|Flower Rosette for Bowed Psaltery|
|Celtic Knot Rosette for Bowed Psaltery|
|Triune Celtic Knot Rosette for Bowed Psaltery|
Fields are essentially forces, varying in strength and direction through space. The notion of a field was introduced back in the early part of the 19th century by Michael Faraday studying magnetic forces. (See picture of the magnetic field–array of iron filings over a bar magnet).
|Magnetic Field: Iron Filings over Bar Magnet|
The electromagnetic field consists of electric and magnetic fields alternating in strength and direction like waves, propagating through space. Light is in fact an electromagnetic field. Now light can also (as Einstein first showed) behave as particles, photons. This is an aspect of the “wave-particle” duality that lies at the heart of quantum mechanics. Since charged particles (electrons, protons) interact via the electromagnetic field, they can also be thought to interact via the exchange of “virtual photons”, as illustrated below.
|electon(e-) / electron(e-)|
virtual photon (gamma)
Symmetry has to do with sameness when things are rearranged or moved. A good illustration of symmetry is shown by the inset figures of bowed psaltery rosettes: if appropriate rotations (about axes perpendicular to the plane or in the plane) or mirror reflections are done, the rosettes will look the same. Now there is another kind of symmetry important for particle physics, permutation symmetry. Let’s consider identical triplets with labels 1,2,3 (see figure). (The triplets aren’t quite identical, but let that pass.)
|Triplets (almost identical)|
SYMMETRY AND PARTICLE PERIODIC TABLES.
I hope at least some of you remember from high school Chemistry how Mendelyev’s Periodic Table helped make sense of the order of elements and their chemical behavior. In a similar way one can arrange fundamental particles into symmetric tables, arrangements which turn out to be explained by group theory, in the same way that the chemical periodic table was explained by quantum mechanics. Below is an example of such a table:
|Quart Octer from Cronodon.com|
What does all this have to do with group theory? Group theory tells us that for a given symmetry, the dimensions of the various possible representations are fixed. So, for example, for SU(3) symmetry there will be a representation of dimension 8 (as in the diagram above) which will have two doublet states (the n and p and the Chi – and Chi 0 rows) a triplet state (the Sigma -, 0, and + row) and a singlet state (the lambda 0 row). There would not be representations of dimensions 7 or 9, for SU(3) symmetry, so that if a septet or nonet state were observed, that would belie an SU(3) symmetry. For SU(3) symmetry there is also a representation of dimension 10, which is realized below in the decuplet of very short-lived baryons (observed as “resonances” in particle physics) with total spin 3/2.
|Baryon Decuplet (10-fold)|
GAUGE INVARIANCE SYMMETRY.
A critical and foundational element in contemporary physical theory is gauge invariance. Gauge invariance is a type of local symmetry, that is to say, symmetry is given at various points, but changes as you move. The symmetry is not global, i.e. is not the same throughout the space (refer to the image below, taken from the Wikipedia article linked above):
Local twisting of up and down coordinates (Cartesian coordinates) to illustrate gauge invariance (taken from linked Wikipedia article). Note that gauge invariant theories (e.g. general relativity, quantum field theory) would be valid at each point, despite the twisting and stretching of the coordinate system.
An important notion in theoretical physics is that of symmetry breaking (follow the link in the heading above for a detailed explanation). Here is a relatively simple example: consider a drop of water (say, as a particle in mist), roughly spherical in shape and therefore highly symmetric; let the temperature fall to the freezing point and the drop will spontaneously turn into a snowflake (or an ice crystal) of lower symmetry. Another example is that of an iron bar that at sufficiently low temperature becomes magnetized (all small magnetic domains in the bar line up in the same direction), giving a direction (that of the magnet N to S poles) and thereby lowering the symmetry (which prior to the magnetization did not have a specified direction). Symmetry breaking plays a fundamental role in the Higgs field; it is the mechanism by which the Higgs field endows fundamental particles with mass. Symmetry breaking is depicted in the “Mexican Hat” potential shown below (a similar diagram was shown in Higgs’ original article proposing this potential and mechanism for symmetry breaking).
|Mexican Hat Potential|
The cornerstone of the Standard Model is Yang-Mills theory. The theory was proposed in 1954 by Chen Yang and Robert Mills as a generalization of gauge-invariant quantum electrodynamics, which had been remarkably successful. The goal was to give a theory for the strong nuclear interaction. However, the theory was not successful: it predicted the existence of massless carrier bosons (in addition to a photon), which were not observed.
THE HIGGS FIELD/BOSON
The Higgs field was proposed by Peter Higgs in 1965 as a way to bring spontaneous symmetry breaking into Yang-Mills theory. The Higgs field generates mass for particles in the following way, as nicely explained by Van Velzen (see link above). Consider the fundamental property of mass, inertia: according to Newton’s Laws of Motion: a particle moving with constant velocity in a straight line will continue to move in a straight line unless acted on by a force; if acted on by a force its velocity will change (acceleration will occur according to the famous equation F=ma). Mass enters by requiring a force to change velocity (accelerate the particle), the force being proportional to mass for given acceleration. If a particle does not have mass, it’s velocity will be constant, as is that of the massless photon, which has the speed c, the speed of light. One can think of the Higgs field acting as a repulsive lattice; that is, one can imagine a regular array of points, which are sources of equal repulsive forces. Lattice points behind a particle will push the mass particle forward; lattice points in front of the mass particle will push it backward with equal force; the effect will then, as the diagram given by Van Velzen shows so well, be for the particle to move with constant velocity. (Note: the explanation given by many popular accounts of the Higgs field, as if a particle were moving in a viscous fluid, is not correct; if that were so then the particle would decelerate and stop moving).
ELECTROMAGNETISM+WEAK INTERACTION=”ELECTROWEAK” THEORY. Sheldon Glashow extended the Yang-Mills theory to cover both electromagnetism and the weak interaction (beta-decay) in 1960. However, his theory was deficient in that it was not gauge-invariant; the masses of the carrier bosons were specified explicitly. In 1967 Steven Weinberg and Abdus Salam independently proposed a truly gauge-invariant version of Glashow’s theory; they achieved this by adding the Higgs field to the theory, thereby yielding spontaneous symmetry breaking with massive carrier bosons. The theory was confirmed experimentally in 1973 by finding “neutral currents” in neutrino scattering and in 1983 by the detection of the carrier bosons, W and Z.
QUANTUM CHROMODYNAMICS = “QCD”
The next extension of Yang-Mills theory in the Standard Model was quantum chromodynamics, QCD, the theory to explain the strong interaction between quarks, the particle constituents of protons, neutrons and other heavy particles (baryons). Glashow, Salam and Weinberg were jointly awarded the Nobel prize in 1979 for this theory.Experimental evidence strongly suggested the existence of three types of quarks, i.e. three types for each of the six different quark “flavors”–up, down, strange, top, bottom, charm; the three types differ in a degree of freedom that is called color, but were otherwise identical in mass, charge, etc, (the quark color has no relation to colors observed in the ordinary world). Thus there are “red”, “blue”, “green” up quarks, “red”, “blue”, “green” down quarks, etc., as shown in the illustration (from the Wikipedia article on Quarks linked above).
|Colors of Up (u) and Down (d) quarks in a proton|
GRAND UNIFICATION THEORY = “GUT”
An obvious next step in the standard model is to give a theory in which electromagnetism, the weak interaction, the strong force and gravity are all derived from a common gauge theory, analogous to the derivation of electromagnetism and the weak interaction from the electroweak theory. There were a number of attempts following the success of QCD to find a theory that would unify all various types of forces and interactions. The first of these was given by Sheldon Glashow and Howard Georgi in 1974, who proposed a gauge invariant theory combining symmetries for electromagnetism, the weak, and the strong interactions. Unfortunately the theory failed a crucial experimental test: it predicted a much shorter decay rate for protons (by a factor of 100 to 10,000) than the lower limit determined experimentally. Other proposed theories also fell short, and the attention of theoretical physicists turned to other “theories of everything” (TOE’s)–supersymmetry, string theory, M-theory–which I won’t discuss.
|Arch with Keystron--The Arch symbolizes the Standard Model|
and the Keystone Symbolizes the Higgs Field/Boson