#41
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SETI
"Brian Tung" wrote in message ... Zane wrote: BUT, we can make the axiomatic statement that if there are enough opportunities, anything not prohibited by the laws of physics will certainly happen. This is so basic a concept that most people, at least, don't think that it has a burden of proof. Davoud wrote: By definition, an axiom has no burden of proof; its truth is taken to be self evident. Thus, what you have asserted is not an axiom. The "will certainly happen" is the problematic part. _May_ happen works, though. The problem is that one can show by quite simple and rigorous math that the probability of occurrence must be either zero or one, provided the trials are independent and identically distributed (i.i.d.). As a simple example, if you have a chance at all of winning the lottery (that is, you play and the game isn't rigged to prevent you from winning), then given an infinite number of trials, you are bound to win. What's more, you are bound to win an infinite number of times. (However, that doesn't mean your expected gain is infinitely positive. On the contrary, it's infinitely negative. But I digress.) So in order to conclude that "may" is preferable to "will," one must show that the trials (of which the Earth is the only one known to yield a successful result) are either not independent or not identicallly distributed. Dependence is difficult to argue. How could we prevent the development of life even several light-years away? Affect it once it had produced a technical civilization, yes; cause it to "commit suicide," even--not likely, but just possible. But prevent it altogether? Hard to see how that could happen. Besides, as far as we know, our influence can only spread out at the speed of light--no faster. The Earth has harbored life for no more than about 4 billion years, and our effects could only extend for 4 billion light-years in any direction. Beyond that is a vast arena for life to develop, and if the question is predicated on an arbitrarily large number of trials, then a 4-billion-light-year-radius sphere is, quite literally, nothing. (Although we'll have to be listening for a very long time to pick up anything more distant!) All right, then, if dependence won't invalidate the principle, we can examine whether or not the trials are really identically distributed. Right away, we can say that they clearly are not. Mercury, for example, is one such trial, and the odds for life developing there (at least in its current state) are clearly not very exciting. However, we know this because we know something a priori about Mercury. In particular, we know that Mercury is very close to the Sun, and that its rotation period is very long, so that the surface temperature undergoes wild swings. (Nor is there, contrary to the hopes of 1950s science fiction stories, a "twilight zone," because the tidal lock acquired by the Sun is not 1:1, but 3:2.) It is not a planet hospitable to life. But if we knew nothing about Mercury other than that it was a planet orbiting a Sun-like star, we would make different estimates altogether about the planet's opportunities for developing life--in fact, we would make it the same as for any other such planet. In other words, in the absence of a priori knowledge about other planets, the trials *do* become essentially identically distributed. To say that they are not would require us to find out that (for instance) Earth-like planets around Sun-like stars can only develop within a finite space in the universe. That goes against the Copernican principle, and so it does not seem to me plausible at all. It's true that the Copernican principle is only a principle and not a hard and fast law, but just the same, we're really arguing philosophy (since the amount of space we can *listen* to is finite, not infinite), so I think it's perfectly applicable. In short, it seems hedging to me to say that given an essentially infinite number of trials, we can only say that life "might" develop. It *must* develop, absent evidence of dependence or a non-uniform distribution, because it *has* happened, demonstrating that the probability per trial is non-zero. Not only that, but it must develop any number of times, no matter how unlikely it might be in any individual case. Brian Tung The Astronomy Corner at http://astro.isi.edu/ Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/ The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt In an infinite universe, everything is not only possible but inevitable. |
#42
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SETI
Tony Turner wrote:
In an infinite universe, everything is not only possible but inevitable. Not quite. There is also the impossible, and that which is possible but can happen only a finite number of times. Brian Tung The Astronomy Corner at http://astro.isi.edu/ Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/ The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt |
#43
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SETI
"Andrew Walker" wrote: I'd call it useful science if they were doing more with the data, like looking for unusual radio emitting stars or galaxies. As far as I can tell if is no detection, which is extremely likely, they'll have nothing to show for it, ie a waste of time. Start searching for interesting astronomical objects and it's a different story. Great news x2! SETI@home data is currently being used in a hydrogen survey of our Galaxy: http://www.planetary.org/html/UPDATE...te_012403.html SETI@home II will use the BOINC platform, which will enable users to choose between many projects (e.g. Folding@home) using a unified client. Besides SETI@home II, one of the first projects will be Astropulse: http://boinc.berkeley.edu/ |
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