The most precise measurement yet of the force that underlies the colors we see and the textures we feel has been unveiled by physicists.

Of the four fundamental forces, which include gravity, electromagnetism is the one that dominates our everyday lives. The fine-structure constant (α) describes the strength of this force, which keeps electrons in place within atoms and molecules, and governs the interaction between light and matter. This constant is approximately 1/137, and is determined by factors such as the charge on an electron, the speed of light and Planck’s constant, which controls the microscopic world.

Physicists have been measuring α with ever-greater accuracy for more than 80 years, in order to test and refine the theories—such as quantum electrodynamics (QED)—that describe how electromagnetism works. The latest determination shows that the inverse of the fine-structure constant is 137.035999710, with an uncertainty of 0.7 parts per billion. This uncertainty is 10 times smaller than in any previous methods¹.

The new measurement depended on collaboration between experimental physicists at Harvard University, US, led by Gerald Gabrielse, and a team of theoretical physicists that included Makiko Nio, from RIKEN's Nishina Center for Accelerator-Based Science in Wako.

Gabrielse and colleagues spent 20 years refining their measurement of a single electron’s magnetism (Fig. 1), and ultimately derived a value that had an uncertainty nearly six times smaller than previous measurements².

Meanwhile, Nio worked with Toichiro Kinoshita of Cornell University in New York, to develop an improved QED calculation that predicted the electron’s magnetism based on α³. Kinoshita worked at RIKEN from 2002 to 2004 as a visiting eminent scientist.

Kinoshita and Nio relied on supercomputers—including RIKEN's Super Combined Cluster (RSCC) system—to interpret 891 Feynman diagrams, which describe the interactions of fundamental particles. “It greatly helped us to improve the precision of the calculation,” says Nio.

So why is it important to determine α with such precision? “If our current understanding of physics is correct, all values ofαshould coincide,” Nio says. “If the test uncovers disagreement among different α's, it would have a profound impact on the foundation of quantum physics.”

The Harvard group will now repeat their measurement, and hope to reduce their uncertainty by a factor of three within a few years. And RIKEN theorists are currently adding 12,672 more Feynman diagrams to their calculations. Combining the two results should allow the team to determine one more digit in the fine-structure constant, says Nio.

References

• 1. Gabrielse, G., Hanneke, D., Kinoshita, T., Nio, M. & Odom B. New determination of the fine structure constant from the electron g value and QED. Physical Review Letters 97, 030802 (2006). doi: 10.1103/PhysRevLett.97.030802
• 2. Odom, B., Hanneke, D., D’Urso, B. & Gabrielse, G. New measurement of the electron magnetic moment using a one-electron quantum cyclotron. Physical Review Letters 97, 030801 (2006). doi: 10.1103/PhysRevLett.97.030801
• 3. Kinoshita, T. & Nio, M. Improved α⁴ term of the electron anomalous magnetic moment. Physical Review D 73, 013003 (2006). doi: 10.1103/PhysRevD.73.013003