In infinite wisdom Joseph Lazio answered:

"R" == Rich  <someone@somewhere.com> writes:

R> In infinite wisdom Joseph Lazio answered:

R> You have the arrow pointing the wrong way, either you have evidence
R> of ET, and hence a reason behind your 'reasonable expectation',

Of course, if we had evidence of ET, there'd be no point in having
an argument about whether looking for ET was justified.  I presume
you mean, Is there any reasonable expectation that ET might exist,
thereby justifying a search?

R> or you do not. So if you have some positive evidence, feel free to
R> post it, or post any other evidence that your expectation is
R> reasonable.  So far all I've seen is emotional arguments and
R> logical arguments, and of course, the ever-present belief argument.

We know planets are widespread.  More than 5% or 10% of solar-type
stars have Jupiter-mass planets.  Serious selection biases against
finding lower-mass planets, but from the current census it appears
that there are more lower-mass planets than Jupiter-mass planets.

R> Which census is this?

Umm, the current one?  By the "current census," I mean the known
extrasolar planets.

Almost exclusively gas giants, only a few oddball terrestrial planets.
I don't see how you can derive that there are "more lower-mass planets
than Jupiter-mass planets" from the data at hand.

An up-to-date listing can be found at the Extrasolar Planet Encyclopedia.

Show me the part where there are more terrestrial planets than gas
giants.

We know of at least two Earth-mass extrasolar planets.

R> [...]  But those planets orbit a pulsar. Seems a rather hostile
R> place for life as we know it.

Note that nowhere did I claim that life would be present on the
planets orbiting PSR B1257+12.

And that's the best you got so far.

Ergo, it is reasonable to expect that Earth-mass planets are
widespread.

R> You extrapolate from "at least two Earth-mass extrasolar planets"
R> straight to "Earth-mass planets are widespread."??

No, I extrapolate from two facts.  First, at least two Earth-mass
planets are known around PSR B1257+12, *and*, second, an analysis of
planetary masses from the current census suggests that the planets
become more frequent at lower masses (or the planetary mass
distribution function is weighted toward low mass planets).

You just pulled the last part out of thin air.

There is no evidence that "planets become more frequent at lower
masses". Such planets cannot yet be detected. And any analysis which
concludes something the exact opposite of what is shown by the
dataset is, to be generous, suspect.

But then, SETI has always been somewhat of a religion. But it annoys
me when beliefs are conflated into data. The current census does not
show that terrestrial planets outnumber gas giants, further, it
cannot. As for the material's that compose terrestrial planets, it
is not clear that first or second generation stars will have such
materials, at least not if you accept that the big bang produced
primarily hydrogen and helium.

A number of organic molecules, some quite complicated, have been
found in interstellar space and comets and are expected on other
solar system bodies (notably Titan).

R> That these are requirements for life it is true, but are they
R> sufficient in and of themselves to create life? I don't know. And I
R> doubt you do either. If you do, please elucidate.

I don't know, but I actually doubt it.  However, I don't have to prove
that life can arise from interstellar molecules.  All I have to do is
establish that it is reasonable that life could exist. 

If the materials you proffer as evidence are not sufficient for
life, you've not shown what you think. Scientists have been tinkering
with the building blocks of life for decades, and they've not
produced life yet. Why do you suppose this is? Perhaps the simple
proximity of amino acids is not sufficient to produce life? What
else does it take? I don't know. Do you? If you don't, you've no
basis for making claims in this regard.

Earth is 4.5 billion years old.  The earliest microfossils appear
to be about 3.5 billion years old, and there is geochemical
evidence suggesting that life was present 3.8 billion years ago.
While we admittedly do not understand the origin of life, one
reasonable (and fairly widely accepted) interpretation of these
data is that life can originate easily, even under potentially
quite harsh conditions.

R> I think you have it the wrong way round. We don't know anything
R> about the robustness of life as it forms. We have no idea what
R> conditions are necessary, much less how common they are (...).

Agreed.  I stated as much.  Nonetheless, as I stated, a both
reasonable and wide spread opinion is that life can form easily, even
under potentially harsh conditions.

Rank speculation. If it was so easy they'd have created life in the
lab. They have not. I'll not accept the beliefs of others as data
(unless data about the beliefs of others :^).

I believe that even Brownlee &
Ward (the authors of _Rare Earth_) would agree to this statement.

It may be sufficient for you. I prefer to go by the data. I'm from
Kansas (philosophically speaking), show me.

Of course, finding ET life via a SETI program would establish quickly
that life is sufficiently robust that it could originate at least twice.

It would say nothing about robustness at all.

We do not know if intelligent life or transmitting civilizations
can/will develop once life has originated.  That's the point behind
SETI.

R> You've shown that you think you can create ET by the sheer force of
R> your logic. It's not a reasonable position, as it has no basis,
R> it's a logical argument. And even as a logical argument is is not
R> compelling.

R> That life 'might' exist elsewhere is supportable, that it 'must' is
R> not.

All I've tried to argue is that intelligent life may exist elsewhere.

I'll accept that it's not impossible. But as to how likely it is, there
remains no data with which to make such an assessment. Life may be
very very rare. There may be only a few life bearing planets in any
galaxy, if that. I doubt that life is as plentiful as in Star Trek.
If it were, perhaps we'd have some better evidence, eh?

However, that's enough to justify SETI.  If you think that intelligent
life may exist elsewhere, the only way to find out is to look.

This does not necessarily follow. Even if life exists everywhere, there
is no guarantee that looking will show it. Perhaps they are more
interested in their religion? Perhaps the notion of wasting resources
on things like SETI is abhorrent to them. Perhaps they have no extra
resources to use? Who knows. But SETI is a two part exercise, you've
stated the first. The second requirement is that ET is out there
pointing a radio dish in our direction. It's not enough that we look,
they must be broadcasting. If SETI finds nothing, it does not prove
that ET does not exist. And I see no basis for a reasonable expectation
that the current SETI will find anything. So what's next?

Rich