Broca's Brain - Carl Sagan [48]
In this chapter I have done my best to analyze critically the thesis of Worlds in Collision, to approach the problem both on Velikovsky’s terms and on mine—that is, to keep firmly in mind the ancient writings that are the focus of his argument, but at the same time to confront his conclusions with the facts and the logic I have at my command.
Velikovsky’s principal thesis is that major events in the history of both the Earth and the other planets in the solar system have been dominated by catastrophism rather than by uniformitarianism. These are fancy words used by geologists to summarize a major debate they had during the infancy of their science which apparently culminated, between 1785 and 1830, in the work of James Hutton and Charles Lyell, in favor of the uniformitarians. Both the names and the practices of these two sects evoke familiar theological antecedents. A uniformitarian holds that landforms on Earth have been produced by processes we can observe to be operating today, provided they operate over immense vistas of time. A catastrophist holds that a small number of violent events, occupying much shorter periods of time, are adequate. Catastrophism began largely in the minds of those geologists who accepted a literal interpretation of the Book of Genesis, and in particular the account of the Noahic flood. It is clearly no use arguing against the catastrophist viewpoint to say that we have never seen such a catastrophe in our lifetimes. The hypothesis requires only rare events. But if we can show that there is adequate time for processes we can all observe operating today to produce the landform or event in question, then there is at least no necessity for the catastrophist hypothesis. Obviously both uniformitarian and catastrophic processes can have been at work—and almost certainly both were—in the history of our planet.
Velikovsky holds that in the relatively recent history of the Earth there has been a set of celestial catastrophes, near-collisions with comets, small planets and large planets. There is nothing absurd in the possibility of cosmic collisions. Astronomers in the past have not hesitated to invoke collisions to explain natural phenomena. For example, Spitzer and Baade (1951) proposed that extragalactic radio sources may be produced by the collisions of whole galaxies, containing hundreds of billions of stars. This thesis has now been abandoned, not because cosmic collisions are unthinkable, but because the frequency and properties of such collisions do not match what is now known about such radio sources. A still popular theory of the energy source of quasars is multiple stellar collisions at the centers of galaxies—where, in any case, catastrophic events must be common.
Collisions and catastrophism are part and parcel of modern astronomy, and have been for many centuries (see the epigraphs at the beginning of this chapter). For example, in the early history of the solar system, when there were probably many more objects about than there are now—including objects on very eccentric orbits—collisions may have been frequent. Lecar and Franklin (1973) investigate hundreds of collisions occurring in a period of only a few thousand years in the early history of the asteroid belt, to understand the present configuration of this region of the solar system. In another paper, entitled “Cometary Collisions and Geological Periods,” Harold Urey (1973) investigates a range of consequences, including the production of earthquakes and the heating of the oceans, which might attend the collision with the Earth of a comet of average mass of about 1018 grams. The Tunguska event of 1908, in which a Siberian forest