The universe remains silent. We’ve never detected messages from non-human life, despite decades of searching. One potential explanation is provided by Dark Forest theory. It posits that the universe is silent because it’s like a dark and shadowy woodland, where hidden predators may lurk. Any intelligent species would therefore be unlikely to reveal itself out of fear that a more advanced or hostile civilization might immediately annihilate it. The theory got its name from Liu Cixin’s 2008 novel The Dark Forest, but the idea predates the book.
Astronomer Vishal Gajjar, who is a researcher with the SETI Institute and a member of the Breakthrough Listen program, the largest scientific search for intelligent life ever undertaken, recently invoked Dark Forest theory when I interviewed him about his new research. Gajjar is co-author of a paper in The Astrophysical Journal that found space weather around stars might be interfering with our ability to pick up signals from extraterrestrials. If we do one day detect communication from ET, I asked him, what should we say back?
For humans, the universe is still a very dark forest, he replied. “If you’re in a forest where it’s completely pitch dark, you shouldn’t make a lot of sound to attract attention to yourself,” said Gajjar. “In the same way, I wouldn’t advocate for transmitting signals to aliens.”
He’s not saying never—just not yet. Not only do we lack the technology necessary to communicate, humans are also a young species, and we’re too divided against one another.
I spoke with Gajjar about his findings, why he remains optimistic that we’ll uncover messages from extraterrestrial life, and what kinds of signals we need to look for. We also talked about UFO conspiracy theories, Independence Day, and what ET might look like.
What is it like to listen for communication from non-human life?
This is definitely an exciting thing to research. What we’re trying to do is answer humanity’s oldest question: Are we really alone in the universe? On the other hand, it’s also a bit frustrating that we haven’t been able to find any evidence of life outside Earth, so far. But I’m very optimistic. I think we’ll at least have some answers about biological life in the next decade or so. With the advancements of new telescopes coming online, we’re able to sniff atmospheres of other exoplanets, which helps us detect signs of biological activities. But evidence of intelligent, technologically advanced life? There’s nothing compared to that. If we find something that’s developed a technology that we’re able to detect across interstellar distances, that would be phenomenal.
Have there been any close calls in your career so far—where you felt you came exceptionally close to finding what you were looking for?
Nothing that was anywhere near what we’d like this signal to look like. We had some false positives in the past, though, that we were quite excited about. There was a signal called BLP-1 that I was involved with through the Breakthrough Listen Program, which was very exciting when we found it. But eventually, we were able to rule it out. There was one more example, which I was also very closely involved with—a signal that we discovered with the FAST telescope in China. If that signal had been generated by human technology, we would’ve seen it in a large number of different directions in the sky, but we found it only in one part of the sky. We all got very excited and sent a lot of messages back and forth.
Many sleepless nights followed. But eventually we were able to rule it out because when we looked at the raw data more carefully, we found that the signal was more likely radio interference from Earth.
Your new paper argues that space weather might be interfering with our ability to detect communications from intelligent life. If you’re an alien on a distant planet somewhere, trying to send a radio message across the galaxy to Earth, what does the weather around that star do to the message?
Our hypothesis is that the signals we’re searching for are going to be extremely narrow in frequency. A single frequency, for example. There’s a very good reason we believe that: All the natural sources that we know produce signals at multiple frequencies—roaring wind, crashing ocean waves, crackling fire—whereas engineered frequencies are typically concentrated, sometimes to a single pure tone. Think about when you tune the dial on your radio and focus on one station, but as you move the dial away, you lose that station, right?
But if that signal originates from a planet whose orbit periodically takes it behind its own star, the signal has to pass through the star’s surrounding plasma and turbulence before it can travel toward us. That environment smears the spike out into something broader and flatter. The signal is still there, carrying the same total energy, but instead of a sharp spike our instruments can recognize, it’s spread across a range of frequencies—and our detection systems, which are specifically tuned to find sharp spikes, simply pass right over it. This is one of the ways we might have missed signals like these in the past.
Can you tell me more about why it’s more likely to be a narrow signal if it’s coming from intelligent life?
That’s just the hypothesis. We have no idea what ET is doing. It’s an educated guess based on our current understanding of radio communication. Narrowband radio signals are the prime candidates because nothing like that exists in nature. If we detect such a signal, there’s a 100 percent chance that you can say, “Oh, this is an artificial signal produced by a form of life.”
But narrowband signals are also useful for communication for a practical reason: You can embed information alongside a pure carrier frequency, and the receiver locks onto that carrier wave like a beacon, using it to decode whatever message is riding next to it. So there are good engineering reasons why a transmitting civilization might use them.
But my opinion is that we shouldn’t concentrate our entire effort on searching only for narrowband signals. We’ve been doing that for the last 60 or 70 years. Researchers have written hundreds of papers on this search over the years, and all of them have focused on just searching for narrow radio signals. But this isn’t the only signal that we should be looking for.
What are some of the other signals that we could look for?
One is a broadband signal, the complete opposite of the narrowband signal. Natural sources in space tend to broadcast across many frequencies at once. But as those signals travel through the interstellar medium, something interesting happens. The medium acts a bit like a crowd: High frequencies are slim and push through easily, while low frequencies are bulkier and take longer to get through. So a broadband signal arriving from a natural source will always show the same pattern—high frequencies arriving first, low frequencies trailing behind.
Here's the clever part: An intelligent civilization would know this, and they could simply reverse it—deliberately sending their signal so the low frequencies arrive first and the high frequencies come later. Nature never does that. It can’t. If we ever detected a broadband signal arriving in reverse order, that alone would be a powerful sign of something engineered.
We could also search for signals that have patterns embedded in them. Natural signals are essentially random. If you measure their statistics hour after hour, nothing repeats, nothing accumulates into structure. But a signal carrying information has an internal consistency to it. Its overall statistical fingerprint stays stable over time in a way that random noise never does. Scientists call this cyclostationarity. It’s the kind of order that only emerges when something is deliberately encoded, but most SETI experiments have largely ignored this possibility.
Read more: “If You Meet ET in Space, Kill Him”
If a civilization is advanced enough to intentionally broadcast across interstellar distances, why wouldn’t they anticipate the interference of space weather?
If their purpose is to intentionally send a beacon, then yes, they’d try to avoid this problem of distortion. And there are many ways to avoid it. You can transmit the signal away from the star, or you can transmit the signal from a location that’s farther away from the star. One other thing they might do is transmit messages at a very high frequency. Most of the searches that we’re doing are at a low frequency, around 1.4 gigahertz. That could be another reason why we haven’t picked up the signals.
There are already efforts ongoing to track higher frequency signals. Part of the Breakthrough Listen program is a search across the full range of frequencies detectable from the ground: from one to 100 gigahertz. In the past we didn’t have enough resources or instrumental power to do that at a large scale. Now we do. I’m very hopeful that in the future we’ll be able not just to search different categories of signal but also higher frequencies.
But when we’re out there searching for signs of life, we don’t hope to only search for deliberate communications. We also try to search for unintentional ones, like leakage radiation.
Is the idea that space weather could be interfering with communications from intelligent non-human life a new idea or a twist on an old idea? What is specifically new here?
We knew that space weather from our sun could interfere, so we’ve targeted our search mostly away from our sun. But nobody had ever considered that the signal that we’re looking for could be originating from a planet, which will revolve around its own star. That was something novel that we did in this paper: bring to everyone’s attention that there’s this effect that might be happening at the source itself, which you have no control over.
It sounds like the most common type of star in the galaxy is the worst place from which to send these kinds of signals. Is that right?
That’s correct. If you look at stars in our Milky Way, 75 percent of these are what we call red dwarfs, or M dwarfs. M dwarf stars are much smaller and dimmer than our sun, but that also means they burn their fuel incredibly slowly—and as a result, they last much longer. Our sun is about 5 billion years old, and it will die in another 5 billion years. But no M dwarf star has ever died yet. They just keep going. That means that these stars are more likely to host life, because it can take a long time for life to emerge, become intelligent, develop technology, and all of that.
On the other hand, M dwarf stars also have the worst space weather environment around them. They produce a lot of plasma, a high magnetic field, and high winds. They have more coronal mass ejection, massive and explosive release of plasma and magnetic fields. And if a planet orbits an M dwarf star, it’s likely sitting much closer to that star than Earth is to the sun. As I mentioned, the closer a planet is to its star, the stronger this effect becomes. Move the planet farther away, and it matters less and less. M dwarf stars are small and low-mass, which means their planets naturally form an orbit much closer to the star. The effect is amplified even further for these systems.
That’s what makes this study particularly important: Roughly 75 percent of all stars are M dwarfs, and they’re exactly where this effect hits hardest. So we have no choice—we need to understand it and correct for it.
How likely do you think, on a gut level, that intelligent life has been trying to contact us, but space weather has disrupted it?
First of all, we truly don’t know the actual prevalence of life in the universe, so we cannot estimate how many civilizations might be transmitting signals. What we did instead was simulate a search for radio signals across 1 million stars. In that simulation, we found that about 30 percent of the star systems—the star and its associated planets and moons—were aligned with Earth in such a way that any narrowband signal originating from them would be spread across a slightly wider range of frequencies, by roughly 10 Hz, making it likely that our current search methods would miss them.
Of course, this doesn’t mean that 1 million stars are actually transmitting signals. Rather, it shows that purely due to the orientation of the star relative to Earth and the way signals spread as they travel through space, there’s roughly a 30 percent chance that signals from a large sample of stars could evade the kinds of searches we’ve been doing over the past 60 years.
What are the main caveats to the hypothesis that space weather is interfering?
Number one is that we’re expecting the signal to come from the habitable zone. This zone includes planets that orbit a certain distance from a star, where water exists. That’s because of our definition of life. We expect life to exist on a planet that has water in liquid form on it. If the planet is too close to the star, the water will simply evaporate. If the plant is too far out, water will never exist in liquid form, it’ll be just ice. That’s one big assumption we’ve taken here, which is probably not much of a stretch.
But it’s possible that the transmitter we’re searching for might be very far away from the star. In that case, this effect of space weather wouldn’t be that significant. One thing that works in our favor is that we’ve only considered the effects of a single star. Half of the stars in the Milky Way are binary stars, where things get even more difficult, even more crazy, because you have these two stars that are going around the planet. There, the space weather is going to be even more chaotic. You’ll have a lot of mass transfer happening due to the gravitational influence. The environment will be full of charge particles and plasma. So we’ve been very conservative in our estimation.
Are radio signals the most likely communication medium, or is that just what we know how to look for?
Again, we’re just making an educated guess. We obviously don’t know what SETI will be doing or which wave band they might select, if they will even select a wave band to transmit. But if you look at the simple principle of energy conservation, everybody in the whole universe will be bound by that. Energy is always finite. And radio band is still one of the best possible and most energy efficient ways by which you can communicate across large distances. If you use anything else, you end up spending a large amount of energy. You won’t gain a lot in terms of its detectability. X-rays and gamma rays, for instance, have much higher energy requirements. Another reason is that high-energy waves will get easily absorbed by the interstellar media.
If we do detect specific communications from non-human life, what’s the best way to communicate back?
We don’t have any way of doing any two-way communication. The distances we’re talking about are on an astronomical scale. Our nearest star is four light-years away. So even if we detect a signal from our nearest star, it’ll take four years for us to catch the signal, and then four years to get any response. That’s not a very efficient way of doing two-way communication. And that’s just for the nearest star.
What we typically search is hundreds of light-years away. In those cases, it’s impossible to do two-way communication. So the astronomers aren’t so keen on responding back. In fact, we try to avoid this kind of transmission. We don’t really understand what’s out there. The universe is quite vast. We’re a very young species. There’s a hypothesis called Dark Forest theory: If you’re in a forest where it’s completely pitch dark, you shouldn’t make a lot of sound to attract attention to yourself. In the same way, I wouldn’t advocate for transmitting signals to aliens.
You can ask this question the other way around: If we’re not doing it, why would ET do it? I’m not saying we should never do it. There’s a difference between doing it right now and doing it later. We shouldn’t do it right now because we don’t have that level of technology, we don’t have the level of globalization where we can all agree on what message to transmit. If at some point the whole human species could agree on that, then it should happen. But before that, it should not.
Are you optimistic that the entire human race could get to consensus on something so colossal?
I’m always optimistic. Currently, things aren’t going in the right direction. But I’m an optimistic person. That’s what keeps our research moving forward.
Do Unexplained Anomalous Phenomena (UAP) have any relevance to your work?
No, we don’t really work on any of it. I have absolutely no expertise in UAPs.
Some of the media coverage of your paper mentioned UAPs as well as these kind of far-out claims about people being injured by aliens and secret programs of UFOs. Why is the subject of non-human intelligent life so prone to conspiracy theories and controversy?
Whenever we’re talking about a subject that humans don’t understand, there will be conspiracy theories. Extraterrestrial life is an intriguing subject. Everybody understands that there’s a high chance of life being out there. That feeds into this mass hysteria, if you want to call it that. But I have absolutely no confidence in any of these theories about UFOs. So far, we haven’t seen any strong evidence for them. Until and unless people who have expertise in this area can really say something about it, it would be very difficult to make any judgment based on just observations from regular people.
As a scientist, if I went out and wrote a paper that said simply, “I saw this,” nobody would believe it, right? That’s not the way science works. Science works on repeatability. You come up with the data, with explanation, and you share the data and then you independently verify that evidence and it becomes a discovery. That’s why I’m really skeptical about any of these claims. But obviously you can’t stop people from thinking about it. It’s definitely a fun idea. I love watching movies about UFOs. I love Independence Day.
Have you tried to imagine what intelligent life might look like? Do you have an image in your head?
There are a lot of people who have started thinking about this question now. Some 10 to 20 years ago, we didn’t know there were so many exoplanets out there, but now we’ve discovered 6,000 of them. We can currently say that almost every star in the night sky has a planet around it. We know planets aren’t very unique. People have started estimating the properties of these planets. The James Webb Space Telescope can characterize atmospheres of planets that are very close to the stars. If no light reaches the surface, is there any reason for any life on that planet to have eyes? Do you even need eyesight? In the upcoming movie Project Hail Mary, the alien comes from a planet which doesn’t get any sunlight, and they don’t have any eyes. They have sonar vision. They use echolocation, things like that.
Of course, we have absolutely no idea what alien life will look like. We also don’t really care much about it. No matter what they look like, if they’re able to build a radio telescope and transmit a signal, that’s good enough for us. 
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