The Doppelgänger Distance

In the latest American Scientist, I came across a book review of Paul Davies’ new book The Eerie Silence. It’s another take on the old Fermi Paradox that bedevils the SETI crowd. Very briefly, it goes like this: if aliens exist, where are they? It sounds flip, but the more you pick at it, the more you realize it’s a significant puzzle. Shouldn’t we at least be able to detect other civilizations, since they’ve had ten billion years or so to get busy? You’d have to guess that we’re arriving late to the party. Only… where is everybody?

In this context, Davies touches on the concept of the Great Filter (which I’ve discussed before). The idea here is that, gosh darn it, intelligent life IS rare. And why? Here’s a quote from the review:

… given that we don’t see any evidence that other intelligent creatures have taken over big chunks of the cosmos, some Great Filter must be operating to prevent life from evolving to the point of colonizing our galaxy. […] Perhaps we’ve already made it through the Great Filter and will go on to colonize the visible universe ourselves. But it may be the case that civilizations as advanced as ours typically go on to destroy themselves before they reach the star-hopping phase, and that we have a Great Filter in our future.

This is why I’m interested in the the Doppelgänger Distance. It’s the distance from earth at which, given our current technology, we could hear the noise made by an exact copy of the current earth, assuming this noise was arriving right now (i.e. don’t worry about the time of transit). The Doppelgänger Distance could grow with either the sensitivity of our ears or the noise of our voices. Assume the listener knows where to listen, but the speaker is making no special attempt to be heard.

As the Doppelgänger Distance gets larger, we can feel better and better about having passed through the Great Filter. This is because, even if we are eventually so foolish as to do ourselves in, at least we will have rocked the neighborhood (which neighborhood being our Twin-o-Sphere, or the spherical volume for which the the Doppelgänger Distance is the radius). Someone clever in the vicinity will have heard us. And more importantly, we will have heard them. So: if we don’t hear anybody at all, then maybe we’re the first. Which would place the Great Filter behind us (in all likelihood).

If we make it this far and then wipe ourselves out, that would totally suck.