| 1. |
- Abdo, A. A., et al.
(författare)
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A limit on the variation of the speed of light arising from quantum gravity effects
- 2009
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 462:7271, s. 331-334
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Tidskriftsartikel (refereegranskat)abstract
- A cornerstone of Einstein’s special relativity is Lorentz invariance—the postulate that all observers measure exactly the same speed of light in vacuum, independent of photon-energy. While special relativity assumes that there is no fundamental length-scale associated with such invariance, there is a fundamental scale (the Planck scale, lPlanck~1.62×10-33cm or EPlanck = MPlanckc2~1.22×1019GeV), at which quantum effects are expected to strongly affect the nature of space–time. There is great interest in the (not yet validated) idea that Lorentz invariance might break near the Planck scale. A key test of such violation of Lorentz invariance is a possible variation of photon speed with energy. Even a tiny variation in photon speed, when accumulated over cosmological light-travel times, may be revealed by observing sharp features in γ-ray burst (GRB) light-curves. Here we report the detection of emission up to ~31GeV from the distant and short GRB090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2EPlanck on the scale of a linear energy dependence (or an inverse wavelength dependence), subject to reasonable assumptions about the emission (equivalently we have an upper limit of lPlanck/1.2 on the length scale of the effect). Our results disfavour quantum-gravity theories in which the quantum nature of space–time on a very small scale linearly alters the speed of light.
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| 2. |
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| 3. |
- Bshary, Redouan, et al.
(författare)
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Pairs of cooperating cleaner fish provide better service quality than singletons.
- 2008
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 1476-4687 .- 0028-0836. ; 455:7215, s. 964-6
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Tidskriftsartikel (refereegranskat)abstract
- Service providers may vary service quality depending on whether they work alone or provide the service simultaneously with a partner. The latter case resembles a prisoner's dilemma(1-4), in which one provider may try to reap the benefits of the interaction without providing the service. Here we present a game- theory model based on the marginal value theorem(5), which predicts that as long as the client determines the duration, and the providers cooperate towards mutual gain, service quality will increase in the pair situation. This prediction is consistent with field observations and with an experiment on cleaning mutualism, in which stable male - female pairs of the cleaner wrasse Labroides dimidiatus repeatedly inspect client fish jointly. Cleaners cooperate by eating ectoparasites(6) off clients but actually prefer to cheat and eat client mucus(7). Because clients often leave in response to such cheating, the benefits of cheating can be gained by only one cleaner during a pair inspection. In both data sets, the increased service quality during pair inspection was mainly due to the smaller females behaving significantly more cooperatively than their larger male partners. In contrast, during solitary inspections, cleaning behaviour was very similar between the sexes. Our study highlights the importance of incorporating interactions between service providers to make more quantitative predictions about cooperation between species.
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| 4. |
- Choi, Minkee, et al.
(författare)
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Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts
- 2009
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 461:7261, s. 246-249
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Tidskriftsartikel (refereegranskat)abstract
- Zeolites-microporous crystalline aluminosilicates-are widely used in petrochemistry and fine-chemical synthesis(1-3) because strong acid sites within their uniform micropores enable size- and shape-selective catalysis. But the very presence of the micropores, with aperture diameters below 1 nm, often goes hand-in-hand with diffusion limitations(3-5) that adversely affect catalytic activity. The problem can be overcome by reducing the thickness of the zeolite crystals, which reduces diffusion path lengths and thus improves molecular diffusion(4,5). This has been realized by synthesizing zeolite nanocrystals(6), by exfoliating layered zeolites(7-9), and by introducing mesopores in the microporous material through templating strategies(10-17) or demetallation processes(18-22). But except for the exfoliation, none of these strategies has produced 'ultrathin' zeolites with thicknesses below 5 nm. Here we show that appropriately designed bifunctional surfactants can direct the formation of zeolite structures on themesoporous and microporous length scales simultaneously and thus yield MFI (ZSM-5, one of the most important catalysts in the petrochemical industry) zeolite nanosheets that are only 2 nm thick, which corresponds to the b-axis dimension of a single MFI unit cell. The large number of acid sites on the external surface of these zeolites renders them highly active for the catalytic conversion of large organic molecules, and the reduced crystal thickness facilitates diffusion and thereby dramatically suppresses catalyst deactivation through coke deposition during methanol-to-gasoline conversion. We expect that our synthesis approach could be applied to other zeolites to improve their performance in a range of important catalytic applications.
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| 5. |
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| 6. |
- Emerson, B. C., et al.
(författare)
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Species diversity can drive speciation
- 2005
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 434:7036, s. 1015-1017
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Tidskriftsartikel (refereegranskat)abstract
- A fundamental question in evolutionary ecology and conservation biology is: why do some areas contain greater species diversity than others? Island biogeographic theory has identified the roles of immigration and extinction in relation to area size and proximity to source areas(1,2), and the role of speciation is also recognized as an important factor(3-6). However, one as yet unexplored possibility is that species diversity itself might help to promote speciation, and indeed the central tenets of island biogeographic theory support such a prediction. Here we use data for plants and arthropods of the volcanic archipelagos of the Canary and Hawaiian Islands to address whether there is a positive relationship between species diversity and rate of diversification. Our index of diversification for each island is the proportion of species that are endemic, and we test our prediction that this increases with increasing species number. We show that even after controlling for several important physical features of islands, diversification is strongly related to species number.
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| 7. |
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| 8. |
- Graversen, Rune Grand, et al.
(författare)
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Reply : Communications arising
- 2008
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 455, s. E4-E5
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Tidskriftsartikel (refereegranskat)
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| 9. |
- Graversen, Rune G., et al.
(författare)
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Vertical structure of recent Arctic warming
- 2008
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 451:7174, s. 53-56
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Tidskriftsartikel (refereegranskat)abstract
- Near-surface warming in the Arctic has been almost twice as large as the global average over recent decades1, 2, 3, 4, 5—a phenomenon that is known as the 'Arctic amplification'. The underlying causes of this temperature amplification remain uncertain. The reduction in snow and ice cover that has occurred over recent decades6, 7 may have played a role5, 8. Climate model experiments indicate that when global temperature rises, Arctic snow and ice cover retreats, causing excessive polar warming9, 10, 11. Reduction of the snow and ice cover causes albedo changes, and increased refreezing of sea ice during the cold season and decreases in sea-ice thickness both increase heat flux from the ocean to the atmosphere. Changes in oceanic and atmospheric circulation, as well as cloud cover, have also been proposed to cause Arctic temperature amplification12, 13, 14, 15, 16, 17. Here we examine the vertical structure of temperature change in the Arctic during the late twentieth century using reanalysis data. We find evidence for temperature amplification well above the surface. Snow and ice feedbacks cannot be the main cause of the warming aloft during the greater part of the year, because these feedbacks are expected to primarily affect temperatures in the lowermost part of the atmosphere, resulting in a pattern of warming that we only observe in spring. A significant proportion of the observed temperature amplification must therefore be explained by mechanisms that induce warming above the lowermost part of the atmosphere. We regress the Arctic temperature field on the atmospheric energy transport into the Arctic and find that, in the summer half-year, a significant proportion of the vertical structure of warming can be explained by changes in this variable. We conclude that changes in atmospheric heat transport may be an important cause of the recent Arctic temperature amplification.
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| 10. |
- Hessa, Tara, et al.
(författare)
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Molecular code for transmembrane-helix recognition by the Sec61 translocon
- 2007
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 450:7172, s. 1026-1030
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Tidskriftsartikel (refereegranskat)abstract
- Transmembrane alpha-helices in integral membrane proteins are recognized co-translationally and inserted into the membrane of the endoplasmic reticulum by the Sec61 translocon. A full quantitative description of this phenomenon, linking amino acid sequence to membrane insertion efficiency, is still lacking. Here, using in vitro translation of a model protein in the presence of dog pancreas rough microsomes to analyse a large number of systematically designed hydrophobic segments, we present a quantitative analysis of the position- dependent contribution of all 20 amino acids to membrane insertion efficiency, as well as of the effects of transmembrane segment length and flanking amino acids. The emerging picture of translocon- mediated transmembrane helix assembly is simple, with the critical sequence characteristics mirroring the physical properties of the lipid bilayer.
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