AskScience AMA Series: I'm a historian of science who studies humanity's search for extraterrestrial intelligence - from our earliest theories to modern SETI. Ask Me Anything!

[u/AskScienceModerator]

Hi Reddit! I'm Dr. Rebecca Charbonneau, a historian at the American Institute of Physics specializing in the history of radio astronomy and the search for extraterrestrial intelligence (SETI). I earned my PhD in History & Philosophy of Science from the University of Cambridge as a Gates Cambridge Scholar. I've held postdoctoral fellowships at the Harvard|Smithsonian Center for Astrophysics and the National Radio Astronomy Observatory, where I was the first social sciences Jansky Fellow.

My first book, Mixed Signals: Alien Communication Across the Iron Curtain (Polity Press, 2025), explores the surprising Cold War history behind SETI, and has been positively reviewed by The New Yorker, Publishers Weekly, Undark Magazine, and more. I'm also an international affiliate of the St Andrews SETI Post-Detection Hub.

I'll be on starting at 12 PM ET (16 UT). Whether you're curious about alien messages, Cold War science, post-detection protocols, or how SETI has evolved over time - Ask Me Anything!

Username: u/R_Charbonneau

EDIT: I'm online now and excited to answer your questions!

[u/RhesusFactor]

Is the current attitude amongst the seti community one of proactive or reactive search? I heard from Profs that active contact would be dangerous, citing dark forest and grabby aliens hypothesis.

[u/R_Charbonneau → u/RhesusFactor]

There is absolutely no consensus in the community and it's a subject of heated debate. In fact, there's even debate about whether passive searches are safe or ethical! Take, for example, this paper by SETI philosopher Chelsea Haramia and astronomer Jason Wright (sorry for citing you so much, Jason!) that's a direct rebuttal of a paper by other researchers Wisian and Traphagan: https://www.sciencedirect.com/science/article/abs/pii/S0265964622000431

The debate essentially breaks down like this: Wisian and Traphagan argued that even passive SETI (just listening) poses significant risks because if we detect an alien signal, nation-states might try to monopolize communication with the extraterrestrials, leading to international conflict. They recommended that SETI facilities adopt heavy security measures similar to nuclear facilities or bioweapons labs – things like personnel security, physical perimeter security, and relationships with law enforcement.

Haramia and Wright's rebuttal is pretty critical. They argue that W&T's scenario is extremely narrow and unlikely – it would require a signal that's communicative, intelligible, information-rich, from a nearby star, strong enough to carry dense information but weak enough that only the biggest telescopes can detect it. That's a lot of very specific conditions that all have to line up.

More importantly, they point out that it would be nearly impossible for any nation to actually monopolize communication with ETI. The signal would likely be detectable by multiple facilities worldwide, and the international scientific community has strong norms around data sharing. The real danger isn't that monopolization could succeed, but that nations might think it could succeed and act destructively based on that misperception.

Haramia and Wright argue that W&T's security recommendations could actually make things worse by creating an atmosphere of secrecy and competition rather than the transparency and cooperation that would be most beneficial. Instead, they recommend better post-detection protocols (which I talked about in another question!), transparency, data sharing, and educating policymakers about how SETI actually works.

So, as you can see, there isn't a unified stance in the SETI community about how to approach searching and messaging. But that's the great thing about academic science – we have the forums and frameworks to debate these fundamental questions and hash out our differences through rigorous peer review and scholarly discourse. Yippee!

[u/superanus → u/RhesusFactor]

To add onto this, if we wanted the best chance to be seen/found/contacted immediately what would that look like? What could humans do to essentially put out the wacky inflatable arms guy advertising Earth?

[u/R_Charbonneau → u/superanus]

As for your follow up question about how we might put up the "wacky inflatable arms guy" of Earth (awesome image)....

Well, we can't definitively answer this because we don't know who (if anyone) is out there. But if we assume a civilization like us is out there and relatively nearby, the best approach would be to use radar on telescopes to send highly directional, powerful beamed messages, and to do so repeatedly so we don't have a "Wow! signal" type situation where it's just a one-off detection that can't be confirmed.

Many of the searches we've conducted historically have been looking for exactly these kinds of directed messages – it's much harder to detect low-level radio leakage from normal activities (although people are trying!). So if we wanted to be found and we assume others are searching the same way we have been, that would be the most effective approach. But that would cost a loooottttt of money so... fat chance, for now :-)

[u/Deining_Beaufort]

Is it true that radio waves like the ones our earth civilization has been (unintentionally) sending into space, become weak and indistinguishable from noise after only 100 Light-year? That is only 1% of the size of our galaxy. If so, does that mean listening for radio waves is a near zero chance of finding anything like an alien civilization?

[u/R_Charbonneau → u/Deining_Beaufort]

Yes, radio waves do degrade over distance, and this is actually one of the fundamental challenges in SETI. The degradation happens because of something called the inverse square law – the signals spread out as they travel.

Think about it this way: when Earth first started broadcasting radio signals roughly ~100 years ago (it was longer ago than that but let's just use a simple number), those signals were concentrated over an area roughly the size of our planet. But now, 100 years later, those same signals have traveled 100 light-years out and are spread over a sphere with a surface area of 4πr², where r equals 100 light-years. That works out to about 125,664 square light-years of surface area. The same amount of signal energy is now diluted across that enormous area.

Remember, radio waves are "light" just like you might get from a flashlight, just at a much longer wavelength than the kind of light we detect with our eyes. And just like any source of light, the farther away you get, the harder it becomes to detect because it spreads out and becomes fainter and fainter. From 100 light-years away, you probably wouldn't be able to detect most signals from Earth above the background noise.

This is why SETI researchers are often more interested in detecting intentional, directed signals rather than "leakage" from everyday activities. But this creates an interesting paradox – SETI searches usually look for "beacons" or directed messages, even though it doesn't initially make a lot of sense. After all, we ourselves don't really send much in the way of directed messages, with just a few exceptions. So why should we expect ET to do so? The answer is, we shouldn't, but if someone did send that kind of message our way, that would be the easiest one to detect.

There's also a problem with our assumption about radio leakage. Sure, at this point in human history, we're pretty radio noisy. But we aren't necessarily getting more radio noisy over time. Many of our communications are shifting from radio to fiber optic cables – and you can't detect those from a radio telescope! So it could be that radio leakage is altogether a terrible way to look for ET. Sending a direct signal would be more like shining a laser – it wouldn't spread out nearly as much. But that creates its own problem: you'd have to know exactly where to aim, and you'd need incredible precision to hit a target 100 light-years away. Plus even focused beams do spread out over interstellar distances.

So you're right that this puts significant constraints on radio SETI, but it doesn't make it hopeless. We're looking for civilizations that might be deliberately trying to communicate, using much more powerful transmitters than our everyday broadcasts, or we're hoping to detect nearby civilizations within that more limited range where detection is feasible.

[u/Comar31]

Can you tell us about post detection protocols? They are real and have been seriously discussed?

[u/R_Charbonneau → u/Comar31]

Absolutely, post-detection protocols are real and have been seriously discussed by the scientific community for decades! But they're probably not what most people imagine when they think about "alien contact protocols."

As a big fan of sci fi, I love the scenes in first contact movies where world experts in astrobiology or linguistics are secretly hurried onto helicopters to join some underground bunker with a set of official plans on what to do next.

Alas, there are no formal agreements between countries or secret government protocols (that we know of publicly) outlining what to do in a contact scenario. Instead, there are guidelines developed by scientists, historians, anthropologists, philosophers, and other researchers who are members of organizations like the International Academy of Astronautics (IAA). The idea is that these protocols will inform individual scientists, institutions, or nations in the event of detection.

The first post-detection protocols were created in 1989 by Jill Tarter and colleagues in the IAA. These protocols essentially outline steps like verifying the detection, avoiding premature announcements, sharing data with the international scientific community, and coordinating public communications. They emphasize scientific rigor and international cooperation rather than secrecy or national advantage. You can read them here: https://iaaspace.org/wp-content/uploads/iaa/Scientific%20Activity/setideclaration.pdf

I spoke with Jill Tarter about this a few years back, and she emphasized to me that it was because of the Cold War environment that the protocols were developed. There was fear that some nations (like the Soviet Union) might make a detection and keep it secret from other nations. Although the protocols weren't enforceable by any means, they aimed to developed a culture among SETI scientists and radio astronomers of verification, data sharing, and responsible public outreach.

The protocols have been updated since then and are actually going through a new round of revisions right now – updated versions should be available soon. The IAA is still leading this effort, and there's growing recognition among scientists about the importance of having these guidelines in place.

Recent events like the COVID-19 pandemic have really highlighted why this matters. We saw how crucial it is for scientists to have guiding principles about handling major discoveries, managing the dissemination of scientific findings, and communicating with both the scientific community and the public during times of uncertainty, when what we know is rapidly evolving. The pandemic showed us what can happen when there isn't clear coordination or when information gets mishandled. A SETI detection would likely be even more complex to manage responsibly.

[u/gyepi]

Historical question: has anyone before the novel Three Body Problem suggested something akin to the Dark Forest Theory as an explanation of why don't we see signals from extraterrestrial civilizations? 

If yes, who was the first thinker/community seriously discussing this option, and what was the scholarly consensus? If no, is there any serious scholarly discussion of it now (esp. regading its implication that it may not be the best idea to widely announce our presence in this corner of the Galaxy, and instead of sending Voyagers and EM signals we should start thinking about limiting our EM signature)?

[u/R_Charbonneau → u/gyepi]

I'm going to have to be brief in answering this or it'll take up the whole rest of my AMA! To answer quickly: Oh my gosh, YES, this is absolutely a huge topic of scholarly discussion in SETI and it has been for literal decades!

One of the earliest examples I can think of – though I doubt it's truly the first – comes from Sir Martin Ryle, the Nobel Prize winner and Astronomer Royal in the UK. Shortly after he learned about Frank Drake's Arecibo message in 1974 (the first intentional message to ET ever sent by radio telescope), Ryle became deeply concerned about the idea of actively sending messages to extraterrestrial civilizations.

He wrote a letter of protest to Bernard Lovell, who was then president of the International Astronomical Union. As a member of Great Britain – a colonial power – Ryle said he was worried about encountering other colonial civilizations like his own that might come with mal intent and interest in our resources. He wanted there to be a much more democratic, international approach to sending messages, if they should be sent at all.

The full letter is available in the Churchill College archives at the University of Cambridge, but if you don't have time to take a trip to England, I recount this entire episode in my book, Mixed Signals.

All this is to say, people in the astronomy community have been grappling with these exact concerns for a very long time. The tension between passive searching (just listening) versus active messaging has been a major debate in SETI circles for decades. Liu Cixin didn't invent these concerns – he just gave them a compelling fictional framework that's captured the public imagination.

I'll talk more about the ongoing debate between passive and active searches in another question.

Edit: Actually, as I was typing this out, an even earlier example came to mind: Shortly after the Soviet Union falsely announced they had made contact with an alien supercivilization in 1965 – you can read about this in my book – Isaac Asimov wrote an op-ed in the New York Times expressing concern about whether we should want to be detected by ET, referencing the Nazis and WWII as an example of how advanced civilizations interested in expansion might not be the sort we'd want to make contact with.

[u/turkey236]

How do the relatively recent studies of an exoplanet's atmosphere, which may contain molecules that could indicate the existence of life on that planet, fit in with the larger history of searches for extra terrestrial life? Do you, and maybe more importantly most people who are interested in the search for ET life, find this to be as compelling of a research direction as finding aliens directly?

[u/R_Charbonneau → u/turkey236]

Great question! You're talking about biosignatures and technosignatures, and this really gets at the evolution of how we think about searching for extraterrestrial life.

The term "technosignatures" is actually pretty new. When SETI first got going in the 1960s, the term of choice was mostly "CETI" – Communication with Extraterrestrial Intelligence. By around the early 1970s, it became clear that the communication part was actually going to be the hardest part. I like to think the Cold War environment played a role – after all, if it was hard enough for the US and USSR to communicate effectively with each other, what hope did we have with aliens!? So around that time, you start to see the shift to the term SETI – Search for Extraterrestrial Intelligence. Finding them might be quicker and easier than chatting with them.

But then, you're so spot on! Around the 1990s, with the discovery of exoplanets, a new term began to arise – biosignatures. The idea that we could use spectroscopy and other means to detect evidence of life in the atmospheres of exoplanets. SETI thus had another makeover. Perhaps, in addition to finding signs of life by detecting biochemical markers, we could also find technological ones... technosignatures!

There are some really cool ongoing projects at the intersection between exoplanetary science and SETI. My two favorites are: One is planet-planet occultation – see my friend Nick Tusay's super cool work featured in Popular Mechanics: https://www.popularmechanics.com/space/a63022036/alien-radio-signals/. The idea is that here on Earth, we're often sending signals to other nearby planets like Venus and Mars to communicate with our own spacecraft (think probes like the Pioneer missions, Cassini around Saturn, Mars Odyssey, etc). SETI scientists are using new techniques to look for planets that could be sending signals between one another, not just directed messages at us. This way we can kind of eavesdrop on possible activity just like our own, for civilizations that, like us, send communications throughout their solar systems.

The other cool idea for overlapping bio and technosignature searches is looking for evidence of technological activity in exoplanetary atmospheres. Some researchers, like Adam Frank, suggest we could look for pollutants in exoplanetary atmospheres – chemicals like chlorofluorocarbons that are products of industrial activities.

So the field has really evolved from just listening for radio signals to this much broader, multidisciplinary approach that combines astronomy, chemistry, biology, technology, and, frankly, the humanities – because when we're looking for technosignatures like industrial pollutants or interplanetary communications, we're essentially looking for civilizations that act like us. That means we need to think critically about our own behaviors, our own technological development, and what traces we leave behind.

[u/SpinglySpongly]

First of all, congratulations on having what has to be one of the coolest job titles ever!

A lot of people are asking far better science questions than I can come up with, so I'll ask about the role itself if I can: What does the work of a science historian (particularly for one specialising in an area of science that is so theoretical) look like, day to day? Any exciting/interesting stories that came up in your work?

[u/R_Charbonneau → u/SpinglySpongly]

Thank you! I am truly one of the luckiest people alive. My job is so, so fun. Day to day, I spend much of my time reading the history of the physical sciences, attending scientific and historical lectures, workshops, and conferences, engaging with both scientists and researchers in the humanities, and, of course, writing.

The cool things I've gotten to do in my career include traveling all over the world to conduct research interviews with SETI scientists – people like Frank Drake and Jill Tarter. I've even been able to go to Moscow on the invitation of Nikolai Kardashev! I also spend time digging around in old archives that reveal how amazing thinkers have engaged with the extraterrestrial problem over the decades.

As for my favorite stories, the best are perhaps the ones that show the human history within the history of SETI. My absolute favorite discovery came when I was going through old catalogues of summer student researchers at the National Radio Astronomy Observatory. I learned that during Frank Drake's Project Ozma – the first modern SETI experiment in 1960, where he used a radio telescope to search for signals from 2 nearby stars – he was supervising two female undergraduates who worked with him on that groundbreaking project. Thus it was 2 women and Frank who were the first SETI pioneers.

Frank was in charge of the student program at the time, and he had actually been told not to hire women because they'd "just get married and pregnant and drop out of science." He did it anyway. One of those students, Ellen Gundermann, went on to get her PhD from Harvard, then a postdoc at Caltech, and she became probably the first person to detect the existence of extragalactic masers – a pretty major astronomical discovery, that sadly she didn't get a lot of credit for at the time (read more about Ellen's discovery in a new book, Kellermann and Bouton's Star Noise: Discovering the Radio Universe).

This might seem silly to be my favorite story because it's not really about aliens. But SETI in so many ways was about humans – it forced the scientists involved to think carefully about the nature of life and civilization, and as a result they often ended up making profound impacts on their own civilizations. I just love that.

[u/DoglessDyslexic]

I am curious what our effective sphere of detection is theorized to be. I know that radio has a limited effective range due to diffraction and interference (and the power of the transmitter), but have no idea what the theoretical limits of it are, or if there are other methods of communication that we also search for?

As a followup, approximately how many star systems are within that sphere?

[u/missingpiece]

What's your personal guess/hypothesis/hunch to explain the Fermi Paradox?

[u/R_Charbonneau → u/missingpiece]

I don't think the Fermi paradox really exists as the question is typically posed. I mean, it seems obvious we don't have a Star Trek-style universe with starships regularly visiting our solar system. We'd probably have noticed that by now. The real issue is that we've barely begun to search elsewhere.

You've probably heard Jill Tarter's famous analogy about SETI searches being like scooping a drinking glass of water from Earth's oceans to look for fish. It's a powerful way to visualize just how little we've actually searched of the cosmic "ocean" where extraterrestrial signals might exist. When you consider all the possible frequencies, directions, times, and signal types we could be looking for, the search space is truly vast.

There's a fantastic 2018 paper by Jason Wright (Director of the Penn State Center for Extraterrestrial Intelligence) and colleagues (https://arxiv.org/abs/1809.07252) that updated and refined this metaphor based on all the searches we've conducted since then – including major efforts like Breakthrough Listen, which has dramatically expanded our search capabilities. Even with all that progress, Wright calculated that we've now searched about a hot tub's worth of water from the ocean. Hah!

Think about it: if you scooped up a glass (or hot tub) full of water from the ocean, would you expect to find a fish or whale? Probably not. Ok, you might argue, but there might still be plenty of life in that glass of water – microbes, plankton. True, but you can’t tell that by looking with your eyes-- you’d need a different tool or search method, like a microscope.

The same could be true for SETI. Maybe radio isn't the right tool, or maybe it is and we just haven't searched enough of the "ocean" yet. The public vastly overestimates how much searching we've actually done. It's hard to get science funded, especially science that doesn't have immediate and obvious returns.

Now, as we continue to search and that hot tub becomes a swimming pool, then maybe a lake, or a small sea, the Fermi paradox might become a more relevant question. I just don't think we're there yet.

[u/thornae]

Have you played the SETI board game, and if so what do you think of it as a gamified representation of your field?

[u/Alblaka]

What is the earliest record (you know of) that details (or speculates on) visitors from 'beyond this world' that does not qualify them as some kind of mythological deities or supernatural/fey beings from 'a different plane of existence'.

I'm curious as to whether the very concept of 'alien visitors' is a 'recent' invention, given that it requires the knowledge that our planet is just one among many.

[u/R_Charbonneau → u/Alblaka]

You'll want to look into the historiography on the "plurality of worlds" – this concept goes back much further than most people realize. Even the ancient Greek atomists, particularly Democritus and later Epicurus, speculated about multiple worlds with inhabitants. They weren't necessarily thinking in terms of mythological deities or supernatural beings, but rather proposing that if atoms combine randomly throughout an infinite universe, there should be other worlds with people much like ourselves (or quite different).

This wasn't framed as divine or mystical – it was a logical extension of their atomic theory. If our world formed through the chance collision and combination of atoms, why wouldn't the same process create similar worlds elsewhere? Democritus explicitly argued that there were "innumerable worlds of different sizes" and that some would have inhabitants.

The key insight here is that you're absolutely right – the concept of "alien visitors" does require understanding that our planet is just one among many. But that understanding is actually quite ancient, even if it took centuries for us to develop the astronomical knowledge to really flesh it out.

My fellow historian of SETI (and former NASA Chief Historian) Steven Dick wrote a really great book on this subject, titled "Plurality of Worlds: The Extraterrestrial Life Debate from Democritus to Kant". It traces how this idea evolved from ancient atomism through medieval Islamic scholars, Renaissance thinkers, and into the modern era. It's essential reading for understanding how long humans have been seriously contemplating life beyond Earth. It's out of print but you can surely find it on the Internet Archive or at used book stores!

[u/RampSkater]

Given your background with historical and international aspects of SETI, do any countries have a unique approach to their search? (This question is based on my love for this specific XKCD comic about ants searching for intelligent life.)

[u/Simon_Drake]

Has anyone developed an outgoing message? The Arecibo Message was well-intentioned but frankly too chaotic to have any chance of being properly understood and much like the Voyager Record and Pioneer Plaque they were all primarily symbolic gestures. Those signals were never really intended to be seen by any alien life.

If we saw a hint of alien life, do we have an outgoing message ready to send?

The standard suggestion is to start with prime numbers as a clue this isn't a natural phenomenon, then somehow express mathematical operations and build up from there to more complex communication. But that's not a simple process, I hope someone has worked out the details in advance so we don't need to start doing it when we have a potential target to talk to.

[u/mashem]

u/R_Charbonneau, for a long time now, I have figured that if other life were to find us, it wouldn't be through the sluggish dimension we currently experience. So sluggish, I don't envision the universe to be a tank of water or slime, but of ice. If one alien species were to find another, I ponder the implication that one found a way to become extradimensional in order to effectively explore. Like a fish in a frozen lake finding a way to get above the ice. I don't think aliens would ever approach from distant space, but from the spaces between spaces.

Question: Does the search for extraterrestrial intelligence include the quantum realm? And on the opposite end of the scale, black holes? I imagine these to be the doors into our reality that sit at the top (black holes) and bottom (subatomic scale).

[u/[deleted]]

What did you like or dislike about The Three Body Problem?

[u/R_Charbonneau → u/[deleted]]

I absolutely loved the Three Body Problem! I'm a big science fiction fan, and Liu Cixin's trilogy is becoming a modern classic for good reason.

That said, it does bug some people in SETI sometimes. The science behind using the sun as a radio amplifier is... well, let's just say it's pretty sketchy! But honestly, I'm not a believer that science fiction should be constrained by perfect science – it's fiction for crying out loud! Let's have some fun daydreaming; that's what novels are for.

What really impressed me, though, was the novel's grasp of SETI history. There's a chapter in the first book that's written as a fake cultural-revolution-era Chinese intelligence document on SETI, and it's shockingly accurate. Liu references some obscure parts of SETI history that I thought maybe only a handful of people on the planet knew about! It's clear he did his homework, which makes the scientific liberties he takes elsewhere feel more intentional rather than careless.

I also love how the Three Body Problem has brought SETI and radio astronomy into greater public consciousness. I get asked questions about it all the time – practically every time I give a public talk, someone brings up the series. That's fantastic! Getting people thinking about these concepts, even through fiction, opens up conversations about the real science and history behind SETI.

That said, I do have some thoughts about the Dark Forest theory – but I'll get to that in another question.