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.
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
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.
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.
Burdanov, and Wright focused their SETI effort on the Wolf 359 star system, 7th
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.
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.
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.
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
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:
editions of John's hard SF novels Twistor
Einstein's Bridge are available online at:
John's new hard-SF novel Fermi's Question, sequel to Einstein's Bridge, will be available soon from Baen Books.
View Columns Online: Electronic reprints of 220 or more of "The
Solar Lens Imaging of Exoplanets:
G. Turyshev, et al, "Direct Multipixel Imaging and Spectroscopy of an
Exoplanet with a Solar Gravity Lens Mission", arXiv:
Galactic Communication Network Hypothesis:
Gillon, "A novel SETI strategy targeting the solar focal regions of the most
nearby stars", Acta
Astronautica 94, 629 (2014).
M., "Interstellar communication network. I. Overview and assumptions", arXiv:1912.02616v2,
M., "Interstellar communication network. II. Deep space nodes with gravitational
lensing", arXiv:2009.01866v1, (2020);
M., "Interstellar communication network. III. Locating deep space nodes", arXiv:2104.09564v1,
Gillon, A. Burdanov, and J. T. Wright, "Search for an alien communication from
the Solar System to a neighbor star", arXiv:
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,
Tusay, et al, "A Search for Radio Technosignatures at the Solar
Gravitational Lens Targeting Alpha Centauri", arXiv: