation between geometry and information is thus uncovered. It has yet to be explained. The holographic principle asserts that its origin must lie in the number of fundamental degrees of freedom involved in a unified description of spacetime and matter. It must be manifest in an underlying quantum theory of gravity. We survey some successes and challenges in implementing the holographic principle.Comment: 52 pages, 10 figures, invited review for Rev. Mod. Phys; v2: reference adde

Variations in surface atemperatures: Part 2. Arctic regions, 1881–1980

We describe annual and seasonal changes in air temperatures over high latitudes of the Northern Hemisphere during the period 1881–1980. Trends (that is, fluctuations on time scales greater than 20 years) in the average temperature of the Arctic are compared with those of the Northern Hemisphere. Seasonal and regional departures from the long-term trends in the average temperature of the Arctic are identified. Spatial patterns of variation in the Arctic temperature field are determined by principal component analysis and the major characteristics of the time series of the dominant patterns are summarized. Trends in Arctic temperatures have been broadly similar to those for the Northern Hemisphere during the study period. The Arctic variations were, however, greater in magnitude and more rapid. The spatial pattern of change associated with the trend in Arctic temperatures is clearly identified by principal component analysis. It shows that the trends have, in general, been Arctic-wide, but that certain regions are particularly sensitive to long-term variations, most notably northwest Greenland and around the Kara Sea. There is some evidence that the pattern of Arctic cooling that occurred after 1940 was more complex than the warming that affected the whole Arctic during the 1920's and 1930's. Warming of the Arctic has occurred during the 1970's, but is not yet of sufficient duration to be considered long term, except, perhaps, in spring. The average temperature of the Arctic during the 1970's was equal to that of the 1960's, indicating a cessation of the long-term cooling trend but not, as yet, a shift to long-term warming. Short-term variations in temperature appear to be most pronounced close to major regions of sea-ice production and decay

Environmental fluctuations effects on the global energy balance

Much effort has been devoted to developing simple energy-balance climatic models. Although consideration of latitudinal energy transfer1-4 gives more complete answers it has become clear that global, 'zero dimensional' models may also provide much useful information5,6. These models have the form: illusr rid="illus1"is the surface temperature, C the thermal inertia coefficient, Q the solar constants, σ the Stefan constant, a (T) the (generally temperature-dependent) albedo, and ε the emissivity of the Earth-atmosphere system. The variability of the climate system rests, therefore, on certain types of change experienced by the solar output, or by such planetary factors as emissivity, albedo, cloudiness and so forth. In addition to some long-term trends of the solar constant7, it has been suggested that the Sun is in an almost-intransitive state8,9. Hence, it may generate large fluctuations around some mean value of its output, which will be perceived by the Earth-atmosphere system as an 'external noise' affecting Q. The fact that the terrestrial atmosphere is likely to be in an almost intransitive state10 can also generate appreciable fluctuations in factors influencing the albedo and the emissivity. In the absence of precise knowledge of the mechanism of these fluctuations, one is again tempted to regard them as an 'external noise' affecting a (T) and ε. We explore here the qualitative effect of such environmental fluctuations in the thermal regime13, at the level of a zero-dimensional planetary model. Previous analyses of nonlinear systems of chemical and biological interest11,12 have shown that external noise can dramatically affect the macroscopic behaviour predicted by the deterministic equations of evolution, if coupled to these equations in a multiplicative way. © 1979 Nature Publishing Group.SCOPUS: le.jinfo:eu-repo/semantics/publishe

From the Proliferation of the Photographic to the Nullification of Truth: Personal and Commercial Narratives of Travel in Britain, 1890s-1930s

This article explores the impact that the democratisation of photography had on notions of photographic truth. It does so by focusing on the proliferation of visual narratives of travel produced by tourist photographers and travel firms in Britain between the 1890s and 1930s, a period that saw the emerging travel industry shift from using lens-based images to mixed-media. The article argues that people’s increasing familiarity with the means of representation displaced the ‘truth’ of the travel photograph from the image itself to one’s own experience of travel, forcing travel marketing to re-invent itself in an attempt to control the responses of customers