Radio and optical SETI look for evidence of extraterrestrial civilizations even though we have no evidence that such exist. The search is eminently worthwhile and opens up the ancillary question: How would a transmitting civilization produce a signal strong enough for us to detect it at interstellar distances? Beacons of various kinds have been considered and search strategies honed to find them. But we’ve also begun to consider new approaches to SETI, such as detecting technosignatures in our astronomical data (Dyson spheres, etc.). To this mix we can now add a consideration of gravitational lensing, and the magnifications possible when electromagnetic radiation is focused by a star’s mass. For a star like our Sun, this focal effect becomes useful at distances beginning around 550 AU.
Theoretical work and actual mission design for using this phenomenon began in the 1990s and continues, although most work has centered on observing exoplanets. Here the possibilities are remarkable, including seeing oceans, continents, weather patterns, even surface vegetation on a world circling another star. But it’s interesting to consider how another civilization might see gravitational lensing as a way of signaling to us. Indeed, doing so could conceivably open up a communications channel if the alien civilization is close enough, for if we detect lensing being used in this way, we would be wise to consider using our own lens to reply.
Or maybe not, considering what happens in The Three Body Problem. But let’s leave METI for another day. A new paper from Slava Turyshev (Jet Propulsion Laboratory) makes the case that we should be considering not just optical SETI, but a gravitationally lensed SETI signal. The chances of finding one might seem remote, but then, we don’t know what the chances of any SETI detection are, and we proceed in hopes of learning more. Turyshev argues that with the level of technology available to us today, a lensed signal could be detected with the right strategy.
Image: Slava Turyshev (Jet Propulsion Laboratory). Credit: Asteroid Foundation.
“Search for Gravitationally Lensed Interstellar Transmissions,” now available on the arXiv site, posits a configuration involving a transmitter, receiver and gravitational lens in alignment, something we cannot currently manage. But recall that the effort to design a solar gravity lens (SGL) mission has been in progress for some years now at JPL. As we push into the physics involved, we learn not only about possible future space missions but also better strategies for using gravitational lensing in SETI itself. We are now in the realm of advanced photonics and optical engineering, where we define and put to work the theoretical tools to describe how light propagates in a gravity field.
And while we lack the technologies to transmit using these methods ourselves (at least for now), we do have the ability to detect extraterrestrial signals using gravitational lensing. In an email yesterday, Dr. Turyshev offered an overview of what his analysis showed:
Many factors influence the effectiveness of interstellar power transmission. Our analysis, based on realistic assumptions about the transmitter, shows that substantial laser power can be effectively transmitted over vast distances. Gravitational lensing plays a crucial role in this process, amplifying and broadening these signals, thereby increasing their brightness and making them more distinguishable from background noise. We have also demonstrated that modern space- and ground-based telescopes are well-equipped to detect lensed laser signals from nearby stars. Although individual telescopes cannot yet resolve the Einstein rings formed around many of these stars, a coordinated network can effectively monitor the evolving morphology of these rings as it traces the beam’s path through the solar system. This network, equipped with advanced photometric and spectroscopic capabilities, would enable not only the detection but also continuous monitoring and detailed analysis of these signals.
We’re imagining, then, an extraterrestrial civilization placing a transmitter in the region of its own star’s gravitational lens, on the side of its star opposite to the direction of our Solar System. The physics involved – and the mathematics here is quite complex, as you can imagine – determine what happens when light from an optical transmitter is sent to the star so that when it encounters the warped spacetime induced by the star’s mass, the diffracted rays converge and create what scientists call a ‘caustic,’ a pattern created by the bending of the light rays and their resulting focused patterns.
In the case of a targeted signal, the lensing effect emerges in a so-called ‘Einstein ring’ around the distant star as seen from Earth. The signal is brightened by its passage through warped spacetime, and if targeted with exquisite precision, could be detected and untangled by Earth’s technologies. Turyshev asks in this paper how the generated signal appears over interstellar distances.
The answer should help us understand how to search for transmissions that use gravitational lensing, developing the best strategies for detection. We’ve pondered possible interstellar networks of communication in these pages, using the lensing properties of participating stellar systems. Such signals would be far more powerful than the faint and transient signals detectable through conventional optical SETI.
Laser transmissions are inherently directional, unlike radio waves, the beams being narrow and tightly focused. An interstellar laser signal would have to be aimed precisely towards us, an alignment that in and of itself does not resolve all the issues involved. We can take into account the brightness of the transmitting location, working out the parameters for each nearby star and factoring in optical background noise, but we would have no knowledge of the power, aperture and pointing characteristics of a transmitted signal in advance. But if we’re searching for a signal boosted by gravitational lensing, we have a much brighter beam that will have been enhanced for best reception.
Image: Communications across interstellar distances could take advantage of a star’s ability to focus and magnify communication signals through gravitational lensing. A signal from—or passing through—a relay probe would bend due to gravity as it passes by the star. The warped space around the object acts somewhat like a lens of a telescope, focusing and magnifying the light. Pictured here is a message from our Sun to another stellar system. Possible signals from other stars using these methods could become SETI targets. Image credit: Dani Zemba / Penn State. CC BY-NC-ND 4.0 DEED.
Mathematics at this level is something I admire and find beautiful much in the same way I appreciate Bach, or a stunning Charlie Parker solo. I have nowhere near the skill to untangle it, but take it in almost as a form of art. So I send those more mathematically literate than I am to the paper, while relying on Turyshev’s explanation of the import of these calculations, which seek to determine the shape and dimensions of the lensed caustic, using the results to demonstrate the beam propagation affected by the lens geometry, and the changes to the density of the EM field received.
It’s interesting to speculate on the requirements of any effort to reach another star with a lensed signal. Not only does the civilization in question have to be able to operate within the focal region of its stellar lens, but it has to provide propulsion for its transmitter, given the relative motion between the lens and the target star (our own). In other words, it would need advanced propulsion just to point toward a target, and obviously navigational strategies of the highest order within the transmitter itself. As you can imagine, the same issues emerge within the context of exoplanet imaging. From the paper:
…we find that in optical communications utilizing gravitational lenses, precise aiming of the signal transmissions is also crucial. There could be multiple strategies for initiating transmission. For instance, in one scenario, the transmission could be so precisely directed that Earth passes through the targeted spot. Consequently, it’s reasonable to assume that the transmitter would have the capability to track Earth’s movement. Given this precision, one might question whether a deliberately wider beam, capable of encompassing the entire Earth, would be employed instead. This is just [a] few of many scenarios that merit thorough exploration.
Detecting a lensed signal would demand a telescope network optimized to search for transients involving nearby stars. Such a network would be capable of a broad spectrum of measurements which could be analyzed to monitor the event and study its properties as it develops. Current and near-future instruments from the James Webb Space Telescope and Nancy Grace Roman Space Telescope to the Vera C. Rubin Observatory’s LSST, the Thirty Meter Telescope and the Extremely Large Telescope could be complemented by a constellation of small instruments.
Because the lens parameters are known for each target star, a search can be constructed using a combination of possible transmitter parameters. A search space emerges that relies on current technology for each specific laser wavelength. According to Turyshev’s calculations, a signal targeting a specific spot 1 AU from the Sun would be detectable with such a network with the current generation of optical instruments. Again from the paper:
Once the signal is detected, the spatial distribution of receivers is invaluable, as each will capture a distinct dataset by traveling through the signal along a different path… Correlating the photometric and spectral data from each path enables the reconstruction of the beam’s full profile as it [is] projected onto the solar system. Integrating this information with spectral data from multiple channels reveals the transmitter’s specific features encoded in the beam, such as its power, shape, design, and propulsion capabilities. Additionally, if the optical signal contains encoded information, transmitted via a set of particular patterns, this information will become accessible as well.
While microlensing events created by a signal transmitted through another star’s gravitational lens would be inherently transient, they would also be strikingly bright and should, according to these calculations, be detectable with the current generation of instruments making photometric and spectroscopic observations. Using what Turyshev calls “a spatially dispersed network of collaborative astronomical facilities,” it may be possible not only to detect such a signal but to learn if message data are within. The structure of the point spread function (PSF) of the transmitting lens could be determined through coordinated ground- and space-based telescope observations.
We are within decades of being able to travel to the focal region of the Sun’s gravitational lens to conduct high-resolution imaging of exoplanets around nearby stars, assuming we commit the needed resources to the effort. Turyshev advocates a SETI survey along the lines described to find out whether gravitationally lensed signals exist around these stars, pointing out that such a discovery would open up the possibility of studying an exoplanet’s surface as well as initiating a dialogue. “[W]e have demonstrated the feasibility of establishing interstellar power transmission links via gravitational lensing, while also confirming our technological readiness to receive such signals. It’s time to develop and launch a search campaign.“
The paper is Turyshev, “Search for gravitationally lensed interstellar transmissions,” now available as a preprint. You might also be interested in another recent take on detecting technosignatures using gravitational lensing. It’s Tusay et al., “A Search for Radio Technosignatures at the Solar Gravitational Lens Targeting Alpha Centauri,” Astronomical Journal Vol. 164, No. 3 (31 August 2022), 116 (full text), which led to a Penn State press release from which the image I used above was taken.
This article is very much along the same line as my original article on this published in Centauri Dreams
Virtual Von Neumann Probes using Self Amplification and Replication of Electromagnetic Signals through Natural Stellar Processes
https://www.centauri-dreams.org/2017/01/30/virtual-von-neumann-probes-using-self-amplification-and-replication-of-electromagnetic-signals-through-natural-stellar-processes/
Has anyone mapped potential focal lines Earth /Hubble may pass through?
the question is interesting but I found nothing. to create this (global) map, I think we should define an area around the earth that could contain the difracted light rays, but also list the different supermassive objects in or at the limit of this area that would form the gravitational lens. (need to do optical calculations). One would thus arrive at a kind of sphere crossed by several luminous rays arriving from different angles or the earth would have a central place. I do not know if it would be of great interest since we generally look at the objects we analyze in one direction only?
some info here : https://www.centauri-dreams.org/2016/04/26/gravitational-lensing-with-planets/
I read the article. It indicates indirectly that there is currently no mapping on the subject insofar as it would be necessary to choose several stars that could serve as gravitational lenses + or – in the ecliptic plane, without disturbing the signal, while selecting certain radio frequencies but also defining or could be placed the relay probes by an ETI and considering that the earth would be the receiving point. The article did two tests: with alpha centauri and another star. To have a mapping, it would be necessary to draw the multiple focal points, for certain frequencies, all in a determined sphere (here 500ua) …if I followed the lesson correctly:) Sorry if there are inconsistencies: I am not a scientist and I have to translate all this into French ;)
Barnard’s Star is really booking…it all but has to pass “over” something of interest sometime…the stars are right :)
This got me wondering what is underway with SETI in the gravitational wave domain. Here’s a nice little article ( https://arxiv.org/pdf/2212.02065.pdf ) suggesting that relativistic Jupiter-sized ships might be found anywhere in the galaxy as detection of lower-frequency gravitational waves is implemented.
I could imagine a truly advanced civilization determined to ring the dinner bell might find or make a system of three black holes, then adjust their trajectories (per the three-body problem) to create a long series of very close encounters. These could be detected anywhere in the Local Group and perhaps beyond, even with current human technology. In concept, the chaotic motions of the holes might be planned by small, well-planned manipulations of nearby asteroids in a Great Orrery, so that the holes pass each other with their poles vs. equators adjacent to encode bits of data based on whether they drag space in the same or opposite directions. Such a signal might be just out of reach of current instruments. Or (because it’s not coming to a crescendo, and perhaps not periodic), could it be sitting somewhere in the LIGO data, but considered as noise?
It would be far easier for ET who wish to specifically contact us to use radio. The sky is pretty quiet at frequencies of the order of a GHz and radio astronomy is a predictable feature of emerging advanced civilizations. They can use stellar gain to create exceptionally bright sources that we would easily stumble across. We haven’t. Or they could signal from much further away, that would otherwise be difficult to impossible for us to detect.
That seems more sensible than to use frequencies where the stellar background of the stars they use as lenses would lessen the SNR and otherwise mask the presence of a signal. The signal would have to be stumbled upon by analyzing a broad spectrogram. Sure, it’s possible, but not better or easier than radio.
For incidental lensing events that occur by happenstance, radio is even better for capturing these brief and rare events. The wider beam width of the antennae is a feature rather than a bug.
@Ron
Are you suggesting replacing the lasers with masers? Doesn’t the change of wavelength make this approach more difficult?
Masers are not practical at such frequencies, nor are they needed. We have plenty of perfectly capable observatories for the low GHz range. Besides, this is about SETI (reception), not METI (transmission).
@Ron
The transmitter has to be monochromatic. If you want an annular beam aimed at the star’s outer edge, a coherent microwave beam would substitute for the laser. This is collimated by the star’s gravity and the same principles focus the beam near our sun’s 1 AU orbit of Earth. Our radio telescopes could receive this signal
ETI is doing METI and we are doing SETI.
If this is not what you are proposing, can you clarify?
Why is this difficult? I really don’t understand your confusion on these points.
Masers and lasers are transmitting devices. Not relevant to SETI (reception).
Monochromatic, maybe. Coherent, no. This is routine RF (radio spectrum) transmitter design. The receiver doesn’t actually need to know whether the transmission is coherent or not, so the transmitter doesn’t require it. Coherence has no value for almost every communication application at RF. You use the antenna to transmit a narrow beam width (e.g. big parabolic reflector). Masers and lasers employ coherence to achieve a similar outcome at higher frequencies (shorter wavelengths) where parabolic reflectors run into limitations; that is, coherence is a tool to narrow the beam width.
By ‘maybe’ above, I am alluding to the modulation employed; only an isolated carrier can be monochromatic, which of course contains no information other than its own presence. An information carrying signal is not monochromatic.
Does this help?
@Ron
We are talking about the whole system, transmitter and receiver, yes?
The whole purpose of this paper is to allow a less technologically advanced species (e.g. humanity) to receive signals from a more advanced species (ETI). The idea is that we can receive signals on our home world and needn’t have to try to reach and position a receiver at our SGL. Even ETI wants to minimize the energy demand which is alluded to in the introduction:
Why does the transmitter need to be a coherent, monochromatic source? I leave that to the author who states:
Can you use a parabolic radio transmitter to do this?
The bulk of the content is calculating the energy received at the reference 1 AU position, i.e. Earth, and importantly the signal-to-noise ratio that would be readily detectable. It assumes an optical telescope that is able to detect the changing light intensity of a star due to microlensing.
I understood you suggested radio signals instead. What I don’t understand is why you suggest that the transmitter need not be a laser equivalent that can emit radio waves, albeit at the shorter end. The receiver doesn’t need to involve masers any more that the optical receiver needs to involve lasers.
Lastly, a monochromatic beam is perfectly suitable for digital information transfer. NASA is experimenting with that now for deep space transmissions to increase the bandwidth.
You watched/read the 3 Body Problem. Flickering stars transmitted a s signal. In the case of a the Earth passing through a focal point at 1 AU, the beam gives hours of detectable SNR. Surely enough to send a “hello” as an unnatural number sequence such as primes, if not a more detailed message depending on the limits of the interference due to the irregularity of the lens and ISM.
Addendum.
Figure 2 in the paper shows that teh transmitter need not be at teh star’s gravitational focal line, but rather can be offset. This implies that a number of transmitters can provide the needed beam that is a synthetic annulus that will be gravitaionally bent to afocus at our suns’ 1 AU point.
@Alex
The beam is very narrow. As Turyshev notes in his paper (and his other papers) one cannot use the geometric optics approximation in this case , because the amplification diverges on the focal axis. The wave optical solution is finite but large , going like one over the wavelength.
In some work I did I sought to make the signal by gravitational lensing sort of ‘isotropic’ by putting a constellation of transmitters about the ‘lens’. (Something I am supposing an advanced civilization could do.)
I suppose a civilization might identify us as a target and arrange to keep us on a focal line, but that’s just a speculation.
One might lens electromagnetic radiation as well as neutrinos. Using a black hole as a lens also implies an advanced civilization with the ability to travel to a black hole or a neutron star.
A. A. Jackson, Black Hole Beacon: Gravitational Lensing, , JBIS, 68, pp.342-346,2015.
A.A. Jackson, A Neutrino Beacon, JBIS, 73, pp.15-20, 2020.
@Al
This is arguably another solution to an unknown problem. The ETI civ has to be near enough to us, and in a state that is advanced, but not by millions of years, is interested in us, can already determine we are a technological one, and want to say “hello, there” at a minimum. This may just be as unlikely as having a deity appear in the sky. But at least our detection of such a signal as part of SETI is nowhere near as expensive as building cathedrals (even if we reciprocate with an SGL transmitter ourselves).
What I think is useful is that the receiver need not be new technology, and this assumed ETI transmission can be detectable with all-sky optical surveys. As we do not know where ETI may be, this is a good strategy on the part of ETI. This reserves SGL telescopes to provide some imaging of selected exoplanets. A signal captured in our orbit and the source star system located could inform our positioning of an SGL telescope and other receivers.
IIUC, the beam reaching our system is not perfectly parallel and requires a receiver at our SGL, but rather converging to focus near our orbit. The SNR is sufficient to be detectable if the beam is monochromatic light.
That is convenient for ETI trying to send us a signal. A digital signal is going to be far easier to receive and analyze compared to the deconvolution of a 2D exoplanet image.
I like the idea that a signal is highly directional, reducing teh power requirement, and does not assume we have the technology of a telescope pre-aligned with teh transmitting system. This means that we can use Gaia-like ‘scopes to detect stars with periodic brightness increases in a single wavelength are perfectly possible to build and deploy to look at the whole sky. At some point, the signal should be detectable.
If the calculations are correct, this seems like a logical next step for SETI.
It seems like this technique assumes ET already knows planet earth is inhabited and the details of our orbit. So their transmissions would be initiating communications. We would have to decide: Answer or ‘Do not answer!’
If all we have to do is observe for stars that grow brighter in a mono-band, it seems this would be a low cost way to listen for a door bell.
If one has a monochromatic light source which means that all the photons are at the same wavelength as in a laser, then why do we need gravitational lensing which is only needed if one has a giant light source like a star or planet’s reflection? I can imagine a giant light in orbit around a star that we could see from a distant star system. We would need the same large light to seen information. The premise seems impracticable. One could turn the light off an on or dim it and make Morse code?
If we are talking about individual photons, then it’s the variations in their amount that makes the code? One would have to figure based on the inverse square law how many individual photon we arrive at the distant location and the strength of the light source, other wise we are back to the Morse code or computer language of zero an one?
Laser light being monochromatic does not spread out over long distances like ordinary light.
Over interstellar distances, laser light does indeed spread, which is why Robert Forward imagined a huge Fresnel lens in the outer system to collimate a laser beam sent from a power station near the Sun. The Turyshev paper explains the problem of ordinary laser communication at these distances in its introductory section. It also discusses the powerful signal that can be produced by gravitational lensing, and the reasons why it would be preferable.
Hi Paul,
in addition to diffraction problems, I told myself that the galactic Internet would be very complex to implement for an ETI since it would be perfectly necessary all the problems of power and direction of the beams of the signals which should be of a formidable precision but in addition calculate the differents positions of stars that could serve as a gravitational lens AND all drifts of probes, the travel time of the signal etc (as specified in the article) In short, all the the celestial mechanism…a such civilization would probably be very advanced ! If we start from a principle of economy of means, I am not sure that it is the best way to communicate…
…as much play a game of pool on a continent scale or it is necessary to place the ball in one – may be two – stroke !
But this topic is exciting…keep on dreaming :)
These may be older, but they contain very useful information on Optical SETI:
http://coseti.org/
http://seti.harvard.edu/oseti/
Then there is Laser SETI:
https://www.indiegogo.com/projects/laser-seti-first-ever-all-sky-all-the-time-search#/
Yesterday I submitted a couple of comments in reply to Alex. They aren’t posted yet I see a comment dated today. Were my comments lost?
Very sorry, Ron. Had a glitch yesterday when installing a needed software fix and the site briefly went down. I lost a message from Al Jackson that I recovered, but I don’t see your two comments. Apologies! If you want to re-submit them, of course I’ll push them right through.
Ok, I understand. Attempting to recreate them will take too much time so I’ll have to pass. My apologies to Alex that I’m letting the discussion drop.
tss tss …another diffraction angle story and the message missed :)
Earth might have been more interesting to ETI during the dinosaur era:
https://www.msn.com/en-us/news/technology/earth-was-more-attractive-to-aliens-back-when-dinosaurs-roamed/ar-AA1jJkAn?rc=1&ocid=winp1taskbar&cvid=13a76b86e16f4acaed5b5c16a2fd2a60&ei=14
The gravitational lens raises another communication question: let’s say A, an ETI that sends a direct message via a light beam to B, another ETI of quite same technological level. A expects a response from B*.
For a reason x, the luminous ray of the message of A is deflected by a gravitational lens and it is the earth, which will be called C, which receives the message.
Civilization A is not informed that its message has not arrived in the right place.
What happens to the three actors in this story? :)
* for example, “Darling, where did you put the keys to the flying saucer, they’re not on the on the kitchen table. Call me back – Kiss” :D
How Scientists Have Surveyed Over 500,000 Sources To Look For Evidence Of Intelligent Life
Story by Georgina Torbet • 2mo •
https://www.msn.com/en-us/news/technology/how-scientists-have-surveyed-over-500-000-sources-to-look-for-evidence-of-intelligent-life/ar-BB1hmfMO?rc=1&ocid=winp1taskbar&cvid=6f35911a3c294ee2e2aba74f4f6148aa&ei=10#
To quote:
To help with the search for technosignatures, researchers are now making use of the Karl G. Jansky Very Large Array (VLA), a set of 28 massive radio dishes located in New Mexico. These multiple dishes, or antennae, work together to form the equivalent of a single, enormous antenna, allowing them to detect faint signals from distant objects. Each dish is 82 feet across, holds eight receivers, and is mounted on a tripod-style mount which allows it to move and tilt to point at desired objects.
The VLA has a cutting-edge detector called the Commensal Open-Source Multimode Interferometer Cluster (COSMIC) from the SETI Institute, which is specifically designed for SETI research. Described in a paper in The Astronomical Journal, this is a digital tool that can search for technosignatures in a much more comprehensive and efficient way than previous tools, allowing researchers to process large amounts of data more quickly. It is also designed to enable the VLA to be used for other science research while performing its SETI work.
“The COSMIC system greatly enhances the VLA’s scientific capabilities. Its main goal of detecting extraterrestrial technosignatures addresses one of the most profound scientific questions ever. This topic was previously not possible with the VLA,” said Dr. Paul Demorest of the National Radio Astronomy Observatory. “By operating in parallel with projects such as the VLA Sky Survey, COSMIC will accomplish one of the largest SETI surveys ever while still allowing the VLA to carry out its usual program of other astronomical research.”
The idea is to use COSMIC to perform a survey of the sky, looking at objects, which are viewed as the array makes its observations, to see if they could be remarkable for SETI. The system has already been used to observe hundreds of thousands of objects, and researchers estimate it will be able to view millions of sources over the next few years.
“COSMIC introduces modern Ethernet-based digital architecture on the VLA, allowing for a test bed for future technologies as we move into the next generation era,” said COSMIC project scientist Dr. Chenoa Tremblay. “Currently, the focus is on creating one of the largest surveys for technological signals, with over 500,000 sources observed in the first six months.”
The researchers will search through observations from the Very Large Array Sky Survey (VLASS), a wide-scale survey which aims to map 80% of the sky across two years of operations. During that time it will catalog approximately 10 million radio sources.
The big plan for COSMIC, however, is to have it adapt to future technologies. It could be upgraded to look for other signals in different areas of astronomy such as searching for fast radio bursts or dark matter.
The paper is online here:
https://iopscience.iop.org/article/10.3847/1538-3881/ad0fe0
Perhaps aliens will use white dwarfs as SETI transmitter/receivers. They could have thousands of these craft in orbit about them using the SFL of the compact mass and the rapid orbital period of the those craft. There is on avergae 1 white dwarf about 8 to 10 light years from another MS star so are fairly common.
https://iopscience.iop.org/article/10.3847/1538-3881/ac8358/pdf
The paper asks two questions:
1- what is an interesting signal? is it its power level after receiving & filtering over a frequency range chooses or its own characteristic, such as a repetition or sequence that would not be natural?
2- in order to listen continuously to a possible signal sent by a probe “behind” a gravitational lens, a series of radio receiving probes could be placed not at the focal point of the earth (Green bank) but on an orbit and in a precise plane, perpendicular to the cone of reception of the signal and the direction of a star chooses? This is the idea of the butterfly net…oriented.