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Summary
Physicists' best chance of spotting an effect of "quantum gravity"—the melding of quantum mechanics and Einstein's theory of gravity—may have evaporated. According to some quantum-gravity theories, the speed of light may change very slightly with the light's wavelength, and experimenters are searching for the effect in radiation from distant stellar explosions. Those searches may be in vain, however. If light's speed varied in this way, then untenable paradoxes would arise, one theorist argues. The speed variations must be at least 23 orders of magnitude smaller than experimental limits set last year, she says.
