Peeking inside an electron's screen.

Standard references and textbooks describe an electron as a stable elementary particle. Typically, they specify values for its mass, electric charge, and spin, and they sometimes mention vaguely that an electron's charge appears concentrated in a point. However, there's both more and less to an electron than such a bare-bones description indicates.

According to modern quantum theory, the space surrounding an electron is not empty but filled with a boiling sea of so-called virtual particles, which continually blink into existence in oppositely charged pairs, then almost immediately disappear again. Since the 1930s, theorists have proposed that these virtual particles cloak the electron, in effect reducing the charge and electromagnetic force observed at a distance.

By forcing electrons and positrons (the oppositely charged, antimatter counterparts of electrons) to collide head-on at sufficiently high energies, researchers have now penetrated the virtual-particle screen and made the first measurements confirming that an electron's electromagnetic influence increases as the distance from the particle's central core decreases.

"As we probe the cloud, getting closer and closer to the core charge, we see less of the shielding effect and more of the core," says David S. Koltick of Purdue University in West Lafayette, Ind. "This means that the electromagnetic force from the electron as a whole is not constant but rather gets stronger as we go through the cloud and get closer to the core."

Koltick and his coworkers report their findings in the Jan. 20 Physical Review Letters.

The experiment was performed by members of the TOPAZ detector group at the TRISTAN particle accelerator of the National Laboratory for High Energy Physics in Tsukuba, Japan. The accelerator was operated at an energy of 57.77 gigaelectronvolts to enable them to penetrate the screen without creating other particles.

From their data, the researchers obtained a value of the fine structure constant, a number that characterizes the inherent strength of the electromagnetic force. As expected theoretically, the newly obtained value of 1/128.5 is significantly larger than the 1/137 observed for a fully screened electron.

"Ours is a clean measurement of the electromagnetic effect," Koltick says. In higher-energy experiments at other accelerators, the effect is swamped by additional factors, including the strong force, which holds neutrons and protons together in an atomic nucleus and binds quarks into protons and neutrons. Those factors make it difficult to distinguish the relative contributions of the nuclear and electromagnetic forces.

"The observed properties of an electron derive from an interplay between the particle and the vacuum," Koltick notes. "We have to go much deeper to learn more about the 'bare' electron." Pulling aside the virtual-particle curtain opens up new possibilities for revealing the naked truth about electrons.