| Subject: Re: What is SETI? was->>Re: How smart are SETI@homers? - ScientificAmerican |
| From: Joseph Lazio |
| Date: 06/05/2004, 00:38 |
| Newsgroups: sci.astro.seti,alt.sci.seti,sci.space.policy |
"R" == Rich <someone@somewhere.com> writes:
R> In infinite wisdom Joseph Lazio answered:
[Regarding the scientific method as described in textbooks and
whether SETI qualifies:]
R> 1. Observation and description of a phenomenon or group of
R> phenomena.
Radio transmissions from the Earth are detectable over interstellar
distances.
R> Do you mean current (or past) emissions, or beamed transmissions of
R> the kind being discussed?
Clearly nobody on Earth is producing the kind of beacons that current
SETI programs are capable of detecting. That's irrelevant. My
statement is correct, as a quick perusal of the FAQ will demonstrate.
R> And the problem remains that in order to detect such a signal, you
R> need a really good receiver, you need to know where to look, and
R> you need to know what frequency at which to look.
Umm, yes. That's what I've been saying. If you don't search at some
frequencies, you can't make broad claims about never having detected
ET transmissions.
R> 2. Formulation of an hypothesis to explain the phenomena. [...]
Hypothesis: There may be other civilizations in the Galaxy also
capable of producing radio transmissions detectable over
interstellar distances.
R> Above you say "interstellar", now you talk about galactic
R> sources. Do you think we could detect civilizations across the
R> milky way?
(Your first sentence is a bit of a tautology.)
Interstellar means between the stars. There may be civilizations only
10 pc away and there may be civilizations 10,000 pc away.
R> 3. Use of the hypothesis to predict the existence of other
R> phenomena, or to predict quantitatively the results of new
R> observations.
Observations at radio wavelengths can be conducted at a certain
sensitivity levels (call this S). These sensitivity levels
translate to a distance for an assumed transmitter power (roughly d
~ sqrt{S/P} for an assumed transmitter power P). If one conducts a
search at a sensitivity level of S and finds no examples of ET
broadcasts, the number of radio transmitting ET civilizations in
the Galaxy cannot be more than roughly (D/d)^2, where D is the
diameter of the Galaxy.
R> Assuming that said civilizations broadcast with the intent to be
R> detected, perhaps.
Well, yes. That's why I was careful to write "radio transmitting ET
civilizations," both here and in my original hypothesis. If you
want to consider other kinds of ET civilizations, one would have to
start at Step 1 again.
R> And lots can happen in the 100,000 years it takes light to traverse
R> the galaxy, there's a bit of a lag. I suggest that the probability
R> of said civilization broadcasting in our direction is inversely
R> related to it's distance from us
That's a hypothesis. The evidence that you have to support it is?
R> (and note that much of the galaxy is occulted and not directly
R> viewable).
This is manifestly not true. At radio wavelengths, there is
effectively no absorption. I have observations of other galaxies seen
through the disk of the Galaxy. Indeed, whole surveys have been
conducted in the radio to try to find background galaxies seen through
the plane of the Galaxy.
R> 4. Performance of experimental tests of the predictions by several
R> independent experimenters and properly performed experiments.
Otherwise known as SETI.
R> Indeed.
R> But with all the radio surveys done of the sky, with 30 years or so
R> of SETI searches of ever greater sensivity, one might question why
R> any nearby ET civilization's broadcasts have not already been
R> picked up.
Well, there are two possibilities, right? As you note above, maybe we
haven't looked at the right frequencies or maybe there are no radio
transmitting ET civilizations nearby.
For the sake of the argument, let's just carry this thought through.
You don't define "nearby," but let's take 10 pc (~ 30 light years) as
a fiducial value. Take the diameter of the Galaxy to be roughly
20,000 pc (~ 60,000 light years). Then from your "datum" and by my
prediction above, the number of radio transmitting ET civilizations in
the Galaxy can be no more than about 4 million.
However, you really need to check your numbers. There haven't been
all that many SETI surveys done of the sky, and the ones that have
been conducted have typically covered only a very small range of
frequencies at limited sensitivity.
R> It has been claimed that a 1 watt transmitter would be sufficient
R> with modern receivers.
Umm, have you read the FAQ?