Ghosts at the Horizon: How Black Holes Whisper with Sound ✛ ✎ ✦

Original: "Acoustic Hawking radiation as a tunnelling effect in Michel accretion"

arXiv:2607.00037 · 2026-06-28 · arXiv non-exclusive · independent analysis · 2 min

When gas falls into a black hole, it forms an acoustic horizon — a mirror of the gravitational one. Sound cannot cross it, but quantum phonons tunnel through like ghosts, and the curvature of space only amplifies this otherworldly whisper.

Imagine a river at a rapid: the current accelerates until no wave can go against the flow. So it is with gas near a black hole: reaching the speed of sound, it locks itself in — an acoustic black hole is born, a twin of the gravitational abyss. Here the event horizon is woven from a flow faster than its own sound.

For a solar mass, the analogue Hawking temperature is a measly ~10⁻⁸ K, but the curvature of spacetime near the black hole acts as a “gravitational amplifier,” boosting it many times over.

But the silence is deceptive. High-frequency quantum phonons — quanta of sound — pull off a trick forbidden by classical physics: they tunnel through the horizon, like ghosts passing through walls. This is the precise analogue of Hawking radiation, predicted by Stephen Hawking for real black holes. In the accreting gas, phonons are born in pairs at the horizon: one particle is swept away by the flow into oblivion, while the other, breaking the ban, breaks free, carrying a scrap of energy — a shadow torn from its master.

In the model of Michel accretion in the curved spacetime of Karl Schwarzschild, the equation for sound waves acquires a term that breaks the symmetry of the acoustic metric. This “gravitational defect” does not hinder but helps tunneling: the radiation frequency and horizon temperature $T_H$ grow as Ω = ω √(1+3a_c²). Curvature gives the metaphorical waterfall an extra boost.

To circumvent the singularity near the horizon, where the wave number undergoes infinite blue shift, the authors use Cauchy's residue theorem — an elegant slice through the complex plane, revealing a finite tunneling probability.

This result is more than just an elegant theory. It reinforces the idea of the universality of Hawking radiation as a kinematic effect in Lorentzian geometries, whether curved spacetime or a turbulent flow. Looking ahead, laboratory experiments with Bose-Einstein condensates or superfluid helium will allow real-time observation of phonon birth at the acoustic horizon. Such analogue systems bring us closer to solving the information paradox: perhaps the singularity does not destroy information but encodes it in quantum correlations at the horizon. And who knows — might the Universe itself be just phonon ripples on the surface of a cosmic black hole?

Whole graph →

The analogue Hawking temperature for a stellar mass is vanishingly small — around 10⁻⁸ K — but in accretion flows near supermassive black holes, the effect can be significantly boosted by the factor √(1+3a_c²), which acts as a 'gravitational megaphone'.

\Omega = \omega \sqrt{1 + 3 a_c^2}

The curvature of spacetime increases the frequency of emitted phonons compared to a laboratory reference frame.

T_H = \frac{\hbar}{2\pi k_B} \left. \frac{d}{dr} \left( a_0 \sqrt{f+u_0^2} - |u_0| \right) \right|_{r_c}

The radiation temperature of the acoustic black hole, determined by the surface gravity at the sonic horizon.

Scientists

George Gamow Leo Esaki Stephen Hawking Jacob Bekenstein Henri Poincaré Fritz Zwicky

Concepts

Hawking radiation hawking radiation mechanism black hole accretion winds sound wave acoustic oscillations black hole resonance wave amplification speed of sound ultrasound baryon acoustic oscillations phononic crystal echo moiré pattern effects Bekenstein-Hawking entropy radio loud quasar wind scalar perturbations overtones Hall effect thermal hall effect

I am the author

Ghosts at the Horizon: How Black Holes Whisper with Sound ✛ ✎ ✦