Gamma ray test shows that GR is still right


来自: CS 2009-11-07 08:10:42

  • CS

    CS 2009-11-07 09:14:05

    quantum gravity effect:
    This, the story goes, could have an effect on the propagation of light — or photons, as they are called in quantum-speak — slowing light with short wavelengths relative to light with longer wavelengths. The higher the energy of a photon, the shorter is its wavelength. One way to think about it is to envision the photons as boats on this choppy sea. The small ones, like tugboats, have to climb up and down the waves to get anywhere, while the bigger ones can slice through the waves and bumps like ocean liners, and thus go a little faster.

  • CS

    CS 2009-11-07 09:21:25

    The photons from GRB 090510, detected on May 9, ranged from 0.010MeV to 31GeV in seven brief bursts over about two seconds.

    The spread in travel time of 0.9 second between the highest- and lowest-energy gamma rays, if attributed to quantum effects rather than the dynamics of the explosion itself, suggested that any quantum effects in which the slowing of light is proportional to its energy do not show up until you get down to sizes about eight-tenths of the Planck length, according to the Nature paper, whose lead author was Sylvain Guiriec of the University of Alabama.

    Here we report the detection of emission up to approx31 GeV from the distant and short GRB 090510. 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 theories3, 6, 7 in which the quantum nature of space–time on a very small scale linearly alters the speed of light.

  • CS
  • CS

    CS 2009-11-07 09:43:07

    Universe's quantum 'speed bumps' no obstacle for light

    * 19:20 28 October 2009 by Rachel Courtland
    * For similar stories, visit the Quantum World Topic Guide

    A hint that quantum fluctuations in the fabric of the universe slow the speed of light has not been borne out in observations by NASA's Fermi telescope. The measurements contradict a 2005 result that supported the idea that space and time are not smooth.

    Einstein's theory of special relativity says that all electromagnetic radiation travels through a vacuum at the speed of light. This speed is predicted to be constant, regardless of the energy of the radiation.

    Yet in 2005, the MAGIC gamma-ray telescope on La Palma in the Canary Islands suggested the speed of light might not be constant after all. The telescope, which measured the light released by a galaxy around 500 million light years away, found that higher energy photons arrived four minutes behindMovie Camera their lower energy counterparts.
    Grainy universe

    The discovery hinted that the speed of light may change depending on its energy. This effect could be a consequence of some theories of quantum gravity, which attempt to unify Einstein's theory of gravity with the laws of quantum mechanics. These models postulate that space and time are not smooth. Instead space-time is inherently grainy, fluctuating rapidly over distances of about 10-35 metres, a length called the Planck scale.

    If space-time is grainy, higher-energy photons would move more slowly than their lower-energy counterparts. That's because higher-energy photons have smaller wavelengths, which makes them more sensitive to tiny fluctuations in space-time.

    However, the MAGIC lag was apparently too large to be easily explained by graininess on the quantum scale. If the delay were caused by fluctuations in space-time, they would have to occur on scales more than 10 times larger than the Planck scale.

    "This intriguing evidence has been wandering around in the quantum gravity community for more than a year now, with hope on the progressive side, and stomach aches on the conservative side," says physicist Giovanni Amelino-Camelia of Sapienza University of Rome in Italy.

    Now new observations suggest quantum gravity cannot be responsible for the time delay observed by MAGIC. The light from a powerful, 7-billion year old gamma-ray burst detected by NASA's Fermi Gamma-ray Space Telescope shows no evidence of a lag between photons of a range of energies.

    "We have fewer stomach aches now," says Amelino-Camelia. "The Fermi data has pushed the limit where it's now proven the MAGIC data cannot be interpreted in that way."
    Light artefact

    Fermi's measurement is the most stringent direct limit on how much the speed of light might vary with energy, says Jonathan Granot of the University of Hertfordshire in the UK, who led the analysis of the burst. "For the first time, we can put the limit [down to] the energy scale in which quantum effects would alter the geometry of space time."

    The MAGIC time delay may be down to an astrophysical process where particles are accelerated to enormous energies within the hearts of galaxies. Follow-up calculations after MAGIC's 2005 result showed that is possible to produce flares that release lower-energy radiation before higher-energy radiation, according to MAGIC collaborator Robert Wagner of the Max Planck Institute of Physics in Munich, Germany. "I think what we can say for the time being is quantum gravity effects cannot be the dominant effect," he says.
    Knockout blow?

    The result does not necessarily strike a blow to quantum gravity. Only a subset of models predict the effect, and "while it seems reasonable to expect that the variation of the speed of light with energy is a sign of quantum space-time, there is no well developed theory of quantum space-time that cleanly makes this prediction," says Lee Smolin of the Perimeter Institute for Theoretical Physics in Waterloo, Canada.

    What's more, it will require even more precise measurements to completely exclude the possibility that light may change its speed depending on its energy. "If there is an effect, the experiment is now at the threshold of scales where the effect is expected, and there is the exciting prospect that it could be discovered over few years," Smolin says.


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