Re: OT: Aliens in the heavans (was Re: seti@home)
On Wed, 2002-03-06 at 01:08, Gary Turner wrote:
> On Wed, 06 Mar 2002 16:46:17 +1030, Tom Cook wrote:
>
> >Ron Johnson wrote:
>
> >
> >This would be true were it an electric field. Actually electromagnetic
> >radiation falls off inversely with distance, not with the cube of
> >distance, since no-one has yet devised an antenna which radiates very
> >well in all directions and there is some fancy explanation of Maxwell
> >that shows that it's 1/r, now 1/r^2 or 1/r^3.
>
> While it is true that the EMF, or voltage is inversely proportional to
> the distance, the power is reduced by the square of the distance.
> (P=e^2/r, P=i^2*r, or P=e*i) Thus a signal with a power density of 1
> Watt per sq meter at 10 meters distance, will have a power density of
> .01 Watts per sq meter at 100 meter's distance (in a lossless system).
Ah. I knew gravity worked that way; I should have realized that EMF
works the same way...
> >> If most stars need telescopes (even the Hubble) to see them,
> >> and they radiate jillions^3 of watts on energy, and still look
> >> like pin-pricks, how could the signal from a 50,000 watt radio
> >> station, or a 250,000 watt TV station (both of whose signals
> >> are absorbed somewhat by the air) reach an antennae 20 light
> >> years away, while passing through all that background noise?
> Which brings us to power density at the receiving end--there ain't much.
Isn't that even more of an understatement than "Hydrogen bombs
go 'Boom'."?
> Given that the W/sq meter is minuscule, an antenna such as the one the
> seti project uses has an effective aperture on the order of 10's of
> thousands of sq meters. And that helps. The small beam angle acts to
> remove all signals not in the desired direction, so the noise level is
> reduced. VLNAs bring sensitivity to a level that a signal energy level
> only a few degrees above abs 0 is detectable.
Is the project SOL if the alien broadcast is from that part of the
galaxy that's only visible from the southern hemisphere?
--
+------------------------------------------------------------+
| Ron Johnson, Jr. Home: ron.l.johnson@cox.net |
| Jefferson, LA USA http://ronandheather.dhs.org:81 |
| |
| 484,246 sq mi are needed for 6 billion people to live, 4 !
! persons per lot, in lots that are 60'x150'. |
! That is ~ California, Texas and Missouri. !
! Alternatively, France, Spain and The United Kingdom. |
+------------------------------------------------------------+
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