Black holes are powerful objects in space made by the collapse of big stars. They have strong gravity and are described only by two things: their mass and how fast they spin. Unlike stars or planets, black holes don’t stretch or change shape when something pulls on them. Scientists measure this kind of stretching using something called “Love numbers.” For black holes, the Love numbers are always zero—but no one really knew why. Now, a scientist named Alexandru Lupsasca has found a reason. He discovered a hidden kind of symmetry in black holes when they are disturbed in a calm and balanced way. This special symmetry is called conformal symmetry, and it changes how we understand black holes. Because of this symmetry, the forces pulling on the black hole and the black hole’s response live in different “worlds” in the math. That means the black hole doesn’t react the way other objects do—it stays firm and doesn’t stretch. This discovery solves a long-standing mystery and helps us better understand black holes and the signals they send out when they merge. It may also lead to new ideas about space, time, and gravity.
Black holes are some of the strangest things in the universe. They’re made when a huge star collapses under its own weight, squeezing all its mass into a tiny space. Their gravity is so strong that nothing—not even light—can escape. But for all their mystery, black holes are surprisingly simple: they can be completely described just by their mass and how fast they spin.
But there’s something even weirder: black holes don’t seem to stretch or change shape when something pulls on them. This goes against what we see in pretty much everything else in the universe. Earth, for example, is stretched slightly by the Moon’s gravity. So are stars and planets. Scientists use something called “Love numbers” to measure how much an object deforms under this kind of pull. And for black holes? Those Love numbers are zero.
For years, no one knew exactly why. But now, physicist Alexandru Lupsasca has found an answer—a hidden symmetry in black holes that protects them from being deformed at all.
Let’s break it down.
What Are Love Numbers?
Think of Love numbers like a “squish factor.” They tell us how much something bulges or stretches when it’s under the gravitational pull of another object.
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Earth’s oceans rise and fall because of the Moon’s pull. That’s a tidal effect.
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The same thing happens to stars when they’re close to each other—they get a little stretched.
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We expect black holes to do the same. But they don’t.
Every time scientists tried to calculate how a black hole responds to tides, the answer was always the same: they don’t. Their Love numbers are zero.
But why? That’s where Lupsasca’s new work comes in.
A New Kind of Symmetry
Lupsasca discovered that when a black hole is gently "poked" or disturbed in a very specific way—without changing over time and staying symmetric around its spin axis—there’s a hidden mathematical structure that appears.
This structure is called a conformal symmetry, and it comes from a group known as SL(2,R). You don’t need to know what that name means—just that it’s a kind of symmetry that’s often seen in very special, highly balanced systems.
The surprising thing is that this symmetry isn’t about space itself—it’s about the solutions to the equations that describe what happens around the black hole. In other words, it’s a hidden rule that connects different ways the black hole could respond to disturbances.
And this hidden rule has a powerful effect: it blocks the black hole from stretching or changing shape when something tries to pull on it. That’s why its Love numbers are zero.
Equations Made Simple
Physicists use something called the Teukolsky equation to figure out how waves and forces behave around a spinning black hole. It’s a very complicated equation that changes depending on how fast the black hole spins and what kind of disturbance is hitting it.
But Lupsasca found something amazing: if you look at time-independent and axis-symmetric disturbances (the simplest kind), the complicated equation becomes much simpler. In fact, it acts just like an equation in flat space—that is, space without any gravity at all!
So in this special case, it’s like the black hole disappears from the math. The equations that describe what’s happening around the black hole become the same as if you were just in empty space. That’s a big clue: maybe the black hole isn’t affected by the disturbance because, mathematically, there’s nothing for it to respond to.
Different Types of Fields Don't Mix
Here’s where the symmetry comes in. The disturbance trying to pull on the black hole and the black hole’s possible responses fall into different categories in the math. These categories are so different that they can’t interact. They’re like puzzle pieces from two different puzzles—they just don’t fit together.
This means the black hole doesn’t respond. It doesn’t stretch. It doesn’t bulge. It just sits there, unaffected. That’s why its Love numbers are zero.
Why This Matters
You might be wondering: why should we care about whether or not black holes stretch?
It turns out this is a big deal for gravitational waves—ripples in spacetime that happen when huge objects like black holes or neutron stars crash into each other. The shape of these waves depends on how the objects deform.
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If it’s a neutron star, the wave has certain features that come from its squishiness.
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But if it’s a black hole, the wave is simpler—because black holes don’t deform.
So, understanding why black holes don’t deform helps scientists read gravitational waves more clearly and figure out exactly what kind of objects are crashing together.
It also helps test whether black holes really behave the way Einstein’s theory says they should.
What’s Next?
This hidden symmetry adds to a growing list of special patterns scientists have found in black holes. In other situations—like when black holes spin very fast or when the disturbance has very high or very low energy—other kinds of symmetries also show up.
Lupsasca’s work fits into this bigger picture. It shows that black holes might be even more orderly and symmetrical than we thought. And it gives physicists a powerful new tool for understanding the strange world near a black hole’s edge.
There’s still a lot to explore:
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Can this new symmetry be connected to the others we already know?
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Does it tell us something deeper about quantum gravity—the theory that tries to mix Einstein’s ideas with quantum mechanics?
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Could it help explain other black hole mysteries, like the information paradox?
Time will tell. But for now, we finally understand why black holes stay perfectly still when the universe tries to stretch them.
In Short
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Black holes don’t stretch when pulled—something called having zero Love numbers.
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A new hidden symmetry explains why this happens.
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The symmetry keeps the forces trying to deform the black hole from having any effect.
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This helps us understand gravitational waves and the deep math of black holes.
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It’s a big step in the ongoing quest to understand how black holes really work.
Reference: Alexandru Lupsasca, "Why there is no Love in black holes", Arxiv, 2025. https://arxiv.org/abs/2506.05298
Technical Terms
🔹 Black Hole
A black hole is a place in space where gravity is so strong that nothing—not even light—can escape. It forms when a very big star collapses at the end of its life.
🔹 Love Numbers
Love numbers are numbers that measure how much an object—like a planet or a star—gets stretched or squished when another object’s gravity pulls on it.
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Zero Love number means the object doesn't change shape at all.
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Black holes have zero Love numbers, meaning they don't stretch or bulge under gravity.
🔹 Symmetry
Symmetry in physics means that something looks or behaves the same even after you change it in certain ways.
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For example, a circle looks the same if you rotate it.
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In this article, symmetry helps explain why black holes don't react to being pulled on.
🔹 Conformal Symmetry
This is a special kind of symmetry that keeps the shape of things the same, even if their size changes.
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Think of blowing up or shrinking a drawing without changing how the objects in it relate to each other.
🔹 SL(2,R)
This is the name of a group of transformations (like rotations, flips, or stretches) that follow certain math rules.
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In the article, it describes a special symmetry that appears in the math around black holes.
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You don’t need to understand the details—just know it’s a powerful hidden pattern.
🔹 Perturbation
A perturbation is a small disturbance or change.
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Imagine gently poking a still pond and watching ripples—those ripples are perturbations.
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In this article, the “poke” is something like gravity from another object pulling on the black hole.
🔹 Time-Independent
Something that doesn’t change over time.
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A time-independent perturbation means the “poke” is constant, not changing or shaking over time.
🔹 Axis-Symmetric
This means the situation looks the same if you spin it around a certain axis (like the center of a spinning top).
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In this case, it means the black hole and the disturbance around it are the same all the way around its spin axis.
🔹 Teukolsky Equation
A complex equation used by physicists to describe how things like light, sound, or gravity behave near a spinning black hole.
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Lupsasca’s discovery showed that, under certain conditions, this complicated equation becomes simple!
🔹 Field
In physics, a field is something that exists in space and can carry energy or force.
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Examples: the magnetic field around a magnet or the gravitational field around Earth.
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Fields can interact with objects like black holes.
🔹 Mode
A mode is like a “vibration pattern” in physics.
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Just like a guitar string can vibrate in different ways to make different notes, fields can vibrate in different modes.
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Some modes interact with the black hole, others don’t.
🔹 Gravitational Waves
These are ripples in space itself, caused by massive objects like black holes moving around or crashing into each other.
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Like waves on a pond, but in space and time.
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