Heisenberg uncertainty derivation

[tungs10]

Hi,

I am trying to teach myself some quantum mechanics and here is something I am stuck on. Various derivations of Heisenberg uncertainty start out with two Hermitian operators, usually called A and B to represent position and momentum. Then they define another two operators ∆A and ∆B as:

∆A = A − < A > ∆B = B − < B >

That appears to me to say that ∆A equals operator A minus the expectation value of A. But how can you subtract a number from a matrix? Is an operator like a variable or is it more like a function? If it is like a function, then how can you plug ∆A and ∆B into the Cauchy-Schwartz inequality? This is probably a dumb question, but I don't think I can move on until I sort it out. Example of said derivation at:

"www.physics.ohio-state.edu/~jay/631/uncert1.pdf"[/URL]

Thanks for any help.

 

[arkajad]

<A> is a short way of writing <A>I - which means a diagonal matrix with the number <A> on the diagonal. Matrix I has number 1 at each entry on the diagonal - the identity matrix. That is similar to when you you see A-0. Here 0 stands for the matrix consisting of 0-s, not just one number 0. In such cases the meaning of the symbol must be read from the context.

 

[jtbell]

Think of a numeric constant as the following operator: "multiply [the following state function or matrix] by this number."

 

[tungs10]

So it is just <A> multiplied by the identity matrix. Thanks, that makes sense. I am glad I found this forum!