The Vast and the Tiny
The Milky Way: An Insider's Guide
by William H. Waller
A Palette of Particles
by Jeremy Bernstein
The British philosopher J.L. Austin coined the handy phrase "medium-sized dry goods" to describe the world of everyday phenomena that the human nervous system is best suited to cope with, phenomena ranging in size from a grain of dust to a landscape. Within that range our senses and cognition are at home. All our intuitions about how objects move, change, and interact arise from our dealings with "medium-sized dry goods."
Much beyond that size range in either direction our senses and understanding are at sea. How we can say anything at all — anything coherent, with predictive power and technological application — about the invisible constituents of matter, or about the universe at large, is a considerable mystery. We certainly can say such things: the device I am using to write this review would not exist if we did not know true facts about the atom and its parts. The only language we have for expressing those true facts, however, is the language of mathematics: a tower of abstractions of abstractions of abstractions, in which everyday intuitions recede in a fog of wave-particle duality and twisted spacetime.
To master that language takes years of specialized training, but citizens who have taken different paths through life, or youngsters wondering whether to take that path, are naturally curious to understand as much as they can of what the specialists know. For those purposes we have popularizers of the physical sciences — heroes of our intellectual culture, in my opinion, though of course sometimes more, sometimes less skillful in the presentation of their material.
Here are two such, working at opposite ends of the size zone, one with the vast, one with the tiny. Both deliver very satisfactory products, although with differences of style and approach.
The astronomer William H. Waller has taken our galaxy as his subject. He is not the first to do so. As he notes in his preface, Bart and Priscilla Bok blazed the trail seventy years ago with their book The Milky Way. The last edition of that book appeared in 1981 (Bok died in 1983), and there have been great advances in astronomy since then, so Waller's book meets a need, and quite a pressing one, to judge from popular usage of the word "galaxy." Nonspecialists probably associate it most closely with the opening "crawl" of the Star Wars movies, which begins: "A long time ago in a galaxy far, far away …" The scriptwriters probably meant "a solar system," not "a galaxy." There is no reason to go to other galaxies for sci-fi scenery, and in any case all galaxies but our own are far, far away. The confusion is a common one: I have seen it committed in broadsheet newspapers.
It is also a forgivable one. Until the 1920s the word "galaxy" could not appear with an indefinite article in front of it. The entire entry for "Galaxy" in my 1911 Encyclopædia Britannica reads: "GALAXY, properly the MILKY WAY, . . .," Milky Way …" followed by an etymology, then a single sentence on the figurative usage ("a galaxy of brilliant scholars"). That's it. The Milky Way, that faint luminous band that can be seen across the sky on clear nights, and which telescopes resolve into a great multitude of stars, was the galaxy. There was nothing beyond. That was the understanding. It found literary expression in H.G. Wells' short story "Under the Knife" (1896).
We now know that the Milky Way is merely a galaxy, the one we happen to live in. The universe contains hundreds of billions, perhaps a trillion, of other galaxies. That makes our Milky Way somewhat inconsequential, to put it very mildly; but we can salve our humbled spirits with municipal pride. The place may not amount to much, but it's ours. That seems to be Mr. Waller's approach, in my view a very commendable one. Let's take significance where we can find it.
The historical path to our present understanding makes a good story. Waller gives over his longest chapter to it.
Until the invention of the telescope in the early 17th century, it was not possible to know that the Milky War was made up of stars. Mythologies took it to be a heavenly river (generally of milk from the breasts of some goddess — whence the etymology), or road (to be traveled by the dead), or tree, or serpent, or robe. It is just conceivable that someone in the ancient world glimpsed the truth through a shaped crystal, but most likely the philosopher Democritus was making a lucky guess — inspired by his a priori preference for atomic solutions to questions of ultimate structure — when he called the Milky Way "a luster of small stars very close together."
Galileo saw the truth of the matter through his telescopes: "The galaxy is … nothing but a congeries of innumerable stars …" From there it was three centuries hard slog to our modern understanding. Problems of the utmost difficulty had to be solved; most notably, the difficulty of calculating the distances of celestial objects.
Pre-Copernican astronomers had solved this problem for the planets, at least to fair approximations, but it was 1838 before anyone measured the distance to a star: sixty trillion miles, according to the German astronomer Friedrich Wilhelm Bessell's observations of 61 Cygni (he was in error by less than ten percent). Though not unexpected, Bessell's result put the universe on a whole new scale. His star was twelve thousand times more distant than anything known in the Solar System of his time; and his method of measuring its distance implied that it was one of the nearest stars.
Further progress depended on better understanding of the nature of stars: of the relations between their luminosity, size, age, chemical composition, and temperature. Key advances here were the invention of the spectrometer (1819) and the Hertzsprung-Russell diagram (1910). The latter relates the intrinsic brightness of a star to its spectral type — its temperature, approximately. Waller calls Hertzsprung-Russell "the Rosetta Stone of stellar deciphering" and relies on it through the rest of his book when touching on the life cycles of stars. However, he fails to give readers the standard astronomy-class mnemonic for remembering spectral types: "Oh, Be A Fine Girl — Kiss Me Right Now, Sweetie!"
(Or according to a different school of thought: "… Right Now! Smack!" That's the mnemonic I learned from Werner Buedeler's book To Other Worlds back around 1960. The R, N, and S spectral classes have since been dropped. The sun, by the way, is spectral type G.)
With good tools for measuring stellar distances, it became possible to estimate how all the stars in the sky are distributed. The Dutch astronomer Jacobus Kapteyn made an impressive attempt at this, reporting, in results published in 1922, that all the stars were gathered in a lens-shaped disk 240 quadrillion miles across, with the Sun near its center. Kapteyn was wrong in some important respects. The Sun, for example, is far from the galactic center. If the I495 Beltway around Washington, D.C. were the galaxy's perimeter and the White House its center, our Solar System would be in Chevy Chase, Maryland. Kapteyn's was, however, the first modern-looking model for the shape of our galaxy.
Even with planets and stars accounted for, there were other celestial objects whose distances needed computing: clusters of stars, clouds of gas, and, most vexing of all, the spiral nebulae. Were these latter faint smudges part of the "island universe" of stars whose shape Kapteyn was trying to work out? Or were they island universes in themselves, separate and far from our own? In 1920 a famous debate was held at the National Academy of Sciences in Washington, D.C., with Harley Shaplow for the single-galaxy case and Heber Curtis for separate-and-far. Both men were respected astronomers with a wealth of data to support their cases.
The issue became moot four years later when Edwin Hubble studied a certain particular type of star in several nebulae. The distances and sizes he deduced showed beyond doubt that these nebulae were autonomous Milky Ways outside our own, as Curtis had argued. Waller:
Shortly thereafter, Hubble wrote a letter to Harlow Shapley at Harvard College Observatory, sharing the epochal news. Upon reading the note, Shapley was heard to say, "Here is the letter that destroyed my universe."
Hubble may have turned Shapley's universe to rubble, but he had given us a whole new class of celestial objects to study — the galaxies (a plural popularized, ironically, by Shapley). The Milky Way is now just one of those galaxies, the one we inhabit.
How can we learn more about our galaxy — about its structure, its age, its mass, its motions? By observation, of course. In a chapter titled "Panchromatic Vistas" Waller give a comprehensive run-down of the different kinds of electromagnetic radiation we can gather, and what each of them tells us about the Milky Way.
Visible light is seriously inadequate here. Much of the space between the stars is filled with dust, especially towards the galactic center. This becomes a serious hindrance to observation at distances greater than 5,000 light years (a light year is six trillion miles). The galactic center is 27,000 light years away. Fortunately modern astronomers can observe in regions of the electromagnetic spectrum far beyond the visible: in radio, microwave, and infrared wavelengths, and in ultraviolet light, X-rays, and gamma rays. Each spectral region has its pluses and minuses. Together, they have given us a good picture of the Milky Way's components and their motions.
Those motions, now known with good precision, have introduced us to a mystery. Our galaxy is rotating, but some parts are moving much faster than they ought under basic physical laws — too fast to be in balance with the known total mass of stars and dust we can see. Why don't those parts fly off away from the galaxy altogether? There must be some mass we are not seeing at any electromagnetic wavelength — a lot of mass. Waller:
The nature of the dark matter remains one of the greatest enigmas in modern astronomy. Evidence for it can be found in the excessively rapid stellar and gaseous motions within the Milky Way and within other galaxies, in the zippy relative motions between galaxies in galaxy clusters [galaxies are gregarious — J.D.] … In the Milky Way, modeling of the orbital velocities and estimates of the stellar and nebular masses indicate that more than 90 percent of the Galaxy's mass is in some dark form. What that form might be remains completely unknown.
Our own solar system is in motion along with everything else, completing a circuit of the galactic center once every 220 million years or so — the "galactic year." This time last year the earliest dinosaurs had just appeared on Earth. The orbit is a precessing ellipse; that is, it traces out a rosette pattern "akin to the designs you can make with a spirograph" (Waller). That's the least of it, though: The Sun's vertical velocity component causes it to bob up and down through the galactic midplane with a period of about 42 million years.
We are currently about 25 light-years above the midplane of the galactic disk. That midplane is itself somewhat warped, "like the brim of a fedora," probably by the gravitational attraction of small satellite galaxies.
Yes, the Milky Way has satellites; and a spherical "halo" of star clusters around the disk's central bulge, and great rings of debris far out, and strange streams, fountains, and eddies of stars, and an oblong central bar of stars, and spiral arms, and bubbles of greater or lesser density … There is an astonishing amount of structure here. But then, the galaxy is a very large object.
Even in our own little neck of the woods there is structure. I was amused to learn that our Solar System is currently passing through "an ornate filigree of tenuous gas and dust" known as the Local Fluff, which is contained within a larger object called the Local Bubble. Let's hope we don't get caught in the Local Lint Trap.
Waller's true passion, I think, is not so much for the Milky Way as for stars. I counted thirteen Hertzsprung-Russell diagrams in the book. He displays a characteristic trait of the true enthusiast, perceiving fine individual differences between the objects of his enthusiasm, like a shepherd who knows every one of his flock by sight. He tells us that even stars of the same brightness and spectral class can differ in interesting ways. He actually speaks of stars having "individual personalities."
In a chapter on the mechanics of stars, Waller describes them as "finely tuned furnaces." A star is a balancing act. Radiation from the nuclear reactions at the core pushes outwards; the gravitational force of the star's mass pulls inward. As those reactions consume the star's substance its composition changes, and the balance is eventually lost. A number of interesting things may then happen, various combinations of implosion and explosion, leaving behind at last a zoo of stellar remnants: planetary nebulae, white dwarfs (nobody, it seems, writes "dwarves") of at least two different types, pulsars, magnetars, and of course black holes — knots in spacetime caused when matter collapses to sensational density behind an "event horizon" at which time stops and from which light cannot escape.
These remnants, including black holes, are scattered throughout the Milky Way. At our galaxy's very center, though, is an exceptionally large black hole, implicated in some way we don't completely understand in the galaxy's origin and deep history. Although we cannot see this "Monster in the Core" — Waller's chapter title — we know it's there from the motions of stars around it. The monster weighs four million times as much as the Sun and is known formally as "Sagittarius A*." (You pronounce the second part "A-star," and I'd like to lodge a protest about this notation with the International Astronomical Union: that asterisk has readers looking for a footnote.)
Many galaxies have black holes at their center, but few of those black holes are as well-behaved as ours. Sagittarius A* is "strangely quiescent," Waller tells us. We don't know why. Probably the thing was more energetic in the past, perhaps passing through a quasar phase, when it blazed brighter than a trillion suns. Perhaps one day it will flare up again. We don't understand much about the dynamics of these strange objects.
A concluding chapter mulls over the possibility of civilization elsewhere in the Milky Way, using the 1961 Drake Equation that everyone employs when discussing this topic. You try to come up with estimates for seven variables — things like the fraction of planets that are habitable, then the fraction of those that actually have living things on them — feed them into the equation, and out pops an estimate for the number N of "intelligent species that are technologically communicative." Most of the estimates are speculative, though, so that perfectly reasonable assumptions can produce a value of N as low as one or as high as a million. Our ignorance here is profound.
Waller concludes with a plea for Homo sapiens to "exercise our natural birthright as full-fledged citizens of the Galaxy."
He explains: "That means taking greater care in our mutual relations, so that other more advanced civilizations might regard us with some measure of respect.
It's a nice thought, but probably one to which the inhabitants, if there are any, of the Perseus Spiral Arm might be more receptive than would Mahmoud Ahmedinejad or Kim Jong-un.
The Milky Way: An Insider's Guide is a very comprehensive and up-to-date survey of its subject. The overall impression it leaves one with is of our galaxy as a very busy place, fizzing with what the author, in a particularly felicitous phrase, calls "vigorous fecundity." Our little burg may be a mere speck in the grand scheme of things, but there's a lot going on here.
The other book under consideration deals with the opposite end of the size range: the realm of subatomic particles. The change of scale is comparable, though of course in the other direction. If we take one kilometer — ten minute's walk — as our "medium sized dry good," the Milky Way is a quintillion times bigger, while a proton is a quintillion times smaller.
There is much more strangeness down among the protons, though, than there is among the stars, black holes excepted. It is actually quite hard to grasp what particle physicists are talking about, even when you understand the math. A physicists once explained (or "explained") to me, in regard to the force-carrying particles called bosons, that, "it sometimes helps if you think of these critters not as nouns, but as verbs." Uh …
Jeremy Bernstein has been immersed in this stuff for sixty years. He has written scores of popular articles on physics, and more than a dozen books. Bernstein knew personally many of the great names in his field, including Wolfgang Pauli, the man who said, after reading a physics paper he thought insufficiently rigorous, that it was "not even wrong."
In writing a book about particle physics, however, Bernstein is handicapped by the fact of there having been not much progress in understanding this past thirty years. Back in the 1970s physicists worked out a comprehensive theory called the Standard Model, uniting particles and forces in quite simple patterns and explaining why they interact as they do. That theory was not complete: gravity, for example, was not included. Within its scope, though, it proved to be very robust. None of it has been falsified by experiments.
Still its incompletenesses remain unresolved; and where there has been progress, it has been of a nature to add even more of the same. We now know, for example, that the expansion of the universe is not slowing, as everyone believed in 1980, but accelerating. The concept of "dark energy" has been cooked up to explain the acceleration; but nobody knows what it is, or how the Standard Model might be extended to include it.
Under these circumstances there isn't much Jeremy Bernstein can do but find some new way to present old material. He has hit on the idea implied in his title, of treating the particles as "colors in a palette that can be used to compose the tableau of the universe."
He accordingly divides the particle palette, and his book, into three parts. Under "Primary Colors" he deals with the six particles that emerged from the great burst of creativity in physics across the first third of the 20th century. Five of these were matter particles ("fermions"): the proton, electron, neutron, positron, and the hypothesized (by Pauli) but not yet observed neutrino. The sixth was the photon, a force-carrying particle ("boson").
Then come the "Secondary Colors": those particles that emerged from the great postwar particle-accelerator experiments, and that were eventually organized into the Standard Model. The most important of these were the quarks, of which there are six, all with whimsical names: Up, Down, Charm, Strange, Top, and Bottom. Quarks combine to make more familiar particles: an Up and two Downs, for example, constitute a neutron.
Unfortunately quarks can only exist in combination: free quarks are in principle unobservable. As Bernstein says: "This raises the question — at least for some people — of the sense in which one can say that quarks 'exist.'" By firing electrons through protons, however, physicists satisfied themselves that lurking inside the proton were objects that walked, talked, and quacked like quarks, so the queries quite quickly quiesced.
Finally, under the heading "Pastels," Bernstein deals with unobserved and hypothetical particles: gravitons, tachyons, squarks, and the fabulous Higgs boson. Gravitons are the force-carrying particles for gravity; tachyons travel faster than light, which is okay so long as they don't try to slow to less than light-speed. You can't cross the boundary. Squarks may, if added to the Standard Model, lift it up into a more comprehensive theory named Supersymmetry.
The Higgs boson, which acts to give other particles their mass, is the one whose existence was tentatively confirmed in March of this year by experiments at the Large Hadron Collider in Europe. It belongs firmly in the Standard Model, so observation of it will be a "conservative event," upsetting no applecarts. For this reason Bernstein, and some other physicists I have seen quoted, have mixed feelings about the claimed observation.
Here's Bernstein: "If they have indeed found it, it will end a chapter in physics. I am reminded of something that a French colleague once said to me. He had had a parameter named after him, and it was much discussed because a theory of the weak interactions hinged on it. Finally the parameter was measured, and it confirmed the theory. I went to congratulate him and he said that he was unhappy 'because now they will speak of it no more.'"
"In physics," Jeremy Bernstein notes elsewhere, "truth does not always equal beauty." Even less often does it equal excitement. It is a mighty precious thing none the less; and in the physical sciences, hard-won, as these two books testify.