Re: How anthropic is the anthropic principle, exactly?

On Mar 9, 11:54 am, John Harshman <jharshman.diespam...@xxxxxxxxxxx>
wrote:
Timberwoof wrote:
In article <kgq2v2tkqfnmkneoroljn0m6gf156mv...@xxxxxxx>,
*** <remdic...@xxxxxxxxxxxxx> wrote:

On Thu, 08 Mar 2007 14:30:36 GMT, John Harshman
<jharshman.diespam...@xxxxxxxxxxx> wrote:

Rolf wrote:

"John Harshman" <jharshman.diespam...@xxxxxxxxxxx> wrote in message
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Rolf wrote:

"John Harshman" <jharshman.diespam...@xxxxxxxxxxx> wrote in message
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Mark Isaak wrote:

Most of the universe is hostile to human life. The proportion that is
not is approximately 10^-n.

My question: What is the value of n in that number?

(Yes, I am admitting that I am too lazy to work it out myself.)

How would you work it out? It depends on the number of earthlike
planets, in the first place. Of course, if we count every star as having
one earthlike planet, we still come up with a tiny fraction of the total
volume of the universe. Is that what you're getting at?

WRT 'earthlike planet' - can a planet be 'earthlike' unless it has life

on

it?
Isn't it so that our planet is 'earthlike' just because it has life?

No, just the opposite. I think our planet has life on it because it's
earthlike.

Hm. is there any good definition of what 'earthlike' is or means?

No. It's an undefined term that can refer to any or all properties of
earth, now or long ago.

Say if life had never evolved on this planet, would it not have been more
'mars-like' than 'earth-like'?

In some ways yes, in other ways no.

But pick your favorite definition. I don't think it really matters. I
have avoided any definition, just assuming that whatever it means,
there's one around every star. I think that comfortably maximizes the
number, don't you? I'm in little danger that anyone would tell me that
there are more than that.

Now obviously since it's developed life it's become even more
earthlike (free oxygen and all that). The question is how hospitable the
universe is to human life, and even in an anthropic-principal, ID sort
of context, this probably means how prevalent are the conditions that
allow human-type life to evolve.

But in fact the calculations below render that distinction moot. I've
assumed a maximum (ridiculously so) amount of hospitality in the
universe and get a tiny percentage. If you would care to reduce the
prevalence of earthlike planets, you can add a few orders of magnitude
to the number. But it's already 10^-31, so why bother?

I find that the mean distance between stars in the galaxy is around 1
parsec. So that's (3 x 10^13 km)^3 or about 3 x 10^40 km^3 of space for
each earthlike planet. Let's be generous and count the entire volume of
the planet as being the human-friendly volume. That's about 10^9 km^3.
So the habitable fraction is about 1 in 3 x 10^31.

Note the several ridiculous assumptions that all make this figure way
too big: every star has a habitable planet, the entire volume of the
planet is counted, and intergalactic space has been ignored entirely.

I think "earth like" is a poor standard. Aren't we talking about
conditions to support "life." Many continue to believe Mars has or
had some form of life.

Defining "life" seems to be a critical consideration.

Since the original question was about human life, I think it's perfectly
fair to talk about earth life.

Doesn't matter. In terms of the universe as a whole, places that could
conceivably be hospitable to any life we can imagine are not much more
plentiful than places hospitable to human life. The main point here is
that the overwhelming proportion of the universe is vacuum, which is not
conducive to life. If I wanted to make a universe in which there was
life everywhere, I wouldn't put in so much empty space. (Consider Larry
Niven's book The Integral Trees, blown up to universe-size, for example.)


This is a fallacy. You're just assuming that life as we know it is
somehow preferred, and not an anomaly.

Even considering our own organic life form, the fact that the majority
of the universe is a vacuum doesn't mean anything. This entire thread
is basically faulty. It is assigning a probability of life based on
volume of space occupied. It's irrelevant. If there are places where
life could begin, the correct probability to evaluate is the chance of
those conditions happening, NOT how empty the space is around them.
People are feeding into this question because it's fun to work out the
math, but ultimately all the answers (which people gave several
different ones) are not correct, and irrelevant to the question of
"how likely is life".

To evaluate the probability of life as we know it properly you would
need to know

1) The probability of a star forming within T seconds. (large
probability, it has happend many times if you consider T = 14 billion
years)
2) The probability of cosmic dust collecting around it within T
seconds. (also large probability for T = 14 billion years)
3) The probability of that cosmic dust coalescing into a planet with T
seconds. (large probability again for T = 14 billion years)
4) The probabilty of that planet being of sufficient characteristics
to support complexity of life as we know it after T seconds
(magnetosphere, outgassing water vapor and C02, the right temperature,
etc) (unknown probability... but it's happened at least once in 14
billion years)
5) The probability of organic carbon based life forming as a
consequence after S seconds of evolution (unknown probability again...
but it's happened at least once in S = 4 billion years).

So steps 1-3 are very probable. Steps 4 + 5 have happened at least
once, in us. We have a huge statistical error on this measurement, so
the probability lies somewhere between 0 and 1 (using poisson
statistics and a Feldman-Cousins type boundary limit fix). Therefore
using arguments of earth's "unlikelihood" are, at the moment, not
useful and contrived. Presumably someday soon we will be able to
answer the question of the probabilities of 4 + 5. I'm going to bet
that they're also large (as are 1-3) because it's not all that
surprising, if you think about it.

So really, the question of the probability of volume occupied is
irrelevant. But fun to think about nonetheless.

.

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