[extropy-chat] cosmic silence

[Christopher Healey]

Spike,

> Well, no.  I'm focused on the galaxy for now.  I'm
> not ready to accept the notion that technological
> civilizations are short-lived.  The Drake equation,
> as I understand it, suggests tech-capable civs must
> be short lived.  Otherwise we are hard up for an
> explanation for why we aren't hearing or seeing
> them.
>
> spike
> 

Dan Clemmensen made a really important point regarding signal efficiency resulting in a signal that approxiamtes noise.  

Any civilization utilizing broadcast radio transmissions is going to quickly become concerned with achieving optimal bandwidth efficiency.  This basically means compressing the data as much as possible.  As the data stream becomes more completely compressed, its bit-length will approach its Kolmogorov complexity.  If there are still regularities in the transmission, then it follows that those regularities could be taken advantage of for further compression.  We are left with what appears to be noise.

That should leave us with 2 possible detection scenarios.  

The first would be the detection of their radio light-sphere.  This doesn't seem likely at all, given the point above.  At the point where advanced compression comes into regular use, the light sphere will appear to terminate, leaving us with a "light-shell" of time during which to detect anything at all.  I think that 300 years from the discovery of radio would be a liberal estimate for that window of opportunity.  The light-shell has either already past us, or is still a long-time-coming.

Even if we can statistically detect a regular signal at less than the strength of the background radiation, the dispersion over stellar distances would also radically weaken the signal to the point where it may be undiscernable.  Further, once the signal is indistinguishable from noise due to compression, a directional power spike in the noise would be the only way to notice anything at all.  In other words, once compressed, unless we're close enough to see a power spike in the noise, it'll all be background.

The second scenario would be receipt of an intentionally broadcast signal, designed to be detected easily.  I'd posit a massively parallel array of directed transmitters, each emitting its directional beam at individual near-field star systems, in order to maximize signal strength.  100 transmitters for 100 stars.  This would be the only type of signal we can HOPE of detecting.  

But would another civilization actually start doing this and sustain the effort for millenia or longer?  I'm skeptical.  If they have survived past their technological infancy, it seems like taking on a lot of risks for few benefits.  I'd expect a civilization that had reached that point of development to be much more coordinated in its actions, and I'm not sure benign curiosity would be a conservative guess on out part; nor would they expect such of us.  It would be a contingency that needed to be addressed before taking reckless action.  The response window to any future actions taken by either side would be very limited, assuming eventual near-c-velocity travel, supporting this stance.  I'd expect this to result in a lot of bias away from emitting EM radiation.

I'm all for radio telescopy in general.  SETI, however, is minesweeping with binoculars.  You'd expect the mines to be buried, but they're hoping one *is* visible.

-Chris Healey