Neoclassical. Electron orbits à la Newton can make quantum problems solvable.

Shedding Light on Light

Newton's laws usually fly out the window in the subatomic realm. The traditional interpretation of quantum theory dictates that, unlike planets around a star, electrons don't loop around their nuclei in nice, elliptical orbits. But now, scientists have shown that a nearly Newtonian set of electron orbits can explain a strange phenomenon that, on the face of it, should be impossible.

The puzzle turned up in the late 1980s. Scientists had long known that if they zap an atom with a photon, its electron can pick up a packet of energy that sends it into an excited state. Like a rock raised on high, the excited electron stores the energy. Zap an atom hard enough, however, and its electron flies free, like a rock boosted beyond Earth's escape velocity. So an electron in an atom should be able to store only so much energy, even if it is hit with a huge barrage of photons.

But then scientists discovered that atoms could absorb hundreds of photons beyond their binding energy and could emit photons with much more energy than should be allowed. "By the 1990s, there was much confusion on how to describe these phenomena," says Gerhard Paulus, a physicist at the Max Planck Institute for Quantum Optics in Garching, Germany. "It was a big controversy."

A mathematical method suggested by Caltech's Richard Feynman seemed to hold the answer. Most quantum theorists had tackled the problem by using the Schrödinger equation to find the distribution of electron wave functions--smeary particle-waves that inhabit a large parcel of space all at one time. Feynman, on the other hand, treated electrons as ordinary point-particles that circle their nuclei just as planets orbit their star. For instance, one path sends the electron looping around, smashing back into the atom and scattering off into the distance, picking up more energy than would normally be allowed. (But unlike Newtonian orbits, all possible electron paths have to be taken into account at the same time.)

In the 4 May issue of Science, Paulus's team reports a test of this theory. They zapped a sample of xenon with a titanium-sapphire femtosecond laser. By using Feynman's theory, the team predicted the energy of the electrons coming off the sample, as well as the high-energy light that gets released in the process--and it matched their observations admirably well.

"I don't think anyone's given a good demonstration before," says Ken Kulander, a physicist at Lawrence Livermore National Lab in California who helped formulate the Feynman-based theory behind the experiment. Kulander hopes that the newly confirmed theory will one day yield powerful extreme-ultraviolet lasers.