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://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://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.
Conclusion
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!
An analogy might be the use of the ionosphere to bounce radio waves around the curvature of the Earth.
Assuming this works, there is an advantage to the broadcasting civilization that its home star is not readily identifiable after several amplifications. This partially removes the predator problem for transmitters. The disadvantage is that this is a one-way transmission only. This might make sense if the possibility of communication is vanishingly small in any case.
One problem addressed is the transient nature of such a signal as the beams sweeps past the target due to relative motion. We might get a signal that says “Encyclopedia Galactica” only to find we only received the front cover and no content. However, that tantalizing hint might be all we need as a civilization to start our stellar beaming and even travel.
The author should contact the model builders for signaling to see how the ideas might impact the results. Instead of various shells randomly appearing, what would happen if the signal was amplified in some directions and also retransmitted in different directions? Does this fill the space more completely, leaving us with a series of noisy, overlapping signals that would be detectable in almost any direction we looked?
This is a remarkable idea. And it makes sense that we could use this development of Maccone’s idea of gravity lensing telescopes and communication channels, to create active EM signaling in precise locations near other stars.
That is, using these potential methods we could use the Sun’s gravity for placing controlled signals of nearly arbitrary strength at controlled locations around other stars and/or celestial phenomena beyond our Solar System.
If that’s possible, the idea of using these stellar Masers as amplifiers won’t be so preposterous, turning them into SETI beacons in our behalf.
Of course, light taking years or more for reaching any target, may turn this into an exercise of extreme patience and deliberation.
Gee, we could even use these methods for creating unmistakable signs of intelligence for other to see, like artificial stars, blinking with unambiguous intelligence over the skies of a living alien world, if we find them to exist.
Paul, should it be pulsars rather than quasars for pricision timing, quasars can be quite changeable.
It all just adds to my growing belief that there simply are no other civilizations that exist past, present or ever. We’re it. Let’s deal with it.
We’re probably it , but there is one more possiblity : Perhaps the only civilisation that survive are the ones that are NOT stupid enough to shout in a dark and unknown forest ….so perhaps we should try to listen very carefully before we use the power of a star to amplify our own stupidity …
I tend to agree. Despite the size and age of the galaxy.
But it’s flabbergasting. First, we find ourselves “here” on earth with no explanations and no reassurances, other than wishful thinking.
Then, upon understanding that we occupy no special place in the universe, not at the “center” of anything, so that it seems there “should be” lots of other civilizations in the same boat with us, there appears to be no one! Let’s face it, anyone slightly advanced could make it clear who they are or were with little effort on our part. They can see us if they look, they know we’re here. They could be here themselves if they cared. We’re only getting started and we are doing careful sky surveys, counting exoplanets, developing models of habitability. Anyone a few steps ahead would know plenty.
So, we get hammered from both ends, we’re not special, and we’re also alone. Great.
Oh well, “all these worlds are ours”, we might as well focus on some better and faster transportation.
“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”.
In the distant future another option might be a more reliable and feasible way.
https://asgardia.space/en/forum/forum/engineering-32/topic/catalysis-project-for-future-500/
Multiple artificial biosphere and appears in these civilizations themselves in this model are functionally the same as Von Neumann probes, only for long periods of time in billions of years.
And “… to be preserved forever”.
I’ve been working on the question of how to maximize the bandwidth of communication between two different star systems (homeworld and colony) and still keep the hardware cheap and simple. I knew about gravitational lens amplification and assumed that’s the right way to go, but now I’m really intrigued by the phased array idea you mentioned. I get how it would work for the sender, but what about the receiver? Could they just rely on the fact that the sender’s signal is directed well enough that it doesn’t need much further focusing on the receiver’s side?
A lot depends on the initial power and coherency of the originating signal. Obviously bandwidth and SNR will increase with increased effective receiver antenna size. So it may not require use of gravitational lens fosuing on the receiver size.
One also must note that sending and receiving signal via gravitational lens from earth restricts direction of transmission and reception in the direction of the orbital plane.
Bill
I read Webb’s book and was very unimpressed by his argument. Essentially, he looked at every “They could exist but we haven’t detected them because…” argument and concluded that each on its own was insufficient. Few of them were exclusive, but he neglected to consider them cumulatively. Then he proceeded with a kind of negative Drake equation in which also each factor was insufficient, but considered them cumulatively. This struck me as logically inconsistent, to put it politely.
I don’t see a strong analogy to von Neumann probes here. Maybe I’m misunderstanding how this is supposed to work. What it sounds like is that the sending civilization plans a very complicated path for the signal, something like a multi-bank pool shot. First we send the signal through this gravitational lens. Then it goes through this gas cloud maser. Then it goes on to another gravitational lens, etc. The transmission has to be aimed very precisely to follow this path.
But the sender plans a finite number of hops and after it passes the last hop it goes off in some random direction. There’s basically no chance of it continuing on to further hops that the sender didn’t plan for.
The fundamental feature of von Neumann probes is that they can go on to further hops that the original sender didn’t have to plan for. That’s what makes their expansion potentially unlimited.
I think these are neat ideas and might be used by civilizations for communication, but they wouldn’t result in the galaxy being filled by self perpetuating signals.
Your analogy of a multi-bank pool shot is correct. But this method would be much more cost effective than an omni-directional beacon which would take a gazillion watts of power.
However I make the anthromorphic argument that an ET may not use these types of signals to communicate with another ET, but instead to signal to the universe that human trait to say “I have been here”
Bill
On the ‘multi-bank pool shot’ analogy… surely this is no big deal as the first signal becomes self replicating in many, many directions. I imagine it as a branching tree where surely some twigs would sweep onto inhabitted systems eventually so you can forget about all the other branches that either fail or fall upon lifeless systems. It’s a numbers game as mentioned in the article… almost every possible multi-bank shot is doable if you have a trillion attempts ; with a single beam replicating and branching off a trillion times the odds become very favourable.
“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.”
Contemplating this: You certainly could create a wavefront from a phased array antenna near Earth that (Neglecting edge effects.) “looked” like it originated at the Sun’s focal point. It would likely have to be a fairly large phased array, but doable.
However, it could only look like this for a very, very small fraction of the area around the Sun that a signal actually originating at that focal point would have passed through. Basically that part of the area that the actual focal point would “see” looking through the phased array.
Anywhere outside that area would “see” the radiation originating from a *different* focal point, and send it in a different direction.
The Sun’s focal ‘point’ is at about 548 AU. A phased array near the Earth would be at 1 AU. A simple analysis of the geometry says that the pencil originating in that phased array would only be 549/548 larger as it passed the Sun. And THAT is how much you’d improve the diffraction limit of the system: By a tenth of a percent.
By contrast, if you start from the actual focal point, your diffraction limit is based on the entire area around the sun, at a minimum several AU wide.
Now, I suppose that, if you launched a large number of phased arrays, each of them could create a different pencil appearing to originate from the same focal point, and with sufficient phase control between them, you’d get the same effect, (Neglecting, again, the sparse array problem.) as transmitting from the focal point.
But that’s just to say that if you create a phased array several AU in size, you can achieve the same performance as using a single transmitter at the focal point. The Sun isn’t really doing any work here, is it?
No, I don’t think there’s actually any shortcut around putting something out at the focal point. Though it could possibly be just a modest sized passive reflector.
That’s a nice argument. Now if we wish to press it into actual service, a receiver for a Proxima b starshot seems a decent candidate. The problem here is that it may well take longer to install and station the receiver out at 550+ AU than it takes the probe to reach the exoplanet – say 40 years.
Shouldn’t; If you can do something like Starshot, you are well positioned to use mass beam propulsion for closer targets with much larger craft. Micro-sails traveling at relativistic speed would make a dandy mass beam.
Yabut you cannot brake
You don’t need to brake on a mission to the Sun’s focal ‘point’; It’s not really a point, it’s a line, anywhere past 548 AU is good. Mind, you don’t want to be going so fast that transmitting data back and forth becomes a bigger than necessary problem. But that shouldn’t be an issue, as you don’t want to spend resources getting there faster than necessary anyway.
Just launch the focal mission to about 3% of the speed of the Starshot mission, and you’ll reach the focus when you need to.
You mentioned the “WOW!” Signal.
Carl Sagan in Pale Blue Dot mentioned 11 other such signals found by Project METI which seemed to come from the plane of the Milky Way. And SETI has seen many such signals since, especially the one from TYC 1220-91-1. Maybe these are such VanNeumann signals?
Bill, a quick shout out of thanks for referencing the article Roger and I penned!
Your concept is wild and I’m going to keep it in mind as I build my parametric models.
Off topic here but I’ve wondered that since Earth’s bio-signature has been “oxygen positive” for almost a billion years, how many alien telescopes, spectrometers, and Kepler-like missions have aimed at us? How many hopeful doorbell rings have we missed? The cheapest and most efficient thing we can do is listen and watch in a extended manner.
ET search: Look for the aliens looking for Earth
Astronomers propose hunting for civilizations on worlds that can see our planet cross the Sun.
Alexandra Witze
01 March 2016
By watching how the light dims as a planet orbits in front of its parent star, NASA’s Kepler spacecraft has discovered more than 1,000 worlds since its launch in 2009. Now, astronomers are flipping that idea on its head in the hope of finding and talking to alien civilizations.
Scientists searching for extraterrestrial intelligence should target exoplanets from which Earth can be seen passing in front of the Sun, says René Heller, an astronomer at the Max Planck Institute for Solar System Research in Göttingen, Germany. By studying these eclipses, known as transits, civilizations on those planets could see that Earth has an atmosphere that has been chemically altered by life. “They have a higher motivation to contact us, because they have a better means to identify us as an inhabited planet,” Heller says.
About 10,000 stars that could harbour such planets should exist within about 1,000 parsecs (3,260 light years) of Earth, Heller and Ralph Pudritz, an astronomer at McMaster University in Hamilton, Canada, report in the April issue of Astrobiology1.
They argue that future searches for signals from aliens, such as the US$100-million Breakthrough Listen project, should focus on these stars, which fall in a band of space formed by projecting the plane of the Solar System out into the cosmos. Breakthrough Listen currently has no plans to search this region; it is targeting both the centre and the plane of our galaxy, which is not the same as the plane of the Solar System, as well as stars and galaxies across other parts of the sky.
The idea of searching for worlds whose inhabitants could see Earth transits dates back to at least the 1980s. But astronomers can now update and revise their ideas thanks to what they have learned from Kepler, Heller says.
Full article here:
http://www.nature.com/news/et-search-look-for-the-aliens-looking-for-earth-1.19439
To quote:
For four days in 2010, the Allen Telescope Array in northern California looked for signals coming from the region of space directly opposite the Sun, says Seth Shostak, an astronomer at the SETI (search for extraterrestrial intelligence) Institute in Mountain View, California. The goal was to test whether extraterrestrials might be timing any transmissions to reach Earth just as they see it transiting the Sun. No signs of aliens were found, and no follow-up is planned.
“Unfortunately, there are more good ideas for SETI experiments than there are SETI experimenters to act on them,” says Andrew Siemion, an astronomer at the University of California, Berkeley.
In the next five or so years, the European Space Agency’s Gaia satellite is likely to discover most of the nearby stars in the Earth transit zone, Heller says. Until then, he and Pudritz plan to use data from K2, the Kepler follow-on mission, to hunt directly for planets in the zone — and to look for aliens who might be looking for us.
The paper online:
The Search for Extraterrestrial Intelligence in Earth’s Solar Transit Zone
To cite this article:
Heller René and Pudritz Ralph E.. Astrobiology. April 2016, 16(4): 259-270. doi:10.1089/ast.2015.1358.
Published in Volume: 16 Issue 4: April 15, 2016
Online Ahead of Print: March 11, 2016
http://online.liebertpub.com/doi/pdf/10.1089/ast.2015.1358
“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 exactly what we did!
https://arxiv.org/abs/1506.00055
It was originally sent back by the reviewer, but we are working on revising it this summer.