How Do Black Hole Jets Form From Magnetic Fields?

How do black hole jets form in the universe? I’m going to go over 7 things to help you understand. While my focus will be on the role of magnetic fields.

I’ve always been utterly fascinated with black holes. But most content I’ve read has either been too complex or incomplete.

So I wrote this article to explain different aspects of black holes in simple words. All without skipping over the finer details we’re all so curious about.

I discuss rotational black holes with the Blandford Znajek Process. More specifically, how to extract the rotational energy of black holes.

This will help us piece together how black hole jets form.

Disclaimer: my explanations are oversimplifications of the process. Plus, a lot of the existing discussion even from experts is theory.

There’s just so much we don’t know about black holes. So, what I share is both theory and my own thoughts with plenty of generalizations. I leave out most complexities that come with general relativity too.

Even more, what we know today as absolute truth may be wrong. Once our understanding of physics improves, we can better understand black holes. 

#1 What’s the Blandford Znajek process? 

A method to extract energy from a rotating black hole. For this method to work, we need two things:

  1. A rotating black hole
  2. A disc of plasma around a black hole

Important Note: when atoms move very fast they knock electrons off each other. Thus, ionized atoms are missing one electron or more. What’s left is electrons and atomic nuclei. 

This makes plasma an electrically charged gas. So, electric and magnetic fields can affect plasma. 

#2 What’s a black hole? 

A region of space where the curvature is so high nothing can escape. Not even light.

I know, sounds confusing. So let’s pick Einstein’s brain to better understand.

Einstein’s general theory of relativity describes gravity as the curvature of spacetime. In other words, anything with a mass will bend the four-dimensional cosmic grid.

This sounds sci-fi, but certain predictions made by Einstein have now been verified.

To explain, gravity is a force of attraction between masses.

Yet, light that is massless bends around massive objects.

So, how does light bend from the force of gravity? It doesn’t!

Light travels on a straight path. For light to bend, spacetime must be distorting.

So mass distorts spacetime. Yes, a confusing subject to wrap your mind around. But let’s keep going.

A supermassive black hole is our culprit here. It bends spacetime enough where light can’t even escape.

Important Note: gravity doesn’t cause massive objects to interact with light. Rather, gravity causes massive objects to interact with space. Then light interacts with space. 

#3 What’s the event horizon of a black hole?

This is the point of no return of a black hole. If anything crosses the event horizon, the outcome is a singularity.

In other words, nothing escapes. Not even light.

Important Note: singularity represents a region of space with infinite curvature. In our case, it’s the center of a black hole.

This point contains a black hole’s mass in an infinitely small space. 

#4 What’s a black hole’s accretion disc?

Massive black hole with accretion disc and plasma jet
Supermassive black hole with accretion disc (Photo Credit: NASA/JPL-Caltech)

Accretion discs are materials that orbit objects with strong gravity. The material includes gas, dust, plasma, and other stellar debris, made of:

  • Protons
  • Electrons
  • Radiation

Most accretion discs form when dense objects suck material off other things. Like a nearby star or a gas cloud.

For example, a black hole and a star may orbit another center. This creates orbital planes that create accretion discs.

More straightforward is a star orbiting a black hole.

formation of a black hole accretion disc

Important Note: accretion disc material can heat up to thousands of degrees. This happens right before the material reaches the event horizon. This produces X-rays.

The material can reach higher temperatures of millions of degrees too! It all depends on the size of the black hole. These extreme temperatures can release gamma rays. 

The disc radiation among other things is what we detect to find black holes.

Accretion material gets close to a black hole. But it doesn’t get close enough to instantly fall inside.

Yes, black holes have very strong gravitational attractions. But, accretion material moves fast with angular momentum. This is what creates the amazing rotating discs we see.

Think of accretion material like the International Space Station (ISS). The ISS needs to travel 17,150 miles per hour or it’ll fall out of Earth’s orbit.

Important Note: if Earth stopped moving it’d fall into the sun. The Sun’s gravity would pull it in. 

But, Earth moves sideways relative to the Sun at around 2 miles/second. This is fast enough for Earth to not fall into the Sun.

Yet not too fast to escape the Sun’s gravity and leave the solar system. Thus, we orbit the Sun.

It’s like a tetherball. Imagine the post is the Sun and the ball is the Earth. Then the rope connecting the ball and post is the force of gravity. 

When you hit the ball, it spins around the post. If we didn’t have air or rope friction, the ball would forever spin around the post. 

This is what Earth does as it orbits the Sun in the vacuum of space. 

#5 How does accretion material fall into a black hole?

As we learned, accretion material wants to stay in orbit around a black hole. Because of its angular moment, it won’t want to spiral beyond the event horizon.

Over time though, more material will enter an accretion disc. Also, material at adjacent radii orbit at different velocities.

So, atoms will collide producing friction and heat.

This causes the material to lose angular momentum. The mass then moves inwards and atoms lose electrons becoming ions.

Even more, magnetic fields form through the accretion disc from charged particles. These magnetic fields then rotate with the black hole.

Important Note: black holes become more magnetically charged over time. Greater material builds up in the accretion disc leading to the increased charge. 

At the same time, induced electric fields speed up charged particles. All these forces cause matter to flow in complex ways.

As a result, the material falls closer to the black hole.

Important Note: a magnetic field forms whenever electric charges move. In the accretion disc, the charges rotate with the black hole. 

Then a changing magnetic field creates an electric field. This is a cyclic feedback process, where one field constantly creates the other. 

In short, we have an electromagnetic field. 

What’s more, material closer to the event horizon travels faster.

Important Note: material near massive objects travel faster to maintain their orbit.

It’s why satellites closer to Earth travel faster to maintain their orbit. 

 Satellite altitudeSatellite speed
Geostationary35,780 km11,100 km/hour
GPS20,200 km13,900 km/hour
Sun synchronous705 km27,500 km/hour

In this entire process, the density of the material increases near the event horizon. Again, the heating effect increases, and atoms collide more violently.

Charged particles then fall closer to the black hole.

So it’s like a domino effect until the material falls into the event horizon.

This entire process is the key to how black hole jets form.

#6 How do black hole jets form from magnetic fields?

Let’s now dive even deeper into our question.

To create these amazing black hole jets, we need 2 things:

  1. Power source: to generate energy for the plasma
  2. Alignment: to keep the plasma flowing in a confined area

The power source is our rotating black hole. It spins the accretion material at high speeds.

The material can heat up to several million degrees celsius. This happens in the inner accretion disc near the event horizon.

Without a doubt, the charged particles here are in a chaotic state traveling near the speed of light.

As we learned, each moving charged particle creates a magnetic field. These magnetic field lines thread all throughout the accretion disc. More so near the event horizon.

In this special region, the magnetic field lines violently reconnect and strengthen.

So, a strong magnetic field forms in the accretion disc by a turbulent dynamo.

Important Note: the dynamo in accretion discs is not fully understood.

The cause may be from the differential rotation of material in the accretion disc. Or maybe new poloidal flux merges with and augments the original poloidal flux.

No different than how we don’t fully understand Earth’s dynamo effect. We know liquid separates the inner core from the mantle. As a result, they spin at different rates.

Charged particles in the outer core then move and generate electric current. This generated current in the liquid metal then creates Earth’s magnetic field.

Even more, a rotating black hole drags space. Imagine how water twists when you flush your toilet.

We call this frame-dragging. An amazing effect from general relativity on spacetime.

Because of black holes’ strong gravity and rotational forces, spacetime locally rotates.

frame dragging in local spacetime around a rotating black hole

Thus, the magnetic field lines become denser closer to the event horizon. They become like a tangled coil of wire.

This further strengthens the magnetic field near the event horizon.

Important Note: electromagnetic fields can’t escape black holes. But when a black hole swallows a charged particle, its field line will remain glued to the black hole. 

The flux conservation law shows field lines disconnected from their source can’t escape. These field lines become squeezed inward and forced to thread the black hole. As a result, they’re held in the accretion disc.

This is because of Maxwell pressure of nearby field lines. But if both ends of the magnetic field land in the black hole, the field will go away. 

For example, see magnetic field line B1 in the image below. This field line remains intact. 

black hole schematic with magnetic field lines

The magnetic field would completely go away in the following scenarios though:

  • The accretion material goes away and charged particles cease to exist
  • The accretion disc’s conductivity stops

What’s more, Mars once may have had a magnetic field. But since it couldn’t maintain its molten core its magnetic field went away. 

Rotation of magnetic field lines

Rotating black holes drag their accretion disc’s magnetic field lines. The rotation of the field lines induces Electromagnetic Forces (EMF).

The magnetic field lines then wrap around the accretion disc’s rotation of axis. This creates a helical magnetic field in the Z direction.

Now, a lot of the material the black hole eats. But some material is magnetically launched in jets at near the speed of light.

magnetic field line stages in a spinning black hole

This shows the strength of the magnetic field. It takes a lot of force to rip charged particles from a black hole’s gravitational grip.

So, charged particles that enter the magnetic field accelerate away in the jet.

This is because ionized matter can’t cross field lines due to the EMF. The mechanism is like a synchrotron.

Important Note: synchrotrons are large machines that accelerate charged particles. Thus, particles follow a set path as they’re deflected through magnetic fields.

Keep in mind, objects that don’t travel in straight lines accelerate. This is important because accelerating electrons emit radiation, such as gamma rays. 

For this reason, the plasma travels on the axis of the magnetic field’s rotation. Again, think of a synchrotron.

Charged particles remain on set paths made by the helix-shaped magnetic field lines.

This explains the perfect alignment of black hole jets. Where we see plasma shooting out both ends of a black hole.

All in all, the twisting magnetic field transfers rotational energy to black hole outflow.

The end result is like a simple electrical circuit.

  • The rotating black hole is the voltage source
  • Magnetic field lines are the wires
  • Plasma is the load carried through the field lines

Important Note: black holes lose rotational energy in this process. They give rotational energy to the charged particles, which slows them down. 

#7 How much energy does black hole radiation release? 

The amount of energy produced by black holes is what’s so appealing.

I’m not talking about what’s inside the black hole. Rather, the material that’s fired out from the accretion disc.

We’re interested in the radiation. To better understand, mass is energy.

Using Einstein’s E = {mc}^{2}, we can see where this energy comes from. In short, energy equals mass times the speed of light squared.

So, the conversion of matter to energy is unworldly from a black hole’s accretion disc. It can reach as high as 42%.

42% may not seem like much, but you need perspective. The Sun only converts 0.7% of the mass of hydrogen into various wavelengths of light. The remaining 99.3% settles to helium.

Even more, our only power source for converting matter to energy is nuclear power. We convert 0.1% of the mass of Uranium-235 into energy.

Further, 1 kilogram of matter converted to 42% energy would yield over 10 terawatts-hours of energy. We could power all of Earth for 30 minutes!

Look over the below table. Compare hydrogen fusion that powers the Sun to gasoline that powers our cars.

Now compare the 0.7% efficiency of hydrogen fusion to the 42% efficiency of black holes. Mind-blown!

Fuel SourceEnergy Density (MJ/kg)
Hydrogen fusion 650,000,000
Natural uranium 81,000,000
Rocket chemical fuel50
Lithium battery1

Now, capturing this power is entirely another story.

But who knows, there may be an alien civilization much more advanced than us. They may already have tapped into this endless energy source.

“How do black hole jets form” wrap up

These astrophysical jets are not only amazing but a potential near-infinite energy source.

This is sci-fi on steroids if we can one day harness this unworldly power.

What are your thoughts on how black hole jets form? Do you think we can ever tap into black holes as an energy source? 

Featured Image Photo Credit (cropped): NASA-JPL-Caltech


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