Structure of Matter and Fundamental Forces

Abstract

The previous section provided a rough description of the large structures in the observable universe. We now go inside matter. 2.1 The Fundamentals at Subatomic Distances When we look inside matter, at sizes much smaller than our natural sizes, as we go deeper and deeper into the subatomic scales, we discover entire new worlds of molecules, atoms, nuclei, quarks, and leptons. This is illustrated in Figs. 2.1 and 2.2. During its early stages right after the Big Bang, the entire universe is imagined to be extremely dense, extremely energetic, and extremely hot. According to our current understanding of cosmology, a tiny speck of the early universe will even-tually develop to become our present visible universe. So, to understand the status of our visible universe at the time of the Big Bang, you need to imagine squeezing all the mass and energy of the present universe into an unimaginably small region. During such early stages of the universe, elementary matter collides with huge ener-gies within volumes 10 20 times smaller than the sizes of nuclei or protons depicted in Figs. 2.1 and 2.2. Therefore the evolution of the universe from the Big Bang into what it is today must have been determined by the fundamental laws of physics that govern the smallest elementary particles, namely quarks, leptons, and force particles, moving in extremely small regions at huge energies. This is well beyond the levels of energies in so-called high-energy physics experiments at modern accel-erators. So, we need to look deep into the structure of matter and understand thoroughly its elementary constituents and the fundamental forces acting on them, in order to explain our origins. It has been experimentally established that all known matter in our environment is made up of quarks, leptons, and " force particles " that hold them together, as illus-trated in Figs. 2.1, 2.3, 2.4 and 2.6. The behavior of these elementary constituents is controlled by four fundamental forces. The strong and weak forces are short range and can be felt only by tiny particles moving at subnuclear distances. However, the electromagnetic and gravitational forces are long range, and act at both small and large distances, including macroscopic distances typical of our everyday life. For this reason we are more easily aware of the electromagnetic and gravitational forces. The strong force, whose influence dominates within matter of the size of a proton (10 −15 m in Fig. 2.1), holds quarks and gluons inside protons and neutrons and other similar strongly interacting particles called hadrons. The strong force is transmitted by the gluons. The weak force has an even smaller range of influence (within 10 −17 m) and it is responsible for the decay of matter, such as neutron → proton+electron + anti-neutrino. It is transmitted by the W ± and Z 0 particles. The electromagnetic force is responsible for holding the electrons around a nucleus in atoms (10 −10 m in Fig. 2.1) and also for the formation of molecules from atoms. The range of the electromagnetic force is infinite; therefore, we can experience it also in the macroscopic world, such as when it acts to pull magnets together, or the attraction/repulsion of electric charges, as well as in all phenomena that involve light. The electromagnetic force is transmitted by photons, which are the smallest bits of light. Photons make up the electromagnetic waves in the entire spectrum of frequencies, including radar, radio, TV, X-rays, and the visible spectrum that is interpreted by the human eye as the colors from red to violet light.