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The professor in my class of mathematical physics introduces the definition of groups and said that group theory is the mathematics of symmetry.

He gave also some examples of groups such as the set of all real numbers under addition.

My question is what symmetry are we talking about here when we consider the set of all real numbers under addition?

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It is translational symmetry. But isn't this question more about mathematics not physics? –  Maksim Zholudev Jan 20 '12 at 17:29
    
@MaksimZholudev How is it translational symmetry. Group theory is taught in physics, I am physics major. –  Revo Jan 20 '12 at 18:57
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@MaksimZholudev perhaps so. But I would argue that although group theory is mathematical, symmetry is physical. Since this question talks about symmetry, it has some physical relevance of sorts. I'd like to wait for further input before deciding to migrate this. (Also, keep in mind that answers to the question should be posted as answers, not comments) –  David Z Jan 20 '12 at 19:21
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@Revo, translations represent an additive group of real numbers because if you compose a translation by distance $L_1$ with a translation by $L_2$, you get a translation by $L_1+L_2$ which is the sum of the two real numbers. –  Luboš Motl Jan 20 '12 at 20:25
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Indeed group theory is the mathematical language of symmetry. Here is a link of a nice article entitled " Symmetry in Physics : Wigners legacy " jptp.uni-bayreuth.de/vorlesungen/symmetry.pdf It was written in 1995 by Nobel prize winner David Gross. –  Serifo Blade Jan 21 '12 at 0:33
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2 Answers 2

  1. On one hand, a group $G$ is a purely mathematical concept.

  2. On the other hand, given a system $S$ on which a group $G$ acts $G\times S \to S$, the system $S$ can in certain cases be invariant under the group action, and in such cases we say the system $S$ possesses a $G$-symmetry.

  3. In physics, $S$ is typical a set of equations, or an action (not to be confused with the notion of a group action).

  4. The Abelian group $G=(\mathbb{R},+)$ that OP mentions is implemented in many physical system. It could be translation symmetry of a physical system in a given direction, as Maksim Zholudev writes in a comment. But this is just one out of many possibilities, and there is not a 1-1 correspondence between groups and symmetries in a strict mathematical sense.

  5. Finally, note that the notion of a group $G$ may be weakened/generalized in several ways, e.g. to a groupoid.

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Let me check if I get this right: So a group $G$ is not identified with a symmetry unless the group elements leave some system invariant, right?. Who is OP?. In case when $G$ is the translation symmetry of a physical system in a given direction, what are the representations of the group elements in this case?and what is the quantity that will be invariant under the system translation? (whenever the word symmetry is mentioned, I try to think of a quantity that is left invariant under the effect of the group elements, like equilateral triangle under rotation by $n\times 120$ degrees) –  Revo Jan 21 '12 at 21:06
    
Well now you have to speak of the difference betweeen actual symmetries, like the symmetry of a circle when the group $\mathbb{Z}_n$ acts by rotations or the dihedral group acting on a regular n-gon (reflections, rotations), with gauge transformations which speak about equations and their solutions (prime example being Electrodynamics). And "OP" means original poster. –  Chris Gerig Jan 22 '12 at 3:57
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There is a one to one correspondence between symmetry and group theory for the simple reason that if A is a symmetry and B is a symmetry, then so is B followed by A. This implies that symmetries form a group, where the group law is composition of maps (a symmetry is a map).

Translation along the real axis is the physical symmetry of time translation. As pointed out by Wiener, it is an important fact that if we begin an experiment at time $ t =0$ and measured the results at time $t = 4$, the answers will be, physically, the same as if we had started the experiment at time $t=21$ and measured the results at time $t=25$.

This is the reason why Fourier analysis is useful... whenever you have a symmetry group, the representations of that group are useful. The representations of the translation group are the exponential functions so Fourier analysis, the decomposition of an arbitrary function into combinations of different exponential functions, is useful.

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