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

• Enrico Fermi is one of the great physicists, legendary Italian physicists who laid many of the foundations of modern 20th-century physics. The Fermi Paradox is probably the thing he spent least time on, actually. It's almost one throwaway remark. 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 galaxy we now know has something like 400 billion suns, and we now know that most of those suns have planetary systems around them.

And so there's a lot of real estate, and there's been plenty of time for civilizations to develop. The Fermi Paradox at its heart is the statement that: notwithstanding 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?

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 the climate, the conditions on Earth were stable enough for long enough for life to go from cell to civilization. If you think about that, that's a big ask. We know that we live in a violent Universe. We know there are supernova explosions all over the place. We know that there have been impacts on the Earth, the famous impact that wipes out the large dinosaurs.

There's been no impact big enough to break the unbroken chain of life for four 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 civilizations. So that will be a property of the planet itself; the so-called 'Rare Earth hypothesis.'

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, a legend, 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 - 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?

Another possibility 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. 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, but if they're the size of an iPhone then we'd have no way of detecting them.

So it could be that 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 that's certainly, entirely possible. Another possibility 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, even if they had the most powerful radio transmitters you could imagine, 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.

Now, perhaps you can build a spacecraft that can hop a few light years away to the nearby solar system, but you can't build a spacecraft that can traverse a galaxy. 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.'

Let's imagine that civilizations, when they get technologically advanced also get intellectually, 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. Maybe that's a logical thing to do.

I find it difficult to believe, given human history, that that's the way that intelligent civilizations behave. We certainly haven't made any attempt to remain hidden so far. We broadcast radio signals out to the stars. We've launched on our space probes, like Voyager, pulsar maps, in that case, which shows the location of our solar system should any other civilization find it.

We've tried at every opportunity to broadcast our existence. Carl Sagan argued that a sufficiently advanced civilization, a civilization that can build interstellar spacecraft and communicate 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. Maybe it's like Star Trek. Maybe it's the "Prime Directive." - "Animals who happen to look like us. Still thank the Prime directives for this planet? - I don't think we have a right or the wisdom to interfere, however a planet is evolving."

• 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 - but it's a hypothesis. There's an idea in this field, in trying to explain the Fermi Paradox, called the 'Great Filter.'

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 spacefaring 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 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.

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? If you look back through our recent history, there have been several occasions that we know about where we came very close to destroying ourselves, or at least setting us back to the Stone Age.

For example, where there could have been nuclear launches and weren't. 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.

So it might just be almost a law of nature. I think my favorite's the other ones, so I'll do the other great filter. 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 been any.

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 three billion years on this planet that there's nothing more complex than a single cell. 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 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. 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 lead the foundations for us.

If it typically is the case that it takes four 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. 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. 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.

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, 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 learned something, and that's the way that knowledge progresses.

So nobody should be worried about making a guess, advancing a hypothesis, an educated guess. 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 learnt something about the Universe, which is that complex civilizations are not as rare as I think they are, or civilizations aren't as rare, so that would be a good thing.

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