Introduction:

• Plan today is to try and solve the paradox from Living Universe E1. If life should be everywhere and we don’t see it, then what happened?
• The Fermi paradox: Why don't we see life everywhere?

Simple Solutions:

• The “Rare Earth” solution just means that in some way, Earth-like planets are very rare. Maybe there is some mechanism that stops rocky planets from forming in the habitable zone.
• The “Rare Chemistry” solution means that maybe Earth got lucky in having just the right mix of chemicals to support life.
• The “Rare Intelligence” solution means that there is some factor which limits the development of intelligent life. 
• Finally, the “Rare Technology” solution means that intelligent life develops but rarely in a way that develops tools/technology. 

Other Mechanism for restricting life:

• “Shielded Earth” Hypothesis in which our Earth is protected by our special solar system. We know that Jupiter has absorbed many asteroids/comets that could have instead hit us and wiped out life on the planet. 
• “Early self-limiting life” where an early stage of life does something that inhibits growth or kills life all together.
• Another possible limit on life is “late annihilation” where a civilization becomes technologically advanced but destroys itself with nuclear weapons.

Fermi paradox with life:

• First, there is the “Firstborn” hypothesis. The idea is that life is as common as we think it is, we are just the first.
• Another possibility is the “zoo world” hypothesis. Where Earth is an experiment by aliens to observe how life evolves. 
• A similar possibility is the “prime directive” hypothesis. This is popularized in star trek, where there is a collaboration of advanced civilizations that hide themselves from new civilizations until they reach a certain technology level.
• A more sinister version is the “Dark forest” hypothesis. This is the idea that there are indeed many civilizations that are technologically advanced, but they are all hiding from each other.
• A less sinister version is the “Secluded world” hypothesis. Where a civilization just isn’t interested in exploring, they develop really good virtual reality technology and end up just playing games and simulations instead of exploring.
• Finally, the least sinister is the “Transcension hypothesis”. Where some technology vastly beyond our understanding allows a civilization to leave our universe altogether and venture into some different plane of existence. 

Links to Science Outreach Material:

• McDonald Institute
• Royal Astronomical Society
• Astronomy on Tap

Special thanks to Colin Vendromin for the music, also thanks to Zac Kenny for the logo!