Details all in one place. Compiled from here.
N = R * Fp * Ne * Fl * Fi * Fc * L
Types of civilizations:
Self-replicating automata. He called them "Universal Assemblers."
1981 someone arguing against ETI said that since there weren't von Neumann machines here, ETI did not exist. Segan and Newman said ETI wouldn't build such machines since it would destroy the galaxy.
You think that's a silly rebuttal. A von Neumann probe could obviously have a density ceiling, e.g. one per solar system.
If ETI exists, they would have taken over the galaxy already be here.
Assumes behavior of ETI to be aggressively expansionist.
First association of "they would be here because of expansion" with Fermi paradox.
Tipler added "self-replicating universal constructor."
Stages of space-faring civilization:
One of these must be really rare. If it's #9 we are in big trouble.
Drake's equation is too optimistic; this adds requirements of big moon, gas giant, etc.
But you say, Drake's equation builds that in with ne.
Also, science does not support the low guesses — e.g. many more planets we have spotted are rocky, Jupiter might do more harm than good, etc.
Basically states that advanced civilizations are waiting for the universe to cool.
All civilization gets wiped out by von Neumann probes.
David Brin says:
There is no need to struggle to suppress the elements of the Drake equation in order to explain the Great Silence, nor need we suggest that no ETIs anywhere would bear the cost of interstellar travel. It need only happen once for the results of this scenario to become the equilibrium conditions in the Galaxy. We would not have detected extra-terrestrial radio traffic – nor would any ETIs have ever settled on Earth – because all were killed shortly after discovering radio.
You think, then it would also just take one instance of berserk-killer probes to counteract.
The simulation hypothesis. What we see is not real.
Note: Baxter, originator of this theory, states that a type III civilization could not simulate 1 100-light-year diameter volume. So it would keep simulation small.
Earth is in a sanctuary and we are intentionally not disturbed.
Civilizations only last 6,500 years.
The state of the universal is not constant. It did not used to be conducive to life. For example, one model suggests there used to be a lot of gamma-ray bursts and only "now" is it hospitable to intelligent life. So we are among the first.
Other life has had its technological singularity and is no longer recognizable. Like brief window.
Look at black holes for signs of ETI?
We have just the right amount of water for life and land for intelligence. Maybe other worlds with water are just mostly water, so very low fi.
Maybe a lot of worlds with water are frozen over like Europa.
Named after a sci-fi story where a ship named the Aurora arrives at a moon to colonize, but finds it's lethal to them because of unidentified, tiny, primitive life form.
Assumes life forms, not robots.
Traveling between stars is hard and expensive. A civilization would expand unevenly, with gaps, and changes in commitment over time.
Destroy the other guy out of self-preservation. Based on the book. Everyone hides on purpose, and tries to kill whatever they run into. From the book:
The universe is a dark forest. Every civilization is an armed hunter stalking through the trees like a ghost, gently pushing aside branches that block the path and trying to tread without sound. Even breathing is done with care. The hunter has to be careful, because everywhere in the forest are stealthy hunters like him.
If he finds other life — another hunter, an angel or a demon, a delicate infant or a tottering old man, a fairy or a demigod — there’s only one thing he can do: open fire and eliminate them. In this forest, hell is other people. An eternal threat that any life that exposes its own existence will be swiftly wiped out. This is the picture of cosmic civilization. It’s the explanation for the Fermi Paradox.
If it is possible to build a von Neumann probe, and if ETI exists, then it must have happened already.
There would be a built-in ceiling as to how many probes would be built. Perhaps one probe per system of interest. Perhaps a million. What is that number?
There could be evolutionary pressure on the probes. For example, if berserker probes existed, pressure to avoid them or to destroy them.
Assumptions:
There are three possible extreme actions for these probes when encountering other probes:
Perhaps there are variations: for example, "negotiate" might be a variation of "do nothing, and "avoid slowly" might be a less pessimistic version of avoiding.
Strategies:
In a galaxy with a random population of these different strategies but equal capability, the do-nothings would get wiped out if there were any berserkers. Avoidance would seem like a lower-risk action than to destroy.
Is there a ceiling for detection range and acceleration/max velocity? That is, either it is or it isn't possible that there is a berserker that cannot be avoided.
What is the benefit of being a berserker? Less competition. If there is no resource being competed for, then there is no benefit (to the probe; there may be a benefit to the creators, e.g. religious prejudice is satisfied). The risk is being destroyed in turn.
What is the benefit of being police? Less competition from berserkers. Less risk from other police. The risk is being destroyed in turn.
What is the benefit of avoiding? Less risk from all. The risk is being overrun by berserkers, that is, they consume all the resources.
What is the benefit of doing nothing? Less cost. The risk is being destroyed by berserker and/or being overrun.