A compass needle points north, but what if space itself has a hidden axis? Physicists have used Earth's magnetic field to test whether the universe's structure is truly the same in all directions.
You can add a term to the equations of electromagnetism that picks out one direction, as if the universe had its own 'north.' This term is tied to a constant vector, and its effect on a magnet's field can be calculated exactly. It turns out that the correction to Earth's usual dipole field would show up in the precise geomagnetic field maps built from data collected by satellites like Swarm and ground-based observatories.
The result is four orders of magnitude stricter than previous estimates from other geophysical phenomena. This means spacetime is, to high precision, the same in all directions, and the compass needle honestly points to the magnetic pole, not to some hidden cosmic axis.
Earth's magnetic field changes over time: the north magnetic pole drifts at about 40 km per year, and over the last 150 years it has moved from Canada toward Siberia.
In Greg Egan's novel Schild's Ladder, physicists encounter changes in fundamental laws across different regions of space — a violation of directional symmetry would look like a local change in physics.