The Basic Forces and Messenger Particles
We turn now from cataloging the particles to considering the forces between them.
The Electromagnetlc Force
At the atomic level, we say that two electrons exert electromagnetic forces on each other according to Coulomb’s law. At a deeper level, this interaction is described by a highly successful theory called quantum electrodynamics (QED). From this point of view we say that each electron senses the presence of the other by exchange ging photons with it. We cannot detect these photons because they are emitted by one electron and absorb~· by the other a very short time later. Because of their undetectable existence, we cafes them virtual ,hotels. Because they communicate Between the two interacting charged particles, we sometimes call these photos messenger particles. If a stationary electron emits a photon and remains itself unchanged, energy is not conserved. The principle of conservation of energy is saved, however, ~y the uncertainty principle, written in the form.
as discussed in Sample Problem .3-10. Here we interpret this relation te mean that you can “overdraw” an amount of energy AE, violates& conservation of ener~¥., provided you “return” it within an interval At given by hlAE so that the violation cannot be detected. The virtual photons do just that. When, say, electron A emits a virtual photon, the overdraw in energy is quickly set right when that electron receives a virtual photon from electron B, and the violation of the principle of conservation of energy for the electron pair is hidden by the inherent uncertainty
The Weak Force
A theory of the weak force, which acts on all particles, was developed by analogy with the theory of the electromagnetic force. The messenger particles that transmit the weak force between particles, however, are not (massless) photons but massive particles, identified by the symbols W anti Z. The theory was so successful that it revealed the electromagnetic force an4 the weak force as being different aspects of a single electroweak r.rce. This accomplishment is a logical elaterin I)f, the work of Maxwell, who revealed the electric anti-magnetic forces being! c1ifferentaspects of a single electromagnetic force.The electroweak theory was specific in predicting the properties of the messenger particles. Their charges and masses, for example, were predicted to be
Recall that the proton mass is only 0.938 GeV/c2; these are massive particles! The 1979 Nobel prize in physics was awarded to Sheldon Airshow, Steven Weinberg, and Abdus Salam for their development of the electroweak theory.The theory was confirmed in 1983 by Carlo Rubbia and his group at CERN, who experimentally verified both messenger particles and found that their masses agreed with the predicted values. lpe 19M Nobel prize in physics went to Rubbia and Simon van der Meer for this brilliant experimental work. Some notion of he complexity of panicle physics in this day and age can befound by looking at an earlier Nobel prize-particle physics experiment-the discovery of the neutron. This vitally important discovery was a “tabletop” experiment, employing particles emitted by naturally occurring radioactive materials as projectiles; it was reported in 1932 under the title “Possible Existence of a Neutron,” the single author being James Chadwick. The discovery of the W and Z messenger particles in 1983, by contrast, was carried out at a large particle accelerator, about 7 krn in circumference and operating in the range of several hundred billion electron-volts. The principal particle detector alone weighed 20 MN. The experiment employed more than 130 physicists from 12 institutions in 8 countries, along with a large support staff.
The Strong Force
A theory of the strong force-that is, the force that acts between quarks to bind hadrons together-has also been developed. The messenger particles in this case are called gluons and, like the photo, they are predicted to be massless. The theory assumes that each “flavor” of quark comes in three varieties that, for convenience,have been labeled red, yellow, and blue. Thus, there are three up quarks, one of each color.rand so on. The antiquarks also come in three colors, which we call antired, anti yellow, and antiblue. You must not think that quarks are actually colored, like tiny jelly beans. The names are labels of convenience but (for once) they do have a certain formal justification, as you shall see. The force acting between quarks is called a color force and the underlying theory, by analogy with quantum electrodynamics (QED), is called quantum chromociynamics (Qed). Apparently, quarks can be assembled only in combinations that are color-neutral.There are two ways to bring about color neutrality. In the theory of actual colors, red + yellow + blue yields white, which is color-neutral; thus, we can assemble three quarks to form a baryon, provided one is a yellow quark, one is a red quark, and one is a blue quark. Antired + anti yellow + antiblue is also white, so that we can assemble three antiquarks (of the proper anticolors) to form an antibaryon. Finally, red + attired, or yellow + anti yellow, or blue + antiblue also yields white.Thus, we can assemble a quark-antiquark combination to form a meson. The color neutral rule does not permit any other combination of quarks, and none are observed. The color force not only acts to bind together quarks as baryons and mesons,but it also acts between such particles, in which case it has traditionally been called the strong force. Hence, not only does the color force bind together quarks to form protons and neutrons, but it also binds together the protons and neutrons to form
The unification of the fundamental forces of nature into a single force-which occupied Einstein’s attention for much of his later life-is very much a current focus of research. We have seen that the weak force has been successfully combined with electromagnetism so that they may be jointly viewed as aspects of a single electroweak force. “1beories that attempt to add the strong force to this combination- called grand unification theories (GUTs)-are being pursued actively. Tbeories that say to complete the job by adding gravity-sometimes called theories of everything (TOE)-are at an encouraJing but speculative stage at this time.