We’ve now had humans in space for 25 continuous years, a feat that made the news last week and one that must have caused a few toasts to be made aboard the International Space Station. This is a marker of sorts, and we’ll have to see how long it will continue, but the notion of a human presence in orbit will gradually seem to be as normal as a permanent presence in, say, Antarctica. But what a short time 25 years is when weighed against our larger ambitions, which now take in Mars and will continue to expand as our technologies evolve.
We’ve yet to claim even a century of space exploration, what with Gagarin’s flight occurring only 65 years ago, and all of this calls to mind how cautiously we should frame our assumptions about civilizations that may be far older than ourselves. We don’t know how such species would develop, but it’s chastening to realize that when SETI began, it was utterly natural to look for radio signals, given how fast they travel and how ubiquitous they were on Earth.
Today, though, things have changed significantly since Frank Drake’s pioneering work at Green Bank. We’re putting out a lot less energy in the radio frequency bands, as technology gradually shifted toward cable television and Internet connectivity. The discovery paradigm needs to grow lest we become anthropocentric in our searches, and the hunt for technosignatures reflects the realization that we may not know what to expect from alien technologies, but if we see one in action, we may at least be able to realize that it is artificial.
And if we receive a message, what then? We’ve spent a lot of time working on how information in a SETI signal could be decoded, and have coded messages of our own, as for example the famous Hercules message of 1974. Sent from Arecibo, the message targeted the Hercules cluster some 25,000 light years away, and was obviously intended as a demonstration of what might later develop with nearby stars if we ever tried to communicate with them.
But whether we’re looking at data from radio telescopes, optical surveys of entire galaxies or even old photographic plates, that question of anthropocentrism still holds. Digging into it in a provocative way is a new paper from Cameron Brooks and Sara Walker (Arizona State) and colleagues. In a world awash with papers on SETI and Fermi and our failure to detect traces of ETI, it’s a bit of fresh air. Here the question becomes one of recognition, and whether or not we would identify a signal as alien if we saw it, putting aside the question of deciphering it. Interested in structure and syntax in non-human communication, the authors start here on Earth with the common firefly.
If that seems an odd choice, consider that this is a non-human entity that uses its own methods to communicate with its fellow creatures. The well studied firefly is known to produce its characteristic flashes in ways that depend upon its specific species. This turns out to be useful in mating season when there are two imperatives: 1) to find a mate of the same species in an environment containing other firefly species, and 2) to minimize the possibility of being identified by a predator. All this is necessary because according to one recent source, there are over 2600 species in the world, with more still being discovered. The need is to communicate against a very noisy background.
Image: Can the study of non-human communication help us design new SETI strategies? In this image, taken in the Great Smoky Mountains National Park, we see the flash pattern of Photinus carolinus, a sequence of five to eight distinct flashes, followed by an eight-second pause of darkness, before the cycle repeats. Initially, the flashing may appear random, but as more males join in, their rhythms align, creating a breathtaking display of pulsating light throughout the forest. Credit: National Park Service.
Fireflies use a form of signaling, one that is a recognized field of study within entomology, well analyzed and considered as a mode of communications between insects that enhances species reproduction as well as security. The evolution of these firefly flash sequences has been simulated over multiple generations. If fireflies can communicate against their local background using optical flashes, how would that communication be altered with an astrophysical background, and what can this tell us about structure and detectability?
Inspired by the example of the firefly, what Brooks and Walker are asking is whether we can identify structural properties within such signals without recourse to semantic content, mathematical symbols or other helpfully human triggers for comprehension. In the realm of optical SETI, for example, how much would an optical signal have to contrast with the background stars in its direction so that it becomes distinguishable as artificial?
This is a question for optical SETI, but the principles the authors probe are translatable to other contexts where discovery is made against various backgrounds. The paper constructs a model of an evolved signal that stands out against the background of the natural signals generated by pulsars. Pulsars are a useful baseline because they look so artifical. Their 1967 discovery was met with a flurry of interest because they resembled nothing we had seen in nature up to that time. Pulsars produce a bright signal that is easy to detect at interstellar distances.
If pulsars are known to be natural phenomena, what might have told us if they were not? Looking for the structure of communications is highly theoretical work, but no more so than the countless papers discussing the Fermi question or explaining why SETI has found no sign of ETI. The authors pose the issue this way:
…this evolutionary problem faced by fireflies in densely packed swarming environments provides an opportunity to study how an intelligent species might evolve signals to identify its presence against a visually noisy astrophysical environment, using a non-human species as the model system of interest.
The paper is put together using data from 3734 pulsars from the Australia National Telescope Facility (ATNF). The pulse profiles of these pulsars are the on-off states similar to the firefly flashes. The goal is to produce a series of optical flashes that is optimized to communicate against background sources, taking into account similarity to natural phenomena and trade-offs in energy cost.
Thus we have a thought experiment in ‘structure-driven’ principles. More from the paper:
Our aim is to motivate approaches that reduce anthropocentric bias by drawing on different communicative strategies observed within Earth’s biosphere. Such perspectives broaden the range of ETI forms we can consider and leverage a more comprehensive understanding of life on Earth to better conceptualize the possible modes of extraterrestrial communication… Broadening the foundations of our communication model, by drawing systematically from diverse taxa and modalities, would yield a more faithful representation of Earth’s biocommunication and increase the likelihood of success, with less anthropocentric searches, and more insights into deeper universalities of communication between species.
The authors filter the initial dataset down to a subset of pulsars within 5 kpc of Earth and compute mean period and duty cycle for each. In other words, they incorporate the rotation of the pulsar and the fraction in which each pulse is visible. They compute a ‘cost function’ analyzing similarity cost – how similar is the artificial signal to the background – and an energy cost, meaning the less frequent the pulses, the less energy expended. The terms are a bit confusing, but similarity cost refers to how much an artificial signal resembles a background pulsar signal, while energy cost refers to how long the signal is ‘on.’
So if you’re an ETI trying to stand out against a background field of pulsars, the calculations here produce a signal background period of 24.704 seconds and a duty cycle of ~0.004 (meaning that the signal is ‘on’ for 0.4 percent of the period). Such signals appear at the edge of the pulsar distribution – they would be signals that stand out by being relatively rare and also brief in contrast to the rest of the pulsar population. They would, in other words, serve as the optimal beacon for ETI attempting to communicate.
I spare you the math, which in any case is beyond my pay grade. But the point is this: A civilization trying to get our attention while broadcasting from a pulsar background could do so with a signal that has a long pulsar period (tens of seconds) and a low duty cycle. This would be sufficient to produce a signal that becomes conspicuous to observers. Now we can think about generalizing all this. The pulsar background is one of many out of which a possible signal could be detected, and the principles can be extended beyond the optical into other forms of SETI. The broad picture is identifying a signal against a background, proceeding by identifying the factors specific to each background studied.
Any time we are trying to distinguish an intentional signal, then, we need to optimize – in any signaling medium – the traits leading to detectability. Signals can be identified by their structural properties without any conception of their content as long as they rise above the noise of the background. Back to the fireflies: The paper is pointing out that non-human signaling can operate solely on a structure designed to stand out against background noise, with no semantic content. An effective signal need not resemble human thought.
Remember, this is more or less a thought experiment, but it is one that suggests that cross-disciplinary research may yield interesting ways of interpreting astrophysical data in search of signs of artificiality. On the broader level, the concept reminds us how to isolate a signal from whatever background we are studying and identify it as artificial through factors like duty cycle and period. The choice of background varies with the type of SETI being practiced. Ponder infrared searches for waste heat against various stellar backgrounds or more ‘traditional’ searches needing to distinguish various kinds of RF phenomena.
It will be interesting to see how the study of non-human species on Earth contributes to future detectability methods. Are there characteristics of dolphin communication that can be mined for insights? Examples in the song of birds?
The paper is Brooks et al., “A Firefly-inspired Model for Deciphering the Alien,” available as a preprint.
A quick skim of teh paper strikes me as somewhat of a poor analogy, with unnecessary assumptions constrained by the firefly model.
It is always nice to see some new “out of the box” thinking about the problem of recognizing technosignatures.
However, I am skeptical about the analogy and model for the following reasons:
Firstly, fireflies are limited by the chemical reaction they use to create the light flash. All fireflies use pretty much the same reaction, which results in the same peak emission of light. Therefore, their only choice of signal differentiation is to change the code. This is like the telegraph system, where unique codes identify the receiving station. However, once we entered the radio era, we could choose different em frequencies to separate signals. Today, the radio spectrum is sliced up for many different purposes, with different frequencies within each bandwidth to carry the content signal.
Secondly, fireflies are using their signaling, as are other methods by other animal species, to indicate location and therefore to allow the individuals to come to the same location where important communication before mating is done. If ETI is in its own home system, there is no need to locate another world with the same species – they are alone. If they have spread out through interstellar travel, wouldn’t they know where their species is, rather than broadcasting a signal to try to find them? The only model that makes sense is that a species has migrated through the galaxy and continues to evolve. The signal recognition is only to find evolved species that are sufficiently similar to be worth communicating with, and to ignore others.
Thirdly, a feature of firefly signaling is that they start randomly, and then slowly synchronize, so that the flashes appear together, rather than randomly. If this model applies to ETI, then one would expect, within the difficulties of time, space, and location, that pulses should synchronize between species that want to communicate with each other. It might therefore make sense to see if the duty cycles of the same signal patterns “synchronize”.
Lastly, given the issues of the limitations of c hinted at above, wouldn’t the signaling model need to use FTL means of communication? If so, as we have no knowledge of such communication modes, wouldn’t this imply that this model has no relevance for our SETI methods?
While this model uses fireflies, it is a common pattern amongst species. Think of the mating calls of birds, using sound instead of light, or similarly, frog croaking. As with any such signaling, predators may mimic the signal to lure their prey within striking distance. Could “Dark Forest” predators do the same for this ETI signaling?
Reading the paper more carefully, it appears that the firefly analogy is a bit of a red herring. The approach is really to create a beacon that will stand out from the background for a particular target world.
IOW, there might need to be a unique signal for each target world, or at least for a close cluster of worlds in space.
It also assumes that the binary signal requires a pre-determined energy level to be considered as “ON”. The question I have is how the receiving world’s intelligences know where this cutoff is to be set to convert an analog signal to a binary one? A wrong setting could make the signal blur into the background.
As the background includes pulsars out to 15,000 ly, this means that the pulsars will move over time. This may require periodic tweaking of the signal parameters to maintain its uniqueness to stand out from the background.
When considering the approach, I really don’t see the value compared to using a narrowband signal with a non-natural pattern, like a series of prime numbers. Why not use a laser to transmit the signal, or perhaps some other medium that is not common in the galaxy, but likely to be detected?
If the signal is just a “hello, can you hear me?”, it needs a reply to trigger the content signal in some way. That could take a very long time. For an advanced ETI, it might be a lot better to just send Bracewell probes to the system, updating them as they wear out waiting to be contacted by the emerging technological civilization. Any communicating strategy – calls, followed by handshakes to find a common “language”, and then a “conversation” or translated galactic history or encyclopedia, would be much more timely, and might be necessary if technological civilizations need some help to pass through some extinction-level great filter.
There is a cloying bias in the paper:
“Though ingenious, these methods are likely still too centered on the human experience and way of understanding the world. These should not be expected to be readily conceived of, or even identifiable, by an alien mind. Likewise, alien communication may be entirely unlike the kinds of communication we find across the human species and human technologies.”
The implication that ETI, though supposedly highly advanced, is incapable of realizing that their natural structure of communication might not be the only one, and therefore that’s how they conduct METI/SETI. But humans (aren’t we the clever ones!) are not so limited.
I would counter that ETI is as least as sophisticated in their thought as we are, and indeed as they must be. They will, like us, consider various communication options in a bid to be heard and understood by sentient, technological species with unknown inherent traits and biases.
To counter the paper more directly, I take aim at their basic premise that the signal ought to differ from natural phenomena. Instead, I suggest, the primary signal should look almost exactly like a natural astrophysical source. After all, it is to be expected that astronomically active species hunt for and study these phenomena. So make it look like a pulsar to ensure that search algorithms find it. Place the communication channel in proximity where it is then easy discovered. For example, a frequency where the “pulsar” signal peaks but between the pulses.
A signal with irregular pulses, per their firefly analogue, could easily escape a search algorithm. I’d be thinking about that were I the one designing the signalling protocol.
I went only as far as the introduction. I’m not sure that it’s worth reading further.
Hi Paul
Something a little different to read, and think about thats for sure.
Interesting comment Alex
Cheers Edwin
I would rather try to orbit copper cables and harvest power from pulsars
Might a network allow a space cursor where intersecting beams pantograph a Leak Myrabo deal around?
Here, low voltages seem to allow electrical energy to be converted into mechanical motion:
https://techxplore.com/news/2025-11-polarization-motion-ferroelectric-fluids-redefine.html
For light relay:
https://www.secretprojects.co.uk/threads/solid-state-laser-news.9380/page-4#post-260788
I think we are overthinking this,
Frank Drake, Carl Sagan and their colleagues already had it figured out in the 1960s. There are very good scientific and technical reasons to use radio telescopes for SETI, and there are equally valid reasons to search in the microwave spectrum for evidence of other civilizations in the cosmos. Just because we haven’t found anything in a scant half-century of searching does not mean the pioneers’ assumptions were hopelessly anthropocentric or provincial.
1) Even if ETIs have long ago abandoned radio as a means of interstellar communication (perhaps replacing it with Q-rays, modulated neutrinos or tachyon beams or some other exotic tech) it is safe to assume that they have an interest in astronomy, and surely they will understand other civilizations will share that interest. They will be listening to radio, and they will assume we will be listening, Should they wish to make their presence known, electromagnetic radiation is an obvious choice. We are astronomers, and they probably are as well.
2) Microwaves have another great advantage, the interstellar medium is transparent to them, so they will be easy to detect at great distances.
3) The ‘waterhole’ is a low-noise region of the spectrum, the few natural sources of radiation there (such as the 21cm band) are narrow band and easily detectable and can be filtered out, so signals transmitted at some harmonic of those frequencies will be a logical way to attract the attention of astronomically adept species.
If an ET civilization wants to make contact, the microwave region is the most reasonable place for a beacon, or to listen for one. If they desire to remain hidden, then they will not transmit in those frequencies (although they will most certainly be listening). If they are engaging in industrial activities that are noisy in the waterhole, they will make every effort to try to keep them silent if they wish to remain anonymous. On the other hand, if they choose to deliberately create a biosignature, then selecting a wavelength likely to be detected would serve both purposes. For example, a planetary defense radar operating at 21 cm could be detectable by even our primitive technology at immense distances. This could very well be the origin of the Wow! signal, a beam that briefly swept across our planet a half century ago.
A case can even be made for the alternate scenario. Assume the optimum frequency for some industrial application would be easily detectable and classed as artificial by a hostile neighbor. Tuning your planetary defense radar to a common sub-optimal “natural” wavelength instead (like 21 cm) might make it likely that it would be missed or otherwise ignored if briefly detected by a potential enemy. It would just fade in to the background hum of galactic hydrogen.
I suspect we, as a species, do not possess the technological sophistication to detect a technosignature from an advanced civilization determined to stay hidden at all costs. On the other hand, if they are trying to get our attention, I’m sure they will make every effort to stick out like a sore thumb.
Of course, if they are trying to advertise their presence, then their beacon would have to be deliberately noticeable, even over long distances. We naturally assume ETI will be far advanced over us, but what if that is NOT the case? Just because our science has advanced enormously over the last two hundred years does not mean it will continue to progress with ever-increasing rapidity indefinitely. Maybe we’ve just been picking up the low-hanging fruit and from now on technical progress will continue at an increasingly slower pace. Perhaps there are natural barriers? We are assuming our distant future will be similar to our recent past and THAT is the anthropocentrism I fear most.
We may already be one of the most advanced civilizations in the cosmos. Perhaps we’ve done all the easy stuff and we are already near some natural limit of technological sophistication, and that suggests they may already be there too.
@henry
Are we really “overthinking” this?
If so, why does SET! have an optical SETI program as well as its traditional radio SETI?
Radio-based SETI made a lot of sense in the 1960s for the reasons you state. It even works for ETI stuck below an optically opaque cloud cover, or in a region that is already optically very noisy. However, for ETI that has eyes that are sensitive in the optical region, from IR to UV, an optical signal works well for a civilization that can respond to civilian observations on a daily basis. This is the most primitive concept of an “all-sky” program. After all, all astronomical observations were by eye before Galileo’s invention of the telescope, and remain the best means of quickly detecting ‘strange” anomalies with or without telescopes.
I would argue that a laser signal that shows a star sending out a pattern is more likely to draw attention, even if fairly transient. A telescope with a spectrum analyzer will even show the signal coming from one or a few narrow bandwidths, which would surely provoke interest among professionals. I daresay that even amateur astronomers could add such analyzers to their ‘scopes as part of a citizen science program, much like bird watchers contribute to bird species surveys.
So why lasers now, rather than back in teh 1960s and a few decades later?
Firstly, the laser was only invented in 1960. It was usually conceived of as an intense, optical “death ray” as depicted in the “Goldfinger” movie and many later SciFi movies. Efficiencies were low, and high-power beams interfere with the atmosphere, which, even today, is a price that is acceptable for phased laser arrays as the beam for a photon sail, because the cost of a space-based facility remains immense and economically infeasible with current launch technologies, even with RLVs.
More fanciful optical approaches could include a solar polar obititing series of sunshades that could create a “barcode-like” dimming of the star’s output, like complex transit data. This would be another “all-sky” approach that sends a complex optical dimming signal over the whole sky in regular repeating transients.
But why assume ETI wouldn’t build such devices, especially if they are approaching KII level status with energy to “burn”?
Radio emissions make a lot of sense if the aim is to transmit information between systems, assuming a “galactic club”, which we might detect by chance from leakage or the occasional deliberate reorienting of the beam. But once we start thinking in terms of “beacons” from advanced ETI to less advanced species, then optical means seem like they would have more success. After all, Gaia and other all-sky observatories use optical imaging, not radio imaging, for obvious reasons.
I rest my case. ;-)
The distinction between microwave and optical (radio vs lasers) is one of degree, not of of kind. After all, OUR physiology has evolved to detect light, not radio, but we still first used radio as a potential SETI methodology in spite of that. Actually, it makes little difference fundamentally; they are both forms of electromagnetic radiation. For all we know ET may be blind as a bat but still find his way around with natural radar.
The key distinction between the two is the opacity of the interstellar medium
at different wavelengths. Dust in the galactic plane obscures ‘optical’ frequencies much more than it does microwaves (the dreaded ‘interstellar reddening’ astronomers are always whining about)–NOT to be confused with ‘red shifting’). This is why infrared and radio astronomy are so useful today for general astronomy, regardless of our biological preferences. Don’t sell radio short. This holds true in deep space as well as at the bottom of our atmosphere. Optical SETI is very promising, and must be fully exploited, but it is not a replacement for ‘old fashioned’ radio. The signal to noise ratio is just much better at microwave wavelengths. It has nothing to do with technology or biology.
The nearest use of radar in animals is the sonic pinging used by bats. But just as resolution is dependent on wavelength and aperture, bats can locate objects in the dark with sound, but not fine details of their moth prey. Wavelength is the reason there is no technology to make microscopes using microwaves or sound. Aperture size is possibly the main constraint that animals probably could never have used radio waves for communication or as a means of vision.
Radio transmissions are not about resolution but reception. Radio proved easy to make, easy to modulate, and was not so limited by obstacles that could block light. Nevertheless, before radio, long-distance communication was achieved with light using fire and semaphores. Ships used light for in-line-of-sight communication that could not be intercepted like radio, even when radio was widely used during war for other communication purposes. Sound has been used to communicate for millions of years. But it requires a medium, hence “In space no one can hear you scream.”
Now that NASA is experimenting with lasers for deep space communication, the higher information transmission rate with current technology, it is quite possible that ETI would use lasers to communicate. Had we known of this as a possibility back in the middle of the last century, we might have suggested laser light communication rather than radio as the best way to transmit information between star systems. To be sure, there are tradeoffs to be had between radio and optical transmissions, but as we know, we can see stars and galaxies at great distances. I find it hard to accept that the ISM causes too much of a problem except under certain circumstances, such as gas and dust clouds obscuring the stars beyond in the line of sight. Unless all transmission must be line of sight only, there are so many ways to provide multiple, redundant pathways that it seems to me that light is still a good way to transmit information across space, with an inherent advantage in data rates compared to radio. Short “blips” might be all that is needed to transmit information-dense signals using light, or even shorter wavelengths.
Regarding SETI explicitly. If ETI was modulating the radio signal from its star using some radio-blocking mechanism to create a slow binary signal like the firefly example, would it work better with radio waves or light? [Hint. We don’t use radio waves to detect transiting planets.] Radio or optical pulses would work well for the firefly example as the information rate is likely to be very low. But as I said earlier, all-sky surveys for these firefly (or other) signals are going be be easier with light, and because light signals are not so easily confused by background noise because of the higher spatial resolution, these elaborate approaches explained in the paper become superfluous. We can see changes in the illumination of individual stars, so that other stars changing output over time do not add noise to the particular star’s signal.
A crude example of the importance of spatial resolution is the famous “Wow!” signal. Spatial resolution was so poor that there was no hope of determining a stellar source. Even the recent Breakthrough Listen analysis of radio “anomalies” could only indicate possible radio sources in the sky, not a definite target based on signal reception. In teh days of analog radio reception, signals from different transmitting stations might “overlap” in frequency/wavelength when turning the dial for reception. With packet-based transmission over the internet, this cannot happen, as each transmission is identified by a unique ID – the equivalent of “perfect spatial resolution”.
@Alex
Everything you say is true, but you’re still missing the point.
Sure, the shorter the wavelength, the higher the bandwidth, an inescapable law of physics. You can cram more information into optical frequencies than you can into radio. An optical laser, or even UV, Xray or gamma would be ideal for communicating, that is, transmitting lots of data. But to make yourself heard, to get someone’s attention over potentially long distances, other considerations come into play, like opacity, extinction, that is, anything that affects the signal to noise ratio. A beacon would best be radio, and the best frequencies to pick would be in the quiet (yet probably intensely monitored by alien astronomers) waterhole.
The universe is practically transparent in the microwave region, and what few natural emissions found there are extremely narrow band, such as the OH- and 21 cm emissions. This suggests alien astronomers would be studying that part of the spectrum intensely, yet there would be little stray natural noise there to obscure an artificial signal.
Other frequency ranges would be obscured by natural extinction, both in planetary atmospheres or in interstellar space. This occurs at all wavelengths, of course, but is minimized in the ‘waterhole’. I suggest you look up interstellar extinction, or ‘reddening’ which is such a problem in optical astronomy. This dimming is from dust in the galactic plane which scatters blue light causing things to appear redder and dimmer than they actually are at lower galactic latitudes. Unfortunately, low latitudes are where most of the stars are!
Any civilization with active SETI ambitions would certainly want its beacons to penetrate as far into this fog as possible and for its signal to be dimmed as little as possible, so optical beacons would underperform compared to microwaves. The same considerations would apply to the passive, or receiving party. The Wikipedia article tells us “For stars lying near the plane of the Milky Way which are within a few thousand parsecs of the Earth, extinction in the visual band of frequencies (photometric system) is roughly 1.8 magnitudes per kiloparsec.[4]”
Of course, for relatively nearby targets (say, several hundred pc) this would not be a problem. But it stands to reason that for someone trying to make their presence known throughout as large a volume of space as possible, it would be. Looking for lasers in the optical region would only make sense for surveys of nearby stars; to sample large volumes you need microwaves.
Once contact is made with an alien civilization, if it isn’t too far away, THEN an intensive exchange of information would favor optical frequencies with their bigger bandwidth. But for a beacon, a simple, “Hey, is anybody out there?”, radio is the way to go.
The argument boils down to signal-to-noise. Wavelengths at some offset of the waterhole maintain a good signal. Because the weather doesn’t impede teh signal, the radio telescope receiver can be situated on the surface of a biosphere supporting planet, like Earth. The downside is that the signal can only be detected by radio telescopes, which requires this technology to have been invented and used by the recipients. In our case, we only really got started in the middle of the 20th century. In addition, the resolution of the source of the signal is poor and requires an optical map of the sky to help locate the source.
Optical signals are detectable by any recipient with sensors in the optical spectrum. Land-based organisms have had eyes for hundreds of millions of years. A series of flashes from a star is readily detectable even by a dinosaur, albeit of no use cognitively. Humans with culture have been around for 10s of millennia. We have had sky observations for millennia – didn’t our ancestors note the supernova that is now the Crab Nebula? We have had optical telescopes for hundreds of years. We still use them more widely than radio telescopes, including the Hubble Space Telescope. So the opportunity to detect what may be a transient is more likely with optical telescopes.
As for how to create a visually detectable signal, that is clearly an issue. The best way is to use a laser so that the wavelength is clearly discernible in the spectrum of a star. For a civilization with lots of energy to use, the light can easily be created to be visible with optical telescopes that have spectral analyzers. With even more energy use, the flashes may be visible even with unaided eyes.
As to dimming. I would argue that receiving any signal 10s of thousands of parsecs distant is interesting, but useless for communication. A beacon can show presence, but that is all. The sender must be close enough to make 2-way communication possible, or, just send out those Encyclopedia Galactica transmissions rather like a galactic history tv broadcast.
If communication is important, then we are back to requiring a nearby transmitter, preferably in our solar system. A radio beacon, of almost any frequency, would be noticeable; no need to restrict it to the waterhole region. Similarly, an optical signal would also be very obvious. A flashing light in sky, whether isolated or on a celestial body would be easily detectable, even with the naked eye. The only question if it was a single wavelength, is whether the local intelligent species can see at that wavelength. We assume our vision is common, able to see from red to blue/purple, but that is not the case for most species, although most can detect light intensity, even if not color.
In summary, optical signals are:
1. more likely to be detectable if the intelligences can see the signal with their naked eyes simply because there are more eyes looking at the sky than instruments.
2. Intelligences will have been able to observe the sky for far longer than they have with instruments.
3. Unless we are unusual, optical instruments being invented earlier also extends the time of possible signal detection. This will matter less as the centuries go by and the ratio of radio vs optical telescope observing capability declines towards 1.0. However, until radio telescopes have the same all-sky observation capability, they will always be less likely to detect a point transient signal than optical instruments.
After all this, we then fall back into the discussions of the transmission economics of the sender. Radio vs optical. Omni-directional beacons vs beams. This is well beyond my knowledge, and I leave it to the experts to make those assessments. All I can speculate is that if we assume ETI is more technologically advanced than we are, then their capabilities will also be far greater than ours. If so, perhaps using far more energy than we contemplate is trivial for them. This is similar to the question of how to send signals back from Proxima with the least mass of equipment and transmitting power. [We are now building multi-GW AI-supporting datacenters that we have never considered in the past. Why assume ETI won’t do something similar in signal transmitting facilities?] Economics is a reasonable domain to consider, and it may well be that radio beams are teh most energy efficient means to send signals. If so, then I accept that, given the technology we know about, radio transmissions may remain the best means of transmitting a signal and for a technological species to receive them, and my arguments for optical are of lesser value.
I don’t believe that it is just about SNR (signal to noise ratio). There is questionable value in a light pulse that is visible to native organisms. What can rabbits do with it? The signal ought to require some technological capability to detect since to filter for potential ETI (us, in this case).
That can be an optical telescope (as Alex suggests) or a radio telescope (perhaps even better since it requires reaching beyond native senses). Sci-fi has often played on this theme, from monoliths on the Moon to beacons directly opposite the Sun from Earth, and to modulated neutrino beams. The sender draws a technological line in the sand as to which ETI they want to receive the message.
After that basic minimum, well, it gets complicated, as we know. For example, is the communication narrowly directed or isotropic? The choice by the sender is very telling.
The problem ETI has is that it doesn’t know the status of intelligent life on the target planet. [Just as we have no knowledge of the recipients of our test METI transmissions.] In an alternative universe without the KT event, would there be intelligent Therapoda or Ornithopoda dinosaurs 10s of millions of years ago? The point is that without knowledge of the intelligence of the target world, othe best chance of success is to use a technology that is detectable by any intelligence that can make use of the “signal”. As I already pointed out, sending a signal that is visible to teh naked eye gives teh best longest period detection by humans. We still get hundreds of years of observational opportunities with the invention of the telescope. But less than a century with a radio telescope. Could any human between Galileo’s invention of the telescope, and out mid-20th century invention of radio telescopes have been able to investigate an optical signal – a star apparently sending out a short sequence of prime numbers as brightening or dimming events? With spectroscopy (possibly invented after Newton) could a laser signal be distinguished from a stellar spectrum? (Astronomers were cataloguing stars by spectral class a 1/4 century before radio telescopes. A bright spectral wavelength rather than an absorption dimming would stand out in spectra.) We were certainly capable of thinking about ETI in the 1st quarter of the 20th century.
Of course, if ETI is using a technology currently unknown to us, then they are still not going to get any hint we are receiving a signal until at some time in the future we can develop that technology.
Clearly, SETI assumed radio technology – that we had developed – was the likely means of ETI transmissions. As we haven’t received a radio signal, optical SETI was also later developed in case that approach yields a signal.
I find the assumption that radio transmissions are best is motivated reasoning based on both some physics and our technological (and economic) capabilities. What if ETI is waiting for a physical object to reach them, a technology beyond our current economic capabilities, but perhaps on the cusp of technological capability? More likely ETI has the capability, and we should be looking for lurkers in our system, as the Benfords have suggested. (There is a story, “The Lurker,” by Greg and Jim Benford in the collection “Tales of the United States Space Force” that may be about this – but I haven’t read it yet.) SETI was predicated on the idea of a “galactic club” that was either leaking signals or searching for new members. Interstellar travel was assumed unlikely/impossible, so look for transmissions as ETI was stuck in its home system. Convenient that we just happened to have radio telescopes available to look for those synals. Also convenient that the original meeting made some “guesses” that there must be 10s of thousands of transmitting ETI civilizations, so that we only need to point our radio telescopes towards promising areas of the sky and we should immediately detect those transmissions. “There’s scientific, and history-making, gold in them thar stars. Just give us a little time on your radio telescopes to look see.” History has proved disappointing…so far.
I’m pleased that SETI has broadened its scope – optical, exo artifacts like Dyson swarms, lurkers, and dead artifacts in our system, signals with different media – neutrinos, gravity waves, etc, etc. As a biologist by training and interests, I am more than happy that we are looking for life more generally, which I hope we will detect, albeit our proxy biosignatures may prove ambiguous to various extents.
As I said, I may be wrong about my assumptions, although I would bet that it will prove to be motivated reasoning that ETI is out there within some detectable range at this time in galactic history. We may be the first, or maybe there is a Great Filter ahead of us that kills off all technological intelligences that cannot live within their biosphere’s limits. We don’t know. I hope life proves ubiquitous on habitable planets, making them inhabited, even if only by unicellular organisms. Maybe we will know relatively soon. What I doubt is that ETI civilizations are anything like ours, with the implicit assumption that technology and economic power extend almost infinitely upward. Human history is replete with gods and stories about trying to emulate them. We happen to live in an age where, as Stewart Brand once said, “We are as gods and might as well get good at it.” I hope so, but history seems to suggest otherwise.
A slight digression…
Discussions of modulated signals designed to attract attention often mention numerical sequences, such as prime numbers, Fibonacci sequences, or some other sophisticated mathematical concepts. Is there a reason for this? Why not just a simple set of arithmetical pulses? The simpler the message, the more likely it is to survive the inverse square law, extinction, attenuation, or any of the other factors which might tend to garble a beacon.
Why not just a simple 1, 2,…n-1, n, perhaps repeated over and over again: simple bursts separated by equal intervals of silence (or darkness, if you prefer optical!). Other simple combinations unlikely to be the result of natural processes present themselves: such as squares (1, 4, 9…n**2).
Or perhaps powerful bursts separated by intervals of silence based on some mathematical ratio like pi or e? Can we be assured that even a technologically advanced civilization would HAVE to be familiar with Fibonacci numbers?
It can be argued that separated civilizations would have no prior knowledge of each other’s ability to resolve time; for example, we are incapable of distinguishing intervals much shorter than about 0.1 sec, but we can easily devise circuits capable of resolving many orders of magnitude shorter than that.
It seems to me that the simpler a signal is structured the easier it would be recognizable as such over long distances, and the easier it would be to generate given a certain level of technology. For example, ET might have the tech capable of producing a very powerful signal but not the ability to modulate it in order to identify it as artificially generated.
I’m talking about characteristics of a simple beacon here, not a complex signal capable of transmitting lots of data. Sailors know lighthouses and other aids to navigation usually flash repeated simple codes so they can be unambiguously identified. These patterns are printed on nautical charts along with the location of the flasher allowing the navigator to triangulate his position. In crowded harbors with many buoys and other nav aids, as well as many other confusing lights on shore or other vessels, these numerical patterns are essential to allow the mariner to determine which one he is looking at.
Again, everything you say is true, but not necessarily relevant.
Due to the nature of cosmic distances and the speed of light, not to mention the potential impenetrability of alien psychology, I’m voting for the beacon, not the com channel. This is especially the case when one considers the brevity of an individual human life. The youngest human is not likely to ever participate in or witness a cosmic dialog, but even I, at age 78, still have a chance of being confronted with evidence of extra-terrestial intelligence. SETI is my personal hobby, I could care less whether my descendants pick up a signal or engage in a conversation. I wan to be there when it happens.
As for omnidirectional broadcasting, I’m afraid that is too energetically expensive for either radio or optical, which will be necessarily limited to line of site targeting. Perhaps the most promising method will be some sort of techno- or biosignature, whether deliberate or as an intentional or accidental by-product of some other type of industrial or cultural activity.
As for the ubiquity or practicality of visual methods, it may be useful to consider that among terrestrial life forms, only a handful of our fellow organisms on Earth are blessed with visual organs superior to ours. Savanna dwellers descended from arboreal herbivores need very good eyes. Intelligent slime molds, social insects and mole rats don’t. Keen optical vision may not be as useful or as inevitable as you might think. On the other hand, a species with no ability to detect or transmit radio is not likely to have sufficient technological ability to develop lasers or orbital telescopes anyway.
How much of our current infatuation with (yes, admittedly very promising!) optical SETI is due to our desperation to pursue anything that might possibly work because nothing else seems to be yielding results?
I concur.
The argument is often made that we have metaphorically only sampled a cup of the ocean’s water, looking for ETI. Yet every cup of ocean water is full of viruses, bacteria, and the debris of life. Hoping to scoop up a fish, or even a human or whale, might be a very long shot, so looking for other evidence of life might well be a better way to go, and hoping that technology-using intelligence naturally emerges from life.
Postscript to my last
An additional consideration:
In addition to extinction in the optical frequencies (vs radio) there is a lot of stray stellar radiation from stars, as you point out. The universe is flooded with optical wavelength light. The waterhole is much darker (except for a few isolated spikes from natural sources). A radio beacon would be much more conspicuous, like a single candle on a dark night.
“Yet every cup of ocean water is full of viruses, bacteria, and the debris of life.”
An yet, it took ~200,000 years to find out that that is the case.
I’m not sure how to interpret this remark. It may be that it took most of H. sapiens species history to discover this, so expect SETI might take just as long – IOW, be patient.
Alternatively, I would argue it couldn’t have been discovered until at least the germ theory of disease was elucidated, to consider microbes (in this case, prokaryotes) existed, and even less time since we could observe them directly with optical and electron microscopes. IOW, we may need new technology. (c.f. Clarke’s suggestion that we may be in the position of indigenous people on an island, thinking they are alone on the planet as they don’t hear any drums beating offshore, yet the “air” is full of radio chatter announcing the presence of other people.)
We don’t need to rehash the “Fermi Question” here.
It is perspective. We may or may not get there, i.e. ever detect ET. 50 yrs is not even a blink in the scheme of things.
At nearly 51 I am resigned to not being around when/if we detect ET.
What is more realistic is exploration of the immediate space – Sol. Find life (or not) and start to draw conclusions.
@Tesh
Back in the early 1960s, Drake thought that there would be perhaps 10,000 transmitting ETI in the galaxy, separated by 1000-2000 ly. That large a number made it seem as if detection would happen quite quickly, assuming there were transmissions and we were listening on the correct wavelength of 21 cm as suggested in Morrison and Cocconi’s 1959 Nature paper,”Searching for Extraterrestrial Intelligence”. It meant that there might be a good chance of an early detection, with an inexpensive, short search using precious radiotelescope time. Today, we might call that motivated reasoning.
60 years later, as that search has proven fruitless, the assumptions have to be reexamined. But as a result of this lack of success, SETI has tried to ensure its continuity by suggesting that the search space is much larger than originally thought (more wavelengths, etc.), that the economics of beacons requires only narrow beams that sweep the skies like a lighthouse, that the beams may only be activated transiently, and of course, that the L terms in the Drake equation may be much shorter than originally envisioned, for whatever reason. This all implies that the search will take far longer than anticipated. As @henry implied with optical SETI, all this may be desperation in order to maintain funding for a continued search. This may be psychologically like a religion expecting some metaphysical event. Predictions are made of its imminent appearance (e.g., Christianity’s “Rapture”), and the congregation is encouraged to donate funds. When the event fails to materialize, an excuse is given, and a new date is set. Rinse and repeat.
One might also argue that astrobiology is a convenient approach for SETI to back into, as this extends the parameters of a search for a less stringent goal that might well be expected to be more common than technological, communicating ETI. This is obvious as the SETI talks have become much broader over time, with more emphasis on the search for life, and on search technologies, both remote and by space probes to reachable targets.
Because of its importance, both scientifically and culturally, we should continue to search for ETI. But that search should not be as dominant as cathedral building in medieval Europe, with the economic deadweight of the imposition of tithes.
@ Alex
I agree with your reply.
My original point was just that we have sampled the metaphorical cup of the ocean’s water but with current knowledge and it may take a while before we have the tools to notice the analogous bacteria and viruses… which is dragging the conversation OT.
I’m disposed to resist this one. Pulsars were once, briefly, spun in the press as celestial lighthouses; but in most ways they seem ill-suited for communication. If the gods can twiddle the spin rate of a pulsar, surely they can send a self-replicating sentient AI to pay us a visit.
There are countless beautiful natural communications that we seem to almost entirely ignore. The radio songs of Jupiter and Saturn, the seething surface of the Sun, the aurora, the movements of wind and sea … any one of these might conceal a hidden message from some extraterrestrial mind. Why don’t we pay more attention to what we have near at hand to speak with?