Coming of Age in the Milky Way - Timothy Ferris (2003)



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.


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 that 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 thunderheads 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.

This, the epoch of “let there be light,” has a significant effect on the structure of matter. Electrons, relieved from constant harassment by the photons, are now free to settle into orbit around nuclei, forming hydrogen and helium atoms. With atoms on hand, chemistry can proceed, to lead, eons hence, to the formation of alcohol and formaldehyde in interstellar clouds and the building of biotic molecules in the oceans of the early earth.

The ambient temperature of the universe rises rapidly as we continue up the stairway. It was less than 3 degrees above absolute zero on the bottom step, reached room temperature by the third step, and by the sixth step has risen to 10,000 degrees Kelvin—hotter than the surface of the sun. By the eleventh step, at which point the universe is a little under one month old, the temperature everywhere surpasses that of the center of the sun, and at the fifteenth step (five minutes ABT) it is fully a billion degrees Kelvin.

Energetic as this may be, the universe at the age of five minutes has already become cool enough for nucleons to stick together to make permanent atomic nuclei. We watch as protons and neutrons adhere to make nuclei of deuterium (a form of hydrogen) and deuterium nuclei pair off to form the nuclei of helium (two protons and two neutrons). In this fashion, one quarter of all the matter in the universe is rapidly combined into helium nuclei—along with traces of deuterium, helium-3 (two protons, one neutron), and lithium. The whole process is over in three minutes twenty seconds.

Above this point—prior to about one minute forty seconds ABT—there are no stable atomic nuclei. The ambient energy level exceeds the nuclear binding energy. Consequently, any nuclei that form are quickly torn apart again.

Between the seventeenth and eighteenth steps, at about one second ABT, we encounter the epoch of neutrino decoupling. Though the universe at this time is denser than rock (and as hot as the explosion of a hydrogen bomb) it has already begun to look vacuous to the neutrinos. Since neutrinos react only to the weak force, which is extremely short in range, they now find that they can escape its clutches and fly along indefinitely without experiencing any significant further interaction. Thus emancipated, they are free hereafter to roam the universe in their aloof way, flying through most matter as if it weren’t there. (Ten million trillion neutrinos will speed harmlessly through your brain and body in the time it takes to read this sentence. By the time you have read this sentence, they will be farther away than the moon.) The flood of neutrinos released at one second ABT therefore persists ever after, forming a cosmic neutrino background radiation comparable to the microwave background radiation produced by the decoupling of the photons. If these “cosmic” neutrinos (as they are called, to differentiate them from neutrinos released later on by supernovae) could be observed by a neutrino telescope of some sort, they would provide a direct view of the universe when it was only one second old.

As we climb on, the universe continues to become hotter and denser, and the level of structure that can exist becomes ever more rudimentary. There are of course no molecules or atoms or atomic nuclei at this early time, and by about the twenty-second step, some 10-6 (0.000001) second ABT, there are no protons or neutrons, either. The universe is an ocean of free quarks and other elementary particles.

If we take the trouble to count we will find that for every billion antiquarks there are a billion and one quarks. This asymmetry is important: The few excess quarks destined to survive the general quark-antiquark annihilation will form all the atoms of matter in the latter-day universe. The origin of the inequity is unknown; presumably it involved the breaking of a matter-antimatter symmetry at some earlier stage.

We are approaching a time when the basic structures of natural law, and not only those of the particles and fields whose behavior they dictate, were altered as the universe evolved. The first such transition comes at the twenty-seventh step, 10−11 second ABT, when the functions of the weak and electromagnetic forces are found to be handled by a single force, the electroweak. There is now enough ambient energy available to support the creation and maintenance of large numbers of W and Z bosons. These particles—the same kind the conjuring up of which in the CERN accelerator verified the electroweak theory—mediate electromagnetic and weak force interactions interchangeably, making the two forces indistinguishable. Prior to the twenty-seventh step the universe is ruled by only three forces—gravity, and the strong nuclear and electroweak interactions.

The next two dozen or so steps of our ascent are clouded in mystery. Some say that they traverse a “desert,” a bleak stretch of time in which little of importance occurred. But it remains to be seen, given further accelerator experiments and the development of more sophisticated theories, whether the desert will prove to have bloomed.

According to the “inflationary universe” theory (about which more in the next chapter) there may here have been a brief period, upward of the fortieth step, during which the universe expanded much more rapidly than it did thereafter. During this inflationary epoch the universe would have been empty, all its latent matter and energy swallowed up by the rapidly expanding vacuum. There would be nothing to write home about (no material structure at all!) other than the vacuum itself, its unfolding fields pregnant with potential but devoid of tangible objects.

Prior to the start of the inflationary epoch—at about the fifty-first step, only 10−35 second ABT—we enter a realm in which cosmic conditions are even less well understood. If the grand unified theories are correct, there here occurred a symmetry-breaking event in which the unified electronuclear force split into the electroweak and strong forces. If supersymmetry theory is correct, the transition may have come earlier, and would have involved gravitation. Writing a fully unified theory amounts to trying to understand what went on at this early time, when the symmetry thought originally to have characterized the universe shattered into the broken symmetries we find around us today.

But until we have such a theory, we cannot expect to understand what went on in the infant universe. We approach the limits of our present conjecture at the sixtieth step, when the age of the universe is but 10−43 second. Here we encounter a locked door. On the other side lies the Planck epoch, a time when the gravitational attraction exerted by each particle was comparable in strength to the strong nuclear force.* The theoretical key that could open the door would be a unified theory that includes gravitation. The person who arrives at that theory will gaze deepest into the dawn of time. What will he or she see?

One possibility, of course, is that there will be more doors. This prospect has been raised by several researchers, among them Michael Turner, an American cosmologist working on early-universe theory at Fermilab and the University of Chicago. “I suspect that we may always find ourselves in this position—that to go the next tiny fraction of a second we will need some further knowledge that we won’t yet have,” Turner suggested in a 1985 interview. “If so, it may be a very long time, if ever, before we can answer the question that everyone would like to know—the question of what caused creation.”4 Another possibility is that we will find the answer, behind the Planck door or the one after that. The conviction that such an outcome is possible was expressed this way by the American physicist John Archibald Wheeler:

To my mind there must be, at the bottom of it all, not an equation, but an utterly simple idea. And to me that idea, when we finally discover it, will be so compelling, so inevitable, that we will say to one another, “Oh, how beautiful. How could it have been otherwise?”5

Suppose that a unified theory is written—a year from now or a century from now—that succeeds in delivering up just such a transcendental vision of perfection. How could we be sure that we could trust it? As Kepler realized after wasting years on his spherical universe of Platonic solids, one needs not only elegance from a theory but the verdict of experimental or observational test as well. A fully unified theory would in all likelihood purport to describe the universe as it was at less than 10−43 second ABT, when the ambient energy level was more than 1019 GeV. To re-create such conditions would require an accelerator far beyond the reach of any foreseeable technology. Experimental verification of such a theory might remain forever out of reach.

The big bang itself, however, can be regarded as a gigantic accelerator experiment, and the universe we live in as its result. Viewed in this way, our microwave radiotelescopes are like Carlo Rubbia’s detectors at CERN, inasmuch as the particles they intercept were hurled off by the first (and still the greatest) experimental run of all time. A proper unified theory ought to specify just how that run turned out, by predicting the existence of all the particles in the present-day universe. Some of these, presumably, would not yet have been detected: One could then test the theory by searching for such “relic” particles in the here and now. Supersymmetry theory, as we saw in the previous chapter, predicts the existence of enormous numbers of as yet undetected particles left over from the early universe.* If the theory ripened to the point that it could specify the masses of these particles, it might be possible to test it by looking for them.

A ghostly clue that there may be such undetected material in the universe today is proffered by what astronomers call the “dark matter” problem. The masses of galaxies and their clusters can be deduced by measuring the velocity at which stars orbit the centers of the galaxies to which they belong, and at which galaxies orbit the centers of clusters of galaxies. In case after case, this turns out to add up to something like five or ten times the mass of all the visible stars and nebulae. The startling implication is that everything we see and photograph in the sky amounts to only a fraction of the gravitationally interacting matter in our quarter of the universe. The unseen matter might, of course, consist of relatively large objects, such as brown dwarf stars or small black holes. But it might also consist of subatomic particles, many of them left over from the high-energy days of the early universe, in which case the identity of the particles would provide an observational test of supersymmetry or of any comparable unified theory of the early universe.

While awaiting the wished-for apotheosis of supersymmetry theory, we may care to reflect on the role played by symmetry in cosmic history. In doing so we soon confront the realization that perfect symmetry, though beautiful in the abstract, is also sterile. If, for instance, the matter-antimatter symmetry thought to have existed at the outset of cosmic evolution had been preserved, the particles of matter and those of antimatter would have mutually annihilated in the big bang, and no matter of either kind would have survived from which to make stars and planets and people. Had the putative primal force not scattered into the four forces, the universe today would be very different, perhaps uninhabitably so. It just may be, then, that we owe our existence, and that of the stars in the sky, to imperfections born of broken symmetry. To investigate the riddle of creation would then involve envisioning a perfectly symmetrical but unlivable universe, then trying to determine how it devolved from that sterile, pristine state toward becoming the less perfect but more variegated and hospitable universe in which we find ourselves today.

*There are exceptions, of course, notably those mathematicians who come into physics with little or no grounding in experimental science. But generally speaking, the best theoretical physicists are willing, if only during their student days, to get their hands dirty in the laboratory. Recall that the young Einstein nearly lost a hand this way.

*At this point gravitons, the carriers of gravitational force, would have decoupled from the other particles, producing a gravitational background radiation much like those generated later by the decoupling of neutrinos and photons. The present-day temperature of the cosmic gravitational background radiation, however, is only 1 degree Kelvin, placing it far below the sensitivity of any conceivable gravitational detector. Still, it is there, and if we could find a way to observe it we could see all the way back to the Planck epoch.

*String theory postulates that there exists and has existed only a single variety of particle, but that this particle has an infinite number of manifestations—as in the innumerable tunes that may be composed on a single string of Pythagoras’s lyre. Thus a single supersymmetric variety of particle shows up in various harmonics as gravitons and gravitini, quarks and squarks, photons and photinos, and so forth. Since, as Gell-Mann noted, “these infinitely many particles all obey a single very beautiful master equation,” the theory suggests how maximum complexity could have arisen from maximum simplicity.

†Recall that, as Newton found, the gravitational force of any object may be regarded as emanating from a point at its center. Each star in a galaxy responds to the total gravitation of the mass of the galaxy that lies within its orbit, as if the gravity were coming from a point source at the galactic center. The orbital velocity of a star lying near the edge of a galaxy therefore constitutes an index of the total mass of the galaxy.