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by Jeffrey Winters
Albert Einstein's epochal insight into the equivalence of matter and energy, elegantly expressed as E=mc2, has been confirmed countless times, most dramatically whenever a nuclear weapon detonates. The process also occurs naturally--a star shines because atoms in its core fuse, transforming a sliver of matter into light. And when particles of matter and antimatter meet, they annihilate each other in a blaze of energy.
But like any equation, E=mc2 works in both directions, at least theoretically. That is, it should be possible to convert energy into matter. Now a team of physicists has accomplished just that: they have transmuted light into matter. "We're able to turn optical photons into matter," says Princeton physicist Kirk McDonald, coleader of the team. "That is quite a technological leap."
Of course, physicists would have been shocked if they couldn't get energy to convert into matter. After all, the entire universe began with an explosion of energy--the Big Bang. And physicists who smash atoms together have witnessed the conversion of energy into matter--"virtual" photons that flit in and out of existence just long enough to spawn the particles of exotic matter routinely observed in particle accelerators. But such virtual photons aren't under the direct control of physicists; these photons arise as part of a complex chain of events starting with a collision of two particles of matter. Until now, no one had directly created matter from light. "Back in 1934 physicists realized that it would be possible to do this in principle," says McDonald, "But it just wasn't technically feasible."
By the early 1990s, McDonald and his colleagues had all the technological pieces in place to conduct such an experiment. The key piece was a laser capable of packing a tremendous amount of energy into a small space. The laser that McDonald and his collaborators use at Stanford generates a trillion watts of power, enough to light every home in North America. But rather than drain the national electric grid, the laser takes a rather ordinary amount of energy and compresses it into a pulse for about a trillionth of a second. By focusing this pulse on an area of just 16- millionths of a square inch, the physicists bathe a spot with an incredibly intense electromagnetic field. But even with this crowd of high-power photons squeezed together, the energy is still only about a millionth of what's needed to make matter.
The problem is that the laser's green-light photons don't pack much of a punch. McDonald needed a way to boost the energy of these photons. He and his colleagues knew that a photon, which is massless, can sometimes siphon off part of the energy of a high-speed particle with mass. This occurs because the total energy of the particle, which includes its mass, may exceed that of the photon, just as a truck moving at 60 miles per hour may have more total energy than a sports car traveling at 70.
At the Stanford Linear Accelerator, a two-mile-long drag strip for subatomic particles, McDonald found just what he needed. The accelerator drives swarms of electrons to speeds close to that of light. When McDonald shot photons from the laser at the racing electrons, the photons ricocheted off, absorbing so much energy that they changed from run-of-the-mill green photons to powerful gamma rays. These gamma-ray photons then merged back into the intense stream of green-laser photons, and when a group of photons with the right energy crowded close enough together, out popped a pair of particles: an electron and its antimatter twin, a positron. The reaction is the reverse of the usual matter- antimatter annihilation: the blaze of energy becomes matter.
The method isn't foolproof. Of about 22,000 beams fired into the Stanford accelerator, just over 100 pairs of particles materialized. With the development of increasingly powerful lasers, McDonald estimates that in another five or ten years this may be an efficient way to make small amounts of antimatter. But the technique will never generate a cheeseburger. For example, even if all the sun's power could be focused on one spot, there still wouldn't be enough energy, says McDonald, to make even an ounce of matter.