VOL. 52, 1964 CENTENNIAL: FIRST SCIENTIFIC SESSION 565 observed forbidden emission lines. No galaxies, or clusters of galaxies, are as- sociated with the apparently stellar images. The sources have small angular diameters, from radio and optical measures-less than a second of arc. If the red shifts are cosmological in origin, the distances of the objects are 2-4 billion light years, so that this upper limit means only that the diameters are less than 10,000 light years. No traces of stellar spectra are seen. Analysis of the spectra provides an acceptable range of space density of gas from 104 to 108 electrons per cubic centimeter, corresponding to masses of 109 to 104 l\1 0 and radii of 300 to less than one light year. The emission lines are broad, indicating internal or expansion velocities of 1500 km/sec. If they are massive and large, the internal kinetic energy is sufficient to maintain their luminosity for 105 years, if an efficient, un- known mechanism converts kinetic energy into magnetic field and relativistic electrons for the synchrotron process. But if they are small, they contain energy for only a few hundred years. Their light is almost certainly variable, suggesting small size. If the small size is proved correct, we require an unknown, enormous supply of energy from a very small volume. Chain reactions of supernovae would barely suffice, even in a very densely crowded group of stars. Thermal energies are 10 electron volts per nucleon; to maintain the luminosity for 500 years requires 30 kilovolts per nucleon, and for 200,000 years over 10 million volts per nucleon. Thus either an unknown new type of super supernova, an enormously massive star, or some unknown storehouse of energy is required to explain these great explosions. In the quasi-stellar radio sources like 3C48 or 3C273 we are not even certain whether the objects are in galaxies or are newly formed in intergalactic space. We do not know whether they represent new matter created by singularities in the geometry of space, or gathered from the intergalactic background, or whether they are common events in old galaxies as are the ordinary intense radio sources. In any case, they promise to carry cosmological studies further and to provide exciting work for astronomers, theoretical physicists, and cosmologists. THE HISTORY OF THE SOLAR SYSTEM BY FRED L. WHIPPLE SMITHSONIAN ASTROPHYSICAL AND HARVARD COLLEGE OBSERVATORIES Introduction.-What has happened in the past appears to be almost as vague and uncertain as what will happen in the future: the story depends upon the teller. Contrast the histories of a war as written by the winner and the loser, or the stories of a courtship as related by the bride, by the groom, or by friends. Note that observers of automobile accidents almost invariably disagree as to the actual se- quence of events. And so it is, even in science. We can reconstruct the history of the solar system with little more confidence than we can predict its future. Actually, we possess only a fragmentary knowledge of the system today and have inadequate theoretical tools to deal with many of the physical processes that have taken place. Since there are many contradictory arguments regarding the history of the solar Downloaded by guest on September 27, 2021 566 CENTENNIAL: FIRST SCIENTIFIC SESSION PROC. N. A. S. a~~~~~~~~~~~~ .............~~ ~ ...... FIG 1 A pleochroic halo system, this will also be a biased account stressing a few of the areas where there is a strong consensus, and presenting only part of the counterevidence where great uncertainties or unusually strong differences of opimion exist. We must make one general assumption to avoid wallowing in the quicksands of sheer speculation (or metaphysics?), viz., that the laws of nature have remained unchanged for some five aeons (an aeon is defined as 1O9 years). To my knowledge only one measurable quantity has certainly remained constant over some three aeons, 1 e , the range of alpha particles from specific radioactive atoms in mica, as evidenced in pleochroic halos1 (Fig. 1). As we shall see, today's measured ages of the Earth and meteorites agree well, although determined from different radioactive processes, thus indicating that certain relationships among the constants of radioactivity have not changed. There is no similar evidence to substantiate the assumption that such fundamental quantities as the velocity of light, the constant of gravitation, or other physical constants have remained unchanged during the past five aeons. Downloaded by guest on September 27, 2021 VOL.,52, 1964 CENTENNIAL: FIRST SCIENTIFIC SESSION 567 TABLE 1 RADIOACTIVE ATOMS TABLE 2 Half-life, AGES (IN AEONS, 109 YEARS) Atom Decay products aeons Method Earth Meteorites U2U Pb2E6 + 8 He4 4.51 U, Th-He -3 Max 4.0 (peak) U235 Pb2E7 + 7 He4 0.71 K-Ar -3 Max 4.3 (peak) Th232 Pb20' + 6 He4 13.9 Rb-Sr -3 Max 4.4 Rb87 se, 46 Lead-Lead 4.55 4.6 i 0.2 K40 (I11%) Ar40 1.25 (Note that P. Pochado and M. Schwarzschild [Ap. J., 139, 587 (1964) ] find for the Sun that g cannot have varied more rapidly than by (time)-02.) History of the Earth.-Assuming, however, that all is well with the physical laws, we can determine an age of the Earth, that is to say, the time interval since the Earth became something like its present self. At least we think we can. Figure 2 shows a few estimates of the age of the Earth. It begins with the adopted biblical value, the smallest value, yet the one that has been believed for the longest time. Following are values determined by Helmholtz from the rate of the contraction of the Sun, by Kelvin from heat conductivity, by Joly from the transport of salt to the oceans, by Holmes from radioactivity measures, and by recent investigators who employed a number of techniques involving radioactivity. On the average, the "age" of the Earth has been doubling every 15 years for the past three centuries; the rate, perhaps, has been somewhat faster during the past century. There are some notable exceptions to these more classical estimates. Some Eastern philoso- phies postulate much greater ages. In the West, James Hutton, a geologist of the late 18th century, could see no evidence of a beginning or an end to geological proc- esses. Perhaps these estimates best illustrate my first point that the past is highly variable. Actually, there is some real basis for thinking that the numbers are now converging. Other lecturers have discussed the facts of radioactivity and nucleogenesis, so I list in Table 1 some of the major processes of radioactivity that are useful in determining the age of the Earth and of meteorites; included are certain end products and the half-lives, or the intervals of time in which half of the atoms spontaneously decay. By measuring the parent atom and its daughter decay products in a sample for which both are preserved, we can now determine the length of time these atoms have been contained in the sample. The analysis is not so straightforward should the material contain some of the decay products initially. Thus age determinations by the strontiunm-rubidium method or by the lead produced from uranium and thorium become too complicated for our exposition. The gases helium and argon, of course, can leak out of some kinds of rocks or meteorites, and oftentimes ages calculated for them are less than the time since the material be- TABLE 3 came solid. THE TITIuS-BODE LAW Table 2 lists ages determined for the Planet Besis Sum/lO Distance, Earth and meteorites. Meteorites, of Mercury 4 + 0 0.4 0.39 are stones and have Venus 4 + 3 0.7 0.72 course, irons that Earth 4 +6 1.0 1.00 fallen from the sky. We have ample evi- Mars 4 + 12 1.6 1.52 dence that to the solar Asteroids 4 + 24 2.8 they belong sys- Jupiter 4 + 48 5.2 5.20 tem. No Earth rocks yet found have re- Saturn 4 + 96 10.0 9.54 mained cool and solid for more than three Uraflnus 4 + 192 19.6 19.18 Neptune ... ... 30.06 aeons. The lead-lead method, however, Pluto 4 + 384 38.8 39.52 Downloaded by guest on September 27, 2021 568 CENTENNIAL: FIRST SCIENTIFIC SESSION PROC. N. A. S. d0,? measures the age since the Earth was HOLMES assembled, regardless of thermal proc- 10'9 HUBBLEY _ esses, mixing, and loss of gases. Claire HUTTON L Patterson2 finds an age of 4.55 aeons K ELVIN 10 ,d° 10 - / 8-JOLY by this method. Many of the meteor- 10o/X-- LHLTZ ites solidified 4.0-4.4 aeons ago,3 but the age since their major consolidation, > to_ as measured by the lead-lead method, 5 10 zoois in/_excellent agreement with that of BISHOPusSHER the Earth. Thus, a mean value of the to4 age of the Earth and meteorites appears IO I/ to be within a few hundred million 1600 1700 DATE1800 1900 2000 years of 4.6 aeons. We am A. Fowler, 4 FIG. 2.-Dated estimates of the age of the Earth. fortunately, synthesizes the elements in time for the Earth to appear. There is, however, a real possibility not to be ignored that the lead-lead method may err appreciably; thus, the consolidation of the Earth and the parent bodies of the meteorites may have occurred earlier than we now believe (see Fig. 2). Newtonian Facts.-Until about the beginning of this century, our knowledge of the solar systemn consisted almost entirely of what I shall call Newtonian facts, or those deductions that could be drawn from geometrical observations interpreted according to Newton's laws of motion and gravitation.