Dear readers and friends,
We're about to do a great calculation,
It will help us think and solve that ultimate problem
- Are humans lonely in the universe?
Now, we invite you to witness this moment!
But don't worry,
Although such calculations may seem extremely spectacular,
But there are no dizzying formulas,
You can understand it just by having a second grade of elementary school
- because we only use multiplication for this calculation.
Having mastered this formula,
You can count how many aliens there are in your favorite galaxy.
But let's start with the galaxy we are most familiar with!
You may need this map when you're traveling around the galaxy :)
(Source: NASA)
In order to figure out how many carbon-based civilizations in the galaxy can communicate with us, that is, the so-called aliens, we first need to know what factors affect the emergence of extraterrestrial civilizations?
Factor 1: How many habitable planets are there in the Milky Way?
Factor 2: What is the probability of intelligent life on habitable planets?
Multiply the two, and you will know how many alien civilizations there are in the galaxy, and it's that simple, have you all followed this step? Okay, let's take a look at how to calculate the answers to these two factors separately.
Kepler-186f, an imaginary diagram of a potentially habitable planet
(Source: NASA)
Factor 1: How many habitable planets are there in the Milky Way?
To roughly estimate the number of habitable planets in our galaxy, it's easy to meet only three conditions:
First, the Milky Way is capable of producing new stars, assuming that this rate is R (pcs/year);
second, that a newborn star can have planets, assuming that this possibility is f;
Thirdly, there are n planets suitable for life among these planets.
Multiply the above three items and we know how many habitable planets there are in the Milky Way, and we denote this number as N. As far as the Milky Way is concerned, it can give birth to stars, so R>0; stars can also give birth to planets, f>0; and most importantly, the average number of habitable planets, n>0, why is it so certain? Because at least the earth is a habitable planet in the Milky Way! Therefore, multiplying three numbers greater than zero, the total number of habitable planets in the Milky Way N must be greater than zero, so let's study factor 2.
The Earth's position in the universe (Source: NASA)
Factor 2: What is the probability of intelligent life on habitable planets?
The probability of intelligent life on a habitable planet can also be broken down into several conditions:
First, these planets are suitable for life generation and reproduction, with a ratio of f1;
second, the resulting life can develop into complex intelligent life, with a ratio of f2;
Based on previous experience, do you feel that you can multiply these two conditions to get the answer we want? In fact, there are two key conditions:
Third, intelligent life can form an interstellar civilization, that is, it can be detected by us, and this ratio is f3;
Fourth, the length of time each civilization lasts is L (after all, we want to send a message and then receive it), and the above four terms are multiplied by the probability that an observable extraterrestrial civilization has appeared on a habitable planet.
The moment to reveal the answer
Drake's formula
Now, we have the Drake formula, the first factor is the number of habitable planets in the galaxy, and the second factor is the possibility of intelligent species appearing on habitable planets, and after multiplying the two, we can know the number of intelligent civilizations that have appeared in the galaxy.
Okay, now that we understand the formula, let's start the real math!
We assume that the Milky Way has been in a relatively stable state, that is, within the Drake formula, that is, the parameters in the Drake formula do not change. The Milky Way generates 10 stars per year (R=10), of which 1/4 of the stars have planets (f=1/4), and on average, 1 of these stars is in the habitable zone (n=1).
Schematic diagram of the habitable zone: too close will cause too high temperature, too far away will lead to too cold
(Source: NASA)
And these planets in the habitable zone (let's be optimistic) will definitely produce life, and these lives will definitely develop into intelligent civilizations (f1=1, f2=1), and 1/4 of these civilizations will develop into interstellar civilizations that want to communicate with other civilizations (f3=1/4), and this civilization can last for 10,000 years (L=10000), then there should be at least 6,250 civilizations in the galaxy that are eager to communicate with us. (The "year" here is based on the length of the Earth's orbit around the Sun for our convenience, i.e., the sidereal year.)
When you wake up on a strange planet, all you see is desolate rocks and daylight......
(Source: NASA)
In fact, the number of 6,250 is only the number of newborn intelligent alien civilizations that will be witnessed within 10,000 years of civilization's existence, and does not even include civilizations that existed before this and civilizations in other galaxy systems, so it should actually be much larger than 6,250 civilizations!
The universe should be filled with noisy voices from various civilizations. But now, the galaxy is dead silent, and humanity can't hear back with the most advanced technology, so, in this silence, that creepy thought asks from behind us, where has everyone gone?
Countless galaxies are like countless pairs of eyes (Source: ESA)
So at this moment, we have a deep understanding of why Luo Ji in "The Three-Body Problem" suffers from severe starry sky phobia - in this boundless starry sky, there should be countless extraterrestrial life, and we don't know anything about it.
So everyone, where have they gone?
This is known as the "Fermi Paradox". There are paradoxes, and naturally there is no shortage of answers, and we will introduce you to the interesting answers, see you next time!
Fermi's paradox
The "Fermi Paradox" was proposed by Enrik Fermi, a great physicist of the 20th century and winner of the Nobel Prize in Physics, in 1950.
On this day, Fermi discussed take-off saucers and aliens with his colleagues Edward Taylor, Emil Konopinski, and Herbert Yorke over lunch. The four of them first discussed the possibility of UFO reports, and then discussed the world beyond Earth, and then Fermi asked, "Where are they?" and then the four of them were full of interest and made simple calculations (this is Fermi's best trick, estimating the huge number, which later became known as the Fermi problem), and came to the conclusion that we should have seen these aliens many times a long time ago!
In fact, in addition to Fermi, Konstantin Tsiolkovsky (1933), a famous cosmonaut and aerospace propulsion expert from the former Soviet Union, David Vieuin (1975), a British engineer, and Michael Hart (1975), a British astronomer, have independently proposed this paradox and given their own explanations.
Editor: Zhao Ruxin
Proofreading: Liu Qijun