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OE Reports 167 - November 1997 Briefly Matter created from pure light A team of 20 physicists from four institutions has created particles of matter from ordinary light for the first time. The experiment was carried out at the Stanford Linear Accelerator Center (SLAC) by scientists and students from the University of Rochester, Princeton University, the University of Tennessee, and Stanford. The team reported the work in the 1 September issue of Physical Review Letters. Converting energy into matter isn't completely new. When smashing together protons and antiprotons in high-energy accelerator experiments, the initial particles are destroyed and release a fleeting burst of energy. Sometimes this energy burst contains virtual photons, short-lived packets of light, which go on to form new particles. In this experiment scientists observed for the first time the creation of particles from real photons, packets of light that scientists can observe directly in the laboratory. The feat was accomplished by dumping an incredible amount of power into an area less than one billionth of a square centimeter. They used high-energy electrons traveling near the speed of light, produced by SLAC's two-mile-long accelerator, and photons from a tabletop terawatt glass laser developed at Rochester's Laboratory for Laser Energetics. The laser unleashed a picosecond of lightjust half a millimeter long. The team synchronized the two beams and sent the electrons head on into the photons. Occasionally an electron barreled into a photon with immense energy, knocking the photon backward with such tremendous energy that it collided with several of the densely packed photons behind it and combined with them, creating an electron and a positron. In a series of experiments lasting several months the team studied thousands of collisions, leading to the production of more than 100 positrons. The energy-to-matter conversion was made possible by the incredibly strong electromagnetic fields that the photon-photon collisions produced. Similar conditions are found only rarely in the universe; neutron stars, for instance, have incredibly strong magnetic fields, and some scientists believe that their surfaces are home to the same kind of light-to-matter interactions the team observed. This experiment marks the first time scientists have been able to create such strong fields using laser beams. By conducting experiments like this scientists test the principles of quantum electrodynamics (QED) in fields so strong that the vacuum "boils" into pairs of electrons and positrons. The scientists say the work could also have applications in designing new particle accelerators. Spokesmen for the experiment, funded by the U.S. Department of Energy, are Kirk McDonald, professor of physics at Princeton, and Adrian Melissinos, professor of physics at Rochester.

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