by Judy Jones
One way of measuring the extent of our knowledge about the universe is to ask how close to the beginning we can get in our description of the events that formed it. In the 1920s, for example, we knew enough about atoms to get to within half a million years of creation. By the Fifties, the development of nuclear physics had allowed us to come within three minutes. Today, thanks to our understanding of elementary particles, we can get to within a fraction of a second—a fraction so small we need a different way of writing numbers to describe it. We can talk about time between 10-36 (that’s a decimal point followed by thirty-five zeros and a one) and 10-43 (forty-two zeros and a one) seconds after the Big Bang. In fact, only one hurdle remains before we get to the moment of creation itself.
The best way to picture the evolution of the universe is to think of a cloud of highly compressed steam. If you let it go, it will expand, cooling off as it does so. When it cools to 212°F it will condense into droplets of water. If it continues to expand and cool it will reach another critical temperature—32°F—at which point the water will freeze into ice. The same thing happened with the universe as would happen with the steam, except that the transitions that correspond to condensation and freezing are both more numerous and more complicated.
Although a few intrepid theorists have speculated on how the initial collection of matter that eventually became the universe came into existence, there is no firm scientific opinion on this subject; right now the question is simply beyond our knowledge. With regard to the first 10-43 seconds of the universe—the time elapsed before the first major “freezing”—we’re in a little better shape. Although there is no generally accepted theory that describes matter at the temperatures that existed then, we can still make out some of its salient features. Most striking was a tremendous simplicity. Instead of the large number of basic particles we see in matter today, there was just one kind—the so called superparticle. Instead of the four basic interactions (more on this later), there was only one way for matter to interact. Everything was as economical and as elegant as it could be. It’s been downhill ever since.
At 10-43 seconds, the first “freezing” occurred. Two broad classes of particles appeared—one group similar to the electron, the other similar to the photon (the “particle” which makes up visible light); quarks first appeared at this time as well. The force of gravity became distinguishable from the other forces. During the period from this freezing down to 10-36 seconds, the universe may or may not have conformed to something called the grand unification theory (GUT). This theory, first developed in the mid-Seventies, is being subjected to intensive experimentation right now. (More on it later, too.)
During freezings at 10-36 and 10-12 seconds, respectively, the universe became steadily more differentiated and more complex. At around 10-5 seconds (a tenth of a millisecond), the free quarks that had been roaming the universe condensed into the elementary particles that are the building blocks of matter as we know it. At three minutes, these elementary particles started to collect to form the nuclei of atoms. Finally, at five hundred thousand years, the final major transition occurred. The first electrons attached themselves to those nuclei to form simple atoms. In each of these cases, the transition to a new form of matter occurred as soon as the temperature had dropped to the point at which the interparticle collisions no longer had sufficient energy to disrupt the fragile new structure.
The fifteen billion years that have elapsed since then have seen the formation of galaxies and stars and the development of life. During this period, all of the chemical elements heavier than helium, including the iron in your blood and the carbon in your DNA, were made in stars that later died, returning their contents to the interstellar medium, where they were taken into the nascent solar system. Still, from the point of view of fundamental physics, all of the interesting events in the history of the universe were over by the time it was a few minutes old. In fact, it takes longer to describe what happened than it took the universe to undergo the actions described.
What Matter Is Made Of
The best analogy for how we are coming to understand matter is the peeling of an onion. We peel off one layer and penetrate to a deeper one, only to discover that there is yet another to work on. The first layer was peeled off our onion in 1805, when British chemist John Dalton published the modern atomic theory of matter. He showed that the enormous variety of substances that surround us is made up of only a few different chemical elements, each of which has its own type of atom. In the early twentieth century the second layer was peeled off when the structure of the atom was revealed. All of the different kinds of atoms, it was argued, were made up of only three kinds of elementary particles—protons and neutrons in the nucleus and electrons orbiting the nucleus; this picture of the atom would become more or less standard. For a while, its inherent simplicity seemed to be clouded when it was found that there were not just two types of particles inside the nucleus, but hundreds. Most of these appeared and disappeared too quickly to be seen, but they could be produced independently in accelerators.
In the mid-1960s, yet another layer was peeled off. It was pointed out that the myriad elementary particles are actually made up of a small number of still more basic entities called quarks. Quarks are held together by particles called gluons— they’re the Elmer’s that keeps the three quarks of a proton or neutron together. Scientists believe that these particles derive from an even more basic form of matter called quark-gluon plasma, a primordial soup that made up the universe ten millionths of a second after the Big Bang. As things cooled down, this liquid went through “hadronization,” hardening into protons and neutrons, which in turn coalesced first into nuclei, then into atoms. After that, the atoms came together to form molecules, which eventually transformed into you, this book, and the easy chair you’re sitting in. At least, that’s the theory.
The Fundamental Forces
Physics is the study of matter and motion. Thus the quest for the fundamental building blocks of matter that led to the quarks is only half the picture. The other half has to do with the way that matter in all stages behaves, i.e., interacts with itself. Up until the beginning of the twentieth century, physicists had been able to find only two fundamental forces in nature—the force of gravity and the force associated with electricity and magnetism. Since then, studies of the nuclear and subnuclear world have produced two more fundamental forces, neither of which is part of our everyday experience. One of these, the strong force, is what holds all the elementary particles together in a nucleus. The other, the weak force, operates in many situations in nature, the most familiar being the slow radioactive decay of some unstable nuclei and particles. These four forces are a varied lot. Some, like electromagnetism and gravity, act over long distances. Others, like the two nuclear forces, act only over distances about the width of a nucleus or less. They also differ markedly in strength, with the strong being the most powerful, followed by the electromagnetic, weak, and gravitational forces.
Despite these apparent differences among the forces, the great advances in physics over the last few decades have been made by theorists who believe that the differences are only apparent, and that the forces are actually identical. There are important historical precedents for this view. Isaac Newton gave us the first in the seventeenth century, when he showed that the same force which, in your backyard, causes an apple to fall to the ground also holds the moon and planets in their orbits. In so doing, he reversed two millennia of Western thought, during which scientists had seen no resemblance between earthly and celestial gravity. Yet Newton’s Theory of Universal Gravitation, which attributes the motions of both planets and apples to a mutual attraction that exists between all objects possessing mass, says that despite apparent differences, the two forces are, at bottom, the same. We say that Newton unified the two forces of gravity and, because of the term “gravitational field,” we say that Newtonian gravity is an example of a “unified field theory.”
In 1968, Steven Weinberg (then at
MIT) took an important step toward the unification of the forces when he theorized that the weak and electrical forces were unified (i.e., essentially identical), and that apparent differences between them were due more to the present low temperature of the universe than to anything fundamental to their natures. The result was a reduction in the number of basic interactions in nature from four to three. Since 1968, enough predictions of this particular unified theory have been confirmed that it is now accepted by everyone.
The new idea that made this particular advance possible is called the gauge principle. In essence, this principle states that the correct theory of nature must be one in which it is not significant if different observers come up with different definitions of things. For example, the principle suggests that the correct theory of gravity is one in which the amount of energy you expend getting to the top of a cliff is the same whether you go straight up its side or take a more gradual, winding path. Theories like that of the electroweak interaction, which incorporates the gauge principle, are said to be gauge theories.
Gauge theories and unification go hand in hand, and once Weinberg had shown the way, a further unification was not long in coming. This was the grand unification theory mentioned above. In the GUT, the electromagnetic and the strong and weak nuclear forces are all considered to be fundamentally the same. In addition to allowing us to retrace the history of the beginnings of the universe, the GUT predicts the end of it. That is, most GUTs hinge on the idea that the proton, hitherto thought to be the absolutely stable building block of the nucleus, actually decays. And if one proton can decay, all of them can—that means no more atoms, no more molecules, no more DNA, no more CNN, no more KFC, no more anything.
The good news—for most laypeople, if not for scientists and prophets of doom—is that after almost three decades of research, not a single proton has decayed. Until that happens, any proof of a grand unification theory is still elusive.
Gravity: Odd Man Out
Given this history, it would seem that the next step is obvious. All we have to do is use the same techniques to bring gravity into the fold and we’ll have the ultimate theory, in which all forces of nature are brought together under the same roof. Such a theory would be called the Theory of Everything (TOE).
Unfortunately, this is one of those easier-said-than-done things. Our current concept of gravity centers on Einstein’s General Theory of Relativity, and the concept of force used in this theory is radically different from the concepts used to describe the other three forces. To understand the frontier problem in theoretical physics these days, then, we have to know something about relativity.
The first thing to realize is that there are actually two theories of relativity.
The simplest, called the special theory, was published in 1905. It is not a theory of gravitation, but most of the other effects associated with relativity are incorporated into it. The equivalence of mass and energy, the twin paradox (which stated that a twin who went on a long space journey at high velocity would be much younger when he returned than his brother who stayed home), and the increase of mass with speed were all predicted by this theory. It is well verified experimentally; in fact, every time an accelerator delivers a burst of particles, every time a reactor produces a kilowatt of energy, the theory is verified anew.
The general theory was published in 1916. It is anomalous in science because it was accepted with only a few experimental tests, the most familiar being the bending of starlight as it passes the edge of the sun.
Both theories are based on a single principle, called, appropriately enough, the principle of relativity. It states that any two observers will discover the same laws of physics in action, regardless of their relative motion, no matter how different the picture may look to the two of them. The special theory is restricted in that the two observers can’t be accelerating, whereas the general theory holds for any observers, accelerating or not. Since gravity produces acceleration, it can be treated only in the context of the general theory.
To grasp the picture of the gravitational interaction that arises we need another analogy. Imagine a rubber sheet held taut at the edges. (This is the image most often used to discuss Einstein’s curved, four-dimensional space-time continuum.) If we drop a heavy weight on the sheet, the sheet will deform, developing a depression in the area of the weight. If we shoot a marble across the sheet, it could well become trapped in this depression, circling round and round the heavier weight. In general relativity, a large object (such as the earth) distorts the fabric of space and time in just the same way that the heavy weight distorts the sheet. Other objects (such as the moon) react to this distortion by going into orbit. The important point about this view of gravity is that it is purely geometrical—it does not involve any dynamic concept of force. It’s this difference between gravity and the other fundamental forces that makes the development of a truly unified field theory so difficult.
There’s one more thing to say about relativity. Not only does it not imply that “everything is relative,” in fact, the whole point is that, even though different observers may see events differently, there is still a firm and unchanging bedrock in nature—a bedrock made up of the laws of physics.
CHECKING IN WITH QUANTUM MECHANICS
Quantum mechanics is the name given to the theory, developed in the 1920s, which describes the behavior of matter on the atomic and subatomic level. It has been the subject of a spate of books and articles, each purporting to show that accepting quantum mechanics requires some basic change in our thinking about the world. Some authors tell us that we have to turn to Eastern mysticism because of it, others that it teaches that the presence of consciousness is required for the world to function. One writer has claimed that quantum mechanics demands the downfall of patriarchal societies.
There’s no question that the world of the electron is different from our own. For starters, in our everyday experience, we’re accustomed to being able to observe something without changing it. We can watch a bowling ball roll along, confident in the knowledge that the light beams bouncing off it and hitting our eyes won’t deflect it from its path. In our everyday world, the possibility of continuous observation allows us to assume that the ball moves smoothly from one point to the next.
In the subatomic world, things are different. The only way to observe a particle is to bounce another particle off it, and this must necessarily disturb the original particle. It’s as if the only way we could determine the position of a bowling ball was by bouncing another bowling ball off it. In that case, its smooth, continuous, predictable motion toward the pins could not take place.
In the subatomic world, the act of measurement changes the system being measured, giving rise to what is known as the Heisenberg Uncertainty Principle. The principle tells us that if we choose to measure one quantity (e.g., the position of an electron), we inevitably alter the system itself and therefore can’t be certain about other quantities (e.g., how fast the electron is moving). Since an interaction is involved in every measurement, and since measurements are involved in observations, physicists sometimes say that the act of observation changes the system. This is a reasonable statement, provided you realize that it’s the interaction of particles, not the conscious observer, that is important. Consciousness has nothing to do with quantum mechanics, although there are plenty of people who misinterpret the principle to suggest that it does.
That probing matter affects its form and behavior is only part of the weirdness inherent in quantum mechanics. Equally bizarre: that some particles exist so briefly that they are not real but “virtual,” and that the universe as we know it is based on chance and randomness at the subatomic level.
Riding Herd on the Life Sciences
They’re an ornery lot and skittish to boot, always fighting among themselves, wandering off into the chaparral, grazing near precipices, or popping up unexpectedly with a brood of wacky hybrid offspring in tow. Nobody warned you back in Biology 101 that once you took the
m on, you’d never be able to turn your back on them again for a minute. (Or that everything you’d learn about them would turn out to be wrong, or at least outdated, just when you needed it most.) Here contributors Judith Stone and Karen Houppert wrestle a few of bio-sci’s meanest subjects to the ground, in an effort to restore some sense—possibly illusory and definitely fleeting—of mastery and control.
ALL IN THE FAMILY
“Descended from monkeys?” the Bishop of Worcester’s wife is said to have cried after hearing about Darwin’s startling new theory of evolution. “My dear, let us hope it isn’t true! But if it is, let us pray it doesn’t become widely known!”
Well, the skeleton’s been out of the closet for almost 150 years now, and today anthropologists have a fairly clear, though unfinished, picture of the human family tree, based on fossil remains. Like all mammals, we ultimately trace our roots back to a tiny, shrewlike creature that inherited the earth after the dinosaurs disappeared abruptly and mysteriously 65 million years ago. As the shrew’s progeny diversified, tree dwellers developed such dandy gimmicks as binocular vision, hands that could grasp, and brains efficient enough to handle all that swinging. This ape line seems to have forked between 10 and 4.5 million years ago; one group stayed apes, the other eventually became us.
About 15 million years ago, the planet’s climate cooled; steamy forests gave way to grassy savanna. Those tree dwellers who came down for a look eventually became the family Hominidae, of which our subspecies, Homo sapiens sapiens, is the only remaining member. A hominid family album would feature the venerable ancestors following, including Toumai, the oldest—and most human-like— hominid fossil found to date (a nearly complete cranium discovered in Chad in 2001); the controversial, Hobbit-like LB1 (found on an Indonesian island in 2003); Turkana Boy (a nearly complete skeleton discovered in Kenya in 1984); and, of course, Lucy, the 3.2-to-3.6-million-year-old grand dame of the hominid fossils, who achieved celebrity status back in 1974.
