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Electromagnetic Coupling

In this artist's rendering, a "bare" electron is denoted by the bright spot at the center of the figure. The wispy white lines represent electric field lines radiating out from the electron. Virtual particle-antiparticle pairs popping into and out of the vacuum are represented by blue-gold ellipses; the blue side, corresponding to a positively-charged particle, is nearer to the electron. This polarization effect reduces the effective charge of the electron that we observe at a large distance.
Dennis Harp/Purdue University

According to modern quantum theory, the empty space near an electron (the "vacuum") is not empty at all but filled with virtual particles blinking into and out of existence. When these particles materialize from virtually nowhere they would seemingly be adding a slight amount of energy to the universe, but that does not violate the conservation of energy. The Heisenberg uncertainty principle, which states that a small amount of uncertainty exists in joint measurements of energy and time, allows for these particles to appear but only if they live for a very short time.

Just as it is difficult to look at the surface of Venus with its thick atmosphere, it is a daunting task to look at a naked electron because of its self-made cloak of virtual particles. But now, physicists at the TRISTAN accelerator in Japan have partly lifted the electron's veil. In high-energy collisions between electrons and positrons, the electron's unadulterated electromagnetic nature can be measured to an unprecedented extent.

The electrons and positrons effectively penetrate each other's clouds in the closest encounters between the particles. In these closest encounters-- corresponding to collisions in which the researchers measured the highest-momentum-squared between the particles, the electron and positron are within 2 billionths of a billionth of a meter (2 x 10^-18 m) from each other. Studying these collisions, the TOPAZ detector collaboration at TRISTAN has shown, in a way that does not depend on assumptions about the other forces of nature, that as expected the electromagnetic coupling constant, the parameter which specifies the inherent strength of the electromagnetic force, actually increases for very high momentum-squared events.

  • Measurement of the Electromagnetic Coupling at Large Momentum Trasfer (Postscript, 440K)

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