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Where Is Everybody? - The Fermi Paradox and the Great Filter Explained

I have been obsessed with space and astronomy since I was a child. I love writing and making videos about topics in astronomy.


NASA estimates that there are more than 10 billion terrestrial planets spread throughout our Milky Way galaxy.1 There are at least 1500 planets within 50 light-years of Earth.2 And the Milky Way is just one of an estimated two trillion galaxies in the observable universe.3 Astronomers came up with this estimate using the Hubble Space Telescope. They took pictures of small parts of the sky, counted the number of visible galaxies in those small regions, and then multiplied those numbers to account for all regions of the sky. If there are 10 billion or so rocky planets in each galaxy, odds are high that intelligent life exists on other worlds.

Despite the odds of intelligent life existing all over the universe, there is no evidence of it. In 1960, astronomer Frank Drake performed the first modern Search for Extraterrestrial Intelligence (SETI) experiment, named Project Ozma. Since then, SETI has been scanning the sky, unsuccessfully searching for signals from alien civilizations. However, decades of failure don't mean no one is out there.4 SETI Astronomer Seth Shostak outlines the difficulties of detecting advanced alien signals in his book Confessions of an Alien Hunter. If there are lots of other intelligent civilizations, why have we detected no evidence of their existence?

A Hubble Space Telescope image containing thousands of galaxies of various ages, sizes, shapes, and colors

A Hubble Space Telescope image containing thousands of galaxies of various ages, sizes, shapes, and colors

What Is the Fermi Paradox?

Fermi’s paradox is the conflict between the lack of evidence for intelligent life on other planets and the probability of their existence.5 In 1950, during a discussion with fellow physicists about reported UFO sightings and the possibility of Faster-than-Light (FTL) travel, Fermi is said to have asked his now-famous question, "But where is everybody?"

There are many possible reasons for the Fermi Paradox. Physicist Stephen Webb lists dozens of possible explanations in his book If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY?, Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life.

Some potential explanations are:

  • Complex intelligent life exists only on Earth
  • Advanced civilizations die out before they develop the capability to engage in interstellar travel
  • The universe is so vast the likelihood of two intelligent civilizations meeting is low. In other words, intelligent civilizations are too far away to detect. The diameter of the Milky Way galaxy is more than 150,000 light-years. A light-year is about 5.88 trillion miles (9.46 trillion kilometers)
  • The zoo hypothesis suggests that intelligent civilizations are observing us without revealing their presence
  • Advanced alien civilizations are sending signals our way, but we do not recognize them, or we are listening at the incorrect frequency
  • We are one of the first intelligent civilizations in the galaxy, and it will be thousands of years before other intelligent civilizations advance sufficiently to converse with us

The Great Filter, an idea developed by Economist Robin Hanson, is another possible explanation.

What Is the Great Filter?

Robin Hanson posits that there are many obstacles to becoming an interstellar species. One of those obstacles, the Great Filter, might be tougher than the rest. According to Hanson, a civilization must go through several critical steps to become interplanetary explorers. One or more of these steps might be highly improbable.6

These are some of the critical steps:

  • A planet capable of supporting life
  • Life must develop on that planet
  • That life must be able to reproduce
  • Simple cells must evolve into complex cells
  • Multi-cellular organisms must develop
  • Sexual reproduction must develop to increase genetic diversity
  • Complex organisms must evolve the capability to use tools
  • Intelligent lifeforms must create the advanced technology needed to engage in space travel and colonization
  • This intelligent species must colonize other star systems without destroying themselves
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Any one of these steps could represent a Great Filter.

  • Planets capable of supporting life are unlikely to be that Great Filter. According to NASA scientists, the recipe for earthly life, water, elements associated with life, and sources of energy, are common on exoplanets (planets that orbit around other stars)
  • Scientists don't know whether life develops easily. Even if single-celled organisms develop easily, multi-cellular organisms might not
  • If intelligent species do develop, they may eventually destroy themselves. Some potential dangers to advanced civilizations are destructive weaponry, malevolent artificial intelligence (AI), and hazardous nanomaterials

An asteroid strike could wipe out an intelligent species if they don't develop technology to divert an incoming asteroid. Supernovae are a bigger danger because there is no way for an advanced species to stop them. A supernova is the explosion of a star. According to the European Southern Observatory (ESO), a supernova explosion within about 25 light-years of Earth could strip away our atmosphere and cause life to perish. X-rays emitted from Supernovae are lethal up to 160 light-years away. These x-rays can destroy the ozone layer, exposing living creatures to harmful UV rays.7 Supernova explosions may have played a role in past extinctions on Earth, like the Pliocene marine megafauna extinction, which killed about a third of Earth’s large marine species.8 However, a very advanced civilization may be able to survive even a nearby supernova if they have spread to multiple planets and space stations or developed the technology to survive long-term in domes.

Has humanity made it through the great filter? Unfortunately, we don’t know. If the great filter is ahead of us, our species may be doomed to extinction. If it is behind us, our descendants may one day colonize worlds around distant suns.


1. There could be up to 10 billion warm and cozy Earth-like planets in our home galaxy, new research reveals

2. The Milky Way's 100 Billion Planets

3. A universe of 2 trillion galaxies

4. If extraterrestrials are out there, why haven't we found them?

5. What is the Fermi Paradox?

6. The Great Filter: a possible solution to the Fermi Paradox

7. How A Star’s Mass Affects Its Supernova Explosion

8. A Supernova 2.6 Million Years Ago Could Have Wiped Out the Ocean’s Large Animals

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2022 LT Wright

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