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September 16, 1997

Scientists Use Light to Create Particles


A trailblazing experiment at the Stanford Linear Accelerator Center in California has confirmed a longstanding prediction by theorists that light beams colliding with each other can goad the empty vacuum into creating something out of nothing.

In a report published this month by the journal Physical Review Letters, 20 physicists from four research institutions disclosed that they had created two tiny specks of matter -- an electron and its antimatter counterpart, a positron -- by colliding two ultrapowerful beams of radiation.

The possibility of doing something like this was suggested in 1934 by two American physicists, Dr. Gregory Breit and Dr. John A. Wheeler. But more than six decades would pass before any laboratory could pump enough power into colliding beams of radiation to conjure up matter from nothingness. The Stanford accelerator finally provided enough energy to do it.

Dr. Adrian C. Melissinos of the University of Rochester, a spokesman for the group, said in an interview that the weaker of the two light beams used in the experiment was produced by a trillion-watt green laser. That in itself fell far short of the needed energy, even though the pulsed green laser is one of the world's most powerful.

But the opposing beam of radiation was another story; boosted by energy drawn from electrons whizzing down the two-mile-long Stanford accelerator, this second beam of radiation was some 10 billion times as powerful as the green laser beam.

The paths of colliding electrons and photons in the experiment were as complicated as those choreographed by an expert pool player planning a difficult shot.

Photons of light from the green laser were allowed to collide almost head-on with 47-billion-electronvolt electrons shot from the Stanford particle accelerator. These collisions transferred some of the electrons' energy to the photons they hit, boosting the photons from green visible light to gamma-ray photons, and forcing the freshly spawned gamma photons to recoil into the oncoming laser beam. The violent collisions that ensued between the gamma photons and the green laser photons created an enormous electromagnetic field.

This field, Melissinos said, "was so high that the vacuum within the experiment spontaneously broke down, creating real particles of matter and antimatter."

This breakdown of the vacuum by an ultrastrong electromagnetic field was hypothesized in 1950 by Dr. Julian S. Schwinger, who was awarded a Nobel Prize in physics in 1965. The creation of matter by colliding photons of radiation is believed to take place in some stars, but it was never observed in laboratory experiments before, largely because the required energy is beyond the reach of conventional laboratory equipment.

With his special theory of relativity, Einstein showed that matter and energy are equivalent and can be transmuted through the equation E equals mc2; that is, energy in ergs is equal to mass in grams times the speed of light squared, in centimeters per second. This accounts for the vast energy released by small amounts of matter in nuclear explosions, but it also means that staggering amounts of energy are required to create even the tiniest particles of matter.

The hardest part of the project, in which scientists employed by the Stanford Linear Accelerator Center collaborated with colleagues from the University of Tennessee in Knoxville, Princeton University and the University of Rochester, was synchronizing the timing of laser and electron pulses, Melissinos said. The green laser pulse, traveling at the speed of light, was only one half millimeter long. That pulse had to be timed to collide with an electron pulse almost as it emerged from the two-mile-long beam line.

The experiment, Melissinos said, is unlikely to have many practical applications, although it might help in the design of a new generation of research accelerators. Existing accelerators use particles of matter as projectiles -- protons, electrons or entire atoms. But a possible future accelerator that physicists call a "gamma-gamma machine" might work by colliding opposing beams of photons, especially gamma-ray photons.

Meanwhile, Melissinos and his colleagues expect to use photon collisions as a way to explore the intricacies of quantum electrodynamics -- a highly successful but complex theory explaining the interactions of electromagnetic fields with matter.

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