The first reason for scepticism is, simply, the tremendous distance involved in any voyage from even the nearest habitable exoplanet (Proxima Centauri, about 4.25 light-years away). If such a distance can be traversed, this is almost certainly not going to happen by a means of transportation that bears any resemblance, or evolutionary connection, to the horse-cart or the school bus, as our own space shuttles still do, and as virtually all reported UFO descriptions do as well. It would simply not be fit for interstellar travel.

Even if these UFOs are often said to be operating according to mechanical principles we do not yet understand — veering off suddenly in a different direction with no apparent acceleration period, for example — they remain conventional vehicles, containers built to transport the living bodies inside them. But when it comes to traversing distances measured in light-years, it is vastly more likely that any intelligent beings that figure out how to do so will not be relying on vehicular motion as we understand it, but on the exploitation of some physical principle, such as wormholes, or some information-theoretical principle, such as one that allows them to dematerialise the unique patterns that constitute their identity, and to “beam” them across galaxies for rematerialisation elsewhere. If there are aliens among us, in short, they almost certainly didn’t come here in spaceships.

And whether they come through wormholes, or teletransportation, or some other means, it is most probable that what will count for them as “arrival” will not be an arrival in an organically embodied form. Indeed, the idea that alien visitors would come in biological bodies such as ours is, I contend, even less plausible than that they would come in artificial contraptions. Organic substrates, as the philosopher and xenobiologist Susan Schneider has argued, may well turn out to be a relatively short-lived host for intelligence whenever and wherever it emerges in the universe, soon to be replaced, wherever a technologically advanced species appears in the cosmos, by robots.

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If intelligent beings are smart enough to get here, they are almost certainly smart enough to get here without dragging their bodies along. They could get around this hitch by transferring their consciousness or personal identity (whatever that is) into some more durable physical substrate — either a technological device of their own making, or, more intriguingly, a pre-existing physical system or process in the universe into which they figure out how to integrate themselves. To be clear, I have no clue how this would work. But neither you nor I have any idea how you could sit your living body down and fly a school-bus-like shuttle across interstellar distances — so the modal operator at work in both cases is not concrete feasibility, but only conceptual possibility.

Our alien visitors could also pull off such an impressive feat simply by means of what we, with our limited technological imagination, would classify as “simulations”. For us, when a high-powered telescope or an unmanned probe sends back images of objects in space, we consider that we are “seeing” and “experiencing” these objects only in a downgraded better-than-nothing sense, as mediated representations. But this may only be because our technologies of representation are still in a primitive stage, while another race of intelligent beings might well succeed in improving these technologies to the point where technologically mediated representation might count for them as “the real thing”. It might be both easier and more desirable for them to avail themselves of such representations than physically to travel and to experience that thing unmediated in a way that we ourselves, in our present technological condition, would consider “real”.

To imagine that one must go to another part of the universe, in one’s own organic body, in order to truthfully claim that one has been there, may turn out to be somewhat like supposing, circa 1920, that in order to participate in a conference with colleagues in Paris, one must actually go to Paris, rather than joining them by Zoom — a possibility that would only come onto the horizon a century or so later.

But let us suppose just for a moment that alien beings did set themselves the goal of making an “actual” voyage to Earth in their standard-issue, carbon-based, biologically evolved bodies. And let us suppose they succeeded. What might these bodies look like? There are at least some compelling arguments that any technologically advanced race of beings is likely to share certain anatomical traits with us: appendages that grasp, for example, or adaptations facilitating gravity-bound terrestrial locomotion. I have even heard it argued that space-travelling aliens would probably be equipped with exactly two eyes, and as with us, these would be found in the front of their faces, facilitating binocular vision.

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We know from convergent evolution in widely different phylogenies on Earth that different species can come to have similar appearances (e.g., placental wolves in the northern hemisphere and marsupial “wolves” in Australia), and in principle, it is possible that xenobiological evolution could yield up bipedal, two-handed beings with eyes in the front of their heads. But wolves and “wolves” started from a common mammalian ancestor, and what you could not expect to find in any interstellar convergence of organic Baupläne is a being such as the one purportedly discovered of late in Mexico, which clearly has not just hands, feet, and a face, but also a ribcage, a spinal column, and an unmistakable pelvis. On Earth only about 3% of all animal species are vertebrates; how strange it would be if our first extraterrestrial visitors turned out to be vertebrates too!

But the really interesting question is why, in the first place, we started thinking about the beings that inhabit the world “up there” (call it the heavens, or the celestial spheres, or outer space, according to the conventions of your era) as similar to us in their bodily conformation, which is to say as organic beings.

This habit of ours has a very precise starting point in history. We could probably find some earlier examples if we tried, but I tend to date the birth of aliens, as we know them today, to 1609, the year Johannes Kepler wrote his Somnium (which would only be published posthumously in 1634). This is a peculiar work, both a proto-science-fiction reverie, as the Latin title suggests, and a work of properly modern relativistic astronomy, in which the author attempts to describe how the orbits of the other celestial bodies would appear if one were observing them from the moon. In the dream-like part of the work, Kepler relates the story not only of the transit of his hero Duracotus to our nearest satellite (evidently with the assistance of some psychoactive plants), but also the various animal-like beings he finds once he arrives there, including some creeping and slithering reptiles, and some sort of giant camel. Some decades later, in his Histoire comique des États et Empires du Soleil of 1662, the French author Savinien Cyrano de Bergerac would describe the different species of birds that inhabit the surface of the sun.

Why are we beginning to find birds, snakes, and camels in a region of the cosmos where previously we could have expected to find only spirituous beings, divinities, and angels? During this same period, natural philosophers such as René Descartes and G.W. Leibniz were intent on arguing for the uniformity of nature. The new mechanical philosophy required its adherents to accept the principle that, as Leibniz would put it: “Always and everywhere, it’s the same as it is here.”

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For Descartes, it made no sense to divide the cosmos into different ontological realms, as the Aristotelians had previously done, where different laws apply to each. On the Aristotelian model, the most significant boundary in the physical universe is the one traced by the orbit of the moon: everything below it is a “sublunar” being, subject to generation and corruption (i.e. birth and death), while everything above it is immortal. In the new Cartesian physics, by contrast, the laws that govern the motions of bodies here on Earth, from the arc of a projectile shot from a cannon, to the rising and lowering of the tides, to the generation of a foetus, are exactly the same as the laws that govern the formation of other star systems, and presumably also of the planets and of the beings inhabiting them.

In other words, the key theoretical innovations of the scientific revolution positively require that, if there are intelligent beings elsewhere in the universe, they are like us. Specifically, this would mean that they depend for their existence not on some fine homogeneous ether or quintessence (as the celestial beings were often conceived in premodern cosmology), but rather on a specific arrangement of internal organs constituting a fragile and perishable organic body. In short, the homogenisation of the physical universe, which had been such a key part of the new mechanical physics, caused a corollary reconceptualisation of the inhabitants of the world “up there”, from quasi-divine beings that were either immaterial or ethereal, to ordinary reptiles or humanoids such as those that have remained the staples of science fiction, and of tabloid claims of alien sightings, to the present day.

Nearly all the great Enlightenment philosophers presupposed that other worlds must be inhabited by organically embodied intelligent beings like us. Thus Immanuel Kant declares in his Critique of Pure Reason of 1781: “I say that it is not merely an opinion but a strong belief (on the correctness of which I would wager many advantages in life), that there are also inhabitants of other worlds.” Kant’s sometimes-adversary, the significantly less rigorous Emanuel Swedenborg, wrote extensively of his own purported voyages to Jupiter and Saturn, where he learned a great deal about the inhabitants’ agricultural methods, the techniques they employ for yoking their beasts of burden, and so on. For more than 200 years, for serious and unhinged thinkers alike, outer space was teeming with a very familiar variety of life, not so different from the sort we know here on Earth.

By the late 19th century, just at the dawn of the era of rocket science, with actual space exploration finally within reach (Konstantin Tsiolkovsky would publish his “ideal rocket equation” in 1903), institutional science entered a sort of “alien winter”. I deploy this term in conscious allusion to the “consciousness winter” that hung over brain science for the better part of a century, scaring researchers away from any acknowledgement of the self-evident reality of internal subjective states of the mind.

In the same fashion, for most of the 20th century — notwithstanding Carl Sagan’s enthusiasm and the small budgets allocated for programmes like SETI — sober-minded experts placed a high premium on the public expression of scepticism about non-terrestrial intelligent life. Naysaying became part of the system of values and presuppositions by which the public identity of an expert was confirmed. In the same period, preoccupation with aliens mostly became the monopoly of fantasists of various sorts, either the creators of fictional entertainments or people who were unbothered by the social stigma of holding beliefs perceived to be “fringe”.

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Why exactly this “winter” set in when it did has much to do with the broader cultural history of science and of the perception of expertise, and indeed seems not entirely separate from the long banishment of consciousness as a category of inquiry. Throughout the 20th century, for the most part, excessive interest in extraterrestrials was the telltale mark of a crank. This attitude had much to do with the reigning positivism of the scientific community, and the general consensus that speculation about things happening beyond the sphere of direct observability is ipso facto unfalsifiable and therefore unscientific.

But this era has decidedly come to an end in the past decade or so, as vast social, economic, and technological transformations have fundamentally realigned the public’s perception of expertise, and of who gets to claim to have it. After the crisis of epistemic authority that experts brought upon themselves throughout the Covid pandemic, and after the replacement of our old media ecosystem by one in which authoritativeness has become more than ever a sort of popularity contest, we are now in a period of history in which extraterrestrials are important if the masses of internet users think they are important, scientific consensus be damned. Under these conditions, the 20th century’s “alien winter” comes to look to many like a government cover-up. The truth is far more banal.

The cultural position of aliens has changed radically in the 21st century, both as a result of changes in the cultural position of science, but also as a result of real and promising scientific breakthroughs. Notably, since the Nineties, there has been a tremendous revolution in the way we observe, catalogue, and estimate the number of habitable exoplanets, and significant discoveries have also been made in astrochemistry, favouring the theory of so-called “soft panspermia”, according to which the chemical compounds that brought about the first life-forms on earth are by no means rare in the cosmos. Given these scientific advances, and given our new information ecosystem, it is safe to say that we will be hearing a lot more about aliens in the coming years. But we would be foolish to believe that this is the result of an actual uptick in sightings, or that our own most recent cultural representations of intelligent life beyond Earth get something uniquely right about the heavens that our ancestors failed to notice.

In an interview with Florence Read, he explains his theory of the objects’ origin, why they were sent here, and how we should interact with aliens if we find them.

Florence Read: When was the first unidentified interstellar object detected in our solar system?

Professor Avi Loeb: It was spotted in October 2017 by a telescope in Hawaii. It was given the name ‘Oumuamua, which means “the scout”. And at first, of course, everyone thought it was a rock from another star that just happened to pass by.

The more we learned about it, the more it looked weird. It was tumbling every eight hours and the amount of sunlight reflected from it changed by a factor of ten as it was tumbling. So that meant that it had a very extreme shape. The best fit for the variation of reflected sunlight was that of a flat object, pancake-like, which is quite unusual.

And then, this object didn’t show any cometary tail — there was no evaporation from it visible — yet it exhibited an excess push away from the sun, by some mysterious force. I suggested that since the force is not the rocket effect of evaporating gases, it could be being pushed by the reflection of sunlight. But for that to be the case, the object had to be very thin, sort of like a sail being pushed by light. And, of course, if it happens to be that way, then it could be artificial in origin.

Nature doesn’t make sails that are propelled by reflecting light. In fact, the same telescope in Hawaii discovered another object which exhibited the same qualities as ‘Oumuamua: it was pushed away from the sun by reflecting sunlight, and had no cometary tail. And within a few weeks, the astronomers realised it was actually a rocket booster that was launched in 1966 by NASA, as part of a lunar lander mission. It was made of stainless steel, so it didn’t evaporate. It had thin walls, and that’s why it was pushed away by reflecting sunlight. Here’s an example of an object we know is artificial, because we made it. The question is: who made ‘Oumuamua?

FR: And your answer was: potentially extraterrestrial life?

AL: It’s a possibility. We don’t have conclusive evidence. My book, Extraterrestrial, describes the anomalies of ‘Oumuamua. But it didn’t come close enough to Earth for our telescopes and even radars to resolve it. We don’t know what, exactly, it looks like.

About a year and a half ago, I established the Galileo Project. One of the goals is to design a space mission that will come close to the next ‘Oumuamua. We have a “dating app” — the Vera Rubin Observatory in Chile — that will start operations within a year. It has a camera of 3.2 billion pixels, 1,000 times as many pixels than you have in your cell phone. It will survey the southern sky every four days, and has a good chance of finding more objects like ‘Oumuamua. We can review them, and swipe most of them to the left on this “dating app” — but if we decide that one of them looks like ‘Oumuamua, we might come close to it and take a close-up photograph. And if we see that it’s a rock, so be it — but it could also show some bolts and screws on its surface, and even a label, made on an exoplanet.

I should say that almost four years before ‘Oumuamua was discovered, the very first interstellar object was discovered, in the form of a meteor. ‘Oumuamua was the size of a football field, but in 2014 an object roughly half a metre in size exploded in the lower atmosphere of the Earth. The US Space Command confirmed that this object is interstellar. The data indicates that this object survived all the way down to the lower atmosphere of the Earth, where it was subjected to very extreme stress — it was tougher than iron meteorites. In fact, it’s 10 times tougher than all meteors ever identified in the same catalogue.

So the question is: was it from some unusual source very different from the solar system? Or was it artificial in origin? Maybe it was made of stainless steel, and was a spacecraft from another civilisation. We’re going to find out the composition — we are going to the Pacific Ocean this summer to look for the fragments leftover from this meteor. By collecting them from the ocean floor, we hope to figure out their composition and tell whether they were just a rock or part of a spacecraft.

FR: If it’s artificial, where might it come from?

AL: Freeman Dyson — a British-born scientist, very well-known about 60 years ago — suggested that a very advanced scientific civilisation would not just rely on the amount of energy coming from its star to the surface of its planet, but would surround the star with a megastructure that would harvest the energy of the star. It is called the “Dyson sphere”: it’s a huge engineering project to imagine. It could be made of tiles that are hovering above the star, just like very thin sails. But there would be very high stresses on the construction, and therefore it would be made of very tough material. Once the civilisation loses interest in the Dyson sphere, it might not maintain it, and then asteroids would hit it quite frequently and could destroy it within a billion years. So the pieces of a broken Dyson Sphere could float in interstellar space and they could account for the materials that we found in the interstellar meteor.

FR: But your theory about ‘Oumuamua is different — in your report you argue: “An artificial interstellar object could potentially be a parent craft that releases many small probes during its close passage to Earth.”

AL: A very good approach, to probe a large region of space, is to release many small probes — like a dandelion seeding its environment. If you have a probe visiting the habitable zone of a star like the Sun, the probe itself might be too big and move too fast to land on a particular planet. If you want to probe a lot of regions and a lot of planets, you might just spread dandelion seeds. In fact, just this year, we founded a new space corporation with my colleague, Dr Frank Laukien, that aims to use this approach for the Moon and Mars, instead of the traditional approach of using just one big probe and visiting one location.

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FR: How do they propel themselves?

AL: It depends on the nature of the engine. The only type of engine that we employ so far for interstellar probes are chemical rockets that can push a probe up to about a thousandth of the speed of light. But there could be much better schemes. For example, a very distinguished British cosmologist named Hermann Bondi wrote a paper in 1957, suggesting that if you have a negative mass — we are used to positive masses, but a negative mass in principle is possible to construct — then the negative mass will push away, if you put the positive mass next to it. And the positive mass will pull the negative mass with it. So together, they would accelerate indefinitely up to the speed of light. That is a type of engine that we have never constructed, and could potentially be engineered, if we developed the physics behind it.

FR: So should we assume that an alien species could have far more sophisticated technology than we do?

AL: Yes, and the way I think of it, it’s just like going on a date. You can learn from the other person. We should look at it as an opportunity; we shouldn’t be fearful of it. We are probably not sophisticated enough to be a threat to whatever comes to visit us. But we can make money from it — if we learn how to replicate [the technology] here on Earth. We can get a leap into our future.

As for what they might be hoping to accomplish — if you stay on your planet, then you might be annihilated or destroyed by a single-point catastrophe — we will not know until we observe them. I don’t think it makes sense to establish a committee of experts, and decide what the protocol is for responding to visitors. We should just try and figure out what they’re seeking. I spoke to Henry Kissinger just last year, and I asked him — because he negotiated with other nations, where the culture is very different — what is the realpolitik of interstellar communication? And he said that first you want to learn what the other side is seeking, and only then develop a dialogue with them.

FR: Is there any evidence from observatories or telescopes that these probes are actually entering our solar system?

AL: The Government collects data as a result of national security concerns and the sensors the government is using are classified. If there is high-quality data, it’s not being released to the public. But an extraterrestrial technological object has nothing to do with national security. It doesn’t adhere to national borders, and therefore any knowledge about it should be shared by all humans. It shouldn’t be the privilege of the President of the United States. We know that we are not at the centre of the universe; we know that there are billions of other Earth-Sun systems in the Milky Way galaxy alone. We are not at the centre of the stage, and we just came to the cosmic play at the end ,over the past few million years. The play is not about us.

FR: Tell me what it’s like being an expert in aliens. It must be quite an odd field to choose as your specialism.

AL: The way to think of me is as a farm boy. I’m connected to nature more than people. I have no social media. I don’t care how many likes I get. I’m just trying to do what sounds like common sense — something today which is not that common (especially in academia). There are many of my colleagues working on extra dimensions of the multiverse who do fancy mathematics — and it’s their way of showing off that they are smart. But if there is an object from another civilisation that could change the future of humanity, it may not take fancy maths to realise that it’s not a rock. When there is data about something unusual, we’re supposed to be curious. There is uncertainty. You can be wrong. That’s part of the job description.

FR: What if you are a young junior professor, at a university like Harvard, when stepping outside the bounds of what’s considered acceptable science is too risky?

AL: If you look at the career of Pablo Picasso, you will find that early on, he was a realist. He tried to draw in the style of traditional painting. After he mastered that, he invented cubism, which was disruptive. So as a young scientist, you don’t want to disrupt the system. You want to learn first of all the basic principles of physics, and apply them along the themes that are defined by senior people in academia, so that you can get a postdoc position after your PhD. Eventually you’re promoted to tenure.

But what happens after people get tenure is they are even more obsessed with their ego. They want to get honours, awards — to be recognised by their peers — and they conform to the beaten path. I say that the road not taken is the most interesting thing to take, because it may have low-hanging fruit. So I advise young scholars: if you’re interested in innovation, once you get tenure, start to be creative: innovate, because that’s the whole purpose of academia. Betray the slogan of the Party in George Orwell’s 1984, “ignorance is strength”. That’s what academia should be about.

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But unfortunately, it’s not. I heard, a few days ago, a very distinguished scientist, who said: “I don’t really want to know whether Covid-19 came from a lab leak, or the wet market in Wuhan.” Why would he say that? Because he doesn’t want the image of science to be tarnished, if it came from a lab leak. This is similar to arguing that “ignorance is strength”. Knowledge is strength, and the same is true about the nature of unidentified aerial phenomena, the nature of dark matter. Everything should be driven by our ambition to be knowledgeable, because then we can adapt even if reality doesn’t look as pretty as it should be. Let us see it.

FR: There is a genuine cohort of your colleagues who would say that there is 0% probability that aliens do exist. What do you think the probability is of life beyond Earth?

AL: Galileo was placed under house arrest just for suggesting that maybe the Earth moves around the Sun, and today he would have been cancelled on social media. If you were to ask those theologians, they would say there is zero probability that the Earth is not at the centre of the universe. Now, why would they say that? Because the church wanted to control people. There are always ulterior motives. I bet you that if we find a piece of technological equipment from another civilisation, people will say: “Of course, this was talked about for decades. There is nothing really new — we knew it all along.”

But if I had to guess, I would say not only that we are not alone, but that there were many technological civilisations before us that reached greater heights. We are probably not even close to the top half of their advances. They would have technologies that look like magic to us — like miracles in religious texts. It’s possible that that civilisation could create life in its laboratories. It would bring the concept of God, or something very capable, into reality — it will unify science and religion in some sense. I think it’s very likely that they were out there billions of years ago, maybe not right now. But they existed, and the only question is, when will we find the conclusive evidence beyond any doubt? And for that, we need to search. If there are objects of technological origin, we will find them.

***

This transcript has been lightly edited.