Speed of light difference in the speeds of photons of different energies, confirming that Lorentz invariance is verified at least down to the scale of the Planck length ("l" = ≈ ) divided by 1.2. In a medium, light usually does not propagate at a speed equal to "c"; further, different types of light wave will travel at different speeds. The speed at which the individual crests and troughs of a plane wave (a wave filling the whole space, with only one frequency) propagate is called the phase velocity "v". An actual physical signal with a finite extent (a pulse of light) travels
Speed of Darkness fiction, named The Science of Discworld, written by Terry Pratchett, Ian Stewart and Jack Cohen. In a 2011 interview on BBC Radio 1, frontman Dave King explained that the title track and album title was taken from a quote of Dino Misetić, the artist who designed the album cover, which appeared in the book Sarajevo Marlboro. Dino, who grew up in the Balkans during the Balkan Wars, is quoted in the book, saying: "They taught us what the speed of light is, but nobody can teach you what the speed of darkness is." Flogging Molly: Additional musicians: Speed of Darkness