Animated particle tracks showing invisible gamma ray photons producing two pairs of electrons (green) and positrons (red) in a bubble chamber at the Lawrence Berkeley Laboratory. The photons come in from the right edge of the picture. In the pair at right, the new electron and positron spiral tightly in the chamber s magnetic field. This is because some of the photon s energy is taken up in displacing an atomic electron, which travels towards the top left of the picture, curving gently. In the pair to the left, all the photon s energy goes into producing the electron and positron and, as a result, these particles are more energetic than the pair to the right and their tracks curve much less in the magnetic field. (The bubble chamber photograph has been cleaned up to remove all but the relevant tracks, coloured to aid identification of the particles, and animated.)