Are we alone? The scary thing about the Drake equation - Fermi Paradox

I have always loved space and was very excited by the Viking Program for the exploration of Mars and followed the various space probes to the bigger outer gas giants and their corresponding moons. The robotic space probes Voyager, Voyager2, New Horizons, etc sent back excellent photographs and terrific data, they expanded our knowledge of the universe. Then probes sent to travelling asteroids and comets, that gave us a fresher experience of these visitors and their elliptic journey within our solar system from slinging close to our sun to the extremely cold temperatures of far beyond. With the discovery of an underground lake of water on Mars, a subsurface ocean on Europa (a moon of Jupiter) and Enceladus (a moon of Saturn). Finding liquid water as essential in our thinking for life, "as we know it", has rekindled our efforts to find life, be it microscopic or primitive in our solar system. Then, there are millions and millions of galaxies in the universe. And each of these galaxies contains billions of stars, like our Sun and each may have its own unique system of planets.  When looking at the movies and TV series Star Trek, Aliens, Thor, Star Wars, Dr Who, etc one gets the idea that the universe should be streaming with life and extraterrestrial aliens. One would ask where is ET?

We have been searching for life beyond our planet since people could look at the heavens.  Giovanni Schiaparelli, who in 1877 reported the appearance of certain long, thin lines he called Canali on Mars. These canals had been postulated by various people to be built by an advanced alien civilization to transfer water and maybe act as a means of transportation from its poles to hotter regions around the equator. But by the early 20th century, the improved astronomical observations revealed that the "canals" to be an optical illusion, and modern high-resolution mapping of the Martian surface by overflying spacecraft shows no canals.  On August 15, 1977, Ohio State University's Big Ear radio telescope when scanning the sky for radio signals discovered an abnormality in an entire signal sequence which lasted for a 72-second window in the 1420 MHz bandwidth from the region of the sky in question lies northwest of the globular cluster M55, in the constellation Sagittarius. As at that time the scientists had to go through the computer output manually, and astronomer Dr Jerry R. Ehman was so excited that he circled the frequency numbers and wrote "WOW!" beside it in red ink. Since then it has been officially known as the "wow signal". This abnormal frequency in the miss of low-level frequencies as at that time could not be explained to be due to any known natural phenomenon. 

We also have SETI (Search for Extraterrestrial Intelligence) this involves an array network of radio telescopes dishes all around the world which is continuously scanning the sky. This was formalized by NASA in 1971 and it is an ongoing investigation. The days of reading a manual output have a long gone, and everything is being done by computers since the data is so much. There is an Internet-based volunteer computer project SETI@home, which allows the University of California to use the processing power of your home computer along with millions of others to analyze the data. So far nothing has been found but only a fraction of the sky has been looked at and also the aliens may have moved away from radio waves and been communicating by other means or beyond our spectrum. With over 5.2 million participants worldwide, the project is the distributed computing project with the most participants to date. The original intent of SETI@home was to utilize 50,000-100,000 home computers. Since its launch on May 17, 1999, the project has logged over two million years of aggregate computing time. You could easily become involved in the search by downloading the BONIC software (Click to download)

System requirements

• There is an initial download of about 10 MB.
• You'll need about 20 MB of free disk space and 64 MB of RAM.
• With a typical computer (such as a 2 GHz Pentium 4), you'll need to let SETI@home run for at least 2 hours per week (slower computers are fine but they'll have to run proportionally more).

Dr Frank Drake an American astronomer and astrophysicist, came up in 1961 with a probabilistic equation to estimate the number of active, communicative extraterrestrial civilizations in our Milky Way galaxy. Using this equation based on our current data with the minimum numbers N should be 20 and maximum be 50 million.

where:

R_∗ = the average rate of star formation in our galaxy

f_p = the fraction of those stars that have planets

n_e = the average number of planets that can potentially support life per star that has planets

f_l = the fraction of planets that could support life that actually develop life at some point

f_i = the fraction of planets with life that actually go on to develop intelligent life (civilizations)

f_c = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space

L = the length of time for which such civilizations release detectable signals into space

Enrico Fermi an Italian American physicist proposed an argument of scale and probability. Considering there are in the range of 400 billion stars in our milky way and there are about 70 sextillions (7×10²²) stars in the observable universe. The universe is about 13.8 billion years old and the earth is 4.56 billion years old and only in the last 50 years been looking. It has been postulated that life might not be so common as thought and there might be "great filters". These are like great barriers and prevent life from developing from simple to complex. These great barriers could have either been behind us which means that we are unique and really special or in front of us which means that we would be destroyed ultimately and when all civilizations reach this stage they all auto-destruct. 

The barriers may be

1. The right star system (including organics and potentially habitable planets)
2. Reproductive molecules (e.g., RNA)
3. Simple (prokaryotic) single-cell life
4. Complex (eukaryotic) single-cell life
5. Sexual reproduction
6. Multi-cell life
7. Tool-using animals with big brains
8. Where we are now
9. Colonization explosion.

The filter if after then we have not reached it yet then either of the scary scenarios could pan out; 

1. Artificial intelligence. We might develop an AI which might decide that we are the problem and wipe us out like in the Terminator movies or the Matrix.
2. Biotechnology. We develop a virus or release a modified airborne virus that kills all people instantly. 
3. Cyber attack - All the systems are down and it makes it impossible for us to communicate. We have grown so independent on technology that all the systems are dependent on one another.
4. Global warming - We tip the Earths global weather system and there are hurricanes and tornadoes, drought and floods. Effects of global warming include loss of biodiversity, stresses to existing food-producing systems, increased spread of known infectious diseases such as malaria, and rapid mutation of microorganisms
5. Environmental disaster - An environmental or ecological disaster, such as world crop failure and collapse of ecosystem services, could be induced by the present trends of overpopulation, economic development, etc. Bees which pollinate a large number of our food source have been mysteriously disappearing.
6. Mineral resource exhaustion - Earth's capacity to sustain human populations and consumption levels — is bound to decrease sometime in the future as Earth's finite stock of mineral resources is presently being extracted and put to use; and consequently, that the world economy as a whole is heading towards an inevitable future collapse, leading to the demise of human civilization itself
7. Experimental technology accident like a nuclear accident or the Large Hadron Collider might start a chain-reaction global disaster involving black holes, strangelets, or false vacuum states. 
8. Nanotechnology - These microscopic Nanomites, nanorobots or nanites which destroy everything and convert it into an useless goo. 
9. Warfare and mass destruction - Nuclear war setting off a chain reaction. With there being up to 14,000 bombs in the world. There have been near misses from misinterpreted signals to computer errors. That of note is during the Cuban missile crisis, Norwegian Rocket Incident, Computer error on 9th of November, 1979 which indicated a nonexistent large-scale Soviet attack, etc.
10. World population and agricultural crisis, as the Green Revolution transformed agriculture around the globe, world grain production increased by 250%. Food population did not keep up initially and people die from lack of food.

(There is a list of corresponding movies)

And this the scary thing about Drake equation the is the letter "L" which denotes, "the length of time for which such civilizations release detectable signals into space". In essence, how long will the civilization exist? Do all civilizations autodestruct at one point or another. So if we come across a dead civilization it would mean disaster for us. As it shows that the great filter is after us and we have to be careful how we proceed. We may want to study them and find out what caused their demise to prevent us from going into that trap. They could have something like Star Trek's, "Prime directive" - not to interfere with the development of a pre-warp civilization. Warp drive is the fictitious faster-than-light (FTL) spacecraft propulsion system that spaceships bend space and time to move.  As these aliens would have been able to travel the vastness of space in a fraction of a second and have far superior technology. They would have been able to conquer various diseases, download their consciousness, expanded beyond their planet, been able to merge their biological bodies with technology and extend their lives. To us, they would be like gods. It will be like us trying to have a relationship with a lower life form like a rat or even a monkey, to a monkey how would we explain the concept of self or even money. The smartest one among us, who understands quantum mechanics, nuclear physics, relativity, etc may be like their children. Another thing to fear, by looking at our experience in our world whenever two civilization meets the weaker one suffers. Look at the Europeans in America, Africa and Asia they decimated the America populations (Maya, Aztec, Teotihuacan, etc) the number of native Americans left are very small. Then, there are the Australian Aborigines who are still a minority in their country. 

Astronomer Seth Shostak stated in 2004 that he expects to get a conclusive signal and proof of alien contact between 2020 and 2025, based on the Drake equation