Quantum Physics

30.16 Problems and Perspectives 1001Radiant energy density per wavelength interval (eV/m 3 /m)10 810 610 410 210 010 –2PenziasandWilson0.01 0.1 1 10 100Wavelength (cm)Figure 30.16 Theoretical blackbody (browncurve) and measured radiation spectra (bluepoints) of the Big Bang. Most of the data werecollected from the Cosmic Background Explorer(COBE) satellite. The datum of Wilson andPenzias is indicated.(b. 1945) at the Lawrence Berkeley Laboratory found that the background was notperfectly uniform, but instead contained irregularities corresponding to temperaturevariations of 0.000 3 K. It is these small variations that provided nucleationsites for the formation of the galaxies and other objects we now see in the sky.30.16 PROBLEMS AND PERSPECTIVESWhile particle physicists have been exploring the realm of the very small, cosmologistshave been exploring cosmic history back to the first microsecond of the BigBang. Observation of the events that occur when two particles collide in an acceleratoris essential in reconstructing the early moments in cosmic history. Perhapsthe key to understanding the early Universe is first to understand the world ofelementary particles. Cosmologists and particle physicists find that they have manycommon goals and are joining efforts to study the physical world at its most fundamentallevel.Our understanding of physics at short and long distances is far from complete.Particle physics is faced with many questions: why is there so little antimatter in theUniverse? Do neutrinos have a small mass, and if so, how much do they contributeto the “dark matter” holding the universe together gravitationally? How can weunderstand the latest astronomical measurements, which show that the expansionof the universe is accelerating and that there may be a kind of “antigravity force”acting between widely separated galaxies? Is it possible to unify the strong andelectroweak theories in a logical and consistent manner? Why do quarks and leptonsform three similar but distinct families? Are muons the same as electrons(apart from their different masses), or do they have subtle differences that havenot been detected? Why are some particles charged and others neutral? Why doquarks carry a fractional charge? What determines the masses of the fundamentalparticles? The questions go on and on. Because of the rapid advances and new discoveriesin the related fields of particle physics and cosmology, by the time youread this book some of these questions may have been resolved and others mayhave emerged.An important question that remains is whether leptons and quarks have a substructure.If they do, one could envision an infinite number of deeper structurelevels. However, if leptons and quarks are indeed the ultimate constituents of matter,as physicists today tend to believe, we should be able to construct a final theoryof the structure of matter, as Einstein dreamed of doing. In the view of many physicists,the end of the road is in sight, but how long it will take to reach that goal isanyone’s guess.