Quantum Physics

30.15 The Cosmic Connection 999protons and antiprotons to energies of 270 GeV, and the world’s highest-energyproton acclerator, the Tevatron, at the Fermi National Laboratory in Illinois, producesprotons at almost 1 000 GeV (or 1 TeV). CERN has started construction ofthe Large Hadron Collider (LHC), a proton–proton collider that will provide acenter-of-mass energy of 14 TeV and allow an exploration of Higgs-boson physics.The accelerator is being constructed in the same 27-km circumference tunnel asCERN’s LEP collider, and construction is expected to be completed in 2005.Following the success of the electroweak theory, scientists attempted to combineit with QCD in a grand unification theory (GUT). In this model, the electroweakforce was merged with the strong color force to form a grand unifiedforce. One version of the theory considers leptons and quarks as members of thesame family that are able to change into each other by exchanging an appropriateparticle. Many GUT theories predict that protons are unstable and will decay witha lifetime of about 10 31 years, a period far greater than the age of the Universe. Asyet, proton decays have not been observed.Applying Physics 30.4Head-on CollisionsConsider a car making a head-on collision with anidentical car moving in the opposite direction at thesame speed. Compare that collision to one in whichone of the cars collides with a second car that is atrest. In which collision is there a larger transformationof kinetic energy to other forms? How does this idearelate to producing exotic particles in collisions?Explanation In the head-on collision with both carsmoving, conservation of momentum causes most, ifnot all, of the kinetic energy to be transformed toother forms. In the collision between a moving car anda stationary car, the cars are still moving after the collisionin the direction of the moving car, but with reducedspeed. Thus, only part of the kinetic energy istransformed to other forms. This suggests the advantageof using colliding beams to produce exotic particles,as opposed to firing a beam into a stationary target.When particles moving in opposite directionscollide, all of the kinetic energy is available for transformationinto other forms—in this case, the creationof new particles. When a beam is fired into a stationarytarget, only part of the energy is available for transformation,so particles of higher mass cannot be created.30.15 THE COSMIC CONNECTIONIn this section we describe one of the most fascinating theories in all of science—the Big Bang theory of the creation of the Universe—and the experimentalevidence that supports it. This theory of cosmology states that the Universe had a beginningand that this beginning was so cataclysmic that it is impossible to look backbeyond it. According to the theory, the Universe erupted from an infinitely densesingularity about 15 to 20 billion years ago. The first few minutes after the Big Bangsaw such extremes of energy that it is believed that all four interactions of physicswere unified and all matter was contained in an undifferentiated “quark soup.”The evolution of the four fundamental forces from the Big Bang to thepresent is shown in Figure 30.14. During the first 10 43 s (the ultrahot epoch, withBig BangUnifiedforceGravitationalStrong andelectroweakTwoforcesQuarks and leptonsElectroweakThreeforcesProtons andneutronscan formNuclei can formAtoms can formElectromagnetic forceFour forces10 –40 10 –30 10 –20 10 –10 10 0 10 10 10 20Age of the Universe (s)GravitationStrong forceWeak forcePresent ageof the UniverseFigure 30.14 A brief history of theUniverse from the Big Bang to thepresent. The four forces becamedistinguishable during the firstmicrosecond. Following this, all thequarks combined to form particlesthat interact via the strong force. Theleptons remained separate, however,and exist as individually observableparticles to this day.