Frank
D. Drake, an active
investigator in SETI
(Search for
Extra-Terrestrial Intelligence) programme,
has said that the large radio telescope in
Arecibo, Puerto Rico,
“can detect a signal which lays down on
the entire surface of the Earth a power of only one picowatt”.
We have to find (a)
the power actually received by the Arecibo
antenna for such a signal. The
antenna diameter is 305 m. (b) We
have to find the power output of a source at the centre of our galaxy
that could provide such a signal. The
galactic centre is
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Solution: Click For PDF Version (a)
Radius of the
Earth,
The surface
of the Earth,
The intensity of the weakest signal that can be detected by the Arecibo radio telescope will therefore be
Diameter of
the Arecibo antenna
Therefore, the power received by the Arecibo radio telescope for the weakest signal will be
Distance of the galactic centre is
where we have used that
Therefore, the power of the source emitting the signal from the centre of our galaxy will be
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away. We
can take the source as radiating uniformly in all directions.
.
.
(b)
