Broadly speaking, group theory is the study of symmetry. When we are dealing with an object that appears symmetric, group theory can help with the analysis. We apply the label symmetric to anything that stays invariant under some transformations. This could apply to geometric figures (a circle is highly symmetric, being invariant under any rotation), but also to more abstract objects like functions: x^{2} + y^{2} + z^{2} is invariant under any rearrangement of x, y, and z and the trigonometric functions sin(t) and cos(t) are invariant when we replace t with t+2π.
Conservation laws of physics are very closely related to the symmetry of physical laws under various transformations. For instance, we expect the laws of physics to be unchanging in time. This is an invariance under "translation" in time, and it leads to the conservation of energy. Physical laws also should not depend on where you are in the universe. Such invariance of physical laws under "translation" in space leads to conservation of momentum. Invariance of physical laws under (suitable) rotations leads to conservation of angular momentum. A general theorem of Emmy Noether says, roughly speaking, that symmetries and conversation laws of a physical system correspond to each other. This dominates how theoretical physicists think about conservation laws when developing new physical theories.
Modern particle physics would not exist without group theory; in fact, group theory predicted the existence of some elementary particles before they were found experimentally. The physicist Richard Feynman discusses this story during 1:20--15:16 in the video here.
The structure and behavior of molecules and crystals depends on their different symmetries. Thus, group theory is an essential tool in some areas of chemistry.
Within mathematics itself, group theory is very closely linked to symmetry in geometry. In the Euclidean plane R^{2}, the most symmetric kind of polygon is a regular polygon. For any n > 2, there is a regular polygon with n sides: the equilateral triangle for n = 3, the square for n = 4, the regular pentagon for n = 5, and so on.
A Regular Pentagon |
The Five Platonic Solids |
Consider another geometric topic: regular tilings of the plane.
This means a tiling of the plane by copies of congruent
regular polygons, with no
overlaps except along the boundaries of the polygons.
For instance, a standard sheet of graph paper illustrates
a regular tiling of R^{2} by
squares (with 4 meeting at each vertex).
Tiling the Plane with Congruent Squares |
Tiling the Plane with Congruent Equilateral Triangles |
Tiling the Plane with Congruent Regular Hexagons |
Some Lines in the Hyperbolic Plane H^{2} |
Tiling the Hyperbolic Plane with Congruent Regular Pentagons |
Tiling the Hyperbolic Plane with Congruent Equilateral Triangles, 7 at a Vertex |
Tiling the Hyperbolic Plane with Congruent Equilateral Triangles, 8 at a Vertex |
Group theory shows up in many other areas of geometry. For instance, in addition to attaching numerical invariants to a space (such as its dimension, which is just a number) there is the possibility of introducing algebraic invariants of a space. That is, one can attach to a space certain algebraic systems. Examples include different kinds of groups, such as the fundamental group of a space. The Euclidean plane with one point removed has a commutative fundamental group, while the Euclidean plane with two points removed has a noncommutative fundamental group. In higher dimensions, where we can't directly visualize spaces that are of interest, mathematicians often rely on algebraic invariants like the fundamental group to help us verify that two spaces are not essentially the same.
Classical problems in algebra have been resolved with group theory. In the Renaissance, mathematicians found analogues of the quadratic formula for roots of general polynomials of degree 3 and 4. Like the quadratic formula, the cubic and quartic formulas express the roots of all polynomials of degree 3 and 4 in terms of the coefficients of the polynomials and root extractions (square roots, cube roots, and fourth roots). The search for an analogue of the quadratic formula for the roots of all polynomials of degree 5 or higher was unsuccessful. In the 19th century, the reason for the failure to find such general formulas was explained by a subtle algebraic symmetry in the roots of a polynomial discovered by Evariste Galois. He found a way to attach a finite group to each polynomial f(x), and there is an analogue of the quadratic formula for all the roots of f(x) exactly when the group associated to f(x) satisfies a certain technical condition. Not all groups satisfy that technical condition, and by this method Galois could give explicit examples of fifth degree polynomials, such as x^{5} - x - 1, whose roots can't be described by anything like the quadratic formula. Learning about this application of group theory to formulas for roots of polynomials would be a suitable subject for a second course in abstract algebra.
The mathematics of public-key cryptography uses a lot of group theory. Different cryptosystems use different groups, such as the group of units in modular arithmetic and the group of rational points on elliptic curves over a finite field. This use of group theory derives not from the "symmetry" perspective, but from the efficiency or difficulty of carrying out certain computations in the groups. Other public-key cryptosystems use other algebraic structures, such as lattices.
Some areas of analysis (the mathematical developments coming from calculus) involve group theory. The subject of Fourier series is concerned with expanding a fairly general 2π-periodic function as an infinite series in the special 2π-periodic functions 1, sin(x), cos(x), sin(2x), cos(2x), sin(3x), cos(3x), and so on. While Fourier series can be developed solely as a topic within analysis (and at first it was), the modern viewpoint of Fourier analysis treats it as a fusion of analysis, linear algebra, and group theory.
Identification numbers are all around us, such as the ISBN number for a book, the VIN (Vehicle Identification Number) for your car, or the bar code on a UPS package. What makes them useful is their check digit, which helps catch errors when communicating the identification number over the phone or on the internet or with a scanner. The different recipes for constructing a check digit from another string of numbers are based on group theory. Usually the group theory is trivial, being just addition or multiplication in modular arithmetic. However, a more clever use of other groups leads to a check-digit construction that catches more of the most common types of communication errors. The key idea is to use a noncommutative group.
On the lighter side, there are applications of group theory to puzzles, such as the 15-puzzle and Rubik's Cube. Group theory provides the conceptual framework for solving such puzzles. To be fair, you can learn an algorithm for solving Rubik's cube without knowing group theory (consider this 7-year old cubist), just as you can learn how to drive a car without knowing automotive mechanics. Of course, if you want to understand how a car works then you need to know what is really going on under the hood. Group theory (symmetric groups, conjugations, commutators, and semi-direct products) is what you find under the hood of Rubik's cube.