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Originally Posted by Johnny Rotten
It depends on who you ask and where you get your information. Some people quantify the subject in terms of the Drake Equation and Fermi's Paradox, and are content with keeping the topic well within philosophical or mathematical constraints.
Take Von Neumann machines, for example. In this hypothesis, a given civilization sends out self-replicating devices. These devices, in multiplying, each branch out in different directions. The end result is a geometric spread that covers a given space, like a galaxy, within ten thousand years. The size of the space is finite while the devices replicate, so it takes only one civilization and one self-replicating probe to blanket an enormous expanse. Given the results of the Drake Equation, it is highly likely that this phenomenon has already occurred in our own cosmic neighborhood.
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The 10,000 years is questionable -- but if it takes 1000 years to move to a nearby star, 20% of probes that are sent make it and find suitable material to produce an industrial base, 10,000 years to bootstrap from a probe to an industrial base capable of producing a probe every 1000 years, and the industrial base collapses after an average of 10,000 years...
Then every 21,000 years the number of probes double.
There are about 100 billion stars in the milky way, which is less than 2^30. That means it takes less than 630,000 to have as many probes as there are stars in the milky way.
The universe is on the order of 10 billion years old. So this is 0.00000063% of the lifetime of the universe -- the blink of an eye.
So what are the odds that an intelligent, spacefaring race would be more advanced than us, but not sufficiently more advanced than us that they have already colonized the entire galaxy?
Of course, the above math could be tweaked -- what if it takes 1 billion years to bootstrap from a probe to an industrial base capable of building probes?
[SNIP misquoted text]