Is Anyone Out There?

There is considerable evidence (according to what i've read, and i claim no expertise) that fertility in industrialized nations had fallen sharply--as though the evolutionary process had "recognized" that many offspring are no longer necessary to assure survival. That is something to which i alluded earlier--a sufficiently sophisticated technological society might easily attain environmentally safe equipoise, and have little reason to pursue interstellar colonization.

The evolutionary process cannot "recognize" anything.

In this article, Jensen et al. challenge the conventional demographic interpretation that reductions in fertility in industrialized nations can be explained completely by changes in the choices that women are making about whether and when to bear children.

Jensen et al. explore instead the possibility that part of the decline in fertility rates in the industrialized world that has occurred over the past several decades is a result of long-term trends in male reproductive health, particularly sperm density and quality. They propose the possibility that fewer babies are born in part because couples are having greater difficulty conceiving.

Jensen et al. do not argue that reductions in male reproductive health are the sole contributor to the demographic trends, nor do they conclude with certainty that reductions in sperm quantity and quality are involved, but they make a plausible case for the possibility that sperm declines are contributing to changes in industrialized country birth rates.

If substantiated by further research, this is a profoundly important reinterpretation of conventional demographic wisdom. Jensen et al.'s provocative arguments warrant further investigation, not just in Denmark but in other industrialized countries that are experiencing similar declines.

Birth rates have declined dramatically in industrialized countries to the point now that in many countries the birth rate is below the level (absent immigration) required to sustain population size, generally accepted to be 2.1 children per couple. In Italy and Spain it has fallen as low as 1.2 children per couple. These declines below replacement are causing significant changes in social conditions because they lead to a change in the age structure of the population. Ageing populations have fewer entry level workers and more senior citizens, putting severe strains on socio-economic systems.

I'm with Setanta. Keen on Drake and iffy on Fermi. I don't think Fermi appreciates the difficulties of interstellar travel and communication.

"If even a very small fraction of the hundred billion stars in the galaxy are home to technological civilizations which colonize over interstellar distances, the entire galaxy could be completely colonized in a few million years. The absence of such extraterrestrial civilizations visiting Earth is the Fermi paradox."

Proposed solutions to the Fermi paradox either deny the possibility of extraterrestrial civilizations [1, 5], an assumption as yet unwarranted, or accept the possibility of extraterrestrial technological civilizations and propose explanations for why such civilizations may nevertheless not have colonized the galaxy. Explanations include suggestions that such civilizations collapse or blow themselves up, run out of resources, choose not to colonize, or chose to colonize but leave us alone. The difficulty with all such explanations is that they must all assume an unwarranted uniformity of motive for extraterrestrial civilizations over extremely long periods of time. If even a single civilization choses to colonize the galaxy, the explanations fail. It is useful, therefore, to try to look for explanations of the Fermi paradox which do not rely on uniformity of motive.

Proposed solutions to the Fermi paradox either deny the possibility of extraterrestrial civilizations [1, 5], an assumption as yet unwarranted, or accept the possibility of extraterrestrial technological civilizations and propose explanations for why such civilizations may nevertheless not have colonized the galaxy. Explanations include suggestions that such civilizations collapse or blow themselves up, run out of resources, choose not to colonize, or chose to colonize but leave us alone. The difficulty with all such explanations is that they must all assume an unwarranted uniformity of motive for extraterrestrial civilizations over extremely long periods of time. If even a single civilization choses to colonize the galaxy, the explanations fail. It is useful, therefore, to try to look for explanations of the Fermi paradox which do not rely on uniformity of motive.

Yes, it only takes one, but my point has been from the outset to examine what the likelihood is that they have been here, and if they have, what is the likelihood that we would know it, or could find it out.

So, even were there a million such civilizations, they have to have visited all of the roughly one hundred billion star systems which might reasonably qualify. And if all of these star systems have been visited, including our own, why, and when, would they come back?

You have pointed out that Drake's equation relies upon too many unknown variables. I'm simply applying the standard to Fermi's paradox.

Like all discussions of the Fermi paradox, solutions based on a percolation approach are dependent on the validity of its assumptions. Until we have either explored the galaxy or contacted extraterrestrial civilizations, all such assumptions can be challanged, however, I argue that the assumptions used are reasonable, and that in any case the assumptions made here are less universal and restrictive than those required by other analyses of the paradox.

Geoffrey Landis wrote:

Like all discussions of the Fermi paradox, solutions based on a percolation approach are dependent on the validity of its assumptions. Until we have either explored the galaxy or contacted extraterrestrial civilizations, all such assumptions can be challanged, however, I argue that the assumptions used are reasonable, and that in any case the assumptions made here are less universal and restrictive than those required by other analyses of the paradox.

By the way, add to that Terry's reasonable hypotheses--that aliens might be hostile, or that they might fear hostility. Again, i find the thesis unrealistically simplistic, and would revert to the point in Ros' source about the assumption of uniformity--it applies to the thesis as well as it applies to the criticisms of the thesis.

Mr. Landis' fondness with his own assumptions is not to be wondered at. Mr. Landis' assumptions, are, however, no better founded than the assumptions of those with whom he disagrees, and he simply chooses to declare them more reasonable.