Nobel Prize-winning physicist Frank Wilczek explores the secrets of the cosmos. Read the previous columns here.
On April 7, the world of physics was caught in a glorious confusion by two announcements about 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, correct at the level of one part per billion, are the product of the multiannual collaborations of large groups of physicists in institutions around the world. You may think that the work of making such accurate measurements and calculations is as tedious as science becomes, but it can make magic happen.
Muons are elementary particles that in many fundamental ways resemble more familiar electrons; 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 are unstable, with an average lifespan of about two microseconds.
As exotic particles go away, muons are unusually easy to use. They are easy to produce in large quantities on high energy accelerators. And while a microsecond may not sound like a long time, fast-moving muons can travel long before they expire, leaving traces that are easy to spot. Although it is often taken for granted, the fact that we can talk about the magnetic “mass” and “moment” of the muon, when in practice we take millions of different particles, is both profound and amazing. Precision measurements so far strengthen our confidence that all muons, like all electrons, have exactly the same properties.
Muons rotate forever – as physicists say, they “rotated” – which is the key to many aspects of their behavior. If a muon is exposed to a magnetic field, its axis of rotation revolves around the direction of that field, similar to how the axis of an inclined circle, which revolves around the vertical. This peak movement is called precession. The rate of precession 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.