What do the massive Earth and the tiny electron have in common? The unusual answer is that both act as if they have a magnet lurking inside them. For the Earth, the magnet is almost aligned with the axis of the Earth's rotation, while the magnet inside the electron seems to point precisely in the same direction as the electron's intrinsic angular momentum or spin.
Measurements of the Earth's magnetism give us a glimpse into the inner structure of our planet, revealing dramatic changes in the otherwise unseen currents of molten iron that are thought to be responsible for the geomagnetic field. Now, after a 20-year quest, we have obtained a similar magnetic glimpse into another place we cannot travel to or see: a single electron.
Over the last two decades, a series of seven graduate students and I at Harvard University in the US have developed new methods and apparatus with which to measure the intrinsic magnetism or "magnetic moment" of the electron. Recently, we announced the first fruits of our labour: a value with a precision better than 8 parts in 1013 that represents the best determination of the magnetic moment of the electron since a celebrated measurement in 1987.
In the February issue of Physics World, Gerald Gabrielse explains his team's ingenious method to measure the magnetic moment of the electron, and how their findings have let them determine the fine-structure constant – a fundamental constant of nature – with an accuracy of about 10 times better than any rival method. The work could even help physicists to test if the electron has any internal structure.
To read the full version of this article – and the rest of the February issue of Physics World – please subscribe to our print edition.