On 6-4-2010 9:03, scott wrote:
>>> Extrapolating from the Voyager power levels and distance, I make it 
>>> that we need a 350 GW signal to be transmitted from 200 ly away.
>>
>> The SETI equipment might even be somewhat more sensitive because they 
>> are using larger disks. Then again they don't know what exact 
>> frequency to tune into.
>>
>> BTW I arrive at something more like 47 MW if the voyager is 
>> transmitting all its power, which is unlikely, so the number could 
>> even be much lower.
> 
> I assumed that the power needed goes up with the square of distance, so 
> used Google to calculate:
> 
> "(200 light years / 16.8e9 km)^2 * 23 W"
> 
> which actually gives 300 GW, but I think that's a big underestimate, as 
> presumably Voyager is directing its 23 W directly at Earth, and not 
> radiating out in a sphere. 

You are absolutely correct of course, I seem to have forgotten the 
square. :(

> If an alien world 200 ly away is radiating 
> in all directions equally, it would need to be outputting much more than 
> 300 GW in total.

That too. Probably the most efficient way to do it is by modulating the 
output of a star. Either directly (controlled starquakes?) or by putting 
things in front of it (jupiter sized arrays of shutters?).
Only, SETI isn't looking for that.

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