# What is quotient?

1. Nov 17, 2006

### leon1127

As one can see, the definition of quotient space, group, ring, field, vector space are very similar. It is similarly defined as an algebraic structure with a ~ on it. I am really having trouble vistualise what a quotient space and group are. My professor told me that we can work more easily with a quotient space since they are divided into mutual disjoint set. Is there another reason why it is defined in such a way?

Thx

Leon

2. Nov 17, 2006

### dextercioby

Lemme give you a physical example. In Quantum Mechanics one uses a (complex, separable) Hilbert space to describe physical (pure) quantum states. A Hilbert space is made up of vectors. There are physical reasons for which the mapping

{vectors}------>{physical (pure) states} is not one-to-one, i.e. injective. Worse, it maps an uncountable infinite set of vectors to one physical state.

So there's too much information in the Hilbert space (too many vectors) that we don't need. The application mentioned above is onto (surjective). To make it bijective, one factorizes the Hilbert space by this equivalence relation:

{vector A} ~ {vector B} iff the physical (pure) quantum state mapped from A is identical to the physical (pure) quantum state mapped from B.

In this way the bijective mapping is accomplished between the equivalence classes of vectors from the Hilbert space and the set of all physical (pure) quantum states.

The space H|~ is called "the projective Hilbert space" and provides the mathematical description of physical (pure) quantum states in Quantum Mechanics.

Daniel.

NOTE: The words in paranthesis: pure and separable are irrelevant for the point i'm trying to make. Just as the other words in paranthesis they're written for completeness.

3. Nov 18, 2006

### leon1127

What about normal subgroup? How does it important in physics?

4. Nov 18, 2006

### matt grime

All kernels are normal. Whenever groups are involved in anything, and maps of groups, you need to know about normal subgroups.

Groups control the symmetries of things. Some physical theories are described as the things that are left invariant by some group operation (one can characterize Galilean/Newtonian and relativistic mechanics this way). The group SU(3)xSU(2)xSU(1) is important in physics - representations of SU(2) correspond to spin states, for instance. A represnetation is a group hom into a vector space, so the kernel, being a normal subgroup is important....