The old trope about signals from Earth reaching other civilizations receives an interesting twist when you ponder just what those signals might be. In his novel Contact, Carl Sagan has researchers led by Ellie Arroway discover an encrypted TV signal showing images from the Berlin Olympics in 1936. Thus returned, the signal announces contact (in a rather uncomfortable way). More comfortable is the old reference to aliens watching “I Love Lucy” episodes in their expanding cone of flight that began in 1951. How such signals could be detected is another matter.
I’m reminded of a good friend whose passion for classical music has caused him to amass a collection of recordings that rival the holdings of a major archive. John likes to compare different versions of various pieces of music. How did Beecham handle Delius’ “A Walk in the Paradise Garden” as opposed to Leonard Slatkin? Collectors find fascination in such things. And one day John called me with a question. He was collecting the great radio broadcasts that Toscanini had made with the NBC Symphony Orchestra beginning in 1937. His question: Are they still out there somewhere?
Image: A screenshot of Arturo Toscanini from the World War II era film ‘Hymn of the Nations,’ December, 1943. Credit: US Office of War Information.
John’s collection involved broadcasts that had been preserved in recordings, of course, but he wanted to know if somewhere many light years away another civilization could be listening to these weekly broadcasts, which lasted (on Earth) until 1954. We mused on such things as the power levels of such signal leakage (not to mention the effect of the ionosphere on AM radio wavelengths!), and the fact that radio transmissions lose power with the square of distance, so that those cherished Toscanini broadcasts are now hopelessly scattered. At least John has the Earthly versions, having finally found the last missing broadcast, making a complete set for his collection.
Toscanini was a genius, and these recordings are priceless (John gave me the complete first year on a set of CDs – they’re received a lot of play at my house). But let’s play around with this a bit more, because a new paper from Reilly Derrick (UCLA) and Howard Isaacson (UC-Berkeley) tweaks my attention. The authors note that when it comes to the leakage of signals into space, a 5 MW UHF television picture has effective radiated power of 5 x 106 W and an effective isotropic radiated power (EIRP) of approximately 8 x 106 W. ERP tells us the strength of an actual signal in a specific direction, while EIRP describes an isotropic ideal antenna.
It’s interesting to see that a much more powerful signal than our TV broadcasts comes from the Deep Space Network as it communicates with our spacecraft. Derrick and Isaacson say that DSN transmissions at 20 kW power have an EIRP of 1010 W, making such signals (103 times higher than leakage, thus more likely to be detected. With this in mind, the authors come up with a new way to identify SETI targets; viz., find stars that are within the background of sky positions occupied by our spacecraft at such times as the transmissions to them from the DSN were active.
Ingenious. Remember that identifying interesting SETI targets has led us to study such things as the ‘Earth Transit Zone,’ which would identify stars so aligned as to be able to see transits of Earth across the Sun. In a similar way, we can study stars whose ecliptic planes align with our line of sight to intercept possible communications in those systems. It turns out that most of our outbound radio traffic to spacecraft occurs near the ecliptic, and the assumption would be that other civilizations might do the same.
Image: With the Pluto/Charon flyby, we have performed reconnaissance on every planet and dwarf planet in our solar system, with the help of the Deep Space Network. The DSN comprises three facilities separated by about 120 degrees around the Earth: Goldstone, California; near Madrid, Spain; and near Canberra, Australia. Above is the 70-meter Deep Space Station 14 (DSS-14), the largest Deep Space Network antenna at the Goldstone Deep Space Communications Complex near Barstow, California. Credit: NASA/JPL-Caltech.
So what Derrick and Isaacson are doing is an extension of this earlier work (see SETI: Knowing Where to Look and Seeing Earth as a Transiting World for some of the archival material I’ve written on these studies). The authors want to know where our DSN signals went after they reached our spacecraft, and that means building an ephemeris for our deep space missions that are leaving or have left the Solar System: Voyagers 1 and 2, Pioneer 10, Pioneer 11, and New Horizons. They then consult the positions of over 300,000 stars within 100 parsecs as drawn from the Gaia Catalogue of Nearby Stars, checking to ensure that the stars they identify will not leave the radius of the search in the time it takes for the cone of transmission to reach them.
The Voyagers were launched in 1977, with both of them now outside the heliosphere. As to the others, Pioneer 10 (launched in 1972) crossed the orbit of Neptune in 1983, while Pioneer 11 (launched in 1973) crossed Neptune’s orbit in 1990. New Horizons, launched in 2006, crossed Neptune’s orbit in 2014. All these craft received or are receiving DSN transmissions, though the Pioneers have long since gone silent. Their ephemerides thus end on the final day of communication, while the other missions are ongoing.
The universe is indeed prolific – the Gaia Catalogue includes 331,312 stars within 100 parsecs, and as the authors note, it is complete for stars brighter than M8 and contains 92 percent of the M9 dwarfs in this range. We learn that Pioneer 11 – I should say the signals sent to Pioneer 11 and thus beyond it – encounters the largest group of stars at 411, while New Horizons has the least, 112. The figures on Voyager 1, 2 and Pioneer 10 are 289, 325, and 241 stars, respectively. Transmissions to Voyager 2, Pioneer 10 and Pioneer 11 have already encountered at least one star, while Voyager 1 and New Horizons signals will encounter stars in the near future. From the paper:
Transmissions to Voyager have already encountered an M-dwarf, GJ 1154, and a brown dwarf, Gaia EDR3 6306068659857135232. Transmissions to Pioneer 10 have encountered a white dwarf, GJ 1276. Transmissions to Pioneer 11 have encountered a M-dwarf, GJ 359. We have shown that the radio transmissions using the DSN are stronger than typical leakage and are useful for identifying good technosignature targets. Just as the future trajectories of the Voyager and Pioneer spacecraft have been calculated and their future interactions with distant stars cataloged by Bailer-Jones & Farnocchia (2019), we now also consider the paths of DSN communications with those spacecraft to the stars beyond them.
The paper provides a table showing stars encountered by transmissions to the spacecraft sorted by the year we could expect a return transmission if a civilization noted them, along with data on the time spent by the star within the transmission beam. DSN transmissions are several orders of magnitude smaller in EIRP than the Arecibo planetary radar (1012 W), but it’s also true that the positions of the spacecraft, and hence background stars during transmissions, are better documented than background stars that would have encountered the Arecibo signals.
So what we have here is a small catalog of stars whose systems are in the background of DSN transmissions, and the dates when each star will encounter such signals. The goal is to offer a list of higher value targets for scarce SETI time and resources, especially concentrating on those stars nearest the Sun where civilizations may have noticed us. I don’t hold out high hopes for our receiving a signal from any of these stars, but find the process fascinating. Derrick and Isaacson offer a new way of considering our position in the galaxy in relation to the stars that surround us.
The paper is Derrick & Isaacson, “The Breakthrough Listen Search for Intelligent Life: Nearby Stars’ Close Encounters with the Brightest Earth Transmissions,” available as a preprint. Thanks to my friend Antonio Tavani for the pointer to this work. The Bailer-Jones & Farnocchia paper mentioned above is likewise interesting. It’s “Future Stellar Flybys of the Voyager and Pioneer Spacecraft,” Research Notes of the AAS Vol. 3, No. 4 (April 2019) 59 (full text).
Alexander Zaitsev did something similar for radar astronomy transmission in the past.
Somewhat off topic, but a publication this morning ( https://arxiv.org/pdf/2304.09895.pdf , https://www.nature.com/articles/s41550-023-01943-9 ) comes down in favor of axionic dark matter because it has a wave structure (10E-22 electron volts?!) that disrupts gravitational lensing by distant galaxies. Will that interfere with the stellar gravitational telescopes we expect our space agencies (and others’) to build?
This is tantalizing context for understanding the Wow! Signal, and underscores why the mere chance of a SETI detection deserves our best attention all the time. Like the transmissions we “receive” from our antecedents here on Earth — in the form of artifacts, broken DNA, fossils — any transmissions we receive through deep space are artifacts deposited without us in mind.
That is, if durable transmissions are most likely leakage between two points doing business while moving rapidly through four dimensions, with no other purpose in mind, that one priceless pottery shard is all we’ll get from an entire civilization. We have to be ready to capture it and glean the most we possibly can.
If so, SETI may be less about “contact” and more about the unfathomable fortune to salvage an artifact of a long-gone species’ ancient daily routine. This is another way that the disciplines of archaeology and anthropology might inform SETI. Observations through deep space are observations through deep time. We are not the masters of the past — we are its lucky ephemeris.
I think you’re right about gleaning the most from a signal. Is it plausible for astronomers to figure out which way the aim of a signal is shifting? The most hopeful case would be a signal (or launching laser) to an interstellar ship, which should be loud, slow-moving (in an angular sense), tightly focused in space, and almost prohibited (due to lightspeed limitations) from making capricious course corrections. If Earth had multiple observatories able to detect the beginning and end of the signal, and very precise localization of the source, could they measure a difference between the timing of peak intensity? (preferably if we had multiple satellites in distant orbits on several axes around the Solar system…) If the direction of the signal sweep could be measured, a rapidly launched probe might line up behind the target and spy on its communications for some time.
These are marvelous questions — I’d love to know the answers!
Some have described the Wow! Signal, and SETI itself, as a triumph of branding over substance. It’s true that our propensity for marvel often leads us in circles without information to match, and that we need far greater tools to meaningfully catch an artificial hiccup in the galactic static.
But by the same token, if the minds at the Big Ear had only had the resources we have today, they likely would have resolved their enigma in 1977. Asking better questions about the nature of a signal in the first place, as these authors do, gives us an even greater leg up on the SETI challenge.
Which all suggests an encouraging promise: ongoing best efforts are always better than not listening at all!
The alien radio atronomers will pick up a brief signal that appears to be of intelligent origin, but then discard that possibility as the signal does not repeat.
Sound familiar?
“Small steps Ellie, Small steps..”
I read the fascinating article but strength of the signals when they passed a star system was not included. Likely extremely faint, so that a large, highly sensitive receiver would be needed.
According to CHATGPT4 the signal flux density is calculated as follows:
EIRP = 10^10 W = 10^13 mW
distance = 100 parsecs = 3.0857 × 10^19 meters
beamwidth = 0.128 degrees = 0.00223 radians
Substituting the values into the formula:
Flux density (in Janskys) = (10^13 mW) / (4 * pi * (3.0857 × 10^19 meters)^2 * 0.00223 radians)
= 1.62 × 10^-28 Jy
Therefore, the flux density of the signal at 100 parsecs would be 1.62 × 10^-28 Jy.
Furthermore, ChatGPT says the FAST telescope is theoretically sensitive enough to receive such a signal but it requires many hours, even days of integration. I believe that means no data could be extracted.
Thus, a civilization at our level or above would need to be on the receiving end, assuming they are creatures who build radio telescopes!
Curious if the AI got the math correctly, it mentioned factors having to do with the signal’s frequency / wavelength that likely impact reception.
Yes, *extremely* faint. More to follow in this.
Now we are using ChatGPT to perform basic engineering calculations? We are truly doomed.
On Wednesday I watched the SETI Insitute webinar talk “Do ETs Watch Us? What Do They See?”. with Molly Bentley asking questions of Seth Shostak and Paul Dalba.
It was depressing to hear such a limited viewpoint about how ET might detect us. Most of the discussion seemed to be about biosignatures. I think Shostak essentially stated that METI/active SETI would not be receivable at proximal without vast radio telescopes. An audience member asking about a probe that might be nearby was effectively dismissed by Shostak.
IIRC, there was no mention of DSN-type signals, and online questions about how far can various radio signals be detected were not addressed or answered.
An issue I have with DSN-type signals is that they are likely to be received as single transients, like the WOW signal. ET will be very lucky to receive the signal assuming they happen to be in existence and looking in the right direction with the right technology. But a single event is not likely to gain much attention beyond “That was interesting, but what can we do with this?” If ET exists within 100 parsecs on one or a few stars and which the signal reaches of the 300,000 stars in that volume, that implies that the galaxy must be teeming with ETs with the appropriate technology to monitor the skies. Very Star Trek, but how realistic is it?
The galaxy has changed a lot since the early 60s when Drake and Sagan assumed signals would be heard from every corner of the sky and all humans needed to do was start listening. 60s shows like Star Trek certainly picked up that spirit and depicted the galaxy as teeming, not just with life, but with every manner of advanced, intelligent life. Now days it feels instead that we are adrift on a tiny life raft in the middle of a vast ocean. Which of course we are. Anybody home? Anybody?
Please , “If you’re out there in the dark with your transistor goin’, call us at the station, the lines are open”!
I agree with this paper that we should look for a suitable target. I would consider sending signals to G class stars. At some point, we will find an Earths sized planet around a G class start in it’s life belt. The idea that our TV signals don’t make it to the nearest star is correct. At least one million watts of power is needed for that and radio stations are not anywhere near that power. Consequently, we can confidently conclude that not any TV shows or music from radio stations have made it as far as the nearest star.
It’s a different story for nearby though as we know that at night there is no ultra violet radiation making ionization and free electrons to reflect radio waves. Someone in orbit could pick up radio waves easier at night. It’s hard for me not to imagine ET critics of our science fiction TV shows of the 1950’s hair brained lack of understanding of physics, rocket science, etc. On the other hand, they would admire our radio broadcasts of classical music. One would have to assume a warp driven, interstellar spacecraft which used stealth technology making it completely undetectable by our radar listening to all of our radio broadcasts and watching our TV shows and even surfing our internet. Since this is impossible to prove with present technology, one would have to use satellites with telescopes that used infra red or lidar, etc. Maybe the JWST. This is completely speculative and based on UAP’s and UFO’s in our atmosphere which could also listen to radio waves, but I only wanted to show we can’t completely ruled out the idea that our classical music is admired by ET’s.
I just glanced at the Table you referenced in the paper. It looks like the soonest we could expect a reply (assuming nothing superluminal) would be 2029, and it’s a “metal” contaminated white dwarf. So, no hurry to check them out.
The interesting thing is the white dwarf could act as a very powerful stellar gravity lens aiding signal gathering. In the far future these WD’s could form the nodes of a very powerful galactic communication system due to their near SGL lines. Sirius A with it’s WD B would be an ideal system to get to.
Great idea, but not just “far future” (let’s hope!).
If tech aliens managed to get around a WD they could scan a section of the entire sky in around a week due to the high orbital velocity and the entire sphere to a fair resolution in thousands of years or less.
Just a thought if the WD going around Sirius A concentrates light via gravity lensing there should be a region near the WD where the light from Sirius A is of very high intensity in the tens of kilowatts per square meter. Enough to power probes to move around the system observing all of space and a great comms instrument back to Earth.
Perhaps they are here to listen and see our art, at least the ones that have similar senses. We have only been making fine art for 500-1000 years and our Contact Era may be short lived…
The Fermi Paradox revisited: Technosignatures and the Contact Era.
Amri Wandel.
“A new solution to the Fermi Paradox is presented: probes or visits from putative alien civilizations have a very low probability until a civilization reaches a certain age (called the Contact Era) after the onset of radio communications. If biotic planets are common, putative advanced civilizations may preferentially send probes to planets with technosignatures, such as radio broadcastings. The contact probability is defined as the chance to find a nearby civilization located close enough so that it could have detected the earliest radio emissions (the radiosphere) and sent a probe that would reach the Solar System at present. It is found that the current contact probability for Earth is very low unless civilizations are extremely abundant. Since the radiosphere expands with time, so does the contact probability. The Contact Era is defined as the time (since the onset of radio transmissions) at which the contact probability becomes of order unity. At that time alien probes (or messages) become more likely. Unless civilizations are highly abundant, the Contact Era is shown to be of the order of a few hundred to a few thousand years and may be applied not only to physical probes but also to transmissions (i.e. SETI). Consequently, it is shown that civilizations are unlikely to be able to inter-communicate unless their communicative lifetime is at least a few thousand years.”
The Fermi Paradox Revisited: Technosignatures and the Contact Era.
https://iopscience.iop.org/article/10.3847/1538-4357/ac9e00
I think we have been making “fine art” for much longer than 1000 years. It seems like that might be defining it as Renaisance onwards, and while western art made great progress from that time, it seems to ignore the many fine works produced by “western” culture earlier, or other cultures.
On the other hand, works will only be detectable via radio/TV since the time of their inception, which would mean our art is effectively only 100 years old!
The exceptions might be large structures, cities, etc, but again we have things like the pyramids, Angkor Wat, the Great Wall, etc, some of which are older than 1000 years, although only detectable from interstellar distances with outlandishly high resolution (maybe stellar focal telescopes run by an alien Percival Lowells?). Such structures would be more easily detectable by a probe stationed in our solar system of course.
The radiosphere expands, but it’s at the mercy of the Inverse Square Law. I will check Amy Wandel’s paper.
Very good point, the great pyramids with their white stone facing reflected giant crosses at certain times of the year. Gobekli Tepe in Turkey a 12,000 year old temple has a fine art structure and symbols. Many megaliths were designed for astronomical fine alignment analysis of the heavens. Any exterrestrial civilization would be shocked to discover from even great distances that the earth is one of the truly rare planets in the galaxy that has a very rare fine art. The total solar eclipse of the sun…
Not even all humans appreciate all works of art. Hence the saying “Beauty lies in the eye of the beerholder”.
I suppose one day we will build huge world ships that spiral around the galaxy approaching other star systems to colonise taking millions of years.
We have one already – the Earth. It has lasted billions of years without intelligent maintenance and survived a number of catastrophic events, including the current one being made by the indigenous dominant species.
Or was the Earth, rather than a manufactured worldship, what you were alluding to?
Ships like these, but they can be much bigger if used for growing crops which would require less gravity. They have their own ‘sun’ for illumination and perhaps collect interstellar hydrogen for fusion and propulsion.
https://www.youtube.com/watch?v=Ev5QG-6rD0I
It was discussed here some time back that radar ranging used to detect and measure solar system objects was powerful. Much more so than DSN I would expect. We could have been signaling ET trying to gauge distance to something. We’re still tracking 60s era rocket boosters, I don’t imagine this is a passive endeavor.
This has been discussed here before, many times in fact. I recall doing a few calculations once on CD so I did a search. I had to pick my jaw off the floor. It was in 2008. I had no idea that I’d been active on CD for so long! I’m feeling old.
https://centauri-dreams.org/2008/02/29/toscanini-through-the-light-years/
Interesting that Toscanini was also a thing back then.
I read the preprint and I had a few problems with it. First, they fail to address spectral density. For example, an NTSC television signal is 6 MHz wide. DSN uses are far narrower bandwidth, depending on the spacecraft and other factors. There’s a reason television transmitters are such high power: the bandwidth; without it the range with an acceptable SNR is very poor.
Second, the conclusion twice refers to spacecraft transmissions. Really? I hope that’s an error that will be corrected since it’s absurd.
I found the analysis in the paper to be superficial. Interesting but hardly noteworthy.
We’re over thinking this. Our accidental signals attenuate quickly–inverse square law. And there is background noise. Not all our transmissions are in the low-noise region of the spectrum. If there was someone out there who might intercept one he would have to be listening deliberately and carefully, and he would have his equipment tuned to detect it; highly unlikely unless we were deliberately transmitting a beacon designed to stand out above the noise and deliberately fashioned to appear artificial.
Perhaps some of our accidental broadcasting might be easily detectable, IF they happened to be listening at just the right time. Maybe our Dew Line radars might reach out for a few light-years, but its highly unlikely anyone nearby has a program in place to look for them. We have an enormous interest in SETI but we really don’t field a search anywhere near what our technol0gy and financed are capable of supporting. And even those famous I Love Lucy episodes would only be in earshot for a few moments, and the TV signals interleaved and coded so that they would not be seen as communication. It would just be static, gone in a flash.
Everything is in motion, spinning around, there may be a lot of Wow! moments out there, but no repetitions. The signals won’t appear to come continuously from one spot in the sky. They will zip by like lighthouse beams. And lets face it, we’ve had radio for only about a century. The wave front has only traversed a tiny fraction of the volume of the Galaxy. Its highly unlikely there’s another radio capable civilization nearby, and even more unlikely they will be listening in our direction at just the right time..
We have a much better chance of hearing THEM. Hopefully there are advanced technologies that routinely generate loud bursts of EM, (for a very long time, perhaps during some kind of industrial activity) frequently enough that we might get lucky and spot one as it flickers past us. If the Wow! is ET, that’s probably what it was, a radar beam or nav beacon that flashed by and was intercepted by sheer luck. Since, presumably, our neighbors have been in the radio business a lot longer than we have, we are more likely to hear them first.
I would suggest monitoring old open clusters for EMR leakage. Any civilizations there would have had the opportunity to colonize other cluster members, and there may be a lot of tight-beam radio or laser communication between cluster stars. Some might escape the cluster and we might get lucky.
If we assume a nearby civilization has the capability to pick up one of our accidental signals easily, and if they are able to readily identify it as an artifact, then they are probably aware of many other radio civilizations already out there. They may not really care that there is yet another in the club. It may turn out that SETI capable civilizations are so numerous that they simply don’t care about cataloging yet one more.
Scott Guerin is quite correct. At the range of the closest of the stars they list, the most powerful transmitter on Earth would produce a power of only 1 watt over a surface area of the entire orbit of earth! Consequently the transmissions they describe are completely unobservable.