*Dirac meant, of course, not that one should ignore the empirical results altogether, but that a beautiful theory need not be abandoned just because it fails an initial test. He had in mind Erwin Schrödinger’s reluctance to publish his estimable equations of wave mechanics merely because they conflicted with experimental data. “It is most important to have a beautiful theory,” Dirac told the science writer Horace Freeland Judson. “And if the observations don’t support it, don’t be too distressed, but wait a bit and see if some error in the observations doesn’t show up.”5
*The ratio is approximate because the numbers generated by the Fibonacci series are “irrational”—i.e., the ratio upon which they converge cannot be expressed exactly in terms of a fraction. The Pythagoreans discovered irrational numbers, and are said to have been so unsettled by them that they prescribed the death penalty to any of their sect who revealed their existence to the untutored multitudes. Hippasus was banished for defying the ban. He drowned at sea, a fate that the Pythagoreans ascribed to divine retribution.
*Richard Feynman, Gell-Mann’s chief competitor for the title of World’s Smartest Man but a stranger to pretension, once encountered Gell-Mann in the hall outside their offices at Caltech and asked him where he had been on a recent trip; “Moon-TRW-ALGH!” Gell-Mann responded, in a French accent so thick that he sounded as if he were strangling. Feynman—who, like Gell-Mann, was born in New York City—had no idea what he was talking about. “Don’t you think,” he asked Gell-Mann, when at length he had ascertained that Gell-Mann was saying “Montreal,” “that the purpose of language is communication?”
*Quantum interactions customarily are depicted as taking place not in the conventional space that makes up the theater for macroscopic events, but in an assessment complex space described in part by the quantum wave functions. Quarks, for instance, are for convenience depicted as existing in a three-dimensional “color” space described by quantum chromodynamics—color is a quantum number that plays a role in the strong force analogous to that of the charge in electrodynamics—while electrons normally occupy a one-dimensional space the two directions of which represent positive and negative electrical charge.
*Connections between the weak and electromagnetic interactions had been noted before; Fermi in 1933 formulated the first model of the weak force by analogy with electromagnetism. But a great many such threads weave their way through the history of physics, and this book is not the place to attempt to trace more than a few of them.
*Compare, for instance, the value of g, the gyromagnetic ratio of the electron, as predicted by the theory of quantum electrodynamics and as tested experimentally:
Theory: £ = 1.00115965241
Experiment: £ = 1.00115965238, ± 0.00000000026
*Electrons, since they also carry an electrical charge, can also be employed; the resulting explosions are cleaner and therefore easier to study, but as electrons are less massive than protons they collide less violently, and so electron accelerators yield weaker collisions relative to their energy consumption.
†After Ernest Walton, an Irish physicist, and John Cockroft, the English physicist who on one fine day in 1932 could be seen stopping strangers on the streets of Cambridge and exclaiming, “We have split the atom! We have split the atom!”
*Since antiprotons have opposite electrical charge, the same sequence of magnetic pulses that kept protons moving clockwise around the ring would keep the antiprotons moving counterclockwise. A somewhat more exotic way of looking at the situation, proposed by Feynman years earlier, was to say that antimatter particles move in reverse time.
17
THE AXIS OF HISTORY
Every present state of a simple substance is naturally a consequence of its preceding state, in such a way that its present is big with its future.
—Leibniz
He who has seen present things has seen all, both everything which has taken place from all eternity and everything which will be for time without end; for all things are of one kin and of one form.
—Marcus Aurelius
The late twentieth century may be remembered in the history of science as the time when particle physics, the study of the smallest structures in nature, joined forces with cosmology, the study of the universe as a whole. Together these two disciplines were to sketch the outlines of cosmic history, investigating the ancestry of natural structures across an enormous range of scale, from the nuclei of atoms to clusters of galaxies.
It was a shotgun wedding between two very different disciplines. Cosmologists tend to be loners, their gaze fixed on the far horizons of space and time and their data tenderly garnered from trickles of ancient starlight; none will ever touch a star. Particle physicists, in contrast, are relatively gregarious—they have to be; not even an Einstein knows enough physics to do it all by himself—and physical: They are by tradition hands-on students of the here and now, inclined to bend things and blow up things and take things apart.* Physicists work hard and fast, haunted by the legend that they are unlikely to have many useful new ideas after the age of forty, while cosmologists are more often end-game players, devotees of the long view, who can expect to still be doing productive research when their hair turns white. If physicists are the foxes that Archilochus said know many things, cosmologists are more akin to the hedgehogs, who know one big thing.
Yet by the late 1970s, particle physicists were venturing to cosmology seminars to bone up on galaxies and quasars, while cosmologists were hiring on at CERN and Fermilab to do high-energy physics at underground installations blind to the stars. By 1985, Murray Gell-Mann could declare that “elementary particle physics and the study of the very early universe, the two most fundamental branches of natural science, have, essentially, merged.”1
Their meeting ground was the big bang. As we saw in the previous chapter, the physicists identified symmetries in nature that today are broken but which would have been intact in a high-energy environment. From the cosmologists came word that the universe was once embroiled in just such a high-energy state, during the initial stages of the big bang. Put the two together, and a picture emerges of a more or less perfectly symmetrical universe that fractured its symmetries as it expanded and cooled, creating the particles of matter and energy that we find around us today and stamping them with evidence of their genealogy. Steven Weinberg, a champion of the new alliance, described the electroweak unified theory in terms of its connection with the early universe:
The thing that’s so special about the electroweak theory is that the [force-carrying] particles form a tightly knit family, with four members: There’s the W+, the oppositely charged W−, the neutral Z, and the fourth member is our old friend the photon, the carrier of electromagnetism. These are siblings of each other, tightly related by a principle of symmetry that says that they’re really all the same thing—but that the symmetry is broken. The symmetry is there, in the underlying equations of the theory, but it’s not evident in the particles themselves. That’s why the W and the Z are so much heavier than the photon.
But there was a time, in the very early universe, when the temperature was above a few hundred times the mass of the proton, when the symmetry hadn’t yet been broken, and the weak and electromagnetic forces were all not only mathematically the same, but actually the same. A physicist living then, which is hard to imagine, would have seen no real distinction between the forces produced by the exchange of these four particles—the Ws, the Z, and the photon.2
Similarly, if less distinctly, the emerging supersymmetry theories suggested that all four forces may have been linked, by a symmetry that evidenced itself in the even higher energy levels that characterized the universe even earlier in the big bang.
The introduction of an axis of historical time into cosmology and particle physics benefited both camps. The physicists provided the cosmologists with a wide range of tools useful in trying to piece together how the early universe developed: Evidently the big bang was not the impenetrable wall of fire tha
t Hoyle had scoffed at, but an arena of high-energy events that might very possibly be comprehensible in terms of relativistic quantum field theory. Cosmology, for its part, lent a tincture of historical reality to the unified theories. Though no conceivable accelerator could attain the titanic energies invoked by the grand unified and supersymmetry theories, these exotic ideas still might be tested, by investigating whether the particle constituency of the present-day universe accords with the sort of early history the theories imply. As Gell-Mann put it, “The elementary particles apparently provide the key to some of the fundamental mysteries in early cosmology…. and cosmology, it turns out, provides a sort of testing ground for some of the ideas of elementary particle physics.”3
Viewed from this new, historical perspective, the proliferation of particle types that had been so discouraging to the physicists (prompting Fermi to muse that he should have been a botanist) began to look less like a burden than a boon. Once it became clear that every particle has arisen from a process of cosmic evolution, about which it can testify, one could regard the variety of particles as evidence of the richness of cosmic history. Physicists no longer needed to feel unhappy about the diversity of the particle world, any more than archaeologists would be disappointed if, say, while excavating the ruins of ancient Herculaneum they unearthed the foundations of an even older city beneath it. Instead, they could consider that nature is complicated and imperfect because it has a past—that, as the American physicist Thomas Gold remarked, things are as they are because they were as they were.
Indeed, one could discern signs of a direct relationship linking the size, binding energy, and age of nature’s fundamental structures. A molecule is larger and easier to break apart than an atom; the same is true of an atom relative to an atomic nucleus, and of a nucleus relative to the quarks that comprise it. Cosmology suggests that this relationship results from the course of cosmic history—that the quarks were bound together first, in the extremely high energy of the early big bang, and that as the universe expanded and cooled the protons and neutrons made of quarks adhered to one another to form the nuclei of atoms, which thereafter attracted electrons to set up shop as complete atoms, which in turn linked up to form molecules.
If so, the more closely we examine nature the further we are peering back in time. Look at something familiar—the back of your hand, let us say—and imagine that you can turn up the magnification to any desired power. At a relatively low magnification you will discern individual cells in the skin, each looming as large and complex as a city, its boundaries delineated by the cell wall. Increase the magnification and you will see, within the cell, a tangle of meandering ribosomes and undulating mitochondria, spherical ly-sosomes and starburst centrioles—whole neighborhoods full of complex apparatus devoted to the respiratory, sanitary, and energy-producing functions that maintain the cell. Here, already, we encounter ample evidence of history: Though this particular cell is only a few years old, its architecture dates back more than a billion years, to the time when eucaryotic cells like this one first evolved on Earth.
To determine where the cell obtained the blueprint that told it how to form, move into the nucleus and behold the lanky contours of the DNA macromolecules secreted within its genes. Each holds a wealth of genetic information accumulated over the course of some four billion years of evolution. Stored in a nucleotide alphabet of four “letters”—made of sugar and phosphate molecules and replete with punctuation marks, reiterations to guard against error, and superfluities accumulated in blind alleys of evolutionary history—its message spells out just how to make a human being, from skin and bones to brain cells.
The relationship between the sizes of basic natural structures and their binding energies (i.e., the forces needed to tear them apart) is thought to reflect their origins at differing stages of cosmic history. Quarks, for example, are said to be smaller than nucleons (i.e., protons and neutrons), and to have higher binding energies, because they were formed earlier in cosmic time, when the universe itself was small and relatively energetic.
Turn up the magnification some more and you can see that the DNA molecule is composed of many atoms, their outer electron shells intertwined and festooned in a miraculous variety of shapes, from hourglasses to ascending coils like lanky springs to ellipses fat as shields and threads thin as cheroots. Some of these electrons are new arrivals, recently snatched away from neighboring atoms; others joined up with their atomic nuclei more than five billion years ago, in the nebula from which the earth was formed. Increase the magnification a hundred thousand times, and the nucleus of a single carbon atom swells to fill the field of view. Such nuclei were assembled inside a star that exploded long before the sun was born; the age of this one might be anywhere from five to fifteen billion years or more. Finally, looking closer still, one can perceive the trios of quarks that make up each proton and neutron in the nucleus. The quarks have been bound together since the universe was but a few seconds old.
In venturing to smaller scales, we have also been entering realms of higher binding energies. An atom can be stripped of its electron shell by applying only a few thousand electron volts of energy, but to split up the nucleons that constitute an atomic nucleus requires several million electron volts, and to liberate the quarks that make up each nucleon would require hundreds of times more energy still. Introduce the axis of history, and this relationship attests to the particles’ past: Smaller, more fundamental structures are bound by higher levels of energy because the structures themselves were forged in the heat of the big bang.
This implies that accelerators, like telescopes, function as time machines. A telescope looks into the past by virtue of the time it takes light to travel between the stars; an accelerator re-creates, however fleetingly, conditions that pertained in the early universe. The 200 KeV accelerator devised in the 1920s by Cockroft and Walton replicated some of the events that transpired at about one day after the beginning of the big bang. Accelerators built in the 1940s and 1950s hovered at around the one-second mark. The Fermilab Tevatron pushed back the boundary to less than a billionth of a second after the beginning. The superconducting super collider (had it been completed, rather than being canceled in mid-construction by Congress) would have provided a glimpse of the cosmic environment when the universe was less than one thousand billionth of a second old.
That’s pretty early: One ten thousand billionth of a second takes a smaller slice out of a second than a snap of the fingers takes out of all recorded human history. And yet, oddly enough, research into the evolution of the newborn universe indicates that a great deal happened even earlier, during that first tiny fraction of a second. The theorists, accordingly, endeavored to piece together a coherent account of the first moments in cosmic history. Their ideas were of course sketchy and incomplete, and many of their conjectures will doubtless turn out to have been distorted or simply wrong, but they constituted a far more enlightening chronicle of the early universe than was available only a decade or so earlier, and hinted at the extraordinary beauty and explanatory power that could be expected from a more advanced theory once one could be worked out.
To review the story of cosmic history as depicted by the early-universe theories, imagine a staircase leading into the past—a stairway to heaven, if you will. We are standing at its base, in the present, at a time when the universe is some ten to twenty billion years old. (Most of the observational evidence suggests that the age of the universe is a little under fourteen billion years.) The first step upward will take us back to when the universe was only one billion years old, and each step higher will turn back the clock to a tenth of its previous reading—to only a hundred million years after the beginning, then ten million years, then one million, and so on.
Suppose that we ascend this staircase. One step, and the date is one billion years after the beginning of time (or ABT for short). The universe looks quite different. The nucleus of the young Milky Way galaxy burns brilliantly, casting the shadows of galactic thunde
rheads out across the murky disk; at its core shines a bright, blue-white quasar. The disk, still in the process of formation, is jumbled and thick with dust and gas; it bisects a spherical halo that will be dim in our day, but currently wreathes the galaxy in a glittering chandelier of hot, first-generation stars. Our neighboring galaxies in the Virgo Supercluster float relatively nearby; the expansion of the universe has not yet had time to carry them away to the distances, typically tens of millions of light-years, at which we will encounter them in our own era. The universe is highly radioactive: Torrents of cosmic rays rain through us every millisecond, and if anything lives at this time, it probably mutates rapidly. Indeed, the pace of most events is hectic, an urban bustle compared to the relative placidity of our more mature epoch.
With the second step, we are plunged into darkness. We have reached a time, one hundred million years ABT, before any but the most precocious stars have yet had time to form. Except for their scarce and smoky beacons, the universe is a dark soup of hydrogen and helium gas, whirlpooling here and there into protogalaxies.
The history of the universe, depicted in terms of a stairway leading exponentially backward in time, displays the evolution of natural structures from quarks to atomic nuclei to atoms and galaxies of stars.
In two more steps, the darkness is replaced by blinding white light. The time is one million years ABT, and the technical term for what has happened is photon decoupling. The ubiquitous cosmic gas has recently thinned sufficiently to permit light particles—photons—to travel for significant distances without colliding with particles of matter and being reabsorbed. (There are plenty of photons on hand, because the universe is rich in electrically charged particles, which generate electromagnetic energy, the quantum of which is the photon.) It is this great gush of light, much redshifted and thinned out by the subsequent expansion of the universe, that human beings billions of years hence will detect with radiotelescopes and will call the cosmic microwave background radiation.
Coming of Age in the Milky Way Page 35
