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Become a member Login The Big Think Interview Why alien civilizations may bloom and die unseen Brian Cox examines why, despite billions of stars and trillions of planets, we have found no evidence of other intelligent life.
Why alien civilizations may bloom and die unseen
Brian Cox
Brian Cox obtained a first class honors degree in physics from the University of Manchester in 1995 and in 1998 a Ph.D. in High Energy Particle Physics at the DESY[…]
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Why alien civilizations may bloom and die unseen
Brian Cox
Brian Cox obtained a first class honors degree in physics from the University of Manchester in 1995 and in 1998 a Ph.D. in High Energy Particle Physics at the DESY[…]
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What if humanity is the galaxy’s only advanced civilization?
Brian Cox examines why, despite billions of stars and trillions of planets, we have found no evidence of other intelligent life.
BRIAN COX: Enrico Fermi is one of the great physicists, legendary Italian physicists who laid many of the foundations of modern 20th century physics, so-called Fermi-Dirac statistics and all sorts of things. If you do a degree in physics, then you will spend a lot of time revising equations and theories that Fermi did. One of the great legends. The Fermi paradox is probably the thing he spent least time on actually, is almost one throwaway remark that he delivered. And the question is, where are they? By they, I mean aliens.
- [Announcer] The Great Filter, Why Civilizations May Not Survive.
- The heart of the Fermi paradox is this. We know that we live in a big old galaxy, in a big old universe. And let's, for the purposes of this discussion, confine ourselves to the Milky Way galaxy. The Milky Way galaxy we now know has something like 400 billion suns. And we now know that most of those suns have planetary systems around them, so trillions of planets. The galaxy's been around for pretty much the age of the universe, 10 billion years plus. And so there's a lot of real estate, and there's been plenty of time for civilizations to develop in the galaxy. The Fermi paradox at its heart is the statement that, not withstanding the fact that there have been billions of years on billions of worlds for civilizations to arise, we see no evidence of any of them in the galaxy at all. So the paradox is why it's a paradox. And I think it's a very good question. It's an extremely good question, and there can be many answers. And the great fun or the great, I would say, the intellectual value of the Fermi paradox. So if we accept that we haven't seen any, so let's accept that. Let's accept that, you know, there isn't a UFO sat in some warehouse in Roswell or something like that. Let's accept that we haven't seen any. Let's accept, well, we have to accept the picture that I've just given you about the Milky Way galaxy and its age 'cause that's a measurement, so we know that. The question is then why, how do we resolve these apparent contradictions and paradoxes? So one answer to the Fermi paradox, this is the idea that we don't seem to see anyone, is that no one ever evolved, right? So life didn't get complex. Why might we think that? Well, what did we need to produce our civilization on this planet? Well, on this planet, one thing we needed was time. We have good evidence that life was present on this planet 3.8 billion years ago, perhaps even earlier. The planet is four and a half billion years old. So we know, as a matter of fact, as an observation on this planet, that life was present 3.8 billion years ago. But it took 3.8 billion years, give or take a few tens of thousands of years, to go from the origin of life to a civilization, let's say, from cell to civilization, 4 billion years. That's one third of the age of the universe. So a possible answer to the Fermi paradox, the question of why there are no civilizations, is because the Earth is pretty much unique in the Milky Way galaxy, in that it was stable enough, the climate, the conditions on Earth were stable enough for long enough for life to go from cell to civilization. If you think about it, that's a big ask. What I'm saying is on this planet, an unbroken chain of life existed for almost 4 billion years, not withstanding the fact that we live in a violent universe. The Sun must have been stable enough for long enough. We know the output of the Sun has changed over those billions of years, but it's not changed so radically that it managed to erase, destroy life on Earth. We know that we live in a violent universe. We know there are supernova explosions all over the place. It turns out that there have been no stars massive enough to explode as a supernova in a way that would damage life on Earth or erase life on Earth in this vicinity for 4 billion years. We know that nothing has happened, like we know that there there've been impacts on the Earth, right? We know that the famous impact that wipes out the large dinosaurs, there's been no impact big enough to destroy the unbroken chain or break the unbroken chain of life for 4 billion years. So maybe, maybe it's the case that whilst there are billions of planets which may have liquid water on the surface, may have oceans that can support life, it may be that none of those planets in the Milky Way galaxy have been stable enough for long enough to produce a civilization. So that would be a property of the planet itself, the so-called rare Earth hypothesis. Actually I should say it's a property of a solar system. It's not really just a property of the planet, it's a property of the parent star. You could ask the question, well, if it was a binary system, for example, a binary star system, many stars are binary star systems, is it possible to have a planet with a stable climate, stable enough orbit in a binary star system to support an unbroken chain of life for 4 billion years? Perhaps not. When we talk about Earth, I think I would like to talk about rare solar system, another possibility with the Fermi paradox is that it's not a paradox, they are here. So there are intelligent civilizations out there, and they are present in the solar system. It's possible. Let's think, for example, what such an intelligence might look like. Well, who knows? They could have sent nano machines to our solar system. There could be probes all over the place in the solar system, but if they're the size of an iPhone, then we'd have no way of detecting them. So it could be the technology of a sufficiently advanced alien species, a civilization, is so beyond anything we can comprehend or detect that we haven't seen it, and we've been fooled into thinking that there are no advanced civilizations in the galaxy. And that's certainly entirely possible. Another possibility, another possible resolution to the Fermi paradox is just that the galaxy is so big, the distances between stars are so great that if you imagine there's another civilization, let's say on the other side of our galaxy, let's say 20 million, 30 million, 40 million light years away, even if they had the most powerful radio transmitters you could imagine, or even if they'd spread out to neighboring solar systems, then it may just be that the distances are so great that the signals are diluted, that we can't detect them because they're too weak. Or that it is just very, very, very difficult in an engineering sense to build interstellar spacecraft. Now, perhaps you can build a spacecraft that can hop a few light years away to the nearby solar system, four light years, in our case to Alpha Centauri. But you can't build spacecraft that can traverse a galaxy. That's a possibility. One of the arguments against that for me is the argument, it's called the space travel argument against the existence of extraterrestrial life. And it's often framed in terms of self-replicating machines, so-called Von Neumann machines. So imagine it's possible for us, for a civilization, to build a machine, some kind of AI that's sufficiently smart and capable, that it can fly to a nearby solar system, reproduce itself, copy itself, and then send the copy out to the next solar system and so on. So you have, if you build a, one, successful replicator, you have an exponentiation of replicators. You have one, and then two, and then four, and then eight and so on, exponential growth. And you can show that, even given our rocketry technology, you can cover a galaxy like the Milky Way in a reasonably short space of time. By reasonably short, I might even mean a hundred million years, right? That's reasonably short on galactic timescales. But the key point is once a single successful replicator has been launched, then it is inevitable that over a few tens of millions of years, the galaxy will be covered with replicators, and we don't see any evidence of them. It's possible that we can infer, that if we assume that we could detect them, then the absence of them may allow us to infer that no civilization has ever got to that point. And I think that's quite a persuasive argument. Now, it's possible that there are many civilizations out there, but the advanced civilizations choose to remain hidden, sometimes called the dark forest hypothesis, the quarantine hypothesis. We've been asked to make moral judgments or judgments on how a civilization will behave, what they will choose to do. And that's, of course, impossible to judge. But let's imagine civilizations, when they get technologically advanced also get intellectually and morally advanced. And let's say that they choose, perhaps for good reason, let's say they choose to remain hidden because they don't want to draw attention to themselves. Let's say it's inevitable that if you think about it carefully and you think there are other advanced civilizations out there, then you choose to remain silent. You hide yourself as best you can. It's possible that that's the way that a civilization would think. Maybe that's a logical thing to do. I find it difficult to believe, given human history, that that's the way that intelligence civilizations behave. We certainly haven't made any attempt to remain hidden so far. We've broadcast radio signals out to the stars, the Arecibo message, for example, albeit weak ones. We've launched on our space probes, like Voyager, maps, pulsar maps in that case, which shows the location of our solar system should any other civilization find it. So at least at the moment, we haven't come to the view, which may be a wise view, but we haven't got there that we should remain silent. Quite the opposite, we've tried at every opportunity to broadcast our existence. Maybe that's 'cause Carl Sagan argued, I think, that a sufficiently advanced civilization, a civilization that can build interstellar spacecraft and communicates across interstellar distances, perhaps is wise enough to have overcome those primitive instincts, the instinct to cause trouble, to fight wars, to colonize, to walk over other civilizations. Perhaps it's inevitable that with technological advance ultimately comes wisdom, but it's hypothesis. Maybe it is. Maybe it's just anybody sufficiently clever to build an interstellar spaceship will be also sufficiently clever to hide it and not draw attention to themselves. Maybe it's a moral, maybe it's like "Star Trek." Maybe it's the Prime Directive. Maybe it's morally certain that if you're sufficiently advanced, you decide to take the position that you will never introduce yourself or interfere with another civilization. Maybe that becomes a kind of law of nature for sufficiently intelligent beings. Maybe that's conceivable as well. Another explanation for the Fermi paradox might be that civilizations live and die. They rise and then they fall. And because of the sheer timescales involved and the sheer size of the galaxy, no two civilizations ever overlap. I once had the great pleasure of meeting Frank Drake, the Drake equation legend, in his house. And he also grows orchids. And I arrived at his house just coincidentally on the day that this rare orchid flowers, and it flowers for I think one or two days. And then goes away again for the year. And then flowers again the next year for one or two days. And he used it as an analogy. He said, "Well, maybe civilizations are like that." So maybe civilizations are like rare orchids. And so they flower and die and flower and die. And just because of the sheer timescales involved, none of them ever overlap. And so there could be the wreckage, the ashes, the fossils of civilizations out there. But of course, we'd have no way of knowing until we explore the galaxy and maybe find the ruins of these other civilizations. Who knows? I mean, it's quite plausible if you think about it. Are we gonna exist in 10,000 years time? It's, to a large extent, in our hands. Maybe we're sufficiently stupid that we won't exist beyond the next century. There's an idea in this field. I'm trying to explain the Fermi paradox called the Great Filter. Now, the Great Filter can lie in our future or our past. So let's think about what it would mean for a Great Filter to lie in our future. That would mean that civilizations do arise in the Milky Way galaxy and get to somewhere like the position that we are at now. So they develop rocketry, they develop nuclear power, nuclear weapons, for example, they industrialize. But then there's a filter in the future that stops them becoming true space-fairing civilizations. So stops them becoming multi-planetary species and stops them ultimately traveling between solar systems to begin to colonize a galaxy. So why might that be? Why might there be a filter waiting for us in our not-too-distant future that's gonna stop us going onto Mars and stop us escaping our solar system? What might stop us from becoming an interplanetary species and ultimately traveling out beyond our solar system? I don't think it's technology, as far as I can see, I don't see anything in the laws of nature in principle that would stop us from becoming an interstellar species. Might be 1,000 years in the future, 10,000 years in the future, might be 100,000 years in the future. Even that, right, 100,000 years, it's a blink of an eye in the lifetime of the universe, in the lifetime of a galaxy. So I don't see any reason in principle why we couldn't become an interplanetary, interstellar species other than potentially our own stupidity. And I think that probably, it could be one of the reasons why we don't see any other civilizations around. It could be that our knowledge, our scientific prowess exceeds our wisdom, exceeds our political skill. It could be that once a civilization develops the means to destroy itself in the form, for example, of nuclear weapons or biological weapons or maybe some kind of a lack of control of AI, who knows? It may be that once a civilization acquires that technical know-how, then it goes ahead and destroys itself essentially inexorably because it's just too difficult politically to run a civilization that has the power to destroy itself. If you look back through our recent history, there've been several occasions that we know about, that I know about and you know about, where we came very close to destroying ourselves, or at least setting us back to the Stone Age, basically. The Cuban Missile Crisis, well-documented events in the 1980s, for example, where there could have been nuclear launches and weren't, and I'm sure there are many others that we don't know about. There's the challenge of climate change. We're completely incapable of coming together at the moment as a global civilization to address that challenge. That could set our civilization back. Biological weapons, the threat of AI, we seem to be completely incapable of regulating those threats. So it might just be almost a law of nature. Things like us, things that can build an industrial civilization are just inherently too stupid to get out there to the stars. And I wouldn't put that past us. My favorite's the other one. So I'll do the other Great Filter. If I was to guess, and this is a guess, right? If I was to guess why we see no evidence of other civilizations out there, the so-called Great Silence is what astronomers call it, is because there aren't any, and there never have beat any. That's my guess. The reason I guess that, and I emphasize it's a guess, is biology. So if you look at the history of life on Earth, then we see that life began 3.8 billion years ago, let's say. But then we see for the best part of 3 billion years on this planet that there's nothing more complex than a single cell, 3 billion years. It's suddenly in the last billion years or so, perhaps a little bit less, that multicellular life has existed on this planet. And there could be good biological reasons for that. One that springs to mind is the evolution of what's called the eukaryotic cell, which is the cell with the cell nucleus and all little organelles and chloroplasts in plants and all those things which form all multicellular living things on the planet. Those cells, which seem to be prerequisite for complex multicellular life, evolved once on this planet as far as we can tell. It's pretty widely accepted. It's called the Fateful Encounter hypothesis. And so it seems that there is a very unusual evolutionary event at some point, maybe a billion, a billion and a half, even 2 billion years ago, that laid the foundations for us. If that's typical, if it typically is the case that it takes 4 billion years from cell to civilization, then I think there may be very few planets in a typical galaxy which are stable enough for long enough for that process to proceed. And we could be, for all we know, on the fortunate end of evolutionary timescales, we don't know. Let's imagine that actually we were on the lucky side and really on a typical planet, if there is such a thing, then it takes three or four times as long. That would exceed the current age of the universe. My guess is that whilst I think there might be microbes all over the place, I wouldn't be surprised, I'd be delighted, but I wouldn't be surprised if we found evidence of microbes on Mars, Europa, Enceladus, even in the subsurface oceans of places potentially even as far out as Pluto, right? Subsurface lakes, if they exist, liquid water below the surface, who knows? I wouldn't be surprised if we find microbes all over the place. But a galaxy full of complex living things, other planets with not only complex life, but sentient life, things as smart as us, things smart enough to build rockets that head out to the stars, my guess is that a typical galaxy may have less than, on average, less than one civilization per galaxy, let's put it that way. But actually, just to say it is a very famous book as strongly recommend, Barrow and Tipler called "The Anthropic Cosmological Principle." It's a great book, one of the books I grew up with. And in that book, Barrow and Tipler say that in their view, there might be one civilization in the observable universe, which should be us, right? So who knows whether we should go that far, but I think civilizations are very rare. And I think it's the biology. So I'm guessing, I'm giving you a guess, I'm saying that in my opinion, I think there's one civilization in the Milky Way galaxy, and there only has ever been one, and there might only ever be one. And that's us, which by the way means that we have a tremendous responsibility not to mess this up, because as I said to the, I was asked to give a video introduction to the climate summit, the COP26 climate summit in Glasgow recently. "Give the world leaders one minute," they said. "You've got a video, say something to them." And I said, "I think, let's assume that we're the only civilization currently in the Milky Way galaxy, perhaps the only civilization there will ever be. That means the Earth is the only island of meaning in a sea of 400 billion suns. And so if we destroy this, we might destroy meaning in a galaxy forever. Discuss." So that's my guess. However, that's a hypothesis. I will be delighted if it turns out that's not true. And that's not just because it removes some responsibility from me and you and everybody else. To preserve intelligence in a galaxy of 400 billion suns, the weight of responsibility is heavy. But also every scientist should be delighted if they are shown to be wrong. Because the moment you are shown to be wrong, it means you've learn something. And that's the way that knowledge progresses. So nobody should be worried about making a guess, advancing a hypothesis, an educated guess, or even an uneducated guess. Don't worry about doing that as long as the moment it turns out you're wrong, and me being wrong, by the way, would constitute a flying saucer landing and some aliens coming out like ET and saying hello. So that would be brilliant. But it would be doubly brilliant because it would turn out that I'd learned something about the universe, which is that complex civilizations are not as rare as I think they are, or civilizations aren't the rare. So that would be a good thing. So there's a lesson.
Overview TranscriptWhat if humanity is the galaxy’s only advanced civilization?
Brian Cox examines why, despite billions of stars and trillions of planets, we have found no evidence of other intelligent life.
BRIAN COX: Enrico Fermi is one of the great physicists, legendary Italian physicists who laid many of the foundations of modern 20th century physics, so-called Fermi-Dirac statistics and all sorts of things. If you do a degree in physics, then you will spend a lot of time revising equations and theories that Fermi did. One of the great legends. The Fermi paradox is probably the thing he spent least time on actually, is almost one throwaway remark that he delivered. And the question is, where are they? By they, I mean aliens.
- [Announcer] The Great Filter, Why Civilizations May Not Survive.
- The heart of the Fermi paradox is this. We know that we live in a big old galaxy, in a big old universe. And let's, for the purposes of this discussion, confine ourselves to the Milky Way galaxy. The Milky Way galaxy we now know has something like 400 billion suns. And we now know that most of those suns have planetary systems around them, so trillions of planets. The galaxy's been around for pretty much the age of the universe, 10 billion years plus. And so there's a lot of real estate, and there's been plenty of time for civilizations to develop in the galaxy. The Fermi paradox at its heart is the statement that, not withstanding the fact that there have been billions of years on billions of worlds for civilizations to arise, we see no evidence of any of them in the galaxy at all. So the paradox is why it's a paradox. And I think it's a very good question. It's an extremely good question, and there can be many answers. And the great fun or the great, I would say, the intellectual value of the Fermi paradox. So if we accept that we haven't seen any, so let's accept that. Let's accept that, you know, there isn't a UFO sat in some warehouse in Roswell or something like that. Let's accept that we haven't seen any. Let's accept, well, we have to accept the picture that I've just given you about the Milky Way galaxy and its age 'cause that's a measurement, so we know that. The question is then why, how do we resolve these apparent contradictions and paradoxes? So one answer to the Fermi paradox, this is the idea that we don't seem to see anyone, is that no one ever evolved, right? So life didn't get complex. Why might we think that? Well, what did we need to produce our civilization on this planet? Well, on this planet, one thing we needed was time. We have good evidence that life was present on this planet 3.8 billion years ago, perhaps even earlier. The planet is four and a half billion years old. So we know, as a matter of fact, as an observation on this planet, that life was present 3.8 billion years ago. But it took 3.8 billion years, give or take a few tens of thousands of years, to go from the origin of life to a civilization, let's say, from cell to civilization, 4 billion years. That's one third of the age of the universe. So a possible answer to the Fermi paradox, the question of why there are no civilizations, is because the Earth is pretty much unique in the Milky Way galaxy, in that it was stable enough, the climate, the conditions on Earth were stable enough for long enough for life to go from cell to civilization. If you think about it, that's a big ask. What I'm saying is on this planet, an unbroken chain of life existed for almost 4 billion years, not withstanding the fact that we live in a violent universe. The Sun must have been stable enough for long enough. We know the output of the Sun has changed over those billions of years, but it's not changed so radically that it managed to erase, destroy life on Earth. We know that we live in a violent universe. We know there are supernova explosions all over the place. It turns out that there have been no stars massive enough to explode as a supernova in a way that would damage life on Earth or erase life on Earth in this vicinity for 4 billion years. We know that nothing has happened, like we know that there there've been impacts on the Earth, right? We know that the famous impact that wipes out the large dinosaurs, there's been no impact big enough to destroy the unbroken chain or break the unbroken chain of life for 4 billion years. So maybe, maybe it's the case that whilst there are billions of planets which may have liquid water on the surface, may have oceans that can support life, it may be that none of those planets in the Milky Way galaxy have been stable enough for long enough to produce a civilization. So that would be a property of the planet itself, the so-called rare Earth hypothesis. Actually I should say it's a property of a solar system. It's not really just a property of the planet, it's a property of the parent star. You could ask the question, well, if it was a binary system, for example, a binary star system, many stars are binary star systems, is it possible to have a planet with a stable climate, stable enough orbit in a binary star system to support an unbroken chain of life for 4 billion years? Perhaps not. When we talk about Earth, I think I would like to talk about rare solar system, another possibility with the Fermi paradox is that it's not a paradox, they are here. So there are intelligent civilizations out there, and they are present in the solar system. It's possible. Let's think, for example, what such an intelligence might look like. Well, who knows? They could have sent nano machines to our solar system. There could be probes all over the place in the solar system, but if they're the size of an iPhone, then we'd have no way of detecting them. So it could be the technology of a sufficiently advanced alien species, a civilization, is so beyond anything we can comprehend or detect that we haven't seen it, and we've been fooled into thinking that there are no advanced civilizations in the galaxy. And that's certainly entirely possible. Another possibility, another possible resolution to the Fermi paradox is just that the galaxy is so big, the distances between stars are so great that if you imagine there's another civilization, let's say on the other side of our galaxy, let's say 20 million, 30 million, 40 million light years away, even if they had the most powerful radio transmitters you could imagine, or even if they'd spread out to neighboring solar systems, then it may just be that the distances are so great that the signals are diluted, that we can't detect them because they're too weak. Or that it is just very, very, very difficult in an engineering sense to build interstellar spacecraft. Now, perhaps you can build a spacecraft that can hop a few light years away to the nearby solar system, four light years, in our case to Alpha Centauri. But you can't build spacecraft that can traverse a galaxy. That's a possibility. One of the arguments against that for me is the argument, it's called the space travel argument against the existence of extraterrestrial life. And it's often framed in terms of self-replicating machines, so-called Von Neumann machines. So imagine it's possible for us, for a civilization, to build a machine, some kind of AI that's sufficiently smart and capable, that it can fly to a nearby solar system, reproduce itself, copy itself, and then send the copy out to the next solar system and so on. So you have, if you build a, one, successful replicator, you have an exponentiation of replicators. You have one, and then two, and then four, and then eight and so on, exponential growth. And you can show that, even given our rocketry technology, you can cover a galaxy like the Milky Way in a reasonably short space of time. By reasonably short, I might even mean a hundred million years, right? That's reasonably short on galactic timescales. But the key point is once a single successful replicator has been launched, then it is inevitable that over a few tens of millions of years, the galaxy will be covered with replicators, and we don't see any evidence of them. It's possible that we can infer, that if we assume that we could detect them, then the absence of them may allow us to infer that no civilization has ever got to that point. And I think that's quite a persuasive argument. Now, it's possible that there are many civilizations out there, but the advanced civilizations choose to remain hidden, sometimes called the dark forest hypothesis, the quarantine hypothesis. We've been asked to make moral judgments or judgments on how a civilization will behave, what they will choose to do. And that's, of course, impossible to judge. But let's imagine civilizations, when they get technologically advanced also get intellectually and morally advanced. And let's say that they choose, perhaps for good reason, let's say they choose to remain hidden because they don't want to draw attention to themselves. Let's say it's inevitable that if you think about it carefully and you think there are other advanced civilizations out there, then you choose to remain silent. You hide yourself as best you can. It's possible that that's the way that a civilization would think. Maybe that's a logical thing to do. I find it difficult to believe, given human history, that that's the way that intelligence civilizations behave. We certainly haven't made any attempt to remain hidden so far. We've broadcast radio signals out to the stars, the Arecibo message, for example, albeit weak ones. We've launched on our space probes, like Voyager, maps, pulsar maps in that case, which shows the location of our solar system should any other civilization find it. So at least at the moment, we haven't come to the view, which may be a wise view, but we haven't got there that we should remain silent. Quite the opposite, we've tried at every opportunity to broadcast our existence. Maybe that's 'cause Carl Sagan argued, I think, that a sufficiently advanced civilization, a civilization that can build interstellar spacecraft and communicates across interstellar distances, perhaps is wise enough to have overcome those primitive instincts, the instinct to cause trouble, to fight wars, to colonize, to walk over other civilizations. Perhaps it's inevitable that with technological advance ultimately comes wisdom, but it's hypothesis. Maybe it is. Maybe it's just anybody sufficiently clever to build an interstellar spaceship will be also sufficiently clever to hide it and not draw attention to themselves. Maybe it's a moral, maybe it's like "Star Trek." Maybe it's the Prime Directive. Maybe it's morally certain that if you're sufficiently advanced, you decide to take the position that you will never introduce yourself or interfere with another civilization. Maybe that becomes a kind of law of nature for sufficiently intelligent beings. Maybe that's conceivable as well. Another explanation for the Fermi paradox might be that civilizations live and die. They rise and then they fall. And because of the sheer timescales involved and the sheer size of the galaxy, no two civilizations ever overlap. I once had the great pleasure of meeting Frank Drake, the Drake equation legend, in his house. And he also grows orchids. And I arrived at his house just coincidentally on the day that this rare orchid flowers, and it flowers for I think one or two days. And then goes away again for the year. And then flowers again the next year for one or two days. And he used it as an analogy. He said, "Well, maybe civilizations are like that." So maybe civilizations are like rare orchids. And so they flower and die and flower and die. And just because of the sheer timescales involved, none of them ever overlap. And so there could be the wreckage, the ashes, the fossils of civilizations out there. But of course, we'd have no way of knowing until we explore the galaxy and maybe find the ruins of these other civilizations. Who knows? I mean, it's quite plausible if you think about it. Are we gonna exist in 10,000 years time? It's, to a large extent, in our hands. Maybe we're sufficiently stupid that we won't exist beyond the next century. There's an idea in this field. I'm trying to explain the Fermi paradox called the Great Filter. Now, the Great Filter can lie in our future or our past. So let's think about what it would mean for a Great Filter to lie in our future. That would mean that civilizations do arise in the Milky Way galaxy and get to somewhere like the position that we are at now. So they develop rocketry, they develop nuclear power, nuclear weapons, for example, they industrialize. But then there's a filter in the future that stops them becoming true space-fairing civilizations. So stops them becoming multi-planetary species and stops them ultimately traveling between solar systems to begin to colonize a galaxy. So why might that be? Why might there be a filter waiting for us in our not-too-distant future that's gonna stop us going onto Mars and stop us escaping our solar system? What might stop us from becoming an interplanetary species and ultimately traveling out beyond our solar system? I don't think it's technology, as far as I can see, I don't see anything in the laws of nature in principle that would stop us from becoming an interstellar species. Might be 1,000 years in the future, 10,000 years in the future, might be 100,000 years in the future. Even that, right, 100,000 years, it's a blink of an eye in the lifetime of the universe, in the lifetime of a galaxy. So I don't see any reason in principle why we couldn't become an interplanetary, interstellar species other than potentially our own stupidity. And I think that probably, it could be one of the reasons why we don't see any other civilizations around. It could be that our knowledge, our scientific prowess exceeds our wisdom, exceeds our political skill. It could be that once a civilization develops the means to destroy itself in the form, for example, of nuclear weapons or biological weapons or maybe some kind of a lack of control of AI, who knows? It may be that once a civilization acquires that technical know-how, then it goes ahead and destroys itself essentially inexorably because it's just too difficult politically to run a civilization that has the power to destroy itself. If you look back through our recent history, there've been several occasions that we know about, that I know about and you know about, where we came very close to destroying ourselves, or at least setting us back to the Stone Age, basically. The Cuban Missile Crisis, well-documented events in the 1980s, for example, where there could have been nuclear launches and weren't, and I'm sure there are many others that we don't know about. There's the challenge of climate change. We're completely incapable of coming together at the moment as a global civilization to address that challenge. That could set our civilization back. Biological weapons, the threat of AI, we seem to be completely incapable of regulating those threats. So it might just be almost a law of nature. Things like us, things that can build an industrial civilization are just inherently too stupid to get out there to the stars. And I wouldn't put that past us. My favorite's the other one. So I'll do the other Great Filter. If I was to guess, and this is a guess, right? If I was to guess why we see no evidence of other civilizations out there, the so-called Great Silence is what astronomers call it, is because there aren't any, and there never have beat any. That's my guess. The reason I guess that, and I emphasize it's a guess, is biology. So if you look at the history of life on Earth, then we see that life began 3.8 billion years ago, let's say. But then we see for the best part of 3 billion years on this planet that there's nothing more complex than a single cell, 3 billion years. It's suddenly in the last billion years or so, perhaps a little bit less, that multicellular life has existed on this planet. And there could be good biological reasons for that. One that springs to mind is the evolution of what's called the eukaryotic cell, which is the cell with the cell nucleus and all little organelles and chloroplasts in plants and all those things which form all multicellular living things on the planet. Those cells, which seem to be prerequisite for complex multicellular life, evolved once on this planet as far as we can tell. It's pretty widely accepted. It's called the Fateful Encounter hypothesis. And so it seems that there is a very unusual evolutionary event at some point, maybe a billion, a billion and a half, even 2 billion years ago, that laid the foundations for us. If that's typical, if it typically is the case that it takes 4 billion years from cell to civilization, then I think there may be very few planets in a typical galaxy which are stable enough for long enough for that process to proceed. And we could be, for all we know, on the fortunate end of evolutionary timescales, we don't know. Let's imagine that actually we were on the lucky side and really on a typical planet, if there is such a thing, then it takes three or four times as long. That would exceed the current age of the universe. My guess is that whilst I think there might be microbes all over the place, I wouldn't be surprised, I'd be delighted, but I wouldn't be surprised if we found evidence of microbes on Mars, Europa, Enceladus, even in the subsurface oceans of places potentially even as far out as Pluto, right? Subsurface lakes, if they exist, liquid water below the surface, who knows? I wouldn't be surprised if we find microbes all over the place. But a galaxy full of complex living things, other planets with not only complex life, but sentient life, things as smart as us, things smart enough to build rockets that head out to the stars, my guess is that a typical galaxy may have less than, on average, less than one civilization per galaxy, let's put it that way. But actually, just to say it is a very famous book as strongly recommend, Barrow and Tipler called "The Anthropic Cosmological Principle." It's a great book, one of the books I grew up with. And in that book, Barrow and Tipler say that in their view, there might be one civilization in the observable universe, which should be us, right? So who knows whether we should go that far, but I think civilizations are very rare. And I think it's the biology. So I'm guessing, I'm giving you a guess, I'm saying that in my opinion, I think there's one civilization in the Milky Way galaxy, and there only has ever been one, and there might only ever be one. And that's us, which by the way means that we have a tremendous responsibility not to mess this up, because as I said to the, I was asked to give a video introduction to the climate summit, the COP26 climate summit in Glasgow recently. "Give the world leaders one minute," they said. "You've got a video, say something to them." And I said, "I think, let's assume that we're the only civilization currently in the Milky Way galaxy, perhaps the only civilization there will ever be. That means the Earth is the only island of meaning in a sea of 400 billion suns. And so if we destroy this, we might destroy meaning in a galaxy forever. Discuss." So that's my guess. However, that's a hypothesis. I will be delighted if it turns out that's not true. And that's not just because it removes some responsibility from me and you and everybody else. To preserve intelligence in a galaxy of 400 billion suns, the weight of responsibility is heavy. But also every scientist should be delighted if they are shown to be wrong. Because the moment you are shown to be wrong, it means you've learn something. And that's the way that knowledge progresses. So nobody should be worried about making a guess, advancing a hypothesis, an educated guess, or even an uneducated guess. Don't worry about doing that as long as the moment it turns out you're wrong, and me being wrong, by the way, would constitute a flying saucer landing and some aliens coming out like ET and saying hello. So that would be brilliant. But it would be doubly brilliant because it would turn out that I'd learned something about the universe, which is that complex civilizations are not as rare as I think they are, or civilizations aren't the rare. So that would be a good thing. So there's a lesson.
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