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One of the three basic forces of nature, along with the strong nuclear interaction and the gravitational interaction. 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 three basic interactions.
Until the early 1970s, it was believed that there were four fundamental forces: strong nuclear, electromagnetic, weak nuclear, and gravity. It was by the work of S. Glashow, S. Weinberg, and A. Salam that the electromagnetic and the weak nuclear forces were 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 J. C. 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 and describe the presently known three basic interactions in a unified way, usually referred to as the grand unified theory (GUT). Whether this is possible remains to be seen. See Electromagnetism, Fundamental interactions, Gravitation, Maxwell's equations, Strong nuclear interactions, Weak nuclear interactions
Some of the properties of the basic interactions are summarized in the table. 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.
|Interaction||Relative strength||Property acted on||Force carrier||Range|
|Strong nuclear||1||Color charge (r, g, b)||Gluon (g)||10-13 cm|
|Electromagnetic||10-2||Electric charge (q)||Photon (γ)||∞|
|Weak nuclear||10-6||Weak charges (t3, y)||Bosons (W±, Z0)||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 come 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 Color (quantum mechanics), Electric charge, Hypercharge, I-spin, Mass
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. 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 Lepton, Quarks
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. See Gluons, Graviton, Intermediate vector boson, Photon
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. 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 T. D. Lee and C. N. Yang 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. 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 Angular momentum, Helicity (quantum mechanics), Parity (quantum mechanics), Spin (quantum mechanics)
Until the early 1970s, the electromagnetic and the weak interactions were believed to be separate basic interactions. At that time the Weinberg-Salam-Glashow model was proposed to understand these two interactions in a unified way. 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 G. `tHooft, M. 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 Higgs boson, Renormalization, Symmetry breaking
The neutral gauge bosons, the W0 and B0, form a quantum-mechanical mixture, which produces the two physically observable gauge bosons, the γ and the Z0, as given by Eqs. (1).
The coupling constants that govern the electroweak interactions of all of the particles can be summarized as:
1. Electromagnetic interactions: gv = q, gA = 0
2. Charged current weak interactions: gv = - gA = t
3. Neutral current weak interactions: gv = t3 - 2q sin2 Θ, gA = -t3
In the above expressions, t stands for the magnitude of the weak isospin, and t3 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 Z0 boson. Until the time of this prediction, the weak interactions were believed to be of charged-current nature only, with no neutral-current component.
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, with masses very close to the values predicted by the theory. At this time the validity of the theory was considered firmly established. See Particle accelerator
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. It represents a landmark achievement of both experimental and theoretical physics. See Quantum chromodynamics, Standard model
In spite of these great successes, two major problems remain to be solved in this field. The first one is that the standard model in its present form cannot 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. There must therefore be an additional piece of the puzzle, usually referred to as the source of the electroweak symmetry breaking, that remains to be found. Hypothetical ideas about this missing element of the model range from the prediction of a single additional particle, the Higgs boson, to complicated models such as supersymmetry that predict dozens of new elementary particles. See Higgs boson, Supersymmetry, Symmetry breaking
The second outstanding problem is the search for a theory that not only describes the strong nuclear and the electroweak interactions but includes gravity as well. 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 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 Elementary particle, Quantum gravitation, Relativity