Weblog
6 October 2025
EHT Collaboration
Making a black gap is straightforward. Simply squeeze a bunch of stuff right into a sufficiently small quantity. It doesn’t even matter what you employ. You’ll be able to collapse stars, planets, previous automobile tires, Labubus, or lacking left socks. The ensuing black gap will solely depend upon the mass, rotation, and electrical cost of the unique materials.
At the least that’s what we predict. The concept that black holes solely have three bodily traits known as the no-hair theorem, although it ought to correctly be referred to as the no-hair speculation as a result of it has by no means been formally confirmed. The excellent news is that we at the moment are gathering direct observations of the black holes Sag A* and M87*. Whereas these observations assist the no-hair theorem, they’re additionally displaying us that issues are a bit extra difficult.
In response to the no-hair theorem, a black gap can not have a magnetic subject. The magnetic subject strains would represent extra construction or “hair,” and that isn’t doable as a result of the sphere strains can’t pierce the black gap’s occasion horizon. However since a black gap has an intense gravitational subject within the area close to its occasion horizon, mud and gasoline within the area are squeezed and superheated. Consequently, there’s a surrounding torus of sizzling plasma surrounding the black gap, and it may have a robust magnetic subject.
Whereas we are able to’t observe this magnetic subject immediately, we are able to examine it by wanting on the polarization of radio mild coming from the area. When mild passes by means of ionized gasoline, it turns into polarized. That polarization aligns with the magnetic subject strains, so by taking a look at polarized mild from M87*, we all know the orientation of its magnetic fields.
We first noticed the supermassive black gap in M87 in 2017, and with a mass of about 6 billion Suns, we wouldn’t count on it to vary shortly. That’s typically true, however observations from 2017, 2018, and 2021 have discovered that the polarization of the area is altering quickly. Over the course of simply 4 years, the orientation of the magnetic subject has reversed, much like the best way Earth’s magnetic subject flips each 200,000 years or so, solely a lot sooner.
The noticed magnetic shift is so dramatic that we aren’t positive the way it occurs. One thought is that charged areas inside the torus may protect currents from one another, permitting shifts to occur shortly. One other is that sturdy turbulence inside plasma flows causes dramatic magnetic shifts. There may also be an interplay between the rotation of the black gap and the rotation of the fabric torus. However regardless of the trigger, it’s clear that whereas black holes themselves don’t have magnetic fields, they’ll result in the creation of sturdy and fast-changing magnetic fields close to them. There may be nonetheless a fantastic deal black holes have to show us.
