On 2-4-2010 0:21, Nicolas Alvarez wrote:
> andrel wrote:
>>> Is that realistic to expect?
>> If you really want to broadcast a message anything is realistic.
> 
> And what if they aim the message directly at us? I assume they can use less 
> power if they are pointing exactly at the Earth.
> 
Then they know we are here which implies that they noticed our radio or 
television broadcasts. So, the answer to your question will not be 
relevant within the next 3 centuries.

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>> 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.  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.

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>>> What if it's not directional but broadcast? How much power do the
>>> 200ly-far aliens need to send a radio signal covering an entire
>>> hemisphere*, so that a reasonable amount still reaches us (or rather
>>> SETI's antenna)?
>>
>> That would be 7 TW. Which is the amount of power you can generate with a
>> solar array 150 km x 150 km* in size (assuming 24% efficiency). That 
>> would
>> be a bit more than our society could afford to spend on trying to
>> broadcast our position.

Good info, thanks!

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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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