[Paper Review] Interstellar communication network. II. Deep space nodes with gravitational lensing

Michael Hippke|arXiv (Cornell University)|Sep 3, 2020

Space Science and Extraterrestrial Life4 citations

TL;DR

This paper proposes a deep space interstellar communication network using gravitational lensing by stars to achieve Gbits/s data rates over interstellar distances. By placing transmitters and receivers at the solar gravitational lens (SGL) focal line (~1,000 au from the Sun), signals are focused with gains up to 10⁹, enabling high-bandwidth, energy-efficient links; optimal receiver and transmitter sizes are found to be on the order of meters to avoid temporal smearing while maximizing signal capture.

ABSTRACT

Data rates in an interstellar communication network suffer from the inverse square law due to the vast distances between the stars. To achieve high (Gbits/s) data rates, some combination of large apertures and high power is required. Alternatively, signals can be focused by the gravitational lenses of stars to yield gains of order $10^{9}$, compared to the direct path. Gravitational lens physics imposes a set of constraints on the sizes and locations of receivers and apertures. These characteristics include the minimum and maximum receiver size, the maximum transmitter size, and the heliocentric receiver distance. Optimal sizes of receivers and transmitters are of order meters. Such small devices allow for the capture of the main lobe in the beam while avoiding the temporal smearing which affects larger apertures. These and other properties can be used to describe the most likely parameters of a lensed communication network, and to determine exact position of communication nodes in the heliocentric reference frame.

Motivation & Objective

• To determine the optimal physical parameters—size, location, and aperture—for transmitters and receivers in a gravitational lens-based interstellar communication network.
• To analyze the constraints imposed by gravitational lensing physics, including beam width, temporal smearing, and signal resolution.
• To evaluate the feasibility of achieving high data rates (Gbps) using lensed links compared to direct free-space transmission.
• To identify the most likely configurations for SGL nodes that would enable stable, high-gain communication between stellar systems.
• To provide a foundation for directing future SETI searches toward specific locations and technological signatures in the heliocentric reference frame.

Proposed method

• Uses the Schwarzschild metric and lensing geometry to model the solar gravitational lens (SGL), deriving the focal line at z₀ ≈ 547.8 au and the Einstein ring at z > z₀.
• Applies the point spread function (PSF) formalism with a Bessel-function-based Airy pattern to describe signal resolution and beam width at the focal plane.
• Derives the effective aperture of the Einstein ring as A_ER ≈ πbw, where b is the impact parameter and w is the receiver aperture, assuming a thin ring approximation.
• Evaluates data rate scaling using the Friis transmission equation modified for lensing gain, showing quadratic dependence on transmitter aperture and linear scaling with receiver aperture.
• Considers temporal smearing effects from extended sources and aperture size, limiting practical receiver and transmitter sizes to ~1–150 m.
• Assesses the heliocentric distance z for optimal lensing performance, finding z ≈ 2,189 au as optimal for resolving the gap between the Sun and Einstein ring.

Experimental results

Research questions

• RQ1 What are the optimal sizes and locations for transmitters and receivers in a gravitational lens-based interstellar communication network?
• RQ2 How does the data rate scale with transmitter and receiver aperture size in the SGL configuration?
• RQ3 What physical constraints—such as temporal smearing, beam width, and PSF decay—limit the performance of lensed communication?
• RQ4 How does the lensing gain compare to direct free-space transmission in terms of energy efficiency and achievable data rates?
• RQ5 What are the most favorable heliocentric distances for placing SGL nodes to maximize link stability and data throughput?

Key findings

• The optimal receiver and transmitter sizes are on the order of meters, as larger apertures suffer from severe temporal smearing, while smaller ones fail to capture the main lobe of the lensed beam.
• Data rate increases quadratically with transmitter aperture size but only linearly with receiver aperture size, making transmitter power and size the dominant factor for high-rate links.
• The optimal heliocentric distance for a receiver in the SGL is approximately 2,189 au, where the gap between the solar limb and the Einstein ring is just resolvable, maximizing signal capture and minimizing alignment difficulty.
• Lensing gains reach up to 10⁹ compared to direct transmission, enabling Gbps data rates with moderate power and aperture sizes, especially when using coherent detection and coronagraphs to suppress coronal noise.
• For a 1 m transmitter and 1 m receiver at 1,000 au, direct free-space transmission at λ = 1 μm and 1 kW power achieves less than 1 Mbit/s, while lensed links can exceed Gbps, improving energy efficiency by a factor of ~10⁵.
• The lensing scheme is most effective when one node is in the SGL focal plane and the other is in free space; the SGL node requires only low Δv for station-keeping due to the small plate scale (z/d).