Re: OT: Aliens in the heavans (was Re: seti@home)
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).
>
>> 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.
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.
And so ends this day's recitation.
--
gt
Yes I fear I am living beyond my mental means--Nash
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