Re: A of E author in alien light signal detection project

don@xxxxxxxxxxxxxx (Don Klipstein) wrote in
news:slrne6ies6.mbe.don@xxxxxxxxxxxxxx:

In article <pan.2006.05.16.06.58.04.837327@xxxxxxxxxxx>, Rich Grise
wrote:

I seriously doubt if anyone who has the power to communicate between
star systems would use electromagnetic radiation to do it. What's the
status on gravity waves these days? How about quantum black holes? ;-)

Well, I could comment a bit on gravity waves vs. lasers.

Consider wavelength, and how that limits narrowness of a beam.

If I wanted to be noticed by someone on a planet 30 lightyears away,
I
would get the biggest Nd:YAG ("YAG") laser that I could get and fire
it through a large telescope.

Let's see what happens if I get a 25 megawatt peak pulse YAG laser
(1064
nm) and fire it out a telescope whose objective is 3 meters in
diameter.
I try Google and find 25 MW YAG lasers have been made, and Earth's
biggest telescope is about 5 meters last time I checked (long ago).

If I don't have things terribly wrong, good optics can get the
beamwidth
in radians down to not much more than the ratio of wavelength to
objective diameter. With a micrometer and 3 meters, that's 1/3
microradian or a bit more, with cross section of the beam being about
1E-13 steradian.
25 megawatts into this is 2.5E20 watts per steradian.

Maybe that will be weak compared to output from the sun... Let's
see...

I am figuring the sun to give us 1380 watts per square meter from
1.497E11 meters away. That's about 3.1E25 watts per steradian...

This does mean that a pulse train fired from a 25 MW peak power YAG
laser through a 3 meter telescope will be about 51 dB below the sun's
output.

Now, suppose aliens are checking us out with a narrowband filter or
having a computer monitor a spectral power distribution of our solar
system for patterned spikes? If we are not doing the same, then I
think we should!
I certainly know that a spectrometer costing only a few thousand $
has
resolution down to a few nanometers. It appears to me that not too
many megabucks are needed to have a computer-monitored spectrometer
with resolution of 1/10 nanometer and checking by the microsecond, and
with alerts beeped out and spectral power distribution curves logged
if a discernably non-random pattern of a spectral spike is detected.
If we are not doing this, I don't think it's much of a waste of
taxpayer
money to get a few of these up and running to monitor at least
parttime the main sequence stars within maybe 30-50 light-years and of
spectral class lower or middle F to upper K or so.
And I also think it's worthwhile to have a setup or a few firing
pulse
trains of laser radiation towards such stars.

But back to calculating numbers:

Portion of solar output in a 1 nm wide band at 1064 nm: .048% of
3.1E25 w/sr, which is about 1.5E22 w/sr. I am proposing 2.5E20 w/sr
competing against that, which is about 17 dB down.

Now, I will assume that better-achievable high power lasers will
have
wavelength known to the .1 nm range and that monitoring of a spectral
power distribution of "optical band" output of a star system can watch
for this. Now we only have to watch for non-random patterns at
selected wavelengths to be monitored having patterns 7 dB below the
output in same bandwidth from a sun-like star, assuming their
capabilities for producing patterned laser bursts are what I mentioned
above.

Now for an alternative spectral region to monitor: Radio bands.
Possibly it might be worthwhile to see if nuclear explosives get
detonated in the outer atmospheres of other planets - for whatever
purpose!

- Don Klipstein (don@xxxxxxxxx)


Thankyou. I can see what I was missing now. The beam of a CW laser diverges
more, fades over distance more than the sun's light does, and I wasn't
taking that into account properly.

I don't know much about extracting signals from noise, but I tried it with
sounds. 1KHz pulsed at 1Hz with 50% on-time, against a background of white
noise. I could hear it down to -30 dB, but only down to -24dB reliably,
which is lower than your -17 dB, so that should work. Shorter pulses would
make it harder though. One thing I noticed was that it didn't matter if I
filtered the mix or not, it had no effect on my ability to pick out the
sine wave, it only made the test a little more comfortable to listen to. :)
If anything though, the wideband background provided a better anchor for my
perception of the narrowband one than the filtered mix did. Whether that
effect would be the same for a mechanical monitor I don't know. It might
have been due to a poor quality filter too.

What this leads me to ask is: could it be better to convert light signals
to sounds or other representations to let human perception do the
filtering, to take advantage of perception we haven't learned to model?
Could that work better than doing it entirely by mechanism? I know that
this must happen anyway with all measurements, but my point is that science
tends to refine as much as possible before human interpretation is allowed,
and this might not be the best way. A human can pick out the tune and the
harmony in music, even in an instrument low in the sound mix. This is
enhanced dramatically in a a two-channel stereo mix. No-one's modelled that
and made a machine do it, the results so far have been a joke, so if human
perception is allowed to have a greater share of detection, we might get
better results.
.

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