Re: OT: Aliens in the heavans (was Re: seti@home)
On Wed, 2002-03-06 at 07:30, John Hasler wrote:
> Tom Cook writes:
> > Actually electromagnetic radiation falls off inversely with distance,...
>
> Field intensity is inversely proportional to distance. Power density is
> inversly proportional to the square of distance (and power is, of course,
> what we can actually detect).
>
> > ...since no-one has yet devised an antenna which radiates very
> > well in all directions...
>
> That's completely irrelevant.
>
> > The direction of propagation is perpendicular to the direction of motion
> > of the exciting charges (aren't they exciting? ;-) and so the wave
> > propagates in the horizontal plane (assuming that your antenna is
> > oriented that way.
>
> The radiation propagates in all directions (though the intensity varies
> around the antenna patern). I think you are confounding polarization and
> propagation.
>
> > I agree with you (except about wave energy density); I believe a 1 MW RF
> > plane radiator is very difficult to demodulate with our technology from
> > the edge of our solar system,...
>
> Just the other day NASA received a response to a signal that they sent to
> Pioneer 10. It is 22 light-hours out and has a low-gain antenna and a
> transmitter power of a few watts.
Isn't that incredible? Maybe slide-rules and armies of women on
adding machines is worth something after all...
According to newscientist.com, the _reply_ came in 22h 6 min, so it
would be 11 light-hours out.
Still, 24*365 = 8,760, so a 10 light-year star is 87,600 l-h away.
That's 7,964x as far away as P10, and given the inverse square law,
imagine how small the power density is.
http://www.newscientist.com/news/news.jsp?id=ns99991997
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