Why haven’t we found aliens? A physicist shares the most popular theories. | Brian Cox

- Enrico Fermi is one of the great

physicist

s, legendary Italian

physicist

s, who laid many of the

found

ations of modern physics of the 20th century. In fact, the Fermi paradox is probably what he spent the least time on. It's al

most

a throwaway comment. The question is: "Where are they?" By "they" I mean

aliens

. We know that we live in a big old galaxy, in a big old Universe. The Milky Way we now know has something like 400 billion suns, and we now know that

most

of those suns have planetary systems around them. That's why there is a lot of real estate and there has been a lot of time for civilizations to develop.

The Fermi paradox in essence is the claim that: despite the fact that it has taken billions of years on billions of worlds for civilizations to arise, we see no evidence of any of them in the galaxy. So the paradox is: why? One possible answer to the Fermi paradox, the question of why there are no civilizations, is that Earth is quite unique in the Milky Way, in that the climate and conditions on Earth were stable enough over time. enough for life to disappear. From the cell to civilization. If you think about it, it's a great question. We know that we live in a violent Universe.

We know that there are supernova explosions everywhere. We know that there have been impacts on Earth, the famous impact that wiped out the large dinosaurs. There has been no impact large enough to break the unbroken chain of life for four billion years. So maybe, maybe it's the case that while there are billions of planets, that 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 has been stable enough for long enough to produce civilizations. Then that will be a property of the planet itself; the so-called 'rare earth hypothesis'.

Another explanation for the Fermi paradox could be that civilizations live and die. They rise and then fall. And because of the time scales 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, a legend, and he also grows orchids. And I came to his house by coincidence on the day that this rare orchid blooms, and it blooms I think for a day or two and then it disappears again during the year, and then it blooms again the next year, 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 bloom and die, bloom and die. And just because of the time scales involved, none of them overlap. And then there could be the remains, the ashes, the fossils of civilizations out there, but of course we would have no way of knowing until we explore the galaxy and perhaps find the ruins of these other civilizations. Who knows? Another possibility is that it is not a paradox. They are here. So there are intelligent civilizations out there and they are present in the solar system.

Let's think, for example, about what that intelligence would be like. Well, who knows? They could have sent nanomachines to our solar system. There could be probes everywhere, but if they are the size of an iPhone then we would have no way of detecting them. So it could be that the technology of a sufficiently advanced alien species, a civilization, is so far beyond anything we can understand or detect that we

haven

't seen it, and that is certainly, entirely possible. Another possibility is that the galaxy is very large. The distances between the stars are so great that if you imagine that there is another civilization, say on the other side of our galaxy, even if they had the most powerful radio transmitters you can imagine, then it may be that the distances are so great that the signals are so diluted that we cannot detect them because they are too weak.

Now, maybe you can build a spaceship that can jump a few light years away to the nearby solar system, but you can't build a spaceship that can fly across a galaxy. Now, many civilizations may exist, but advanced civilizations choose to remain hidden, sometimes called the 'dark forest hypothesis' or the 'quarantine hypothesis'. Let's imagine that civilizations, when they advance technologically, also advance intellectually and morally. And let's say they choose, maybe for good reason, let's say they choose to stay 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 believe that other advanced civilizations exist, you'll choose to remain silent.

You hide as best you can. Maybe that's a logical thing. I find it hard to believe, given human history, that this is the way intelligent civilizations behave. Certainly so far we have made no attempt to remain hidden. We transmit radio signals to the stars. We have launched into our space probes, such as Voyager, pulsar maps, in that case, showing the location of our solar system in case another civilization finds it. We have tried at every opportunity to spread our existence. Carl Sagan argued that a sufficiently advanced civilization, a civilization that can build interstellar spaceships and communicate across interstellar distances, may be wise enough to have overcome those primitive instincts: the instinct to cause trouble, fight wars, colonize, walk on other civilizations.

Perhaps it is inevitable that with technological advancement ultimately comes wisdom. Maybe it's like Star Trek. Maybe it's the "Prime Directive." - 'Animals that look like us. Do you still thank the Prime Directives for this planet? - I don't think we have the right or the wisdom to interfere, whatever the evolution of the planet.' -Maybe it is morally safe that if you are advanced enough, you decide to take the position that you will never introduce yourself to or interfere with another civilization, but that is a hypothesis. There is an idea in this field, when trying to explain the Fermi Paradox, called the 'Great Filter'.

So let's think about what it would mean if a great filter existed in our future. That would mean that civilizations arise in the Milky Way and reach somewhere like the position we are in now; So they develop rockets, they develop nuclear energy, nuclear weapons, for example. They industrialize. But then there is a filter in the future that prevents them from becoming true space civilizations, thus preventing them from becoming multiplanetary species, and ultimately preventing them from traveling between solar systems to begin colonizing a galaxy. So why might that be? Why might there be a filter waiting for us in the not-too-distant future that will prevent us from becoming an interplanetary species and ultimately traveling beyond our solar system?

I don't think it's technology. As far as I can see, I see nothing in the laws of nature, in principle, that would prevent us from becoming an interstellar species. It could be that our knowledge, our scientific skill exceeds our wisdom, exceeds our political skill. It could be that once a civilization develops the means to destroy itself in the form of, say, nuclear weapons or biological weapons, or perhaps some kind of lack of AI control, who knows? If you look back in our recent history, there have been several occasions when we know we came very close to destroying ourselves, or at least returning ourselves to the Stone Age.

For example, where there could have been nuclear launches and there were not. I'm sure there are many others we don't know about. There is the challenge of climate change. We are completely unable to come together right now as a global civilization to address that challenge that could set our civilization back. So it could almost be a law of nature. I think my favorites are the other ones, so I'll make the other big filter. If I had to guess why we don't see evidence of other civilizations, the so-called "great silence" as astronomers call it, it's because there aren't any and there never have been any.

The reason I assume that, and I emphasize that it is an assumption, is biology. So if we look at the history of life on Earth, we see that life began 3.8 billion years ago, let's say. But then we see for almost three billion years on this planet that there is nothing more complex than a single cell. And there could be good biological reasons for this: one that comes to mind is the evolution of what is called the "eukaryotic cell," which makes up all multicellular living things on the planet. Those cells, which appear to be a prerequisite for complex multicellular life, once evolved on this planet, as far as we know.

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 that sets the stage for us. If it normally takes four billion years from a cell to a civilization, then I think there may be very few planets in a typical galaxy that are stable enough for long enough for that process to take place. I believe there is one civilization in the Milky Way, and there has only been one, and there may ever be only one, and that is us. Which, by the way, means we have a tremendous responsibility not to screw this up.

That means Earth is the only meaningful island in a sea of ​​400 billion suns. And if we destroy this, we could destroy the meaning of a galaxy forever. However, that is a hypothesis. I'll be delighted if that turns out not to be true. And that's not just because it takes some responsibility off of me, you, and everyone else, but every scientist should be delighted if he's proven wrong. Because the moment you are proven wrong, it means you have learned something, and that is how knowledge progresses. So no one should worry about making a guess, proposing a hypothesis, an educated guess.

And if I'm wrong, by the way, it 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 had learned something about the Universe, which is that complex civilizations are not as rare as I think, or civilizations are not as rare, so it would be a good idea. stuff. So there is a lesson. - Do you want to go deeper? Become a Big Think member and join our members-only community. Watch videos in advance and unlock full interviews.