In infinite wisdom David Woolley answered:
In article <40990158.7040305@somewhere.com>,
"Rich <someone@somewhere.com>" <> (improbable address) wrote:
Do you mean current (or past) emissions, or beamed transmissions of the
Planetary radar is detectable to something like 1,000 LY with current
systems and relatively short integration times. We've been doing that
sort of transmission for quite some time,
But very, very intermittantly. And I should expect that a large
percentage of the emmitted energy is absorbed or reflected by the
target.
and even decades before that,
a reasonable extrapolation of technology would have predicted that it
would be possible. Planetary radar will tend to give false negatives at
the confirmation stage because it won't be repeatable.
While I agree with this, it's also not aimed at the stars. What are the
chances of it intercepting a star somewhere down the line? Who knows. It
does not seem highly likely.
The Allen array is claimed to have the ability to detect the equivalent of
our analogue TV carriers from local stars. (I suspect this is with
a long observation time.)
And LIGO is claimed to be able to detect gravity waves. Physicists are
calling it a success although it's detected nothing yet. But this is
a bit too pessimistic an observation, as radio detectors do work, and
the existence of radio waves is not at test. We shall see I suppose.
Now that I've looked more closely at the construction, and learned a
bit more, the Allen Array looks like a really elegant solution, and
teaming with Berkeley gives Berkeley an extra instrument as well as
solving operating cost problems for SETI, which seems an elegant
solution as well.
to look, and you need to know what frequency at which to look.
Most searches cover a wide range of frequencies, so frequency is not
critical.
There seems to be some disagreement on this then. From a recent post
by Lou Scheffer...
---
From: Louis Scheffer (
lou@cadence.com)
Subject: Re: How smart are SETI@homers?
Newsgroups: sci.astro.seti, alt.sci.seti, sci.space.policy
Date: 2004-05-05 09:43:32 PST
Rich
<someone@somewhere.com> writes:
>In infinite wisdom Joseph Lazio answered:
>Only if you assume that the "looking" we can do is of sufficient
>sensivity to detect ET life. And indeed, if ET is out there
>trying to contact us, would we not have detected them already?
This is not at all clear. Let's suppose ET does exist and is
at this very moment trying to catch our attention by sending us
an enormously strong (by radio astronomy standards) monochromatic
signal on 23.456789 GHz (to pick a random example). We've never
even looked at this frequency range, to my knowledge, much less
done any sort of systematic search. Almost surely we would not
have detected them already, even though they are trying to contact us.
Once we've searched the whole sky, in all likely frequency bands,
then we can make the statement you made above. But this is
exactly what the SETI searches are trying to do!
[...]
---
In any case that they are difficult to detect, whilst
possible, is a justification for continuing, as it is reasonable
grounds for assuming that no detection doesn't indicate no ETI's.
But as has been said, it places lower bounds on their potential
frequency, transmit power, and or attempts to communicate, should
they exist. That they would do so at all is, IMHO, a very
anthromorphic viewpoint.
Above you say "interstellar", now you talk about galactic sources. Do
you think we could detect civilizations across the milky way?
Arecibo is detectable by Arecibo to the best part of 1000LY, in a quite
conservative processing configuration. Detecting itself at the nearest
point on the effective edge of the disk ought to require significantly
less than 1 day's (around an hour, I think) integration time (we are
nearly on the centre plane), so current technology is theoretically
capable of making detections over galactically significant distances.
Although there is an inverse relationship between distance and
detectability, for a number of reasons.
Finding something within about 100LY is, of course, more attractive,
and what targetted SETI aims for.
There are quite a few stars in this range.
It has been claimed that a 1 watt transmitter would be sufficient
with modern receivers.
I don't know where you got that figure.
From a current thread. I believe the posts were by Lou Scheffer,
but with the way everyone's speaking for other posters it may have
been Joseph Lazio.
You need very approximately
1GW EIRP from the nearest star to get the S@H threshold of 22 times mean
noise power in 0.075Hz and time * bandwidth = 1. To detect 1W EIRP at
that distance, you would need an observation time of about 1E19 seconds!
That's only 3x10^11 years. :-)
It's not my claim however.
1 Watt feed point power is detectable from local stars if the transmit
aperture is similar to Arecibo, although it will require around 20
minutes observation.
Perhaps that's the idea.
Rich