Spanning 13.7 billion years of cosmic history, across more than 90 billion light years, countless galaxies and more stars than there are grains of sand on a million Earths, the Universe is a wondrous, beguiling, curious, dramatic, sometimes frightening, always awe-inspiring and mostly mysterious creation that, despite all our knowledge, all our tools, all our scientific prowess, continues to resist our understanding. Of all the enigmas of existence, the one that most intrigues us is that of life. Not just on this planet, but especially on other planets.

Earth, cradle of life. Image: NASA.

The search for extraterrestrial intelligence (SETI) is the manifestation of our will to seek an answer to the question, is anybody out there? It is a question that some would have you believe is unscientific. How so? That life exists elsewhere in the Universe is a simple hypothesis, and SETI is the experiment that is attempting to prove it, but SETI cannot provide a definitive negative answer to disprove the hypothesis – all we can do is search, and keep searching until we find something. However, SETI is not a question of belief or faith; no judgement has been made on whether there is life out there or not. The trouble is that we could keep looking forever and still be none the wiser. That is why SETI alone may not be enough to do the job; astronomy, physics, chemistry, biology – they all have a role to play in understanding how life, particularly intelligent life, began here on Earth, and whether it can begin elsewhere. It turns out that the search for extraterrestrial intelligence isn’t just an obsession amongst a cult of scientists; in some sense it is the ultimate goal of all our attempts to understand the Universe.

We study other life on Earth because we want to understand its abilities, and make inevitable comparisons with human life. We delve deep into Earth’s past to trace the vital tree of life, to piece together the steps of evolution from the most primitive bacterium to radio telescope-building homo sapiens so we can learn how intelligent life on Earth came to be. We explore how our world and its neighbours in the Solar System formed, accumulating from trillions of pieces of flotsam and jetsam that swarmed around the young Sun 4.6 billion years ago, to better understand how planets arise in habitable zones conducive to life. We hunt out scraps of rock that have fallen to Earth, which record the construction history of the Solar System and, along with primitive comets, these meteorites may have brought the ingredients of life – among them amino acids and water – to our blue planet. We cast our telescopes further afield, taking in stars and planets beyond our Solar System, comparing them to our own, searching for stars that can sustain worlds with life, and probe deep into the darkest star-forming nebulae to watch the process of stellar creation in action, as though we were witnessing the birth of our own Sun whose energy and stability is essential to life.

The rate at which stars are born, the number of worlds they harbour, and amongst them the number of Earth-like worlds, are all factors of the Drake Equation, that famous formula for predicting the number of intelligent civilisations in the Milky Way. Think what you will to the accuracy of its results, but the Drake Equation succeeds in making us more aware of what is required of the Universe to give life a chance. Today, exoplanet-finding missions such as NASA’s Kepler and ESA’s French-led CoRoT spacecraft, as well as ground-based instruments such as the enormously successful HARPS spectrograph at the European Southern Observatory, lead the planet-seeking charge. Within a few years, Kepler will be able to give us a handle on not just how many planets are out there, but what percentage of them may be Earth-like. Meanwhile, spectroscopic and astrometric studies of the stellar population of stars in the Galaxy, and infrared detections of stars being born in vast nebulae, provide an accurate assessment of the star-formation rate, both today and in the past. We watch the life cycle of stars, from birth to death, whereupon they cast the heavy elements needed for building terrestrial planets and their life forms out into the Universe, like pollen drifting on a breeze of stellar radiation and supernova shock waves.

Our search to understand why life exists doesn’t stop there, for the forces of Galaxy evolution shape what we know about star, and thus planet, formation. We study other galaxies, frozen in time at varying distances from us, each one describing a step along the path of the history of our Galaxy, and also its future. Collisions, small and large, sculpt galaxies irrevocably, building up spirals that evolve into giant dusty ellipticals. And how did these galaxies form in the first place? We turn now to cosmology, and the beginning of the Universe, and the formulation of the fundamental constants of nature that seem oddly pre-determined to be suitable for the transcendence of intelligent life. If gravity were slightly stronger, the Universe would have collapsed in on itself as soon as it began. If dark energy were more energetic, it would rip the cosmos apart in no time at all. If the charge on the electron, the mass of a proton, the speed of light, or the nuclear forces that bind together atoms, were all just slightly out one way or another, stars could not form, nucleosynthesis could not create the elements of the periodic table, and life would be impossible. The difference between a flourishing Universe, and a dead Universe, is a scarily fine line. And we want to understand all this so that we can know why life has evolved on Earth, and whether it has evolved on other planets too.

Our attempt to understand the Universe is driven by our wish to appreciate why it is hospitable to life. We’d quite like to meet some of that alien life, to try and understand its thoughts, and compare its form of life to our own. Broadening into philosophical issues, it would be a comfort to know we are not alone on this gargantuan stage, and some respected physicists and theoreticians have speculated that perhaps life is not just a fortunate accident, a lucky by-product, but that perhaps the purpose of the Universe itself is to create life. At this juncture we begin to steer away from SETI’s original scientific hypothesis, but perhaps future discoveries will allow us to make scientific inroads here too. Complexity theorist James Gardner, author of Biocosm and The Intelligent Universe, has speculated that the purpose of the Universe is to create intelligent life that will go on to create more baby universes in the laboratory or through black holes, which in turn will spawn their own intelligent life and continue the cycle, echoing the idea of an evolution of universes espoused by physicists such as Lee Smolin of the Perimeter Institute for Theoretical Physics. The Anthropic Principle – that the constants of nature are fine-tuned to allow for life – has a possible solution in the multiverse, where in a multiplex of universes the probabilities are such that some have to meet the requirements for life. Thus if life-giving universes can reproduce, passing on their finely-tuned fundamental constants like good genes, they can come to dominate the multiverse. As Professor Paul Davies of the University of Arizona recently commented in an article on the website Science and Religion Today, “If the emergence of life and mind are part of the great outworking of the laws of the Universe, then we would expect to find that life is widespread. Life would not be just some sort of irrelevant, meaningless, side issue, but integral to the whole great cosmos.”

And so we come full circle. To understand the origin of life and whether it is widespread, we have to understand the Universe better, and to truly understand the Universe, we have to seek out life. And if there is no life, if we are truly alone in this great, vast cosmos, then that leaves us with a terrible responsibility. As the sole beacon of consciousness amongst the stars, we will inherit the Universe, and represent the one, single chance to unlock the secrets of the cosmos. And it all hinges on that single scientific hypothesis: we are not alone. So the next time someone tells you that the quest for ET is unscientific, remind them that SETI represents the reason we are exploring the Universe in the first place.

Stay tuned throughout April for more SETI articles.