Recent activity in sending signals to the stars has caught the attention of plasma physicist Jim Benford. The CEO of Microwave Sciences and chairman of the Sail Subcommittee for Breakthrough Starshot, Jim has more than a few doubts about the efficacy of these signals, and questions the rush to send them. Is the recent EISCAT signal detectable at interstellar distances? A look at the science of such signals follows, and thoughts on the caution with which we ought to proceed.
By James Benford
Yet another ‘Message’
Recently, advocates of METI (Messaging to Extraterrestrials) sent a ‘message’ consisting of prime numbers followed by 36 music pieces to Luyten’s Star. It was a collaboration of METI International, led by Doug Vakoch, with the Catalonia Institute of Space Studies. This star is 12.4 light years from Earth and has a potentially habitable exoplanet (GJ 273b).
This was sent from the EISCAT facilities near Tromsø, Norway, using a microwave antenna. The music pieces are 10 seconds long, therefore contain only 1500 bits, so are quite simple.
Can this deliberate transmission from Earth be detected at the distances of nearby stars? What is the reality of claims that the low power messages sent to date are ‘practically detectable’? Such qualitative statements are not useful in a quantitative science. We will see that the message is faint and very unlikely to be detected, even if aimed at nearer stars.
How detectable will the message actually be?
Image: EISCAT Tromsø site with the EISCAT3D test facility in the foreground. Credit: Craig Heinselman.
Can it be heard?
No. This group is in fact whispering at the stars.
I base this on what we know about Vakoch’s transmission: power 2MW, 32 meter dish, frequency 929-930.2MHz, bit rate 125 bits/s, encoded to 8 bits PCM and to a frequency of 6.4 kHz, repeating the 33 minute signal three times over three days.
Analysis of this EISCAT (European Incoherent Scatter Scientific Association) METI, accounting for differences in power, aperture and frequency, is that the power density at long range (EIRP, effective isotropic radiated power) is only 1/100th of Arecibo, and a 10th of that used by Alexander Zaitsev in his messages. Therefore, the several observations and conclusions that John Billingham and I made 3 years ago about the unobservable Zaitsev ‘messages’ are true in spades for this failed transmission as well (“Costs and Difficulties of large-scale METI, and the Need for International Debate on Potential Risks”, John Billingham and James Benford, JBIS 67, pg. 17, 2014).
The conclusion is: This will not be detectable as a message by radio telescopes such as we have on Earth. The energy might be detected with radio telescopes larger than any we have, integrating the signal, but there isn’t much integration time, and integration would destroy the content of Vakoch’s transmission. So it will not be recoverable as a message by ETI if their radio telescopes are comparable to or substantially greater than ours. (One can of course assume a Supercivilization only 12 light years away with vastly larger radio telescopes. But if our leakage were detectable by them, as the METI-ists claim, then their greater leakage radiation would surely be detectable by us. But we do not see it.)
Dave Messerschmitt, who is in METI International’s Advisory Council, but wasn’t consulted about this message, observes:
“This METI signal is a simple on-off keying scheme, which dates to the 1837 invention of the telegraph. It has the virtue of extreme simplicity and transparency. However, there are modulation and coding techniques known today that operate near the fundamental limits of data-rate vs energy, such as is described in my paper “Design for Minimum Energy for Interstellar Communications.
“For the same average power (and energy consumption) such signal designs can considerably increase the distance over which information can be reliably extracted. They have other benefits, such as easier discovery and less susceptibility to local sources of radio-frequency interference. However energy-efficient signals will be essentially unobservable by long-term spectrum analysis. Rather, transient (short-term energy) analysis is more effective for such signals. They also require a transmitter implementation capable of high peak-to-average power ratios.”
Note also that the following must all occur for ETI to detect this weak signal:
1) Their system must stare in the very small part of the sky where our sun is, i.e., they must be interested in our system. (To get high sensitivity, the antenna area must be large, so the targeting angle is very small.) This could be because they’ve detected our out-of-equilibrium atmosphere, thus possible life here. This has been true for billions of years.
Or they could have detected our leakage radiation. But the bandwidth of incoherent leakage sources, such as TV and radar, is too wide and the power too unfocused for such signals to escape the Solar System, let alone reach other stars, before it’s indistinguishable from noise.
2) They would have to guess the bit rate of the message. Processing the stored signal with successive assumed rates and seeing which gives the best signal could do this.
3) They would have to deduce that we’re using simple on-off keying instead of another of our many modulation methods, so must analyze the received signal against a list of such stratagems.
While all the above could occur, this is by no means certain.
A decent respect for the opinion of mankind…
They are Star Whisperers. They show no decent respect for the opinion of mankind, to quote a certain historical document, in speaking for Earth.
What we see here is yet another attempt by the METI-ists to announce ourselves to the stars with weak signals that have no serious possibility of being received and interpreted. After several such transmissions in the last decade, they continue to make false claims and send silly signals, paying no attention to the scientific fact that their messages cannot be heard. And they continue to advertise such matters as the following:
“I would say, on behalf of the Klingons, that I prefer to listen to some good music than to the empty whistle of SFO’s radar.” – Seth Shostak
Seth Shostak surely knows that the San Francisco Airport short-range radar, being weak in power with a very low gain antenna, cannot be heard beyond the moon. He certainly knows this if he does any quantitative calculation. They claim, quite falsely, that we have announced ourselves by leakage radiation or intentional transmissions in recent years. This is not true. Therefore these are simply claims to excite the public. This is not an intellectually defensible position.
I advise the METI-ists to restrain themselves from trying to signal ETI. They are not being given access to seriously high power facilities such as Arecibo because they have no rationale for sending messages. They have no claim to speak for Earth.
In 2014 John Billingham and I made several suggestions in our paper referenced above. The time has come to address the METI issue on an international scale by establishing international symposia on transmitting from Earth to ETI. I advocate a moratorium on METI until an international consensus has been reached about announcing ourselves to the stars.
Methinks METItes is a better term…
Even as I am sceptical that the signals will be received by a nearby eti(given my scepticism that there are nearby eti), I am personally of the opinion that such broadcasts should be outlawed in lieu of a broad consensus on the use-risk axis which admittedly will remain largely a realm for mere speculation for the foreseeable future.
The worst that can happen is that METI is wasting their time and money sending a signal that is too faint. They certainly might do better than an on-off keying scheme. In theory, a large enough ETI radio telescope might hear it, but only if there actually is an ETI at that location. We don’t yet any spectroscopic evidence of biosignature gases and their waste products.
Conservatively, I am biased against the idea that there might be indigenous ETI on any exoplanet other than an exact Earth twin which has a Moon that might be needed for a planetary magnetic field and a small wobble of it’s axes for stable climate over a long period. The ETI solar system must also be very similar to our own with a G class star with high metallicity and far away enough from the star and a large gas giant like Jupiter to deflect asteroids.
If ETI have interstellar travel, why colonize a planet with a harsh environment which is difficult to live if you can go anywhere and pick a suitable planet.
Well, that burns. While any critical thinker is free to doubt a premise, few are qualified to challenge one in radio astronomy. And some premises sound sufficiently obvious coming from those qualified to state them, that we’re just willing to move on to the next part of the argument.
Dr. Shostak has indeed presented the “ETI cat’s out of the bag” premise many times in his role as a science educator. Several instances come to mind from SETI’s regular podcast, Big Picture Science. Particularly with regard to aviation radar.
In light of his and Dr. Molly Bentley’s years of work as SETI science educators ostensibly promoting public interest, skepticism and critical thinking, I accepted that premise (irony now noted) without much technical explanation. This casts gloom on the many other ideas I’d come to accept and support as a longtime BiPiSci listener. It also clouds what I crucially believed was SETI’s official agnosticism toward METI.
I’d like to hear Dr. Shostak’s explanation, albeit more accountably peer-moderated than what he offers on his podcast.
Bright side: let Isaac Arthur know that some extra donor money has probably freed up.
Te U.S. has been sending regular strong radar signals into interstellar space since the late 1940’s. https://en.wikipedia.org/wiki/SAGE_radar_stations
This was earlier then the Ballistic Missile Early Warning System (BMEWS) in the far north. By the late 1950’s NORAD (North American Aerospace Defense Command) had complete coverage around the Continental US as well as Alaska, this system was also used by the FAA for there AIR ROUTE TRAFFIC CONTROL CENTERS (ARTCC). These ARSR have a peak power of five megawatts and have a fortyfive foot diameter parabolic dish, the ones you see in the geodesic domes.
Any nearby exosolar system would see signals coming from these units like a Christmas tree lights blinking on and off as the earth rotated, even to the effect of different colors from the different freqs used by the radar sites. These radars rotated at 5 rpm and had several features that would make them very obviously artificial. The signals from them has now reached out 70 light years, with any return contact to us coming from 35 light years. Here is a very good article on history United States development of Radar going back to 1935.
http://www.dtic.mil/dtic/tr/fulltext/u2/a331231.pdf
Reading my copy of the book “Life in the Universe”*, edited by John Billingham, one of the articles is on “Eavesdropping Mode and Leakage from Earth” by Woodruff T. Sullivan III, here he discusses the distance BMEWS could reach.
“Keeping the above factors in mind, an examination of all radio services reveals two categories of strong signals escaping Earth that might be of interest to an extraterrestrial observer.
An acquisition signal merely announces our presence over a large region of space by its very existence but is not generally useful for careful study because it fails to meet one or more of the criteria given above. An information signal, however, satisfies all three criteria. At the present time on Earth, some of the most important acquisition signals originate from a half-dozen or so U.S. military radars (and their presumed Soviet counterparts). These Ballistic Missile Early Warning System (BMEWS) radars sweep out a large fraction of the local horizon with extraordinarily powerful transmitters. The result is that this “radio service” provides by far the most intense signals that leak from our planet to a large fraction of the sky.
While BMEWS radars pass criterion (2) above, they fail (3) and partially fail (1) because they are so few and often change their frequency of operation to avoid being jammed. Nevertheless, if an external observer used equipment comparable to the most sensitive radio telescope on Earth (the 305-m diameter dish at Arecibo, Puerto Rico), we calculate that a BMEWS-type radar could be detected as far away as 15 light years. This distance includes only about 40 stars, but, of course, it is possible that our eavesdropper possesses a much more sensitive radio telescope than we do. If “he” had something like the largest one ever proposed for Earth, namely, the array of a thousand 100-m dishes called for by Project Cyclops (Oliver and Billingham, 1973), he could detect a BMEWS-type radar at a distance of 250 light years. In this case at least 100,000 stars are possible candidates for such an eavesdropper’s location. But note that radio waves travel at the finite speed of 1 light year per year, and thus it will take until the 23rd century, or 250 years from now, before all these stars have had a chance to be bathed in the radiation of our defense system radars!
After picking up a BMEWS (or other) acquisition signal, the observer needs at least 100 times more sensitivity in his equipment to reach the rich lode of information signals emanating from Earth. It turns out that television broadcast antennas (or stations) are the most intense sources of such signals.
[380] All other services either have their transmitter power spread over too broad a Frequency band (for instance, FM broadcasting and most radars) or do not transmit continuously (ham radio operators) or from the same location on Earth each day (taxis, aircraft). Many signals, such as medium-wave AM broadcasting and almost all short-wave communications, never even penetrate the ionosphere, the reflective layer of charged particles that surrounds Earth. We thus concentrate on TV broadcasting; all other services that leak from Earth are less intense and merely add to the background noise a distant observer would measure in the direction of our Sun as seen in his sky.
But again note that TV broadcasting from Earth has been in existence for only 40 years. Figure 1 illustrates the phenomenal growth in intensity of the resultant ever-expanding “power bubble.” On a cosmically infinitesimal time scale, Earth has indeed become a very bright planet, outshining the Sun by orders of magnitude in certain narrow frequency ranges”.
I found a complete copy online:
https://history.nasa.gov/CP-2156/contents.htm
and the article by Woodruff T. Sullivan III here:
https://history.nasa.gov/CP-2156/ch5.4.htm
*From the NASA Ames Research Center participants in the June 1979 Life In The Universe Conference.
Space Surveillance Sensors: The PAVE PAWS and BMEWS Radars (April 12, 2012)
https://mostlymissiledefense.com/2012/04/12/pave-paws-and-bmews-radars-april-12-2012/
BMEWS
Background
“The two BMEWS (Ballistic Missile Early Warning System) are collateral sensors in the SSN’s network (as noted above, a third radar in Clear, Alaska is officially classified as a BMEWS radar, although its characteristics are same as a PAVE PAWS radar), and are essentially larger versions of the PAVE PAWS. Located at Fylingdales in Britain (54.36?N, 0.67?W) and Thule in Greenland (76.57?N, 68.32 W?), these radars’ primary mission is early warning of ballistic missile attack. The radar in Greenland has two faces, providing 240? degree azimuthal coverage, while the radar in Britain has three faces, providing 360? coverage. Both of these radars have received upgrades that allowed them to be incorporated into the United States’ ground-based national missile defense (GMD) system.
Technical Characteristics
Each radar face is bore-sited at 20? above the horizon, and can provide elevation coverage from 3? to 85?. Each octagonal BMEWS radar face has a diameter of about 25.6 m and contains 2,560 active transmit/receive modules (the total number of elements, including passive ones, is 3584). Each face has a peak power of 850 kw and an average power of 255 kw (corresponding to a duty factor of 0.3). The radar’s beam width is about 2.0?
The BMEWS transmit/receive modules have a maximum duty factor of 30% and can produce pulses with lengths between 0.25 and 16 msec. The BMEWS radars use pulse lengths of 0.3 and 6 (typical) msec. in search and pulse lengths between 1 and 16 msec. in track.[10] Search bandwidth is 0.3 MHz (Britain) and 0.6 MHz (Greenland) and track bandwidths are 5-10 MHz respectively. The radars operate using a 54 msec. resource periods”.[11]
BMEWS Performance Claims
3,000 nmi range coverage.[12]
Who remembers hearing the Russian Woodpecker on shortwave?
“The Russian Woodpecker”
Duga Radar Array, Chenobyl, Ukraine
http://www.thelivingmoon.com/45jack_files/03files/Russian_Bases_Woodpecker_Duga_Radar_Ukraine.html
Abandoned Radar Bases: 10 Defunct Early Warning Systems & Monitoring Stations.
https://www.urbanghostsmedia.com/2014/12/10-abandoned-radar-stations-early-warning-spy-bases/
I do not think much of the public or even the SETI community realize just how much noise we were transmitting into space during the cold war. The amount of money and energy that went into these projects was much more then the military budget for space. The history of what was built and used in the these black projects on both sides during this time period will most likely be what will by pick up by Exo Civilizations first.
I keep thinking of Contact’s image of Hitler as our greeting to them.
This is helpful information, thank you. I won’t pretend to be able to digest the implications at once. Which, indulging your patience, is closer to the point I intended. Apologies to anyone if I made it unkindly.
Unfortunately, I’m not really qualified to agree on what might constitute interstellar Christmas lights. And Mr. Benford makes a compelling argument that METI has a devastating technical, not just ethical, burden to overcome.
Given Dr. Shostak’s very public role as a scientist and critical thinker, I would have expected to have heard those arguments first as part of his discussion. Especially if he is able to lay them to rest.
I agree wholeheartedly, Centauri Dreams is the ideal space for that to happen. I’ll follow the ensuing dialog with great interest, and will mind my reaction.
*DR. Benford
(Apologies for the typo, too! Time for glasses.)
A long time ago, many people used smoke signals or other methods to communicate with creatures living on the Moon or Mars. Why even bother?
Is the EISCAT EIRP at long range *in comparison* to isotropic, or is it said *to be* isotropic at long range? The distinction is an important one. Antenna gain is expressed in decibels, usually *in comparison* to what an isotropic antenna would put out at the same transmitter output power (dBi–the isotropic antenna is an “ideal” theoretical antenna that emits RF energy equally in all directions, in a spherical pattern); sometimes–although not in this case–an antenna’s gain is compared to a dipole antenna’s performance (dBd), but:
Even at a distance of light-years, no Earth-based or space-based interstellar radio antenna system would have an isotropic pattern, for multiple reasons):
The isotropic antenna is a theoretically perfect one that doesn’t exist (a canted turnstile antenna, or an electrically very short [at the operating frequency], inductor-loaded antenna, has a more-or-less 3D omni-directional pattern), but even these antennas’ patterns have lobes in which more power is radiated (and electrically-short antennas are inefficient radiators). While canted turnstile antennas are commonly used on satellites, they aren’t often used on deep space probes (except as auxiliary low-gain antennas for near-Earth operations) because they have no–or very little–gain on either transmit or receive.
When one is listening to–or transmitting to–points in space that are light-years distant, *some* type of directional beam antenna, whether a parabolic dish or an array of reflector-backed dipoles, must be used. If one tried to transmit omni-directionally in all directions to the galaxy, such a “quasi-isotropic” antenna system, having no gain, would require a transmitter with an enormous output power, and it would be equally lousy at hearing faint signals. (Broadcasting with a “pancake-shaped” beam that covered the galactic disc [as the Pioneer 6 – 9 probes’ medium-gain antennas covered the ecliptic, and thus always the Earth] would be more sensible, but its transmitter power output requirement would still be very high.) Also:
Thanks to Pioneers 10 & 11, Voyagers 1 & 2, and (if a sufficiently durable message can be recorded onto its memory chips) New Horizons, as well as the 1974 Arecibo Message, the “Who speaks for Earth?” argument has already been mooted… If those spacecraft’s messages–as well as the Arecibo Message–had to wait until consensus was reached concerning their content, I doubt if they would have been launched aboard the probes (or transmitted) at all, and:
As long as METI projects are carried out with the projects’ own money (and/or by other countries), I have no objections to them. I see them as fancier variations of the “name a star” companies that sell star maps (which are often given as gifts [as are lunar and Martian property deeds]) with buyers’ named stars listed on them. Their claims that the star names “are officially registered with the U.S. Copyright Office” are regularly demonstrated to be untrue, but the buyers don’t care; the “personalized” star maps make inexpensive, off-beat gifts and conversation-starters. In addition:
Since we know absolutely nothing regarding the existence, let alone the technological capabilities, of the intended (“hoped-for” is probably more realistic) recipients of METI messages, we can only make educated guesses about what they could–and couldn’t–detect and decode (based on current and projected human capabilities in this area, and on the theoretical limits of radio propagation and detection). While I agree that the METI folks are likely “whispering [too softly to be heard] in the dark,” I find the calls to legally ban METI transmissions far *more* troubling than any potential risks from alerting hostile aliens or misrepresenting humanity to peaceful aliens…
I have nothing against METI sending a signal to (GJ 273b), but one has to admit it is not the best choice of targets for potential habitable planets since it has 2.89 Earth masses. I don’t think ET’s like being squashed by a high atmospheric pressure and greater gravity. How about sending a signal to a more Earth like planet instead of a super Earth?
Also, the wattage of radar power is way too low to go to the nearest star. TV signals can be powerful but they are aimed at any point in the sky and the leakage will be defeated by inverse square law and rotation of the Earth. A hundred million watts would be needed to get a strong signal to the nearest star so it would be easily detected which is far above the power TV signals. In other words, it is highly improbable that our radio and TV signal leakage could ever be picked up by another star system so it is more like science fiction.
The idea that biosignature Exoplanet atmospheric gases could be seen for hundreds of millions to billions of years sound. Some kind of ET square kilometer array or larger of radio telescopes might pick up of signal.
This discussion of radar and TV leakage signals gives me an opportunity to weigh in. I have not looked into radar signals, but I have studied the detectability of TV signals. The argument for their detectability at great distances is based on the large component of single-frequency carrier, which allows ET to apply a very narrowband filtering with longish integration times. See for example Sullivan’s paper “Eavesdropping: The Radio Signature of the Earth” in 1978. He does a thorough and credible job based on the technology of the era. However, this is predicated on the old analog TV signals that were designed/standardized with 1930’s technology. The reason for including a large carrier component was simple and cheap receivers constructed from the vacuum tube technology of the time.
For the past 10+ years we have moved to a new digital TV broadcast system. The signals are specifically designed to be statistically indistinguishable from white noise spread over a wide bandwidth, so as to reduce interference with nearby cities. Such signals would not be detectable by ET. Or if by some hook or crook they were able to “tune in”, the signals are constructed with such complicated algorithms in multiple layers of compression and error-control coding that there is no way they could extract any useful information (like a picture), absent access to the thousands of pages of standards documents that our own receiver designers use. We did transmit a credible leakage signal for about 40 years, but no longer. Sullivan’s analysis concludes that the old analog signals could only be detected in our nearby region.
As far as the argument that ET may have much more advanced technology, this is true with respect to antenna size/area. For example, if ET had a planet- or solar-system-size antenna they could pick up some of our higher-power transmissions. However, it is not true with respect to extraction of information from such signals. We well understand the fundamental limits on such process, and our current technology is able to approach those limits. Those limits are technology-independent, based on the statistics of the noise accompanying the signal. ET could do no better.
Yes, but you are forgetting the rest of the world, many second and third world countries are still using analog open air broadcast and many analog channels are being broadcast via satellites. (mostly religious, to save the Aliens from themselves! :-) On top of that, most of the military and aviation radar equipment is being past down to these countries for a small price. So as to the USAs tunnel vision (TV) there is still many stray signals leaving this Earth for more then 70 years.
Digital TV is being rapidly adopted by all countries, because of its greater efficiency and because equipment to transmit and receive the old analog standards is not being manufactured anymore.
According to Wikipedia “A switchover from analog to digital broadcasting began around 2006 in some countries, and many industrial countries have now completed the changeover, while other countries are in various stages of adaptation.”
Direct-broadcast satellite TV is aimed at the Earth, not away from the Earth. There will be some diffraction around the Earth, but small since this will be a sidelobe of the transmit antenna.
Good point about radar, but I explicitly excluded radar from my comments. I have not studied their detectability.
Well here in Bohol Philippines are cook and maids still receive analog TV with the old style tube television and antenna, we have direct satellite with HD digital but many people in the poorer areas of the world can not afford the switch to digital. I do not have a current copy of the World Radio TV Handbook but it would be interesting to see how many channels are broadcast in both analog and digital for just this reason.
As I mentioned before in an earlier comment Direct-broadcast satellite TV sends the signal from the earths stations to the geostationary satellites with much of it leaking into space. These earth stations extend far north and somewhat south of the equator (Alaska to southern Australia) and would cover nearby stars that are 10 degrees north and south of where the geosatellites orbit.
As for extraterrestrial civilizations being able to pick up these weak signals I give you an example from Spocks home Vulcan around the triple star system 40 Eridani some 16.5 light years away in the constellation Eridanus. This consists of 40 Eridani A,B and C, A is a K1 star that Vulcan orbits, B is a white dwarf and C a red dwarf. (https://en.wikipedia.org/wiki/40_Eridani) Now the white dwarf B and the red dwarf C orbit each other in about 248 years but orbit the K dwarf A star is 7000 to eight thousand years. What does this have to do with signals from earth? The white dwarf would be a perfect giant radio and optical telescope with a very short focal length, since it density is so high and easy to reach from the K1 star. (SEE The FOCAL Mission: To the Sun’s Gravity Lens – Centauri Dreams) If civilizations developed in this system it would also be able to reach relativistic speeds from flybys of the three stars but especially from the white dwarf.
So there you have it, Spock could observe and transmit/receive via the white dwarf. Because white dwarfs are relatively common it would be used by advanced civilizations to communicate and observe as soon as they are capable of traveling short distances in interstellar space.
Our closest is Sirius B at only 8.6 light years away from earth.
Just found out that the Philippines shuts down analog broadcast in 2023 going completely digital.
By the way love your “Search for intelligent life in the Milky Way: A challenge for science and engineering”, Cal Day program, University of California, Berkeley, CA, April 7, 2010; especially page 3!
http://www.eecs.berkeley.edu/~messer/Talks/10/CalDay2010.pdf
Hi Paul & Jim
Another METI presumption is that ETIs aren’t already here watching us via their proxies, or even in person as post-biological entities. Of course, to remain undetected, They’d be using something better encoded than PCM signals blabbed all over the sky.
Deliberate transmission without the approval of pretty much everyone living on earth is an unconscionable act. Essentially these people are using scientific concepts and tools, but for what amounts to religious belief. They do not speak for mankind.
Yes I understand the concept that sufficiently advanced ETI could detect our presence passively; we humans are even now developing many of these tools. This however does not excuse the arrogance of taking it upon themselves to send messages.
Well stated. Like many, I have no objection to considering messaging, but I do think it should come as the result of a deliberative process with international input. What bothers some of us is the rushed, ramshackle feel of current attempts. It’s helpful to know that they’re too weak to travel far.
Jim Benford shows the virtue of applying real world science to the claims of those transmitting messages to the stars. The vast majority of those messages will never be heard by an alien civilization.
The transmitters with sufficient power are the real issue, which is why the 1974 transmission from the Arecibo radio telescope raised alarms. Nearly all those transmitters (including Arecibo) were built with taxpayer money. The funding government has the right to approve or disapprove how such a transmitter is used.
METI advocates should drop the premise that it is too late to regulate transmissions. As Benford makes clear, that argument fails to meet scientific standards.
It is time for a serious international discussion of how the most powerful transmitters should be regulated.
Cixin Liu’s Three-Body Problem trilogy, of which I see Paul is now reading the final volume, is a must-read fictional exploration of how METI might not be a great idea.
Peter Watts’s Blindsight is definitely worth a read for its take on talking to alien intelligence. Which reminds me, I need to get around to reading Echopraxia at some point.
There are many aspects to the METI argument, which I summarize at http://www.davidbrin.com/meti.html … Jim Benford and his colleagues have done a fine job showing that the METI-ists are dishonest in their claims of physical detectability. Michael Michaud has been cogent that these efforts aren’t scientific but attempts to do DIPLOMACY in all our names, without giving the nations or peoples of Earth any choice in the matter. Thanks to Liu Cixin’s influence, the Chinese may (for now) refrain from METI stunts.
There is some hope we can finally organize an “Asilomar-type” conference on METI, that would (at last) invite experts from other fields, like history, anthropology, evolutionary biology, diplomacy etc., for an eclectic and thorough examination of assumptions. If televised, such a conference might enthrall a billion people… which makes one wonder why the METI cultists avoid such a gathering, so strenuously.
In this confined pace, I’ll only add one remark Jim left out, re: the “Barn Door Argument.” With typical disbelief in his listeners’ sapience, Seth Shostak calls a coherent-narrow METI beam equivalent to the San Francisco airport radar, except that the METI signal will be more musical. Jim Benford shows that the levels of detectability are very different. So, dishonest. Try this comparison: communicating with a Scout Camp on the other side of a wide lake. You could slap Morse Code on the surface of the water… or aim a laser pointer right at the campers. That is very close to the difference in scale! Yes, with advanced tech, those scouts MIGHT decipher your Morse water-slaps… but…
I have left for last the crucial rebuttal to the Barn Door Argument. Indeed, it is the only one that’s needed, because it reveals the stunning sophistry of this deeply dishonest talking point. Here it goes:
However detectable we have been until now, the METI guys want to change it by multiplying our detectable luminosity at-target by millions of times. That’s no small increment over “I Love Lucy.”
Truly, ask these fellows: “Um, if aliens know about us already, why are you trying so hard to be a million times louder, shouting ‘Yoohoo! We’re here’?”
Alas, these are not honest people. They are transforming the scientific field pioneered by Shklovskii, Sagan, Drake, Tarter, Morrison etc into manifestly a cult.
See http://www.davidbrin.com/meti.html
I agree with most of David’s points but dislike the dig at the honesty of METI advocates. To be useful, the discussion needs to focus on issues and not the motivations of one side, which cannot in any case be known. A collegial atmosphere is best — we need to treat both sides with respect, as they’re trying to work out a complicated issue.
Agreed. And because I’ve received several far more extreme comments from other readers that I won’t publish, I want to note again the comments policy, published on the front page. Civility counts. Let’s stay issue oriented and get something accomplished.
I think METI people should be given a chance for a rebuttal but the arguments presented above seem very logical and fact based. We do hopefully want to stick to fact based arguments as certain elements of society who now have political power are demonstrating the results of abandoning this approach. The future literally depends on it.
Rebuttal invitations already sent.
Trying to establish an official moratorium is useless. How would we know who have already sent such messages in the past or who will do so again in the future? Although I don’t think it’s a great idea, for now, our standing in the cosmos amounts to living in the Wild West. Unless the messages are blatantly offensive, I don’t think it’s a matter worth pursuing.
Please invite Chinese scientists and astronomers, given the new Chinese facility’s impressive capabilities.
We – the earth people – are trying to contact beings almost just like us. Heaven forbid that they are, for exploration of our own distant lands led to the near or total extermination of indigenous populations. I can imagine E.T. getting our message, and replying, “We hear you, but we just don’t give a damn.” It’s probably better for Earth that way.
Why play with radio or microwave, just go for a phased array laser system, much more powerful but a little more expensive.
Dr. Benford and Dr. Shostak both start from the premise that ETI is located on another star, far, far away.
As we have discussed many times, ETI is likely to be either very far behind or very far advanced from our current technological capability, probably by millions of years. Their capabilities are therefore likely to be vastly underestimated.
I find this particularly interesting as Dr. Benford is participating in the Breakthrough Starshot mission, where we might be sending a probe to Proxima, without any indication of ETI present, in less than a century. We might consider that advanced ETI has already sent probes out to monitor interesting [HZ?] planets and even have self-replicating probes that are monitoring hundreds of billions of stars already. It is possible there is such a probe already in the solar system, possibly cloaked, and possibly on Earth. If so, that invalidates the argument that our radio presence has not been detected. If on Earth, they have monitored congregated, religious prayer for millennia!
Dr. Brin makes the same error. In all likelihood, ETI across the lake can see the campfire, the tents, listen to the campfire tales, and note the use of trees as urinating posts.
Before this century is out, I would be surprised if we cannot detect industrial pollutants in the atmospheres of exoplanets, or large scale artifacts on an exoplanet’s surface. If so, we should assume that any ETI can already do the same. Signaling our presence is just icing on the cake.
Within a millennium, we will probably have our own probes sending back information from other star systems within hundreds of light years, making the “keep silent” strategy futile for any ETI within range. Within a million years, we may be the intelligence monitoring the whole galaxy and searching other galaxies for signs of ETIs.
Trying to corral and align all nations, institutions, and individuals with respect to METI, strikes me as very difficult, even futile. It is like all animals in an ecosystem insisting on not doing any mating signaling because there might be predators about, and their whole ecosystem could be wiped out. [Meanwhile we humans wipe them out anyway to make way for economic developments, like sugar plantations and cattle ranches. Let’s hope there are no Vogon constructor fleets on their way ;) ]
If sending out a targeted signal is such a problem, doesn’t the same issue accompany sending out photon launched sails, heavier probes, and even crewed starships? Any high energy drive may prove detectable by advanced ETI with the right detectors.
In the Starship Century Book, which Jim Benford co-edited, brother Greg wrote a wonderful story The Man Who Sold the Stars about a maverick who rode a starship to the planet of a nearby brown dwarf star. Should that fictional trip have been stopped as it might have signaled Earth’s presence to ETI? I would hope not. That would seem to foreclose the idea of interstellar flight until all concerns about galactic predators have been eliminated. That might take how long? Centuries? Millennia? Eons?
Alex –
A quick analogy. Think of an isolated rainforest tribe. They don’t know about us until they venture a bit further out from their mud huts. Maybe they come across our ‘great machines’ knocking down the rainforest, or see our aeroplanes fly overhead. We ignore them, or maybe study them from a distance, but the moment they come out and try and make contact with us, there’s the threat that their culture will ultimately be subsumed by the rest of the world. For good or bad, there’s no going back from that first contact.
It’s the same with METI. Maybe ET does know we’re here, and either are not sufficiently bothered about us or are studying us from a distance. But the moment we provoke contact (and military radar and the laser beams driving photon-powered sails are not efforts to provoke contact) there’s no going back and yet we haven’t given the slightest bit of thought about the consequences.
At least until our computational power reaches 10^60 cps. If interstellar war happens, we want to be more like the Culture not the Idirans.
Good one ;)
The “Barn door of incidental signaling” argument is METI’s best response to critics. Is there a list of incidental and intentional signaling that includes the probability of detecting each example?
I am on the fence concerning METI. On one hand I think Cixin Liu’s theses is very weak. If every star star system can be considered an unique civilization, then the process of colonization would create competing civilizations. He presents a theses that would have civilizations casually destroying other civilizations to make room for other civilizations.
On the other hand, METI will never be a useful experiment. A negative result is worthless. METI will always be diplomacy first, a signal that is received but not responded to is still diplomacy. Unless we can develop a message that is guaranteed not to alienate or prolong their silence, then no messaging is better than messaging.
I think METI risks being the pursuit of people trying to prove themselves right in a world that thinks they are nuts. I get it, but I think they need to be realistic about the need to message. ET may not casually destroy us but will they be willing to share tech with us that may make us a threat. As well, how could one conduct trade in ideas and concepts between stars and maintain fair trade practices. How would we trade Newton’s laws for Relativity without explaining what they are first? As well, given the time it would take to develop a trade relationship and the time it takes to communicate, parties risk trading for things that end up being developed while they wait.
If ETIs hear us and aren’t reaching out it is likely because there is little to no return on the investment. I do not mean we are worthless, just that are most valuable resource, our otherness, would be eroded by contact.
Obviously, you don’t have to be a Supercivilization to use the gravitational lens of a star. How loud would humans be from the gravitational lens of a star light years away?
I hope there is a rebuttal from the METI people and I also agree that calling people dishonest is not helpful. There is too much of that going on right now and look what it has led to. A huge schism has developed now between those leaning left and those leaning right with nothing but angry words and accusations.
J. Watson Wentworth: The EIRP I’m referring to is the standard term: the product of antenna gain and transmitted power. By that measure the recent emissions are only 1% of Arecibo and therefore have a range that’s only 1% of Arecibo. (Because range at which a signal can be detected is directly proportional to the EIRP.)
You find advocacy of banning transmissions troubling. But that’s not what I’m proposing. I’m proposing a moratorium on such emissions until we’ve had sufficient opportunity to discuss it as a civilization. Specifically, I’m proposing the following:
Step 1: I suggest that all those radiating ‘messages’ provide clear documentation along these lines:
All such radiations, past and future, should be described completely enough to ascertain their delectability as a function of assumed ETI technologies.
Such descriptions should be required to meet peer-reviewed publication standards, which past radiation events have not done.
This information should include both transmitter parameters (power, frequency, aperture, bandwidth, frequency stability) and ‘message’ parameters (bit rate, keying method, error correction coding, number of message repeats, etc.).
There should be an on-line database containing such descriptions in a standard uniform format.
Step 2: Conduct international symposia on the METI issues, with experts and non-experts from many disciplines, would cover a wide range of topics related to METI and SETI, including:
Technical: science and engineering of beacons; detection probabilities; standard “passive” SETI as it exists now, and in the future, as compared to METI.
Sociological: study of historical analogs, sociological issues, national and international law, education and the media.
Geoffrey Hillend: Indeed, the planet we see around GJ 273b has about 3 Earth masses. That’s at the limit of what chemical rockets can escape from. Therefore the creatures there will probably not have a space program until they can devise high exhaust velocity nuclear rockets.
I don’t agree that biosignatures are observable at millions or billions of light-years away. In fact an out-of-equilibrium atmosphere is what you need but it’s not entirely clear that there is an unambiguous smoking gun indicator. They could be caused by other means. Oxygen is the most commonly used example. But the exoplanet community doesn’t entirely agree that it is a sign of life. And it certainly isn’t a sign of civilization.
Harold Daughety: the large drop in indigenous populations in many areas of the world after contact with European explorers certainly occurred. Most of it was due to particular diseases. That can’t happen with radio transmissions.
Michael: Laser systems cost approximately 10,000 times as much as millimeter and microwave systems today. (Laser advocates tend to disguise this fact in their presentations.) Fiber lasers today cost about $100/watt and microwaves are available at a few cents/watt If you want to spend 10,000 times as much money on such system, go right ahead and buy lasers.
Michael Fidler: It’s true that, decades ago, it was thought that ETI would have an easier time spotting signals from over-the-horizon radars built during the Cold War, which directed much of their power into space. Sullivan quantified that. But those have long since been superseded by frequency-hopping ‘spread-spectrum’ broadband radars that are undetectable by ETI.
Alex Tolley: You make several interesting points in your essay. However, several physical realities are different from what you expect.
One is that industrial pollution is observable at great range. Almost all “pollution” by civilization is in the form of oxides of carbon, sulfur and nitrogen. All of these occur the atmosphere naturally; our contribution to it is this not seriously observable. These oxides were present in long before civilization. For example, carbon dioxide is an essential catalyst in the reactions that make a life on earth possible. But geological processes also emit it. Carbon dioxide is present at 400 ppm in the atmosphere and only part of that is due to us. There are times in the past when CO2 was greater than it is today and their times in the recent past, the ice ages, when it was very low. Volcanoes, lightning and other natural phenomena produce other atmospheric constituents that might be thought of as industrial. They have always been here and always will be.
True industrial pollutants in our atmosphere are extremely small and not easy to measure even when you’re in the atmosphere. We cannot make infinitely sensitive detectors and you always have to consider the background noise in any detection. It’s the signal-to-noise ratio that matters.
Starshot probes sent to nearby stars would contain in their velocity vector an indication of where they came from. But that really says the solar system was the launch point; it doesn’t tell them anything about us. Capturing a Starshot probe would tell them something about our technology level. They’d have to catch it at 0.2 light speed, which would not be a trivial matter.
You say that we can be observed by a supercivilization at great range, therefore why bother to oppose deliberate emissions. You are committing a category error. Unintended emissions or observables are just that, inadvertent. Deliberate emissions such as the METI-ists do and advocate is deliberate, a different category of activity. For example the law recognizes the difference between an unintended crime and deliberate crime. They have very different moral implications as well.
Dr. Benford:
We need to be just a little bit smarter, and detect macro effects. For example, chlorofluorocarbons although in very small amounts do have teh macro effect of creating polar “ozone holes”. These ozone holes may be detectable by scopes that can image worlds (and hence do spectroscopy at different places on the planet) using gravitational lensing, a technique that is not so far off technologically.
We might detect glints off reflective metal surfaces, or the regular lines of political borders (e.g. The US-canada border can be seen from space quite clearly, as can the border between Haiti and the Dominican Republic).
Hunters don’t need to understand anything about ducks either, just locate their presence to shoot them. DB says we shouldn’t shout “YooHoo” in te dark. It also is nothing but a meaningless sound, that gives away a location.
Hard for us maybe, but a million years advanced ETI? They might deploy a laser to slow the craft much as we launched it. Or tease it into orbit around a black hole to observe it. Perhaps they could send a probe to match its velocity and analyze it. So many options that we can already think of. Once they have its point of origin, they can deploy telescopes to observe our world, or send probes of their own.
Being killed deliberately or accidentally may be different, but the victim is still dead. The anti-METI argument is about fear of the consequences of discovery. The super-predator bogey-ETI may be out there, so we should stay silent. It is even used as one explanation for the Fermi question. Your argument boils down to small noises may be considered natural, but a big bang, or series of bangs, will attract unwanted attention. I argue that this is a failure of imagination. ETI will likley be as advanced over us, as we are over animals. We can detect sounds in the jungle over a wide range of decibels. We can see with a much wider range of the EM spectrum than animals. It is getting close to where we can detect volatiles as well as dogs. Animals hoping to evade us can only do so as individuals in groups. Is our strategy really to hope that our world is hidden amongst hundreds of billions in the galaxy? That we hope to avoid predation by not attracting attention in a herd, or staying motionless to avoid movement detection?
If ETI has starflight capability, then we may well have a Clarkian monolith waiting to detect our presence fairly locally. If they don’t then what are they going to do? Trick us into doing something dangerous? Run a Rigellian Bank scam on us? Are we really all going to suffer culture shock with contact? If so, then SF authors haven’t done a good enough job innoculating us.
Although microwave/millimetre systems are much, much cheaper the cost of laser systems will drop in time as the communication industry requires faster and faster transmitting rates. I in a personnel opinion would rather we not transmit in a directionally powerful way until we know more about our local star systems and we spread out more. I am also of the opinion as other have stated here that METI is more to do with publicity and money than scientific endeavour.
Hello Jim,
Thank you for responding and answering my question (and those of the other readers as well). I thought you’d meant the regular EIRP (referenced to an isotropic radiator), but I wasn’t sure. (Another “rule of thumb” [because it’s not exact] involves dividing a radio/radar telescope dish’s diameter–in feet–by 10, which yields the range at which an alien instrument of the same size, with a high-power transmitter, could be detected and “read.” Thus Arecibo could hear its “twin” up to 100 light-years away, Jodrell Bank could hear its counterpart at 25 light-years, and so on.) Also:
Your other proposed steps sound perfectly sensible to me, although compliance would necessarily, due to the very nature of this activity, be voluntary and hard to enforce. The difficulty of detecting surreptitious METI transmissions by dishes and other highly-directional antennas would make it hard to detect any moratorium violators, unless a satellite happened to cross the transmitting antenna’s beam at just the right moment and was briefly “swamped,” or if “side-lobe” reflections off the Moon happened to be heard–but anyone with the expertise to attempt such transmissions could easily take steps to avoid both situations, but:
Hopefully Kim Jong Un won’t take a notion to achieve interstellar immortality with gigantic Morse code dots and dashes blasted into space via artificial lightning bolts (as Nikola Tesla did it), or with a multi-megawatt spark-gap transmitter and a huge foil-lined dish hole in the ground (or a reflector-backed dipole array), as either would spread “RF pollution” here on Earth! :-) Regarding METI messages themselves:
Just as the Pioneer plaques (parts of which–“design-wise”–are also on the Voyager spacecraft’s record covers) are a sort of de facto “minimum-content template” for graphics-based interstellar messages, maybe the discussions within and between the SETI and METI communities could result in the creation of a similar “METI template” message (that could be expressed in multiple forms, depending on the signal modulation method used), which all parties would consider okay to transmit (and which, later, could also serve as a “cipher” or a “key” for longer METI messages). One portion of it might be as follows:
I had described to Paul (and to Douglas Vakoch) a possible “key” that could be included in interstellar messages (whether they be “delivered” directly from Earth via radio or laser, or—in the future—by means of ‘local’ transmissions from Bracewell-type interstellar messenger probes). It is possible that my idea, because it is so simple, may already have been thought of by others, but I have never read about anything similar to it in the METI and SETI literature (although my reading of this literature is admittedly not exhaustive). The message “key” would be composed as follows:
In Arthur C. Clarke’s book “The Promise of Space” (and in the classic collection of interstellar communication papers by Drake, Morrison, Oliver, and others), pictorial “dot-and-dash” (or “1” and “0”) messages are discussed, such as Frank Drake’s 551-digit (the product of two prime numbers, 19 and 29) message which—when the 0’s and 1’s are arranged in a 19 x 29 grid (or a 29 x 19 grid)—yields either a meaningless jumble, or a clear-cut picture of a biped surrounded by geometric symbols. This raises a question:
Has anyone ever suggested including a “key” to interstellar messages, at the beginning of such messages (and repeated to ensure that it was received “intact,” of course), that would be a square (perhaps containing an “X” of 1’s or 0’s from corner to corner, or maybe a “+” sign of such digits, running between opposite sides)? Such a square “key,” made up of equal (prime) numbers of digits per side, would *always* show a regular, symmetrical appearance regardless of which way the grid was oriented, which would show the alien signal recipients what the multi-digit “strings” of numbers are for. Then:
This could be followed up with rectangular pictorial grids like the 551-digit, 19 x 29 (or vice-versa) one that Dr. Frank Drake once sent to his friends to see if they could decipher it. Even if the signal recipients arranged the rectangular grid-pictures the wrong way at first, they would soon discover that arranging the grids the only other possible way would show coherent images. Later, such grid-pictures could be used to convey crude, slow animation, or the shapes of our alphanumerical symbols, or many other things.
Two points seem obvious:
1. An air of secrecy surrounds these and previous METI events: no announcement before they do it. Why? No response to objections, either. This makes more concrete the spreading impression that this is simply a publicity campaign.
2. There is a very real possibility that grandstanding METI events will discredit the more sober SETI field. Stunts do that. So do their false claims about detectability. They do very real damage to an emerging scientific field. This is deplorable.
I’m a bit late to the discussion, it seems. Here is the video of Dr. Kelly Smith’s presentation at the most recent Tennessee Valley Interstellar Workshop symposium, in Huntsville, AL, that addresses this exact issue:
https://youtu.be/dmbhWM0FD_c?t=7
Tongue-in-cheek response.
The corollary of Clarke’s 3rd Law implies that magic is really high technology. Theists believe that there is an omniscient and omnipotent entity that observes all events and can intervene. Therefore we can infer that this is the same as an ETI with very advanced technology.
Religions offer prayers to the entity. As they are sonic, the entity must be observing within our atmosphere. As the entity can observe all events, its senses must be everywhere, even possibly interpreting thoughts. Is individual prayer low-level noise, while congregations engaged in prayer a high power signal?
Are organized religions attempting to ensure the acceptable communication with the entity, or are they rogue communications? Is prayer by an individual or congregation a non-diplomatic communication when ignoring other religions’ communications?
The parallels with the METI argument should be clear. Is there even a danger that organized, “acceptable” METI messaging (or banned messaging) becomes religious over time?
The Invisible Pink Unicorn just giggled at that, Alex… at least I think she did.
;)
There must be a limited number of facilities that can make enough noise to attract interstellar attention. Let the kids play around with their cracker-box shortwave sets. What harm can that bring?
I’m not a proponent of METI but if there were a technical civilisation in this planetary system, I don’t think its logical to assume the potential recipients of these messages will have the same technological limitations that we do. For example, we are already beginning to build radio telescopes with much larger fields of view than traditional paraboloids permit via the adoption of Phased Array Feeds and Aperture Arrays (see Benford’s point [1]). In addition, the Figure of Merit of radio telescopes as survey instruments must have increased by about a factor of 1 million since the early 1960’s, it would be folly to consider that further gains will not continue to be made by us (and by other civilisations). However, I personally belive that the rarity of such technical civilisations probably means we have absolutely nothing to worry about anyway.
If the thought of a radio transmission to the stars makes you tremble, then you better not read what the Soviet father of the hydrogen bomb had planned in 1971 for getting the attention of alien minds…
http://lnfm1.sai.msu.ru/SETI/eng/articles/sakharov.html
Arthur C. Clarke mentioned “H-bomb METI” too, as well as “labeling” a star’s atmosphere with artificial elements. Maybe Dr. Sakharov wanted to expend the materials that might have gone into more Tsar Bombas, while also putting the explosions to scientific use, but:
My only objection (since thermonuclear bombs could be launched into space safely, in disassembled form) is that the bombs that–in his proposal–would be used for interstellar signalling could be used to power Orion spaceships, starprobes, and starships instead. In fact, with multiple Orion ships, both purposes could conceivably be served at once, although the ships’ synchronized thermonuclear thrust pulses wouldn’t be visible in quite all directions, because their pusher plates would obscure part of each expanding fireball.
I am all for Orion starships, but others are less so. For those who have issues with METI consisting of benign radio messages, I can only imagine the reaction to a full loaded starship powered by nuclear bombs being shot out its rear every three seconds plowing through the galaxy and then entering an alien solar system.
Whatever course it takes, as with most things in human history, our first encounters with ETI will probably not be as expected, nor go as nicely as hoped, simply due to human nature and probably the nature of the aliens we encounter.
People want to control everything, but the Universe usually has other plans. Better to prepare for that kind of reality if we want First Contact to succeed.
(“… the reaction to a full loaded starship powered by nuclear bombs being shot out its rear every three seconds plowing through the galaxy and then entering an alien solar system.”
I would imagine if the system was inhabitted by machine artilects they would be most distressed wondering what their visiting cousin must’ve eaten for all THAT to be shooting out its rear-end!)
“That is the last time we make a pit stop at Taco Bell!”
I think these messages to ETI serve some sort of psychological need. The purpose is not to communicate and be heard, it is to shout into the unknown; the fact that their voice is swallowed up by the wind is beside the point. I can’t explain the psychology exactly, but I’m sure that’s what this is about.
We don’t know much about ETI, except for vague projections based on universal constants. Even if we could successfully communicate to ETI, we are not certain as to whether this is wise or unwise. It is also improper for a select group to elect themselves as representatives of the entire human race. As you said, there should be no more star whispering until a broader consensus is established.
I wonder how many people oppose dispatching interstellar probes–particularly Bracewell probes–as well, and if those people also oppose signals-based METI (and vice-versa). Even Ronald Bracewell suggested that we should just listen out (“do SETI”) for some time–decades to a century or more, if memory serves–but that if we hear nothing after that time, we should take the initiative to seek out any neighbors (who might not have technologies like ours, or any technology at all [he quoted Freeman Dyson’s comment that it is easy to conceive of intelligent civilizations that have no particular interest in technology]), and:
One type of interstellar probe that might appear hostile to any locals would be a Von Neumann-type, self-replicating probe. (If we found that some such spacecraft had arrived here and was “eating” a moon or asteroid in our solar system to make copies of itself, we would certainly be justified in wondering if its purposes were hostile, no matter how friendly any communications from it might sound, and even more so if it ignored us.) As attractive as Von Neumann probes are to many starflight enthusiasts (because we only need to pay for and launch *one* in order to–eventually–explore indefinitely many stars and planets), I’d be leery of launching that one self-replicating probe, because it probably wouldn’t give any aliens who encountered it a positive first impression of humanity.
I think probes are a different matter. I agree that there is a need for caution in that pursuit, especially in the case of self-replicating probes. However, the main purpose of a probe is to extend our observations by sending out mechanical eyes and ears, whereas signals are primarily for communication. While both signals and probes reveal the presence of our civilization, they must be considered separately.