Out Of Pure Light, Physicists Create Particles Of Matter

Scientists have long been able to convert matter to energy; the most spectacular example is a nuclear explosion, where a small amount of matter creates tremendous energy. Now physicists have succeeded in doing the opposite: converting energy in the form of light into matter -- in this experiment, electrons and their anti-matter equivalent, positrons.

Converting energy into matter isn't completely new to physicists. When they smash together particles like protons and anti-protons in high-energy accelerator experiments, the initial particles are destroyed and release a fleeting burst of energy. Sometimes this energy burst contains very short-lived packets of light known as "virtual photons" 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.

Physicists accomplished the feat by dumping an incredible amount of power -- nearly as much as it takes to run the entire nation but lasting only for a tiny fraction of a second -- into an area less than one billionth of a square centimeter, which is far smaller than the period at the end of this sentence. They used high-energy electrons traveling near the speed of light, produced by SLAC's two-mile-long accelerator, and photons from a powerful, "tabletop terawatt" glass laser developed at Rochester's Laboratory for Laser Energetics. The laser unleashed a tiny but powerful sliver of light lasting about one trillionth of a second (one picosecond) -- just half a millimeter long. Packed into this sliver were more than two billion billion photons.

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, "like a speeding Mack truck colliding with a ping pong ball," says physicist Adrian Melissinos of the University of Rochester. That knocked 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 Melissinos, professor of physics at Rochester. Also taking part in the experiment were William Bugg, Steve Berridge, Konstantin Shmakov and Achim Weidemann at Tennessee; David Burke, Clive Field, Glenn Horton-Smith, James Spencer and Dieter Walz at SLAC; Christian Bula and Eric Prebys at Princeton; and seven other physicists from Rochester, including Associate Professor David Meyerhofer; graduate students Thomas Koffas, David Reis, Stephen Boege, and Theofilos Kotseroglou; research associate Charles Bamber; and engineer Wolfram Ragg.