I think a more credible hypothesis would be if certain STDs induce promiscuity in humans, as that would make a hell of a lot more sense.

The only reason I can think of to travel to another solar system(in person) would be to escape intolerable conditions at home(which is the main reason people left europe for the Americas). Even if all we wanted was to ensure the propagation of our species in a distributed environment, we would only need to send DNA patterns and a few clonable cells and raise them via computer/robotic tending. As long as our “perfect” virtual reality holds out, there would be no need for US to go in person.

On the other hand, I am insatiably nosy, so if I had an unlimited lifespan and lots of money, I’d go,,,

,,,and here’s a sample of a planet that might be an example of a world sized, virtual reality generating computer. It absorbs nearly all incident light. Perfect for powering the darned thing.

http://io9.com/5830089/strange-alien-planet-is-impossibly-black

GAry 7

Dutch Railroader makes a very good case. Niven and Pournelle essentially reduced the argument to the “crazy Eddy” hypothesis: no matter how unlikely interstellar travel is, some civilization, somewhere has done it.

Yes, but that civilization is only 50,000 years old, located 20,000 light years away, and even at a brisk pace of 2% of the speed of light isn’t going to reach us for another 950,000 years.

Or they were only 10,000 light years away, and they swung by Sol 500,000 years ago, and are currently 10,000 light years past us, which is how long it will take for them to see our radio signals and realize Earth now is home to a civilization of its own.

The universe is very big. “Okay but at least one civilization must be exploring the galaxy” does not justify the “no or very few aliens” interpretation of the Fermi Paradox.

If you have N civilizations and no one’s come by, then you must concoct a set of reasons such that when you put them all together, you credibly make a case that this is the most likely outcome.

No, you don’t! It does not have to be more likely that aliens wouldn’t/couldn’t visit us than they would. It only has to be likely enough that you can’t reject the possibility. For concluding that alien civilizations don’t exist or are exceedingly rare, I’d say you’d need at the very least a 95% confidence interval, and there’s no way you or anyone else can put forward figures solid enough to do that.

Concluding that alien civilizations must be rare based on the evidence that none have made themselves apparent to us yet is simply an unsupported conclusion.

If N=10, say, then sure, it’s easy to concoct reasons for no visitation. But for N=10^5, it’s much harder to make a convincing argument that allows for no leakage. Sure, some don’t like space travel, some don’t figure it out, some blow themselves up, some think it’s best to leave us alone, and so on. But when you’re all done *no one* has ever stopped by?

Okay, I was wondering what you thought of as “very, very rare”. But if just 10,000 civilizations in a galaxy 100,000 light years across with several hundred billion stars counts as “not exceptionally rare” (and I’d tend to agree) then this is hardly a mystery at all. In this case, I’m sure it could be shown that the current state of affairs is not improbable even assuming that every alien race could and would exponentially explore the galaxy and say “Hi” to every sentient race they meet.

Here, all you need is the probability that an arbitrary species based on location and the time since they began their expansion and estimated rate of expansion that they would have reach earth before now, and also reached earth after there were humans sophisticated enough to understand what was happening and record it. “No one has ever stopped by” (emphasis mine) is just an unsubstantiated assumption.

It could easily take millions of years for a species to cross a significant portion of the galaxy, and this fact alone makes it quite likely that alien explorers either haven’t reached us yet, or they did reach us, came down to say “hi”, but there was nobody but primitive humans to greet them, who recorded the event in cave paintings as a god descending from the sky. Or they were greeted by dinosaurs. Compared to the time span of the endeavor, the window in which aliens
would have to have come by for us to be certain today that they did visit is very, very small.

Multiply in the odds for every other factor that could get in the way, and even with millions of civilizations I don’t think there’s anything particularly surprising about the apparent non-existence of aliens.

It’s when N gets large that I’m more and more forced to adopt blanket assumptions about the unknown…

Well, that’s the problem, you have to make assumptions one way or another to show that the odds of visitation are so high that the only conclusion we can make is that they don’t exist to come visit.

To draw no such conclusion requires no assumptions.

The Fermi Paradox is only a paradox if you make a bunch of bad assumptions.

You should read Octavia Butler’s ‘Lilith’s Brood’ trilogy for an extremely interesting take on your premise.

About the only thing it implies is that if advanced alien civilizations are common, then FTL is probably either literally or practically impossible.

I’m perfectly happy to resolve the Fermi “paradox” with the reasoning that “advanced” alien civilisations are probably very rare, and FTL travel is literally impossible.