How smart are SETI@homers?

Andrew Nowicki wrote:

If the ET lives near a sun-like star and beams to us 1 watt
of microwave signals, his star makes so much microwave noise
that we cannot read the signal unless one beep lasts at least
10^10 seconds (about 300 years).

I have assumed non-directional transmitter (It radiates
microwaves in all directions.) A directional transmitter
has much better energy density and signal-to-noise ratio,
but it may be aimed in a wrong direction at a wrong time.
The latest Scientific American article about the microwave
SETI ends with a pessimistic statement -- interstellar
distances are too vast for this kind of communication.

"Andrew Nowicki" wrote in message
...

add Scientific American to subj line, in order to delineate new thread

[...]

The latest Scientific American article about the microwave
SETI ends with a pessimistic statement -- interstellar
distances are too vast for this kind of communication.

Andrew - I'm interested in reading this article. I just checked out the
Scientific American website, but couldn't find it. I'm going to try my local
newstand, when I get a chance.

Would you know if the article is in the May issue?

Thx.
--
StratR

Andrew Nowicki wrote in
:

Andrew Nowicki wrote:

If the ET lives near a sun-like star and beams to us 1 watt
of microwave signals, his star makes so much microwave noise
that we cannot read the signal unless one beep lasts at least
10^10 seconds (about 300 years).

I have assumed non-directional transmitter (It radiates
microwaves in all directions.) A directional transmitter
has much better energy density and signal-to-noise ratio,
but it may be aimed in a wrong direction at a wrong time.
The latest Scientific American article about the microwave
SETI ends with a pessimistic statement -- interstellar
distances are too vast for this kind of communication.

You refute the previous poster's argument that a focussed transmission of 1
watt would be sufficient, by counter-arguing that an omnidirectional 1 watt
system is not good enough?

Are you malicious, or just plain stupid?

For you information, a 1 watt, systemwide-focused, monochromatic
transmission will outshine the sun at anything more than about half a
lightyear. At 100 lightyears distance, that 1 watt will outshine the sun's
radiation at the same frequency by 5 magnitudes! The only real limit
applicable here is how well they can focus the transmission, and this is
directly related to how big they are willing to make the transmitter. A
reasonably advanced civilisation using a solar-system scale transmitter
(easily done witha few dozen coordinated emitters) can achieve solarsystem-
scale focus up to some 50 000 ly.

Do yourself a favour, and read up some about the subject before you start
spouting bull again.

StratcatR wrote:

Andrew - I'm interested in reading this article. I just checked out the
Scientific American website, but couldn't find it. I'm going to try my local
newstand, when I get a chance.

Ian Crawford, "Where Are They?" Scientific American, July 2000

Look for microwave SETI at the end of the article.

"Andrew Nowicki" wrote in message
...
StratcatR wrote:

Andrew - I'm interested in reading this article. I just checked out the
Scientific American website, but couldn't find it. I'm going to try my
local
newstand, when I get a chance.

Ian Crawford, "Where Are They?" Scientific American, July 2000

Look for microwave SETI at the end of the article.

Thx for the quick reply...I was just leaving for the newstand, & decided to
do a quick n/g check on my way out-the-door! Guess now, I'll swing by my
local library too...thx for saving me some time.
--
StratR

Andrew Nowicki wrote in message ...
StratcatR wrote:

Andrew - I'm interested in reading this article. I just checked out the
Scientific American website, but couldn't find it. I'm going to try my local
newstand, when I get a chance.

Ian Crawford, "Where Are They?" Scientific American, July 2000

Look for microwave SETI at the end of the article.


Here's a link to the article:

http://www.sciam.com/article.cfm?art...81809EC588EF21

Keep searching, Jason H.

Marvin wrote in message ...

For you information, a 1 watt, systemwide-focused, monochromatic
transmission will outshine the sun at anything more than about half a
lightyear. At 100 lightyears distance, that 1 watt will outshine the sun's
radiation at the same frequency by 5 magnitudes! The only real limit
applicable here is how well they can focus the transmission, and this is
directly related to how big they are willing to make the transmitter. A
reasonably advanced civilisation using a solar-system scale transmitter
(easily done witha few dozen coordinated emitters) can achieve solarsystem-
scale focus up to some 50 000 ly.

You don't need a solar system scale transmitter - it's entirely within
the technology we have today. 50,000 ly = 5x10^20 meters, more or
less. 10 AU is roughly 1.4x10^12 m. So you need a transmitter 3x10^8
wavelengths across to do this. With a 1 cm wavelength, that's only
3000 km, so the array of transmitters could easily fit on any of our
continents.

Another way to see this is that our radio telescope arrays can resolve
features 10 AU big at 50,000 light years. So if run in reverse as
transmitters, they could create a beam of the same size (assuming, of
course, that we solved lots of practical problems such as UV plane
coverage and difficulties of phase referencing through the ionosphere
while transmitting).

Lou Scheffer

(Lou Scheffer) wrote in
om:
You don't need a solar system scale transmitter - it's entirely within
the technology we have today. 50,000 ly = 5x10^20 meters, more or
less. 10 AU is roughly 1.4x10^12 m. So you need a transmitter 3x10^8
wavelengths across to do this. With a 1 cm wavelength, that's only
3000 km, so the array of transmitters could easily fit on any of our
continents.

Erf, you are right, of course.
I slipped a digit (three actually) in my calc on baseline length required.

The point i tried to make is even more valid than I thought: It is *easy*
to focus a microwave transmission sufficiently so that one can transmit
"louder" than your parent sun at interstellar distances, in a chosen
frequency, without anything like prohibitive transmission power usage. As
long as you know exactly where to point your transmitter at.

The only limitations in being an *active* participant in SETI, rather
than a passive listener, a
Finding a target that looks a likely candidate.
Waiting the rather huge timedelay while your speed-of-light message crawls
to it.
Ensuring the content of your transmission is unmistakeably artificial.

Meaning: There are a heap of logistical issues, especially time-related
ones, that hamper a practical SETI program. But there are NO fundamental
physics preventing your search.

You just need a reasonably big reciever, a methodical search pattern, and a
lot of a lot of a heck of a lot of patience. Sending out your own signals
would likely trim down the search time from millions to mere thousands of
years. Yes, years!. Lightspeed is *so* very slow, when you are
communicating over interstellar distances. And there are *so* many targets!
Even if there were a million system-bound civilisations in our galaxy at
this time, and their distribution was random, the average closest distance
between neighbours would be some 800 years.

In article ,
Andrew Nowicki " wrote:

If we treat the Sun as a black body, intensity of its
microwave radiation at 100 GHz is about 10 million times
smaller than intensity of its visible radiation. The total
visible output of the Sun is on the order of 10^17W, so the
total microwave output of the Sun is on the order of 10^10W.

This is getting power densities and integrated powers confused.
In particular, the power density issue means that it is using a figure
for the power integrated over the whole of the microwave band when what
matters for SETI is typically the power integrated over a band of about
1Hz, and down to about 0.05 Hz.

The quiet sun component of the microwave noise at the earth, at about
1.4Ghz, is about 5E-21 watts / square metre / Hz (5E5 Janskies). The Sun
is about 1.5E11 metres away, so the illuminated surface is 4 pi times
the square of this (2.83E23 square metres) giving a total isotropic
power output of about 1,400 watts per Hz. Over the 0.1Hz of a stable
carrier subject to the Drake-Helou limit, that is 140 watts. (Note
this is a lot more than 10E10 watts integrated over any reasonable
definition of the whole microwave band.)

If the ET lives near a sun-like star and beams to us 1 watt
^^^^^
of microwave signals, his star makes so much microwave noise

But as already pointed out, they are beaming. Current interstellar
capable transmitters have antenna gains of over 80dB, so that 1 watt
represents an effective isotropic radiated power of 100,000,000 watts,
meaning that the signal outshines the sun by a factor of almost
700,000.

Quite a lot of TV transmitters in the USA transmit more than 1MW EIRP,
so even if only 1% of the power leaks to space, their carriers will
outshine the sun in achievable bandwidths by a factor of more than 70
(actually more like 700, because the solar output is about 10 times less
at UHF TV frequencies).

(Note that, even wideband signals can be detected well below the noise if
you know how they are structured, although this doesn't help for initial
detection.)

that we cannot read the signal unless one beep lasts at least
10^10 seconds (about 300 years).

The only way I can interpret this figure is to assume that there are two
mistakes here, which have opposite effects on the result. Firstly, it
seems to be assuming a bandwidth of 1Hz rather than the whole microwave
band, that has been assumed in getting the 1E10 watts. Secondly it fails
to account for the fact that non-coherent integration only gives a signal
to noise ratio gain in proportion to the square root of the integration
time. If the source star were the dominant noise source (it isn't),
a 1 watt isotropic, 0.1Hz signal would reach 1:1 SNR in about 2.3 days.

Note that an effective 1:1 signal to noise ratio is too poor for initial
detection, as the false positive rate is worse than 1 in 10.

The signal-to-noise ratio improves 3 orders of magnitude
when the ET's microwave transmitter is moved away from
their star.

Even the cosmic microwave background will dominate it for any reasonably
achievable antenna antenna gain, at interstellar distances.

If the ET replaces his microwave transmitter
with a laser and moves it away from his star, the

But then you have suddenly allowed antenna gain! Incidentally, the limit
on laser systems is set by quantum shot noise, not by Planckian noise.
Realistically achievable optical bandwidths are of the range of 10s of
kHz. On the other hand, because of absorption lines in the star's
spectrum there are lots of quiet places for a close in transmitter
to use.

Solar output figures are taken from the 1982 edition of Handbook of
Space Astronomy and Astrophysics, by Martin V Zombeck, and are taken
from a graph, so only read approximately. Disturbed sun figures are
about 200 times higher than the quiet sun figures at that frequency.

Lou Scheffer wrote:

Why look for a signal that no one is
motivated to send?

It's impossible to extrapolate any potential ETI's motivations based on what
motivates humans.