Bad Hair Day

EHT Collaboration

Making a black hole is easy. Just squeeze a bunch of stuff into a small enough volume. It doesn’t even matter what you use. You can collapse stars, planets, old car tires, Labubus, or missing left socks. The resulting black hole will only depend on the mass, rotation, and electric charge of the original material.

At least that’s what we think. The idea that black holes only have three physical characteristics is called the no-hair theorem, though it should properly be called the no-hair hypothesis because it has never been formally proven. The good news is that we are now gathering direct observations of the black holes Sag A* and M87*. While these observations support the no-hair theorem, they are also showing us that things are a bit more complicated.¹

According to the no-hair theorem, a black hole cannot have a magnetic field. The magnetic field lines would constitute additional structure or “hair,” and that isn’t possible because the field lines can’t pierce the black hole’s event horizon. But since a black hole has an intense gravitational field in the region near its event horizon, dust and gas in the region are squeezed and superheated. As a result, there is a surrounding torus of hot plasma surrounding the black hole, and it can have a strong magnetic field.

While we can’t observe this magnetic field directly, we can study it by looking at the polarization of radio light coming from the region. When light passes through ionized gas, it becomes polarized. That polarization aligns with the magnetic field lines, so by looking at polarized light from M87*, we know the orientation of its magnetic fields.

We first observed the supermassive black hole in M87 in 2017, and with a mass of about 6 billion Suns, we wouldn’t expect it to change quickly. That’s generally true, but observations from 2017, 2018, and 2021 have found that the polarization of the region is changing rapidly. Over the course of just four years, the orientation of the magnetic field has reversed, similar to the way Earth’s magnetic field flips every 200,000 years or so, only much faster.

The observed magnetic shift is so dramatic that we aren’t sure how it happens. One idea is that charged regions within the torus might shield currents from each other, allowing shifts to happen quickly. Another is that strong turbulence within plasma flows causes dramatic magnetic shifts. There might also be an interaction between the rotation of the black hole and the rotation of the material torus. But whatever the cause, it is clear that while black holes themselves don’t have magnetic fields, they can lead to the creation of strong and fast-changing magnetic fields near them. There is still a great deal black holes have to teach us.