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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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