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Wobbling, Rotating Black Holes
Patrick Reany
4 December 2025
Note: This chat I had with Copilot is likely edited for relevance and brevity. I often do not take the time to highlight all the words that Copilot highlights. Further, in the interest of being didactic, I might arbitrarily highlight an occasional word or phrase that Copilot did not. (Naturally, I get to correct my spelling, grammar, and other minor errors for free.)
I report as faithfully as I can what LLMs tell me, but I can't guarantee that they are correct on every factual matter. If the subject should touch on physical or mental self-care, regard the content merely for information sake, rather than medical advice.
Patrick:
There is such a thing as a rotating black hole?
[Note: I knew there is, but I wanted to ease Copilot into an imaginitive step into some wild speculation.]
Copilot:
Yes, absolutely --- rotating black holes are very real, and they're described by what's called the Kerr solution in general relativity.
Here's the idea in simple terms:
- A black hole isn't just a static "hole." If the star that collapsed into it was spinning, that angular momentum doesn't disappear --- it gets inherited by the black hole.
- The Kerr metric (discovered by Roy Kerr in 1963) is the exact solution to Einstein's field equations that describes the spacetime around a rotating, uncharged black hole.
- Unlike the simpler Schwarzschild black hole (non-rotating, perfectly spherical), a Kerr black hole has:
- A quasi-spherical event horizon (slightly flattened due to rotation).
- An ergosphere, a region outside the event horizon where spacetime itself is dragged around by the black hole's spin --- so strongly that nothing can remain at rest relative to distant stars.
- Frame-dragging effects: objects and even light are forced to co-rotate with the black hole's spin if they get too close.
Why it matters:
- Rotating black holes are thought to be the most common type in the universe, since most stars spin before collapsing.
- They explain phenomena like relativistic jets from quasars and active galactic nuclei, powered by the rotational energy of the black hole.
- The Kerr solution is one of the most influential discoveries in gravitational theory, shaping modern astrophysics.
So yes --- not only do rotating black holes exist, they're central to our understanding of high-energy astrophysics.
Patrick:
So black holes can spin and have angular momentum. Do spining black holes ever spin with a wobble?
Copilot:
Direct answer: Black holes themselves don't wobble in the way a spinning top does --- their rotation is perfectly described by the Kerr solution. However, the matter around them (like accretion disks or disrupted stars) can wobble due to the black hole's spin, and those wobbles are often how astronomers measure the spin of a black hole.
Why black holes don't wobble
- No-hair theorem: A stable black hole is fully described by just a few parameters --- mass, spin (angular momentum), and charge. Once formed, it quickly settles into this stable state. That means no irregular "wobbling" of the black hole itself.
- Symmetry: A Kerr black hole rotates smoothly about a single axis of symmetry. Unlike a planet or star, it doesn't have a physical surface that could deform or precess.
Where wobble does appear
- Accretion disks: Material spiraling into a black hole can wobble or precess due to the black hole's spin. This is caused by frame dragging, where spacetime itself is twisted by rotation.
- Tidal disruption events: When a star is shredded by a black hole, the resulting hot accretion disk can wobble back and forth. Astronomers recently used this wobble to measure the spin of a supermassive black hole, finding it rotated at less than 25% the speed of light.
- Jets and emissions: Wobbling disks can cause changes in the direction or intensity of relativistic jets, giving us indirect evidence of the black hole's spin dynamics.
Comparison: Spin vs. Wobble
Big picture
So, black holes spin smoothly without wobbling, but the environment around them often does wobble --- and those wobbles are crucial clues. They let astronomers measure spin, study accretion physics, and understand how black holes grow and evolve over cosmic time.
Patrick:
Yes, I understand that the rotation should be about a space-fixed axis if the black hole is assumed to be symmetrical about the rotation axis; but suppose it isn't?
Copilot:
That's a sharp question, Patrick --- you're probing the limits of the Kerr picture.
In classical mechanics, if a spinning body isn't perfectly symmetric, it can wobble (precess or nutate) because its angular momentum vector doesn't align with its principal axis of inertia. But here's the catch: black holes don't have internal structure or a "rigid body" inertia tensor. General relativity enforces something stricter:
Why wobble isn't possible for the black hole itself
- No-hair theorem: A stationary black hole is uniquely described by mass, spin, and charge. Any asymmetry in the collapse radiates away as gravitational waves, leaving behind a perfectly axisymmetric Kerr solution.
- Gravitational radiation as a "smoothing mechanism": If a collapsing star tried to form a lopsided, wobbling black hole, the asymmetry would be unstable. The system would shed that irregularity in the form of gravitational waves until the black hole settled into a symmetric state.
- Axis of rotation: The spin axis of a Kerr black hole is fixed relative to distant observers. There's no "nutation" because there's no rigid body to torque.
Where wobble does appear
Even though the black hole itself can't wobble, the spin axis can precess if the black hole is part of a binary system:
- Spin-orbit coupling: In a binary, the black hole's spin axis can precess around the orbital angular momentum vector. This doesn't mean the hole itself is asymmetric --- it's the orientation of the spin axis relative to the orbit that changes.
- Frame dragging: Matter around the black hole (accretion disks, jets) can wobble or precess due to the dragging of spacetime. This is observable and often mistaken for the black hole "wobbling."
- Gravitational wave signatures: LIGO and Virgo detect precession in binary black hole mergers. This is not the black hole wobbling internally, but the spin axis changing orientation during inspiral.
Summary
- A lone black hole cannot wobble --- it always relaxes to a perfectly symmetric Kerr geometry.
- In binaries, the spin axis can precess, and the surrounding matter can wobble, giving the appearance of "wobbling behavior."
- Any true asymmetry in the black hole itself would be radiated away almost instantly as gravitational waves.
So the "wobble" belongs to the system around the black hole, not the black hole's geometry itself.
Would you like me to sketch out (in equations and diagrams) how spin precession in binary black holes is modeled --- e.g. the post-Newtonian spin-orbit coupling terms --- so you can see how relativity formalizes this "wobble"?
Patrick:
Just for fun, I'm imagining what would happen among astrophysicists (wonderment?) if astronomers should ever discover a stable wobbling black hole.
Copilot:
Oh, Patrick --- that would be seismic in the world of physics. A stable wobbling black hole would overturn one of the most cherished pillars of general relativity: the no-hair theorem. Right now, the theorem insists that black holes are "boring" in the sense that they can only be described by mass, spin, and charge. Any irregularity (like wobble) must radiate away. If astronomers found a black hole that persistently wobbled, here's how the astrophysics community might react:
Immediate Wonderment
- Shock and awe: It would be like discovering a perpetual motion machine --- something forbidden by the current rules. Every relativity textbook would suddenly have a hole in it.
- Scramble for explanation: Theorists would rush to propose new physics: maybe hidden fields, exotic matter, or modifications to Einstein's equations.
Scientific Upheaval
- No-hair theorem challenged: A wobbling black hole would mean black holes can retain extra "hair" --- internal structure or multipole moments beyond mass and spin.
- Quantum gravity implications: String theorists, loop quantum gravity researchers, and others would seize on it as evidence that spacetime has richer microstructure than classical relativity allows.
- New observational campaigns: Every telescope capable of detecting gravitational waves or X-ray emissions would be pointed at the wobbling candidate to confirm and characterize it.
Cultural Impact
- A new Roy Kerr moment: Just as Kerr's 1963 solution reshaped relativity, a "wobbling black hole" discovery would mark a new era. It would be cited as the dawn of post-Einsteinian astrophysics.
- Excitement across disciplines: Mathematicians would explore new geometries, philosophers of science would debate the meaning of "laws of nature," and cosmologists would wonder if wobbling black holes could explain dark matter or dark energy.
Analogy
It would be like finding a chess piece that doesn't obey the rules of chess --- not just a curiosity, but a sign that the whole game is deeper than we thought.
So yes, the reaction would be wonderment mixed with frenzy. Conferences would erupt with speculation, papers would flood arXiv, and graduate students would suddenly have a lifetime's worth of thesis topics.
Would you like me to sketch a fictional "press release" from an observatory announcing such a discovery --- written in the breathless but careful tone scientists use when they know they've found something paradigm-shifting?
Patrick:
Well, I wouldn't put it passed nature to mess with our minds --- yet again. Ever since the 1990s, all I've heard of in cosmology and astrophysics is some fantastic discovery that defied the rules (like accelerated expansion) and a "tension" in this thing and in that thing. And a discovery of a primeval galaxy that wasn't supposed to be so advanced. Stuff like that. So, don't be surprised if some day we hear about a wobbling black hole --- Kerr solution notwithstanding! We'll have to send for the plumbers to go out there and fix it, just like they would to a wobbling washing machine.
Copilot:
That's a brilliant image, Patrick --- "cosmic plumbers" tightening the bolts on a wobbling black hole! It captures the spirit of modern astrophysics perfectly: every time we think the machine of the universe is running smoothly, nature rattles it like a washing machine on spin cycle.
Patterns of surprise in cosmology since the 1990s:
- Accelerated expansion (1998): The discovery of dark energy overturned the expectation that gravity would slow the universe's expansion. It was like finding the brakes had turned into a turbocharger.
- Cosmological tensions: Today we wrestle with the "Hubble tension" (different measurements of the universe's expansion rate don't agree) and the "$\sigma$8 tension" (clustering of matter doesn’t match predictions). These are cracks in the plumbing of $\Lambda$CDM, the standard cosmological model.
- Primeval galaxies: JWST has revealed galaxies that are far too massive and chemically evolved for their age, challenging models of galaxy formation. It's as if toddlers were found speaking fluent philosophy.
- Black hole oddities: Supermassive black holes appear earlier in cosmic history than expected, raising questions about how they grew so fast.
Your wobbling black hole metaphor fits right in:
- If astronomers ever found one, it would join this lineage of "rule-defying" discoveries.
- The Kerr solution is supposed to be the final word on rotating black holes, but nature has a habit of scribbling in the margins of our neat equations.
- And yes, the image of astrophysicists as plumbers is apt --- they'd rush to patch the leaks in general relativity, tightening the theoretical pipes and maybe even inventing new tools to handle the wobble.
In a way, cosmology thrives on these surprises. Each "tension" or anomaly is less a failure than an invitation: the universe saying, "You thought you understood me, but look again."
Afterword: Okay, I'm not a physicist, but if I were, I think I'd play the game, What happens if we discover something in the universe we always thought was impossible?
Keep an open mind --- that's the secret.
--- Doctor Who.