One of the first triumphs of Albert Einstein's 1915 general theory of relativity (GR) was the observation, performed in 1919 by Arthur Eddington, Frank W. Dyson, and their collaborators during the total solar eclipse of May 29, that light is bent by the Sun's gravitational field.  The observed light deflection agreed well with Einstein's GR prediction and was twice the deflection predicted by Newtonian gravity.

The deflection of light demonstrates that the Sun acts as a kind of lens, bending light from distant stars.  However, while the familiar double-convex glass lenses used in optics bend light at an angle that increases with distance from the central axis, a gravitational lens bends light at an angle that decreases with distance from the central axis.  Therefore, instead of bringing all parallel rays to a point focus, a gravitational lens focuses them on a focal line.

For the Sun considered as a lens, light rays in a ring just beyond the outer edge of the solar disc are focused to a point at 547.8 AU (astronomical units) behind the Sun.  (For scale, Uranus orbits the Sun at 20 AU, and in 42 years Voyager 1 has reached about 150 AU.)  Rays in successively larger-radius rings will be focused on successively more distant points along the focal line, so the focal-line region of interest extends from about 550 to 1,000 AU.  As viewed from any point on the gravitational focal line, an imaged object appears to be a bright circle called an "Einstein ring".  The Sun's gravitational lens, because of its huge light-gathering area, provides light-intensity amplification on the focal line of up to a factor of 10¹¹ and angular resolution approaching 10⁻¹⁰ arcseconds.

Light rays from a distant star system can be focused by the Sun, detected along the focal line, and mathematically transformed to high resolution optical images of exoplanets.  Indeed, NASA's NIAC program, for which I consult, has funded a project led by Dr. Slava G. Turyshev of JPL that, for any system within 100 light-years of Earth, would use solar gravitational lensing to reconstruct an exoplanet's surface image with 10-km resolution.

The proposed mission would use a solar-sail spacecraft to closely orbit the Sun in an Oberth maneuver and then travel the 550 AU out to the Sun's gravitational focus line for a particular star system, reaching its gravitational focus line in less than 25 years.  There it would deploy a "string of pearls" optical detector system strung out along a section of the focal line to provide the data needed for the high-resolution image reconstruction.

The huge light amplification factor of at least 10⁹ provided by the Sun's gravitational lensing also has interesting implications in the search for extraterrestrial intelligence (SETI).  In particular, in 2013 Michaël Gillon of the University of Liège proposed a novel SETI strategy.  Suppose that, as suggested by Tipler in 1980, our galaxy has been colonized by self-replicating "Von Neuman" probes.  Such probes would need to communicate with each other and with their home base at interstellar distances.  Because of the inverse-square law, point-to-point communication between them would be essentially impossible, requiring enormous transmission power and huge receiver arrays.

A viable alternative would be to construct a network of relay receiver-transmitters spaced a few parsecs apart across the galaxy.  The efficiency of such a network would be greatly enhanced by locating the relays within the gravitational focal lines of network stars, thereby using the intensity amplification factor of ~10⁹.

Gillon labels such units "Focal Interstellar Communication Devices" (FICDs) and argues that the residual electromagnetic radiation from such FICDs operating in transmission mode in our solar system might allow us to detect them, if we look in the right places.  There is also the possibility that hypothetical FICDs on solar focal lines are communicating with sub-units on the Earth's surface to collect information on our civilization.  If that is the case, electromagnetic waves originating on the FICDs, probably in the microwave region, should be beamed toward the Earth's surface, so that an Earth-based observer could detect such communications.

Michael Hippke of Germany's Sonneberg Observatory and UC Berkeley has studied in detail the optimum configuration of FICDs.  He estimates that FICDs should have an optimum size on the order of 1 meter.  He finds that in using solar gravitational focusing for communication, the longer-wavelength radio region of the electromagnetic spectrum is troubled by the plasma near the Sun, which absorbs signals and creates fluctuations in refractive index that blur the focus.  To avoid such effects, the FICD would optimally communicate in the shorter wavelength range between 100 micrometers (in the far infrared) to 1 nanometer (in the soft x-ray region), a range that includes the visible light spectrum.

Hippke estimates for our Solar System the optimum location for the FICD would be at about 1,000 AU from the Sun along the targeted star's gravitational focus line.  For Earth-based observations of electromagnetic radiation from FICDs, the sky position of the object moves around as Earth orbits the Sun.  Thus, one would need to aim at the calculated moving position of a hypothetical FICD.

 Others have concluded that single stars like the Sun (rather than binary star systems) would have the smallest FICD station-keeping costs because of the reduced action of gravitational perturbations.  Hippke has provided a priority list of candidate star system focal lines that should be searched.  It favors non-binary stars and systems with detected planets.  Here is his list of stars, in priority order with light-year distances from Earth in parentheses: Proxima Centauri (4.2), Tau Ceti, (11.7), Sigma Draconis (18.3), Epsilon Eridani (10.4), Delta Pavonis (19.6), Barnard's Star (5.9), Wolf 359 (7.8), Lalande 21185 (8.2), Alpha Centauri A&B (4.2), and Sirius A&B (8.5).

Hippke's list also includes Sagatarius A*, the 4.2×10⁶ solar-mass black hole at the center of our galaxy, which is 26,673 light-years from Earth.  Since gravitational light scales with the mass of the lensing object, Sagatarius A* would offer a light amplification factor perhaps 10⁶ times larger than that of a star and closer in.  Therefore, Sagatarius A* might be surrounded by FICDs communicating in many directions, including ours.

To check on Gillon's hypothesis, it is of considerable SETI interest to train observational capabilities in the infrared, visible, and ultraviolet regions on points on the solar focal lines pointing back to selected star systems and also to look for microwave communications from the hypothetical FICD to sub-units on the Earth.  Several groups, including one led by Gillon himself, have begun to work on this SETI program.

Gillon, Burdanov, and Wright focused their SETI effort on the Wolf 359 star system, 7^th on Hippke's priority list, because that star happens to lie in the extended plane of Earth's orbit around the Sun.  That means that if there is a communication beam between Wolf 359 and the Sun, Earth should pass through it in early September of each year, making it possible to detect direct star-to-star transmissions.  Since Wolf 359 is in the sky of the Northern Hemisphere, such a search would need to use a Southern Hemisphere telescope.

The group used the TRAPPIST-South robotic telescope in Chile to search the 300 to 950 nm wavelength region on September 4, 5, and 8, 2015, as the Earth was passing through the hypothetical FICD beam aimed at Wolf 359.  They would have been able to detect a signal transmitted with only 1 W of power, but after analysis they observed no signal.  They repeated the search using the robotic SPECULOOS-South telescope, also in Chile, to search for FICD emissions to Wolf 359 for about three hours on the night of September 4, 2019.  Again, after analysis they observed no signal.

Marcy, Trellis, and Wishnow argued that lasers would likely be used by FICDs because the laser's narrow bandwidth is ideal for communication.  They comment that many astronomers have already searched for any indication of laser-generated light in our galaxy and have observed none.  However, to be detectable without gravitational lensing at interstellar distances, a laser beam would need a source power of 50 kW to 10 MW.

In 2020-21 they used two prism-objective telescopes to search for both pulsed and continuous laser-based signals of wavelengths 380 to 950 nm with source power 100 W or more from the solar gravitational focal lines for Proxima Centauri and Alpha Centauri A&B.  They used 0.25 s exposure durations over a six month period, recording 88,000 exposures for Proxima Centauri and 47,000 exposures for Alpha Centauri A&B.  They report that no signals were detected.

Since the Earth was not in a hypothetical beam between the FICD and the target stars, any laser emissions detected would presumably be from communication between the FICD and sub-units on Earth or from secondary emissions produced in forming the main beam.

The large collaboration of Nick Tusay, et al, based mainly at Penn State University, has taken a somewhat different approach to the SETI search for FICDs.  They assume that while optical communication of the FICD with a nearby star system may be necessary, the FICD would likely communicate with its sub-units based elsewhere in solar system by using radio waves.  Therefore, existing radio telescopes can be used for the search.  To implement this search concept, they have used the Green Bank Telescope with the Breakthrough Listen back end to search for such sources in the L and S bands (1-2 GHz and 2-4 GHz) originating on the gravitational focal line for Alpha Centauri A&B.  They report no observation of signals that would be indicative of non-human artificial origin.  They would expect to detect any transmission with a source strength of at least 2 to 23 W, the lower limit depending on band and focal line location.

These three attempts to detect FICDs are not the end of such SETI searches.  There are many other star systems on Hippke's priority list that have not been studied, and many regions of electromagnetic emission that have not been covered.  Watch this column for further developments.

John G. Cramer's 2016 nonfiction book describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at:
        http://www.springer.com/gp/book/9783319246406 or
        https://www.amazon.com/dp/3319246402 .

SF Novels:  editions of John's hard SF novels Twistor and Einstein's Bridge are available online at:
https://www.amazon.com/Twistor-John-Cramer/dp/048680450X  and
https://www.amazon.com/Einsteins-Bridge-John-Cramer/dp/0380788314 .  
John's new hard-SF novel Fermi's Question, sequel to Einstein's Bridge, will be available soon from Baen Books.

Alternate View Columns Online: Electronic reprints of 220 or more of "The Alternate View" columns written by John G. Cramer and previously published in Analog are currently available online at:  http://www.npl.washington.edu/av .

References:

Solar Lens Imaging of Exoplanets:

Slava G. Turyshev, et al, "Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission", arXiv: 1802.08421 (2018).

Galactic Communication Network Hypothesis:

M. Gillon, "A novel SETI strategy targeting the solar focal regions of the most nearby stars", Acta Astronautica 94, 629 (2014).

FICD Characteristics:

Hippke, M., "Interstellar communication network. I. Overview and assumptions", arXiv:1912.02616v2, (2019);

Hippke, M., "Interstellar communication network. II. Deep space nodes with gravitational lensing", arXiv:2009.01866v1, (2020);

Hippke, M., "Interstellar communication network. III. Locating deep space nodes", arXiv:2104.09564v1, (2021).

FICD Searches:

M. Gillon, A. Burdanov, and J. T. Wright, "Search for an alien communication from the Solar System to a neighbor star", arXiv: 2111.05334, (2021).

G. W. Marcy, S. K. Tellis, and E. H. Wishnow, "Laser Communication with Proxima Centauri using the Solar Gravitational Lens", Monthly Notices of the Royal Astronomical Society 509-3, 3798–3814 (2022).

Nick Tusay, et al, "A Search for Radio Technosignatures at the Solar Gravitational Lens Targeting Alpha Centauri", arXiv: 2206.14807v1 (2022).

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