Nobel Prize-winning physicist Frank Wilczek explores the secrets of the cosmos. Read previous columns here.
On April 7, the physics world is startled in glorious confusion by two announcements of a new measurement and a new calculation of the magnetic moment of the muon, published in the journals Physical Review Letters and Nature. The new results, accurate to the level of one part per billion, are the product of collaboration by more groups of physicists at institutions around the world. You would think that the work of making such precise measurements and calculations is as dull as science, but it can make magic happen.
Muons are elementary particles that resemble the more well-known electrons in several fundamental ways; both carry, for example, exactly the same amount of electric charge. But there are two major differences: Muons are about 200 times heavier than electrons, and they are unstable, with an average lifespan of about two microseconds.
As exotic particles go, muons are unusually user-friendly. It is easy to manufacture in large numbers with high-energy accelerators. And while a microsecond may not sound like it for a long time, fast-moving mice can move a long way before it expires, and this is easily observable. Although often taken for granted, the fact that we can talk about ‘the’ mass and ‘the’ magnetic moment of the muon, when we test millions of different particles in practice, is profound and astonishing. Precision measurements so far strengthen our confidence that all muons, like all electrons, have exactly the same properties.
Muons are forever on the move – as physicists say, they “turn around” – which is the key to many aspects of their behavior. When a muon is exposed to a magnetic field, its axis of rotation circles around the direction of the field, just like the axis of a tilted, rotating circle around the vertical circle. This top-moving motion is called precession. The rate of precision of a muon in a magnetic field is equal to the product of the strength of the magnetic field, some known physical constants and a number called the magnetic moment.