Who said scientists don’t like to speculate? In Aliens: The World’s Leading Scientists on the Search for Extraterrestrial Life, an anthology edited and introduced by Jim Al-Khalili, they do almost nothing but. The result is fascinating, not least because the book’s contributors utilize the available facts to launch themselves, figuratively, into outer space, where certain planets and/or their moons just may play host to organic life. In other words, the speculation animating “Aliens” ranks as the most responsible kind – that informed by science.
Al-Khalili is an Iraq-born British theoretical physicist whose previous works include “Quantum: A Guide for the Perplexed” and “The House of Wisdom: How Arabic Science Saved Ancient Knowledge and Gave Us the Renaissance.” In shepherding his latest book into existence – it was first published under a slightly different title in the UK last year – he handpicked the members of what he calls “Team Aliens.” Of the group’s 20 men and women, many of whom are renowned in their respective fields (which include astrobiology, astrophysics, biochemistry, cosmology, neuroscience, zoology, and more), he observes, “You will find that each and every one of them offers his or her own unique perspective on the subject.”
Indeed, precisely because the contributors to “Aliens” cannot (yet) come up with more than educated guesses regarding the existence of extraterrestrial life, they differ to a great degree in their approach to the issue, and even in their conclusions. This, perhaps counterintuitively, stands as one of the book’s strongest suits. The main purpose of “Aliens” isn’t to argue for or against the proposition that we are not alone, but to examine the oft-ignored side of a subject that has long served as fodder for outlandish popular entertainment and wacky conspiracy theories. This means discussing the conditions necessary for life, the possibility that such conditions exist on this or that planet, and how we would go about determining as much.
Although there may be billions of planets, this doesn’t mean that any can sustain life – at least as we know it. (In “Home Sweet Home: What Makes a Planet Habitable?” Chris McKay explains that life’s desiderata are generally reckoned to be liquid water, energy from sunlight or chemicals, and carbon and associated molecules.) Not only that, but as Paul C.W. Davies (“A Cosmic Imperative: How Easy Is It for Life to Get Started?”) points out, “just because a planet is habitable does not mean it is inhabited.”
It’s also possible that there was life at some point on a certain planet before it got snuffed out by, say, atmospheric change or the impact of a crashing asteroid. In our search for aliens, we could end up finding dead ones.
Bear in mind, too, that the aliens in question – living or dead – might not qualify as intelligent beings. More than one contributor to this anthology is at pains to tamp down expectations that extraterrestrial life will consist of anything more than single-celled microorganisms such as bacteria. “If there are aliens, it is most likely that hundreds of thousands of them could fit on the head of a pin,” is how Matthew Cobb puts it. This is “most likely,” according to Cobb, only in the exceedingly unlikely event that aliens exist; his entry is titled “Alone in the Universe: The Improbability of Alien Civilisations.”
For the opposite take, try “What Are We Looking For? An Overview of the Search for Extraterrestrials,” by Nathalie Cabrol. For Cabrol, who serves as head of the SETI Institute’s Carl Sagan Center for the Study of Life in the Universe, “the discovery of an abundance of exoplanets [planets outside our Solar System] has revolutionised our concept of how many habitable worlds could exist in a very small fraction of our Galaxy alone. Astronomy and astrophysics are also opening wide the potential for habitable worlds in the 100 billion galaxies now estimated in our Universe. The idea that we could be alone is simply completely at odds with statistics.”
If at least a portion of any putative extraterrestrial life qualifies as intelligent, the chances of contact increase, as the aliens themselves may be trying to find the likes of us. However, as Martin Rees notes, there remains a distinct possibility that, should such aliens have been around longer than humans, they will have begun a transition to inorganic forms. After all, intelligent life – at least here on Earth – seems to crave yet more intelligence. In Rees’s view, “This suggests that if we were to detect ET, it would be far more likely to be inorganic: we would be most unlikely to ‘catch’ alien intelligence in the brief sliver of time when it was still in organic form.”
Tantalizingly, the discovery of something right here on Earth would make extraterrestrial life more probable, as explained by Nick Lane in “Electric Origins in Deep-Sea Vents: How Life Got Started on Earth” and Davies in his aforementioned chapter. The ever-elusive holy grail in question is often dubbed “Life 2” or “weird life,” because all life on Earth (that we’re aware of) is believed to be descended from a single source. If all of Earth’s creatures, in their incredible diversity, have a common ancestor, it is easy to see that, but for propitious circumstances a very long time ago, we and every other living thing on this planet might never have come into existence. Discovering a second source of Earthly life – even for nothing more than a kind of microbacteria – would not only prove exhilarating, but bolster the possibility of extraterrestrials roaming a habitable, Earth-like planet somewhere out there.
Presumably in a bid to ensure that “Aliens” wouldn’t alienate (pun intended) enthusiasts of the subject who aren’t scientifically inclined, Al-Khalili wisely solicited several contributions on extraterrestrials in popular culture. In a sense, Dallas Campbell (whose chapter concerns UFO sightings), Ian Stewart (science fiction: novels and short stories), Adam Rutherford (movies), and even Chris French (the psychology of purported alien abductees), all of whose entries are informative yet brisk, bagged the sweet assignments. Most of the other contributors had to contend with making hard science accessible to the lay reader.
It doesn’t always work. For example, Andrea Sella’s “Randomness versus Complexity: The Chemistry of Life” will prove heavy going even for readers who pride themselves on plowing through abstruse material. Meanwhile, Johnjoe McFadden does a fair job of explaining quantum physics in “Quantum Leap: Could Quantum Mechanics Hold the Secret of (Alien) Life?” but its applicability to the birth of self-replicating life on Earth (and, by extension, elsewhere) remains largely theoretical. Even more tenuous in its connection to the book’s overall theme is Anil Seth’s entry, though it admittedly boasts a cool title – “Aliens on Earth: What Octopus Minds Can Tell Us about Alien Consciousness.”
At the other end of the spectrum, you’ve got Giovanna Tinetti (“Good Atmosphere: Identifying the Signs of Life on Distant Worlds”) and Sara Seager (“Are They Out There? Technology, the Drake Equation, and Looking for Life on Other Worlds”). Both authors do an admirable job of an unenviable task: explaining how any initial detection of alien life would probably come about not as a result of interstellar travel or even radio transmission, but through the comparatively unglamorous business of using telescopes and satellite imagery to study changes in far-off exoplanets’ atmospheric composition (Tinetti) and pinpointing such planets’ biosignature gases (Seager).
Perhaps the most mind-boggling aspect of this book (and that’s saying something, given that it’s about extraterrestrial life) concerns just who at our end would make contact with aliens, should they turn out to exist. Remember Rees’s point about prospective aliens having evolved into something inorganic by the time we find them? Well, it works both ways. Indeed, Rees’s chapter is titled “Aliens and Us: Could Post-humans Spread through the Galaxy?” Both Rees and Seth Shostak (“What Next? The Future of the Search for Extraterrestrial Intelligence”) argue that by the time Earthlings and aliens locate each other, let alone meet, “we” – or some of us, at any rate – will essentially have ceased to be human. Not only that, but our (artificially engineered) evolution toward cyborgs or something silicon-based is precisely what will enable physical contact, as opposed to mere electronic communication. “Interstellar travel will only be a realistic possibility for post-humans,” asserts Rees.
It sounds far-fetched, but if Rees and Shostak are correct, their view has at least one serious implication for the here and now. Actually, it’s equal parts serious implication and major bummer. For those of you taking succor in the belief that, even if you fail to arrange that much-desired powwow bringing together humans and aliens, your progeny will manage the feat, think again. Far from being your grandkids or great grandkids, any Earthlings who eventually rendezvous with aliens may well constitute replacements for the human race. Shostak muses that “it’s at least possible that once GAI [General Artificial Intelligence] establishes a presence on Earth, it may so dominate the planet’s resources – material, energetic and geographic – that Homo sapiens will be marginalised in the way that great apes are.”
To you and me, such GAI-guided Earthlings would be almost as alien as any denizens of distant planets with whom they come into contact.