The Milky Way is the galaxy in which the Earth resides. Its one of the 100 billion galaxies in the universe. The Milky Way is a barred spiral galaxy that is about 100,000 light-years across. Our solar system is one third from the edge of the Milky way. The closest galaxy to our Milky Way is the Andromeda Galaxy, which is bright, larger and away from Earth, at 2.3 million light-years distance.
There are estimated about 350 billions planets in our Milky Way. Is there Extraterrestrial Civilizations in our Milky Way. Or we commonly known as aliens or ET. To first answer this puzzle, we need mathematic and probability calculation. Let us deep dive into the Drake Equation. This equation estimates the number of detectable Extra-terrestrials Civilizations in our Milky Way, supposing our present electromagnetic detection methods propagate seamlessly well.
- The Drake equation states:
- N = Ns x fp x ne x fl x fi x fc x fL
- N = number of alien civilizations in the Milky Way
- Ns = estimated number of stars in the Milky Way;
- fp = fraction or percentage of these stars with planets on its orbits;
- ne = average number of these planets with potential to host life as we know it;
- fl = percentage of these planets that actually develop life;
- fi = percentage of these planets that actually develop intelligence on human level;
- fc = percentage of these civilizations that actually develop electromagnetic radiation emitting technologies;
- fL = percentage of these civilizations that keep emitting electromagnetic signals to space. This factor is extremely dependent on the lifetime a civilization remains electromagnetic communicative.
The L term is considered the most important one in Drake equation. We have no idea how long a technological civilization can last. From here we need to understand further the Galactic selection and the Creation of God. Anyway, we resorted to the traditional astrophysics theory for fundamental debriefing. As a result, Statistical Drake Equation (SDE) was introduced further to calculate the ETC. It is mathematically more complex and robust than the Classical Drake Equation (CDE).
The SDE is based on the Central Limit Theorem, which states that given the enough number of independent random variables with finite mean and variance, those variables will be normally distributed as represented by a Gaussian or bell curve in a plot. In this way, each of the seven factors of the Drake Equation become independent positive random variables.
SDE method will incorporate the standard variation concept, the sigma, it said how much variation exists from the average value. In this case the standard variation concept can be very high or mediumly high. We can derived the results as such besides human on earth, zero to 15,000 advanced technological societies could exist in the Milky Way. The turning key here is zero. But would not be likely true, or unlikely be true.
The universe is more than 13 -15 billion years old. Many other planets throughout the universe probably hosted intelligent life long before our Earth did. That means that even if there have been 1,000 civilizations in our own galaxy, if they live only at certain period of times. 1000 is typical most correct and most probable mathematic calculation theoretically true. Along the long pipeline of time traverse over last 15 billion years universe age. Some others won’t evolve until we are not as advanced to contact them. For us to have much chance of success in finding another contemporary active technological civilization, on average they must last much longer than our present lifetime. The average distance we should expect to find any alien intelligent life form may be 2,700 light-years from Earth. Human technologies need these light years speed to get the electromagnetic wave to the nearest civilized solar system and get a reply about the same length of time travel.
Bottom line, the human technological civilization must be able to sustain at least for more than 10,000 years in order to detect an ETC (外星人).