Plurality Enters the Scientific Era

Abstract

It is when we consider recent discoveries in their relation to the existence of other worlds, when we attempt to form a conception of the immense varieties of the forms of life corresponding to the innumerable varieties of cosmical structure disclosed by modern researches, that we recognise the full significance of those discoveries.

Appendix: The Drake Equation

This equation was proposed by Frank Drake in 1961 as an incentive for SETI. It expresses the number N of planets in the Galaxy that house a technologically advanced life form and are able and willing to communicate as the product of a number of factors:

N = n∗ f_p n_h f_l f_t f_c (t/10¹⁰),

where:

n∗ is the number of stars in the Galaxy;f_p is the percentage of stars with planets;n_h is the average number of habitable planets among them;f_l is the fraction of the preceding planets where life actually appeared;f_t is the fraction of the preceding planets where life is technically advanced;f_c is the fraction of the preceding planets wishing to communicate;t/10¹⁰ is the ratio of the average life span t of the corresponding civilizations, in years, to the average lifespan of the stars and their planets, estimated roughly at 10¹⁰ (10 billion) years.In 1961, the quantities f_p and n_h were totally unknown, as were all the other factors but n∗. In his original article, Drake adopted the following values:n∗ = 10¹¹ (all the Galaxy)f_p = 0.5;n_h = 2;f_l = 1;f_t = 0.01;f_c = 0.01;t = 10,000 years,which gives N = 10.

Given the immensity of the Galaxy, in which the Sun is just an ordinary star that distant aliens have no reason to chose as a target, this figure does not leave much hope. But of course many of the values above are eminently debatable! Let us propose some anew, just as a game. Every reader is free to contemplate alternatives.

Limiting to 1000 light years, a very optimistic value, the distance of stars such that their inhabitants have noticed the Solar system as a target, we take n∗ ≈ 15,000.

All stars have planets, so that f_p ≈ 1.

If we suppose that only systems more or less similar to the Solar system contain really habitable planets, the product n_h f_l is smaller than 10⁻⁴ as we have seen. But it might be somewhat larger.

The probabilities f_t and f_c are even more problematic. In a later study, Drake took very optimistically both of them to be equal to 0.3; let us keep this value.

Finally, we are in complete darkness as to the average life span of an advanced civilization. The example of our own civilization does little to encourage optimism: less than two centuries after the appearance of the first machines, man has built nuclear bombs capable of destroying himself, and the overpopulation and global warming that we are responsible for can also destroy our civilization someday. The 10,000 years of Drake, if they look short, could be an upper limit of the life span of our civilization. We do not believe like Stephen Hawking in a migration of the man towards Mars, where life would pose insurmountable problems. However, if we are wise enough not to destroy ourselves, we could be impacted by an asteroid or a cometary nucleus, which would eliminate all evolved life on Earth. The last of the great extinctions of advanced life, that of dinosaurs, goes back to 65 million years, and it is estimated that similar phenomena occur on average every 100 million years. Some attribute these extinctions to volcanism, against which nothing can be done; if, as others think, they are due to the fall of a projectile, perhaps we will be able to divert this projectile and avoid them in the future. There is also another limit: the inevitable increase in the brightness of the Sun is such that the Earth will emerge from the habitable zone in about 1.5 billion years. This would be a strict upper limit for the life-span of man, but it seems more reasonable to take it as 100 million years. Even so, we arrive at N = 13. Pretty small, but is it not still optimistic?