B Berkeley G Ganapathy {Baker Co) M: Moscow LA UCLA S Swiss Group F: French

O Los Alamos N Netherlands C; Canadian or 100 million years has a reasonable probability^—rather than the one chance in a million that the iridium layer is unrelated to the foram extinction. Figure 4 was available to me in several pieces, in 1979.7 It shows the average time between collisions with the earth, of bolides of different sizes over the enormous diameter range of ten orders of magnitude, which translates directly into 30 orders of magnitude in bolide mass. The data for this figure come from many sources, including lunar and terrestrial craters as well as tiny craters found on the surfaces of recovered spacecraft, pins telescopic observations of earth orbit-crossing asteroids. Eugene Shoemaker is recognized as the leading expert in this field. There is of course a good explanation for this simple power law, and it is called comminution—the process by which collisions of two objects produce smaller objects. Figure 4 describes the "integral spectrum" of objects orbiting the sun in the neighborhood of the earth, and it effectively "runs out of rocks" on the large diameter end near 20 kilometers. If we look at the numbers carefully, we see that the earth should be hit by a bolide 10 kilometers or greater in diameter, about once every 100 million years. So one should not be surprised to learn that the earth was hit by a 10 km bolide 65 million years ago.

I'll now speak briefly on how theories become accepted; most laymen feel that theories can be proved or disproved, but with very few exceptions, theories can't be proved, but only disproved. For example, Newton's still extremely accurate theory of gravity was disproved by Einstein's theory (called General Relativity), but Einstein's theory wasn't proved in that process—some new theory may prove that Einstein was wrong. So how do some theories gain nearly universal acceptance, when proofs are so rare? The answer is that every useful theory explains all known observations, and makes predictions, and if the predictions turn out to be true, particularly if some of them are very surprising, then that theory becomes an accepted theory, even though someone may later find that one of

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