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Present quests to show that the four basic forces are different manifestations of a single unified force follow a long tradition. In the 19th century, the distinct electric and magnetic forces were shown to be intimately connected and are now collectively called the electromagnetic force. More recently, the weak nuclear force has been shown to be connected to the electromagnetic force in a manner suggesting that a theory may be constructed in which all four forces are unified. Certainly, there are similarities in how forces are transmitted by the exchange of carrier particles, and the carrier particles themselves (the gauge bosons in (Figure)) are also similar in important ways. The analogy to the unification of electric and magnetic forces is quite good—the four forces are distinct under normal circumstances, but there are hints of connections even on the atomic scale, and there may be conditions under which the forces are intimately related and even indistinguishable. The search for a correct theory linking the forces, called the Grand Unified Theory (GUT), is explored in this section in the realm of particle physics. Frontiers of Physics expands the story in making a connection with cosmology, on the opposite end of the distance scale.

(Figure) is a Feynman diagram showing how the weak nuclear force is transmitted by the carrier particle $Z^0$, similar to the diagrams in (Figure) and (Figure) for the electromagnetic and strong nuclear forces. In the 1960s, a gauge theory, called electroweak theory, was developed by Steven Weinberg, Sheldon Glashow, and Abdus Salam and proposed that the electromagnetic and weak forces are identical at sufficiently high energies. One of its predictions, in addition to describing both electromagnetic and weak force phenomena, was the existence of the $W^{+},W^{-}$, and $Z^0$ carrier particles. Not only were three particles having spin 1 predicted, the mass of the $W^{+}$ and $W^{-}$ was predicted to be $\text{81 GeV/}{c}^2$, and that of the $Z^0$ was predicted to be $\text{90 GeV/}{c}^2$. (Their masses had to be about 1000 times that of the pion, or about $\text{100 GeV/}{c}^2$, since the range of the weak force is about 1000 times less than the strong force carried by virtual pions.) In 1983, these carrier particles were observed at CERN with the predicted characteristics, including masses having the predicted values as seen in (Figure). This was another triumph of particle theory and experimental effort, resulting in the 1984 Nobel Prize to the experiment’s group leaders Carlo Rubbia and Simon van der Meer. Theorists Weinberg, Glashow, and Salam had already been honored with the 1979 Nobel Prize for other aspects of electroweak theory.

Although the weak nuclear force is very short ranged ( ${<10}^{–18}\text{ m}$, as indicated in (Figure)), its effects on atomic levels can be measured given the extreme precision of modern techniques. Since electrons spend some time in the nucleus, their energies are affected, and spectra can even indicate new aspects of the weak force, such as the possibility of other carrier particles. So systems many orders of magnitude larger than the range of the weak force supply evidence of electroweak unification in addition to evidence found at the particle scale.

Gluons ($g$) are the proposed carrier particles for the strong nuclear force, although they are not directly observed. Like quarks, gluons may be confined to systems having a total color of white. Less is known about gluons than the fact that they are the carriers of the weak and certainly of the electromagnetic force. QCD theory calls for eight gluons, all massless and all spin 1. Six of the gluons carry a color and an anticolor, while two do not carry color, as illustrated in (Figure)(a). There is indirect evidence of the existence of gluons in nucleons. When high-energy electrons are scattered from nucleons and evidence of quarks is seen, the momenta of the quarks are smaller than they would be if there were no gluons. That means that the gluons carrying force between quarks also carry some momentum, inferred by the already indirect quark momentum measurements. At any rate, the gluons carry color charge and can change the colors of quarks when exchanged, as seen in (Figure)(b). In the figure, a red down quark interacts with a green strange quark by sending it a gluon. That gluon carries red away from the down quark and leaves it green, because it is an $R\stackrel{-}{G}$ (red-antigreen) gluon. (Taking antigreen away leaves you green.) Its antigreenness kills the green in the strange quark, and its redness turns the quark red.

The strong force is complicated, since observable particles that feel the strong force (hadrons) contain multiple quarks. (Figure) shows the quark and gluon details of pion exchange between a proton and a neutron as illustrated earlier in (Figure) and (Figure). The quarks within the proton and neutron move along together exchanging gluons, until the proton and neutron get close together. As the $u$ quark leaves the proton, a gluon creates a pair of virtual particles, a $d$ quark and a $\stackrel{-}{d}$ antiquark. The $d$ quark stays behind and the proton turns into a neutron, while the $u$ and $\stackrel{-}{d}$ move together as a ${\pi }^{+}$ ((Figure) confirms the $u\stackrel{-}{d}$ composition for the ${\pi }^{+}$.) The $\stackrel{-}{d}$ annihilates a $d$ quark in the neutron, the $u$ joins the neutron, and the neutron becomes a proton. A pion is exchanged and a force is transmitted.

It is beyond the scope of this text to go into more detail on the types of quark and gluon interactions that underlie the observable particles, but the theory (quantum chromodynamics or QCD) is very self-consistent. So successful have QCD and the electroweak theory been that, taken together, they are called the Standard Model. Advances in knowledge are expected to modify, but not overthrow, the Standard Model of particle physics and forces.

How can forces be unified? They are definitely distinct under most circumstances, for example, being carried by different particles and having greatly different strengths. But experiments show that at extremely small distances, the strengths of the forces begin to become more similar. In fact, electroweak theory’s prediction of the $W^{+}$, $W^{-}$, and $Z^0$ carrier particles was based on the strengths of the two forces being identical at extremely small distances as seen in (Figure). As discussed in case of the creation of virtual particles for extremely short times, the small distances or short ranges correspond to the large masses of the carrier particles and the correspondingly large energies needed to create them. Thus, the energy scale on the horizontal axis of (Figure) corresponds to smaller and smaller distances, with 100 GeV corresponding to approximately, ${10}^{-18}\text{m}$ for example. At that distance, the strengths of the EM and weak forces are the same. To test physics at that distance, energies of about 100 GeV must be put into the system, and that is sufficient to create and release the $W^{+}$, $W^{-}$, and $Z^0$ carrier particles. At those and higher energies, the masses of the carrier particles becomes less and less relevant, and the $Z^0$ in particular resembles the massless, chargeless, spin 1 photon. In fact, there is enough energy when things are pushed to even smaller distances to transform the, and $Z^0$ into massless carrier particles more similar to photons and gluons. These have not been observed experimentally, but there is a prediction of an associated particle called the Higgs boson. The mass of this particle is not predicted with nearly the certainty with which the mass of the $W^{+},\phantom{}{W}^{-},$ and $Z^0$ particles were predicted, but it was hoped that the Higgs boson could be observed at the now-canceled Superconducting Super Collider (SSC). Ongoing experiments at the Large Hadron Collider at CERN have presented some evidence for a Higgs boson with a mass of 125 GeV, and there is a possibility of a direct discovery during 2012. The existence of this more massive particle would give validity to the theory that the carrier particles are identical under certain circumstances.

The small distances and high energies at which the electroweak force becomes identical with the strong nuclear force are not reachable with any conceivable human-built accelerator. At energies of about ${\text{10}}^{\text{14}}\phantom{}\text{GeV}$ (16,000 J per particle), distances of about ${\text{10}}^{-\text{30}}\phantom{}\text{m}$ can be probed. Such energies are needed to test theory directly, but these are about ${\text{10}}^{\text{10}}$ higher than the proposed giant SSC would have had, and the distances are about ${\text{10}}^{-\text{12}}$ smaller than any structure we have direct knowledge of. This would be the realm of various GUTs, of which there are many since there is no constraining evidence at these energies and distances. Past experience has shown that any time you probe so many orders of magnitude further (here, about ${\text{10}}^{\text{12}}$), you find the unexpected. Even more extreme are the energies and distances at which gravity is thought to unify with the other forces in a TOE. Most speculative and least constrained by experiment are TOEs, one of which is called Superstring theory. Superstrings are entities that are ${\text{10}}^{-\text{35}}\phantom{}\text{m}$ in scale and act like one-dimensional oscillating strings and are also proposed to underlie all particles, forces, and space itself.

At the energy of GUTs, the carrier particles of the weak force would become massless and identical to gluons. If that happens, then both lepton and baryon conservation would be violated. We do not see such violations, because we do not encounter such energies. However, there is a tiny probability that, at ordinary energies, the virtual particles that violate the conservation of baryon number may exist for extremely small amounts of time (corresponding to very small ranges). All GUTs thus predict that the proton should be unstable, but would decay with an extremely long lifetime of about ${\text{10}}^{\text{31}}\phantom{}\text{y}$. The predicted decay mode is

which violates both conservation of baryon number and electron family number. Although ${\text{10}}^{\text{31}}\phantom{}\text{y}$ is an extremely long time (about ${\text{10}}^{\text{21}}$ times the age of the universe), there are a lot of protons, and detectors have been constructed to look for the proposed decay mode as seen in (Figure). It is somewhat comforting that proton decay has not been detected, and its experimental lifetime is now greater than $5\times {\text{10}}^{\text{32}}\phantom{}\text{y}$. This does not prove GUTs wrong, but it does place greater constraints on the theories, benefiting theorists in many ways.

From looking increasingly inward at smaller details for direct evidence of electroweak theory and GUTs, we turn around and look to the universe for evidence of the unification of forces. In the 1920s, the expansion of the universe was discovered. Thinking backward in time, the universe must once have been very small, dense, and extremely hot. At a tiny fraction of a second after the fabled Big Bang, forces would have been unified and may have left their fingerprint on the existing universe. This, one of the most exciting forefronts of physics, is the subject of Frontiers of Physics.