The problem with black holes is that the most interesting bits are hidden. The gravity of a black hole is such that everything—including light—is trapped if it gets closer than a certain distance. As a result, everything we observe about black holes comes from their effect on the world beyond that distance. Any closer and you pass the event horizon—the distance at which nothing escapes the black hole.

Jets are a common feature of black holes. They're narrow streams of high-energy particles that stream away from outside the event horizon. Their energy is such that the jets glow brightly, revealing themselves to us.

These jets may be able to tell us something about the black holes themselves, but that requires two things. One is observations that require a globe-spanning telescope. The second, more difficult requirement is that we need models that help us understand the observations, as we haven't built detailed models of the physics near the event horizon yet.

That is changing now, as astrophysicists start to beg, borrow, and steal code from plasma physicists everywhere.

Black holes: creators of plasma

Picture a lonely black hole, spinning in space. Thanks to its spin and charge, it has both a magnetic and an electric field. Without quantum mechanics, nothing happens; roll credits. But quantum mechanics tells us that space is not empty—it is filled with virtual particles. Virtual particles (in this case, electrons and positrons) pop up in pairs, fall in love, and consummate their love in fire—they collide and annihilate, bringing things back to where they started.

In a strong electric field, however, the love birds are torn apart before the critical moment. As a result, the strong fields around our black hole generate a growing population of electrons and positrons, accelerated by the electric and magnetic fields. Their very presence—both particles have an electric charge—modifies the fields that separated them. The result is a highly dynamic plasma (a plasma is a fluid of charged particles) that is hard to understand.

Predictions about it are hard, because modeling plasmas is computer intensive and the best code (for a given value of best) uses what are called magneto hydrodynamics. That basically means that the plasma is treated as a fluid without any particles. The code makes assumptions to get rid of the particles, and those assumptions are simply not valid near a black hole.

For instance, approximating a plasma as a fluid implies that there are lots of collisions between the particles that make up the fluid. But, in these plasmas, the spacing between individual particles makes collisions exceedingly unlikely.

To get around this, physicists adapted another type of code, called the particle-in-cell model. Essentially, the plasma is modeled as a bunch of particles that are moving around. This, as you might imagine, can take even more computational power. Each particle (a computational particle might consist of many real particles) must be moved individually. After moving, the electric and magnetic fields need to be recalculated. If you have to worry about collisions, you need to search for collisions and compute their results. The more particles you have, the longer this takes.

The advantage is that, for each time step, new particles can be dropped into the model without much difficulty. This allows the particle-production process to be included in the model. You can also turn off collisions to save some CPU cycles.

Promising results

A group of physicists has been attempting to build one of these particle-in-cell models for black holes. Admittedly, the physicists fudge the pair-creation process. But, even with this short cut, they get a good picture of how the plasma around a black hole builds up and of the creation of jets. The code even predicts the generation of negative-energy electrons that fall into the black hole, slowing its rotation and reducing its energy.

The researchers also showed that the nature of the jets is highly dependent on the particle-production rate. Particle production is dependent on the local electric and magnetic fields. That means that global observations of jets and the plasma around a black hole might be able to be mapped back to particle production. In turn, particle production tells us about local electromagnetic fields. That, finally, tells us something about the event horizon of the black hole.

Admittedly, I don't think this is going to be happening very soon. I think the astronomers will take a few years to gather sufficient data to compare with models. And, it will take quite a bit of time to include particle production in the model properly. So, all being well, I'll report back in five years.

Physical Review Letters, 2019, DOI: 10.1103/PhysRevLett.122.035101 (About DOIs)