Читать книгу Possible Worlds and Other Essays - J. B. S. Haldane - Страница 5

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FIVE hundred years ago the human mind was limited to a tiny patch of space, and the universe must have seemed even smaller after Magellan’s men had girdled the earth. The heavenly bodies were known to be distant, but it was not clear that celestial distances were so much greater than terrestrial. So Cassini’s proof, in a.d. 1672, that the sun was nearly a hundred million miles away was at first too shocking a fact for the mind to accept. Only eighty-nine years ago Bessel measured the distance of one of the nearest fixed stars, 700,000 times greater than that of the sun, and to-day Hubble and other astronomers are estimating distances several million times those that staggered our great-grandfathers.

The range of our minds in time is also increasing, but the process has been slower, partly because time is harder to measure than space, and partly because the chronology of the Old Testament is more precise than its astronomy. So when it was admitted that the earth was older than the six or eight thousand years which the biblical record allowed, scientific men were at first very moderate in their estimates of geological time. Twenty-five years ago geologists and physicists would not admit that the earth could be more than twenty million years old, although the biologists were asking for hundreds of millions for the process of evolution.

In the last generation, however, evidence has accumulated along at least five different lines which allow us to measure the past with complete accuracy for nearly four thousand years, and with tolerable exactitude for over a thousand million. We may conveniently begin with the nearer dates which fall within the range of history. A generation ago the earliest date known with any certainty was that of the first Olympic Games, 776 b.c. Even if the accuracy of the ages of the patriarchs in the book of Genesis was accepted, the length of time between the births of Jacob and David was very uncertain; and the dates fixed by Archbishop Ussher were to that extent at least conjectural.

But there were certain records of eclipses on historic occasions whose dates were known within a few years. Now a total eclipse of the sun visible from any given place is a very rare event; indeed only five have been visible in any part of the British Isles since a.d. 1433. So if we know the place where the eclipse was total, and the date within a century or so, we can calculate the latter with great accuracy. Every one, for example, has heard of Tweedledum and Tweedledee, whose battle was interrupted by a monstrous crow as big as a tar-barrel. The true story of these heroes is as follows: King Alyattes of Lydia, father of the celebrated Croesus, had been engaged for five years in a war with Cyaxares, king of the Medes. In its sixth year, on May 28, 585 b.c., as we now know, a battle was interrupted by a total eclipse of the sun. The kings not only stopped the battle, but accepted mediation. One of the two mediators was no less a person than Nebuchadnezzar, who in the preceding year had destroyed Jerusalem and led its people into captivity. Other eclipses recorded by the Assyrians enable us to date their kings who were contemporary with the kings of Judah and Israel, and incidentally show us that Archbishop Ussher was forty-six years out in his chronology of that period. This is no discredit to the learned prelate, but is highly disgraceful to the publishers who continue to print bibles containing it, and the clergy who continue to use them. Whoever else may have been inspired, Archbishop Ussher was not, and we need not pay much attention to clergymen who protest their reverence for Scripture, and yet continue to use, or permit their flocks to use, bibles adorned with the conjectures of an Irish divine whose political talents were at least as marked as his intellectual.

Readers of Homer will remember that Odysseus’ return to Ithaca was marked by an eclipse of the sun which portended the doom of Penelope’s suitors. As early as a.d. 1612 the attempt was made to date the fall of Troy by this means. But it was only in 1925 that Dr. Schoch of Munich, using far more exact tables of the moon’s motion, arrived at the startling result that in the year 1178 b.c. there actually was a total eclipse of the sun in or very near to Ithaca at 11.41 a.m. on April 10th. Since the track of an eclipse is only 120 miles broad at most and generally less, and Ithaca is only 15 miles long, the sun has probably not been totally eclipsed in Ithaca since Odysseus’ time, or for thousands of years before. Now the most probable date for the siege of Troy was generally given at about 1200 b.c., so presumably both this date and Homer’s story of the eclipse were approximately correct. One need not suppose that the suitors were actually killed on the day of the eclipse; but for the hero’s return and the darkening of the sun to become connected in local tradition, as they apparently were, they must have occurred within a few years of one another.

The first date which is known with nearly complete certainty is 1915 b.c. We have accurate tables of the appearances and disappearances of the planets in the reign of King Ammizaduga, tenth king of the first dynasty, who reigned in the city of Babylon from 1922 to 1902 b.c. In the sixth year of his reign, for example, we read on a cuneiform tablet recently discovered, ‘In the month Arahsamnu on the 28th day Venus disappeared in the west (i.e. as an evening star). Three days she tarried in heaven, and rose in the east on the first day of Kislev.’ On the basis of such data as these, Father Kugler, a German Jesuit, and Dr. Fotheringham of Oxford, have been able to arrive at the only possible system of dates which will fit the facts.

It is indeed fortunate that King Assurbanipal, who reigned in Nineveh from 668 to 626 b.c. approximately, was so addicted to astrology as to have copies made of the observations of predecessors who, when he lived, were already as remote as are King Penda of Mercia or the Caliph Ali to-day. The goodness of the agreement between dates found astronomically and those derived from lists of Mesopotamian dynasties has augmented the faith of historians in the latter, and by their use Professor Langdon of Oxford has calculated back to about 3357 b.c. as the date of the beginning of the second dynasty in the city of Ur. Unfortunately the kings who are recorded as having lived before this date are often alleged to have reigned for many centuries. If we allow them lives of a reasonable length we arrive at a date for the great Mesopotamian flood somewhere between 5000 and 6000 b.c. This event, which is probably historical, though greatly exaggerated, will not be fully explained till Iraq and Armenia have been studied by competent geologists. At this time a good deal of Scandinavia was still covered by an ice-sheet left over from the last glacial epoch, and the same was probably true of Armenia. Noah’s flood may well have been due to an abnormal thaw, perhaps accompanied by the bursting out of a lake or lakes pent up behind a glacier or moraine.

For in Scandinavia and Canada the melting ice has left very exact records, which Baron de Geer and his pupils have investigated. The whole of Scandinavia, 12,000 years ago, was covered by ice. Then the covering of its southern tip began to melt, and each year the thaw water from it deposited a layer of mud. At any given spot a number of such layers may be found wherever a road or railway cutting or a pit allows the examination of the subsoil. The thick layers due to warm years which thawed much ice can easily be identified. As one travels northward each layer is gradually overlaid by fresh ones and finally disappears. As lately as 9000 years ago the site of Stockholm was still covered by ice, but now the ice-fields are restricted to high ground. The final 7000 years in de Geer’s calculations were reached by the counting of annual layers of clay laid down in a lake. In Canada the northern ice-sheet probably reached the great lakes less than 20,000 years ago, though here the evidence is not so complete. De Geer’s counting of the mud bands gives us an idea of the geological time scale. There were four ice-ages during the Pleistocene period. The last of them was already waning 20,000 years ago, and as there were lengthy warm periods between them, the whole Pleistocene period must have lasted for some hundreds of thousands of years, perhaps the best part of a million. Similar bands, if they consist of mud laid down in annual floods, record the work of a great river in Burma in mid-tertiary times during about a million and a half years.

But the principal evidence for the geological time-scale is of a different kind. Uranium and thorium break down into a series of short-lived radio-active elements which end up as lead. If the rate of decay has always been the same as at present, half of any given mass of uranium is transformed in the course of about 4,600,000,000 years. The fixity of this rate may seem a large assumption. But it is justifiable for two reasons. Firstly no chemical or physical treatment has the slightest effect on it. Secondly the speed with which α-particles are shot out from radio-active atoms depends on their rate of decay. Now particles of radio-active matter in mica and other rocks are surrounded by definite spheres of discoloration where the α-particles from them have stopped. If the velocity and hence the range of these particles had altered during geological time these spheres would not be definite. Assuming then that the ‘clocks’ have not slowed down or speeded up one can use them to calculate the age of the rocks in the following way. Many volcanic minerals contain uranium or thorium but very little lead. But there is always some lead; and the older the rock, as judged by ordinary geological standards, the more lead is present. From its quantity we can calculate how long the change has been going on. This gives us the following ages for various strata.

(B.M. means before man. It does not matter what individual man we consider!):—

Eocene (London Clay)	60 million B.M.
Carboniferous (British coal measures)	260-300 million B.M.
Upper Pre-Cambrian	560 million B.M.
Oldest known rock	about 1500 million B.M.

These dates may be as much as ten per cent. out, but can hardly be a great deal more.

That is to say, 60,000,000 years ago our ancestors were mammals, probably not unlike lemurs, 300,000,000 years ago amphibians somewhat resembling newts or mud-puppies, and 500,000,000 years ago very primitive fish, combining some of the characters of sharks and lampreys. The origin of life on our planet was probably over a thousand million years ago, so that the record furnished by fossils only refers to half—perhaps much less than half of the time during which life has existed.

If all the lead in our planet is of radio-active origin, which is rather unlikely, it can hardly be more than eight thousand million years old. Astronomical evidence points to a somewhat smaller age.

As the earth goes round, the moon, and to a lesser extent the sun, raise tides in the sea. The energy used in raising them comes from the earth’s rotation, hence they slow it down and lengthen the day. The moon thus acts as a brake on the earth, and by so doing is pushed onwards in its orbit, and moves further away. If we calculate backwards instead of forwards we find both the day and the month becoming shorter, until at a sufficiently early date they possessed the same length of about four hours, and the moon was so near to the earth as to be practically touching it. It is fairly clear that the moon is a portion of the earth thrown off as the result of excessive rotation, almost certainly before the earth’s crust had solidified. Unfortunately the frictional effect of the tides depends on the detailed form of the sea’s bed. At present the main retarding action takes place in the Bering Sea. At a geological epoch characterised by many shallow and partly land-locked seas tidal friction must have been greater than now, at other times less. So we can only say that the moon was born somewhere about four thousand million years ago, but the true figure might be as low as one thousand million, or as high as twenty thousand.

The birth of the moon was only one event in a greater catastrophe. Our sun, after a relatively brief period, probably a few thousand million years or less, of youthful exuberance as a giant star radiating energy at thousands of times its present rate, settled down as a respectable dwarf, which it now is, and has been throughout geological time.

For many thousands of millions of years it probably shone as a lonely star unaccompanied by planets. Then it appears to have passed near to another, probably heavier star, which raised tidal waves in it. The detached crests of these waves, or one of them, formed the planets, and it is fairly clear that the moon broke off from the earth within a few years of its formation. So the approximate dating of the moon’s birth gives us that of the earth’s. This is further confirmed by the eccentricity of Mercury’s orbit, which is still far less circular than the earth’s, but is gradually settling down towards circularity. It can be calculated that it has not been going round the sun for more than ten or less than one thousand million years. Various other lines of evidence converge to a date somewhere between 8,000,000,000 and 1,500,000,000 B.M. for the origin of the solar system. If science continues we shall arrive at the exact date in the following way. The relative motions of the various ‘fixed’ stars will be determined, and on calculating backwards it will be found that one passed very near to our sun at a certain date in the remote past. The star in question must be very far away by now. It is a wise child that knows its own father, and we shall probably not know ours for thousands, perhaps hundreds of thousands of years.

It is possible to penetrate still further into the past and to arrive at a very rough date for the origin of the sun. But any such date depends on some particular hypothesis as to the origin of stellar energy, and there are several such hypotheses, leading to very different dates. On the other hand a number of independent arguments, based on well-ascertained facts, converge to the same date for the origin of the earth. There are, of course, respectable scientific theories, such as the planetesimal, which lead to different conclusions. The reason for rejecting such theories, and the detailed evidence for many of the dates here given, are to be found in such books as Jeffrey’s The Earth (Cambridge University Press). In a popular exposition it has been necessary to be dogmatic. If I have been so it is because I consider it unlikely that any of the figures I have given will be very seriously upset in the future.

In a few generations it is probable that these dates will meet with general acceptance and their meaning will gradually penetrate the human imagination. As the earth has lasted for at least a thousand million years in a condition not very unlike the present, it will probably continue habitable for a future period of at least the same order of magnitude, possibly for very much longer. An acceptance of such a future is bound to affect human thought. It will be realized that the things which seem to us most stable, such as human nature and the facts of geography, are really not only changeable but certain to change. On the other hand it will be realized that remarkably little change can occur within a lifetime.

Such a world-view leaves room for optimism in the most desperate circumstances, but yet reduces the probable effects of the vastest human efforts to the tiniest dimensions. As it is accepted, people will probably become more and more prone to devote themselves to their own affairs and those of their immediate neighbours. And when they turn their attention to greater things, they will perhaps be less occupied with institutions as ephemeral as nations. They will be more disposed to serve Man than England or America. A just law may outlive the state in which it was made, a scientific discovery the civilization which brought it forth.

And religion will inevitably alter its standpoint, even if some of its fundamental beliefs survive. On a planet more than a thousand million years old it is hard to believe—as do Christians, Jews, Mohammedans, and Buddhists—that the most important event has occurred within the last few thousand years, when it is clear that there were great civilizations before that event. It is equally difficult to doubt that many events as significant for humanity will occur in the future. In that immeasurable future the destiny of humanity dwarfs that of the individual. If our planet was created a few thousand years ago to end a few years or a few thousand years hence, it is conceivable that the main purpose to be worked out on it is the salvation and perfection of individual human beings. No religion which accepts geology can regard such a purpose as anything but subsidiary.

If we define religion as our attitude to the universe as a whole, the new time-scale will make us humbler as individuals, but prouder as a race. Our individual lives are the merest spangles of existence. The life of our ancestors goes back for a thousand million years. That of our descendants may last very much longer. And we cannot say with any certainty that it will not endure for ever.