The known fundamental forces of nature are:
– The electromagnetic force
– The strong nuclear force
– The weak nuclear force
– The force of gravity
Gauge Field Theory
In quantum gauge field theory, a field corresponding to each of the fundamental forces is considered to exist at every point in space, throughout the Universe, even in the absence of light or a charged particle. Electromagnetic radiation, for example, is described as a ripple of energy travelling through the electromagnetic gauge field. A two-dimensional analogy would be the surface of a pond, where energy can be introduced by throwing in a pebble, causing a ripple to spread across the surface. Please note, however, that this does not make a field equivalent to the 19th century concept of the aether – a material substance that was proposed to permeate all of space – as an aether would be at rest in only one inertial reference frame (see Relativity Theory). It might be that a field is locally created by the presence of particles rather than existing across all space in the absence of matter; however, the concepts are mathematically equivalent, and we current have now way to distinguish between the two possibilities.
A gauge boson is a quantum of energy that can be considered to be a ripple in a gauge field. Through the concept of wave-particle duality, the gauge bosons can also be identified with the fundamental spin 1 particles from the standard model of particle physics. Each of the fundamental forces (with the possible exception of gravity) can be considered to be mediated by exchanges of virtual gauge bosons between particles. Different gauge bosons mediate the different fundamental forces as follows:
– Photons – carrying the electromagnetic interaction
– W and Z bosons – carrying the weak interaction
– Gluons – carrying the strong interaction
It has also been proposed that gauge field theory might be extended to include the graviton – a hypothetical particle that mediates the force of gravity. However the theories of general relativity and quantum mechanics have yet to be successfully combined to form a workable theory of quantum gravitation.
Quantum Electrodynamics (QED) is the theory describing the electromagnetic interaction through the disturbances in the electromagnetic gauge field.
Quantum Chromodynamics (QCD) describes the strong nuclear force between quarks and gluons, using the concept of colour charge.
The Electroweak interaction is a unification of the electromagnetism and weak nuclear interactions.
The Higgs Field and the Higgs Boson
The foundations of gauge theory suggests that all gauge bosons should have zero rest mass. However, the W and Z bosons, that mediate the weak nuclear interaction, all possess relatively large rest masses of around 80 GeV/c2. In 1964, three different papers were published suggesting a solution to this problem, using related but slightly different approaches. They proposed the existence of a field that has a non-zero constant value throughout the whole of space, and that the W and Z particles could gain mass by interacting with this field. This became known as the Higgs field, after the author of one of the three papers, Peter Higgs.
Higgs also proposed in his 1964 paper that a quantum excitation of this field should be detectable as a scalar boson (scalar meaning that the particle would have a spin of 0), which later became known as the Higgs boson. In 2012, the Large Hadron Collider discovered a new particle with all the properties predicted for the Higgs boson. This discovery is now considered to be experimental confirmation of Higgs’ theory. However, it should be noted that the Higgs particle is not directly responsible for the Higgs mechanism that couples particles to the Higgs field, although it’s discovery is important because it provides evidence for the field’s existence.
In addition to providing an explanation for the masses of the W and Z bosons, Steven Weinberg in 1967 showed that interaction with the Higgs field, through a mechanism known as Yukawa coupling, could also provide an explanation for the masses of the fundamental fermions – the quarks and charged leptons of the standard model of particle physics.
However, not all mass is derived via the Higgs mechanism. Einstein’s theory of relativity shows that all energy possesses mass through the relation E = mc2 (or more fully E2 = p2c2 + m2c4 including a term for an object’s momentum p). The Higgs mass from the three quarks that make up a proton, for example, contribute only around 1 per cent of the proton’s rest mass. The remainder is due to the quantum chromodynamic binding energy, which includes the kinetic energy (from momentum) of the quarks as well as the energy of the gluons that bind the quarks together.
Popular explanations of how the Higgs mechanism imparts mass to particles often invoke analogies such as a pearl moving through syrup or a celebrity trying to make their way through a crowd of fans. However, the Higgs mechanism also endows fundamental particles with gravitational mass, rather than just inertial mass akin to a resistance from moving through a viscous fluid. If this were not the case, then the principle of equivalence, which states that inertial mass and gravitational mass are identical would not apply to mass generated via the Higgs mechanism. However, the principle of equivalence has been tested to a high degree of accuracy, and any difference in the gravitation and inertial mass of the quarks and charged leptons would already have become evident. This suggests that even the mass derived from the Higgs mechanism ultimately has the same origin in Einstein’s mass energy relationship. The energy derived from coupling with the Higgs field is essentially what gives the Z and W bosons as well the fundamental fermions their masses, similarly to the how the chromodynamic binding energy accounts for the majority of the proton’s mass.