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Sökning: L773:0028 0836 OR L773:1476 4687 > (2000-2009)

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1.
  • Abdo, A. A., et al. (författare)
  • A limit on the variation of the speed of light arising from quantum gravity effects
  • 2009
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 462:7271, s. 331-334
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>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, l(Planck) approximate to 1.62 x 10(-33) cm or E-Planck = M(Planck)c(2) approximate to 1.22 x 10(19) GeV), 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(1-7). Even a tiny variation in photon speed, when accumulated over cosmological light-travel times, may be revealed by observing sharp features in gamma-ray burst (GRB) light-curves(2). Here we report the detection of emission up to similar to 31GeV from the distant and short GRB090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2E(Planck) 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 l(Planck)/1.2 on the length scale of the effect). Our results disfavour quantum-gravity theories(3,6,7) in which the quantum nature of space-time on a very small scale linearly alters the speed of light.</p>
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2.
  • Abdo, A. A., et al. (författare)
  • A limit on the variation of the speed of light arising from quantum gravity effects
  • 2009
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 462:7271, s. 331-334
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>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, l(Planck) approximate to 1.62 x 10(-33) cm or E-Planck = M(Planck)c(2) approximate to 1.22 x 10(19) GeV), 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(1-7). Even a tiny variation in photon speed, when accumulated over cosmological light-travel times, may be revealed by observing sharp features in gamma-ray burst (GRB) light-curves(2). Here we report the detection of emission up to similar to 31GeV from the distant and short GRB090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2E(Planck) 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 l(Planck)/1.2 on the length scale of the effect). Our results disfavour quantum-gravity theories(3,6,7) in which the quantum nature of space-time on a very small scale linearly alters the speed of light.</p>
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3.
  • Abdo, A. A., et al. (författare)
  • A limit on the variation of the speed of light arising from quantum gravity effects
  • 2009
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 462:7271, s. 331-334
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>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.</p>
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4.
  • Adriani, O., et al. (författare)
  • An anomalous positron abundance in cosmic rays with energies 1.5-100 GeV
  • 2009
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 458:7238, s. 607-609
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>Antiparticles account for a small fraction of cosmic rays and are known to be produced in interactions between cosmic-ray nuclei and atoms in the interstellar medium(1), which is referred to as a 'secondary source'. Positrons might also originate in objects such as pulsars(2) and microquasars(3) or through dark matter annihilation(4), which would be 'primary sources'. Previous statistically limited measurements(5-7) of the ratio of positron and electron fluxes have been interpreted as evidence for a primary source for the positrons, as has an increase in the total electron+positron flux at energies between 300 and 600 GeV (ref. 8). Here we report a measurement of the positron fraction in the energy range 1.5-100 GeV. We find that the positron fraction increases sharply overmuch of that range, in a way that appears to be completely inconsistent with secondary sources. We therefore conclude that a primary source, be it an astrophysical object or dark matter annihilation, is necessary.</p>
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5.
  • Ahlberg, Per E. (författare)
  • Birth of the jawed vertebrates
  • 2009
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 457, s. 1094-1095
  • Tidskriftsartikel (övrigt vetenskapligt)abstract
    • <p>The discovery of embryos in certain fossil fishes not only shows that internal fertilization and live birth evolved early in vertebrate history, but also raises questions about the origin of jawed vertebrates.</p>
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6.
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7.
  • Ahlberg, Per Erik, 1963-, et al. (författare)
  • Pelvic claspers confirm chondrichthyan-like internal fertilization in arthrodires
  • 2009
  • Ingår i: Nature. - Nature Publishing Group. - 0028-0836 .- 1476-4687. ; 460:7257, s. 888-889
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>Recent finds(1,2) demonstrate that internal fertilization and   viviparity (live birth) were more widespread in the Placodermi, an   extinct group of armoured fishes, than was previously realized.   Placoderms represent the sister group of the crown group jawed   vertebrates (Gnathostomata)(3,4), making their mode(s) of reproduction   potentially informative about primitive gnathostome conditions. An   ossified pelvic fin basipterygium discovered in the arthrodire   Incisoscutum ritchiei was hypothesized to be identical in males and   females, with males presumed to have an additional cartilaginous   element or series forming a clasper. Here we report the discovery of a   completely ossified pelvic clasper in Incisoscutum ritchiei (WAM   03.3.28) which shows that this interpretation was incorrect: the   basipterygium described previously(1) is in fact unique to females. The   male clasper is a slender rod attached to a square basal plate that   articulates directly with the pelvis. It carries a small cap of dermal   bone covered in denticles and small hooks that may be homologous with   the much larger dermal component of the ptyctodont clasper.</p>
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8.
  • Ahlberg, Per, 1963-, et al. (författare)
  • Ventastega curonica and the origin of tetrapod morphology
  • 2008
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 453:7199, s. 1199-1204
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>The gap in our understanding of the evolutionary transition from fish to tetrapod is beginning to close thanks to the discovery of new intermediate forms such as <em>Tiktaalik roseae</em>. Here we narrow it further by presenting the skull, exceptionally preserved braincase, shoulder girdle and partial pelvis of <em>Ventastega curonica</em> from the Late Devonian of Latvia, a transitional intermediate form between the 'elpistostegids' <em>Panderichthys</em> and <em>Tiktaalik</em> and the Devonian tetrapods (limbed vertebrates) <em>Acanthostega</em> and <em>Ichthyostega</em>. <em>Ventastega</em> is the most primitive Devonian tetrapod represented by extensive remains, and casts light on a part of the phylogeny otherwise only represented by fragmentary taxa: it illuminates the origin of principal tetrapod structures and the extent of morphological diversity among the transitional forms</p>
9.
  • Andrés, E., et al. (författare)
  • Observation of high-energy neutrinos using Čerenkov detectors embedded deep in Antarctic ice
  • 2001
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 410:6827, s. 441-443
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>Neutrinos are elementary particles that carry no electric charge and have little mass. As they interact only weakly with other particles, they can penetrate enormous amounts of matter, and therefore have the potential to directly convey astrophysical information from the edge of the Universe and from deep inside the most cataclysmic high-energy regions. The neutrino's great penetrating power, however, also makes this particle difficult to detect. Underground detectors have observed low-energy neutrinos from the Sun and a nearby supernova2, as well as neutrinos generated in the Earth's atmosphere. But the very low fluxes of high-energy neutrinos from cosmic sources can be observed only by much larger, expandable detectors in, for example, deep water3,4 or ice5. Here we report the detection of upwardly propagating atmospheric neutrinos by the ice-based Antarctic muon and neutrino detector array (AMANDA). These results establish a technology with which to build a kilometre-scale neutrino observatory necessary for astrophysical observations1.</p>
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10.
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