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Scientists Discover Way to Send Information into Black Holes Without Using Energy

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How Rotating Black Holes Eat Gas?

Black holes are some of the most fascinating and mysterious objects in the universe. They have such strong gravity that nothing — not even light — can escape once it gets too close. But how exactly do black holes pull in and “eat” the gas and matter around them?

This process, called accretion, is not as simple as you might think. Gas doesn’t just fall straight in like a rock dropping into a well. The gas can swirl, heat up, and behave in surprising ways depending on how the black hole is spinning and what kind of gas it is.

Recently, scientists Mach, Momennia, and Sarbach studied one special case of this process: how a rotating black hole (called a Kerr black hole) pulls in a special kind of gas known as a collisionless relativistic kinetic gas. That’s a big phrase, but don’t worry — we’ll break it down step by step.

What Kind of Black Hole Are We Talking About?

Black holes are defined by just a few properties:

  1. Mass (M) – how much stuff is packed into them.

  2. Spin (a) – how fast they rotate.

  3. (Sometimes) Electric charge – but in most astrophysical cases, this is basically zero.

The black holes in this study are rotating — which means they twist the space and time around them. This is called frame dragging. If you got close enough, space itself would be swirling around with the black hole.

What Kind of Gas Are We Talking About?

The gas in this study is collisionless, which means:

  • The particles in the gas don’t bump into each other very often.

  • They mostly just move under the influence of gravity.

  • This is different from everyday gases like air, where particles collide all the time.

The gas is also relativistic, meaning:

  • The particles can be moving at speeds close to the speed of light.

  • Their behavior must be described using Einstein’s theory of relativity.

This kind of gas could represent things like:

  • Very hot plasma in space.

  • Streams of dark matter particles.

Bondi-Type Accretion — The Spherical Feeding Style

When scientists talk about Bondi-type accretion, they mean a simple, idealized feeding process:

  • The black hole is surrounded by a huge, uniform “cloud” of gas.

  • The gas slowly drifts inward from all directions.

  • The flow is steady and roughly spherical (not a flat disk).

This is different from the glowing, spinning disks you often see in black hole illustrations. In Bondi accretion, there’s no big swirling disk — the gas just comes in from all sides.

Why Rotation Matters

If the black hole is spinning, it changes the way gas moves toward it. Rotation:

  • Twists the paths of particles.

  • Can fling some particles away.

  • Can change how much mass, energy, and angular momentum the black hole gains.

Scientists have known for a long time that black hole spin affects accretion, but until now, the exact details for a collisionless relativistic gas weren’t fully worked out.

A Brief History of the Problem

Over the years, researchers have tried different ways to solve the accretion problem:

  • In the 1980s, some analytic solutions were found for perfect fluids in a Kerr black hole’s gravity, but only under special assumptions.

  • Collisionless gases had been studied for non-rotating black holes (Schwarzschild type).

  • Planar flows and equatorial disk models had been examined for rotating black holes, but not full 3D spherical flows.

The work by Mach, Momennia, and Sarbach fills this gap. They found an exact stationary solution for Bondi-type accretion of collisionless gas into a Kerr black hole — without simplifying it to just a thin disk or a restricted plane.

How They Did It

To solve the problem, the researchers:

  1. Made some simplifying assumptions:

    • The gas particles are all the same mass.

    • They have no electric charge and no spin of their own.

    • They don’t pull on each other gravitationally (no self-gravity).

    • Only particles that come from infinity and fall in are counted (bound orbits aren’t considered).

  2. Described the gas at infinity:

    • Two cases:

      • Monoenergetic: all particles have the same energy.

      • Maxwell–Jüttner distribution: a range of energies based on a set temperature (like a relativistic version of the Maxwell–Boltzmann law).

  3. Solved the Vlasov equation:

    • This is the main equation for describing collisionless particles in a gravitational field.

    • They expressed key physical quantities — like particle current density, mass accretion rate, and energy accretion rate — as mathematical integrals.

  4. Evaluated the results:

    • They calculated exact numbers using computers.

    • They also made an approximate formula for slowly rotating black holes and compared it to the exact results.

What They Found

1. Zero Angular Momentum Accretion

One of the most surprising results was that the total angular momentum accretion rate is zero.

In simple terms:
Even though the black hole is spinning and the gas particles have all kinds of different paths, the total “spin” delivered to the black hole by the gas cancels out.

This is very different from thin disk models, where angular momentum transfer is the main reason the disk evolves.

2. Spin Reduces Mass and Energy Capture

As the black hole’s spin increases:

  • The mass accretion rate (how much mass it gains per unit time) decreases.

  • The energy accretion rate also decreases.

This makes sense physically: a fast spin makes it harder for the black hole to capture particles because spacetime is being twisted, and some particles are deflected.

3. Hotter Gas = Less Accretion

For both the monoenergetic and Maxwell–Jüttner cases, increasing the temperature of the gas (which means particles move faster) actually reduces the accretion rates.

This is different from some planar models, where hotter gas can lead to more capture. Here, in a spherical flow, faster-moving particles are more likely to escape than fall in.

4. Flow Patterns Change with Spin

The researchers plotted the particle current density near the black hole. The shape of the flow changes slightly as the spin increases, showing how rotation distorts the trajectories.

Why the Zero Angular Momentum Is Interesting

If a black hole gains mass but no angular momentum from the gas it swallows, something important happens:

  • The spin parameter a/Ma/M (spin divided by mass) gets smaller over time.

  • This means a spinning black hole would slowly spin down as it feeds on this kind of gas.

In astrophysics, the spin of black holes is a hot topic — it affects how they interact with their environment, how jets are launched, and how gravitational waves look when two black holes merge.

Slow-Rotation Approximation Works Well

The team also checked whether a slow-rotation approximation (pretending the black hole spins only a little) could match the exact results.

Surprisingly, it worked very well even for larger spins. This is good news because it means future researchers can use simpler math in many cases without losing much accuracy.

What’s Next?

The authors suggest several directions for future work:

  1. Include self-gravity
    If the gas is dense enough, its own gravity could affect the flow.

  2. Study moving black holes
    If the gas at infinity has a bulk velocity, especially in a direction not aligned with the black hole’s spin, interesting drag effects could appear.

  3. Look inside the black hole
    Explore what happens near the Cauchy horizon, where strange effects like mass inflation could occur.

  4. Add bound orbits
    Particles trapped in long-term orbits could add complexity.

  5. Compare with simulations
    Full computer simulations of Einstein’s equations and particle motion could test the accuracy of the analytic results.

Why This Matters

This research:

  • Gives an exact solution for a problem that had only partial answers before.

  • Improves our understanding of black hole feeding in realistic 3D conditions.

  • Shows that rotation reduces accretion rates for collisionless gases.

  • Reveals the surprising zero angular momentum transfer result.

  • Offers practical tools for future studies in astrophysics.

It also bridges the gap between simple spherical models and more complex disk-like models, helping scientists see the full range of black hole feeding behaviors.

The Takeaway

Black holes are not just simple “cosmic vacuum cleaners.” How they feed depends on what they’re eating and how they’re spinning.

For a certain type of gas — one made of non-colliding, relativistic particles — a spinning black hole will:

  • Swallow less mass and energy as it spins faster.

  • Gain no spin from the gas at all.

  • Alter the shape of the incoming flow in subtle but measurable ways.

This means that, over cosmic timescales, such a black hole could gradually slow its spin even as it grows in mass.

Mach, Momennia, and Sarbach’s work gives us the clearest mathematical picture yet of this process — and opens the door to exploring even more complex and realistic scenarios in the future.

Reference: Patryk Mach, Mehrab Momennia, Olivier Sarbach, "Accretion of a Vlasov gas by a Kerr black hole", Arxiv, 2025. https://arxiv.org/abs/2508.04783


Technical Terms

1. Kerr Black Hole

A black hole that spins.

  • Named after mathematician Roy Kerr, who found the exact solution for it in Einstein’s equations.

  • Spin twists space and time around the black hole — like stirring honey with a spoon. This twisting is called frame dragging.

2. Frame Dragging

The effect of a spinning black hole pulling space and time along with it.

  • Imagine placing a spoon in a cup of thick syrup and turning it — the syrup swirls around with the spoon.

  • In space, the “syrup” is spacetime itself.

3. Bondi-Type Accretion

A simple model of how a black hole eats gas.

  • The gas comes in evenly from all directions like water going down a drain.

  • There’s no big swirling disk — just steady, roughly spherical inflow.

  • Named after the physicist Hermann Bondi.

4. Collisionless Gas

A gas where the particles don’t bump into each other much.

  • Instead of colliding like air molecules, they mostly move freely under gravity.

  • Examples: dark matter, very thin hot plasma in space.

5. Relativistic Gas

A gas where particles move close to the speed of light.

  • Requires Einstein’s theory of relativity to describe their motion.

  • At such high speeds, strange effects happen — like time slowing down for the particles.

6. Maxwell–Jüttner Distribution

A way to describe the speeds and energies of particles in a hot relativistic gas.

  • Similar to the Maxwell–Boltzmann distribution you learn in basic physics, but adjusted for particles moving near light speed.

7. Vlasov Equation

A mathematical rule that describes how particles in a collisionless gas move in space and time under forces like gravity.

  • Instead of tracking each particle, it tracks the distribution of particles in position and velocity space.

8. Particle Current Density

A measure of how many particles pass through a certain area in space per second.

  • Like counting how many cars pass a road checkpoint every second.

9. Accretion Rate

How fast a black hole is “eating” matter or energy.

  • Mass accretion rate → how much mass per second goes into the black hole.

  • Energy accretion rate → how much energy per second is swallowed.

  • Angular momentum accretion rate → how much “spin” is transferred into the black hole.

10. Angular Momentum

A measure of how much something is rotating.

  • For a spinning black hole, angular momentum tells you how fast it spins and how hard it would be to stop it.

  • In accretion, if gas has angular momentum, it can change the black hole’s spin.

11. Cauchy Horizon

A strange inner boundary inside some rotating (or charged) black holes.

  • Beyond it, predictions about the future become impossible because physics equations break down.

  • This is deep inside the black hole, past the event horizon.

12. Slow-Rotation Approximation

A math shortcut where you pretend the black hole spins only a little.

  • Makes equations simpler.

  • Sometimes still gives accurate answers even for faster spins.

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