Your other experimental platform will look for axions. How?

The original point of axions was to solve a mystery involving the strong force. To solve that problem, axions need to interact with protons and neutrons. And one way that axions would interact with protons and neutrons is basically to tip the spins of those particles. This tipping is the defining interaction of axions, and we can search for it with magnetic resonance.

How does magnetic resonance work?

You start with a particle that has spin. A spinning particle is basically like a compass needle; it likes to point “north” along any magnetic field in the background. But there’s one big difference between a compass needle and a quantum spin. If you tip a compass needle away from north, it swings straight back to north. Whereas if you tip a quantum spin, it will kind of spiral back toward north — it wants to “precess” about the magnetic field.

There are various games you can play with magnetic resonance, but the key idea is this: First, you tip the needle with a magnetic field, specifically one that “resonates” with the needle. Then, after the tip, you watch the needle precess as it returns to north. It might do this faster or slower than you’d expect, which tells you that there are more magnetic fields in the vicinity, created by other nearby atoms with spin. And you can tell what is making them.

For example, in magnetic resonance imaging, the compass needles are protons in your blood. The MRI machine lines the needles up, tips them, and lets them spiral back. When there are oxygen molecules around, their magnetic fields make these protons spiral up faster, so the MRI machine can map out the oxygen in your body. In the case of axions, the hope is that we can get atoms to resonate with any axion fields that might be out there.

I recently had an MRI due to a foot injury. Why didn’t my MRI find any axions?

Even if the MRI magnet was calibrated to get your protons precessing at the right frequency to resonate with axions, the machine wouldn’t be nearly sensitive enough to detect the tiny tipping that might result.

One little fact I like is the following. Suppose I had a single spin. It’s sitting there, and I have everything set up just right so the axions are interacting with this spin, and their field is tipping the compass needle away from north. It would take 1 million years to tip the spin all the way south. That’s how weak this interaction would be.

That’s a long time to spend in an MRI machine. Or doing an experiment.

We don’t want to wait for a million years. In the lifetime of the experiment, the tipping angle will be very small, perhaps 0.00000000001 degrees.