Billy Brown wrote:
>I've got two different lines of commentary on the heavy-planet scenario as a
>candidate for the Great Filter:
>First, allow me to point out that it isn't enough. Even if one accepts it
>at face value, it would catch only a modest majority of civilizations. Even
>if life develops faster on heavy planets, we know it is possible for it to
>develop on Earth. That means that unless the preference is extremely
>strong, this effect could only reduce the population of spacefaring races by
>1-2 orders of magnitude. Given that what we're looking for is more like
>12 - 14 orders of magnitude, that doesn't help much.
This line of commentary seems very scientifically germane to the suggestion of most or all ET's residing on heavier planets than the Earth. Robin Hanson's paper on the Great Filter made it clear that one plausible reason for a shortage of ET's would be if the typical development of life takes much *longer* than here on Earth. Now, I've read articles that suggest that our own sun will be too hot to support large animal life on our own planet at some point within the next half billion years or so. In that context, it seems we're already "near" the end of the 5 billion year "window" for our sort of life to evolve. If intelligence usually takes a great deal longer than that to get going, you either have to say that it's extremely rare, or look to stars that are less massive and therefore longer lived than our sun, stars that are stellar class "K" or even "M", rather than our sun's "G" class. Among main sequence stars, the smaller and dimmer the star, the longer its life span, by quite a large factor, I believe -- and I understand that the smaller stars are more prone to dangerous stellar flares, too.
Given those general points, what I'm asking about the smaller stars is, does the advantage of longer stellar life balance off against the flare radiation danger in such a way as to strongly discourage life on any but the largest terrestrial -type planets that may be orbiting those stars? This is a highly technical question, no doubt! If someone is able to crunch the numbers, maybe even complete with a good model of atmosphere thickness for different sized planets, *that* would be interesting, certainly beyond anything that I'm prepared to do. Admittedly, it's possible that this "ET's favour large planets" idea might be killed on some technicality in fairly short order, in the sense of disproving it as a statistical trend. For instance, possibly there just isn't all that much difference in radiation dangers for stellar life spans of interest? I have some sense that the uncertainties about such things as "frequency of stellar flaring" might enough to leave an idea like this on the back burner for a long time, before enough is found out to either disprove it, or to solidly encourage it?
>Second, the argument takes a very short-term view of a problem that is
>properly examined from a cosmological perspective. It doesn't matter if
>space travel takes a few hundred years longer, or is a few orders of
>magnitude more expensive. What we're looking for is something that makes
>space travel utterly impossible.
>Perhaps this would be a good time to point out that the problem of escaping
>from a deep gravity well (escape velocity maybe 15 MPS) is trivial compared
>to the problem of interstellar colonization (required velocity at least 0.01
>C = 1,860 MPS). We can't rely on the first problem to prevent people from
>solving the second one.
In terms of making a suggestion about why ET's might not have gotten off-planet, this seems not so much of a concern to me, since no one really knows what it would take to discourage your typical ET civilization from developing space settlements! If, as I've outlined, even the highest tech rocketry could arguably be too expensive or too dangerous for an ET civilization, who knows if they'd ever get around to building the really large-scale launchers that they'd need to surpass those limits? Note that sending cheap little "seed" AI's into space might not be the answer, either; if small, cheap AI's tend to get fried by radiation, maybe our ET's always have to give up on that as well.
What I really notice here is that the above comment from Billy Brown seems to suggest that an obstacle of something being "a few orders of magnitude more expensive" would necessarily be overcome eventually, maybe taking only a "few hundred years" at most before the cost barrier is reduced to something manageable. The trouble with cost barriers is that, if the cost of breaking through the barrier is too high, maybe no one will do it! As the old saying says, "you have to walk before you can run"; what if most would-be space travellers never learn to "walk"?
David Blenkinsop <firstname.lastname@example.org>