[Splitting off this topic since it is a bit specific]

"R" == Rich  <someone@somewhere.com> writes:

*We* can be detected over interstellar distances.  Look at the FAQ.
Some of the transmissions we produce already can be detected over
interstellar distances. 

R> You know, I've always wondered about this. Is the interstellar
R> medium transparent to radio waves? I've run across an interesting
R> factor.

Yes, the interstellar medium is transparent.  Observing in the radio,
we can see other galaxies through the Milky Way.

(Strictly, I should say that the interstellar medium is transparent
for frequencies above about 1 MHz.  Below that, it does become opaque.
This is irrelevant for SETI programs, as they are conducted at
frequencies of about 1000 MHz.)

R> This is from your web-page in fact.

R> http://patriot.net/~jlazio/turbulence.html
[...]

This describes radio-wave scintillation.  Note that scintillation
*implies* transparency.  Scintillation is a change in a signal *as a
result of* having passed through a medium.

Anybody who has ever seen a star twinkle is familiar with
scintillation.  Twinkling = scintillation.

In the case of stars, they twinkle (or scintillate) because their
light passes through the Earth's atmosphere.  In the case of compact
radio sources, they scintillate because their signals pass through the
interstellar plasma.


R> And while it's noted that scintillation can sometimes magnify a
R> signal, it usually does not. I'd suspect that over any long
R> distances, it would defocus any beam heading in our direction, and
R> indeed, it will also affect the radio energy that has already left
R> earth. The general effect would seem to be to lower detectability,

That's actually not true.  Scintillation does not magnify and
demagnify signals equally.  With scintillation, one can probe to
larger distances or detect weaker signals than one could otherwise.
This is an important effect in pulsar observations.

R> and the effect would seem proportional to distance, and of course
R> dependent on local variables.

It is true that the amount of scintillation is roughly dependent on
distance.  However, the effect is weak, particularly at the
frequencies relevant for most SETI programs, thus one has to look to
large distances.  For searches of nearby stars (<~ 100 pc),
scintillation isn't really much of an issue.

-- 
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