present at this time and this ratio will continue about thesame to the present day. There are no atoms formed yetbecause the temperature is still too high. What is presentis called a plasma.

6. From 30 min to 1 Billion YearsFurther expansion and cooling now allows the hydrogenand helium nuclei to capture electrons and the firstchemical elements are born. Large clouds of hydrogenand helium are formed.

7. From 1 Billion Years to 10 Billion YearsThe large rotating clouds of hydrogen and helium matterbegin to concentrate due to the gravitational force. As theradius of the cloud decreases, the angular velocity of thecloud increases in order to conserve angular momentum.(Similar to the spinning ice skater discussed in section9.7.) These condensing, rotating masses are the beginningof galaxies.

Within the galaxies, gravitation causes more andmore matter to be compressed into spherical objects, thebeginning of stars. More and more matter getscompressed until the increased pressure of that matter

causes a high enough temperature to initiate the fusionprocess of converting hydrogen to helium and the firststars are formed. Through the fusion process, more andmore chemical elements are formed. The higher chemicalelements are formed by neutron absorption until all thechemical elements are formed.

These first massive stars did not live very long anddied in an explosion—a supernova—spewing the matterof all these heavier elements out into space. Thefragments of these early stars would become the nuclei ofnew stars and planets.From 10 Billion Years to the PresentThe remnants of dead stars along with hydrogen andhelium gases again formed new clouds, which were againcompressed by gravity until our own star, the sun, and theplanets were formed. All the matter on earth is the leftover ashes of those early stars. Thus, even we ourselvesare made up of the ashes of these early stars. Assomebody once said, there is a little bit of star dust ineach of us.

The Language of Physics

LeptonsParticles that are not affected by the strongnuclear force (p. 1036).HadronsParticles that are affected by the strongnuclear force (p. 1036).BaryonsA group of hadrons that have half-integralspin and are composed of three quarks(p. 1036).MesonsA group of hadrons that have integral spin,that are composed of quark-antiquark pairs(p. 1036).AntiparticlesTo each elementary particle in nature therecorresponds another particle that has thecharacteristics of the original particle butopposite charge. Some neutral particleshave antiparticles that have opposite spin,whereas the photon is its own antiparticle.The antiparticle of the proton is theantiproton. The antiparticle of the electronis the antielectron or positron. If a particlecollides with its antiparticle both areannihilated with the emission of radiationor other particles. Conversely, photons canbe converted to particles and antiparticles(p. 1037).

AntimatterMatter consists of protons, neutrons, andelectrons, whereas antimatter consists ofantiprotons, antineutrons, and antielectrons(p. 1037).QuarksElementary particles that are the buildingblocks of matter. There are six quarks andsix antiquarks. The six quarks are: up,down, strange, charmed, bottom, and top.Each quark and antiquark also comes inthree colors, red, green, and blue. Eachcolor quark also has an anticolor quark.Baryons are composed of red, green, andblue quarks and mesons are made up of alinear combination of colored quark-antiquark pairs (p. 1038).Quantum electrodynamics (QED)The merger of electromagnetic theory withquantum mechanics. In QED, the electricforce is transmitted by the exchange of avirtual photon (p. 1042).Weak nuclear forceThe weak nuclear force does not exert thetraditional push or pull type of force knownin classical physics. Rather, it is responsiblefor the transmutation of the subatomicparticles. The weak force is independent ofelectric charge and acts between leptonsand hadrons and also between hadrons andhadrons. The weak force is the weakestforce after gravity (p. 1043).

Electroweak forceA unification of the electromagnetic forcewith the weak nuclear force. The force ismediated by four particles: the photon andthree intermediate vector bosons called W+,W", andZ°(p. 1043).The strong nuclear forceThe force that holds the nucleons togetherin the nucleus. The force is the result of thecolor forces between the quarks within thenucleons. At relatively large separationdistances within the nucleus, the quark-antiquark pair (meson), which is created bythe gluons, is exchanged between thenucleons. At shorter distances within thenucleus, the strong force can be explainedeither as an exchange between the quarksof one proton and the quarks of anotherproton, or perhaps as a direct exchange ofthe gluons themselves, which give rise tothe quark-quark force within the nucleon(p. 1044).Quantum chromodynamics (QCD)In QCD, the force holding quarks togetheris caused by the exchange of a new particle,called a gluon. A gluon interacting with aquark changes the color of a quark(p. 1044).

1 0 5 4 Modern Physics

Grand unified theoryA theory that merges the electroweak forcewith the strong nuclear force. This forceshould be able to transform quarks intoleptons and vice versa. The theory predictsthe existence of 12 new particles, called Xparticles that are capable of convertinghadrons into leptons by changing quarks toleptons. This theory also predicts that anisolated proton should decay. However, nosuch decays have ever been found, so thetheory may have to be modified (p. 1045).

GravitonsThe quanta of the gravitational field. Sincegravitation is a warping of spacetime, thegraviton must be a quantum of spacetime(p. 1047).SuperforceAn attempt to unify all the forces under asingle force. The theories go under thenames of supersymmetry, super gravity, andsuperstrings (p. 1050).

The Big Bang theoryThe theory of the creation of the universethat says that the universe began as a greatbundle of energy that exploded outwardabout 15 billion years ago. It was not anexplosion of matter into an already existingspace and time, rather it was the verycreation of spacetime and matter (p. 1052).

Questions for Chapter 34

tl. Discuss the statement, "A graviton isa quantum of gravity. But gravity isa result of the warping of spacetime.Therefore, the graviton should be aquantum of spacetime. But just as aquantum of the electromagnetic field,the photon, has energy, the gravitonshould also have energy. In fact, wecan estimate the energy of agraviton. Therefore, is spacetimeanother aspect of energy? Is thereonly one fundamental quantity,energy?"

t2. Does antimatter occur naturally inthe universe? How could you detectit? Where might it be located?

3. When an electron and positronannihilate, why are there two photonsformed instead of just one?

4. Murray Gell-Mann first introducedthree quarks to simplify the numberof truly elementary particles presentin nature. Now there are six quarksand six antiquarks, and each cancome in three colors and threeanticolors. Are we losing some of thesimplicity? Discuss.

5. Discuss the experimental evidencefor the existence of structure withinthe proton and the neutron.

6. How did the Pauli exclusion principlenecessitate the introduction of colorsinto the quark model?

t7. If the universe is expanding from theBig Bang, will the gravitational forceof attraction of all the masses in theuniverse eventually cause a slowingof the expansion, a complete stop tothe expansion, and finally acontraction of the entire universe?

t8. Just as there are electromagneticwaves associated with a disturbancein the electromagnetic field, shouldthere be gravitational wavesassociated with a disturbance in agravitational field? How might suchgravitational waves be detected?

t9. Einstein's picture of gravitationalattraction is a warping of spacetimeby matter. This has been pictured asthe rubber sheet analogy in chapter30. What might antimatter do tospacetime? Would it warp spacetimein the same way or might it warpspacetime to cause a gravitationalrepulsion? Would this be antigravity?Would the antiparticle of thegraviton then be an antigraviton?Instead of a black hole, would therebe a white hill?

10. Discuss the similarities anddifferences between the photon andthe neutrino.

Problems for Chapter 34

Section 34.2 Particles andAn t ipa r t i c les

1. How much energy is released whenan electron and a positron annihilate?What is the frequency andwavelength of the two photons thatare created?

2. How much energy is released when aproton and antiproton annihilate?

3. How much energy is released if 1.00kg of matter annihilates with 1.00 kgof antimatter? Find the wavelengthand frequency of the resulting twophotons.

4. A photon "disintegrates," creating anelectron-positron pair. If thefrequency of the photon is 5.00 X1024 Hz, determine the linearmomentum and the energy of eachproduct particle.

Section 34.4 Quarks5. If the three quarks shown in the

diagram combine to form a baryon,find the charge and spin of theresulting particle.

6. If the three quarks shown in thediagram combine to form a baryon,find the charge and spin of theresulting particle.

0 ©Chapter 34 Elementary Particle Physics and the Unification of the Forces 1055

7. If the three quarks shown in thediagram combine to form a baryon,find the charge and spin of theresulting particle.

O O

10. If the two quarks shown in thediagram combine to form a meson,find the charge and spin of theresulting particle.

13. Why are the two particles in thediagram impossible?

Find the charge and spin of thebaryon that consists of the threequarks shown in the diagram.

11. Find the charge and spin of themeson that consists of the two quarksshown in the diagram. 0

0If the two quarks shown in thediagram combine to form a meson,find the charge and spin of theresulting particle.

12. Which of the combinations ofparticles in the diagram are possibleand which are not. If thecombination is not possible, state thereason.

MJR J 0JR j 0jg j (uR J (dG ") ( sB

(a)

( 5ar )

(b)

(sR J (dGJ 0B j 0jr j (uARN

(c) (d)

14. A baryon is composed of threequarks. It can be made from a totalof six possible quarks, each in threepossible colors, and each with eithera spin-up or spin-down. From thisinformation, how many possiblebaryons can be made?

15. A meson is composed of a quark-antiquark pair. It can be made froma total of six possible quarks, each inthree possible colors, and each witheither a spin-up or spin-down, and sixpossible antiquarks each in threepossible colors, and each with eithera spin-up or spin-down. Neglectinglinear combinations of these quarks,how many possible mesons can bemade?

16. From problems 14 and 15 determinethe total number of possible hadrons,ignoring possible mesons made fromlinear combinations of quarks andantiquarks. Could you make a"periodic table" from this number?Discuss the attempt to attainsimplicity in nature.

17. Determine all possible quarkcombinations that could form abaryon of charge + 1 and spin V2.

(e) (f)

1056 Modern Physics