1. The Fermi Paradox and the Great Filter
Or, Why Isn't Anybody There?
By Kenny Bolster

2. The Great Filter
This term is taken from the paper “The Great Filter – Are We Almost Past It?” (1998) by Robin Hanson
A paper expressing a similar idea with somewhat better readability is Nick Bostrom's “Where Are They? - Why I Hope the Search for Extraterrestrial Life Finds Nothing” (2008)

3. What is the Great Filter?
We don't see evidence of aliens
According to our calculations, we should have seen evidence of aliens already
The Great Filter is whatever unknown factor(s) is/are preventing civilizations from colonizing the galaxy

4. Advantages of Colonization
You may notice the picture in the bottom right as our own dearly beloved “Bold Traveler”.
The green specks in the picture of the center are Pseudomonas syringae, a bacteria that lives in clouds.

5. How to Take Over the Galaxy
This is an artist's rendering of a von Neumann probe.
Chief resource needed by an interstellar civilization is energy/negentropy
Best source of that is stars
Solar panels, Dyson spheres, Star Lifting
All the light shining off of the stars in our galaxy is being “wasted”
Why can we still see those stars?

6. Colonization Speed Average distance between stars: 5.4112 light- years
Acceleration possible using solar sails for interstellar travel: 1 mm/s^2
Total diameter of the galaxy: 120,000 ly
Typical estimates for the rate of acceleration possible using solar sails can be used to calculate the rate at which it would be possible to colonize the galaxy
Average distance between stars: light-years
Acceleration possible using solar sails for interstellar travel: 1 mm/s^2
Total diameter of the galaxy: 120,000 ly

7. Colonization Time
10,077, years

8. In 13,000,000,000 Years...

9. The Drake Equation

10. Hanson's Argument The right star system (including organics)
Reproductive something (e.g. RNA)
Simple (prokaryotic) single-cell life
Complex (archaeatic & eukaryotic) single-cell life
Sexual reproduction
Multi-cell life
Tool-using animals with big brains
Where we are now
Colonization explosion

11. How to Find a Filter Left is Nick Bostrom Right is Hanson
Bostrom proposed two ideas: development should have taken a long time, and only happened once
Hanson pointed out a flaw in one of them: Assuming that there is a limited window in which a development can occur, observing how long it took doesn't give you much evidence, as long as you know it happened.
This assumes the window, which is questionable, but the “only happened once” thing works fine

12. How Right is the Right Star?

13. That First Magical Molecule
Russel's white smoker theory
Primoridial soup, which could have formed under a layer of ice, which would protect it from UV rays
RNA World, but it's difficult to see how it could start
Persperia, which makes Great Filter even worse

14. Little Bugs We're not sure exactly how first cells formed
Important thing to note is that if the white smoker hypothesis is correct, formation of single celled organisms happened multiple times, in different ways
It also seems pretty easy to have some sort of gene in an RNA which would cover that RNA and give it some sort of protection

15. The Incompetent Predator
Key things: seems objectively improbable, and it only happened once
Or maybe not. Chloroplasts?

16. Why Do We Get Naked, Again?
If you go to the wiki page on “Evolution of Sexual Reproduction”, you find lots of discussion of how sex is advantageous from a natural selection perspective, but there are very few plausible ideas about how sex could have actually gotten started
Best theory says that a bacteria engulfed an archaea, that had been infected by a virus at a certain stage of its life cycle, etc
Cambrian Explosion happened basically right afterwards

17. Something Like Actual Aliens
Multicellular animals appeared more than once
It looks like it happened all over the place once we had eukaryotic cells and sex

18. Cave-Aliens with Clubs
Has happened a lot
Basically anytime you have social pack hunters, you get intelligence
A lot of the time, they use tools

19. Is It Safe to Come Out Now?

20. We need something on the order of:
So What's the Total?
We need something on the order of:
If the evolution of life on earth involved 1 in a million events, two would be more than enough
Assuming that the earth pathway is the only pathway, so our filter is probably smaller than we would think
Also assuming we don't find out that sex and eukaryotes were really easy developments
Also don't forget origin of life

21. What Else Could Go Wrong?

24. Final Thoughts