For example, a radioactive isotope such as nickel-63 releases so-called beta particles a highly energetic form of electrons.

ECONOMIST: Micromachines: The engines of Lilliput | The

Because radiation from the Crab nebula is so energetic, the electrons that created it must have been travelling very close to the speed of light so close that the difference between the two speeds would bump up against Planck-scale graininess, and show up in the spectrum of the radiation.

ECONOMIST: Planck-scale physics