Alpha -- one of the fundamental constants of physics -- determines the strength of interactions between charged particles and electromagnetic fields. It equals *e ^{2}/c h-bar* -- where

*e*is the charge on the electron,

*h-bar*is the Planck constant divided by 2π, and

*c*is the speed of light -- and is about 1/137. As a dimensionless number, it is even more fundamental than other constants such as the strength of gravity, the speed of light or

*e*itself.

Most attempts to calculate alpha involve measuring the magnetic moment of the electron, *g*, which relates the size of the electron's magnetism to its intrinsic spin. A value for alpha can then be obtained by inserting this value of *g* into equations from quantum electrodynamics (QED) -- the theory that describes the electromagnetic interactions between electrically charged particles and the virtual particles of empty space. If such interactions did not exist, *g* should be 2, but precise measurements over the years have shown that it differs slightly from this value, as predicted by QED itself.

Until now, the best measurement of *g* had an uncertainty of 4 parts per trillion. Now, Gerald Gabrielse and colleagues at Harvard University have increased this precision by a factor of almost six to 0.76 parts per trillion (*Phys. Rev. Lett.* **97** 030801). By inserting this new value of *g* into new and improved QED equations, the Harvard physicists, with colleagues from Cornell University and RIKEN in Japan have determined a new value for alpha that is ten times more accurate than the next most accurate value (*Phys. Rev. Lett.* **97** 030802).

Gabrielse and colleagues measured *g* by studying the motion of a single electron held inside a trap made of charged electrodes and magnetic coils. The combined electric and magnetic forces keep the electron moving in a circular "cyclotron" orbit. On top of this planar motion, the electron also wobbles vertically up and down in the direction of the magnetic field. This set-up allowed the researchers to cleverly tweak the electron's motion and measure its energy levels with great precision. The value of *g* was determined by observing transitions between the lowest spin and cyclotron energy levels of the electron.

Gabrielse thinks a better value of alpha could help in plans to redefine the kilogram that do not rely on using an actual weight kept in a vault in Paris.