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If Aliens Exist, Here’s How We’ll Find Them

Two esteemed astrophysicists peer into the future of space exploration.

Suppose aliens existed, and imagine that some of them had been watching our planet for its entire four and a half billion years. What…By Martin Rees & Mario Livio

Suppose aliens existed, and imagine that some of them had been watching our planet for its entire four and a half billion years. What would they have seen? Over most of that vast timespan, Earth’s appearance altered slowly and gradually. Continents drifted; ice cover waxed and waned; successive species emerged, evolved, with many of them becoming extinct.

But in just a tiny sliver of Earth’s history—the last hundred centuries—the patterns of vegetation altered much faster than before. This signaled the start of agriculture—and later urbanization. The changes accelerated as the human population increased.

Then came even faster changes. Within just a century, the amount of carbon dioxide in the atmosphere began to rise dangerously fast. Radio emissions that couldn’t be explained by natural processes appeared and something else unprecedented happened: Rockets launched from the planet’s surface escaped the biosphere completely. Some spacecraft were propelled into orbits around the Earth; others journeyed to the moon, Mars, Jupiter, and even Pluto.

If those hypothetical aliens continued to keep watch, what would they witness in the next century? Will a final spasm of activity be followed by silence due to climate change? Or will the planet’s ecology stabilize? Will there be massive terraforming? Will an armada of spacecraft launched from Earth spawn new oases of life elsewhere?

LASER POWER: A crucial impediment to space flight is the inefficiency of chemical fuel. One day a laser power station, located on Earth, might generate a beam to “push” a craft through space.NASA / Pat Rawlings (SAIC)

Let’s think specifically about the future of space exploration. Successful missions such as Viking, Cassini, New Horizons, Juno, and Rosetta were all done with last-century technology. We can realistically expect that during this century, the entire solar system—planets, moons, and asteroids—will be explored by flotillas of robotic craft.

Will there still be a role for humans in crewed spacecraft?

There’s no denying that NASA’s new Perseverance rover speeding across the Jezero crater on Mars may miss some startling discoveries that no human geologist could reasonably overlook. But machine learning is advancing fast, as is sensor technology. In contrast, the cost gap between crewed and autonomous missions remains huge.

We believe the future of crewed spaceflight lies with privately funded adventurers like SpaceX and Blue Origin, prepared to participate in a cut-price program far riskier than western nations could impose on publicly supported projects. These ventures—bringing a Silicon-Valley-type culture into a domain long-dominated by NASA and a few aerospace conglomerates—have innovated and improved rocketry far faster than NASA or the European Space Agency have done. The future role of the national agencies will be attenuated—becoming more akin to an airport rather than to an airline.

We are near the end of Darwinian evolution, but technological evolution of intelligent beings is just beginning.

We would argue that inspirationally led private companies should front all missions involving humans as cut-price high-risk ventures. There would still be many volunteers—a few perhaps even accepting one-way tickets—driven by the same motives as early explorers and mountaineers. The phrase “space tourism” should be avoided. It lulls people into believing such ventures are routine and low-risk. If that’s the perception, the inevitable accidents will be as traumatic as those of the space shuttle were. These exploits must be sold as dangerous, extreme sports, or intrepid exploration.

The most crucial impediment to space flight stems from the intrinsic inefficiency of chemical fuel, and the requirement to carry a weight of fuel far exceeding that of the payload. So long as we are dependent on chemical fuels, interplanetary travel will remain a challenge. Nuclear power could be transformative. Allowing much higher in-course speeds would drastically cut the transit times in the solar system, reducing not only astronauts’ boredom, but their exposure to damaging radiation. It’s more efficient if the fuel supply can be on the ground; for instance, propelling spacecraft into orbit via a “space elevator”—and then using a “star-shot”-type laser beam generated on Earth to push on a “sail” attached to the spacecraft.

By 2100, thrill seekers in the mold of Felix Baumgartner (the Austrian skydiver who in 2012 broke the sound barrier in free fall from a high-altitude balloon) may have established bases on Mars, or maybe even on asteroids. Elon Musk has said he wants to die on Mars—“but not on impact.” It’s a realistic goal, and an alluring one to some.

But don’t expect mass emigration from Earth. It’s a dangerous delusion to think that space offers an escape from Earth’s problems. We’ve got to solve those here. Coping with climate change or the COVID-19 pandemic may seem daunting, but it’s a piece of cake compared to terraforming Mars. There’s no place in our solar system that offers an environment even as clement as the Antarctic or the top of Mount Everest. Simply put, there’s no Planet B for ordinary risk-averse people.

Still, we (and our progeny here on Earth) should cheer on the brave space adventurers. They have a pivotal role to play in spearheading the post-human future and determining what happens in the 22nd century and beyond.


Pioneer explorers will be ill-adapted to their new habitat, so they will have a compelling incentive to re-design themselves. They’ll harness the super-powerful genetic and cyborg technologies that will be developed in coming decades. This might be the first step toward divergence into a new species.

Organic creatures need a planetary surface environment on which life could emerge and evolve. But if post-humans make the transition to fully inorganic intelligence, they won’t need an atmosphere. They may even prefer zero-gravity, especially for constructing massive artifacts. It’s in deep space that non-biological brains may develop powers that humans can’t even imagine.

There are chemical and metabolic limits to the size and processing power of organic brains. Maybe we are close to these limits already. But no such limits apply to or constrain electronic computers (still less, perhaps, quantum computers). So, by any definition of “thinking,” the amount and intensity that can be achieved by organic human-type brains will be swamped by the cerebrations of AI.

We are perhaps near the end of Darwinian evolution, but technological evolution of intelligent beings is only just beginning. It may happen fastest away from Earth—we wouldn’t expect (and certainly wouldn’t wish for) such rapid changes in humanity here on the Earth, though our survival may depend on ensuring the AI on Earth remains “benevolent.”

SPACE TOURISM: Private space companies should avoid the phrase “space tourism,” write Martin Ress and Mario Livio. It lulls people into believing such ventures are routine and low-risk. They’re more like extreme sports.NASA / Jet Propulsion Laboratory

Few doubt machines will gradually surpass or enhance more and more of our distinctively human capabilities. Disagreements are only about the timescale on which this will happen. Inventor and futurist Ray Kurzweil says it will be just a matter of a few decades. More cautious scientists envisage centuries. Either way, the timescales for technological advances are an instant compared to the timescales of the Darwinian evolution that led to humanity’s emergence—and more relevantly, less than a millionth of the vast expanses of cosmic time ahead. The products of future technological evolution could surpass humans by as much as we have surpassed slime mold.

But, you may wonder, what about consciousness?

Philosophers and computer scientists debate whether consciousness is something that characterizes only the type of wet, organic brains possessed by humans, apes, and dogs. Would electronic intelligences, even if their intellects would seem superhuman, lack self-awareness? The ability to imagine things that do not exist? An inner life? Or is consciousness an emergent property that any sufficiently complex network will eventually possess? Some say it’s irrelevant and semantic, like asking whether submarines can swim.

Extraterrestrial civilization might consist of a swarm of microscopic probes that could have evaded notice.

We don’t think it is. If the machines are what computer scientists refer to as “zombies,” we would not accord their experiences the same value as ours, and the post-human future would seem rather bleak. On the other hand, if they are conscious, we should welcome the prospect of their future hegemony.

What will their guiding motivation be if they become fully autonomous entities? We have to admit we have absolutely no idea. Think of the variety of bizarre motives (ideological, financial, political, egotistical, and religious) that have driven human endeavors in the past. Here’s one simple example of how different they could be from our naive expectations: They could be contemplative. Even less obtrusively, they may realize it’s easier to think at low temperatures, therefore getting far away from any star, or even hibernating for billions of years until the cosmic microwave background cooled down far below its current 3 degrees Kelvin. At the other edge of the spectrum, they could be expansionist, which seems to be the expectation of most who’ve thought about the future trajectory of civilizations.

Even if life had originated only on Earth, it need not remain a marginal, trivial feature of the cosmos. Humans could jump-start a diaspora whereby ever-more complex intelligence spreads through the galaxy, transcending our limitations. The “sphere of influence” (or some would envisage a “frontier of conquest”) could encompass the entire galaxy, spreading via self-reproducing machines, transmitting DNA or instructions for 3-D printers. The leap to neighboring stars is just an early step in this process. Interstellar voyages—or even intergalactic voyages—would hold no terrors for near-immortals.

Moreover, even if the only propellants used were the currently known ones, this galactic colonization would take less time, measured from today, than the more than 500 million years elapsed since the Cambrian explosion. And even less than the 55 million years since the advent of primates, if it proceeds relativistically.

The expansionist scenarios would have the consequence that our descendants would become so conspicuous that any alien civilization would become aware of them.


The crucial question remains: Are there other expansionists whose domain may impinge on ours?

We don’t know. The emergence of intelligence may require such a rare chain of events and happenstance contingencies—like winning a lottery—that it has not occurred anywhere else. That will disappoint SETI searchers and explain the so-called Fermi Paradox—the surprise expressed by physicist Enrico Fermi over the absence of any signs for the existence of other intelligent civilizations in the Milky Way. But suppose we are not alone. What evidence would we expect to find?

WE’RE LISTENING: The Allen Telescope Array, located at the Hat Creek Observatory in the Cascade Mountains, about 300 miles north of San Francisco, makes astronomical observations and stays attuned to signs of extraterrestrial life.Seth Shostak / SETI Institute

Suppose that there are indeed many other planets where life emerged, and that on some of them Darwinian evolution followed a similar track to the one on Earth. Even then, it’s highly unlikely that the key stages would be synchronized. If the emergence of intelligence and technology on a planet lags significantly behind what has happened on Earth (because, for example, the planet is younger, or because some bottlenecks in evolution have taken longer to negotiate) then that planet would reveal no evidence of ET. Earth itself would probably not have been detected as a life-bearing planet during the first 2 billion years of its existence.

But around a star older than the sun, life could have had a head start of a billion years or more. Note that the current age of the solar system is about half the age of our galaxy and also half of the sun’s predicted total lifetime. We expect that a significant fraction of the stars in our galaxy are older than the sun.

The history of human technological civilization is measured in mere millennia. It may be only a few more centuries before humans are overtaken or transcended by inorganic intelligence, which will then persist, continuing to evolve on a faster-than-Darwinian timescale for billions of years. Organic human-level intelligence may be, generically, just a brief interlude before the machines take over, so if alien intelligence had evolved similarly, we’d be most unlikely to catch it in the brief sliver of time when it was still embodied in that form. Were we to detect ET, it would be far more likely to be electronic where the dominant creatures aren’t flesh and blood—and perhaps aren’t even tied to a planetary surface.

Astronomical observations have now demystified many of the probability factors in the so-called Drake Equation—the probabilistic attempt traditionally used to estimate the number of advanced civilizations in the Milky Way. The number of potentially habitable planets has changed from being completely unknown only a couple of decades ago to being directly determined from the observations. At the same time, we must reinterpret one of the key factors in the Drake equation. The lifetime of an organic civilization may be millennia at most. But its electronic diaspora could continue for billions of years.

It’s in deep space that non-biological brains may develop powers that humans can’t even imagine.

If SETI succeeded, it would then be unlikely that the signal would be a decodable message. It would more likely reveal a byproduct (or maybe even a malfunction) of some super-complex machine beyond our comprehension.

The habit of referring to “alien civilizations” may in itself be too restrictive. A civilization connotes a society of individuals. In contrast, ET might be a single integrated intelligence. Even if messages were being transmitted, we may not recognize them as artificial because we may not know how to decode them, in the same way that a veteran radio engineer familiar only with amplitude-modulation (AM) transmission might have a hard time decoding modern wireless communications. Indeed, compression techniques aim to make the signal as close to noise as possible; insofar as a signal is predictable, there’s scope for more compression.

SETI so far has focused on the radio part of the spectrum. But we should explore all wavebands, including the optical and X-ray band. We should also be alert for other evidence of non-natural phenomena or activity. What might then be a relatively generic signature? Energy consumption, one of the potential hallmarks of an advanced civilization, appears to be hard to conceal.

One of the most plausible long-term energy sources available to an advanced technology is starlight. Powerful alien civilizations might build a mega-structure known as a “Dyson Sphere” to harvest stellar energy from one star, many stars, or even from an entire galaxy.

The other potential long-term energy source is controlled fusion of hydrogen into heavier nuclei. In both cases, waste heat and a detectable mid-infrared signature would be an inevitable outcome. Or, one might seek evidence for massive artifacts such as the Dyson Sphere itself. Intriguingly, it’s worth looking for artifacts within our own solar system: Maybe we can rule out visits by human-scale aliens, but if an extraterrestrial civilization had mastered nanotechnology and transferred its intelligence to machines, the “invasion” might consist of a swarm of microscopic probes that could have evaded notice. Still, it would be easier to send a radio or laser signal than to traverse the mind-boggling distances of interstellar space.


Finally, let’s fast forward not for just a few millennia, but for an astronomical timescale, millions of times longer. As interstellar gas will be consumed, the ecology of stellar births and deaths in our galaxy will proceed more gradually, until jolted by the environmental shock of a collision with the Andromeda galaxy, about 4.5 billion years hence. The debris of our galaxy, Andromeda, and their smaller companions (known as the Local Group) will aggregate into one amorphous (or perhaps elliptical) galaxy. Due to the accelerating cosmic expansion, distant galaxies will move farther away, receding faster and faster until they disappear—rather like objects falling into a black hole—encountering a horizon beyond which they are lost from view and causal contact. But the remnants of our Local Group could continue for a far longer time. Long enough perhaps for what has been dubbed a “Kardashev Type III” phenomenon, in which a civilization is using the energy from one or more galaxies, and perhaps even that released from supermassive black holes, to emerge as the culmination of the long-term trend for living systems to gain complexity and negative entropy (a higher degree of order).

The only limitations set by fundamental physics would be the number of accessible protons (since those can in principle be transmuted into any elements), and the total amount of accessible energy (E=mc2, where m is mass and c is the speed of light) again transformable from one form to another.

Essentially all the atoms that were once in stars and gas could be transformed into structures as intricate as a living organism or silicon chips but on a cosmic scale. A few science-fiction authors envisage stellar-scale engineering to create black holes and wormholes—concepts far beyond any technological capability that we can imagine, but not in violation of basic physical laws.

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If we want to go to further extremes, the total mass-energy content in the Local Group isn’t the limit of the available resources. It would still be consistent with physical laws for an incredibly advanced civilization to lasso the galaxies that are receding because of the cosmic expansion of space before they accelerate and disappear over the horizon. Such a hyper-intelligent species could pull them in to construct a segment resembling Einstein’s original idea of a static universe in equilibrium, with a mean density such that the cosmic repulsion caused by dark energy is precisely balanced by gravity.

Everything we’ve said is consistent with the laws of physics and the cosmological model as we understand them. Our speculations assume that the repulsive force causing cosmic acceleration persists (and is described by dark energy or Einstein’s cosmological constant). But we should be open-minded about the possibility that there is much we don’t understand.

Human brains have changed relatively little since our ancestors roamed the African savannah and coped with the challenges that life then presented. It is surely remarkable that these brains have allowed us to make sense of the quantum subatomic world and the cosmos at large—far removed from the common sense, everyday world in which we have evolved.

Scientific frontiers are now advancing fast. But we may at some point hit the buffers. There may be phenomena, some of which may be crucial to our long-term destiny, that we are not aware of any more than a gorilla comprehends the nature of stars and galaxies. Physical reality could encompass complexities that neither our intellect nor our senses can grasp. Electronic brains may have a rather different perception of reality. Consequently, we cannot predict or perhaps even understand the motives of such brains. We cannot assess whether the Fermi paradox signifies their absence or simply their preference.

Conjectures about advanced or intelligent life are shakier than those about simple life. Yet there are three features that may characterize the entities that SETI searches could reveal.

• Intelligent life is likely not to be organic or biological.

• It will not remain on the surface of the planet where its biological precursor emerged and evolved.

• We will not be able to fathom the intentions of such life forms.

Two familiar maxims should pertain to all SETI searches. On one hand, “absence of evidence isn’t evidence of absence,” but on the other, “extraordinary claims require extraordinary proof.


Martin Rees is Astronomer Royal for the United Kingdom and author of On the Future.

Mario Livio is an astrophysicist and author of Galileo and the Science Deniers.

Lead image: paulista / Shutterstock

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