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CHAPTER 5 ELEMENTARY PARTICLES AND THE FORCES OFNATURE
Aristotle believed that all the matter in the universe wasmade up of four basic elements - earth, air, fire, and water.
These elements were acted on by two forces: gravity, thetendency for earth and water to sink, and levity, the tendencyfor air and fire to rise. This division of the contents of theuniverse into matter and forces is still used today. Aristotlebelieved that matter was continuous, that is, one could divide apiece of matter into smaller and smaller bits without any limit:
one never came up against a grain of matter that could not bedivided further. A few Greeks, however, such as Democritus,held that matter was inherently grainy and that everything wasmade up of large numbers of various different kinds of atoms.
(The word atom means “indivisible” in Greek.) For centuriesthe argument continued without any real evidence on eitherside, but in 1803 the British chemist and physicist John Daltonpointed out that the fact that chemical compounds alwayscombined in certain proportions could be explained by thegrouping together of atoms to form units called molecules.
However, the argument between the two schools of thoughtwas not finally settled in favor of the atomists until the earlyyears of this century. One of the important pieces of physicalevidence was provided by Einstein. In a paper written in 1905,a few weeks before the famous paper on special relativity,Einstein pointed out that what was called Brownian motion -the irregular, random motion of small particles of dustsuspended in a liquid - could be explained as the effect ofatoms of the liquid colliding with the dust particles.
By this time there were already suspicions that these atomswere not, after all, indivisible. Several years previously a fellowof Trinity College, Cambridge, J. J. Thomson, had demonstratedthe existence of a particle of matter, called the electron, thathad a mass less than one thousandth of that of the lightestatom. He used a setup rather like a modern TV picture tube:
a red-hot metal filament gave off the electrons, and becausethese have a negative electric charge, an electric field could beused to accelerate them toward a phosphor-coated screen.
When they hit the screen, flashes of light were generated. Soonit was realized that these electrons must be coming from withinthe atoms themselves, and in 1911 the New Zealand physicistErnest Rutherford finally showed that the atoms of matter dohave internal structure: they are made up of an extremely tiny,positively charged nucleus, around which a number of electronsorbit. He deduced this by analyzing the way in whichalpha-particles, which are positively charged particles given offby radioactive atoms, are deflected when they collide withatoms.
At first it was thought that the nucleus of the atom wasmade up of electrons and different numbers of a positivelycharged particle called the proton, from the Greek wordmeaning “first,” because it was believed to be the fundamentalunit from which matter was made. However, in 1932 acolleague of Rutherford’s at Cambridge, James Chadwick,discovered that the nucleus contained another particle, called theneutron, which had almost the same mass as a proton but noelectrical charge. Chadwick received the Nobel Prize for hisdiscovery, and was elected Master of Gonville and Caius College,Cambridge (the college of which I am now a fellow). He laterresigned as Master because of disagreements with the Fellows.
There had been a bitter dispute in the college ever since agroup of young Fellows returning after the war had votedmany of the old Fellows out of the college offices they had heldfor a long time. This was before my time; I joined the collegein 1965 at the tail end of the bitterness, when similardisagreements forced another Nobel Prize - winning Master, SirNevill Mott, to resign.
Up to about thirty years ago, it was thought that protonsand neutrons were “elementary” particles, but experiments inwhich protons were collided with other protons or electrons athigh speeds indicated that they were in fact made up ofsmaller particles. These particles were named quarks by theCaltech physicist Murray Gell-Mann, who won the Nobel Prizein 1969 for his work on them. The origin of the name is anenigmatic quotation from James Joyce: “Three quarks forMuster Mark!” The word quark is supposed to be pronouncedlike quart, but with a k at the end instead of a t, but isusually pronounced to rhyme with lark.
There are a number of different varieties of quarks: thereare six “flavors,” which we call up, down, strange, charmed,bottom, and top. The first three flavors had been known sincethe 1960s but the charmed quark was discovered only in 1974,the bottom in 1977, and the top in 1995. Each flavor comes inthree “colors,” red, green, and blue. (It should be emphasizedthat these terms are just labels: quarks are much smaller thanthe wavelength of visible light and so do not have any color inthe normal sense. It is just that modern physicists seem tohave more imaginative ways of naming new particles andphenomena - they no longer restrict themselves to Greek!) Aproton or neutron is made up of three quarks, one of eachcolor. A proton contains two up quarks and one down quark;a neutron contains two down and one up. We can createparticles made up of the other quarks (strange, charmed,bottom, and top), but these all have a much greater mass anddecay very rapidly into protons and neutrons.
We now know that neither the atoms nor the protons andneutrons within them are indivisible. So the question is: whatare the truly elementary particles, the basic building blocks fromwhich everything is made? Since the wavelength of light ismuch larger than the size of an atom, we cannot hope to“look” at the parts of an atom in the ordinary way. We needto use something with a much smaller wave-length. As we sawin the last chapter, quantum mechanics tells us that all particlesare in fact waves, and that the higher the energy of a particle,the smaller the wavelength of the corresponding wave. So thebest answer we can give to our question depends on how higha particle energy we have at our disposal, because thisdetermines on how small a length scale we can look. Theseparticle energies are usually measured in units called electronvolts. (In Thomson’s experiments with electrons, we saw thathe used an electric field to accelerate the electrons. The energythat an electron gains from an electric field of one volt is whatis known as an electron volt.) In the nineteenth century, whenthe only particle energies that people knew how to use werethe low energies of a few electron volts generated by chemicalreactions such as burning, it was thought that atoms were thesmallest unit. In Rutherford’s experiment, the alpha-particles hadenergies of millions of electron volts. More recently, we havelearned how to use electromagnetic fields to give particlesenergies of at first millions and then thousands of millions ofelectron volts. And so we know that particles that were thoughtto be “elementary” thirty years ago are, in fact, made up ofsmaller particles. May these, as we go to still higher energies, inturn be found to be made from still smaller particles? This iscertainly possible, but we do have some theoretical reasons forbelieving that we have, or are very near to, a knowledge of theultimate building blocks of nature.
Using the wave/particle duality discussed in the last chapter,every-thing in the universe, including light and gravity, can bedescribed in terms of particles. These particles have a propertycalled spin. One way of thinking of spin is to imagine theparticles as little tops spinning about an axis. However, this canbe misleading, because quantum mechanics tells us that theparticles do not have any well-defined axis. What the spin of aparticle really tells us is what the particle looks like fromdifferent directions. A particle of spin 0 is like a dot: it looksthe same from every direction (Fig. 5.1-i). On the other hand,a particle of spin 1 is like an arrow: it looks different fromdifferent directions (Fig. 5.1-ii). Only if one turns it round acomplete revolution (360 degrees) does the particle look thesame. A particle of spin 2 is like a double-headed arrow (Fig.
5.1-iii): it looks the same if one turns it round half a revolution(180 degrees). Similarly, higher spin particles look the same ifone turns them through smaller fractions of a completerevolution. All this seems fairly straightforward, but theremark-able fact is that there are particles that do not look thesame if one turns them through just one revolution: you haveto turn them through two complete revolutions! Such particlesare said to have spin ?.
All the known particles in the universe can be divided intotwo groups: particles of spin ?, which make up the matter inthe universe, and particles of spin 0, 1, and 2, which, as weshall see, give rise to forces between the matter particles. Thematter particles obey what is called Pauli’s exclusion principle.
This was discovered in 1925 by an Austrian physicist, WolfgangPauli - for which he received the Nobel Prize in 1945. He wasthe archetypal theoretical physicist: it was said of him that evenhis presence in the same town would make experiments gowrong! Pauli’s exclusion principle says that two similar particlescan-not exist in the same state; that is, they cannot have boththe same position and the same velocity, within the limits givenby the uncertainty principle. The exclusion principle is crucialbecause it explains why matter particles do not collapse to astate of very high density under the influence of the forcesproduced by the particles of spin 0, 1, and 2: if the matterparticles have very nearly the same positions, they must havedifferent velocities, which means that they will not stay in thesame position for long. If the world had been created withoutthe exclusion principle, quarks would not form separate,well-defined protons and neutrons. Nor would these, togetherwith electrons, form separate, well-defined atoms. They would allcollapse to form a roughly uniform, dense “soup.”
A proper understanding of the electron and other spin-?
particles did not come until 1928, when a theory was proposedby Paul Dirac, who later was elected to the LucasianProfessorship of Mathematics at Cambridge (the sameprofessorship that Newton had once held and that I now hold).
Dirac’s theory was the first of its kind that was consistent withboth quantum mechanics and the special theory of relativity. Itexplained mathematically why the electron had spin-?; that is,why it didn’t look the same if you turned it through only onecomplete revolution, but did if you turned it through tworevolutions. It also predicted that the electron should have apartner: an anti-electron, or positron. The discovery of thepositron in 1932 confirmed Dirac’s theory and led to his beingawarded the Nobel Prize for physics in 1933. We now knowthat every particle has an antiparticle, with which it canannihilate. (In the case of the force-carrying particles, theantiparticles are the same as the particles themselves.) Therecould be whole antiworlds and antipeople made out ofantiparticles. However, if you meet your antiself, don’t shakehands! You would both vanish in a great flash of light. Thequestion of why there seem to be so many more particles thanantiparticles around us is extremely important, andI shall return to it later in the chapter.
In quantum mechanics, the forces or interactions betweenmatter particles are all supposed to be carried by particles ofinteger spin - 0, 1, or 2. What happens is that a matterparticle, such as an electron or a quark, emits a force-carryingparticle. The recoil from this emission changes the velocity ofthe matter particle. The force-carrying particle then collides withanother matter particle and is absorbed. This collision changesthe velocity of the second particle, just as if there had been aforce between the two matter particles. It is an importantproperty of ‘ the force-carrying particles that they do not obeythe exclusion principle. This means that there is no limit to thenumber that can be exchanged, and so they can give rise to astrong force. However, if the force-carrying particles have ahigh mass, it will be difficult to produce and exchange themover a large distance. So the forces that they carry will haveonly a short range. On the other hand, if the force-carryingparticles have no mass of their own, the forces will be longrange. The force-carrying particles exchanged between matterparticles are said to be virtual particles because, unlike “real”
particles, they cannot be directly detected by a particle detector.
We know they exist, however, because they do have ameasurable effect: they give rise to forces between matterparticles. Particles of spin 0, 1, or 2 do also exist in somecircumstances as real particles, when they can be directlydetected. They then appear to us as what a classical physicistwould call waves, such as waves of light or gravitational waves.
They may sometimes be emitted when matter particles interactwith each other by exchanging virtual force-carrying particles.
(For example, the electric repulsive force between two electronsis due to the exchange of virtual photons, which can never bedirectly detected; but if one electron moves past another, realphotons may be given off, which we detect as light waves.)Force-carrying particles can be grouped into four categoriesaccording to the strength of the force that they carry and theparticles with which they interact. It should be emphasized thatthis division into four classes is man-made; it is convenient forthe construction of partial theories, but it may not correspondto anything deeper. Ultimately, most physicists hope to find aunified theory that will explain all four forces as differentaspects of a single force. Indeed, many would say this is theprime goal of physics today. Recently, successful attempts havebeen made to unify three of the four categories of force - andI shall describe these in this chapter. The question of theunification of the remaining category, gravity, we shall leave tilllater.
The first category is the gravitational force. This force isuniversal, that is, every particle feels the force of gravity,according to its mass or energy. Gravity is the weakest of thefour forces by a long way; it is so weak that we would notnotice it at all were it not for two special properties that it has:
it can act over large distances, and it is always attractive. Thismeans that the very weak gravitational forces between theindividual particles in two large bodies, such as the earth andthe sun, can all add up to produce a significant force. Theother three forces are either short range, or are sometimesattractive and some-times repulsive, so they tend to cancel out.
In the quantum mechanical way of looking at the gravitationalfield, the force between two matter particles is pictured as beingcarried by a particle of spin 2 called the graviton. This has nomass of its own, so the force that it carries is long range. Thegravitational force between the sun and the earth is ascribed tothe exchange of gravitons between the particles that make upthese two bodies. Although the exchanged particles are virtual,they certainly do produce a measurable effect - they make theearth orbit the sun! Real gravitons make up what classicalphysicists would call gravitational waves, which are very weak -and so difficult to detect that they have not yet been observed.
The next category is the electromagnetic force, whichinteracts with electrically charged particles like electrons andquarks, but not with uncharged particles such as gravitons. It ismuch stronger than the gravitational force: the electromagneticforce between two electrons is about a million million millionmillion million million million (1 with forty-two zeros after it)times bigger than the gravitational force. However, there aretwo kinds of electric charge, positive and negative. The forcebetween two positive charges is repulsive, as is the forcebetween two negative charges, but the force is attractivebetween a positive and a negative charge. A large body, suchas the earth or the sun, contains nearly equal numbers ofpositive and negative charges. Thus the attractive and repulsiveforces between the individual particles nearly cancel each otherout, and there is very little net electromagnetic force. However,on the small scales of atoms and molecules, electromagneticforces dominate. The electromagnetic attraction betweennegatively charged electrons and positively charged protons inthe nucleus causes the electrons to orbit the nucleus of theatom, just as gravitational attraction causes the earth to orbitthe sun. The electromagnetic attraction is pictured as beingcaused by the exchange of large numbers of virtual masslessparticles of spin 1, called photons. Again, the photons that areexchanged are virtual particles. However, when an electronchanges from one allowed orbit to another one nearer to thenucleus, energy is released and a real photon is emitted -which can be observed as visible light by the human eye, if ithas the right wave-length, or by a photon detector such asphotographic film. Equally, if a real photon collides with anatom, it may move an electron from an orbit nearer thenucleus to one farther away. This uses up the energy of thephoton, so it is absorbed.
The third category is called the weak nuclear force, which isresponsible for radioactivity and which acts on all matterparticles of spin-?, but not on particles of spin 0, 1, or 2,such as photons and gravitons. The weak nuclear force wasnot well understood until 1967, when Abdus Salam at ImperialCollege, London, and Steven Weinberg at Harvard bothproposed theories that unified this interaction with theelectromagnetic force, just as Maxwell had unified electricity andmagnetism about a hundred years earlier. They suggested thatin addition to the photon, there were three other spin-1particles, known collectively as massive vector bosons, thatcarried the weak force. These were called W+ (pronounced Wplus), W- (pronounced W minus), and Z? (pronounced Znaught), and each had a mass of around 100 GeV (GeVstands for gigaelectron-volt, or one thousand million electronvolts). The Weinberg-Salam theory exhibits a property known asspontaneous symmetry breaking. This means that what appearto be a number of completely different particles at low energiesare in fact found to be all the same type of particle, only indifferent states. At high energies all these particles behavesimilarly. The effect is rather like the behavior of a roulette ballon a roulette wheel. At high energies (when the wheel is spunquickly) the ball behaves in essentially only one way - it rollsround and round. But as the wheel slows, the energy of theball decreases, and eventually the ball drops into one of thethirty-seven slots in the wheel. In other words, at low energiesthere are thirty-seven different states in which the ball canexist. If, for some reason, we could only observe the ball atlow energies, we would then think that there were thirty-sevendifferent types of ball!
In the Weinberg-Salam theory, at energies much greater than100 GeV, the three new particles and the photon would allbehave in a similar manner. But at the lower particle energiesthat occur in most normal situations, this symmetry betweenthe particles would be broken. WE, W, and Z? would acquirelarge masses, making the forces they carry have a very shortrange. At the time that Salam and Weinberg proposed theirtheory, few people believed them, and particle accelerators werenot powerful enough to reach the energies of 100 GeVrequired to produce real W+, W-, or Z? particles. However,over the next ten years or so, the other predictions of thetheory at lower energies agreed so well with experiment that, in1979, Salam and Weinberg were awarded the Nobel Prize forphysics, together with Sheldon Glashow, also at Harvard, whohad suggested similar unified theories of the electromagnetic andweak nuclear forces. The Nobel committee was spared theembarrassment of having made a mistake by the discovery in1983 at CERN (European Centre for Nuclear Research) of thethree massive partners of the photon, with the correct predictedmasses and other properties. Carlo Rubbia, who led the teamof several hundred physicists that made the discovery, receivedthe Nobel Prize in 1984, along with Simon van der Meer, theCERNengineer who developed the antimatter storage systememployed. (It is very difficult to make a mark in experimentalphysics these days unless you are already at the top! )The fourth category is the strong nuclear force, which holdsthe quarks together in the proton and neutron, and holds theprotons and neutrons together in the nucleus of an atom. It isbelieved that this force is carried by another spin-1 particle,called the gluon, which interacts only with itself and with thequarks. The strong nuclear force has a curious property calledconfinement: it always binds particles together into combinationsthat have no color. One cannot have a single quark on itsown because it would have a color (red, green, or blue).
Instead, a red quark has to be joined to a green and a bluequark by a “string” of gluons (red + green + blue = white).
Such a triplet constitutes a proton or a neutron. Anotherpossibility is a pair consisting of a quark and an antiquark (red+ antired, or green + antigreen, or blue + antiblue = white).
Such combinations make up the particles known as mesons,which are unstable because the quark and antiquark canannihilate each other, producing electrons and other particles.
Similarly, confinement prevents one having a single gluon on itsown, because gluons also have color. Instead, one has to havea collection of gluons whose colors add up to white. Such acollection forms an unstable particle called a glueball.
The fact that confinement prevents one from observing anisolated quark or gluon might seem to make the whole notionof quarks and gluons as particles somewhat metaphysical.
However, there is another property of the strong nuclear force,called asymptotic freedom, that makes the concept of quarksand gluons well defined. At normal energies, the strong nuclearforce is indeed strong, and it binds the quarks tightly together.
However, experiments with large particle accelerators indicatethat at high energies the strong force becomes much weaker,and the quarks and gluons behave almost like free particles.
Fig. 5.2 shows a photograph of a collision between ahigh-energy proton and antiproton. The success of theunification of the electromagnetic and weak nuclear forces led toa number of attempts to combine these two forces with thestrong nuclear force into what is called a grand unified theory(or GUT). This title is rather an exaggeration: the resultanttheories are not all that grand, nor are they fully unified, asthey do not include gravity. Nor are they really completetheories, because they contain a number of parameters whosevalues cannot be predicted from the theory but have to bechosen to fit in with experiment. Nevertheless, they may be astep toward a complete, fully unified theory. The basic idea ofGUTs is as follows: as was mentioned above, the strongnuclear force gets weaker at high energies. On the other hand,the electromagnetic and weak forces, which are notasymptotically free, get stronger at high energies. At some veryhigh energy, called the grand unification energy, these threeforces would all have the same strength and so could just bedifferent aspects of a single force. The GUTs also predict thatat this energy the different spin-? matter particles, like quarksand electrons, would also all be essentially the same, thusachieving another unification.
The value of the grand unification energy is not very wellknown, but it would probably have to be at least a thousandmillion million GeV. The present generation of particleaccelerators can collide particles at energies of about onehundred GeV, and machines are planned that would raise thisto a few thousand GeV. But a machine that was powerfulenough to accelerate particles to the grand unification energywould have to be as big as the Solar System - and would beunlikely to be funded in the present economic climate. Thus itis impossible to test grand unified theories directly in thelaboratory. However, just as in the case of the electromagneticand weak unified theory, there are low-energy consequences ofthe theory that can be tested.
The most interesting of these is the prediction that protons,which make up much of the mass of ordinary matter, canspontaneously decay into lighter particles such as antielectrons.
The reason this is possible is that at the grand unificationenergy there is no essential difference between a quark and anantielectron. The three quarks inside a proton normally do nothave enough energy to change into antielectrons, but veryoccasionally one of them may acquire sufficient energy to makethe transition because the uncertainty principle means that theenergy of the quarks inside the proton cannot be fixed exactly.
The proton would then decay. The probability of a quarkgaining sufficient energy is so low that one is likely to have towait at least a million million million million million years (1followed by thirty zeros). This is much longer than the timesince the big bang, which is a mere ten thousand million yearsor so (1 followed by ten zeros). Thus one might think that thepossibility of spontaneous proton decay could not be testedexperimentally. However, one can increase one’s chances ofdetecting a decay by observing a large amount of mattercontaining a very large number of protons. (If, for example,one observed a number of protons equal to 1 followed bythirty-one zeros for a period of one year, one would expect,according to the simplest GUT, to observe more than oneproton decay.)A number of such experiments have been carried out, butnone have yielded definite evidence of proton or neutron decay.
One experiment used eight thousand tons of water and wasperformed in the Morton Salt Mine in Ohio (to avoid otherevents taking place, caused by cosmic rays, that might beconfused with proton decay). Since no spontaneous protondecay had been observed during the experiment, one cancalculate that the probable life of the proton must be greaterthan ten million million million million million years (1 withthirty-one zeros). This is longer than the lifetime predicted bythe simplest grand unified theory, but there are more elaboratetheories in which the predicted lifetimes are longer. Still moresensitive experiments involving even larger quantities of matterwill be needed to test them.
Even though it is very difficult to observe spontaneousproton decay, it may be that our very existence is aconsequence of the reverse process, the production of protons,or more simply, of quarks, from an initial situation in whichthere were no more quarks than antiquarks, which is the mostnatural way to imagine the universe starting out. Matter on theearth is made up mainly of protons and neutrons, which inturn are made up of quarks. There are no antiprotons orantineutrons, made up from antiquarks, except for a few thatphysicists produce in large particle accelerators. We haveevidence from cosmic rays that the same is true for all thematter in our galaxy: there are no antiprotons or antineutronsapart from a small number that are produced as particle/antiparticle pairs in high-energy collisions. If there were largeregions of antimatter in our galaxy, we would expect to observelarge quantities of radiation from the borders between theregions of matter and antimatter, where many particles wouldbe colliding with their anti-particles, annihilating each other andgiving off high-energy radiation.
We have no direct evidence as to whether the matter inother galaxies is made up of protons and neutrons orantiprotons and anti-neutrons, but it must be one or the other:
there cannot be a mixture in a single galaxy because in thatcase we would again observe a lot of radiation fromannihilations. We therefore believe that all galaxies arecomposed of quarks rather than antiquarks; it seemsimplausible that some galaxies should be matter and someantimatter.
Why should there be so many more quarks thanantiquarks? Why are there not equal numbers of each? It iscertainly fortunate for us that the numbers are unequalbecause, if they had been the same, nearly all the quarks andantiquarks would have annihilated each other in the earlyuniverse and left a universe filled with radiation but hardly anymatter. There would then have been no galaxies, stars, orplanets on which human life could have developed. Luckily,grand unified theories may provide an explanation of why theuniverse should now contain more quarks than antiquarks,even if it started out with equal numbers of each. As we haveseen, GUTs allow quarks to change into antielectrons at highenergy. They also allow the reverse processes, antiquarksturning into electrons, and electrons and antielectrons turninginto antiquarks and quarks. There was a time in the very earlyuniverse when it was so hot that the particle energies wouldhave been high enough for these transformations to take place.
But why should that lead to more quarks than antiquarks? Thereason is that the laws of physics are not quite the same forparticles and antiparticles.
Up to 1956 it was believed that the laws of physics obeyedeach of three separate symmetries called C, P, and T. Thesymmetry C means that the laws are the same for particlesand antiparticles. The symmetry P means that the laws are thesame for any situation and its mirror image (the mirror imageof a particle spinning in a right-handed direction is onespinning in a left-handed direction). The symmetry T meansthat if you reverse the direction of motion of all particles andantiparticles, the system should go back to what it was atearlier times; in other words, the laws are the same in theforward and backward directions of time. In 1956 twoAmerican physicists, Tsung-Dao Lee and Chen Ning Yang,suggested that the weak force does not in fact obey thesymmetry P. In other words, the weak force would make theuniverse develop in a different way from the way in which themirror image of the universe would develop. The same year, acolleague, Chien-Shiung Wu, proved their prediction correct. Shedid this by lining up the nuclei of radioactive atoms in amagnetic field, so that they were all spinning in the samedirection, and showed that the electrons were given off more inone direction than another. The following year, Lee and Yangreceived the Nobel Prize for their idea. It was also found thatthe weak force did not obey the symmetry C. That is, it wouldcause a universe composed of antiparticles to behave differentlyfrom our universe. Nevertheless, it seemed that the weak forcedid obey the combined symmetry CP. That is, the universewould develop in the same way as its mirror image if, inaddition, every particle was swapped with its antiparticle!
However, in 1964 two more Americans, J. W. Cronin and ValFitch, discovered that even the CP symmetry was not obeyedin the decay of certain particles called K-mesons. Cronin andFitch eventually received the Nobel Prize for their work in1980. (A lot of prizes have been awarded for showing that theuniverse is not as simple as we might have thought!)There is a mathematical theorem that says that any theorythat obeys quantum mechanics and relativity must always obeythe combined symmetry CPT. In other words, the universewould have to behave the same if one replaced particles byantiparticles, took the mirror image, and also reversed thedirection of time. But Cronin and Fitch showed that if onereplaces particles by antiparticles and takes the mirror image,but does not reverse the direction of time, then the universedoes not behave the same. The laws of physics, therefore, mustchange if one reverses the direction of time - they do not obeythe symmetry T.
Certainly the early universe does not obey the symmetry T:
as time runs forward the universe expands - if it ranbackward, the universe would be contracting. And since thereare forces that do not obey the symmetry T, it follows that asthe universe expands, these forces could cause moreantielectrons to turn into quarks than electrons into antiquarks.
Then, as the universe expanded and cooled, the antiquarkswould annihilate with the quarks, but since there would bemore quarks than antiquarks, a small excess of quarks wouldremain. It is these that make up the matter we see today andout of which we ourselves are made. Thus our very existencecould be regarded as a confirmation of grand unified theories,though a qualitative one only; the uncertainties are such thatone cannot predict the numbers of quarks that will be left afterthe annihilation, or even whether it would be quarks orantiquarks that would remain. (Had it been an excess ofantiquarks, however, we would simply have named antiquarksquarks, and quarks antiquarks.)Grand unified theories do not include the force of gravity.
This does not matter too much, because gravity is such a weakforce that its effects can usually be neglected when we aredealing with elementary particles or atoms. However, the factthat it is both long range and always attractive means that itseffects all add up. So for a sufficiently large number of matterparticles, gravitational forces can dominate over all other forces.
This is why it is gravity that determines the evolution of theuniverse. Even for objects the size of stars, the attractive forceof gravity can win over all the other forces and cause the starto collapse. My work in the 1970s focused on the black holesthat can result from such stellar collapse and the intensegravitational fields around them. It was this that led to the firsthints of how the theories of quantum mechanics and generalrelativity might affect each other - a glimpse of the shape of aquantum theory of gravity yet to come.
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