A magnetic compass in a test tube sounds like a magic trick, but it's pure physics. In a solution of fumarate (a salt of malic acid), carbon-13 nuclei behave like tiny spinning tops. When the external magnetic field drops to 400 nanotesla, a hundred thousand times weaker than Earth's field, the two energy levels of these tops nearly merge. Physicists call this point an 'avoided crossing.' At that point, the tops start to rock in unison, and the solution emits a rhythmic signal.
The signal isn't just noticeable: it's long-lived. The magnetization oscillations run at about 2 hertz and only decay after 25 seconds. For comparison, a single nuclear spin in a liquid loses its rhythm in 8 seconds. The secret is that in the 'nearly merged' state, the nuclei stop feeling small disturbances, like a choir that doesn't get thrown off by noise in the hall.
But the most interesting part is the crowd of nuclei. When there are many spins and they're all polarized, their own magnetic fields start to influence each other. Near the avoided crossing, this influence becomes measurable. That gives us a direct way to peek into the nonlinear dynamics of spins — how the tops 'jostle' each other and change the collective rhythm.
Such a solution is a ready-made sensor for ultra-weak magnetic fields. You could inject it into a body or a chemical reactor, and it would report on the magnetic environment around it. Perhaps this is how we'll monitor brain activity or chemical reactions in the future — without bulky cryostats and lasers.
400 nanotesla is about 100,000 times weaker than Earth's magnetic field. Even the magnetic field of your heart is dozens of times stronger.
In Robert Wilson's novel 'Spin,' magnetic fields become a tool for manipulating time. Here, in a test tube, nuclei also catch ultra-weak fields, but for now, only for measurements.