Magnetic fields can map the universe—here’s how

Who knew that magnetic fields could be so useful? Astronomers are able to use magnetic fields to map our environment within the Milky Way using a technique called Faraday rotation.

It works like this. There’s a bunch of dust—literal dust grains—floating within the galaxy.

Well, I say there’s a lot of dust, but it’s at very, very low densities. Thankfully, the volumes within interstellar space are so vast that the total amount of dust can really add up. And all these little dust grains have little magnetic fields associated with them, because all the grains are made of electric charges and they’re spinning around themselves.

When light from distant sources passes through the dust, that light encounters all these little magnetic fields. The magnetic fields in turn affect the light.

This is possible because light is really waves of electricity and magnetism, a realization first made by Scottish physicist James Clerk Maxwell when he unified the theories of electricity and magnetism into a single whole. Electric and magnetic fields are perfectly capable of influencing each other and responding to each other. For example, a changing electric field produces a magnetic field, and vice versa.

When an electromagnetic wave encounters a dust grain and its tiny magnetic field, that magnetic field will be oriented in a particular orientation depending on the grain’s spin. The part of the wave aligned with the magnetic field of the grain will get enhanced, and the part moving perpendicular will be suppressed.

This changes the polarization of the electromagnetic wave, which refers to the orientation angle of its electric and magnetic fields. The more that light travels through dust, the more this angle, its polarization, shifts.

This is an incredibly tiny effect, hardly detectable with a single dust grain. It takes light-year upon light-year of travel through dust to make an appreciable shift.

This is Faraday rotation, named in honor of Michael Faraday, who was the first to demonstrate the effects of magnetism on light. And we can use Faraday rotation to make maps of the distribution of dust in the interstellar medium.

As we look at distant bright sources in various directions, we’ll see more Faraday rotation in some directions and less in others. Like shining a flashlight in the fog, we can get a gauge on where the fog is thicker and where it’s thinner.