Lasers Could Make Virtual Particles RealS

Next-generation lasers will have the power to create matter by capturing ghostly particles that, according to quantum mechanics, permeate seemingly empty space.

The uncertainty principle of quantum mechanics implies that space can never be truly empty. Instead, random fluctuations give birth to a seething cauldron of particles, such as electrons, and their antimatter counterparts, called positrons.

These so-called "virtual particles" normally annihilate one another too quickly for us to notice them. But physicists predicted in the 1930s that a very strong electric field would transform virtual particles into real ones that we can observe. The field pushes them in opposite directions because they have opposite electric charges, separating them so that they cannot destroy one another.

Lasers are ideally suited to this task because their light boasts strong electric fields. In 1997, physicists at the Stanford Linear Accelerator Center (SLAC) in Menlo Park, California, used laser light to create a few electron-positron pairs. Now, new calculations suggest next-generation lasers will be able to create such pairs by the millions.Chain reaction

In the SLAC experiment, only one electron-positron pair was created at a time. But with more powerful lasers, a chain reaction becomes probable.

The first pair is accelerated to high speed by the laser, causing them to emit light. This light, combined with that of the laser, spawns still more pairs, say Alexander Fedotov of the National Research Nuclear University in Moscow and colleagues in a study to appear in Physical Review Letters.

"A large number of particles will spill out of the vacuum," says John Kirk of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, who was not involved in the study.

In lasers that can concentrate about 1026 watts into a square centimetre, this runaway reaction should efficiently convert the laser's light into millions of electron-positron pairs, the team calculates.Antimatter factory

That kind of intensity could be reached with a laser to be built by the Extreme Light Infrastructure project in Europe. The first version of the laser could be built by 2015, but it could take a few years after that to complete upgrades necessary to reach 1026 per square centimetre, says study co-author Georg Korn of the Max Planck Institute for Quantum Optics in Garching, Germany.

The ability to generate large numbers of positrons could be useful for particle colliders like the proposed International Linear Collider, which will smash electrons and positrons together, says Kirk McDonald of Princeton University in New Jersey.

But Pisin Chen of National Taiwan University in Taipei says the cost of the very powerful laser might make this method more expensive than the alternative. The standard way to create large numbers of positrons today is to fire a beam of high-energy electrons at a piece of metal to produce electron-positron pairs.

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