Can Black holes Reflect Light? (Astronomy)

Afshordi and Wan Chenv Zhua found that quantum black holes reflect low frequency light or photons

From the theory of General Relativity, black hole is a spacetime with vacuum region around its horizon and mass concentrated at the singularity. Classically, no signal can be transmitted to an outside observer since nothing can propagate faster than the speed of light. However, it is well known that General Relativity breaks down near the energy of Planck scale, and thus quantum effects may modify the near horizon behavior drastically.

In order to describe quantum black holes, a number of conjectures have been proposed, including black holes being horizonless (fuzzballs). The fuzzball proposal states that black holes are composed of microstates that do not possess horizons. This implies fuzzballs radiate information like a blackbody in the absence of Hawking radiation, leading to a resolution of information paradox.

“This solves the information paradox naturally as there is no horizon, and thus nothing can be trapped within the surface.”

— said Wan Zhen Chua, author of the study

Now, Afshordi and Wan Zhen Chua in their recent paper, computed the reflectivity of electromagnetic waves off the electron-positron Hawking plasma that surrounds the horizon of a Quantum Black Hole (i.e. Fuzzballs). They adopted the “modified firewall conjecture” for fuzzballs, where they considered significant electromagnetic interaction around the horizon.

“While prior work has treated this problem as an electron-photon scattering process, we find that the incoming quanta interact collectively with the fermionic excitations of the Hawking plasma at low energies.”

— told Afshordi, lead author of the study

They derived this via two different methods: one using relativistic plasma dispersion relation, and another using the one-loop correction to photon propagator. Both methods found that Hawking plasma reflects the long wavelength/low frequency photons. This leads to the enhancement of the electromagnetic reflectivity for frequencies comparable to the Hawking temperature of black hole horizons in vacuum. While in the large frequency limit, they found that the photons can fall through the horizon unimpeded.

“Our findings verifies the “modified firewall conjecture” in the context of Fuzzball proposal. The photons with energies comparable to Hawking temperature (i.e. wavelength comparable to horizon radius) can have significant interaction with the fermionic excitations of the fuzzball.”

— told Wan Zhen Chua, author of the study

Now, given that they predicted a significant reflection (or at least RQED ∼ O(α) ∼ 10¯2) for low frequency photons, should we expect to see reflections from black hole horizons?

No.. Unfortunately, the plasma frequency of the ambient interstellar medium provides a frequency cutoff of f_p(kHz) ∼
10ne(cm¯3), which would be the primary hindrance for detecting low frequency radio waves. The Hawking frequency of a 10 solar mass non-spinning black hole is 10² Hz, and
only decreases for larger mass or spin.

“Nevertheless, the potential for observing similar quantum effects for radio pulsars orbiting black holes was entertained in some papers and deserves further exploration. Beyond radio astronomy, our findings provide further moral support for searches for gravitational wave echoes from quantum black holes, which are not hindered by interstellar plasma, and can be successfully carried out at frequencies comparable to those of Hawking radiation. This is an already vibrant field of study.”

— Concluded authors of the study

Reference: Wan Zhen Chua, Niayesh Afshordi, “Electromagnetic Albedo of Quantum Black Holes”, ArXiv, pp. 1-16, 2021. https://arxiv.org/abs/2103.05790


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