Given that interstellar communications have been on my mind recently, I was delighted to receive this essay from Don Wilkins. Based in St. Louis, where he is a now-retired aerospace engineer, Don has plenty of experience in avionics and has the chops to know how to make widely-dispersed aircraft talk to each other. Here his scope is a bit wider: What are the implications of ‘lurker’ probes, the conceivably ancient (or newer) technologies from an extraterrestrial civilization that might be monitoring our planet? If such exist, their communications become a SETI target, and the question of how their network might operate is an intriguing one. I had no idea, for example, that the idea of gravitational lensing for such communications had made its way into the SETI field, but Don here acquaints us with several studies that tackle the concept, along with other insights as found below.
by Don Wilkins
If an expansionist star faring civilization exists, it is likely to construct an interstellar communications and control network, beaming information and commands across interstellar depths.[1-7] Information concerning discoveries along with status of developments within a system, to avoid duplication of effort, as an example, are transmitted at high data rates using a star’s gravitational field as a focusing element. Such concepts are familiar to readers of Centauri Dreams.
Failure to find “von Neumann probes” in the Solar System can be related to a hypothesis and a fact. The hypothesis posits aliens hide their presence from a developing civilization until the appropriate time to formally contact the younger society – or perhaps the aliens only want to observe, rather than interact with primitives.
A von Neumann probe is difficult to locate in the vast, uncharted spaces of the Solar System. The Solar System volume is approximately 5 × 1014 cubic astronomical units (AU3) when measured to the outer boundaries of the Oort cloud. Eight planets, hundreds of moons, asteroids, comets and “empty” space provide a multitude of nooks and crannies shy aliens might use to avoid curious neighbors.
An interstellar communications network is characterized by very long delay paths, frequent network partitions needed for delay tolerance and error correction.
The inverse square law mandates a reduction in data rates by three-fourths if the distance between a receiver and transmitter is doubled. If a mesh network links interstellar probes, data rates can be increased orders of magnitude over a communications system relying on direct messaging among the probes.
Consider: The home world of the aliens may be tens of thousands of parsecs distant from a hypothetical node in the Solar System. A mesh of nodes woven through the stars and, located at convenient points in interstellar space, such as nebulae, provide three advantages. Reduced distances between nodes enable faster transmission rates using the same power. A multitude of dispersed nodes provides enhanced reliability through alternate routes to the preferred destination. The mesh network is self-routing avoiding slow, risky centralized communications architectures. These advantages come at the cost of message complexity but are worth the trade-off.
As an aside, if communications relays are located to optimize bandwidth, it may be difficult, if not impossible, for a civilization such as ours to intercept tight beamed transmissions among the probes. Even if a transmission is intercepted, and if the aliens use store and forward architectures and use multiple frequencies to transmit portions of the messages, only combining transmissions at the receiver, it may be impossible for us to identify signals as of intelligent origin or translate them into a coherent message. Transmissions are burdened with metadata, routing information, further complicating understanding of interceptions, Figure 1.
Figure 1 Message Encapsulation for Network Transmission
Given unknown compression techniques, error correction methods, message structure, and splitting messages among different frequencies and routes, intercepting and understanding traffic on the alien network may be impossible.
Frank Drake and others proposed direct communications using simple messages among the stars. “Hello” messages are sequences of bit strings easily reformatted into images depicting the Solar System and humans. If alien probes established interstellar mesh networks, it is highly probable those networks ignore Drake-type messages.
Von Neumann probes could explain the Great Silence. If robots are spreading into every nook and cranny of the Galaxy, the efficiency of radio transmissions as the contact medium is open to doubt. The physical presence of a probe in a distant star system removes any doubts about the nature of the signal. An alien spaceship provides instantaneous interchanges rather than slow century long conversations. Information density as represented by the probe is considerably greater than a radio signal could provide. Aliens may wait for contact through probes rather than relying on energy-hungry beams subject to misunderstanding launched into the unknown.
Researchers hypothesize signals intended for solar gravitational lens receivers could serve as technosignatures of advanced alien civilizations. Figure 2 diagrams a link between a transmitter in orbit about an alien star and a receiver within the Solar System.
Figure 2 Architecture of a communications network based on an Einstein Ring [8].
In addition to detecting stray communications from distant transmitters, other detection methods are possible. A possible technosignature is light reflected from a station-keeping light sail. Assuming the communications node uses a light sail to maintain the system’s position, the sail reflects the Sun’s light back into the Solar System. A “star” whose spectrum suspiciously matches that of the Sun could point to the existence of an alien communications node.
Another technosignature possibility is an area of space which is slightly warmer than expected. This could be a clue that something is trying to hide by spreading its waste heat into a low temperature, innocuous blob.
If an alien probe is located, we could message the robot. “Active ” SETI, where humans transmit to aliens, rather than listen for alien communications, has roused protests from those who worry aliens are hostile. Communicating with alien nodes within the Solar System does not provide aliens with information which they do not already possess and avoids the concerns of active SETI opponents.
Researchers avoid the issue of identifying a signal by content. An alien relay using gravitational lensing is effectively motionless in relation to the Sun. Monochromatic signals should present Doppler shifts resulting from the Earth’s orbit and rotation assuming terrestrial based sensors.
Kerby and Wright generated criteria for a long-duration node using gravitational lensing [8]:
1. Close or moderately-distanced companion stars impart huge delta-v costs on a relay spacecraft, and thus close binary or multiple-star systems should be avoided.
2. The presence of gas giant planets either limits the maximum gain total of the stellar relay (depending on the reflex semimajor axis imparted on the host star) or imparts a delta-v cost of a few m s−1 per year to maintain alignment.
3. A more massive host star requires proportionally higher delta-v costs to resist its inward gravity.
4. A host star that is rapidly rotating will be deformed away from a spherical shape, resulting in a lens that does not have an exact focal point from which to transmit, resulting in significantly lowered gains.
The search for alien communications nodes orbiting nearby stars has already begun,
Table 1. Nearest network node: location priority list (top 10)
Note: In game theory, a Schelling point is a choice made without communication among the involved parties. Sagittarius A* may be chosen as a communications hub by aliens simply because it is such a visible feature located in the center of the Galaxy.
M. Gillon and other researchers examined Wolf 359 and TRAPPIST–1 for evidence of communications nodes in the vicinity of those stars.[5] No evidence of alien communications nodes was uncovered although there are any number of possible explanations for failure. The search continues and, contingent on funding, will expand to between ten and twenty stars.
For six months, a team led by G.W. Marcy searched for continuous and short period laser emissions from Proxima and Alpha Centuri. [6] Search equipment can detect power levels as low as one hundred Watts. No artificial laser light was observed in frequencies between 380 to 950 nanometers.
From the paper:
The intended recipient of the transmission is located at an unknown location near Proxima Centauri, leaving uncertain the most likely location of the local transmitter. Thus, during a year of observations, a transmitter located near the SGL focus line could appear within a region of roughly 400 arcsec from the coordinates given above. Parallax and aberration can be predicted for any instant, but the unknown location of the receiver near Proxima Centauri leaves the SGL location unknown within roughly 60 arcsec. We search within the full 400 arcsec domain for both sub-second pulses and long-lived emission of monochromatic sources. Our total field of view of 2.2 x 2.2 deg easily includes that 400 arcsec domain. Exploring the relatively large area around the anti-solar position of Proxima Cen expands the survey to include emitters located off-center that target only a fraction of the focal ring surrounding the Sun.
Two telescopes were used in the search. Candidate detections in the first were checked against exposures taken in the second telescope. No correlations were found.
Finally, a team used the Green Bank Telescope (GBT) and Breakthrough Listen (BL) backend to listen in the L and S bands for nodes using the Sun as for gravitational focusing [7]. The search covered possible nodes for Alpha Centauri AB system and HD 13908. This search was also unsuccessful although the work was regarded as proof-of-concept.
Searching for nodes in an interstellar network with a terminus within the Solar System has just begun. Earth based sensors can make relatively low-cost searches for Lurkers. Even if the probability of success is low, the enormous rewards of success merit the investment.
References
1. M. Gillon, A Novel SETI Strategy Targeting the Solar Focal Regions of the Most Nearby Stars, Acta Astronautica 94, 629 (2014)
2. M. Hippke, Interstellar Communications I Network, Overview and Assumptions, arXiv 1912.02616v2 (2019)
3. M. Hippke, Interstellar Communications II Deep Space Nodes with Gravitational Lensing, arXiv 2009.01866v1 (2020)
4. M. Hippke, Interstellar Communications III Locating Deep Space Nodes, arXiv 2104.09564v1 (2020)
5. 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)
6. G.W. Marcy, S.K.Tellis, and E.H. Wishnow, Laser Communications with Proxima Centauri using the Solar Gravitational Lens, Monthly Notices of the Royal Astronomical Society, 509-3, 3798-3814, https://arxiv.org/ftp/arxiv/papers/2110/2110.10247.pdf, (2022)
7. Nick Tusay, et al, A Search for Radio Technosignatures at the Solar Gravitational Lens Targeting Alpha Centauri, arXiv 2206.14807v1 (2022)
8. Stephen Kerby and Jason T. Wright, Stellar Gravitational Lens Engineering for Interstellar Communication and Artifact SETI, The American Astronomical Society, 2021 November 19, https://iopscience.iop.org/article/10.3847/1538-3881/ac2820
Our fixed nodes have somewhat conditioned us to assume that the interstellar relays need to be as fixed as possible as their tight beams from star to star and the SGL is tightly defined, requiring the relay to follow it as it moves with the star.
But can we use that movement as a feature? For a star with a lower SGL start and therefore a higher orbital velocity for the relay, the relay can target a number of destination star nodes as it orbits its star. By using a number of relays in different orbits, these small relay constellations might serve as nodes for many stars in the network. High-frequency em signals in the gamma/x-ray wavelengths could ensure high data rates coupled with short pulses. The closer orbits would also facilitate energy collection as the star’s energy flux would be greater.
For destination nodes like our sun with a distant SGL start, the signal source would have to be fairly fixed and so it might receive parts of a message from several of the source star’s relays in the constellation, and similarly, that destination would need to piece together a longer message partially received by a number of the faster-orbiting relays.
To create a mesh network where each star can send messages to several stars to ensure transmission and receipt reliability, I would expect any node in our outer system to have a number of relays to handle this need.
Lastly, we always assume that the transmitter/receiver must be on the SGL for any given transmission. Is this necessary? Suppose we use a very large mirror somewhere on the SGL. The transmitter/receiver could be much closer to the sun. The signal to be received is reflected by the mirror that tracks the relay in the inner system to send the reflected signal. Similarly, to transmit, the relay in the inner system would send the signal to the mirror that would then use the star’s gravitational field to allow the beam to be efficiently sent to the destination star.
IDK if the mirror needs to be conventional, i.e. a reflective surface for the em tradition, a mirror using metamaterials, an active surface that collects the incoming signal and retransmits it, or possibly a swarm of tiny reflectors much like the early space experiment (Project West Ford that dispersed copper needles into MEO to reflect radio signals. A sphere of such reflectors surrounding our sun could be used to send signals to any target star emitted by a transmitter in the inner solar system that targeted the point on the sphere best suited to use the gravity lens of the sun to send the message to the destination node. Such a constellation of “reflectors” would be between the outer edge of the Kuiper Belt and the inner edge of the Oort Cloud and could be indistinguishable from natural objects by our telescopes.
I am reminded that by using computation one can reassemble images from many imperfectly reflecting surfaces, not unlike deconvolving an image from the Einstein Ring.
As our technologies improve and demonstrate more innovative approaches, so should the possibilities of the architecture of these “lurker relays” expand.
What I want to know is if Earth passed through any star system’s focal lines during the WOW signal.
Three phys.org stories of interest:
Researchers develop neutron-shielding film for radiation protection
New technique could speed up the development of acoustic lenses, impact resistant films and other futuristic materials
Laser pulse compression by a density gradient plasma for exawatt to zettawatt lasers
The focal line becomes noisier as the Einstein Ring radius becomes larger to focus further from the lens. AFAIK, the usable SGL for the solar system is about 550-1000 AU.
The smaller the star, the less lensing and the longer the focal line. However, there are no stars that generate a focal line light-years long.
However, planets have much longer focal lines, which I think was addressed in a CD article some time ago. Now it is starting to appear as though “rogue planets” are more numerous in space than we thought. Suppose one came close enough to our system so that its focal line did sweep past Earth? Purely speculative as the WISE IR telescope did not detect any such planets near our system, but “absence of evidence is not evidence of absence”.
The most likely explanation of the Wow! signal is RFI, the same explanation that was hunted down for the recent Breakthrough Listen signal.
This is why white dwarfs will be so important, they have huge amounts of energy around the start of their GFL’s, many times the flux at earths distance and can scan sections of the sky in less than a week that would take thousands of years around the sun. An issue will be high orbital velocities which will tend to blur the signals to very short durations. This issue can be resolved with constellation probes as you have stated. Also you don’t have to have sun to sun comms, a deep space probe could also send to the suns GFL but the signal is greatly reduced.
I need to correct this as white dwarfs can be quite dim, Sirius b is a lucky exception, their luminosity is heavily dependant on the their mass and age. But having said that they still give out on average a lot more power than going to the suns GFL.
Heartening to see that they are beginning to “look around”. Decades were spent in looking at severely constrained regions of space and time in the hopes of hitting a jackpot. But sense is now beginning to prevail.
Regardless of results, one can derive some satisfaction from the methods.
Just interesting if these ring lasers would be better for transmission as most of the power goes into the ring to be bent around the gravity focus.
https://www.holoor.co.il/application/optical-vortex-phase-plate-application-notes/
Using a lens implies your beam is not perfectly aimed to begin with. But an array of decentralized probes can do “beamforming” to aim a signal very precisely, if it is large enough, coordinating their minuscule solar-powered transmitters to reach a distant target in phase with each other. How well have the Stardust samples been examined for potential logic circuitry or transmitters?
The beam emanating from the Einstein ring is nearly perfectly parallel – better than a laser in terms of dispersion. The rest of the signal from a low-power transmitter would rapidly become noise and quickly indistinguishable from the background.
I would be very surprised if a signal could be made that would travel light years without much loss using other means.
Quantum entanglement would work even better – but it cannot be used to send information AFAIK.
I certainly don’t know much about communications, but here’s what I’m thinking. https://www.youtube.com/watch?v=MYvV9LomUIM (15:34) shows a gain of 1991174 = 63 dB for Barnard’s Star. Using two stars, that is perhaps tens of trillions (I didn’t spot the gain numbers for the others). According to https://www.commscope.com/globalassets/digizuite/542044-Beamformer-Explained-WP-114491-EN.pdf page 10, we can get a gain of 10 log10 N from a half-wavelength phased array. So if our aliens have 1991174 transmitters squared, that is like a Barnard’s Star-Barnard’s Star bridge, at least.
This is still daunting for humans. But picture an array of 100 trillion probes, each weighing a microgram (I’ll assume Moore’s Law isn’t quite tapped out yet). They would require only 100,000 tonnes of material overall, about an aircraft carrier’s weight. I would assume that the aliens don’t need to space out their probes regularly nor rely on reflecting a light source emitted in phase – instead they have a swarm of probes, each of which talks regularly to its neighbors and knows where it is in the solar system to within a fraction of a micrometer. The probe could simply emit an EM wave with the right phase to add up at the destination. Alternatively, if there is transmitter gain, that factors into the equation given by the chapter I linked above. The alien probes could emit in any direction without any need to move, and probably would also sample and deconvolute EM radiation coming from all directions continually. The most intriguing part is, if our alien visitors were ambitious and placed exponentially more probes than I suggested, perhaps some of them might be available for us to find in any decent sample of interplanetary dust.
The beamforming reference is just a variant of any phased array of signals from radar to lasers. There is just interference patterns that creates lobes of intensity that can increase the signal strength in a lobe that is greater than what would be the case with a single radiating signal. Note that none of the interfering waves create a parallel beam, just a transient interaction at some point away from the transmitter.
While the number of transmitters increases the beam strength in the main lobe, it seems like an unreasonable extrapolation of transmitters to hope for a high lobe strength light years away, as each beam is subject to 1/r^2 signal decline.
What you are hoping for is some N photons to meet in phase at some point in space, i.e. a target stellar system, that [partially] overcomes their signal strength loss. This is in comparison to an ER annulus that is nearly exactly a parallel beam over the same distance and can be refocused by the same means.
I agree that trillions of highly miniaturized transmitters could, in principle, radiate lobes in all directions in a serial manner, obviating the limitation of the SGL directionality, but whether this makes up for what is like a loss of signal strength seems doubtful to me.
Despite the practical difficulties of aiming the gravitationally focused beam at the target star, this still strikes me as the most clever method of overcoming signal loss over interstellar distances. I would certainly deploy the trillions of relays around the SGL minimum distance radius sphere instead of as a beam-forming phased array.
I will defer to a comms expert to correct my understanding.
White dwarfs may be the best option but we should keep in mind that SGL communication satellites don’t need a lot of power. Dormant black holes may be a contender. A SGL closer to the host body means orbital velocities will be lower than at Earth’s SGL but skew distances to remain on target or change targets will be shorter.
I would add inscription to the list of reasons why an artificial signal may be indistinguishable from natural signals. The list is long enough to make it unlikely that we can discover anything but a signal intentionally sent to us.
I tend to agree. With encrypted, spread spectrum, transmission, we hope to see frequencies with higher strength. But SGL transmission spreads the signal across the Einstein ring and is therefore best detected by a receiver at the focal point. We would need very sensitive detectors to intercept and detect the signal before it was focused.
OTOH, a signal that was meant to be detected could be retransmitted from the receiver (possibly using other transmitters in our system) and aimed at Earth. The signal source would be obfuscated protecting the origin of the transmitting planet.
Signal detection would not be accidental, but deliberate.