A radio telescope picks up a signal from the Crab Nebula, and instead of a smooth line, the screen shows bright and dark bands, like on a zebra's hide. This is the emission from the pulsar, a rapidly spinning neutron star left over from a supernova explosion. The bands appear because the radio waves don't travel to us along a single path but along several, and they interfere, like ripples from two stones dropped into water.

One path is bent by the pulsar's gravity, which acts like a lens, focusing the light. The second path is distorted by plasma — hot gas of charged particles around the star. This gas, in contrast, scatters the rays. When the two paths meet at our telescope, their waves alternately reinforce and cancel each other, creating the bands. The spacing between the bands reveals how much plasma lies along the ray's path.

The authors calculated that at higher frequencies, from 42 to 650 gigahertz, the banded pattern should change. These frequencies are accessible to the ALMA and SMA telescopes, and observations at these bands could peer into the very heart of the pulsar's magnetosphere. This is a rare opportunity to test how gravity works in the ultra-strong field near a neutron star.