Catching a whisper in the middle of a noisy train station: that's roughly the challenge facing physicists searching for the neutron's electric dipole moment. This is a tiny shift of charge inside the particle. If it exists, it breaks the symmetry between matter and antimatter and could explain why our world exists at all. But the signal is so weak that any external noise, even the magnetic field from a passing tram, can drown it out.
To shield the experiment, a room with passive magnetic protection was built at the Paul Scherrer Institute — a multilayer screen that dampens external fields. But that's not enough: residual fluctuations still seep through. So an active system of eight coils was installed around the room, working like noise-canceling headphones. They measure the magnetic field outside and create an opposing field to keep the inside quiet. The system can suppress fields up to 50 microtesla (roughly the Earth's field) and their gradients, leaving only a few microtesla in the low-frequency range.
For the system to work accurately, the sensors that tell the coils what the field is outside must be positioned correctly. The authors modeled the magnetic fields of the entire setup using the finite element method and achieved agreement between calculations and real measurements. Then they used a genetic algorithm, a method that mimics natural selection, to find the optimal number and placement of the sensors. It turned out that a few sensors are enough to keep the field under control.
This approach allows you to calculate the shielding in advance rather than guess, saving time and money. And perhaps this magnetic silence will let us hear that very whisper that tells us why we exist.
The Earth's magnetic field is about 50 microtesla, and the active compensation system can suppress it to a few microtesla — ten times quieter.