what is the intensity of stellar radiation?

[[I _think_ I've unwrapped the nested quoting properly, but if I've
erred and misattributed A's words to B, my apologies to all.]]
In article ,
robert bristow-johnson writes:
guys, i am just trying to figure out how powerful a transmitter ET
will need to create a signal that can well compete against what is out
there naturally.

If our esteemed moderator will permit the re-posting, here's a usenet
posting from 1999 that nicely synopsises SETI detection distances. I
don't think the answers have gotten that much better in the intervening
11 years, although the Allen Telescope array (coming online now) will
help a bit with sky coverage, and the Square Kilometer Array (which
if funded, might come online somewhere around 2020) could help a lot
more with improved sensitivity.

----- begin old usenet posting -----
From:
Newsgroups: sci.space.science
Subject: SETI: What negative results are there?
Date: Tue, 16 Mar 1999 22:45:43 GMT
Approved:
Message-ID:
References:

In article ,
Graham Nelson wrote:

SETI (Search for Extraterrestrial Intelligent life, I suppose it
must stand for) has been in the news again lately.

Suppose we call a planet "civilized" if it produces the same
radio and television signals as Earth and for the same reason,
i.e., if it is not especially aiming to broadcast but is emitting
signals on many wavebands by accident, just as we are. Just
how big a radio source is a "civilized" planet? More specifically:

(i) With present technology, how far away could a "civilized"
planet be detectable?

Not very far - about 1 to 10 light years. With today's technology, all we
can look for are carriers and simple pulses (looking for anything more
complex in wide bandwidths takes more computer power than we have).
The calculation goes like this:
A strong UHF TV transmitter has about 10 MW of radiated power
(counting antenna gain).

About 10% of that power is in the carrier, which we can detect.

The most sensitive search uses big telescopes and narrow bandwidths.
20K system temperature implies 3x10^-22 watts needed in a 1 Hz
bandwidth. Arecibo has about 50,000 m^2 of collecting area.
So we need 6x10^-27 w/m^2 for a S/N of 1.

Now solve for the range = sqrt(power/(4*pi*sensitivity)). It's
only about 3.6x10^15 meters, or about .3 light years.

You can do a little better (slightly lower system temperatures,
narrower bandwidths, longer integrations), but not too much. So
10 light years is a safe upper estimate. Note that the fact that
there are a lot of these transmitters does not help us with our
current detection technology.

Of course, if someone is sending us a signal deliberately it could be
much, much stronger. The Arecibo and JPL planetary radars are about
20 million times stronger, IF you are in the beam (a big if). We could
detect one of these from about 5000 light years if it happened to be
aimed at us. (Of course we have never looked at the frequency of the
JPL radar, but that's another problem.)

(ii) It must now be known that no star within X light-years of
Earth has a "civilized" planet, for some value of X. Can anyone
comment on what value X roughly now has?

X is very small, about 1 light year, as above. A more realistic statement
would be:
- No one is trying to signal us from the nearest few hundred stars
(up to perhaps 100 light years) in the 1-3 GHz band. This is the
result from Project Phoenix of the SETI institute.
- There is no one trying to signal us with super strong transmitters at
the water hole frequencies. (Project BETA and SERENDIP). These
projects survey the whole sky, or as much of it as they can, but they
are about 3-4 orders of magnitude less sensitive.

Note also that all of these experiments assume the transmitter is on almost
all the time. Someone could be sending a very strong signal every so often,
and no existing experiment will see it. Also, they could be sending IR or
optical pulses, and we have not looked for these very thoroughly yet. The
SETI institute is starting to address these gaps.

So the negative results are not as strong as could be hoped. On the other
hand this means the current negative results should not be disappointing,
we haven't tried very hard or very thoroughly yet.
-Lou Scheffer

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----- end old usenet posting -----

--
-- "Jonathan Thornburg [remove -animal to reply]"
Dept of Astronomy, Indiana University, Bloomington, Indiana, USA
"Washing one's hands of the conflict between the powerful and the
powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam

Jonathan Thornburg [remove -animal to reply] a écrit :
Of course, if someone is sending us a signal deliberately it could be
much, much stronger. The Arecibo and JPL planetary radars are about
20 million times stronger, IF you are in the beam (a big if). We could
detect one of these from about 5000 light years if it happened to be
aimed at us. (Of course we have never looked at the frequency of the
JPL radar, but that's another problem.)

Aliens (if they exist) KNOW there is life here since the atmosphere is
NOT at equiliobrium. SOMETHING must be producing all that oxygen.

This atmospheric signal can be deduced from a spectrum of earth
atmosphere, and needs technology slightly more advanced than what humans
master now (10-20 years only)

So, they know this planet has life since millions of years that this
signal has been going on. We are an "interesting" target.

THEN

It is probable that they would point signals to us, if they want to
communicate with us at all...

Problem is that they could use other, much better ways of communicating
than what we know NOW, for instance modulated neutrino beams. Photons
are deviated by many things, neutrinos can pass through a whole planet
without losing signal strength. Problem for us:

we have no neutrino iPhone (yet).

Obviously too, if they live in our immediate vicinity (50 light years)
they have received the radio signal already that we are emitting all
the time.

Conclusion:

After watching our TV shows they lost interest in us

:-)

In article , Eric Flesch
writes:

On Sat, 17 Jul 10, Phillip Helbig wrote:
As a counter-example, it is not a coincidence in the big-bang model that
the age of the oldest known objects in the universe is of the same order
of magnitude as the Hubble time. In the steady-state model, however,
this would be a complete coincidence. This is actually a pretty good
argument against the steady-state model

"Order of magnitude"? So if globular clusters are estimated with ages
of 18 Gy, that is a confirmation of FRW's 14 Gy age of the universe?

No, because there is no such thing as "FRW's 14 Gy age of the universe".
The age depends inversely on the Hubble constant and also on the
parameters lambda and Omega. For otherwise realistic values, the age is
of the order of magnitude of the inverse of the Hubble constant, but can
be somewhat larger. All the stuff about "universe older than the
objects in it" was based on a too large value for the Hubble constant or
(more often) assuming lambda=0 and Omega=1, which, unsurprisingly, is
ruled out by observations.

On Fri, 23 Jul 10 07:18:16 GMT, Phillip Helbig wrote:
The age depends inversely on the Hubble constant and also on the
parameters lambda and Omega.

One has to wonder how there can be a Hubble "constant" in an
"accelerating expansion" universe. Isn't it time to put the "Hubble
constant" out to pasture?

[Mod. note: even in a traditional decelerating universe, H is not a
constant. In describing H_0 as a constant one is referring to its role
as a constant of proportionality in the observed relationship between
velocity and distance, not implying that it remains constant over
cosmic time. Some people prefer to talk about the Hubble 'parameter'
to avoid this confusion -- mjh]

On 7/17/10 8:02 AM, Phillip Helbig---undress to reply wrote:
As a counter-example, it is not a coincidence in the big-bang model that
the age of the oldest known objects in the universe is of the same order
of magnitude as the Hubble time. In the steady-state model, however,
this would be a complete coincidence. This is actually a pretty good
argument against the steady-state model, but as far as I know no-one
thought of it before the steady-state model was no longer a viable
candidate for other reasons.

Check George Lemaitre

Lemaitre predicted the Big Bang based on Einstein's equations
before Edwin Hubble's observed universe expansion data
http://en.wikipedia.org/wiki/Georges_Lema%C3%AEtre

Richard D Saam

Thus spake Phillip Helbig---undress to reply
ax.de
As a counter-example, it is not a coincidence in the big-bang model
that the age of the oldest known objects in the universe is of the same
order of magnitude as the Hubble time. In the steady-state model,
however, this would be a complete coincidence. This is actually a
pretty good argument against the steady-state model, but as far as I
know no-one thought of it before the steady-state model was no longer a
viable candidate for other reasons.

I dare say people did think of the argument, but as there were no
accurate age estimates for the oldest objects in the universe until
quite recently, no accurate estimate of Hubble's constant or of the
other cosmological parameters, it was not of much use.

Regards

--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)

http://www.rqgravity.net

In article , Oh No
writes:

Thus spake Phillip Helbig---undress to reply
ax.de
As a counter-example, it is not a coincidence in the big-bang model
that the age of the oldest known objects in the universe is of the same
order of magnitude as the Hubble time. In the steady-state model,
however, this would be a complete coincidence. This is actually a
pretty good argument against the steady-state model, but as far as I
know no-one thought of it before the steady-state model was no longer a
viable candidate for other reasons.

I dare say people did think of the argument, but as there were no
accurate age estimates for the oldest objects in the universe until
quite recently, no accurate estimate of Hubble's constant or of the
other cosmological parameters, it was not of much use.

The age of the oldest stars was rather accurately know, certainly to
within an order of magnitude. The steady-state universe has the Hubble
time as the only parameter with the dimension time, but there is, within
the steady-state framework, no relation between this and the age of the
universe, so even an order-of-magnitude coincidence should appear
strange.

"robert bristow-johnson" wrote in
message ...
On Jul 15, 2:46 am, Steve Willner
wrote:
In article ,
robert bristow-johnson writes:


For number use a noise temperature of ~18,000 for your
calculations use the following.

at 30 MHz use 18,000 K
at 150 MHz use 450 K
at 420 MHz use 47 K.

for other frequencies around those I gave you assume the noise
temperature goes as 1 over the frequency squared.

The noise power from space is the bandwidth times the Boltzmann's
constant times the noise temperature. You will have to add the
noise power from the other sources. I would make an assumption of
the signal noise ratio you would like to have is 10 to 20 since
you'll probably will only be doing modulation and no coding. The
signal to noise ratio you use will depend on the bit error rate
you want the others to be able to detect.