In the vast silence of the cosmos, one unsettling theory lingers: What if we’re not alone, everyone else is hiding? The Dark Forest Hypothesis offers a chilling answer to the Fermi Paradox: intelligent civilizations may be out there, but they stay silent out of fear.

Astronomer David Kipping explores the risks of shouting into the void, the psychology of cosmic survival, and whether alien civilizations might already know we’re here.

DAVID KIPPING: - SETI, S-E-T-I, is the search for extraterrestrial intelligence. It is essentially an effort to try to detect the radio signals or possibly even more broad than that, laser signals, whatever signals you might wanna look for, from another civilization out there in the galaxy. Now, there is a strange aspect of SETI. We have this kind of paradox that we are listening for radio signals from beyond, from these other planets, but we don't really transmit much. I mean very, very little. So we are kind of assuming that there will be a civilization out there, a benevolent civilization, that is interested in spending huge amounts of energy and electricity, powering their giant radio transmitter saying, "Hey, we are here. Look out for us." And yet, we have no systematic program of doing that. We've sent out one or two little messages, but we certainly are investing billions of dollars shouting out into the cosmos saying, "Hey, we are here. Come say hi."

- [Narrator] The dark forest hypothesis.

- SETI, S-E-T-I, has always had an interesting relationship with the idea of transmission of communication. In fact, when the field first began, it was actually called C-E-T-I, and that C stood for communication. And very quickly they changed their minds and realized that the idea of a two-way communication was maybe a bit ambitious and it might be better just to focus on searching for the signal rather than truly trying to have a dialogue. So on the other side we have the idea of METI, messaging extraterrestrial intelligence, also called active SETI. So this is where we actually send out a message hoping to get a reply at some point in the distant future. Of course, that could be hundreds or even thousands of years given the vast expansive space beyond us. So this is a very common concern. Stephen Hawking had the concern that, you know, maybe we should be very careful about communicating to the civilizations because whenever you've had a more advanced civilization engage with a less advanced civilization on the earth, that has usually ended very badly for the less advanced one. And so if you extend this out to the galaxy, maybe we should be careful about sending out our presence into deep space. And certainly this is a trope that has been played with in science fiction as well. If you look at Liu Cixin's book, "The Three-Body Problem," that's where we get the idea of the dark forest hypothesis. The idea that civilizations could be out there, dangerous, marauding ones who want to take over and colonize your planet. And so it's much better to just be quiet. Don't let anybody know that you are here. I do have a bit of an issue with this idea of resistance to METI and resistance to communication. And that's that we, humanity, are already engaged in the activity of trying to build telescopes and future facilities that can detect life whether they want to be detected or not. So you don't have to send out a radio wave for me to be able to detect your civilization. We could potentially detect their satellite systems, their star links. We could detect their solar panels on their surface. We could detect their industrial space presence, even the chemical pollutants in their atmosphere from industrial processes. There are so many ways that we could imagine being able to tell there was a planet that had a civilization on it, an advanced civilization on it, without the need for a radio message. That it's a little bit archaic to assume that, that's the only way that a civilization will be able to know of another one's presence. I suspect if there is another advanced civilization out there in the galaxy, they already know that we are here. Maybe not the presence of earth's current level of technology because simply the finite travel time of light is, you know, gonna limit us to about 200-year radius, essentially 200-light year radius where another civilization would know that we're doing industry. But certainly beyond that, in the broader galaxy, they would know we had an oxygen-rich atmosphere. They would know we had photosynthesis. They would know that the earth was inhabited by creatures. And so I think it would be difficult to argue that we should be very, very quiet because otherwise they'll never know we're here. I suspect they are well aware that this planet is inhabited with or without our radio signals. So when we talk about the dark forest theory or resistance to METI, I think an interesting way to look at this is what's called a risk quadrant, often used in financial analysis or insurance companies, as a way of evaluating the pros and cons of any activity, any behavior that you engage in. So in our case we have two possible activities we could do. One would be to transmit, other one would be to stay quiet, to hide ourselves. And then on the other axis we'd have the two possible outcomes. One would be that they're friendly and the other one would be that they're hostile. Now, if we transmit and they're friendly, then there's gonna be some net benefit. Let's call that a plus one benefit. If we transmit and they're hostile, that's gonna be a very bad situation for us. That could be a minus infinity, some very large number, a bad direction that they would take as civilization 'cause it could be an extinction level event, ultimately. On the hide axis, we have two possibilities. They're friendly and they're hostile, but it really doesn't make any difference 'cause the outcome is zero-zero in both cases. So when you average out these two rows in this risk quadrant, the activity of transmitting, you have plus one, added onto negative a very large number, which is gonna be another very large negative number. And then on the bottom axis you have the hiding, well, hiding's just zero plus zero's gonna lead to another zero. So there's no net gain or benefit of doing that activity. And when you compare those two on this risk quadrant, in this game theory type approach, you would end up concluding that it makes no sense to transmit. Because if you transmit, there is yes, a possibility of some gain, but there's also a very real possibility of a huge loss, which is your very existence. And so on that axis, a lot of people would claim it's just not worth it. You know, there's no reason why we should even take that gamble. But, of course, the way you might be critical of this naive analysis is that yeah, maybe they actually already know we are here anyway. This whole idea of transmitting is maybe a archaic way of thinking about how they would look for other civilizations. They don't need to listen to our radio signals. They can just take their giant telescopes and take a beautiful image of the earth and see our satellite systems, and know there's in fact, civilization here, regardless of whether we are trying to transmit or not. So I understand the common argument as to why we should be careful, thinking about almost the sociology, the game theory approach of thinking about their behaviors. But it's a little bit more nuanced than that. When you really think about the capabilities of one of these advanced civilizations. I think there's two parts to communicating with alien civilization. The first part is simply making a loud signal that they can see that somebody is there. Doesn't necessarily have any information embedded within it, it's just a loud hello. But the second part and the more interesting part is how do you actually communicate some kind of story of who you were, or what you know, or some request, or ask the civilization, that requires language. Now, of course, they're not gonna, unlike "Star Trek," they're not gonna have the same language that we have. They're not all gonna be aliens speaking English or have universal translators that we can easily make sense of. So it has been a real problem to think about what we might encode within these messages. And certainly there have been early attempts to do this. The Arecibo message sent in 1974 was an attempt at trying to build a universal message that anyone can understand. And it really worked within the language of mathematics. The idea being that an advanced civilization would surely have to have discovered mathematics if it was to get to a certain level of sophistication. And so that is the language that we could attempt to communicate with. Another example that was made in the early 1970s was the Pioneer plaque. This was a replica right here of a physical plaque that was put on board the Pioneer 10 and 11 spacecraft. That was an attempt at communicating to an alien civilization. Perhaps it will never be found, but it was an effort to do so that communicates really using pictorial messages and a little bit of math. For example, on this side of the plaque, we've got a representation as to where the solar system is relative to 14 nearby pulsars. These are like clocks in the universe. Each of these lines represents the distance from those pulsars. And the binary code that is stitched along there represents the frequency of those pulsars. Actually, another interesting bit is that in the future, the pulsars location and their timing, the frequency will slightly drift. And so an alien civilization who discovered this plaque could not only figure out where the solar system is, but the year in which this plaque was launched because this will change slightly over time. So this simple message, even though it's pretty small, already tells a civilization, "Hey, this is where we are. This is the third planet from the sun that we came from. This is what we look like on this planet and this is the vehicle that we used to get to you." It even has a little depiction of the hyperfine transition of hydrogen, letting 'em know that we understand quantum mechanics as well. So a certain level of sophistication was required to make this plaque. And these are the kind of efforts that we're engaged in is thinking about what universal things would they know. Surely there would know quantum mechanics, surely there would know maybe general relativity. Surely there would no advanced mathematics. And it's those that we have to use as a common basis of language. Otherwise, we might be stuck in a situation, like in the film "Arrival," where it would require some linguist to come in and think really hard about how to translate these patterns which are maybe non-mathematical, but might have some cultural connection to those civilizations as well. So I'm a big proponent of technologies for communication that are passive, if you like, that don't require an active power system. If we try to build a communication program that requires federal funding, and energy, and maintenance, it's inevitably gonna fail after 10, 20 years, whatever, when someone decides that's too expensive to do. We need something which can last for not only hundreds of years, but potentially billions of years. For we have to accept that even if we are pessimistic about the odds of cohabiting the galaxy with another civilization right now, into the far, far future, tens, even hundreds of billions years into the future, there could very well be a large number of civilizations which emerge and explore the galaxy and eventually come and arrive at the solar system, and wonder who might have lived here in the past. And so for me, one of the most interesting ideas for communication is to leave something physical behind for them to find. And, of course, this is an idea that was played with, with the film "2001: A Space Odyssey," where there's these monoliths which are left within the solar system, especially beneath the lunar surface. And I think that is a really terrific place to leave something. The moon has no active geology, it has no atmosphere, it has no weathering. So anything that you leave on the lunar surface, is gonna preserve for a very, very long time, millions of years. Think about Neil Armstrong's footprints. They're gonna be there for at least a million years. And if you build something physical, a large structure on the surface, that's gonna be even more robust. And the extra benefit would be, of course, to bury it, maybe a meter beneath the surface where you're now protected from micro meteorites as well. This would be probably the safest place in the solar system, in my opinion, to leave something that we could potentially encode a message, maybe an LLM, an AI agent that can communicate, teach another civilization about who we were, our mathematics, our accomplishments. And I think to some people this idea can be a little bit depressing. We maybe have the idea that we want to actually meet them on the White House lawn, shake their hand like we see in sci-fi films, and have a communication, a dialogue in real time. But if we are being more realistic about it, maybe that's just never gonna happen. And if we concede that, that might be an improbable circumstance, perhaps the next best thing we can hope for is to communicate, not in a simultaneous sense, but through time. We could leave something behind. In the same way that our ancestors speak to us through the monuments they left, be it the pyramids or the Stonehenge, these structures which are left behind and speak to us from the past. We too have an opportunity to build something on the moon, I would claim, that could last for billions of years and be a record. And perhaps, perhaps the most likely advanced civilization that is going to discover that relic that we leave behind will be a future descendant of the earth. For the Earth has another billion years left to go in its evolutionary history of chemical biology still taking place. And think about all the advanced evolution that has occurred in just half a billion years. We've gone from single celled organisms to us in half a billion years on this planet. What's gonna happen in another billion years? We will probably disappear at some point. And I can imagine not just one, but perhaps multiple advanced civilizations rearising on this planet, having their own space ages. And they will go to the moon, find this relic, and perhaps know something about who we were. To me, that is the most likely alien encounter we're gonna have.