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> The major things we have been sending is FM (and AM) modulated radio and
> TV. This is effectively a big signal in a very small bandwidth. If you
> make a spectrum it will turn up as an unexpected peak above the
> background. If you then become interested and look closer you will see
> that the spectral contend is not compatible with a line originating from
> an exited atom or molecule. So, if these aliens are at the right time and
> place and have a SETI project running they will detect us in a couple of
> centuries when the signal reaches them.
OOC how sensitive is the SETI equipment and project? I mean how big must a
signal be from 200 ly away that we are able to detect it with that system?
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. Is that realistic
to expect?
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And lo On Mon, 29 Mar 2010 18:17:10 +0200, Darren New <dne### [at] san rrcom>
did spake thusly:
> Phil Cook v2 wrote:
>> Depends on the setup. Off the top of my head imagine a situation with a
>> night-guard on reception. The building is locked, but the alarms aren't
>> on. Now if the building can't be locked without the alarms being
>> activated that's a pointless function of the card.
>
> Yes, but that's not the problem. Indeed, that's how it works now.
>
> The problem is that the card unlocks the door but does *not* turn off
> the alarm.
Yes I would consider that a flaw :-)
--
Phil Cook
--
I once tried to be apathetic, but I just couldn't be bothered
http://flipc.blogspot.com
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On 1-4-2010 8:56, scott wrote:
>> The major things we have been sending is FM (and AM) modulated radio
>> and TV. This is effectively a big signal in a very small bandwidth. If
>> you make a spectrum it will turn up as an unexpected peak above the
>> background. If you then become interested and look closer you will see
>> that the spectral contend is not compatible with a line originating
>> from an exited atom or molecule. So, if these aliens are at the right
>> time and place and have a SETI project running they will detect us in
>> a couple of centuries when the signal reaches them.
>
> OOC how sensitive is the SETI equipment and project? I mean how big
> must a signal be from 200 ly away that we are able to detect it with
> that system?
I don't know.
> 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.
> Is that realistic to expect?
If you really want to broadcast a message anything is realistic.
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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.
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scott wrote:
> OOC how sensitive is the SETI equipment and project? I mean how big must
> a signal be from 200 ly away that we are able to detect it with that
> system? 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. Is that
> realistic to expect?
Eric Korpela, one of SETI@Home admins, replies:
> SETI@home has a sensitivity of about 1e-25 W/m^2, so if I'm doing my math
> right, a transmitter at 200 ly would need to be about 7e+12 W EIRP
> (effective isotropic radiated power). If the transmitter was attached to a
> telescope the size of AO, it would have a gain of about 2 million, so the
> actual transmitter would need to be about 3.5 MW total power.
>
> Of course for a directional transmitter ET would need to suspect we are
> here.
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And more info:
Eric Korpela wrote:
> Nicolas Alvarez wrote:
>> Eric Korpela wrote:
>>> SETI@home has a sensitivity of about 1e-25 W/m^2, so if I'm doing my
>>> math right, a transmitter at 200 ly would need to be about 7e+12 W EIRP
>>> (effective isotropic radiated power). If the transmitter was attached to
>>> a telescope the size of AO, it would have a gain of about 2 million, so
>>> the actual transmitter would need to be about 3.5 MW total power.
>>>
>>> Of course for a directional transmitter ET would need to suspect we are
>>> here.
>>
>> 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.
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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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