# Few questions on Dirac's Principles of QM.

1. Sep 8, 2011

### DiracRules

Hi there,

I'm reading Dirac's Principles of QM, but I think I miss something...

1)When he derives the Poisson quantic bracket, he states that $u_1v_1-v_1u_1=i\hbar[u_1,v_1]$ and says that hbar must be real since we introduced the imaginary unit. The thing is, since uv-vu is real, because we are talking about dynamics variable that are real linear operator, this means that [u,v] must be pure imaginary. But that doesn't fit well with what Dirac says a few lines below, where he hypothesizes that [u,v] in QM has the same value than in classical mechanic (and I derive from here that it is real).
What am I missing?

2)Why is so important a representation where the observables are diagonal? And why it is so important to build Schroedinger's representation?

3)I can't follow his derivation of the conjugate transpose of d/dq :
giving that $\frac{d}{dq}\left.\psi\right>=\left.\frac{d\psi}{dq}\right>$
$<\psi\frac{d}{dq}=-<\frac{d\psi}{dq}$
he makes these consideration: the conjugate transpose of $$\frac{d}{dt}\psi>\,\,\mathrm{is}\,\,<\frac{d\bar{\psi}}{dq}$$ and, from the previous equation, $<\frac{d\bar{\psi}}{dq}=-<\bar{\psi}\frac{d}{dq}$ hence the conjugate transpose of d/dq -d/dq.
My question is:why $\bar{\frac{d\psi}{dq}>}=<\frac{d\bar{\psi}}{dq}$? I thought that $\forall |P>, \bar{|P>}=<P|$and this gives $\bar{\frac{d\psi}{dq}>}=<\frac{d\psi}{dq}$
Again, what am I missing?

4) What does it mean that $\mathbf{p}=i\hbar\mathbf{d}$, where d is the translation operator? Better, what is the physical importance and meaning?

Thank you in advance, more questions coming :D

2. Sep 8, 2011

### Bill_K

I think much of your question is a result of terminology. Where you say a physical quantity needs to be represented by a "real linear operator", what you mean is Hermitian or self-adjoint. The Hermitian conjugate involves a complex conjugate and an order reversal, probably what you're calling conjugate transpose. Note that if u and v are Hermitian, then uv - vu will be anti-Hermitian. This explains the appearance of i in question 1.

It also explains question 3, since ∂/∂q is anti-Hermitian also. When you take the adjoint of ∂/∂q |ψ> you'll pick up a minus sign.

3. Sep 8, 2011

### DiracRules

Ok, I get (probably) the answer to question (1), but not to the others.

Just to clarify: is the equation $\overline{\alpha|P>}=<P|\overline{\alpha}$(*) correct, the bar meaning taking the complex conjugate and alpha being an hermitian operator?
If so, then I can't understand why the following passage is right
$\overline{\frac{d}{dq}\psi>}=\overline{\frac{d\psi}{dq}>}=<\frac{d\overline{\psi}}{dq}$

If we consider in (*) |P>=dP/dq>, then it should be $\overline{\frac{d\psi}{dq}>}=<\frac{d\psi}{dq}$ w/o the complex conjugate on psi.

Can it be this way:
$\overline{\frac{d}{dq}\psi>}=<\psi\overline{\frac{d}{dq}}$
$\overline{\frac{d}{dq}\psi>}=\overline{\frac{d\psi}{dq}>}=<\frac{d\psi}{dq}=-<\psi\frac{d}{dq}$ hence $\overline{\frac{d}{dq}}=-\frac{d}{dq}$? (This demonstration is different from Dirac's but arrives to the same conclusion).