An ocean permeates all of space, yet remains invisible to us. This ocean is dark matter, and the waves on its surface are particles called axions. They are so light and elusive that they pass through Earth without noticing it. But if these waves crash against a shore made of a special material, they might leave a trace — a tiny electric current.
It is exactly such a shore that scientists propose to build. They take a crystal of Weyl semimetal, a material where electrons move as if they were massless, and place it in a strong magnetic field. When an axion field passes through the crystal, it gently nudges the electrons, causing them to flow in one direction. A current arises, similar to what runs through a wire when a voltage is applied. But here there is no voltage — only the invisible hand of dark matter.
To catch such a signal, you need an instrument as sensitive as a microphone that can hear a whisper at the center of a hurricane. That microphone is a SQUID — a superconducting quantum interference device. It picks up magnetic fields created by even the tiniest current. Calculations show that for a crystal the size of a postage stamp in a magnetic field of 10 Tesla (about 200,000 times stronger than a fridge magnet), the signal would be distinguishable from the noise.
The new method opens a window into regions where axions might have been hiding from astronomical observations. Previously, they were searched for by how they cool stars, but that approach has its limits. Now, sitting in a lab, one can probe the dark matter ocean directly, and perhaps finally understand what it is made of.
A femtoampere is one billionth of a microampere. If one ampere were a stream of water pouring from a bucket in a second, then a femtoampere would be a single drop every 30,000 years.
In Liu Cixin's novel "The Dark Forest," invisible particles of dark matter are used as a weapon, but here they serve as a key to understanding the universe's structure.