The age of the atoms
Before we can discuss the basic problem of the origin of our universe, we must ask ourselves whether such a discussion is necessary. Could it not be true that the universe has existed since eternity, changing slightly in one way or another in its minor features, but always remaining essentially the same as we know it today? The best way to answer this question is by collecting information about the probable age of various basic parts and features that characterise the present state of our universe.
For example, we may ask a physicist or a chemist: "How old are the atoms that form the material from which the universe is built? Only half a century ago, before the discovery of radioactivity and its interpretation as the spontaneous decay of unstable atoms, such a question would not have made much sense. Atoms were considered to be basic indivisible particles and to have existed as such for an indefinite period of time. However, when the existence of natural radioactive elements was recognized, the situation became quite different. It became evident that if atoms of radioactive elements had been formed far back in time, they would by now have decayed completely and disappeared. Thus the observed relative abundances of various radioactive elements may give us some clue as to the time of their origin. We notice first of all that thorium and the common isotope of uranium (U238) are not markedly less abundant than the other heavy elements, such as for example, bismuth, mercury, or gold. Since the half‑life periods of thorium[1] and of common uranium are respectively 14 billion[2] and 4.5 billion years, we must conclude that these atoms were formed not more than a few billion years ago. On the other hand, as everybody knows nowadays, the fissionable isotope of uranium (U235) is very rare, constituting only 0.7% of the main isotope U238. The half‑life of U235 is considerably shorter than that of U238, being only about 0.9 billion years. Since the original amount of fissionable uranium has been divided by two every 0.9 billion years, it must have taken about seven such periods[3], or about 6 billion years, to bring it down to its present rarity, if both isotopes were originally present in comparable amounts.
Similarly, in a few other radioactive elements, such as naturally radioactive potassium, the unstable isotopes are also always found in very small relative mounts. This suggests that these isotopes were reduced quite considerably by slow decay taking place over a period of a few billion years. Of course, there is no a priori reason for assuming that all the isotopes of a given element were originally present in exactly equal amounts. But the coincidence of the results is significant, inasmuch as; it indicates the approximate date of the formation of these atoms. Furthermore, no radioactive elements with half‑life periods shorter than a substantial portion of a billion years are found in nature, although they can be produced artificially in atomic piles. This also indicates that the formation of atomic species must have taken place not much more recently than a few billion years before the present time. Thus there is a strong argument for assuming that radioactive atoms and, along with them, all other stable atoms, were formed under some unusual circumstances which must have existed in the universe a few billion years ago.
The age of the rocks
As the next step in our inquiry, we may ask a geologist: "How old are the rocks that form the crust of our globe?" The age of various rocks‑that is, the time that has elapsed since their solidification from the molten state can be estimated with great precision by the so‑called radioactive‑clock method. This method, which was originally developed by Lord Rutherford, is based on the determination of the lead content in various radioactive minerals such as pitchblende and uranite The significant point is that the natural decay of radioactive materials results in the formation of so‑called radiogenic lead isotopes. The decay of thorium produces the lead isotope, Pb208, whereas the two isotopes of uranium produce Pb207 and Pb208 These radiogenic lead isotopes differ from their companion Pb204, natural lead, which is not the product of decay of any natural radioactive element.
As long as the rock material is in a molten state, as it is in the interior of the earth, various physical and chemical processes may separate the newly produced lead from the mother substance. However, after the material has become solid and ore has been formed, radiogenic lead remains at the place of its origin. The longer the time period after solidification of the rock, the larger the amount of lead deposited by any given amount of a radioactive substance. Therefore, if one measures the relative amounts of deposited radiogenic lead isotopes and the lead‑producing radioactive substances (that is, the ratios: Pb203/Th232, Pb207/U235, and Pb208/U238) and if one knows the corresponding decay rates, one can get three independent (and usually coinciding) estimates of the time when a given radioactive ore was formed. By applying this method to radioactive deposits that belong to different geological eras, one gets results of the kind shown in the following table. The last two minerals are the oldest yet found, and from their age we must conclude that the crust of the earth is at least two billion years old.
