Can Black Holes Rotate at the Speed of Light?

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🌀 The Short Answer

No — according to Einstein’s theory of relativity, nothing with mass can reach the speed of light, not even the edge of a rotating black hole. However, black holes can rotate very close to that ultimate limit, and the faster they spin, the stranger their effects on space-time become.

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1️⃣ Black Hole Spin: The Kerr Black Hole

Most black holes in nature are not static — they rotate. In physics, a rotating black hole is called a Kerr black hole. The spin is measured using a dimensionless parameter called "a"* (pronounced a-star), which ranges from:

• 0 → No spin (static Schwarzschild black hole)

• 1 → Maximum possible spin (extremal Kerr black hole)

If a were exactly 1, the black hole’s event horizon would shrink to its minimum size, and the outer edge would move at the speed of light. But this is theoretical — nature prevents it from happening in reality.

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2️⃣ Why They Can’t Hit Light Speed

Two main reasons stop a black hole from spinning at a = 1:

1. Energy Loss via Radiation

   • As matter falls in, it releases gravitational energy in the form of light, X-rays, and jets. This robs the black hole of some spin energy before it reaches the theoretical max.

2. Cosmic Censorship Hypothesis

   • If a black hole exceeded the limit, it would lose its event horizon and expose a naked singularity — something relativity predicts but nature seems to forbid.

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3️⃣ How Fast Can They Actually Spin?

The record-holders spin incredibly close to the limit:

• GRO J1655-40 → a ≈ 0.93

• Cygnus X-1 → a ≈ 0.99

• Supermassive Black Hole in NGC 1365 → a ≈ 0.97

At these spins, the space-time near the black hole’s equator is being dragged around at over 90% of light speed. This effect, called frame-dragging, twists everything — even light — in the direction of rotation.

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4️⃣ The Weird Effects of Near-Light-Speed Spin

A rapidly spinning black hole can:

• Create super-energetic jets shooting out near its poles.

• Pull material into an accretion disk that orbits dangerously close to the event horizon.

• Generate an ergosphere — a region where nothing can remain still, and even light is forced to move.

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🌌 Conclusion

Black holes can’t rotate at the exact speed of light, but they can get so close that their outer edges appear to move at almost that speed. At that point, they become some of the most extreme objects in the universe, capable of bending reality in ways no other object can.

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