The Fermi ParadoxWacko argumentFermiAll relatedBehind the FridgeIs this typical?TimescalesFirst come, first servedThe ConditionDoes our Galaxy Meet the Condition?Are we first?Interstellar ColonizationSpace is BigTake it Slowly1% cHow would it workRobots or HumansBudgetSmall ChangeRealistic?MotivationConvicts or PilgrimsThe New WorldThe Australian ExampleRepeatExponential GrowthSpeed of LightBlink of an EyelidLower limitSo - why haven’t aliens visited Earth?MavericksDifferent NumbersPrime DirectiveSelf DestructionTwo AlternativesThe Fermi ParadoxSo is there life in space? In addition to the issues we’ve already discussed, there is a rather interesting philosophical argument.Wacko argument•Bear in mind that this argument is supported by a lot of very intelligent people, so do not dismiss it out of hand.•But also bear in mind that a lot of equally intelligent people think that this argument is nonsense.•You may think that it is nonsense, but you may slowly begin to see the force in it.Fermi•This paradox was proposed by Enrico Fermi, he was the man who built the world’s first nuclear reactor. •Consider the question - why did life only start once on Earth?All related•Life got started on Earth pretty quickly. To some, this suggests that life forms easily, whenever conditions are right.•So why are all creatures on Earth descended from the same microbe?•You can tell from the similarities in our DNA and cells that all living things come from the same ancestors. Why?Behind the Fridge•Let’s say the first self-reproducing chemical (whatever it was) appeared in a pool of slime in Africa. Why couldn’t a second self-reproducing chemical have appeared in say the North America, and a third in the pool of slime behind your fridge, for example.•Then there might be three completely unrelated families of life-forms on Earth.Is this typical?•Is this likely to be typical of all planets? What do you think? Will most inhabited planets have only a single family of life-forms?•One explanation for this is as follows: life forms take hundreds of millions of years to evolve.•But how long will it take one to spread?Timescales•Even the most primitive organism will probably be carried around the world in only a few thousand years, by the wind and ocean currents.•So, the time needed to spread throughout the world is far less than the time needed to evolve.First come, first served•So - any new life that forms, unless it is first, probably has to compete with previously existing life.•The new life will probably lose, the old life has had time to evolve and is probably far more capable than something newly created by chance.•Hence - we are all descended from the first life form to evolve. Other subsequent life will have been wiped out by our ancestors.The Condition•So - here is the condition for there to be more than one family of life in some region:The average time needed to spread throughout the region is much greater than the average time to evolve.•This condition is not met on Earth. If a planet were much bigger than Earth, it might be met, and you would get different families of life in different regions.Does our Galaxy Meet the Condition?•Does our galaxy meet this condition, specifically for intelligence?•Once intelligence evolves on some planet, how long would it take to spread throughout the Milky Way?•Is this greater or less than the time it takes to evolve intelligence?Are we first?•If intelligent species can spread through the galaxy fast enough, then you would expect whichever species evolved first to completely colonize the galaxy before the second species even gets started.•So what are the two timescales?•Evolution (if Earth is typical) seems to take billions of years.Interstellar Colonization•How does this compare with the time needed for a typical alien species to colonize the galaxy?•The galaxy is BIG! If we scaled the Earth down to the size of a pea, the Sun would be 300 meters away, while Pluto would be orbiting 12 kilometers away.Space is Big•On this scale, the nearest other star (Proxima Centauri) would be twice around the world.•Consider that humanity’s fastest spacecraft, Voyager. It’s taken 27 years to get where it is now.•The gaps between the stars seem very intimidating. Could we ever cross them?Take it Slowly•Luckily, we have lots of time. Distances that seem impossible to humans used to dealing with weeks and months seem pretty easy when you have thousands of years at your disposal.•Consider what we could do even with current technology.1% c•With nothing much more than current technology, we could accelerate a spacecraft up to 1% of the speed of light.•At this speed, we would take about 1000 years to reach the nearest likely locations of habitable planets.•The only drawback - it would cost trillions of dollars.How would it work•A thousand years sounds like forever. But this may not be a barrier.•People could do the trip in something like suspended animation.–Frozen fertilized eggs could be sent out, implanted in artificial wombs and raised by robots.•The spacecraft could be really big and inhabited for generations.Robots or Humans•Perhaps we’d just send robots.•However; 100 years from now, medical technology may be able to do something about aging. A 1000 year trip may not sound so bad if you live to be 10,000 years old.Budget•So the time may not be an issue (and we may well figure out much faster methods of travel).•How about that colossal budget?•A few trillion dollars may sound like a lot today. But if the world’s economy continues to grow, this may become quite affordable.Small Change•If we assume 3% per year economic growth, then by the year 3000, the world economy will be 6,000,000,000,000 times bigger than it is now.•A few trillion dollars may be “small change.”Realistic?•Is this realistic? It’s not as silly as it seems. Imagine that the medieval world had wanted to build a modern oil tanker. In principle they could - they knew how to smelt iron and shape it.•But doing something like an oil tanker using medieval blacksmiths would have bankrupt the world back then. 500 years later it is easy.•Even 100 years from now, building an interstellar spacecraft may seem routine.Motivation•So - 1000 years from now, we will be able to travel to other stars quite easily. But will we?•It would be rash to speculate on what will motivate our descendents (if any) 1000