11 MUSTER MARK’S QUARKS
《万物简史英文版》章节: 11 MUSTER MARK’S QUARKS,宠文网网友提供全文无弹窗免费在线阅读。!
The device worked very well, but had an additional, unexpected benefit. When heaccelerated an alpha particle through the chamber to seed his make-believe clouds, it left avisible trail—like the contrails of a passing airliner. He had just invented the particle detector.
It provided convincing evidence that subatomic particles did indeed exist.
Eventually two other Cavendish scientists invented a more powerful proton-beam device,while in California Ernest Lawrence at Berkeley produced his famous and impressivecyclotron, or atom smasher, as such devices were long excitingly known. All of thesecontraptions worked—and indeed still work—on more or less the same principle, the ideabeing to accelerate a proton or other charged particle to an extremely high speed along a track(sometimes circular, sometimes linear), then bang it into another particle and see what fliesoff. That’s why they were called atom smashers. It wasn’t science at its subtlest, but it wasgenerally effective.
As physicists built bigger and more ambitious machines, they began to find or postulateparticles or particle families seemingly without number: muons, pions, hyperons, mesons, K-mesons, Higgs bosons, intermediate vector bosons, baryons, tachyons. Even physicists beganto grow a little uncomfortable. “Young man,” Enrico Fermi replied when a student asked himthe name of a particular particle, “if I could remember the names of these particles, I wouldhave been a botanist.”
Today accelerators have names that sound like something Flash Gordon would use inbattle: the Super Proton Synchrotron, the Large Electron-Positron Collider, the Large HadronCollider, the Relativistic Heavy Ion Collider. Using huge amounts of energy (some operateonly at night so that people in neighboring towns don’t have to witness their lights fadingwhen the apparatus is fired up), they can whip particles into such a state of liveliness that asingle electron can do forty-seven thousand laps around a four-mile tunnel in a second. Fearshave been raised that in their enthusiasm scientists might inadvertently create a black hole oreven something called “strange quarks,” which could, theoretically, interact with othersubatomic particles and propagate uncontrollably. If you are reading this, that hasn’thappened.
Finding particles takes a certain amount of concentration. They are not just tiny and swiftbut also often tantalizingly evanescent. Particles can come into being and be gone again in aslittle as 0.000000000000000000000001 second (10-24). Even the most sluggish of unstableparticles hang around for no more than 0.0000001 second (10-7).
Some particles are almost ludicrously slippery. Every second the Earth is visited by 10,000trillion trillion tiny, all but massless neutrinos (mostly shot out by the nuclear broilings of theSun), and virtually all of them pass right through the planet and everything that is on it,including you and me, as if it weren’t there. To trap just a few of them, scientists need tanksholding up to 12.5 million gallons of heavy water (that is, water with a relative abundance ofdeuterium in it) in underground chambers (old mines usually) where they can’t be interferedwith by other types of radiation.
Very occasionally, a passing neutrino will bang into one of the atomic nuclei in the waterand produce a little puff of energy. Scientists count the puffs and by such means take us veryslightly closer to understanding the fundamental properties of the universe. In 1998, Japaneseobservers reported that neutrinos do have mass, but not a great deal—about one ten-millionththat of an electron.
What it really takes to find particles these days is money and lots of it. There is a curiousinverse relationship in modern physics between the tininess of the thing being sought and thescale of facilities required to do the searching. CERN, the European Organization for NuclearResearch, is like a little city. Straddling the border of France and Switzerland, it employsthree thousand people and occupies a site that is measured in square miles. CERN boasts astring of magnets that weigh more than the Eiffel Tower and an underground tunnel oversixteen miles around.
Breaking up atoms, as James Trefil has noted, is easy; you do it each time you switch on afluorescent light. Breaking up atomic nuclei, however, requires quite a lot of money and agenerous supply of electricity. Getting down to the level of quarks—the particles that make upparticles—requires still more: trillions of volts of electricity and the budget of a small CentralAmerican nation. CERN’s new Large Hadron Collider, scheduled to begin operations in 2005,will achieve fourteen trillion volts of energy and cost something over $1.5 billion toconstruct.
1But these numbers are as nothing compared with what could have been achieved by, andspent upon, the vast and now unfortunately never-to-be Superconducting Supercollider, whichbegan being constructed near Waxahachie, Texas, in the 1980s, before experiencing asupercollision of its own with the United States Congress. The intention of the collider was tolet scientists probe “the ultimate nature of matter,” as it is always put, by re-creating as nearlyas possible the conditions in the universe during its first ten thousand billionths of a second.
The plan was to fling particles through a tunnel fifty-two miles long, achieving a trulystaggering ninety-nine trillion volts of energy. It was a grand scheme, but would also havecost $8 billion to build (a figure that eventually rose to $10 billion) and hundreds of millionsof dollars a year to run.
In perhaps the finest example in history of pouring money into a hole in the ground,Congress spent $2 billion on the project, then canceled it in 1993 after fourteen miles oftunnel had been dug. So Texas now boasts the most expensive hole in the universe. The siteis, I am told by my friend Jeff Guinn of the Fort Worth Star-Telegram, “essentially a vast,cleared field dotted along the circumference by a series of disappointed small towns.”
1There are practical side effects to all this costly effort. The World Wide Web is a CERN offshoot. It wasinvented by a CERN scientist, Tim Berners-Lee, in 1989.
Since the supercollider debacle particle physicists have set their sights a little lower, buteven comparatively modest projects can be quite breathtakingly costly when compared with,well, almost anything. A proposed neutrino observatory at the old Homestake Mine in Lead,South Dakota, would cost $500 million to build—this in a mine that is already dug—beforeyou even look at the annual running costs. There would also be $281 million of “generalconversion costs.” A particle accelerator at Fermilab in Illinois, meanwhile, cost $260 millionmerely to refit.
Particle physics, in short, is a hugely expensive enterprise—but it is a productive one.
Today the particle count is well over 150, with a further 100 or so suspected, butunfortunately, in the words of Richard Feynman, “it is very difficult to understand therelationships of all these particles, and what nature wants them for, or what the connectionsare from one to another.” Inevitably each time we manage to unlock a box, we find that thereis another locked box inside. Some people think there are particles called tachyons, which cantravel faster than the speed of light. Others long to find gravitons—the seat of gravity. Atwhat point we reach the irreducible bottom is not easy to say. Carl Sagan in Cosmos raised thepossibility that if you traveled downward into an electron, you might find that it contained auniverse of its own, recalling all those science fiction stories of the fifties. “Within it,organized into the local equivalent of galaxies and smaller structures, are an immense numberof other, much tinier elementary particles, which are themselves universes at the next leveland so on forever—an infinite downward regression, universes within universes, endlessly.
And upward as well.”
For most of us it is a world that surpasses understanding. To read even an elementary guideto particle physics nowadays you must now find your way through lexical thickets such asthis: “The charged pion and antipion decay respectively into a muon plus antineutrino and anantimuon plus neutrino with an average lifetime of 2.603 x 10-8seconds, the neutral piondecays into two photons with an average lifetime of about 0.8 x 10-16seconds, and the muonand antimuon decay respectively into . . .” And so it runs on—and this from a book for thegeneral reader by one of the (normally) most lucid of interpreters, Steven Weinberg.
In the 1960s, in an attempt to bring just a little simplicity to matters, the Caltech physicistMurray Gell-Mann invented a new class of particles, essentially, in the words of StevenWeinberg, “to restore some economy to the multitude of hadrons”—a collective term used byphysicists for protons, neutrons, and other particles governed by the strong nuclear force.
Gell-Mann’s theory was that all hadrons were made up of still smaller, even morefundamental particles. His colleague Richard Feynman wanted to call these new basicparticles partons, as in Dolly, but was overruled. Instead they became known as quarks.
Gell-Mann took the name from a line in Finnegans Wake: “Three quarks for MusterMark!” (Discriminating physicists rhyme the word with storks, not larks, even though thelatter is almost certainly the pronunciation Joyce had in mind.) The fundamental simplicity ofquarks was not long lived. As they became better understood it was necessary to introducesubdivisions. Although quarks are much too small to have color or taste or any other physicalcharacteristics we would recognize, they became clumped into six categories—up, down,strange, charm, top, and bottom—which physicists oddly refer to as their “flavors,” and theseare further divided into the colors red, green, and blue. (One suspects that it was not altogethercoincidental that these terms were first applied in California during the age of psychedelia.) Eventually out of all this emerged what is called the Standard Model, which is essentially asort of parts kit for the subatomic world. The Standard Model consists of six quarks, sixleptons, five known bosons and a postulated sixth, the Higgs boson (named for a Scottishscientist, Peter Higgs), plus three of the four physical forces: the strong and weak nuclearforces and electromagnetism.
The arrangement essentially is that among the basic building blocks of matter are quarks;these are held together by particles called gluons; and together quarks and gluons formprotons and neutrons, the stuff of the atom’s nucleus. Leptons are the source of electrons andneutrinos. Quarks and leptons together are called fermions. Bosons (named for the Indianphysicist S. N. Bose) are particles that produce and carry forces, and include photons andgluons. The Higgs boson may or may not actually exist; it was invented simply as a way ofendowing particles with mass.
It is all, as you can see, just a little unwieldy, but it is the simplest model that can explainall that happens in the world of particles. Most particle physicists feel, as Leon Ledermanremarked in a 1985 PBS documentary, that the Standard Model lacks elegance and simplicity.
“It is too complicated. It has too many arbitrary parameters,” Lederman said. “We don’t reallysee the creator twiddling twenty knobs to set twenty parameters to create the universe as weknow it.” Physics is really nothing more than a search for ultimate simplicity, but so far all wehave is a kind of elegant messiness—or as Lederman put it: “There is a deep feeling that thepicture is not beautiful.”
The Standard Model is not only ungainly but incomplete. For one thing, it has nothing at allto say about gravity. Search through the Standard Model as you will, and you won’t findanything to explain why when you place a hat on a table it doesn’t float up to the ceiling. Nor,as we’ve just noted, can it explain mass. In order to give particles any mass at all we have tointroduce the notional Higgs boson; whether it actually exists is a matter for twenty-first-century physics. As Feynman cheerfully observed: “So we are stuck with a theory, and we donot know whether it is right or wrong, but we do know that it is a little wrong, or at leastincomplete.”
In an attempt to draw everything together, physicists have come up with something calledsuperstring theory. This postulates that all those little things like quarks and leptons that wehad previously thought of as particles are actually “strings”—vibrating strands of energy thatoscillate in eleven dimensions, consisting of the three we know already plus time and sevenother dimensions that are, well, unknowable to us. The strings are very tiny—tiny enough topass for point particles.
By introducing extra dimensions, superstring theory enables physicists to pull togetherquantum laws and gravitational ones into one comparatively tidy package, but it also meansthat anything scientists say about the theory begins to sound worryingly like the sort ofthoughts that would make you edge away if conveyed to you by a stranger on a park bench.
Here, for example, is the physicist Michio Kaku explaining the structure of the universe froma superstring perspective: “The heterotic string consists of a closed string that has two types ofvibrations, clockwise and counterclockwise, which are treated differently. The clockwisevibrations live in a ten-dimensional space. The counterclockwise live in a twenty-six-dimensional space, of which sixteen dimensions have been compactified. (We recall that inKaluza’s original five-dimensional, the fifth dimension was compactified by being wrappedup into a circle.)” And so it goes, for some 350 pages.
String theory has further spawned something called “M theory,” which incorporatessurfaces known as membranes—or simply “branes” to the hipper souls of the world ofphysics. I’m afraid this is the stop on the knowledge highway where most of us must get off.
Here is a sentence from the New York Times, explaining this as simply as possible to a generalaudience: “The ekpyrotic process begins far in the indefinite past with a pair of flat emptybranes sitting parallel to each other in a warped five-dimensional space. . . . The two branes,which form the walls of the fifth dimension, could have popped out of nothingness as aquantum fluctuation in the even more distant past and then drifted apart.” No arguing withthat. No understanding it either. Ekpyrotic, incidentally, comes from the Greek word for“conflagration.”
Matters in physics have now reached such a pitch that, as Paul Davies noted in Nature, it is“almost impossible for the non-scientist to discriminate between the legitimately weird andthe outright crackpot.” The question came interestingly to a head in the fall of 2002 when twoFrench physicists, twin brothers Igor and Grickha Bogdanov, produced a theory of ambitiousdensity involving such concepts as “imaginary time” and the “Kubo-Schwinger-Martincondition,” and purporting to describe the nothingness that was the universe before the BigBang—a period that was always assumed to be unknowable (since it predated the birth ofphysics and its properties).
Almost at once the Bogdanov paper excited debate among physicists as to whether it wastwaddle, a work of genius, or a hoax. “Scientifically, it’s clearly more or less completenonsense,” Columbia University physicist Peter Woit told the New York Times, “but thesedays that doesn’t much distinguish it from a lot of the rest of the literature.”
Karl Popper, whom Steven Weinberg has called “the dean of modern philosophers ofscience,” once suggested that there may not be an ultimate theory for physics—that, rather,every explanation may require a further explanation, producing “an infinite chain of more andmore fundamental principles.” A rival possibility is that such knowledge may simply bebeyond us. “So far, fortunately,” writes Weinberg in Dreams of a Final Theory, “we do notseem to be coming to the end of our intellectual resources.”
Almost certainly this is an area that will see further developments of thought, and almostcertainly these thoughts will again be beyond most of us.
While physicists in the middle decades of the twentieth-century were looking perplexedlyinto the world of the very small, astronomers were finding no less arresting an incompletenessof understanding in the universe at large.
When we last met Edwin Hubble, he had determined that nearly all the galaxies in our fieldof view are flying away from us, and that the speed and distance of this retreat are neatlyproportional: the farther away the galaxy, the faster it is moving. Hubble realized that thiscould be expressed with a simple equation, Ho = v/d (where Ho is the constant, v is therecessional velocity of a flying galaxy, andd its distance away from us). Ho has been knownever since as the Hubble constant and the whole as Hubble’s Law. Using his formula, Hubblecalculated that the universe was about two billion years old, which was a little awkwardbecause even by the late 1920s it was fairly obvious that many things within the universe—not least Earth itself—were probably older than that. Refining this figure has been an ongoingpreoccupation of cosmology.
Almost the only thing constant about the Hubble constant has been the amount ofdisagreement over what value to give it. In 1956, astronomers discovered that Cepheidvariables were more variable than they had thought; they came in two varieties, not one. Thisallowed them to rework their calculations and come up with a new age for the universe offrom 7 to 20 billion years—not terribly precise, but at least old enough, at last, to embrace theformation of the Earth.
In the years that followed there erupted a long-running dispute between Allan Sandage, heirto Hubble at Mount Wilson, and Gérard de Vaucouleurs, a French-born astronomer based atthe University of Texas. Sandage, after years of careful calculations, arrived at a value for theHubble constant of 50, giving the universe an age of 20 billion years. De Vaucouleurs wasequally certain that the Hubble constant was 100.
2This would mean that the universe wasonly half the size and age that Sandage believed—ten billion years. Matters took a furtherlurch into uncertainty when in 1994 a team from the Carnegie Observatories in California,using measures from the Hubble space telescope, suggested that the universe could be as littleas eight billion years old—an age even they conceded was younger than some of the starswithin the universe. In February 2003, a team from NASA and the Goddard Space FlightCenter in Maryland, using a new, far-reaching type of satellite called the WilkinsonMicrowave Anistropy Probe, announced with some confidence that the age of the universe is13.7 billion years, give or take a hundred million years or so. There matters rest, at least forthe moment.
The difficulty in making final determinations is that there are often acres of room forinterpretation. Imagine standing in a field at night and trying to decide how far away twodistant electric lights are. Using fairly straightforward tools of astronomy you can easilyenough determine that the bulbs are of equal brightness and that one is, say, 50 percent moredistant than the other. But what you can’t be certain of is whether the nearer light is, let ussay, a 58-watt bulb that is 122 feet away or a 61-watt light that is 119 feet, 8 inches away. Ontop of that you must make allowances for distortions caused by variations in the Earth’satmosphere, by intergalactic dust, contaminating light from foreground stars, and many otherfactors. The upshot is that your computations are necessarily based on a series of nestedassumptions, any of which could be a source of contention. There is also the problem thataccess to telescopes is always at a premium and historically measuring red shifts has beennotably costly in telescope time. It could take all night to get a single exposure. Inconsequence, astronomers have sometimes been compelled (or willing) to base conclusionson notably scanty evidence. In cosmology, as the journalist Geoffrey Carr has suggested, wehave “a mountain of theory built on a molehill of evidence.” Or as Martin Rees has put it:
“Our present satisfaction [with our state of understanding] may reflect the paucity of the datarather than the excellence of the theory.”
This uncertainty applies, incidentally, to relatively nearby things as much as to the distantedges of the universe. As Donald Goldsmith notes, when astronomers say that the galaxy M87is 60 million light-years away, what they really mean (“but do not often stress to the generalpublic”) is that it is somewhere between 40 million and 90 million light-years away—not2You are of course entitled to wonder what is meant exactly by "a constant of 50" or "a constant of 100." Theanswer lies in astronomical units of measure. Except conversationally, astronomers dont use light-years. Theyuse a distance called the parsec (a contraction of parallax and second), based on a universal measure called thestellar parallax and equivalent to 3.26 light-years. Really big measures, like the size of a universe, are measuredin megaparsecs: a million parsecs. The constant is expressed in terms of kilometers per second per megaparsec.
Thus when astronomers refer to a Hubble constant of 50, what they really mean is "50 kilometers per second permegaparsec." For most of us that is of course an utterly meaningless measure, but then with astronomicalmeasures most distances are so huge as to be utterly meaningless.
quite the same thing. For the universe at large, matters are naturally magnified. Bearing allthat in mind, the best bets these days for the age of the universe seem to be fixed on a range ofabout 12 billion to 13.5 billion years, but we remain a long way from unanimity.
One interesting recently suggested theory is that the universe is not nearly as big as wethought, that when we peer into the distance some of the galaxies we see may simply bereflections, ghost images created by rebounded light.
The fact is, there is a great deal, even at quite a fundamental level, that we don’t know—notleast what the universe is made of. When scientists calculate the amount of matter needed tohold things together, they always come up desperately short. It appears that at least 90 percentof the universe, and perhaps as much as 99 percent, is composed of Fritz Zwicky’s “darkmatter”—stuff that is by its nature invisible to us. It is slightly galling to think that we live ina universe that, for the most part, we can’t even see, but there you are. At least the names forthe two main possible culprits are entertaining: they are said to be either WIMPs (for WeaklyInteracting Massive Particles, which is to say specks of invisible matter left over from the BigBang) or MACHOs (for MAssive Compact Halo Objects—really just another name for blackholes, brown dwarfs, and other very dim stars).
Particle physicists have tended to favor the particle explanation of WIMPs, astrophysiciststhe stellar explanation of MACHOs. For a time MACHOs had the upper hand, but not nearlyenough of them were found, so sentiment swung back toward WIMPs but with the problemthat no WIMP has ever been found. Because they are weakly interacting, they are (assumingthey even exist) very hard to detect. Cosmic rays would cause too much interference. Soscientists must go deep underground. One kilometer underground cosmic bombardmentswould be one millionth what they would be on the surface. But even when all these are addedin, “two-thirds of the universe is still missing from the balance sheet,” as one commentatorhas put it. For the moment we might very well call them DUNNOS (for Dark UnknownNonreflective Nondetectable Objects Somewhere).
Recent evidence suggests that not only are the galaxies of the universe racing away fromus, but that they are doing so at a rate that is accelerating. This is counter to all expectations. Itappears that the universe may not only be filled with dark matter, but with dark energy.
Scientists sometimes also call it vacuum energy or, more exotically, quintessence. Whatever itis, it seems to be driving an expansion that no one can altogether account for. The theory isthat empty space isn’t so empty at all—that there are particles of matter and antimatterpopping into existence and popping out again—and that these are pushing the universeoutward at an accelerating rate. Improbably enough, the one thing that resolves all this isEinstein’s cosmological constant—the little piece of math he dropped into the general theoryof relativity to stop the universe’s presumed expansion, and called “the biggest blunder of mylife.” It now appears that he may have gotten things right after all.
The upshot of all this is that we live in a universe whose age we can’t quite compute,surrounded by stars whose distances we don’t altogether know, filled with matter we can’tidentify, operating in conformance with physical laws whose properties we don’t trulyunderstand.
And on that rather unsettling note, let’s return to Planet Earth and consider something thatwe do understand—though by now you perhaps won’t be surprised to hear that we don’tunderstand