Nobel laureate Frank Wilczek explores the secrets of the cosmos. Read previous columns here.
On April 7, the physics world was startled by two announcements of a new measurement and a new calculation of the muon’s magnetic moment, 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 multi-year collaborations of large groups of physicists at institutions around the world. You might think that making such accurate measurements and calculations is as boring as science can be, but it can make magic possible.
Muons are elementary particles resembling the more familiar electrons in several fundamental respects; For example, both carry exactly the same amount of electrical charge. But there are two big differences: Muons are about 200 times heavier than electrons, and they are unstable, with an average lifespan of about two microseconds.
Because exotic particles disappear, muons are unusually user-friendly. They are easy to produce in large numbers with high-energy accelerators. And while a microsecond may not sound that long, fast-moving muons can go a long way before decaying, leaving easy-to-detect traces. While it is often taken for granted, the fact that we can talk about “the” mass and “the” magnetic moment of the muon when we actually sample millions of different particles is both profound and astonishing. Precision measurements to date reinforce our confidence that all muons, like all electrons, have exactly the same properties.
Muons always rotate – as physicists say, they have ‘spin’ – which is 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 that field, similar to how the axis of a tilted spinning top circles the vertical. This apex-like movement is called precession. The speed of a muon’s precession 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.