Coming of Age in the Milky Way (32 page)
Read Coming of Age in the Milky Way Online
Authors: Timothy Ferris
Tags: #Science, #Philosophy, #Space and time, #Cosmology, #Science - History, #Astronomy, #Metaphysics, #History
Today the assertion that we live in an expanding universe rests upon three fundamental lines of research. The first is the Hubble law: The relation between the distances of galaxies and the redshift of their light appears to pertain to the limits of present-day observation—out to hundreds of millions of light-years—and the only known consistent explanation for such a state of affairs is that the redshifts are produced by the recession velocity of galaxies in an expanding universe. The second piece of evidence is the cosmic background radiation: It traces out the “black-body” curve that would characterize the spectrum of photons released in the big bang, and it is received at equal strength from all directions, except for a small an isotropy (or “hot spot”) introduced by the absolute motion of the earth within the overall cosmic framework. The third piece of data is chronological: The age of the universe inferred from the expansion velocity, some ten to twenty billion years, fits with the ages of the oldest known stars, some twelve to sixteen billion years, and with the temperature of the cosmic background radiation itself.
Whatever its other implications for human thought—and there are many—the expansion of the universe had the tremendous advantage of investing cosmology with a dimension of cosmic history. The structure of the universe, from that of atomic nuclei to the vast superclusters of galaxies that stretch across hundreds of millions of light-years of space, may now be seen to have evolved from prior structures; to explain their present disposition clearly requires that we gain a better understanding of their history. Even natural laws themselves may prove to have a mutable past. These considerations will be discussed in
Part III
of this book. But first we need examine how our species came to comprehend the depths of terrestrial and cosmic history. It’s time for time.
*
The one theory Hubble
did
know about that predicted the redshift-distance relation was that of the Dutch astronomer Willem de Sitter, who had published a model in which redshifts were generated, not by velocity in an expanding universe, but by the “De Sitter effect,” a bit of mathematical arcana with no known physical basis. The chilly reception afforded Hubble’s efforts to link his observations to the De Sitter effect did little to encourage Hubble’s already hesitant ventures into the theoretical side of cosmology.
*
Einstein was referring to Lemaître’s contention that cosmic rays, high-energy subatomic particles from space, had been generated in the primordial fireworks. This did not hold up in its specifics, but it anticipated aspects of George Gamow’s subsequent prediction that the universe might be suffused by a cosmic background radiation composed of ancient photons released by the big bang.
IME
The leading idea which is present in all our researches, and which accompanies every fresh observation, the sound which to the ear of the student of Nature seems continually echoed in every part of her works, is—Time!—Time!—Time!
—George Scrope
Change is my theme. You gods, whose power
has wrought
All transformations, aid the poet’s thought,
And make my song’s unbroken sequence flow
From earth’s beginnings to the days we know.—Ovid
S
ERMONS IN
S
TONES
We aspire in vain to assign limits to the works of creation in
space
, whether we examine the starry heavens, or that world of minute animalcules which is revealed to us by the microscope. We are prepared, therefore, to find that in
time
also the confines of the universe lie beyond the reach of mortal ken.—Charles Lyell
And this our life, exempt from public haunt,
Finds tongues in trees, books in the running
brooks,
Sermons in stones, and good in everything.—Shakespeare
T
he conception of time that held sway in ancient Greece was cyclical, and as closed as the crystalline spheres in which Aristotle imprisoned cosmic space. Plato, Aristotle, Pythagoras, and the Stoics all espoused the view, inherited from an old Chaldean belief, that the history of the universe consisted of a series of “great years,” each a cycle of unspecified duration that ended when the planets all came together in conjunction, unleashing a catastrophe
from the ashes of which the next cycle began anew. This process was thought to have been going on forever: As Aristotle reasoned, with a logic as circular as the motions of the stars, it would be paradoxical to think of time as having had a beginning
in time
, and so the cosmic cycles must eternally recur.
The cyclical view of time was not without its charms. It possessed a world-weary, urbane fatalism of the sort that so often appeals to the philosophically inclined, a tincture indelibly preserved by the Islamic historian Ahmad ibn Muhammad ibn ’Abd al-Ghaffar, al-Kazwini al-Ghifari, who recited the following parable of eternal recurrence:
I passed one day by a very ancient and wonderfully populous city, and asked one of its inhabitants how long it had been founded.
“It is indeed a mighty city,” he replied. “We know not how long it has existed, and our ancestors were on this subject as ignorant as ourselves.”
Five centuries afterwards, as I passed by the same place, I could not perceive the slightest vestige of the city. I demanded of a peasant, who was gathering herbs upon its former site, how long it had been [since the city was] destroyed.
“A strange question!” he replied. “The ground here has never been different from what you now behold.”
“Was there not once a splendid city here?” I asked.
“Never,” he replied, “so far as we have seen, and never did our fathers speak to us of any such a city.”
On my return there five hundred years afterwards, I found the sea in the same place. On its shores was a party of fishermen. I enquired how long the land had been covered by the waters.
“Is this a question for a man like you?” they said. “This spot has always been what it is now.”
Again I returned, five hundred years afterwards, and the sea had disappeared. I inquired of a man who stood alone upon the spot how long ago this change had taken place, and he gave me the same answer as I had received before.
Finally, on coming back again after an equal lapse of time, I found there a flourishing city, more populous and more rich in beautiful buildings than the city I had seen the first time, and when I would have informed myself concerning its origin,
the inhabitants answered me, “Its rise is lost in remote antiquity: We are ignorant how long it has existed, and our fathers were on this subject as ignorant as ourselves.”
1
Taken literally, cyclical time even proffered a species of immortality: As Aristotle’s pupil Eudemus of Rhodes told his students, “If you believe the Pythagoreans, everything will eventually return in the selfsame numerical order and I shall converse with you staff in hand and you will sit as you are sitting now, and so it will be in everything else.”
2
Whether for these or other reasons, cyclical time is still popular today, with many cosmologists arguing for “oscillating universe” models in which the expansion of the universe is envisioned as eventually coming to a halt, to be followed by cosmic collapse into the cleansing fires of the next big bang.
But for all its felicities, the old doctrine of infinite, cyclical history had the pernicious effect of tending to discourage attempts to gauge the genuine extent of the past. If cosmic history consisted of an endless series of repetitions punctuated by universal destruction, then it was impossible to determine what the total age of the universe might actually be: An infinite, cyclical past is by definition immeasurable—is “time out of mind,” as Alexander the Great used to say. Nor did cyclical time leave much room for the concept of evolution, the fruitful idea that there can be genuine innovation in the world.
The Greeks knew that the world changes, and that some of its changes are gradual. Living as they did with the sea at their feet and the mountains at their backs, they appreciated that waves erode the land, and were acquainted with the strange fact that seashells and fossils of marine creatures may be found on mountaintops far above sea level.
*
At least two of the realizations essential to the modern science of geology—that mountains can be thrust up from what was once a seabed, and that they can be worn down by wind and water—were mentioned as early as the sixth century
B.C
., by Thales of Miletus and Xenophanes of Colophon. But they tended to regard these transformations as mere details, limited to the current cycle of a cosmos that was in the long run eternal and unchanging.
“There is necessarily some change in the whole world,” wrote Aristotle, “but not in the way of coming into existence or perishing, for the universe is permanent.”
3
For science to begin to assess the antiquity of the earth and the wider universe—to locate humanity’s place in the depths of the past as it was to chart our location in cosmic space—it had first to break the closed circle of cyclical time and to replace it with a linear time that, though long, had a definable beginning and a finite duration. Curiously enough, this step was initiated by a development that was in most other respects a calamity for the progress of empirical inquiry—the ascent of the Christian model of the universe.
Initially, Christian cosmology diminished the scope of cosmic history, much as it shrank the spatial dimensions of the empirically accessible universe. The grand, impersonal sweep of the Greek and Islamic cycles of time were replaced, in Christian thought, by an abbreviated and anecdotal conception of the past, in which the affairs of men and God counted for more than the inhuman workings of water on stone. If history for Aristotle was like the turning of a giant wheel, for the Christians it was like a play, with a definite beginning and end, punctuated by unique, singular events like the birth of Jesus or the giving of the law to Moses.
Christian scholars estimated the age of the world by consulting scriptural chronologies of human birth and death—by adding up the “begats,” as they say. This was the method of Eusebius, Chairman of the Council of Nicaea convened by the emperor Constantine in
A.D
. 325 to define Christian doctrine, who determined that 3,184 years had elapsed between Adam and Abraham; of Augustine of Hippo when he estimated the date the Creation at about 5500
B.C.;
of Kepler, who dated it at 3993
B.C
.; and of Newton, who arrived at a date just five years earlier than Kepler’s. Its apotheosis came in the seventeenth century, when James Ussher, bishop of Armagh, Ireland, concluded that “the beginning of time … fell on the beginning of the night which preceded the 23 rd day of October, in the year … 4004
B.C.
”
4
Ussher’s spurious exactitude has made him the butt of many a latter-day scholarly snigger, but, for all its absurdities, his approach—and, more generally, the Christian approach to historiography—did more to encourage scientific inquiry into the past than had the lofty pessimism of the Greeks. By promulgating the idea that the universe had a beginning in time, and that the age
of the earth was therefore both finite and measurable, the Christian chronologists unwittingly set the stage for the epoch of scientific age-dating that followed.
The difference, of course, was that the scientists studied not Scriptures but stones. This was how the naturalist George Louis Leclerc expressed the geologists’s creed, in 1778:
Just as in civil history we consult warrants, study medallions, and decipher ancient inscriptions, in order to determine the epochs of the human revolutions and fix the dates of moral events, so in natural history one must dig through the archives of the world, extract ancient relics from the bowels of the earth, [and] gather together their fragments…. This is the only way of fixing certain points in the immensity of space, and of placing a number of milestones on the eternal path of time.
5
To learn from the stones, however, geologists had first to be able to see them, and here the steam engine, prime mover of the Industrial Revolution, played a key role. Steam-driven pumps evacuated water from coal mines in Germany and the north of England, making it possible to dig deeper than ever before; steam-driven hoists brought the coal to the surface; coal from the mines was then transported on barges through canals, and on railroad trains pulled by steam locomotives, to fuel the steam engines of the ships and factories of the industrially developing world. Canal water and steel rails have in common that both work best when level, and the engineers who dug the canals and laid the tracks dealt with hills that blocked their way by cutting through them whenever possible. In doing so they “opened the veins of the earth,” as the builders of the Great Wall of China had put it, exposing previously unseen layers of geological strata deposited over hundreds of millions of years. Budding geologists put to work in the field to help supervise these excavations found themselves presented with a gift as bounteous as a library—evidence of the long history of our planet, inscribed in the strata as if on the corrugated pages of an ancient book.
Among the first to learn to read the language of the stones were Abraham Gottlob Werner, a German mining geologist, and William Smith, an English canal surveyor and consulting engineer who helped excavate the Somersetshire Coal Canal in 1793. Werner
noted that the same strata could be found in the same order at widely separated locations, indicating that the mechanism that laid them in place had operated on a large scale. This implied that local strata might hold evidence of how the planet as a whole had changed. Smith, for his part, observed that the strata—laid out, as he put it, like “slices of bread and butter”—could be identified not only by their gross composition but also by the various sorts of fossils they contained. Crisscrossing the English countryside day and night, by coach on a company pass, Smith observed that “the same strata were found always in the same order and contained the same fossils.”
6
This, then, was a key to deciphering the hieroglyphics of the rocks—the realization that the world’s history could be read in the sequence of fossils the rocks contained.