On Saturday, February 3, 2018 at 8:03:47 PM UTC-6, agrays...@gmail.com 
> On Saturday, February 3, 2018 at 6:03:33 PM UTC-7, John Clark wrote:
>> On Sat, Feb 3, 2018 at 5:43 PM, Lawrence Crowell <
>> goldenfield...@gmail.com> wrote:
>>> ​>​
>>> I suspect planets with complex life above that of prokaryotic-like life 
>>> are few in number per galaxy. It is hard to know how even prokaryotic-like 
>>> life starts.
>> The fact that it took 2 billion years for Prokaryotes to evolve 
>> into Eukaryotas gives some support that your suspicion may be correct. And 
>> even after complex animals have evolved on a planet that doesn't mean it 
>> has a civilization. Richard Dawkins notes that flight evolved 
>> independently 4 times and the eye at least 40 times and perhaps as many as 
>> 60, but intelligence, defined as the ability to make something as complex 
>> as a radio telescope, evolved only once, and in the nearly 4 billion year 
>> history of life that ability has only existed on this planet for about a 
>> century. And yet when we use our telescopes to listen for sounds of 
>> intelligence in the cosmos we hear only an eerie silence. Why? 
> *Because any distinguishable intelligible signal is attenuated to noise 
> when it traverses a few light years in distance. IIRC, LC once posted a 
> limit of 10 light years is sufficient for dissipation.  If so, SETI is a 
> waste of time and energy. AG*

An enormously powerful signal can get through this. Interstellar space has 
ionized hydrogen and this sets up a Debye length. So a fairly weak signal 
in the megawatt range that we generate in most transmissions they will only 
reach 10 or a few 10s of light years out. A gigawatt signal then reaches 
about 3 to 4 times the distance. The Debye  attentuation is exponential, so 
increasing power by 10 will roughly only double the distance. A terrawatt 
signal could then reach 100s of light years, and so forth. This is why we 
are able to receive radio signals from distant galaxies or pulsars that 
generate 10^{20} watts or more. These radio signals from another galaxy can 
travel rather unimpeded through intergalactic space and then due to their 
power make it through the interstellar ionized H^+ in the Milky Way. 
Similarly strong sources can make it through the galaxy. However, a pulsar 
emits radio waves with trillions of time more power than the entire energy 
economy of us humans. The magnetic field of a pulsar has as much energy per 
cubic meter as the sun produces in light within a few minutes.

So if ETI is say 100 light years away and they are casually emitting 
megawatt radio frequency radiation we may not be able to detect it. An ET 
that intentionally sends a collimated radio signal in the terrawatt range 
to us might be detectable within the 100s of light year range. If ETI is 
trying to listen in on us at say some 10s of light years out they might 
detect an excess of radio waves, and maybe a bit of modulation due to the 
Earth's rotation. They would not I think be able to pick out a 
transmissions of *Dallas*, or a radio broadcast of *Led Zeppelin*. 


>> As Enrico Fermi famously asked, where is everybody?   ​
>> ​
>> Maybe we're the first, after all somebody has to be, or maybe some 
>> catastrophe always happens to a civilization whenever it gets much beyond 
>> the point we're at now. 
>>   ​John K Clark​

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