4137 / Time (seconds): Caesium-FontЉnen-Atomic clock at the PTB in Braunschweig: EUROPA, DEUTSCHLAND, NIEDERSACHSEN, BRAUNSCHWEIG (BRAUNSCHWEIG, GERMANY), 18.04.2007: `Second`, SI base unit of time: Caesium fountain atomic clock at Physikalisch-Technische Bundesanstalt PTB in Braunschweig, Germany. At the center of the structure at right caesium atoms are trapped and cooled by six laser beams in a vacuum to be accelerated vertically and to fall back down again. During this flight an internal oscillation is excited to achieve a pacemaker of great accuracy. Physicists Dr. Robert Wynands, at left, and Dr. Stefan Weyers. Copper lining on the walls shields against electromagnetic interference.) - Marc Steinmetz / VISUM - Keywords: Germany, science, technology, physics, metrology, base, unit, international, system, of, units, SI, metrical, measure, standard, accurate, accuracy, lab, experiment, time, second, caesium, cesium, atomic, clock, clocks, people