Nick Bostrom wrote an interesting article for the MIT Technology Review about how he hopes that the search for life on Mars finds nothing. In it, he reasons that inasmuch as we haven't come across any signs of intelligent life in the universe yet, advanced life must be rare. But since conditions for life aren't particularly stringent, there must be a "great filter" that prevents life from evolving beyond a certain point. If we are indeed alone, that probably means that we have made it through the filter. But if life is found nearby, like in our solar system, then the filter is probably ahead of us, or at least ahead of the evolutionary stage of the life that we find. And the more advanced the life form that we find, the more likely that we have yet to hit the filter, which implies ultimate doom for us.

But I wonder about some of the assumptions in this argument. He argues that intelligent ETs must not exist because they most certainly should have colonized the galaxy via von Neumann probes but apparently have not done so because we do not observe them. It seems to me, however, that it is certainly plausible that a sufficiently advanced civilization can be effectively cloaked from a far less advanced one. Mastery of some of those other 6 or 7 spatial dimensions that string theory predicts comes to mind. Or invisibility via some form of electromagnetic cloaking. And those are only early 21st century ideas. Imagine the possibilities of being invisible in a couple hundred years.

Then there is the programmed reality model. If the programmers placed multiple species in the galaxy for "players" to inhabit, it would certainly not be hard to keep some from interacting with each other, e.g. until the lesser civilization proves its ability to play nicely. Think about how some virtual reality games allow the players to walk through walls. It is a simple matter to maintain multiple domains of existence in a single programmed construct!  More support for the programmed reality model?...

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