by Isaac Asimov
And then something new and completely unexpected turned up, and the physicists’ case began to crumble.
In 1896, the discovery of radioactivity made it clear that the earth’s uranium and other radioactive substances were liberating large quantities of energy and had been doing so for a very long time. This finding made Kelvin’s calculations meaningless, as was pointed out first, in 1904, by the New Zealand-born British physicist Ernest Rutherford in a lecture—with the aged (and disapproving) Kelvin himself in the audience.
There is no point in trying to decide how long it would take the earth to cool if you do not take into account the fact that heat is being constantly supplied by radioactive substances. With this new factor, it might take the earth billions of years, rather than millions, to cool from a molten mass to its present temperature. The earth might even be warming with time.
Actually, radioactivity itself eventually gave the most conclusive evidence of the earth’s age (in ways that will be described later in chapter 6) for it allowed geologists and geochemists to calculate the age of rocks directly from the quantity of uranium and lead they contain. By the clock of radioactivity, some of the earth’s rocks are now known to be over 3 billion years old, and there is every reason to think that the earth is somewhat older than that. An age of 4.6 billion years for the earth in its present solid form is now accepted as likely. And, indeed, some of the rocks brought back from our neighbor world, the moon, have proven to be nearly that old.
THE SUN AND THE SOLAR SYSTEM
And what of the sun? Radioactivity, together with discoveries concerning the atomic nucleus, introduced a new source of energy, much larger than any known. In 1930, the British physicist Sir Arthur Eddington set a train of thought working when he suggested that the temperature and pressure at the center of the sun must be extraordinarily high: the temperature might be as high as 15 million degrees. At such temperatures and pressures, the nuclei of atoms could undergo reactions that could not take place in the bland mildness of the earth’s environment. The sun is known to consist largely of hydrogen. If four hydrogen nuclei combined (forming a helium atom), they would liberate large amounts of energy.
Then, in 1938, the German-born American physicist Hans Albrecht Bethe worked out two possible ways in which this combination of hydrogen to helium could take place under the conditions at the center of stars like the sun: one way involved the direct conversion of hydrogen to helium; the other involved a carbon atom as an intermediate in the process. Either set of reactions can occur in stars; in our own sun, the direct hydrogen conversion seems to be the dominant mechanism. Either brought about the conversion of mass to energy. (Einstein, in his special theory of relativity, advanced in 1905, had shown that mass and energy were different aspects of the same thing and could be interconverted; and, furthermore, that a great deal of energy could be liberated by the conversion of a small amount of mass.)
The rate of radiation of energy by the sun requires the disappearance of solar mass at the rate of 4,200,000 tons per second. At first blush, this seems a frightening loss, but the total mass of the sun is 2,200,000,000,000,000,000,000,000,000 tons, so the sun loses only 0.00000000000000000002 per cent of its mass each second. If the sun has been in existence for 5 billion years, as astronomers now believe, and if it has been radiating at its present rate all that time, it would have expended only 1/33,000 of its mass. It is easy to see, then, that the sun can continue to radiate energy at its present rate Ior billions of years to come.
By 1940, then, an age of nearly 5 billion years for the solar system as a whole seemed reasonable. The whole matter of the age of the universe might have been settled, but astronomers had thrown another monkey wrench into the machinery. Now the universe as a whole seemed too youthful to account for the age of the solar system. The trouble arose from an examination of the distant galaxies by the astronomers and from a phenomenon first discovered in 1842 by an Austrian physicist named Christian Johann Doppler.
The Doppler effect is familiar enough; it is most commonly illustrated by the whistle of a passing locomotive, which rises in pitch as it approaches alld drops in pitch as it recedes. The change in pitch is due simply to the fact thai the number of sound waves striking the eardrum per second changes because of the source’s motion.
As Doppler suggested, the Doppler effect applies to light waves as well as to sound. When light from a moving source reaches the eye, there is a shift in frequency—that is, color—when the source is moving fast enough. For instance, if the source is traveling toward us, more light waves are crowded into each second, and the light perceived shifts toward the high-frequency violet end of the visible spectrum. On the other hand, if the source is moving away, fewer waves arrive per second, and the light shifts toward the low-frequency red end of the spectrum.
Astronomers have been studying the spectra of stars for a long time and are well acquainted with the normal picture—a pattern of bright lines against a dark background or dark lines against a bright background showing the emission or the absorption of light by atoms at certain wavelengths, or colors. They have been able to calculate the velocity of stars moving toward or away from us (radial velocity) by measuring the displacement of the usual spectral lines toward the violet or red end of the spectrum.
It was the French physicist Armand Hippolyte Louis Fizeau who, in 1848, pointed out that the Doppler effect in light could best be observed by noting the position of the spectral lines. For that reason, the Doppler effect is called the Doppler-Fizeau effect where light is concerned (figure 2.4).
Figure 2.4. The Doppler-Fizeau effect. The lines in the spectrum shift toward the violet end (left) when the light source is approaching. When the source recedes, the spectral lines shift toward the red end (right).
The Doppler-Fizeau effect has been used in a variety of ways. Within our solar system, it could be used to demonstrate the rotation of the sun in a new way. The spectral lines originating from that limb of the sun being carried toward us in the course of its vibration would be shifted toward the violet (a violet shift). The lines from the other limb would show a red shift since it was receding.
To be sure, the motion of sunspots is a better and more obvious way of detecting and measuring solar rotation (which turns out to have a period of about 26 days, relative to the stars). However, the effect can also be used to determine the rotation of featureless objects, such as the rings of Saturn.
The Doppler-Fizeau effect can be used for objects at any distance, so long as those objects can be made to produce a spectrum for study. Its most dramatic victories, therefore, were in connection with the stars.
In 1868, the British astronomer Sir William Huggins measured the radial velocity of Sirius and announced that it was moving away from us at 29 miles per second. (We have better figures now, but he came reasonably close for a first try.) By 1890, the American astronomer James Edward Keeler, using more accurate instruments, was producing reliable results in quantity; he showed, for instance, that Arcturus was approaching us at a rate of 3.75 miles per second.
The effect can even be used to determine the existence of star systems, whose details cannot be made out by telescope. In 1782, for instance, an English astronomer, John Goodricke (a deaf-mute who died at twenty-two—a first-rate brain in a tragically defective body), studied the star Algol, whose brightness increases and decreases regularly. Goodricke explained this effect by supposing that a dark companion circles Algol, periodically passing in front of it, eclipsing it, and dimming its light.
A century passed before this plausible hypothesis was supported by additional evidence. In 1889, the German astronomer Hermann Carl Vogel showed that the lines of Algol’s spectrum undergoes alternate red and violet shifts that match its brightening and dimming. First it recedes while the dark companion approaches and then approaches while the dark companion recedes. Algol was seen to be an eclipsing binary star.
In 1890, Vogel made a similar and more general discovery. He found tha
t some stars were both advancing and receding: that is, the spectral lines showed both a red shift and a violet shift, appearing to have doubled. Vogel concluded that the star was an eclipsing binary, with the two stars (both bright) so close together that they appeared as a single star even in the best telescopes. Such stars are spectroscopic binaries.
But there was no need to restrict the Doppler-Fizeau effect to the stars of our galaxy. Objects beyond the Milky Way could be studied in this way, too. In 1912, the American astronomer Vesto Melvin Slipher found, on measuring the radial velocity of the Andromeda galaxy, that it was moving toward us at approximately 125 miles per second. But when he went on to examine other galaxies, he discovered that most of them were moving away from us. By 1914, Slipher had figures on fifteen galaxies; of these, thirteen were receding, all at the healthy clip of several hundred miles per second.
As research along these lines continued, the situation grew more remarkable. Except for a few of the nearest galaxies, all were fleeing from us. Further more, as techniques improved so that fainter, and presumably more distant, galaxies could be tested, the observed red shift increased further.
In 1929, Hubble at Mount Wilson suggested that there was a regular increase in these velocities of recession in proportion to the distance of the particular galaxy. If galaxy A was twice as far from us as galaxy B, then galaxy A receded at twice the velocity of galaxy B. This relationship is sometimes known as Hubble’s law.
Hubble’s law certainly continued to be borne out by observations. Beginning in 1929, Milton La Salle Humason at Mount Wilson used the 100-inch telescope to obtain spectra of ever dimmer galaxies. The most distant galaxies he could test were receding at 25,000 miles per second. When the 200-inch telescope came into use, still more distant galaxies could be studied; and by the 1960s, objects were detected so distant that their recession velocities were as high as 150,000 miles per second.
Why should this be? Well, imagine a balloon with small dots painted on it. When the balloon is inflated, the dots move apart. To a manikin standing on any one of the dots, all the other dots would seem to be receding, and the farther away from him a particular dot was, the faster it would recede. It would not matter on which particular dot he was standing; the effect would be the same.
The galaxies behave as though the universe were expanding like the three-dimensional skin of a four-dimensional balloon. Astronomers have now generally accepted the fact of this expansion, and Einstein’s “field equations” in his general theory of relativity can be construed to fit an expanding universe.
THE BIG BANG
If the universe has been expanding constantly, it is logical to suppose that it was smaller in the past than it is now; and that, at some time in the distant past, it began as a dense core of matter.
The first to point out this possibility, in 1922, was the Russian mathematician Alexander Alexandrovich Friedmann. The evidence of the receding galaxies had not yet been presented by Hubble, and Friedmann worked entirely from theory, making use of Einstein’s equations. However, Friedmann died of typhoid fever three years later at the age of thirty-seven, and his work was little known.
In 1927, the Belgian astronomer, Georges Lemaître, apparently without knowledge of Friedmann’s work, worked out a similar scheme of the expanding universe. Since it was expanding, there was a time in the past when it was very small and as dense as it could be. Lemaître called this state the cosmic egg. In accordance with Einstein’s equations, the universe could do nothing but expand; and, in view of its enormous density, the expansion had to take place with superexplosive violence. The galaxies of today are the fragments of that cosmic egg; and their recession from each other, the echo of that long-past explosion.
Lemaître’s work also went unnoticed until it was called to the attention of scientists generally by the more famous English astronomer Arthur Stanley Eddington.
It was the Russian-American physicist George Gamow, however, who, in the 1930s and 1940s, truly popularized this notion of the explosive start of the Universe. He called this initial explosion the big bang—the name by which it has been everywhere known ever since.
Not everyone was satisfied with the big bang as a way of starting the expanding universe. In 1948, two Austrian-born astronomers, Hermann Bondi and Thomas Gold, put forward a theory—later extended and popularized by British astronomer, Fred Hoyle—that accepted the expanding universe but denied a big bang. As the galaxies move apart, new galaxies form between them, with matter being created from nothing at a rate too slow to detect with present-day techniques. The result is that the universe remains essentially the through all eternity. It has looked as it does now through countless eons in the past and will look as it does now through countless eons in the future, so that there is neither a beginning nor an end. This theory is referred to as continuous creation and results in a steady-state universe.
For over a decade, the controversy between big bang and continuous creation went on heatedly, but there was no actual evidence to force a decision the two.
In 1949, Gamow had pointed out that, if the big bang had taken place, the radiation accompanying it should have lost energy as the universe expanded, and should now exist in the form of radio-wave radiation coming from all parts of the sky as a homogeneous background. The radiation should be characteristic of objects at a temperature of about 5° K (that is, 5 degrees above absolute zero, or −268° C). This view was carried farther by the American physicist Robert Henry Dicke.
In May 1964, the German-American physicist Arno Allan Penzias and an American radio astronomer, Robert Woodrow Wilson, following the advice of Dicke, detected a radio-wave background with characteristics much like those predicted by Gamow. It indicated an average temperature of the universe of 3° K. The discovery of this radio-wave background is considered by most astro nomers to be conclusive evidence in favor of the big-bang theory. It is now generally accepted that the big bang did take place, and the notion of continuous creation has been abandoned.
When did the big bang take place? Thanks to the easily measured red shift, we know with considerable certainty the rate at which the galaxies are receding. We need to know also the distance of the galaxies. The greater the distance, the longer it has taken them to reach their present position as a result of the recession rate. It is not, however, easy to determine the distance.
A figure that is generally accepted as at least approximately correct is 15 billion years. If an eon is 1 billion years, then the big bang took place 15 eons ago, although it might just possibly have taken place as recently as 10 eons ago or as long as 20 eons ago.
What happened before the big bang? Where did the cosmic egg come from?
Some astronomers speculate that actually the universe began as a very thin gas that slowly condensed, forming stars and galaxies perhaps, and continued to contract until it formed a cosmic egg in a big crunch. The formation of the cosmic egg was followed instantaneously by its explosion in a big bang, forming stars and galaxies again, but now expanding until some day it will be a thin gas again.
It may be that, if we look into the future, the universe will be expanding forever, growing thinner and thinner with a smaller and smaller overall density, approaching nearer and nearer to a vacuum of nothingness. And if we look into the past, beyond the big bang, and imagine time moving backward, again the universe will seem to be expanding forever and approaching a vacuum.
Such a “once in, once out” affair, with ourselves now occupying a place close enough to the big bang for life to be possible (were it not so, we would not be here to observe the universe and attempt to draw conclusions) is called an open universe.
There is no way now (and there may never be a way) to obtain any evidence for what happened before the big bang, and some astronomers are reluctant to speculate on the matter. Recently there have been arguments to the effect that the cosmic egg formed out of nothing, so that rather than a “once in once out” universe, there is simply a “once out” universe—still a
n open universe.
On this assumption, it may be that, in an infinite sea of nothingness, an infinite number of big bangs may occur at various times, and that ours is therefore but one of an infinite number of universes, each with its own mass, its own point of development, and, for all we know, its own set of natural laws. It may be that only a very rare combination of natural laws make possible stars, galaxies, and life, and that we are in one such unusual situation, only because we cannot be in any other.
Needless to say, there is no evidence yet for the appearance of a cosmic egg out of nothing or for a multiplicity of universes—and there may never be. It would, however, be a harsh world indeed if scientists were not allowed to speculate poetically in the absence of evidence.
For that matter, can we be sure the universe will expand forever? It is expanding against the pull of its own gravity, and the gravity may be sufficient tn slow the rate of recession to zero and eventually impose a contraction. The universe may expand and then contract into a big crunch and disappear back into nothingness—or expand again in a bounce and then some day contract again in an endless series of oscillations. Either way we have a closed universe.
It may yet be possible to decide whether the universe is closed or open, and I shall return to this matter later, in chapter 7.
The Death of the Sun
The expansion of the universe, even if it continues indefinitely, does not directly affect individual galaxies or clusters of galaxies. Even if all the distant galaxies recede and recede until they are out of range of the best possible instruments, our own galaxy will remain intact, its component stars held firmly within gravitational field. Nor will the other galaxies of the local group leave us. But changes within our galaxy, not connected with universal expansion and possibly disastrous to our planet and its life, are by no means excluded.