James Benford (president of Microwave Sciences, Lafayette CA) has just published a note in the Journal of the British Interplanetary Society that has relevance to our ongoing discussion of the Wow! Signal. My recent article was in the context of new work at Arecibo, where Abel Mendez and the Arecibo Wow! research effort have refined several parameters of the signal, detected in 1977 at Ohio State’s Big Ear Observatory. Let me slip in a quick look at Benford’s note before we move on from the Wow! Signal.
Benford has suggested both here and in other venues that the Wow! event can be explained as the result of an interstellar power beam intercepting our planet by sheer chance. Imagine if you will the kind of interstellar probe we’ve often discussed in these pages, one driven by a power beam to relativistic velocities. Just as our own high-powered radars scan the sky to detect nearby asteroids, a beam like this might sweep across a given planet and never recur in its sky.
But it’s quite interesting that in terms of the signal’s duration, bandwidth, frequency and power density, an interstellar power beam would be visible from another star system if this were to occur. All the observed features of such a beam are found in the Wow! Signal, which does not prove its nature, but suggests an explanation that corresponds with what data we have.
An interesting sidenote to this is, as Benford has discussed in these pages before, that if the Wow! Signal were an attempt to communicate, it should at some point repeat. Whereas a power beam from a source doing some kind of dedicated work in its system would never recur. We have had a number of attempts to find the Wow! Signal through the years but none have been successful.
Image: The Ohio State University Radio Observatory in Delaware, Ohio, known as the Big Ear. Credit: By Иван Роква – Own work, CC BY-SA 4.0, via Wikimedia Commons.
The note in JBIS comes out of one of the Breakthrough Discuss meetings, where Michael Garrett (Jodrell Bank) asked Benford why, if a power beam explanation were the answer to the Wow!, a technical civilization would limit their beam to a narrow band of less than 10 kHz. It turns out there is an advantage in this, and that as Benford explains, narrow bandwidth is a requirement for high power-beaming systems in the first place.
Here I need to quote the text:
High power systems involving multiple sources are usually built using amplifiers, not oscillators, for several technical reasons. For example, the Breakthrough Starshot system concept has multiple laser amplifiers driven by a master oscillator, a so-called master oscillator-power amplifier (MOPA) configuration. Amplifiers are themselves characterized by the product of amplifier gain (power out divided by power in) and bandwidth, which is fixed for a given type of device, their ‘gain-bandwidth product.’ This product is due to phase and frequency desynchronization between the beam and electromagnetic field outside the frequency bandwidth.
Now we come to the crux. Power beaming to power up, say, an interstellar sail demands high power delivered to the target. To produce high power, each of the amplifiers involved must have small bandwidth, with the number of amplifiers used being determined by the power required. Benford puts it this way: “…you get narrow bandwidth by using very high-gain amplifiers to essentially ‘eat up’ the gain-bandwidth product.’
Thus we have a bandwidth limit for amplifiers, one that would apply both to beacons and power beams, which by their nature would be built to project high power levels. Small bandwidth is the physics-dictated result. None of this proves the nature of the Wow! Signal, but it offers an explanation that resonates with the fact that the four Wow! parameters are consistent with power beaming.
The Wow! signal was in the microwave radio band. We seem to have focused on monochromatic phased array lasers as the preferred power beam technology for beamed sails, not to mention planetary defence from PHAs. Laser emissions have a very narrow bandwidth that allows the receiver/sail to be carefully tuned to avoid heating. Their dispersion is also lower, allowing the energy to be more effective over a longer range.
AFAIK, the advantage of microwaves is that they are [currently] cheaper to produce and have a higher conversion efficiency than lasers.
So why would the Wow! signal be a radio emission rather than an optical one? What advantage would the bandwidth detected as the Wow! signal have over optical power beams?
As for repetitions, of the few METI signals we have produced, what was the repeat cycle, if any?
If we are using some type of propulsion from an interstellar space craft like beamed power, solar sails etc, the optical power is better based on todays technology. If it a land based antenna then one can have a lot of power. I still think that such an interstellar signal would have to have some kind of code or language that has to be deciphered. We would do it that way with a METI which may be limited to civilizations with today’s technology. It does not rule out mischievous aliens as random, short signals without any information are still signals which won’t violate any prime directive or non interference ethics and those might even have a nearby origin.
It depends on what you want to do with the signal: if you want to communicate or exchange information, logic dictates that you repeat the signal because statistically, the more you repeat it, the more likely it is to be picked up. Repetition also makes the message more intelligible in relation to noise, distortion, and transmission interference: mayday mayday mayday / CQ DX CQ DX CQ DX…
Now let’s assume that the sender does not wish to communicate but just leave a trace of their presence. They then send a simple signal into space and time that is powerful enough to say “we existed.” Think of lovers who have carved their initials into tree bark. In this case, the target is irrelevant—everyone can read it…or no one—and the response is not what matters because the transmitter will probably have disappeared for some reason. In this case, repetition is not necessary.
In a way, we would have captured proof that we are not alone.
What bothers us is that we don’t know if the Wow is artificial or natural, and it is impossible for us to confirm this, i.e., either by repetition or by examining the source of the signal. At best, we can only analyze it again and again with our new technology to extract information from it. But in itself, Wow! IS information.
Professor M. KAKU posits that we cannot explain 10% of “UFO” phenomena because we lack data. We can transpose this postulate to Wow!, in other words, reduce the uncertainty. Not easy…
https://www.youtube.com/watch?v=DpkOFD8HYgI
“Now let’s assume that the sender does not wish to communicate but just leave a trace of their presence.”
Reminding me of perhaps the best TNG episode, the bittersweet “The Inner Light.” Every viewing brings me to tears.
(“The Chase” being a very close second, IMHO).
If they sent the beam, they have communicated.
>Signal were an attempt to communicate, it should at some point repeat.
That’s the sentence that caught my attention ;)
To answer Alex Tolley:
Microwave, millimeter waves and lasers are all serious candidates for beaming to sails. Lasers have the tremendous advantage of short wavelength; therefore can be focused to a smaller sail from a smaller Beamer aperture. This is the approach that Breakthrough Star Shot has taken. The important distinctions between the types of radiation sources are:
1. Lasers are quantum devices, so at high power nonlinear effects in the laser make the coherence length very short. The practical limit in today’s fiber lasers, the current favorite in high-power laser technology, is about 1 kW per laser. For Breakthrough Star Shot, each GW of radiated power requires about a million individual lasers built into a coherent array. Microwave and millimeter wave individual sources are available at MW levels; therefore each GW of radiated power requires ~1000 sources. So, laser systems are much more complex.
2. Microwave equipment such as sources, anechoic rooms, antennas and diagnostics are commonly available than the emerging technology of high power lasers. That’s because microwave and millimeter wave sources, waveguides and supporting equipment, are a developed industry, available at 1-10 $/W. That means it is cheaper and faster to build systems. Lasers are developing fast, but at present are still expensive, ~100 $/W, and are produced in small numbers at slow rates.
3. Microwaves are more efficient than lasers, typically 50-90%. Millimeter wave generation technologies now make it possible to generate wavelengths as short as 0.1 cm with relatively high efficiency (>40%).
4. Microwave phased arrays of transmitters and apertures are relatively easily done and are widely used, while phased arrays of laser beams, although possible in principle, subject to the coherence length constraint related above, are thus far little developed in practice.
Something that I have not seen published is what the high power beams of the three BMEWS sites in Alaska, Greenland and England would cover in the heavens. BMEWS radars operate in the UHF (420-450 MHz) frequency range and had an azimuth of about 200 degrees that point toward the horizon toward Russia. These beams are now reaching out to 65 light-years. It would be very interesting to see how much of the heavens was covered and what known exoplanets within 65 light-years may be picking up these high power microwave beams now.
The elephant in the room is the frequency (or if you prefer, wavelength) of the WoW! signal. The signal is very close to the 21 cm line of hydrogen, a frequency that is manufactured by natural sources in the atomic hydrogen clouds of the interstellar medium. If the Wow! was meant to be intercepted, why pick a wavelength where it might easily be overwhelmed or obscured by the natural noise of the galactic disk? It makes no sense.
If the signal was generated for some other purpose (propelling star sails, planetary defense radars, or some other as-yet-unknown industrial reason) why pick a frequency at precisely the 21 cm line? Perhaps it is because the sender is hoping it be NOT intercepted, and if it is it will be mistaken for the natural background or an anomaly and ignored.
There is another possibility, the signal is either a terrestrial hoax, or is being used by some terrestrial military or intelligence agency who feels it is entitled to ignore the reservation of some frequencies for scientific use. I was once told by Dr Charles Seeger that he had to personally call authorities in our own country’s intelligence services that astronomical receivers often picked up transmissions of emissions on “protected” frequencies coming from their satellites. Its as if the spooks did not really understand just how sensitive astronomical receivers were.
Again, it might even be deliberate. If someone accidentally picked up your clandestine transmission they might simply dismiss it as “the hydrogen line” and ignore it. Perhaps forbidden radio astronomy frequencies are popular for intelligence work because a lot of on-the-shelf hardware and employee expertise is already available around those frequencies.
We don’t have enough information to definitively address any of these questions,
but if I had to make a guess I would say the most likely origin of the WoW! is an extra terrestrial civilization transmitting on a very tight beam that accidentally swept by the earth in 1977 and for a few moments we had an instrument on line capable of detecting it. Even if the signal is eternal, we will never pick it up again. We will never be able to confirm it.
Another characteristic of the WoW! is that it isn’t precisely at 21 cm, but at a slightly Doppler-shifted frequency. The amount of blue shift is consistent with a relative motion between transmitter and receiver of several tens of km/sec. It is probably coming from another system in orbit around the galactic center, perhaps at a distance of several thousand parsecs. The amount of relative motion suggests another star in the galactic disk.
We simply do not have enough information to either rule out or confirm whether this transmission is earth-generated, of a natural physics deep space origin, or whether it is the product of alien technological activity. But until we do acquire that information, I believe our best guess is that it is technosignature.
@henry
This is the reason for the selected wavelength:
The waterhole wavelength
https://en.m.wikipedia.org/wiki/Water_hole_(radio)
I would want to rule out a terrestrial origin of the technosignature before calling it alien.
Isn’t more likely a terrestrial transmission at the edge of the reserved wavelengths? It was transient, either because the transmission was short or it was transmitted from a moving platform like an aircraft. Rather than fetishizing this curiosity, we should accept it is likely inexplicable and move on.
Yes, but…you knew that was coming, didn’t you?
The waterhole has obvious advantages for interstellar microwave transmission, it is transparent, wide and deep, but it also has very loud “spikes” in it caused by gases and plasmas in the interstellar medium. These include the H+ and OH- radicals (hence the name), as well as the monatomic H at 21 cm. And these emitters tend to concentrate at low galactic latitudes where most of the gas and plasma is. Radio astronomers, both ours and (presumably) theirs, will monitor these freqs. Any message or beacon would most logically be originating in a portion of the spectrum where few natural processes emit EM.
It has been suggested that broadcasting at some harmonic of one of these spikes would attract attention from any species interested in radio astronomy.
Just multiply the frequency or wavelength by some number that will
keep the product in the waterhole dip in the spectrum. Typical dimensionless numbers might be an integer, pi, e or 137.
No, I’m not really convinced this is evidence of ETI. More than likely further research will come up with some natural phenomenon responsible for what we saw.
But for now, ETI is the best of a bunch of bad choices.
isn’t that the argument Mendez{?} for the Wow! signal to be a natural, but rare, phenomenon (natural lasing?) as an alternative explanation for the artificial emission, and briefly noted in Benford’s well-reasoned post on the emission being a power beam?
Do natural emissions in the “waterhole” wavelength have any effect on the beaming? As beaming can use any suitable wavelength, why choose the “waterhole” unless other wavelengths absorb the beam and reduce its effective power?
I am at a loss to understand the reasoning behind ETI trumps RFI as a probability of correct explanation. The Breakthrough Listen research ruled out possible ETI signals as RFI. In the documentary about the Wow! signal that I recently watched, it was noted that radio astronomers often received such unexpectedly strong signals. Were they also ETI and dismissed? The only recent example I can think of where a transient proved to be real is the fast radio bursts, some of which do repeat. Interestingly, a CD post about these signals at the Parkes station proved to be a leaky microwave oven, even though elsewhere these, or similar, emissions were celestial.
Given that we are a technological species that uses most of the radio wavelengths, and RFI is going to be strong given proximity, it seems to me that an errant terrestrial signal in the reserved wavelength band is more likely to be produced by terrestrial equipment than ETs.
[When I was young, many devices caused radio interference that created noise in the commercial radio and TV frequencies, most notably, electric shavers. Were devices all properly shielded and radio emissions prevented in that period?]]
Why beaming in the microwave range?
Answer to Michael Fidler on BMEWS radars: They are far weaker than other Earth emitters. BMEWS radars have a gain-power product of only 0.5% of the Arecibo emitter, so they have short range. Their emissions will be undetectable at 65 light-years.
I had a conversation with Grok about it, and after exploring various interpretations, it appears to be uncovering some fascinating insights.
1. Analysis of the Claim
2. Explanation of the Difference in Assessments
3. Detection Range of a 300-Meter Aperture Extraterrestrial Telescope
4. Recalculation of Detection Range for a 300-Meter Aperture Extraterrestrial Telescope
https://grok.com/share/c2hhcmQtMw%3D%3D_aa3e7f27-6db6-40b6-bc58-e41fb6c1c416
I was taken aback to discover that there was a BMEWS station nestled in Trinidad, British West Indies. It was a fascinating revelation that added an unexpected layer to the island’s rich tapestry.
https://www.flickr.com/photos/46494551@N04/13431341833/
There was also a Soviet system!
I would like to conduct further research on this subject. Are there any books or articles that you would recommend exploring?
My father was involved in radar technology during World War II, and I served at the SAGE 24th NORAD Region at McChord Air Force Base in Tacoma, Washington. I was also stationed in Alaska on the DEW Line and at the Tonopah Radar Range in Nevada.
I agree that the signal not repeating makes power beaming more likely than messaging but I also don’t see how we can determine by how much. I don’t see an argument for the lack repetition making power beaming more likely than a natural explanation. We can develop rigorous and semi-rigorous probabilities for natural phenomenon but not for SFI phenomenon. What logic do we use to compare a confined probability with unconfined? The natural explanation isn’t impossible so we can’t use Sherlock Holmes’s logic.
I do think there is a messaging strategy that would use transients without embedded data. An easily detected flash draws attention to a location where a message is transmitted with a more difficult to detect method. This strategy would set a minimum technological threshold for detecting the message.
There may a limited window where an SFI uses messaging as a broad search. Since a receiver doesn’t have to respond, the strategy can not deliver a conclusive result. Any SFI that wants a conclusive result will employ other search methods. Messaging may be part of the overall strategy but is eventually and, in galactic time frames, quickly rendered obsolete by probes. This time frame is much shorter than that employed Fermi pessimists for the colonization of the galaxy. Imho, it is so short we should never expect a “searching message”, only bespoke.
@Harold
If a transient were to draw attention, then why not use optical signaling? The only reason not to would be to exclude pre-technological species. However, I don’t see why that matters.
We use repeating signals to draw attention, notably lighthouses that sweep a light beam across the ocean to warn ships. A non-repeating, 1-minute, transient radio signal doesn’t strike me as very useful, as the receiver has to be fortuitously able to receive that message. In our case, pointing a receiver in the right direction at the right time.
In that regard, a power beam seems to make more sense if it were a true, ET, technosignature.
What we need is an all-sky 24/7 watch. If such transients do occur randomly, we have a greater chance of capturing the event using this more expensive approach.
I do think that if we are sort of correct that ET civilizations are few in the galaxy, then it seems that once we have a “signal”, it it best to keep looking in the same [few] direction[s] once a transient is captured, simply to wait for another from the same direction. They may still not repeat. Consider a scenario where we use a beamed sail or swarm of sails to launch light sails to Proxima. It may be effectively a single event if viewed from Proxima, even if we reuse the approach to send lightsails to other stars. We may send another lightsail to Proxima, offering a repeat event, or we may have better technology that doesn’t require beamed propulsion, and therefore the event becomes a non-repeating transient for any hypothetical observer in the Proxima system.
For the strength of the Wow! signal, what is the reasonable range of transmitting energies by distance, from Proxima, to the center of the galaxy? IIRC, the Benford post did do that calculation to estimate a likely distance to the transmitting system.
“If a transient were to draw attention, then why not use optical signaling? The only reason not to would be to exclude pre-technological species. However, I don’t see why that matters.” Alex Tolley
While it is impossible to predict the motivations of a specific SFI, I think we can safely assume the motivations behind communication will shape the engineering of a transmission. An attention getting signal that can be confused with a natural phenomenon offers utility not provided by one that is obviously artificial. The former should select for greater curiosity and allow the transmitter to remain hidden until the receiver is able to detect the primary signal.
To answer Alex Tolley, asking what strength to reach to the center of the galaxy: In my first Wow paper, I estimated the range which various sources could reach. Arecibo could be detected at two light years. Interstellar precursors estimated by myself and Greg Matloff could be seen at ~100 ly. The Starwisp beam of Bob Forward could be detected at a million light years!
To answer your question, to be detected at Galactic center would require about 4×10^12 watts. That would be half of all the electrical power now generated on Earth!
I wonder if 21cm could be just the carrier…could the rest of the message be hidden elsewhere ?
Another question: how is it that we haven’t been able to deduce any information from the 72 seconds of the Wow signal in half a century, even with the computing power we have today? I’d like to find R. Gray’s book…
Because there was little to deduce from the Wow! Signal. All we really got were a jumble of numbers and letters on a printout and they only indicated that it was a stronger signal than what else was being detected at the time.
There was neither message nor data recorded, only that the signal existed and was picked up by the Big Ear radio telescope – which the Ohio State University (OSU) later sold to developers who scrapped the venerable instrument and put up condos and a golf course in its place. So much for intelligence.
One again I highly recommend this excellent documentary on the subject…
https://www.youtube.com/watch?v=TjQUucV83w4
I bet I know what you were thinking in terms of a more sophisticated signal being hidden within the introductory message…
https://www.youtube.com/watch?v=uhIEfxRLiPI
This might be the case for future ETI transmissions, but I do not see any indications of such a message for the Wow! Signal. Even if there were, the Big Ear antenna sadly did not have the tech to pick it up.
We can either keep playing with a signal that was poorly recorded to begin with, thanks to 1970s technology and minimal staffing at the Big Ear facility, has never reappeared and would likely have been rejected by more modern SETI efforts, or we can focus on much more promising targets…
https://www.universetoday.com/articles/galaxies-with-high-radio-emissions-could-be-home-to-many-advanced-civilizations
I feel like SETI keeps fishing in a water bucket while barely dipping in to the much wider ocean of space around it. The parent of Breakthrough Listen has already stopped working on Breakthrough Starshot, so how soon before they walk away from their SETI efforts – which I would like to add aren’t all that different from the SETI projects we have seen since 1960.
I really wish there were more devoted – and continuous – efforts to search for alien intelligences, but we keep on getting the sporadic projects and token moments that have gone on since Project Ozma.
Don’t believe me? Just check any modern history of SETI. Most recent SETI efforts are still sporadic. Worse, they are overwhelmed with data that no one is checking due to limited funds and staffing. It is almost the Wow! Signal era again, just with better computers. At least Big Ear really was scanning the skies 24/7.
The search for technosignatures is a good idea and a very long time in coming. The odds that someone is beaming a message directly at us for us are very slim. We have better chances of finding an advanced society conducting major astroengineering projects.
Yes, I know that also has its limitations, but considering how limited human SETI is in terms of technology, budgets, personnel, and overall support, it is the best hope we have at the moment. And with the current state of science and society on this planet, I am concerned things will not get better, at least not in certain nations.
https://www.universetoday.com/articles/setting-bounds-on-seti
Setting Bounds On SETI
By Andy Tomaswick
September 30, 2025 at 10:25 PM UTC | Astrobiology
The Search for Extraterrestrial Intelligence (SETI) has a data scale problem. There are just too many places to look for an interstellar signal, and even if you’re looking in the right place you could be looking at the wrong frequency or at the wrong time.
Several strategies have come up to deal narrow the search given this overabundance of data, and a new paper pre-print in arXiv from Naoki Seto of the Kyoto University falls nicely into that category – by using the Brightest Of All TIme (BOAT) Gamma Ray Burst, with some help from our own galaxy.
When searching for SETI signals, a civilization has to choose three important factors: where to look, what frequency signal to look for, and when to do so. The same problem is faced by the transmitting side – sending signals strong enough to reach other stars coherently in all directions is extraordinarily energy-intensive.
In other words, no sane civilization would do that intentionally for long periods of time. And what if you send the wrong frequency? Or worse yet hop between frequencies? How would a receiving civilization ever know how to find your signal?
According to multiple papers on this very subject, the answer lies in game theory, a sub-discipline of economics. In game theory, there is a concept called a Schelling point, which is a solution that a player would default to when they aren’t able to communicate with other players. Typically this would represent something unique, or a “natural” fit for whatever information they are trying to convey.
This concept had previously been used to attempt to suggest we should send/listen to signals via an “anchor event” in our galaxy such as historical supernovae or future neutron star mergers. However, each of these anchor events has its own flaws. There aren’t any binary neutron star systems that are close enough to merging that they could act as an anchor. And distinct supernovae, such as the one that created the Crab Nebula, don’t have well-defined distances, so it wouldn’t constrain the search area very well.
To solve these problems, Dr. Seto, who has also published paper on anchoring events, suggests a “hybrid” strategy. Instead of using only one event, use two – a “spatial” reference and a “temporal” reference. In the paper, he suggests the spatial reference be the center of the Milky Way, while the temporal reference would be an extremely bright event somewhere outside the galaxy.
The underlying idea is to have a “search ring” centered on the event, in the case that your civilization is looking for signals, and a “transmit ring” exactly opposite the event in the case that your civilization is intending to send them. The diameter of each of these rings grows based on the time since the original burst and the distance from the location to the galactic center. Importantly, the angle between the burst and the galactic center is used to “normalize” the time delay at which a signal would be sent to a specific star system.
A civilization in the target star system would also then know when to look for a signal, given the same parameters of when the burst started, its distance to the galactic center, and the angle at which the burst came from towards the galactic center. The reason for using the galactic center as a spatial Schelling point is because civilizations of our level of technological sophistication have very accurate distance measurements about how far Sgr A* (the black hole at the center of the galaxy) is from their home system.
For a “bright event”, Dr. Seto notes that an ideal one was recently found in the form of gamma ray burst GRB 221009A. It was nicknamed the BOAT because it was 40 times brighter than the next brightest GRB on record. It is also ideally positioned in the sky – low “galactic latitude” meaning many of the stars in the Milky Way would be in the search radius of the original ring. Dr. Seto calculates that this combination of brightness and ideal sky position likely only happens once in 100,000 years.
Unfortunately all of this spatial and temporal timing doesn’t necessarily coordinate the other variable – frequency. There are some theories that a Schelling point for frequency, such as the Hydrogen Line of 1,420 MHz, where hydrogen shines when undergoing a frequency transition, could be used, but realistically the receiving civilization would still have to search multiple frequencies over that time.
The hybrid technique itself could limit the space needed to search for sending civilizations by a factor of 100. But, it is entirely based on the assumption that the other civilization would come up with the same methodology. Given the uniqueness of the opportunity represented by GRB 221009A, maybe it’s worth it to take a look at some potentially interesting star systems – we might not get another chance like this for 100,000 years.
The paper here:
https://arxiv.org/abs/2509.20718
https://arxiv.org/abs/2509.23632
[Submitted on 28 Sep 2025]
Blink and you’ll miss it – How Technological Acceleration Shrinks SETI’s Narrow Detection Window
Michael Garrett (University of Manchester, JBCA and Leiden University)
The search for extraterrestrial intelligence (SETI) has historically focused on detecting electromagnetic technosignatures, implicitly assuming that alien civilisations are biological and technologically analogous to ourselves.
This paper challenges that paradigm, arguing that highly advanced, potentially post-biological civilisations may undergo rapid technological acceleration, quickly progressing beyond recognisable or detectable phases. We introduce a simple model showing that the technological acceleration rate of such civilisations can compress their detectable phase to mere decades, dramatically narrowing the temporal “detection window” in which their technosignatures overlap with our current capabilities.
This framework offers a plausible resolution to the “Great Silence”: advanced civilisations may be abundant and long-lived, but effectively invisible to present-day SETI methods. Consequently, our efforts must include but also evolve beyond the search for narrow-band communication signals in the radio and optical domains.
Instead, we require an expanded, technology-agnostic strategy focused on persistent, large-scale manifestations of intelligence, such as broadband electromagnetic leakage, waste heat from megastructures, and multi-dimensional anomaly detection across extensive, multi-wavelength and multi-messenger datasets.
Leveraging advanced artificial intelligence for unsupervised anomaly discovery, recursive algorithm optimisation, and predictive modelling will be essential to uncover the subtle, non-anthropocentric traces of advanced civilisations whose technosignatures lie beyond our current technological and cognitive frameworks.
Comments: 15 pages, 2 figures, accepted by Acta Astronautica
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Popular Physics (physics.pop-ph)
Cite as: arXiv:2509.23632 [astro-ph.IM]
(or arXiv:2509.23632v1 [astro-ph.IM] for this version)
https://doi.org/10.48550/arXiv.2509.23632
Focus to learn more
Submission history
From: Mike Garrett [view email]
[v1] Sun, 28 Sep 2025 04:15:10 UTC (688 KB)
As shown in Contact, Carl Sagan thought that advanced altruistic ETI might “talk down” to less sophisticated civilizations, such as with radio beams, to get their attention and bring them up into the Galactic Federation. Unless someone here has been blocking their invitations, I believe we are still waiting on that one.
Actually, no one needs to deliberately censor ETI transmissions: Our current SETI efforts are worse than a using a sieve to catch alien signals.
I like this quote from the paper…
In conclusion, the search for extraterrestrial
intelligence is also a search for our own future. As
humanity itself approaches potential post-biological
transitions, SETI’s greatest challenge, and
opportunity, lies in recognising that the universe’s
most advanced civilisations may not conform to our
expectations. By embracing this uncertainty, we
open the door to discoveries that could redefine not
only our place in the cosmos but our understanding
of the breadth of intelligence itself. Critically, this
expanded and open approach does not discard
traditional SETI but complements it,
acknowledging that while some civilisations might
still emit recognisable electromagnetic signals,
others may operate in ways we cannot yet imagine.
This exponential growth in technological
advancement should significantly enhance our own
chances of detecting extraterrestrial intelligence.
However, this optimism is somewhat tempered by
the uncertain trajectory of human intelligence in the
face of rapidly evolving AI. One sobering question
is whether human intelligence will still be around to
witness all of this. In the end, we may need to be
resigned to the possibility that it will be our
intelligent machines, searching for theirs.
https://astrobiology.com/2025/09/starshades-as-technosignatures-in-direct-imaging-phase-curves-application-to-the-habitable-worlds-observatory-targets.html
Starshades As Technosignatures In Direct Imaging Phase Curves: Application To The Habitable Worlds Observatory Targets
By Keith Cowing
Status Report
Habitable Worlds Observatory
September 29, 2025
A star’s luminosity increases as it evolves along the Main Sequence (MS), which inevitably results in a higher surface temperature for planets in orbit around the star.
Technologically advanced civilizations may tackle this issue by installing artificial structures — starshades — which can reduce the radiation received by the planet. Starshades, if they exist, are potentially detectable with current or near-future technology.
We have simulated phase curve signatures in direct imaging of hypothetical starshades in systems targeted by the upcoming Habitable Worlds Observatory (HWO), which will be tasked with searching for Earth-like exoplanets orbiting nearby stars. The starshade is assumed to be a circular, reflecting surface placed at the inner Lagrange point between the star and the planet.
Our results show that the phase curve of a starshade has a distinct shape compared to that of a typical planet. The phase curve signature lies above the expected 1σ=10−11 single-visit precision in contrast ratio of the telescope for 70.8% of the target stars for the expected inner working angle (IWA) of around 60 mas.
If the IWA can be reduced to 45 mas, the percentage of stars above the 1σ limit increases to 96.7%. With a sufficiently small IWA, HWO should be able to detect anomalies in light curves caused by starshades or similar highly-reflective surfaces — which could serve as key indicators for technologically advanced civilizations.
Claudia I. Skoglund, Alexander J. Mustill
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2509.22301 [astro-ph.EP] (or arXiv:2509.22301v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.22301
Focus to learn more
Submission history
From: Claudia Skoglund
[v1] Fri, 26 Sep 2025 13:02:03 UTC (2,330 KB)
https://arxiv.org/abs/2509.22301
Astrobiology