For millennia humans have looked to the stars in search of meaning. We have reconsidered our notions about our place in the cosmos with each breakthrough in astronomy, from the astrolabe to the infrared space telescope, but no amount of technological refinement has satisfied our yearning to understand things beyond what our instruments or our eyes can tell us.
In the observable universe, there are around 200 billion galaxies. The Milky Way alone contains between 100 and 400 billion stars and at least that many planets. Data from NASA’s Kepler Space Telescope conservatively suggest that 300 million of those planets orbit their star in its habitable zone — the distance from a star conducive to the presence of liquid water, which may be a prerequisite for life. It would take nearly eighty thousand years to reach the nearest “exoplanet” — a planet in orbit around a star that is not our sun. In 1975 NASA began funding its Search for Extraterrestrial Intelligence (SETI) program, which used then-recent developments in radio astronomy to monitor radio signals coming from space. NASA’s program was canceled by Congress in 1993, at which point the SETI Institute — a nonprofit research organization based in Mountain View, California — took over the program.
Seth Shostak is the senior astronomer for the SETI Institute. He is the cohost (with Molly Bentley) of the weekly Big Picture Science radio show and podcast and the author of four books, including Confessions of an Alien Hunter: A Scientist’s Search for Extraterrestrial Intelligence. Shostak was the 2004 winner of the Klumpke-Roberts Award of the Astronomical Society of the Pacific for his contributions to the popularization of astronomy. He has written more than sixty journal articles documenting SETI’s efforts and advocating for a more expansive search that includes looking for not just radio signals but also extraterrestrial artifacts. In addition to serving as a science adviser for film and television, Shostak frequently appears in documentary television series, including the History Channel’s Ancient Aliens.
Shostak’s interest in aliens was stoked by the science-fiction movies he watched as a child in the 1950s — he and a friend were inspired enough to make their own movies. For his PhD thesis in astronomy at the California Institute of Technology, he used radio telescopes to measure the way galaxies spin. When conditions weren’t favorable to observe galaxies, he would point the telescope at nearby star systems and hope to find signals that might suggest an attempt at communication. He joined the SETI Institute twenty years later, in 1991.
I spoke with Shostak for several hours via Zoom, just after the first images from the James Webb Space Telescope had been released. We talked about aliens, conspiracy theories, and science fiction. Our full conversation was peppered with many Star Trek references, a common shibboleth among SETI aficionados.
Mahaffey: I’ll start with the big one: Are we alone in the universe?
Shostak: It’s possible. We haven’t found any life, let alone intelligent life, beyond Earth. I mean, we haven’t found any yet. There are many, many planets. If it turns out that Earth is the only one with beings that run around and think about stuff, that would make us some sort of miracle, and I can’t bring myself to believe that. If the explanation for your scientific hypothesis is “It’s a miracle,” you’re not going to get published, and you shouldn’t.
You might look at a million solar systems and fail to find a signal, but that doesn’t prove they’re all sterile.
Mahaffey: So the scale of the universe makes it difficult to produce a definitive answer.
Shostak: Yes. One hundred years ago astronomers figured that the sun and the various planets orbiting it were close to the center of this one and only galaxy. But all that changed in the mid-1920s, when people like Edwin Hubble figured out that the little smudges on photos of the night sky are not objects in our galaxy at all; they’re other galaxies. It turned out that ours was just one of many. As of today we can see at least 200 billion galaxies. There are many more we can’t see because they are so far away that the light from them hasn’t yet reached us. In the Milky Way, our galaxy, there are 200 to 300 billion solar systems. And one out of every two or three has a planet the right distance from its star to have the kind of temperatures that you enjoy in North Carolina.
Mahaffey: Since we can’t prove a negative — that there is no life elsewhere in the universe — what will it take to prove we are not alone?
Shostak: It’s difficult to think of an experiment that would definitely decide, “OK, there’s some intelligent life there.” And this actually has consequences for funding. The search for intelligent life beyond Earth is not a government project — hasn’t been since 1993. It’s funded by people who think it’s an interesting question — Are we alone? — and they send a check for twenty dollars or whatever. So there isn’t very much money to support finding an answer. All we can say is that we’re going to continue looking for signals. I think there are other things we can do, though, besides looking for a signal.
Mahaffey: Say you get a promising signal. What is the process for checking it out?
Shostak: Getting a signal is nothing remarkable. In just a few minutes’ time you might get two dozen signals or more. The source of most of them is simply our own activities on Earth or in orbit. But if you get a signal that’s not coming from our technology, it might be something interesting. So the first thing you do is point the telescope away from what you’re looking at and see if the signal goes away. If it’s from a nearby satellite, it won’t go away: you’ll hear it no matter where you point the antenna. So you just “nod” the antenna back and forth, and if the signal still looks good and certain characteristics are right — the Doppler shifts and all that — then you call up somebody at another observatory and ask, “Would you mind checking this out?”
Mahaffey: You’ve been part of this search for a long time. What keeps you from giving up?
Shostak: The fact that the instrumentation keeps getting better, for one thing. The public may think we’re doing the same thing today we did ten years ago, but we’re not, because computers keep getting faster. Faster computers mean we can quickly check billions of radio channels. So instead of going through one solar system per day, we might go through two or five or even ten. Computer speed doubles about every two years. That’s a very optimistic development. We look back now on the experiments of the 1970s and say, “How quaint.”
There are many, many planets. If it turns out that Earth is the only one with beings that run around and think about stuff, that would make us some sort of miracle, and I can’t bring myself to believe that.
Mahaffey: You said there are other methods we could use besides looking for signals. What are some newer frontiers in the search for extraterrestrial life?
Shostak: One is to look for flashing laser lights. Nobody’s done much of that. Another is searching for artifacts. Societies that are a million years more advanced than ours might build things so big that we can find them. And that’s where the James Webb Space Telescope (JWST) comes in. It may find planets with oxygen, but if it found a Dyson sphere, that would be much more interesting.
Mahaffey: Could you explain what a Dyson sphere is?
Shostak: Physicist Freeman Dyson theorized decades ago that a really advanced society would probably need a lot more energy than we get from burning natural gas or coal, or from wind farms or solar. We run at about 18 trillion watts a year. That’s the total annual energy consumption of Homo sapiens. But suppose you had technology that required a quadrillion watts per person. How could you get that kind of energy?
Dyson suggested you might build a large number of orbiting structures that formed a shell around the sun. These structures would be lined with solar cells. The Earth intercepts less than one-billionth of the sun’s energy, but these structures could collect essentially all the sun’s energy and beam it down to the planet. You would then have all the energy you could possibly use.
Mahaffey: How would a Dyson sphere register on the James Webb Space Telescope?
Shostak: If anybody’s built anything like this, the sphere will warm up a little bit. If the telescope registers some unnatural infrared radiation coming from a star system, if it shows too much infrared, that could indicate that there’s something big there. The problem is that there’s a lot of dust in space, and dust also warms up. So you might find a lot of dust and think it’s a Dyson sphere.
Mahaffey: What else can the JWST, which is a government-funded project, tell us about our place in the cosmos?
Shostak: The James Webb is an infrared telescope, and that means it’s perfectly suited for studying galaxies that are far away. Through it we can see the universe the way it looked shortly after the Big Bang — “shortly” meaning maybe a couple of hundred million years after. That’s probably the biggest attraction of that instrument for astronomers. But it can also be used to look at nearby stars in our own galaxy and actually see planets around them, rather than just detecting the planets’ presence through their gravitational influence. And if we can see them, if we can get some light reflected from their surface — even one pixel on a photo — we can put the light through a spectrometer and check the atmosphere of that planet for things like oxygen or water vapor. Our atmosphere has oxygen only because of photosynthesis. So if you found oxygen in the atmosphere of one of these planets, that would tell you life exists there.
Mahaffey: Water is, as far as we know, a prerequisite for life, right?
Shostak: Life is just a form of chemistry, and you usually need some liquid to facilitate chemical reactions. If you take a whole bunch of chemicals, just raid your local chemical-supply warehouse, and throw them all on the living-room rug, it might aggravate your spouse, but not much else will happen. If you pour water over them, however, suddenly the chemicals will start reacting, because they can get to one another on a molecular level. The liquid doesn’t necessarily have to be water. Saturn’s moon Titan has liquefied natural gas flowing over its surface — methane and ethane, like you might buy for your barbecue grill. But Titan has lakes of it. That might facilitate life, too. We don’t know.
Mahaffey: NASA recently reported that the JWST has detected carbon dioxide in an exoplanet for the first time. It’s a massive, hot planet — 900 degrees Celsius, or 1,600 degrees Fahrenheit. Does the presence of carbon dioxide suggest life might be able to exist in more-extreme conditions than we now imagine, or is that pretty consistent with what we know already?
Shostak: I don’t know if it’s consistent, but it’s not surprising. If you look at Mars and Venus, the majority of their atmospheres are carbon dioxide. As you know, that doesn’t indicate the presence of life on Mars or Venus. Mars’s atmosphere is mostly carbon dioxide, but it has very little oxygen, because there aren’t any plants. So finding carbon dioxide in the atmosphere of a planet doesn’t tell you much about the presence of life. It does tell you that the planet has an atmosphere.
Mahaffey: What is our best evidence to date of the existence of extraterrestrial life? I’m thinking of things like the Pentagon’s 2021 report on UFOs — which we’re now calling UAPs, unidentified aerial phenomena — or Oumuamua, the unusual object observed by the University of Hawaii’s Pan-STARRS1 telescope in 2017.
Shostak: Avi Loeb at Harvard thinks that maybe some of these phenomena could be artificially constructed vehicles. All we know about Oumuamua is that it’s longer than it is wide and didn’t come from our solar system. We also have some idea how fast it tumbles through space. One of the arguments Loeb has made is basically “What are the chances that something randomly kicked out of another solar system would ever get close to ours?” It’s like if somebody went outside my office and put a nickel on the sidewalk somewhere — I don’t know where — and I threw a tennis ball and randomly hit it: unlikely, although not impossible. It’s not a bad argument, but we have also found that there are a lot more nickels on that sidewalk than we thought. So hitting a nickel might not be so hard. We will never know whether Oumuamua was a spacecraft or just a rock, because it’s now so far away we can’t see it anymore.
One out of every three Americans believes that Earth is being visited by extraterrestrials who have come here to annoy Navy pilots or haul people out of their bedrooms for novel experiments. I don’t believe we’re being visited. I think if we were being visited, there would be unambiguous evidence of it, such as from the many satellites that every day are making images of Earth.
Mahaffey: Or the millions of smartphone cameras that we now carry.
Shostak: The folks who believe we’re being visited continue to be very excited about that Pentagon report. Suddenly, from their point of view, the government is coming clean on this topic. They almost universally believe there’s been a massive cover-up. But I don’t know. I’ve worked for the government. I grew up in Arlington, Virginia, and needed security clearance for my summer job. I was never impressed with the ability of the government to keep secrets. And if we were being visited, the U.S. government could not cover up all the evidence there would be. Those thousands of satellites in Earth’s orbit are not all the property of the U.S. government.
Even if they could cover it up, my question is: Why? Why not just present the evidence to the world? The answer I often hear is that they think the public couldn’t handle the knowledge — which strikes me as bizarre, given that one-third of the public already believes it’s true.
One out of every three Americans believes that Earth is being visited by extraterrestrials who have come here to annoy Navy pilots or haul people out of their bedrooms for novel experiments.
Mahaffey: Assuming that someday we get confirmation of a signal from extraterrestrials, would we be able to communicate with them?
Shostak: In Hollywood it’s a given, because the aliens often speak English. But there are people who have thought seriously about this. In 1960 a Dutch mathematician, Hans Freudenthal, invented a math-based language he thought we might use to communicate with aliens.
My proposal would be to send them the Internet. Just take a Google server and send its entire contents to the aliens. If they have the kind of technology that can pick up our signal, they also have computers that can analyze Internet data. It would be like decoding hieroglyphics, but a computer would look for redundancies and ultimately make sense of it. Of course, if we send them the Internet, they might think cats run the planet. [Laughs.]
Mahaffey: You wouldn’t be concerned at all about how another intelligence might interpret the full, uncensored content of the Internet? It’s grown quite toxic.
Shostak: That’s by our standards. I don’t think aliens would care any more about that than you care about the toxic relationships among the ants in your backyard. I mean, ants have a pretty brutal existence. What do you care? I am sure the aliens would be more advanced than we are and would regard whatever we do — war and all that — as just another interesting aspect of our behavior. It’s as if you were studying Neanderthals.
Mahaffey: Has SETI ever broadcast messages into space?
Shostak: We have done a little bit of “active SETI,” where we deliberately send signals, but mostly as a sort of demonstration. Some people think that we shouldn’t transmit anything without international consultation. The whole earth would have to agree. But the idea never went anywhere. We couldn’t have enforced it anyhow.
Mahaffey: So we don’t regularly send signals like the ones SETI looks for?
Shostak: We do it every day, just not deliberately. Radar, in particular, is high frequency and in most cases high power. It zooms right off the edge of the Earth. Those signals don’t carry much information, except to tell you where the airports are. But if any aliens picked it up, they would know it had come from an advanced society — or, at least, one clever enough to build a radio transmitter.
A guy I know who used to work at the Institute, Douglas Vakoch, has an office up in San Francisco for what he calls METI — Messaging Extraterrestrial Intelligence. He uses radio telescopes around the world to broadcast short messages. But, honestly, I leave that to ABC, CBS, and NBC, because they’re broadcasting 24-7 with more-powerful transmitters. And though the jokes aren’t always funny on television, at least there’s a signal coming off the planet all the time. Detecting those signals would require the aliens to use a considerably more sensitive listening device, but if the radar first tells the aliens that we’re here, they may focus in and learn a whole lot more from television — and FM radio, too.
Mahaffey: If we could communicate, what questions would you ask?
Shostak: Even assuming they could understand and would want to reply, I’d never hear the answer. If they’re a hundred light years away, you’re not going to get an answer for two hundred years — by which point I suspect our questions will have changed anyway.
Personally I doubt any aliens we might encounter are going to be biological. I think they’re all going to be machines.
Mahaffey: Why is that?
Shostak: In the movies the aliens are almost always biological, which means they’re vulnerable, too. If you hit them over the head with a telephone pole or something, you might take them out. But when you consider what humanity is doing in this century, possibly the most important thing will be the invention of generalized artificial intelligence. I don’t just mean a machine that can beat anybody at chess. We’ve got that. I mean a machine that can write poetry, compose music, make Oscar-winning films — do anything a human can do. And once we have that machine, the first task we’ll give it is to design a machine that’s even smarter. Very quickly we’ll end up with a machine that’s smarter than all humans. And if we’re heading toward that, then maybe the aliens have already done it, and they’ve gotten away from messy, vulnerable biology.
Mahaffey: One of the reasons pop-culture aliens tend to be humanoid, as famously put by Gene Roddenberry when he created the original Star Trek series, is that it’s cheaper. [Laughs.] But it’s also because humanoids are easier for an audience to empathize with.
Shostak: Exactly — empathize with or hate because they’re destroying Los Angeles. I consult for films occasionally, and there’s another big advantage to having aliens always be short, thin, and gray with big eyeballs and no ears. Then the producers and directors don’t have to tell people: This is an alien. The viewer can recognize the creature as an alien. So it has advantages for Hollywood, but that doesn’t mean real aliens are likely to look like that.
Mahaffey: You mentioned that the James Webb Space Telescope lets us see light produced within a few hundred million years of the Big Bang. Can you talk a little about the constraints of the physical universe as we understand it, and how long it would take for any information — even a single bit of data — to make it to us?
Shostak: Albert Einstein said you can’t send information faster than the speed of light, and that seems to be correct, because if it isn’t, you immediately run into problems in physics like losing causality: you might see a fried egg turn back into an egg in a shell. So this is one constraint. People often ask, “Can’t we just use quantum entanglement?” [The phenomenon in which two subatomic particles share the same quantum state, and measuring one affects the other, regardless of the distance between them. — Ed.] But even with quantum entanglement, you can’t just start sending information faster than the speed of light. You would first need to put entangled particles everywhere you want to communicate, and the total time required to set the system up is still always bounded by the speed of light. The distance just from one side of our galaxy to the other is around a hundred thousand light-years.
Mahaffey: Your work with SETI must bring you into contact with some colorful characters. How do you distinguish between the credible and the credulous?
Shostak: The UFO crowd is certainly trying to do the same thing we are: find the aliens. The difference is, they think they already have. I’ve been invited to talk at Ancient Aliens conventions, and many of the people making presentations there have a direct interest in the idea that we’re being visited. In other words, they have books to sell. It may not be a lot of money, but it’s significant to them. One of them, Richard Hoagland, once worked over at NASA Ames, about two miles from where I’m sitting, and whenever I’ve mentioned his name to people at Ames, they’ve just rolled their eyeballs. If you listen to these ufologists talk about the pyramids on Mars or the face on Mars, and you have a background in science, you know it’s all baloney. But it’s still entertaining.
Mahaffey: Do their activities make serious scientific work in this field more difficult?
Shostak: It doesn’t affect SETI terribly much. In the past I’ve sometimes wondered about the effects of pop culture. When The X-Files came on, I thought, OK, maybe SETI will no longer have funding problems. There will be people lining up to support us. But the only difference was that we had more visitors to our website.
Mahaffey: What’s the most convincing account of an interaction with aliens that you’ve heard?
Shostak: I’ve never heard one that convinced me. But more than 100 million people think I’m wrong, and that aliens are here, and the government is covering it up. You can’t disprove the claim, so people will continue believing it. I think people just like the idea. It’s kind of nifty to think you know something all these pointy-headed academics over at the university don’t.
Mahaffey: You have disproved the experts.
Shostak: Exactly. And you didn’t have to go through the trouble of getting all those advanced degrees. It’s empowering in a way.
Mahaffey: I want to talk a little about the Fermi paradox. As I understand it, it asks why, given the vast scale of the universe, we have no proof that there is sentient life beyond Earth.
Shostak: The Fermi paradox assumes that, if there are advanced species out there, we should see them. Maybe. But we could be looking in the wrong place. I walk around here and don’t see any elephants. That doesn’t mean there aren’t any. It’s just that I’m not in a place where I could see one. Aliens also might have an interest in not being seen. They don’t want to shout in a dark forest and let everybody know they’re around. They would all have to keep hidden, though. Otherwise we’d see something.
Another possibility is that we don’t currently know what to look for, but once we find one alien society, we’ll know how to find the rest. That’s often the case in science. People have believed for centuries that there are planets around other stars, but nobody found evidence of it until 1992. As soon as scientists knew how to find them, they started finding planets all over the place. Every week there would be another list of newly discovered planets. Once you know what you’re looking for, you can optimize your search.
Another possibility is that our instruments aren’t sensitive enough. If a planet fifty light-years away — which isn’t very far in astronomy — had big transmitters with more energy consumption than all the transmitters on Earth, we still wouldn’t see them unless they were aiming their signal at us, and it’s unlikely they would ever do that, because they don’t know we’re here. So it could be that we’re surrounded by intelligent life in the galaxy, the same way this building is surrounded by ants. I look out the window and don’t see any ants, but they’re out there.
Mahaffey: What do you think about the Great Filter hypothesis?
Shostak: My understanding of the Great Filter is that if a society develops enough technology, it will eventually wipe itself out, or the technology will wipe it out. So the fact that we don’t see aliens means, if you will, that some guy with a big scythe and black cape comes along and takes them out. I think this is kind of a contrived idea. If you talk to young people, they figure we’re all doomed. Climate change, another pandemic, a nuclear war — there are so many threats that they’re never going to have to worry about Social Security, because humanity won’t last that long. But I don’t agree with that at all. I actually made a sort of back-of-the-envelope calculation about how many people could be wiped out by a really intense and deadly virus; how many people could be wiped out by climate change; how many people could we wipe out with nuclear war. And, sure, particularly in the case of nuclear weapons, we could lose a few billion people, which is a lot, but it isn’t even half the world population. It’s sort of like how the Plague devastated parts of Europe, but it didn’t wipe out Homo sapiens. In fact, our population on Earth just kept going up.
Mahaffey: So life finds a way.
Shostak: Life has been around for nearly 4 billion years on this planet. There have been bad events. The asteroid that did in the dinosaurs wiped out three-quarters of all species. But it wasn’t enough to wipe out life, which is why we can sit here and talk about the dinosaurs.
Mahaffey: This concern seems to be what motivates people like Elon Musk to try to take human civilization beyond our planet and maybe colonize Mars. What do you think of that ambition? Is it a good way to spend our resources?
Shostak: Eventually I think that going beyond Earth is fundamental. If you look at statistics on, for example, how much easily minable copper we have left — or zinc, or a whole long list of other resources — we will start running low in another fifty years. We already don’t make solid-copper pennies anymore. [Laughs.] And we need copper for things like electronics. These resources are essential. Eventually we’re going to use up all we have here on Earth. Salvation lies in the fact that there’s plenty of copper and other minerals in rocks that are flying around the solar system. Mining an asteroid sounds kind of odd, but you might get a lot of precious metals from one, such that it will pay to mine them. If we don’t do that, then I think the human population is going to be strictly limited in the future, and presumably smaller than it is today.
Mahaffey: What can private citizens who don’t happen to be billionaires do to help with space exploration?
Shostak: Try to be generous toward basic research. The JWST is a very expensive telescope, and the average person on the street might say, “Look, that will cost me a hundred dollars in taxes over the years. I would rather have a hundred dollars to spend on tires for my car or food for my kids.” To them the telescope might be a very expensive way to make pretty pictures. They would rather spend money on applied research, such as a cure for cancer. But the cure for cancer is just as likely to come from basic research as it is from applied research. If we can understand cellular chemistry at a deep level, then maybe we can beat cancer. Basic research always pays off, but that may not be so clear to the voting public. I think that’s where the danger lies.
Mahaffey: How do we focus the public’s attention and imagination on scientific pursuits in general?
Shostak: I was in Europe last week and gave four talks trying to do just that. People seemed interested in the subject. They liked my jokes, in any case. But I think the biggest bang for the buck to get people interested in, for example, mining asteroids would be for Hollywood to make a movie about it. A movie reaches an audience of many millions and gets them emotionally involved, and that’s the best kind of involvement. You can write an editorial for the paper, and not many people will read it, and those who do will forget it the next day. But a movie can make a lasting impression.
As I mentioned, I occasionally consult for films and try to correct the science. But, honestly, the science doesn’t matter. What matters is that films give people the impression that a particular problem can be ameliorated.
Mahaffey: Why do you think people are so fascinated by science fiction?
Shostak: It gives you the chance to see something you’re never going to see, right? What will life on Earth be like a hundred years from now or a thousand years from now? It can cause you to think more about the whole question of aliens — although usually the kind that are interested in flattening Los Angeles.
Mahaffey: Stephen Hawking was famously cautious about any sort of alien contact.
Shostak: He made a comment that was essentially “Watch out what you do, because you don’t know whether any aliens out there will be hostile.” It’s just his opinion. But, you know, Stephen Hawking could have discussed his favorite place to get his clothes dry-cleaned, and people would have believed it was the best, because he continues to have a cachet that few others have.
Mahaffey: So you don’t think we should be concerned about making Earth a target?
Shostak: There are some who have that point of view. Many of the people in the SETI business over at the University of California, Berkeley, are very wary about the idea of broadcasting. But I think that’s a misplaced fear. Because, as I noted, our radar and television are being broadcast all the time. If you’re really worried about alerting aliens to our existence, then let’s shut down all television and all the radar around the world.
Mahaffey: Cat’s already out of the bag, basically.
Shostak: Yeah. The cat’s out of the bag, and the bag continues to leak cats.