The planet is populated by some 9 million species, and at least one already belongs to intelligent life. There are countless planets in the universe that are habitable for life. Therefore, we naturally infer that extraterrestrial life (even advanced civilization) should exist in large numbers. For more than half a century, we have also exhausted all means to find them, but we have still found nothing.
Alien life, aliens, why is it so hard to find?
After a long search to no avail, some astronomers have recently turned their attention back to Earth to revisit the question. Coincidentally, the results of their scrutiny coincided with a point made by biologists.
Formula for calculating the number of alien civilizations
For decades, astronomers searching for extraterrestrial intelligent life have scanned the sky with radio telescopes in hopes of intercepting signals from another advanced civilization.
Radio telescopes search for radio waves. To estimate how much intelligent life in the Milky Way is capable of emitting or receiving radio signals, we often go back to a formula developed by American astronomer Frank Drake in 1961. The Drake formula is the product of seven variables, and the final result is the number of civilizations in the galaxy that might be able to communicate with us.
At the time Drake came up with his formula, each of these seven variables had large uncertainties. However, today, more than half a century later, thanks to the thousands of exoplanets discovered by the Kepler space telescope, the first three items in the Drake formula, R* (the average rate of star formation in the Milky Way), fp ( The proportion of stars with planets) and ne (the average number of terrestrial planets in each star system with planets), the values have been relatively determined. There are only two variables involved in biology, fl (probability that life can evolve in habitable planets) and fi (probability of evolving into advanced intelligent life if life evolves), which are more difficult to determine. For this, we can only Do some guesswork.
As things stand, people’s understanding of these two variables (two probabilities) is so disparate that the final calculation results are varied. It is said that we are alone in the galaxy, and there are millions of civilizations like us in the galaxy. Faced with these prophecies, we are still as confused as before.
Statistically, the conventional way to narrow these two probabilities is to enlarge the sample size. For example, look at a large number of terrestrial planets over billions of years to see the probability of life appearing and the probability of life evolving into intelligent life. But the problem is, the only sample we can have is currently the Earth, so this is no way to go.
Approaching from conjecture to objective truth
However, there is another way to approach the problem of probability that could change the way we think about the probability of finding aliens.
The 18th-century British mathematician Thomas Bayes proposed a method that allows us to first give a more subjective guess with a limited number of samples, and then continuously update the probability as new information emerges, in order to be closer to the objective. reality. This method is called “Bayesian method”.
for example. There is a bucket of black and white balls. Now you want to know what the proportion of black and white balls is.
This problem needs to be solved by constantly taking the ball from the bucket. In principle, when you have taken all the balls out of the bucket, you will know the answer. But it’s not until all the balls are taken that we don’t have a clue about it.
Before we get our hands on the balls, we might as well guess that the bucket is 50% white and 50% black. As we keep getting the ball, we update our guesses based on the observations we get. Assuming 4 white balls and 6 black balls after 10 draws, then our estimate of the percentage of white balls is downgraded from the original 50%. Assuming 15 white balls and 35 black balls after 50 draws, our estimate of the proportion of white balls will continue to decrease. As more and more observations are made, we continue to update our guess and become more and more confident about that guess.
In principle, as long as the guess can be updated infinitely, the result of the Bayesian method is independent of the original guess. Your initial guess of “50% white balls and 50% black balls” or “100% white balls” will always come close to the truth. But in practice, since the information we currently have is limited after all, the guess cannot be updated infinitely, so the result of the Bayesian method often depends a lot on the initial guess. A reasonable guess can avoid many detours and approach the objective truth faster than an unreasonable guess.
Even if aliens exist, communication is a hindrance The
text mentions that there are two biological variables in the Drake formula that prevent us from making more reliable predictions about the possibility of alien intelligent life. In fact, there is also a way of ignoring these two variables – but this method seems too arbitrary to many people – that is to use the situation on Earth to measure other planets.
Intelligent life on Earth did not appear until about 4.5 billion years after the formation of the earth, and we can also claim that intelligent life also appeared 4.5 billion years after its birth on other habitable terrestrial planets. In June 2020, shortly after Kipping published his research, two astronomers in the United Kingdom used the Drake formula to estimate according to this assumption, and concluded that “there are at least 36 intelligent civilizations in the Milky Way that can communicate with us”. in conclusion. In their calculations, they also assumed that once an intelligent civilization emerged, it would last at least 100 years. If it lasts longer, then the number of civilizations will also increase.
However, even if there are really 36 alien civilizations in the Milky Way. In this case, the average distance between them is about 17,000 light-years, which is a huge impediment to communicating back and forth with us. You know, it takes 17,000 years for any signal they send to reach us. The signals we send back also take 17,000 years to reach them, and then another 17,000 years for them to reply. In this way, it takes at least 51,000 years to complete an exchange. But we have only learned radio communication for more than 100 years, and each generation has a very limited life. How long civilization can last is unknown. In addition, in the presence of uncertain factors such as natural disasters and large-scale wars, we cannot know whether human civilization can last for more than 50,000 years.
In addition, what is the language or writing used by alien civilizations for communication and communication? Can we communicate with them? Don’t say anything else, different creatures on earth are unable to communicate with each other, and “chicken and duck talk” is a common phenomenon. It is us humans, people from different countries and regions, if there is no translation, communication is very difficult. The texts of different periods are also difficult to decipher. The oracle bone inscriptions in our country have not been fully deciphered yet. So even if we get in touch with them, it’s still a question of whether we can communicate.
The most likely form of alien life
In 2020, astronomer David Kipping of Columbia University in the United States used the Bayesian method when studying the possibility of life in the universe.
First, he made an assumption. He believes that the probability of life appearing on habitable planets and the probability of life evolving into intelligent life should be either close to 0 (meaning it will never happen) or close to 1 (meaning it will always happen), not somewhere between some arbitrary value in between. Because in his opinion, if a certain intermediate value is taken, for example, the probability of life appearing on a habitable planet is 0.5, it means that 50% of the terrestrial planets with the same conditions as the earth can evolve life , 50% cannot evolve life. This is like an experiment under the same conditions, conducted in the United States and in China, and will reach opposite conclusions. In his view, it is too baffling.
Based on this assumption, there are four possibilities: life and intelligent life are rare; life and intelligent life are common; life is rare, but once it appears, it can almost always evolve into intelligent life; life is common, But few can evolve into intelligent life. Before doing the calculations, he also assumed that the initial likelihood of all four scenarios was the same.
Combining geophysical and paleontological evidence, Kipping used a Bayesian approach to calculate the probabilities of the four scenarios. In the end, he found that “life is common” is nine times more likely than “life is rare.” This means that the existence of life on Earth-like planets is a high-probability event. He also found that “intelligent life is rare” is favored over “intelligent life is common” by a ratio of 3:2.
In a word, according to Kipping’s calculation results, “life is very common, but rarely evolves into intelligent life”. Among the four cases, the possibility is the highest.
This is in line with the prevailing view among biologists: it is relatively easy to evolve life from nothing, but there are many obstacles for simple life to evolve into advanced intelligent life. They say that although multicellular life is more advanced than single-celled life, and intelligent life is more advanced, evolution is completely random and blind, and there is no hand that points life to an evolutionary path from low to high. Therefore, the evolution of advanced life and intelligent life on the earth is very contingent. Even if the history of the earth repeats itself in the past, these good things may not happen again.
For example, the evolution from simple unicellular organisms to complex multicellular organisms may have been a fluke. Single-celled life relies on itself to survive and reproduce. In contrast, cells in multicellular organisms have all given up their independence and are powerfully combined to undertake different divisions of labor. How these cells came together so tacitly, and how the hurdles were overcome, remains a mystery. The intellectual and cultural development is equally incredible. Fish have been on Earth for about 450 million years, but they don’t seem to show signs of intellectual growth over that long period of time. So we can also easily imagine that on another terrestrial planet, life has never arisen, or the single-celled organisms that have arisen have never evolved into complex multicellular organisms…
Such a conclusion is indeed a little depressing. None of this conclusively proves that we are alone in the universe, though. The reasons are as follows:
First of all, the universe is very large after all. In the words of the famous British biologist Haldane, “the universe is not only larger than we think, but also larger than we can imagine.” There are at least 100 billion stars in the Milky Way. , and trillions of planets. The history of the universe is so long. So, despite all the obstacles in the evolution from simple life to intelligent life, we can still expect some advanced civilizations to appear on some planets.
Second, the Bayesian method is only a statistical method, and its results are not a substitute for the truth. Or take the ball out of the bucket, you never know what the real situation is until you get it all out. For example, out of a total of 100 balls, you take 80 balls, and the ratio of black and white is almost 1:1. You use the Bayesian method to deduce that the whole bucket of balls is also half black and half, but later you find that the remaining 20 balls are black and white. All are white balls. This overturns your original conclusion. Because in the end, we don’t know how this bucket of balls was put in, and whether it was shaken and stirred well after being put in.
Furthermore, when astronomers apply Bayesian methods, some of the data they input has a lot of uncertainty. For example, we don’t know when the Earth became habitable for life. When it emerged, the first molecules of this life were amino acids, RNA, or lipid membranes. After life first emerged, was it killed by some catastrophic event early in Earth’s history and had to be restarted in a possibly different way? Also, we don’t know if all life must be built solely on carbon. Numerous uncertainties on these issues also affect the reliability of forecasts.
So, the conclusion, while a bit depressing, doesn’t stop astronomers from continuing to search for extraterrestrial intelligent life.