into two photons with an average lifetime of about 0。8 x 10…16seconds； and the muonand antimuon decay respectively into 。 。 。鈥潯nd so it runs on鈥攁nd 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； 鈥渢o restore some economy to the multitude of hadrons鈥濃�攁 collective term used byphysicists for protons； neutrons； and other particles governed by the strong nuclear force。
gell…mann鈥檚 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： 鈥渢hree quarks for mustermark！鈥潯。╠iscriminating 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鈥攗p； down；strange； charm； top； and bottom鈥攚hich physicists oddly refer to as their 鈥渇lavors；鈥潯nd 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鈥檚 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。
鈥渋t is too plicated。 it has too many arbitrary parameters；鈥潯ederman said。 鈥渨e don鈥檛 reallysee the creator twiddling twenty knobs to set twenty parameters to create the universe as weknow it。鈥潯hysics is really nothing more than a search for ultimate simplicity； but so far all wehave is a kind of elegant messiness鈥攐r as lederman put it： 鈥渢here is a deep feeling that thepicture is not beautiful。鈥�
the standard model is not only ungainly but inplete。 for one thing； it has nothing at allto say about gravity。 search through the standard model as you will； and you won鈥檛 findanything to explain why when you place a hat on a table it doesn鈥檛 float up to the ceiling。 nor；as we鈥檝e 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： 鈥渟o 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 leastinplete。鈥�
in an attempt to draw everything together； physicists have e 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 鈥渟trings鈥濃�攙ibrating 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鈥攖iny enough topass for point particles。
by introducing extra dimensions； superstring theory enables physicists to pull togetherquantum laws and gravitational ones into one paratively 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： 鈥渢he 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 pactified。 （we recall that inkaluza鈥檚 original five…dimensional； the fifth dimension was pactified by being wrappedup into a circle。）鈥潯nd so it goes； for some 350 pages。
string theory has further spawned something called 鈥渕 theory；鈥潯hich incorporatessurfaces known as membranes鈥攐r simply 鈥渂ranes鈥潯o the hipper souls of the world ofphysics。 i鈥檓 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： 鈥渢he 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。鈥潯o arguing withthat。 no understanding it either。 ekpyrotic； incidentally； es from the greek word for鈥渃onflagration。鈥�
matters in physics have now reached such a pitch that； as paul davies noted in nature； it is鈥渁lmost impossible for the non…scientist to discriminate between the legitimately weird andthe outright crackpot。鈥潯he 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 鈥渋maginary time鈥潯nd the 鈥渒ubo…schwinger…martincondition；鈥潯nd purporting to describe the nothingness that was the universe before the bigbang鈥攁 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。 鈥渟cientifically； it鈥檚 clearly more or less pletenonsense；鈥潯olumbia university physicist peter woit told the new york times； 鈥渂ut thesedays that doesn鈥檛 much distinguish it from a lot of the rest of the literature。鈥�
karl popper； whom steven weinberg has called 鈥渢he dean of modern philosophers ofscience；鈥潯nce suggested that there may not be an ultimate theory for physics鈥攖hat； rather；every explanation may require a further explanation； producing 鈥渁n infinite chain of more andmore fundamental principles。鈥潯 rival possibility is that such knowledge may simply bebeyond us。 鈥渟o far； fortunately；鈥潯rites weinberg in dreams of a final theory； 鈥渨e do notseem to be ing to the end of our intellectual resources。鈥�
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 inpletenessof 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鈥檚 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鈥攏ot least earth itself鈥攚ere 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 e up with a new age for the universe offrom 7 to 20 billion years鈥攏ot 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 g茅rard 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鈥攖en 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鈥攁