Thinking about SETI

[resqgeek]

A couple of weeks ago, there was a flurry of excitement when it was reported that “Russian SETI researchers are pursuing a promising signal”, as one headline put it.  It turns out that the hype was overblown, as it has been every time one of these stories has surfaced to date. While I haven’t seen that they’ve settled on a clear explanation for the signal they detected, the consensus is that it does not represent evidence of extraterrestrial intelligent life.  While that’s certainly disappointing to those, like me, who find the idea of life elsewhere in the universe fascinating, it shouldn’t be surprise.

I support the efforts of the Search for Extraterrestrial Intelligence (SETI), but I think I’m being realistic when I say that I don’t expect this effort to find any concrete evidence of any life elsewhere in my lifetime (and probably not in my daughter’s lifetime, either).  To understand why I don’t think it’s likely that we’ll detect such signals, let’s consider how signals propagate.  Isotropic signals propagate equally in all directions, so their signal strength is inversely proportional to the square of the distance from the source.  The random noise emitted by a technological society would be essentially isotropic, so that by the time the signal reached us, it would be incredibly weak, unless the signal was almost inconceivably powerful to begin with.  This is what is happening to all the radio noise we’ve been emitting into space for the last century or more...it is propagating into space more-or-less equally in all directions, getting weaker at a rate that is related to the square of the distance from Earth.  However, signal strength can be improved by focussing signals in a specified direction.  The strength of such signals will decrease much less quickly than the isotropic signals, but there’s a catch.  The signal has to be directed almost directly at you for you to detect it.  In fact, the further away you are, the more precisely it must be aimed.  If we were to detect such a signal, it would either mean we had drifted across a signal intended for someone (something) else, or else that our presence was known.  Since our own signals have only propagated out for a bit more than a century, an intentional signal beamed back at us would almost certainly have to originate from less than about a hundred light years away.  There simply aren’t very many stars within that distance.

Even if, through some miracle, we did happen to detect a signal that clearly originated from an extraterrestrial source, the almost impossible to conceive distances of interstellar space raise all kinds of other problems.  Depending on how far away the signal source is from us, the civilization that created the signal could easily have disappeared since the signal was created.  A signal from a thousand light years away is going to take a thousand years to reach us.  It would not be a greeting from a current inhabitant of that point in space, but a time capsule of those who lived there back in time.  This limitation means that even if we detect such a signal, we have no ability, using our current knowledge and tools, to engage in any meaningful conversation.  And if there is life out there that has figured out how to overcome Einstein’s universal speed limit, we simply don’t know how to detect any message they might be transmitting.

Is there life out there?  There’s no way to know, at least so far.  Personally, I think it is likely, but I will readily admit that there is zero evidence to support that belief.  Would I be excited to hear that we’d discovered such evidence?  Absolutely.  Am I holding my breath in anticipation?  Ummm...not so much.