# Boomerang photons around spherically symmetric black holes

## Abstract

The gravitational field of celestial bodies can be strong enough to deflect photons or even trap photons at the photon sphere. This paper is a study of boomerang photons in the spacetimes of spherically symmetric black holes. These are photons that start at some position, go around a black hole, and return to its starting point. The motion of a massless particle is completely specified by its impact parameter *b* which is just the ratio of its conserved angular momentum and energy. The impact parameter can be adjusted to obtain a photon orbit making $n$ revolutions about a black hole for boomerang orbits. We numerically compute impact parameter spectra, discrete sets of impact parameters {*b*_{1}; *b*_{2}; : : : ; *b _{n}*; : : : ;

*b*} for boomerang orbits, bounded by a critical impact parameter,

_{∞}*b*, which corresponds to a trapped photon orbit. The spectrum is sensitive to black hole parameters: it expands as the photon is emitted further from the Schwarzschild black hole; shrinks as its charge

_{∞}*Q*increases; expands when we positively increase the cosmological constant Λ of a (anti-)de Sitter universe; and shrinks as we consider higher spacetime dimensions. Boomerang photons may potentially find use in the pursuit of fuel-free interstellar travel such as a halo drive.

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## How to Cite

*Proceedings of the Samahang Pisika ng Pilipinas*

**39**, SPP-2021-PC-05 (2021). URL: https://proceedings.spp-online.org/article/view/SPP-2021-PC-05.