Production of two electron-positron pairs. In this photo from the 15-foot bubble chamber at Fermilab, USA, gamma-ray photons turn into electron-positron pairs, matter & antimatter. A photon enters unseen at bottom & creates the forked white tracks of one electron-positron pair. The positron curves left in the bubble chamber's magnetic field. Its track ends where it annihilates with an atomic electron in the bubble chamber's liquid to produce two new photons, which head unseen in the same direction. One of these photons creates the second electron & positron in the top left corner of the picture. Non-relevant tracks have been coloured grey.