There is a famous passage in A Study in Scarlet where Sherlock Holmes explains to Watson why, whenever he is told a fact about astronomy, he does his best to forget it.
“But the solar system!” Watson exclaims.
“What the deuce is it to me?” Holmes interrupts. “You say that we go round the sun. If we went round the moon it would not make a pennyworth of difference to me or to my work.”
I would guess that most Europeans felt the same way during the great debates over the Copernican and Ptolemaic theories, and that even today most laymen have a similar indifference toward the debates over contemporary models of the universe. No branch of physical science is more remote from the practical. What does it matter to you and me whether spacetime is infinite, or finite and closed like the surface of a sphere? What difference does it make if the universe expands forever until it dies from the cold, or if, after many billions of years, it starts to contract? Who cares whether the contraction will end in a black hole or whether the universe will bounce back into existence, as foretold by Hindu myths, to start another cycle in an endless round of cosmic rebirths?
Well, astronomers, physicists, and philosophers care, and it has always been impossible for them not to consider such questions. Why does a chicken cross the road? Because it’s there. Why do astrophysicists build models of the universe? Because the universe is there, and because they have in their heads the materials and the mathematical tools for constructing such models.
Indeed, the materials and tools are available in an abundance far exceeding that of earlier centuries. By “materials” I mean, of course, the entire body of scientific knowledge—never certain, always changing, but steadily improving in its power to explain and predict the peculiar behavior of the outside world. There are, therefore, excellent reasons to believe that today’s models of the universe “fit” reality better than the old ones.
Modern cosmology began with Einstein’s model of a finite yet unbounded universe. Although it cannot be visualized, it is easy to understand. A straight line is infinite and unbounded, but bend it through a space of two dimensions and it can form a circle. This “closed universe” is still one-dimensional, with no boundaries, but now it has a finite size. A plane is infinite and unbounded. Bend it through “three-space” (i.e., three-dimensional space) and it can be the closed surface of a sphere. Perhaps, said Einstein, our three-space bends through four-space to form the “surface” of a hypersphere. Like the circle and the sphere’s surface, such a space is unbounded in the sense that you can travel through it as far as you like, in any direction, and never reach an end. Nevertheless it is finite in size. To prevent gravity from collapsing the universe, Einstein posited an unknown repulsive force that preserves the cosmos in static equilibrium. Such serious flaws were found in this model that Einstein reluctantly abandoned it even before the evidence for an expanding universe became overwhelming.
Since then hundreds of books and thousands of papers have explored a bewildering variety of other models. Twenty years ago the two most fashionable were the big bang, skillfully defended by George Gamow in his widely read book, The Creation of the Universe, and the steady state, ably defended by Fred Hoyle in his equally popular book, The Nature of the Universe. The titles differ significantly in a single word. For Hoyle the universe had no moment of creation. It has always been the way it is, perpetually expanding, preserving its over-all structure by a continual creation of hydrogen in empty space.
To the great astonishment of Hoyle and his friends, the steady state theory was suddenly shot down in the mid-Sixties by one of the major astronomical discoveries of the century. The universe was found to be permeated with a microwave radiation that is extremely difficult to explain unless one assumes it to be an electromagnetic glow produced by a primeval explosion that gave birth to the cosmos. The big bang theory at once became the preferred “standard” model.
It is as hard today to find two astrophysicists who agree on all aspects of this model as it is to find two economists who agree on how to model the nation’s economy, but almost all astronomers now believe that the universe began, ten to twenty billion years ago, with a monstrous fireball that flung out matter and energy in all directions. The resulting universe has been expanding ever since.
Will the expansion ever stop? This depends on whether the amount of matter in the universe does or does not exceed a certain “critical density.” In relativity theory space is “bent” by the presence of matter, and the denser the matter the greater the curvature. Beyond the critical density the curvature is strong enough to close space back on itself as in Einstein’s static model. In such a closed cosmos gravity would be strong enough to slow the expansion and eventually reverse it. At the moment the density seems far short of the critical amount. There may, however, be enough matter hidden somewhere to alter the picture, so work is still being done by cosmologists who favor oscillating models.
What will be the ultimate fate of the universe if it goes into a contracting phase? Here again there is little agreement among experts. It could become a super black hole that would just sit there, wherever “there” is, doing nothing much except maybe rotate and radiate. It might go through what John Wheeler calls a worm hole to emerge as a white hole in some other spacetime. It might explode, producing another fireball that would start the whole show over again, perhaps (as Wheeler likes to think) with new kinds of particles and laws that would make a universe utterly unlike the one we know.
In spite of these wild controversies there is now surprising agreement on the nature of the last (if not only) fireball, and that is what Steven Weinberg’s excellent book is all about. He has not tried to write another book about the present structure of the universe. He has tried instead to put down in detail, but in a way that laymen can understand, what the best minds among today’s astrophysicists believe to be the most probable history of the universe during its first three minutes after the zero moment of explosion.
The standard model rests on a wondrous mix of astronomical data, relativity theory, and particle physics. Dr. Weinberg, a physics professor at Harvard and senior scientist at the Smithsonian Astrophysical Observatory, has a distinguished record of prize-winning work in all three fields. His book is science writing at its best. There is no sacrifice of accuracy for sensational effects, at the same time the difficult mathematics is kept to a minimum. The back of the book has a useful glossary, formulas for those who can understand, a selected list of references, and a good index.
Four preliminary chapters give the reader what he needs to know to follow the second part of the book in which Weinberg reconstructs what may have happened in the first few minutes of the universe’s existence. The reconstruction is in the format of a motion picture. At intervals the picture is stopped, and the frame is carefully examined.
The film begins (how could it be otherwise?) in mystery. What was the nature of the fireball at zero time? Weinberg postpones this question to a later chapter where he takes a few tentative peeks “behind the veil.” His first frame assumes a beginning, then stops the film at one-hundredth of a second after zero time. The temperature of the universe is 100,000 million degrees Kelvin. We see an undifferentiated soup of primitive particles and radiation, expanding with unimaginable rapidity, and in a state of almost perfect thermal equilibrium. “The universe is simpler and easier to describe than it ever will be again.”
The particles are electrons, positrons (antiparticles of electrons), and the massless particles: photons, neutrinos, and antineutrinos. (There are no “anti-photons” because the photon is its own antiparticle.) These particles are so closely packed that the soup’s density is almost 4,000 million times that of ordinary water.
This picture of the universe and the film of what happens next rest on such technical reasoning that most readers will find this the hardest part of the book to follow. The deductions draw on the laws of thermodynamics, relativity theory, and quantum mechanics, but in essence the reasoning is very much like that of Holmes when he reconstructed a crime.
The procedure is first to get all the relevant observational evidence one can. Holmes had his hand magnifier. The cosmologist has optical and radio telescopes, bubble chambers, and cyclotrons. To this evidence he applies logic, mathematics, and physical laws. For example, a well-established law says that the net electric charge of the universe cannot vary. Charged particles can be created and destroyed, but only in pairs of equal and opposite charge. Similar “conservation laws” apply to other “quantum numbers.” They put severe restraints on the composition and behavior of the universe in its first three minutes.
Slowly, bit by bit, like the investigators of a bombing, the cosmologist fits the broken pieces together and tries to reconstruct what must have happened. There is no way to identify the Mad Bomber, but shrewd guesses can be made about the bomb’s construction and what it did. It is, of course, a bizarre kind of bomb. The big explosion is not something that takes place in a universe. The explosion is the universe. From the observed rate of the universe’s expansion, the present state of its matter, the temperature of the microwave radiation, and a thousand other things, the cosmologist does his best to run the movie backward. His deductions are so complicated, so dependent on laws that could be scrapped tomorrow, that the wonder is that a standard model can be constructed at all. But back to our film.
Eleven hundredths of a second after the first frame the temperature has dropped to 30,000 million degrees and the density to thirty million times that of water. A second later the temperature is down to 10,000 million degrees and the density to 380,000 times that of water. After thirteen more seconds the temperature is 3,000 million degrees. Electrons and positrons are now furiously annihilating each other, and nuclei of deuterium (heavy hydrogen) are starting to form.
The fifth frame stops the action about three minutes after the first frame. The temperature is now a mere 1,000 million degrees, or seventy times as hot as the sun’s center. Tritium and helium nuclei are shaping up. A half hour later the soup is still too hot for electrons and nuclei to unite in stable atoms. This doesn’t happen until 700,000 years later. At that time the soup—mostly hydrogen and a smaller amount of helium—is starting to form galaxies and stars. “After another 10,000 million years or so, living beings will begin to reconstruct this story.”
One must not suppose that Weinberg’s breathtaking documentary, given in more vivid detail than my skimpy synopsis suggests, is put forth as “true.” Many earlier cosmologists, when they wrote for laymen, had an embarrassing habit of describing their favorite model as if they were describing astronomical facts, only to have the model fall apart a decade or so later. Weinberg is too good a scientist to harbor such illusions. He makes it quite clear that he is doing no more than giving an account of the first few minutes of the best model now available. He does maintain that for the first time in history there is enough data and sufficiently adequate theory to take this reconstruction seriously.
The reconstruction could not have been made had not the microwave radiation been detected in 1965 and found to have a temperature of three degrees Kelvin. This raises a puzzling question: Why was this historic discovery not made earlier? In 1948 Gamow and his associates Ralph Alpher and Robert Herman had predicted a radiation of about five degrees, and the details had been refined by them and others in 1953. Yet nobody thought it worthwhile to look for this dim remnant of the ancient fireball.
In what he calls a diversionary chapter Weinberg speculates on why the search was delayed. He finds three reasons: difficulties in the early big bang theories, a woeful lack of communication between theorists and experimentalists (radio astronomers simply did not know how easily the microwave radiation could be detected), and finally, physicists were in no mood to take seriously any theory about the universe’s origin.
The situation was indeed curious. Here was a respectable theory, along with a relatively simple way to confirm it, but radio astronomers did not bother to make the test. What happened was a reversal of the usual sequence. The observations were made casually by two teams of scientists working near each other geographically (one at Bell Telephone Laboratories, one at Princeton University), neither aware of what the other was doing or of the previous calculations by Gamow, Alpher, and Herman. Not until the results were announced did radio astronomers suddenly become interested in a theory they should have tested fifteen years before. We are so accustomed, writes Weinberg, to think of science history as “the great magical leaps of a Newton or an Einstein” that we forget “how easy it is to be led astray, how difficult it is to know at any time what is the next thing to be done.”
As to what happened at zero time, one scientist’s guess is as good as another because the temperature and density of the fireball would have been beyond the point at which quantum laws apply. There may have been a zero moment before which time itself has no meaning, a singularity about which nothing can be said. Perhaps, as one irreverent aphorism has it, the “big blast was produced by our farter who art in heaven.” Or the blast may have been the universe rebounding from a previous contraction, the fireball never reaching a temperature and density at which quantum theory is irrelevant.
On such mysteries let me quote a once popular writer before disclosing his name: “How their [modern cosmologists’] theories conflict is soon apparent. Next door neighbors? No, they are miles apart…. Some say the world had no beginning, and cannot end; others boldly talk of a creation… though it is by no means obvious how there could be place or time before the universe came into being…. Some circumscribe the All, others will have it unlimited.”
This is from an essay by Lucian, written in Greek in the second century! Today’s models of the universe are more complicated than those of the old Greeks and Romans, and presumably better confirmed, but they remain swathed in ultimate questions as familiar to the ancients as to us. We are no closer to answering them.
This brings me to my only caveat. Weinberg closes his valuable little book with a touch of metaphysics. Whatever cosmic model survives the rapidly expanding data, he writes, there is little comfort for us. “It is almost irresistible for humans to believe that we have some special relation to the universe, that human life is not just a more-or-less farcical outcome of a chain of accidents reaching back to the first three minutes, but that we were somehow built in from the beginning.”
After a brief description of how beautiful the earth looks below the airplane in which he is riding while he writes his epilogue, Weinberg continues: “It is very hard to realize that this all is just a tiny part of an overwhelmingly hostile universe. It is even harder to realize that this present universe has evolved from an unspeakably unfamiliar early condition, and faces a future extinction of endless cold or intolerable heat. The more the universe seems comprehensible, the more it also seems pointless.”
What does that last sentence mean? I take it to mean that in earlier ages, when little was known about natural laws, it was easier to suppose that the gods or God had designed the universe, including us, with some beneficent end in view. As a corollary, the more we learn about the universe, without finding any evidence of such purpose, the more meaningless the whole thing seems.
This certainly is how Weinberg feels. His final paragraph about science lifting human life above the level of farce and giving it the “grace of tragedy” is in the spirit of Bertrand Russell’s famous essay, “A Free Man’s Worship.” But if Weinberg is suggesting that the new cosmologies make the universe seem more pointless to scientists and thinkers in general, I believe he is mistaken. Of those great physicists and philosophers who thought the universe had a point, I am unable to think of a single one who would have been dismayed by any current model. A Russian astronaut may bring back the news that he failed to see God in outer space, but surely this observation, or rather the lack of one, has nothing to do with serious philosophy or theology. Scientists like to imagine that advances in their specialty somehow have grave metaphysical consequences. The dull truth is that the great eternal questions are unaffected by oscillating cosmological fashions.
If God or the gods, or the Old One (as Einstein liked to call Everything), had a transcendent reason for bringing us into existence, what does it matter whether the first man and woman were formed in one day from the dust of the ground, as Genesis has it, or evolved over billions of years from the dust of a primeval fireball? The fact that we are here proves that we derive, in some crazy sense, from the fireball, and I for one find this more miraculous than the Genesis story. We know we are fated to die and to return to the dust. That is a stark fact that does indeed seem pointless, and the writers of Ecclesiastes and Job understood it as well as any modern physicist.
As a layman I like to keep up with the latest developments in cosmology, and I am fascinated by the arguments of rival authorities. Weinberg’s book, to quote Isaac Asimov on the back cover, “is a tremendous service to us all.” But when it comes to deciding on the basis of the latest model whether the universe has a point or not, I find myself in sympathy with the youthful Holmes.