Is it possible to use natural phenomena to boost signals to the stars? In the essay below, Bill St. Arnaud takes a look at the possibilities, noting that civilizations that chose to broadcast information might select a method that mimics by electromagnetic means what the classic von Neumann probe would achieve with physical probes. St. Arnaud is an optical communications engineer, a network and green IT consultant who works with clients on a variety of subjects such as next generation research/education and Internet networks. His interest in practical solutions — free broadband and dynamic charging of electric vehicles — to reduce greenhouse gas emissions is matched by a fascination with interstellar matters, particularly SETI.
By Bill St. Arnaud
In their recent post on Centauri Dreams Roger Guay and Scott Guerin (https://www.centauri-dreams.org/?p=36802) make a compelling argument that fading electromagnetic halos may be all that’s left for us to discover of an extraterrestrial civilization. They argue that there is only a short window in the evolution of a sufficiently intelligent species in which it will broadcast its presence through inefficient electromagnetic transmission of radio, TV and radar signals.
Current SETI searches assume that an advanced civilization will use extremely powerful omnidirectional transmitters or highly directional and focused beacons targeted at our solar system. The challenge with either approach is the fact that successful one-way communication between intelligent species is dependent on the “L” term in the Drake equation. L represents the length of time for which such civilizations release detectable signals into space. If L is relatively short then the possibility of two separate intelligent civilizations being coincident in time to send and receive a signal is very small, as demonstrated by Guay-Guerin. So even though there may have been many intelligent civilizations we will probably never be aware of their existence.
On the other hand, Stephen Webb, in his book If the Universe is Teeming with Aliens .. Where Is Everybody? argues that we may be the only intelligent civilization in our galaxy, if not perhaps the known universe. This is often referred to as the Rare Earth Hypothesis. Given the large multiplier of improbabilities from the creation of simple life, through the prokaryotic-eukaryotic transition, and the many divergent evolutionary pathways in human evolution leading to technology savvy beings, Webb argues that the odds of this being replicated elsewhere in the universe are extremely low.
The bottom line from both Guay-Guerin and Stephen Webb is that regardless of whether the universe is teeming with advanced civilizations or limited to only a very few it would seem that the probability of detecting a SETI signal by conventional means is very limited.
In another line of reasoning in SETI exploration it has been suggested that we look for physical artifacts as well as electromagnetic signals. These artifacts could include devices like Von Neumann probes and physical remains of past advanced civilizations.
A Von Neumann probe is a self replicating spacecraft that would travel from stellar system to stellar system through a galaxy where it would seek out raw materials extracted from planets to create replicas of itself. These replicas would then be sent out to other stellar systems.
Searching for small physical artifacts such as Von Neumann probes would seem to be even more daunting than looking for the proverbial “needle in the haystack” electromagnetic signals. If a civilization were advanced enough to launch artifacts through space you would think at some earlier stage in its existence it would have initially deployed electromagnetic beacons or omnidirectional broadcasts. Of course this is assuming that they do want to make their presence aware to other advanced civilizations.
Von Neumann Signaling
But perhaps there is another approach to SETI that avoids many of the challenges of looking for physical artifacts and the limitation of the possibility of a small L in the Drake equation. Maybe we can look for “electromagnetic artifacts.” Electromagnetic artifacts can be thought of as “virtual” Von Neumann probes where instead of having physical devices replicate and propel themselves through the galaxy, electromagnetic signals are initially transmitted where their signaling properties are designed to be amplified and replicated using natural stellar and physical processes. Such natural processes might include gravitational lensing to refocus signals and using stellar lasers or masers to amplify a given signal.
Such self amplifying and replicating electromagnetic signals are different than normal transmissions used in beacons in that they are not intended to be point to point communications. Like Von Neumann probes they are expected to randomly propagate through a galaxy using passing stellar systems to amplify and replicate the original transmission. This capability might allow electromagnetic Von Neumann probes to propagate throughout a galaxy much faster than physical probes. The advantage of a low cost self amplifying and replicating signal is that you only need one replicant signal in a billion or trillion to multihop many hundreds of stars and be detected by another civilization.
Gravitational lensing has been used in astronomy for some time. In an interesting post (https://www.centauri-dreams.org/?p=10123), Claudio Maccone calculates that by using gravitational lens and with satellites at the appropriate focal points of each solar system a detectable radio signal could be sent from our solar system to Alpha Centauri that uses less than 10-4 watts, i.e. one tenth of a milliwatt!!
Normally a signal sent from earth would not benefit from gravitational lensing as the focal point would be too far out (past Pluto). The signal might be bent but otherwise it will quickly suffer dispersion like any other signal. To achieving lensing a signal must pass both sides of the sun at roughly the same time and the wavefront recombine coherently on the far side.
One solution would be to put satellites at the focal point as Moccone has suggested. But an easier earthbound solution is to use earth-bound phased array antenna. A signal generated by a phased array could be made with a wavefront that looks like it originated from the Sun’s focal point or even a more distant point. You may want to use a more distant (or closer) artificial focal point in order to use gravitational lens of a more distant star; i.e launch our signal so that it is deliberately out of focus (but collimated) by our sun but comes into focus at a distant star. These are the same principles used in multi-lens cameras or telescopes. The converging point of the signal could be at the focal point of a distant star or perhaps even a multi-hop star.
Once you have a collimated signal with a coherent wavefront and wide aperture (i.e. the Sun’s diameter) you could in theory hop many stars that are in line with our orbital plane (or will be by the time the signal gets there). You could also steer the signal up and down a little bit from the orbital plane with a phased array antenna.
Additional amplification could use natural masers/lasers in our sun or distant star. There are several suitable natural maser frequencies – the choice of appropriate frequency will depend on the types of stars we are aiming at.
Stellar masers have been known for some time. A maser emission may be created in molecular clouds, comets, planetary atmospheres, and stellar atmospheres. They are frequently used in radio astronomy as they provide important information on distant stellar objects, such as temperature, velocity, etc. The first “natural” laser in space was detected by scientists on board NASA’s Kuiper Airborne Observatory (KAO) in 1995 as they trained the aircraft’s infrared telescope on a young, very hot, luminous star in the constellation Cygnus. Since then many other examples of both planetary and stellar lasers have been found.
The problem with natural masers/lasers is their noise level. The same is true of gravitational lensing if the signal passes through, or close to the corona. To extract the signal one would need a reference clock – and this would be the tell tale signal that it is artificial. I would theorize a good reference clock would be a distant highly regular timed quasar.
In effect we have created a beacon, but rather than looking in the water hole, a receiving civilization would have to look at the known natural maser/laser frequencies and then auto-correlate the signal to see if they can extract a reference clock. This is the same technology we use in very long baseline interferometry used in deep space radio dishes.
Now the interesting thing about natural masers/lasers is that they can amplify a given signal not only in the line of propagation but in other orthogonal directions as well. If the signal maintains its coherent wavefront ( still needs to be verified) then a given signal can be replicated in many directions from a given star. If it is still collimated then it would also look like a beacon pointed in some unknown random direction. A single star could produce many beacons like a disco ball based on the original transmission.
On a small scale, real world examples of self amplifying and replicating electromagnetic signals already exist. They are called Long Delayed Echoes (LDEs). They were first discovered in 1927 by amateur radio enthusiasts who noticed echoes of their original radio transmissions delayed by up to 40 seconds.
Up to now I have only been talking about signalling from our limited knowledge. I suspect there are other stellar phonemena, like with long delay echoes, that could be used to replicate and amplify signals.
There is no clear agreement on what causes LDEs, but there are several hypotheses on some possible natural phenomena that may enable electromagnetic echoes. These include such things as reflections from distant plasma clouds originating from the sun, magnetosphere ducting, mode conversion and four wave mixing, etc. While these natural phenomena may not be suitable processes for interstellar electromagnetic transmission they do demonstrate the possibility that perhaps equivalent stellar processes could be used on a larger scale to amplify and replicate electromagnetic signals much greater distances.
Given that they depend on natural physical processes for amplification and replication, the originating transmission will likely not need to be that powerful or directional. It is conceivable that low power transmissions are that all is required to launch a self amplifying and replicating electromagnetic probe. Most importantly, with electromagnetic replication a single instance of the signal may be replicated thousands or millions of times as it propagates through the galaxy or the universe. Compared with physical probes replication could accelerate on an exponential scale increasing the probability of detection, particularly in a ‘rare Earth’ situation.
Issues to Be Surmounted
Although using self amplification and replication sounds like an interesting idea there are a number of theoretical and physical challenges that still must be addressed. How, for example, to account for the proper motion of our sun versus distant stars? Will any such signal just sweep by like a beam from a lighthouse and make detection near impossible (e.g., the ‘Wow’ signal?) Other issues include accuracy and phase noise in phased array antenna – how precisely can we control a given signal?
Thermal noise in stellar atmospheres that are to be used for laser/maser gain is clearly a major issue. The “gain” of a stellar laser or maser is also very limited as there is no resonant cavity. There are also a host of well known problems with current inter-stellar electromagnetic signaling such as attenuation, dispersion, group delay, etc etc. In addition, the proper motion of our solar system and that of any intermediate amplifying and replicating stellar system would seem to make detection difficult.
To address these limitations in detecting such a signal it would be useful to explore how we might deploy a self replicating electromagnetic signal given our current technology limitations. Many techniques currently being used in modern radio and optical communication systems could be deployed to launch a self replicating and amplifying electromagnetic probe.
Clearly an external reference clock or coding reference would be required to extract any signal that was amplified by a stellar laser/maser as the inherent stellar noise would mask any external signal. A quasar may provide such a reference signal. Phased array antennae and signal preconditioning could be used to take advantage of gravitational lensing without placing transmitters at the lens focal point.
Gravitational lenses also act as gradient amplifiers and with time delay from two phased array sources it might be possible to regenerate a given signal (such as timing, shaping etc) using all electromagnetic techniques – a process now largely done by electronics. By constantly steering the phased array transmitter(s) a signal could be directed like a beacon at nearby stars that are aligned with our orbital plane to take advantage of the sun’s gravitational lens. Similarly steering of the phased array might allow a given signal to converge at the gravitational focal point of a nearby star where it could be amplified and replicated by that star to be propagated to even more distant stars.
With a little imagination and speculation on the future direction of these technologies a self amplifying and replicating electromagnetic Von Neumann probe might be within our technology capability. Once we have identified a plausible approach on how we would deploy such signals, the obvious next step would be to see if we can detect such signals.
Up to now we have always assumed that a distant civilization would want to send a direct beam at us and so we have been exploring that part of the electromagnetic spectrum that has the least absorption and attenuation. But if a distant civilization discovered it could use natural low cost processes to amplify and replicate a signal that would be its preferred route, especially if intelligence is a rarity in our galaxy. With laser/maser replication millions or billions of signals could be traversing the galaxy, of which only one needs to be detected by another civilization. This would be a much cheaper approach from an energy perspective than building omni-directional antennas, Dyson spheres, or aiming a beacon at our Sun, etc.
The assumption here is that a distant civilization wants to make contact with us, but I suspect that self replicating and amplifying signals will be transmitted for much more mundane reasons. If a self amplifying and replicating signal can be transmitted practically forever, really cheaply, then forget about contacting other civilizations, I want knowledge of my brief presence here on earth to be preserved forever. Paradoxically, religion and belief in the hereafter may be a driving force to transmit such signals!