The universe is approximately 13.8 billion years old, contains an estimated 2 trillion galaxies, each with hundreds of billions of stars, and many of those stars host multiple planets. Given these astronomical numbers, the question is not whether extraterrestrial life could exist — but why we haven't found it yet. This is the heart of the Fermi Paradox, named after physicist Enrico Fermi who in 1950 famously asked: «Where is everybody?»
Drake Equation (1961): N = R* × fp × ne × fl × fi × fc × L. Estimates for N vary from <1 to billions — demonstrating both the scale of the question and the depth of our uncertainty.
The Kardashev Scale
Soviet astronomer Nikolai Kardashev proposed a framework in 1964 classifying civilizations by energy use. A Type I civilization harnesses all energy from its home planet (~1016 W), Type II from its star (~1026 W), and Type III from its entire galaxy (~1036 W). Humanity today is at approximately 0.73 on this scale. A Type II civilization could construct a Dyson sphere around its star — something we might detect as anomalous infrared emission from distant stars.
The Arecibo Message (1974)
On November 16, 1974, scientists led by Frank Drake and Carl Sagan transmitted the Arecibo Message toward globular cluster M13 — roughly 25,000 light-years away. The 1,679-bit binary message encoded basic mathematics, atomic numbers, DNA structure, a human figure, and the Arecibo telescope itself. It was largely symbolic (any response would take 50,000 years), but represented humanity's first deliberate attempt to contact alien intelligence.
«If we ever hear something from space, I think we should be very cautious about answering back.»
— Stephen Hawking, on METI (Messaging Extraterrestrial Intelligence), 2010The Voyager Golden Record
In 1977, both Voyager spacecraft carried a golden phonographic disc with 115 images, greetings in 55 languages, 90 minutes of music, and sounds of Earth. Chosen by a committee chaired by Carl Sagan, the Golden Record serves as both a cosmic message in a bottle and a cultural testament. Voyager 1, now in interstellar space, carries this record indefinitely.
The WOW! Signal and Its Successors
On August 15, 1977, astronomer Jerry Ehman at Ohio State University detected a 72-second narrowband radio signal from the direction of Sagittarius, matching all expected characteristics of an extraterrestrial transmission. He circled it on a printout and wrote “Wow!” — and it was never detected again. In 2016, the BLC1 signal from Proxima Centauri caused brief excitement before being identified as terrestrial interference. SETI has been systematically surveying the sky since 1960 with no confirmed result.
The “Great Filter” and Our Future
The Great Filter hypothesis (Robin Hanson, 1998) proposes that some catastrophic barrier exists on the path to advanced civilizations. If this filter is behind us — the emergence of complex life was extraordinarily improbable — we may be exceptionally rare. If it's ahead, the outlook for long-surviving civilizations is grim. Finding primitive life elsewhere would actually be terrible news, as it would mean the filter is still ahead of us.
- Drake & Sagan – Science, 1975: Arecibo Message
- Kardashev – Soviet Astronomy, 1964: Transmission of Information by Extraterrestrial Civilizations
- SETI Institute – Search for Extraterrestrial Intelligence
- Hanson – The Great Filter, 1998
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