Key Concepts
A fundamental force representing unification of the electromagnetic and weak nuclear interactions. Until the early 1970s, the four fundamental forces — strong nuclear, electromagnetic, weak nuclear, and gravity — could only be described as distinct entities. The work of U.S. particle physicist Sheldon Lee Glashow, U.S. theoretical physicist Steven Weinberg, and Pakistani theoretical physicist Abdus Salam showed that the electromagnetic and the weak nuclear forces can be unified and understood as a single interaction, called the electroweak interaction. This unification was a major step in understanding nature, similar to the achievement of Scottish physicist James Clerk Maxwell and others a century earlier in unifying the electric forces and magnetic forces into the electromagnetic interactions. A goal of theoretical physics is to achieve a further simplification in understanding nature by describing the presently known basic interactions in a unified way. Attempts to unify the strong force with the electroweak interaction are usually referred to as grand unified theories (GUTs). Various frameworks that seek to further unify with the fourth fundamental interaction, gravity, are known as M-theory, superstring theory, or a "theory of everything." Physicists broadly theorize that initially, all forces were united in the extreme energy and temperature at the beginning of the universe in the big bang. The forces then separated out as the temperature decreased (Fig. 1). See also: Electromagnetism; Fundamental interactions; Grand unification theories; Gravitation; M-theory; Maxwell's equations; Strong nuclear interactions; Superstring theory; Weak nuclear interactions
Properties of basic interactions
Some of the properties of the basic interactions are summarized in Table 1. The strong nuclear forces are the strongest, electroweak is intermediate, and gravity the most feeble by a huge factor. The ranges, that is, the distances over which the forces act, also differ greatly. The strong nuclear and the weak interactions have a very short range, while electromagnetism and gravity act over very large distances. Thus, at very short subatomic distances the strong nuclear force, which holds the atomic nucleus together and governs many interactions of the subnuclear particles, dominates. At larger distances the electromagnetic forces dominate, and hold the atom together and govern chemical and most mechanical forces in everyday life. At even larger scales, objects such as planets, stars, and galaxies are electrically neutral (have an exact balance of positive and negative electric charges) so that the electromagnetic forces between them are negligible, and thus the gravitational forces dominate in astronomical and cosmological situations. (Note that the terms “force” and “interaction between particles” are used interchangeably in this context. All of the known forces, such as atomic, nuclear, chemical, or mechanical forces, are manifestations of one of the four basic interactions.) See also: Atom; Atomic nucleus; Atomic physics; Galaxy; Planet; Star
Interaction | Relative strength | Property acted on | Force carrier | Range | |
---|---|---|---|---|---|
Strong nuclear | 1 | Color charge (r, g, b) | Gluon (g) | 10^{−13} cm | |
Electroweak | |||||
Electromagnetic | 10^{−2} | Electric charge (q) | Photon (γ) | ∞ | |
Weak nuclear | 10^{−6} | Weak charges (t_{3}, y) | Bosons (W^{±}, Z^{0}) | 10^{−16} cm | |
Gravity | 10^{−40} | Mass (m) | Graviton (G) | ∞ |
Each of the basic forces acts on, or depends on, different properties of matter. Gravity acts on mass, and electromagnetic forces act on electric charges that come in two kinds, positive and negative. The strong nuclear forces act on a much less well-known property, called color charge, which comes in three kinds, r, b, and g (often called red, blue, and green). The weak nuclear forces act on equally esoteric properties called weak isospin t and hypercharge y. While the mass and the electric charge are properties that are recognized in everyday situations, the color charge and the weak isospin and hypercharge have no correspondence in the large-scale everyday world. See also: Color (quantum mechanics); Electric charge; Hypercharge; I-spin; Mass
Fundamental constituents of matter
All known forms of matter are made of molecules and atoms, which are made up of the nucleus (protons and neutrons) and orbital electrons. These in turn can be understood to be made up of the fundamental constituents, the quarks and the leptons. Each of these comes in six kinds (Table 2).
Weak charges | ||||||
---|---|---|---|---|---|---|
Particle | Symbol | Mass | Electric charge (q) | Color charge | Isospin (t_{3}) | Hypercharge (y) |
Electron neutrino | ν_{e} | < 22 eV | 0 | 0 | +1/2 | −1 |
Electron | e^{−} | 0.51 MeV | −1 | 0 | −1/2 | −1 |
Muon neutrino | ν_{μ} | < 0.17 MeV | 0 | 0 | +1/2 | −1 |
Muon | μ^{−} | 105 MeV | −1 | 0 | −1/2 | −1 |
Tau neutrino | ν_{τ} | < 16 MeV | 0 | 0 | +1/2 | −1 |
Tau | τ^{−} | 1777 MeV | −1 | 0 | −1/2 | −1 |
Down quark | d | 4.7 MeV | −1/3 | r, b, g | −1/2 | 1/3 |
Up quark | u | 2.2 MeV | +2/3 | r, b, g | +1/2 | 1/3 |
Strange quark | s | 96 MeV | −1/3 | r, b, g | −1/2 | 1/3 |
Charm quark | c | 1.3 GeV | +2/3 | r, b, g | +1/2 | 1/3 |
Bottom quark | b | 4.2 GeV | −1/3 | r, b, g | −1/2 | 1/3 |
Top quark | t | 173 GeV | +2/3 | r, b, g | +1/2 | 1/3 |
The six leptons are usually organized into three families:
Similarly, the six quarks can be organized into three families:
The masses of the leptons and quarks are frequently specified in units of electronvolts (1 MeV = 10^{6} eV, and 1 GeV = 10^{9} eV). In more usual units, the electron mass of 0.5 MeV is equal to 9 × 10^{−31} kg. For many years the masses of the three neutrinos (ν_{e}, ν_{μ}, and ν_{τ}) were believed to be zero. However, an effect called neutrino oscillations indicates that they have small but nonzero masses. The values of these masses are not yet known; only experimentally measured upper limits on them are available. See also: Neutrino
All of the quarks and leptons have gravitational and weak interactions since they have nonzero values of mass and weak isospin and hypercharge. The particles with zero electric charge have no electromagnetic interactions, and the leptons have no strong nuclear interactions since they carry no color charge. See also: Lepton; Quark
Exchange forces and gauge bosons
The present understanding is that the basic forces are not contact forces but act over distances larger than the sizes of the particles (action at a distance). In this picture, based on field theory, the forces are carried or mediated by intermediate particles that are called gauge bosons. For example, the electromagnetic force between an electron and a proton is carried by the quantum of the electromagnetic field called the photon (γ). The strong nuclear force is carried by the gluon (g), and the gravitational force is carried by the graviton (G). The weak nuclear force comes in two categories: the charge-changing (charged-current, for short) mediated by the W^{±} bosons, and the neutral-current weak interactions mediated by the Z^{0} boson (Fig. 2). See also: Gluons; Graviton; Intermediate vector boson; Photon
Helicity and parity violation
All of the fundamental constituents, the quarks and the leptons, carry one-half unit of angular momentum (spin = 1/2) as if they were spinning around their own axis. (Such particles are called fermions.) By the rules of quantum mechanics, the direction of this spin is quantized to be either parallel or antiparallel to the direction of motion of the particle. Particles with spin direction parallel to their direction of motion have helicity +1 and are called right-handed, and particles with antiparallel spin have helicity −1 and are called left-handed. See also: Angular momentum; Helicity (quantum mechanics); Spin (quantum mechanics)
One of the symmetries of nature is called parity, which is a symmetry between right-handed and left-handed coordinate systems. If parity symmetry holds, left-handed and right-handed particles must have the same interactions. In 1956, Chinese-born U.S. physicists Lee Tsung-Dao and Yang Chen Ning proposed that parity symmetry is violated in the weak interactions, and this proposal was soon verified experimentally. It was found that the left-handed and the right-handed particles have different weak interactions. The weak isospin and hypercharge assignments given in Table 2 are for left-handed quarks and leptons. The right-handed particles have somewhat different assignments. In particular, the right-handed particles have no weak isospin, and thus only the left-handed particles participate in the charged-current weak interactions. See also: CP symmetry and its violation; Parity (quantum mechanics)
Electroweak unification
Until the early 1970s, the electromagnetic and the weak interactions were believed to be intrinsically separate basic interactions. At that time, the Weinberg-Salam-Glashow model was proposed to understand these two interactions in a unified way. The model was based on an SU(2) × U(1) gauge symmetry in which the SU(2) part corresponds to a weak isospin triplet of gauge bosons, the W^{+}, W^{0}, and W^{−}, and the U(1) corresponds to a singlet, the B^{0}. The W's couple to the property called weak isospin, and the B^{0} couples to weak hypercharge. See also: Gauge theory
In its original form, this model, based on an unbroken gauge symmetry, led to some physically unacceptable features such as zero masses for all the constituent particles and predictions of infinities for some measurable quantities. Through the pioneering work of Dutch theoretical physicists Gerard 't Hooft, Martinus Veltman, and others, it was shown that the theory can be made renormalizable, removing the infinities and providing masses to the particles, by spontaneous breaking of the gauge symmetry and the introduction of one new particle, the Higgs boson. See also: Higgs boson; Renormalization; Symmetry breaking
The neutral gauge bosons, the W^{0} and B^{0}, form a quantum-mechanical mixture, which produces the two physically observable gauge bosons, the γ and the Z^{0}, as given by Eqs. (1).
The γ is the well-known photon that mediates the electromagnetic interactions. The Z^{0} mediates the neutral-current weak interactions, and the W^{±} mediate the charged-current weak interactions. In this way, all of these interactions are described by a common unified theory. The mixing angle θ in Eqs. (1), forming the γ and the Z^{0}, is called the weak mixing angle and is the fundamental parameter of the theory. The strength and nature of the interactions of the particles are determined by the vector and axial vector coupling constants g_{v} and g_{A}. In the electroweak model all of these couplings are given in terms of the single parameter of the theory, the weak mixing angle, and the properties of the leptons and quarks given in Table 2. The model also gives a relationship between the electric charge q and the weak charges t_{3} and y, Eq. (2). See also: Nonrelativistic quantum theory; Quantum mechanics
The coupling constants that govern the electroweak interactions of all of the particles can be summarized as:
- Electromagnetic interactions: g_{v} = q, g_{A} = 0
- Charged current weak interactions: g_{v} = −g_{A} = t
- Neutral current weak interactions: g_{v} = t_{3} −2q sin^{2} θ, g_{A} = −t_{3}
In the above expressions, t stands for the magnitude of the weak isospin, and t_{3} is its projection along an axis of quantization.
The electroweak theory has great predictive power. Its first and most striking prediction was the existence of neutral-current weak interactions mediated by the Z^{0} boson. Until the time of this prediction, the weak interactions were believed to be of charged-current nature only, with no neutral-current component. Some examples of neutral-current processes predicted by the theory were reactions (3). See also: Neutral currents
The experimental discovery of such neutral-current weak interaction processes was considered very strong support for the validity of the theory. From the experimental measurements of the cross sections (that is, interaction probabilities) of such processes, an early estimate of the weak mixing angle was derived to be sin^{2} θ ≈ 0.23. With this value of sin^{2} θ, the theory was able to predict the masses of the hypothetical W^{±} and Z^{0} gauge bosons to be approximately M_{W} ≈ 81 GeV and M_{Z} ≈ 92 GeV.
A second major triumph for the electroweak theory was the discovery of the W and Z bosons in 1983 at the proton-antiproton collider at the CERN Laboratory in Geneva, Switzerland, with masses very close to the values predicted by the theory. At this time, the validity of the theory was considered to be firmly established. A great deal of experimentation followed the discovery of the W and Z bosons that carried out detailed precision tests of the electroweak theory, with all of the experimental results in excellent agreement. See also: Particle accelerator
Prospects
The successful electroweak theory, combined with quantum chromodynamics (QCD), the theory describing the strong nuclear interactions, forms the so-called standard model of particle physics. This standard model has been brilliantly successful in accurately predicting and describing all experimental results over a huge energy range, varying from the electronvolt energies of atomic physics to the 100-GeV energy range of the largest existing particle colliders. As such, it represents a landmark achievement of both experimental and theoretical physics. See also: Quantum chromodynamics; Standard model
One of the major problems within the standard model was its inability to explain the masses of the fundamental constituents, the quarks and leptons. These masses vary over a large range, from a few electronvolts to 174 GeV. The basic gauge symmetry on which the standard model is based would indicate that these masses should all be the same. An additional piece of the puzzle, usually referred to as the source of the electroweak symmetry breaking, was necessary. Physicists discovered this missing element, the Higgs boson, in 2012 at the Large Hadron Collider (LHC) at CERN, a particle accelerator primarily built for finding the long-sought Higgs. See also: Electronvolt; Higgs boson; Large Hadron Collider (LHC); Supersymmetry; Symmetry breaking
However, other outstanding problems with the standard model exist. The model provides no explanation for dark matter, for instance. Nor can the model yet describe a unification of the strong nuclear and the electroweak interactions, let alone gravity. The standard model is based on the principles of quantum mechanics, while the current understanding of the gravitational forces is based on Einstein's theory of general relativity. No one so far has been able to combine these two theories; that is, a complete quantum theory of gravity does not, as yet, exist. The search for such a grand unified theory is a major focus of activity in theoretical physics. See also: Dark matter; Elementary particle; Quantum gravitation; Relativity