Read an Excerpt
THE CREATION OF THE UNIVERSE
By George Gamow
Dover Publications, Inc.Copyright © 1989 R. Igor Gamow
All rights reserved.
Evolution Versus Permanence
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 characterize the present state of our universe.
The age of the atoms
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 the atoms of the radioactive elements had been formed too 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 and of common uranium are 1.4 10110 and 4.5 109 years, respectively, 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 per cent of the main isotope; otherwise the Manhattan Project would have been as easy as fishing in a barrel. The half-life of U235 is considerably shorter than that of U238, being only about 0.9 109 years. Since the amount of fissionable uranium has been cut in half every 0.9 109 years, it must have taken about seven such periods, or about 6 109 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 amounts. 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 produced in exactly equal amounts. But the coincidence of the results is significant, inasmuch as it indicates the approximate date of the formation of these nuclei. Furthermore, no radioactive elements with half-life periods shorter than a substantial portion of 109 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 uraninite. The significant point is that the natural decay of radioactive materials results in the formation of the so-called radiogenic lead isotopes. The decay of thorium produces the lead isotope Pb208, whereas the two isotopes of uranium produce Pb201 and Pb206. 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: Pb208/Th232, Pb207 /U235, and Pb206/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 mineral in the table is the oldest yet found, and from its age we must conclude that the crust of the earth is at least 2.7 109 years old.
A much more elaborate method was proposed recently by the British geologist Arthur Holmes. This method goes beyond the formation time of different radioactive deposits and claims an accurate figure for the age of the material forming the earth. Perhaps the simplest way to illustrate it is by way of a story about an absent-minded Western rancher. This rancher remembers that one day in the spring he let all his cattle out to graze on his pastures, but he cannot recall the exact date on which he did so. He also remembers that at various dates during the summer he was collecting the cattle from different pastures and locking them into newly built corrals (one corral on each pasture), but these dates he has forgotten too. Is there any way for him to reconstruct the sequence?
Yes, there is, provided that he does not mind handling the dung produced by his cattle in the corrals and on the pastures. The reader has probably guessed that the dung produced by the cattle serves here as a symbol of the lead produced by decaying uranium, and that locking up the cattle in corrals represents the formation of radioactive deposits in solidifying rocks. It would be easy, of course, to find out at what approximate dates the different corrals were occupied by measuring the total amount of accumulated dung in each corral and then dividing that amount by the dung productivity of the corresponding herd. (This is exactly like the radioactive-clock method for determining the age of rocks.) But what about the date on which the cattle were first let out into the pastures—the date that radioactive atoms were formed?
At first glance it might seem possible to apply here a similar method, by collecting all the dung produced by the cattle while they were grazing in the open. However, this might not give a correct answer, since there could have been some "primordial" dung in the field which was present before the cattle were first let out (representing the lead which originated simultaneously with uranium during the epoch when all atoms were formed, and which was not produced by uranium decay at a later time). Of course, the same objection could be made against the use of the dung method for estimating the ages of the separate corrals; but there, because of the small area of the corrals, the amount of primordial dung can be readily discounted, compared with what the cattle would produce in the course of even a few days. In the open field, on the other hand, the situation is entirely different and the existence of primordial dung may influence the result quite appreciably.
Considering the problem in more detail, we must realize, assuming the amount of primordial dung on all pastures to be the same (hypothesis of the uniformity of the original atom-making), that there would be less dung in those pastures from which the cattle were driven into corrals at the earlier dates. (Indeed, geologists do find less radiogenic lead in rocks of higher geological age.) For each pasture we can start with that amount of dung which did not change from the time the cattle were put into the corral and go back in time, subtracting the day-by-day dung production. In this way we shall come to a day (in the spring) when the dung in the fields was at zero. If no primordial dung had been present, that day would represent the first date forgotten by the rancher. But if primordial dung were present (its amount, of course, being unknown), this procedure, applied to specific pastures, would fail to give us the answer. However, the situation is entirely different if we compare the data supplied by several pastures. The curves representing the past history of dung deposits on different pastures will be, generally speaking, different, since they depend on the size of the fields, the number of cattle, the dates of corral building, etc. But if we plot all these curves on one diagram (as in Fig. 1) they should intersect in the same point, giving us both the forgotten date on which the cattle were let out to pasture and the amount of primordial dung present at that date. By applying this method (amplified by the introduction of flocks of sheep and sheep dung, to account for the two uranium isotopes U238 and U235 and the two radiogenic lead isotopes Pb206 and Pb207) to the relative amounts of lead isotopes found in rocks of different geological ages, Holmes found that all curves intersect near the point corresponding to a total age of 3.35 109 years, which must represent the correct age of our earth. "Elementary, my dear Watson!"
The age of the oceans
Having received so much help from the geologists as regards solid deposits, let us turn to them once more with the question: "How old are the oceans that cover so much of the earth?" Here the answer is not quite so exact. The method was first proposed more than two centuries ago by Edmund Halley, who predicted the periodicity of the comet that bears his name. His method is based on the fact that the salinity of ocean water is mainly due to salts brought in by the rivers. We know that river water contains small amounts of salts in solution, which make it taste different from rain water. These salts are washed out of the rocky surface of the earth, mostly by fast rivulets and streams that run down mountain slopes. The river water brought into the ocean basins evaporates; the vapor collects in clouds and then falls again as rain on the continents in a steady cycle. The salts do not evaporate; they continue to accumulate in the oceans, gradually increasing their salinity. Dividing the known total amount of salt at present dissolved in the oceans by the known amount of salt brought in yearly by rivers, we find that the salinity of the oceans increases by one-millionth of 1 per cent each century. It follows that, if conditions do not change in the future, all oceans will be saturated with salts (36 per cent) in about 3.5 109 years and will then be similar to the Dead Sea or Great Salt Lake. It also follows that the rivers must have been at work for about 3 108 years for the present amount of oceanic salts (3 per cent) to have accumulated.
This figure, however, is on the short side, as the present rate of deposit of salt is known to be exceptionally high. The reason is that during the greater part of the history of our globe the surface of the continents was quite flat. The old mountains had been completely washed away into the oceans and new mountains had not yet been formed by the gradual contraction of the earth's crust. (Geologists count at least ten such successive mountain- raising periods.) It is estimated very roughly that during these flatland periods the erosive action of rivers must have been no more than one-tenth of what it is at present. This would result in an estimated age of our oceans of a few billion years, a figure which agrees with the estimated age of the oldest rocks.
The age of the moon
After thanking geology for all this valuable information, let us turn to astronomy and ask about the age of various celestial bodies, starting with the question: "How old is the moon?" We learn first that our Queen of the Night has not always been where it is now, that in the distant past it was so close to the earth one could almost touch it by stretching one's hand above one's head (if at that early epoch there had been any animals possessing hands and heads). As was shown by the work of the British astronomer George Darwin (son of the biologist Charles Darwin), the moon is constantly receding from the earth. Its distance from the earth increases at the rate of about 5 inches every year. It goes without saying that even the most precise instruments could not possibly measure such a slight increase in the distance to the moon, and that this conclusion was reached in a roundabout but nevertheless perfectly reliable way.
To understand the argument we must remember that the interaction between the moon and the earth is most markedly displayed in the phenomenon of the tidal waves raised by the moon's attraction on the oceans of the earth. Tidal waves running around and around our globe encounter resistance in the form of the continents that stand in their way. If we could look at the earth-moon system from some fixed point in space, we should see the body of the earth rotating inside the two tidal bulges, much as the axle of a wheel rotates between two brake shoes. Thus we should expect that the rotation of the earth would be slowed down gradually and that this, in turn, would cause a gradual increase in the length of our day. According to a fundamental law of mechanics, known as the law of conservation of angular momentum, this lengthening of the day must result in a lengthening of the rotation period of the moon (month) and in a gradual increase in its distance from the earth.
It has been estimated that tidal friction will lengthen the day by about one-thousandth of a second per century, and will increase the length of the month by one-eighth of a second per century, besides causing the increase mentioned in the moon's distance from the earth. Small as they may seem, these estimated changes in the length of the day and of the month can be checked by direct astronomical observation. In fact, these changes advance the position of the sun among the fixed stars by 0.75 seconds of arc, and the position of the moon by 5.8 seconds of arc, every century. Actual observations give the values of 1.5 ± 0.3 and 4.3 ± 0.7, which are in reasonable agreement with the estimated effect. Consequently there can be little doubt about the accuracy of the estimated increase in the distance between the earth and the moon.
By means of mathematical calculations George Darwin reached the conclusion that the moon must have been practically in contact with the earth about 4 109 years ago. One surprising result of these calculations is that, at that time, the length of a month (moon's orbital period) was equal to the length of a day (earth's diurnal period), both being equal to 7 present-day hours!
At that early epoch, the moon must have hung motionless above the same point of the earth's surface, the point at which it was born by being drawn out from the mother body by the tidal forces of the sun. We may appropriately call this early state of our satellite a Hawaiian Moon, since in all likelihood its birthplace was the middle of the Pacific Ocean. In fact, there is evidence to support the assumption that the Pacific Basin is nothing but a giant scar in the granite skin of Mother Earth, a constant reminder of the birth of her first and only daughter.
Excerpted from THE CREATION OF THE UNIVERSE by George Gamow. Copyright © 1989 R. Igor Gamow. Excerpted by permission of Dover Publications, Inc..
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.