The Alpina Publishing House has published a book by the cosmologist John Willis "All These Worlds Are Yours." We publish an excerpt from scientific non-fiction, which tells how to search for aliens.
Search for extraterrestrial intelligence
Ah, SETI (Search for Extraterrestrial Intelligence, the general name of projects and activities to search for extraterrestrial civilizations and possible contact with them - Ed.)! So we met again. No other branch of astrobiology is so controversial between scientists and those who fund their research. But, perhaps, no other branch of astrobiology attracts such public attention as the search for brothers in mind.
Between the frequencies of the familiar radio stations we listen to on Earth, lies a world of electrostatic noise. This hiss and clicking is the joint sound of radiophysical processes taking place on our planet, in the nearest space and in the entire Universe. But what if, behind this turmoil of radio waves, we cannot distinguish the simplest artificial signal: beep-beep-beep?
The younger generation of radio astronomers, who grew up in the era of rapid technological development that came after World War II, suddenly realized that they had the ability to transmit and receive signals over distances exceeding the size of our Galaxy. If we, a technically young civilization, have learned to transmit messages over such huge distances, maybe there are other civilizations in the Universe seeking to establish contact with us? Maybe there are lively conversations in the interstellar ether and we just need to learn to recognize them?
While our search for elemental life outside of Earth was progressing in small steps, efforts to find highly advanced alien civilizations propelled us far ahead. Throughout this book, I have assured you that our first contact with alien life is likely to be microbial contact, not alien mind. We have focused all our attention on the foundational experiments to detect the metabolism or biochemical structures that are inherent in the most elementary forms of life. Now we will set ourselves higher and more technological goals - the discovery of life in its most highly developed form.
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Conversation a thousand light years away
In 1959, an unusual article was published in the journal Nature. Between the articles on the swarming of bees and the effects of radiation on red blood cells was a publication with an unusual title: "Search for Interstellar Messages". In just two pages, Giuseppe Cocconi and Philip Morrison outlined the overall roadmap that nearly all SETI projects followed over the next 50 years.
Cocconi and Morrison concluded that radio technology gives humankind the practical ability to transmit signals between stars, and this makes them an excellent tool for detecting technologically advanced and therefore intelligent alien beings.
Radio telescopes can not only receive signals, but also transmit them. If we, on Earth, transmit a powerful signal into space, then this electronic message can be received and recognized by an alien telescope with the same characteristics at a distance of tens, and maybe even hundreds of light years. Since radio waves are just long-wavelength photons, they travel at the speed of light. So while this messaging will not be instantaneous, it could very well happen in a single human lifetime.
But even in the radio frequency band of the electromagnetic spectrum, there is an unimaginable array of frequencies that can be transmitted. Is it possible to somehow narrow the boundaries of the search?
Cocconi and Morrison focused their attention on the 1420 MHz frequency. This frequency is associated with the emission of hydrogen, the most abundant element in the universe.
Hydrogen atoms predominate in our Galaxy and to a greater or lesser extent in all other galaxies that we have observed. When an excited hydrogen atom returns to a stable state, one quantum of light is emitted - a photon. The photon radiation frequency is equal to 1420 MHz (astronomers call radiation with a wavelength of 21 cm, corresponding to a frequency of 1420 MHz, the neutral hydrogen radio link). Much like the audience in cinemas, fidgeting in their seats with excitement during a session, hydrogen atoms constantly go into an excited state as a result of collisions of atoms, while emitting radio waves with a wavelength of 21 cm.
Thus, hydrogen serves as an ubiquitous spectral beacon, the radiation of which should be familiar to all observers of the starry sky, be they humans or aliens.
Moreover, the frequency of 1420 MHz lies between relatively free regions of the electromagnetic spectrum, where the background noise of our Galaxy is almost inaudible. Later, astronomers called this range the "space watering hole": radiation with a wavelength of 21 cm, surrounded by areas of relative radio silence, can become an excellent meeting place where civilizations scattered across the galaxy can communicate and exchange ideas.
Exactly like us?
Listening to individual stars and open regions of the sky in the radio range close to 1420 MHz has become a common practice for almost all SETI projects carried out over the past 50 years. Unfortunately, there is one rather obvious problem that has been present in all SETI projects from the beginning. The considerations presented by Cocconi and Morrison could just as well be phrased differently: aliens will use radio telescopes because we use them; the aliens will be transmitting on a frequency of 1420 MHz, because we transmit on that frequency; the aliens will point their transmitters towards the sun-like stars, because we would have done that too.
It seems that we know almost everything about how the aliens are going to report their existence in the universe. We have encountered similar considerations before: this is called anthropocentrism - the belief that man is the focus of the universe. But we no longer live in the center of the solar system, galaxy or universe. Then why should our understanding of interstellar communication be the only correct one? The short answer is they shouldn't, no matter how much we want to. While I don't think SETI will ever be able to make contact in the foreseeable future, when we finally get the message, anthropocentrism will be dealt another blow - at least I hope so.
However, despite all these reservations, interest in SETI has not diminished over time. In all fairness, we have to admit that we are looking because we can and because we are interested in it. I said earlier that our current attempts to discover life in the universe - highly advanced or rudimentary - are very limited compared to the unimaginable number of possibilities that exist. Cocconi and Morrison were among the first astronomers and astrobiologists to understand that you have to start with what you have today.
Perhaps the principles of SETI are idealistic and anthropocentric, but is there a fundamental difference between what SETI does and trying to find elementary life forms on “habitable” exoplanets? This seems to be the main question, especially if two similar projects are competing for funding. According to the good old tradition, I will answer it with another question: given 50 years of searching for extraterrestrial intelligence, has anyone calculated what our chances of success are?
Drake's equation
In 1961, two years after the publication of the paper by Cocconi and Morrison, a small conference was held in the premises of the Green Bank radio telescope, West Virginia. The topic of the conference was the activities of SETI, and was attended by only 10 people from such diverse fields of science as neurobiology, chemistry and astronomy. Among them were Philip Morrison and then young Carl Sagan.
When they got together, they probably discussed many of the ideas that you read about in this book.
Astronomer Frank Drake attended the meeting, among others. He has just completed Project Ozma, the first attempt to detect radio transmissions from the Tau Ceti and Epsilon Eridani star systems with the Green Bank radio telescope.
Wishing to summarize the main issue that was discussed at the conference that day - what you need to know to determine the number of intelligent civilizations ready to make contact in the Milky Way galaxy - Frank wrote on the board a deceptively simple formula:
N = R * × fp × ne × fl × fi × fc × L.
The meaning of the equation was conveyed by a rather long sentence: the number of extraterrestrial civilizations present in our Galaxy today (N) is equal to the product of the number of stars formed per year in our Galaxy (R *), the fraction of stars with planets (fp), the average number of per one star of planets with suitable conditions for life (ne), the probability of the origin of life on a planet with suitable conditions (fl), the probability of the emergence of intelligent life forms on a planet on which there is life (fi), the probability of the existence of a technical possibility to make contact (fc) and the lifetime of such a civilization (L).
As I said, the equation is deceptively simple, but also deceptively scary. If you start on the left side, you can substitute values that are relatively well known. For example, the number of stars forming in the Milky Way galaxy per year is about 4 sun-like stars per year.
We can refine this value if we take into account that most new stars are smaller than the Sun, but when I say that this number is well known, I mean that our estimate is no more and no less than 100 times the true value.
This is good news. But as we move from left to right, our knowledge of each parameter in the equation will rapidly diminish. In fact, if we consider such issues as the lifetime of technological civilizations, we can only assume that it is more than 80 years (approximately how long we are at this stage) and, probably, less than the lifetime of the Universe. Any other number can be named within these values as well. Given such great uncertainty, Drake's equation has been criticized more than once, as all attempts to count the number of civilizations ready to make contact end in fruitless disputes.
Proponents of SETI and the Drake Equations often argue that Frank Drake derived this formula only to outline the range of issues that were supposed to be discussed at that 1961 conference. In that case, we can consider it as a scientific wishlist - topics that deserve further study if we want to assess our chances of establishing contact with alien civilizations sometime in the future. Critics of the Drake equation indicate that we do not know the value of most of the parameters included in the equation, and even question the very possibility of establishing the values of these quantities sometime in the future.
So who's right? Is the original SETI concept fundamentally wrong, or even worse, unscientific? In short, it is not. Drake's Equation is a perfectly true - even too true - scientific statement that demonstrates the extent of our ignorance. Moreover, Drake's equation can and should be regarded as an attempt to determine what factors determine the assessment of civilizations ready for contact in our Galaxy. Do not mask the existence of such a problem with evasive answers. If we really want to discover these civilizations, we need to realize the limits of our ignorance and then start taking steps to close the gaps in our knowledge. Yes, at present most of these parameters are unknown to us, but this does not mean that they are not significant or unknowable in principle.
Take, for example, the success of Kepler's mission. To understand the significance of the discoveries made by Kepler, it should be noted that 20 years after the discovery of the first exoplanet, we received a relatively reliable estimate of the fp parameter - the proportion of stars with planets. It's impressive that it took us only 20 years. Fundamental questions take a lot of time and effort to answer, and Drake's Equation is a powerful way to split one fundamental question into several easier ones.
Which parameter of the Drake equation seems most important to you? In my opinion, the one that lies just beyond the limit of our modern knowledge. Given what we know about Drake's variables today, the next unknown parameter is the number of habitable planets per star. Kepler has already allowed us to make a preliminary estimate of its meaning, and TESS and PLATO will refine it in the future.
We can take it a step further and consider how much of the habitable planets life might arise on. This is one of the most fundamental questions - perhaps even the most fundamental - because in order to solve the Drake equation, we need to discover life beyond Earth. This is the question for all astrobiologists, whether they are looking for biomass on Titan or waiting for tweets from Tau Ceti. The likelihood of success or failure in both cases is determined by the same Drake equation, even if more or less truncated compared to the original formulation.
I myself would very much like to understand how you can use the Drake equation to estimate the number of at least elementary forms of life, not to mention counting the number of potential users of "Twitter" in our Galaxy.
But it never occurs to anyone to criticize other areas of astrobiology simply on the grounds that we do not know what the likelihood of the birth of life on a planet with suitable conditions for it is. In fact, attempts to discover extraterrestrial life are attracting so much attention to themselves because it is a very important and completely unexplored issue.
It must be admitted that, in relation to SETI, several more coefficients appear in the Drake equation, which require further anthropocentric assumptions. However, when Frank Drake wrote his equation, he followed a clear scientific tradition. It would be ungrateful on our part to complain that he raised a whole series of fundamental questions, the answers to which elude us. The agenda, which he set in 1961, remains significant today and contains in a concise form the full range of astrobiology problems.
Excerpt for publication provided by Alpina Publisher
