Presto! Light Creates Matter

As nuclear bombs and many physics experiments show, turning matter into light, heat, and other forms of energy is nothing new. Now a team of physicists has demonstrated the inverse process--turning light into matter. In the 1 September Physical Review Letters, the team describes how they collided large crowds of photons together so violently that the interactions spawned particles of matter and antimatter: electrons and positrons (antielectrons). Physicists have long known that this kind of conjuring act is possible, but they have never observed it directly.

Working at the Stanford Linear Accelerator Center (SLAC), the 20-physicist collaboration focused an extremely intense laser beam at a beam of high-energy electrons. When the laser photons collided head-on with the electrons, they got a huge energy boost, much like ping-pong balls hitting a speeding Mack truck, changing them from visible light to very high-energy gamma rays. These high-energy photons then rebounded into the path of incoming laser photons, interacting with them to produce positron-electron pairs.

Such particle pairs are often spawned in accelerator experiments that collide other particles at high energies, and photons produced in the collision are the immediate source of the pairs. But in those experiments, at least one of the photons involved is "virtual"--produced only for a brief moment in the strong electric field near a charged particle of matter. The SLAC experiment marks the first time matter has been created entirely from ordinary photons.

Princeton University physicist Kirk McDonald, a spokesman for the multi-institution collaboration, says the result, which was completely expected, is only the first step in using powerful lasers and electron beams to explore the interactions of electrons and photons, described by the theory known as quantum electrodynamics (QED). "We're exploring new regimes and trying to map out the basic phenomena," he says. Physicist Tom Erber of the Illinois Institute of Technology is pleased at the prospect of such experiments. "Hopefully, this will open the door to future experiments which will approach [new] tests of QED."

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