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A Little Trick for bra-ket notation over the Reals

  1. Aug 2, 2011 #1
    We know that < \phi | \psi >* = < \psi | \phi > where * denotes the complex conj.
    so if \psi and \phi are ordinary real valued functions (as opposed to matrices or complex valued whatevers) can we also say:

    < \phi | \psi > = < 1 |\phi \psi > = <\phi \psi | 1>

    Or what if \phi = \psi, then above = < 1|\psi^2>=<\psi^2|1>

    or if we have the position operator,R:

    < \phi | R| \psi > = < 1 |R| \phi \psi > = < R| \phi \psi >= <\phi \psi | R > were we assume that the positions must be real because the (wave)functions are real valued.
  2. jcsd
  3. Aug 2, 2011 #2


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    First of all, this isn't really bra-ket notation. You're just talking about a form <|> that takes two members of the set of real-valued functions on [itex]\mathbb R[/itex] (or [itex]\mathbb R^3[/itex]) to a real number. The equalities you're asking about hold if that form is defined by [tex]\langle f|g\rangle=\int f(x)g(x) dx[/tex] for all f,g. However, the vector space that's interesting in QM is the vector space of complex-valued square-integrable functions on [itex]\mathbb R[/itex] (or [itex]\mathbb R^3[/itex]), and the constant function 1 isn't square-integrable.

    In bra-ket notation, the members of the vector space would be written as |f> instead of f, and linear functionals that take those functions to complex numbers would be written as <f|.
  4. Aug 2, 2011 #3
    Your notation doesn't really make sense. To be precise, kets are vectors in the Hilbert space and bras are their duals (that is, operators that map vector to a real (or complex) number). Their relation to wave functions becomes clear when you expand a ket in terms of states which are eigenstates of the position operator:
    \begin{equation}|\psi> = \int \psi(x) |x>\end{equation}
    or equivalently
    \begin{equation}\psi(x) = <x|\psi> .\end{equation}

    The crucial thing here that you probably hadn't realized is that when we write for example |\psi>, the \psi there is just some symbol to label the state (vector in the Hilbert space). Thus your notation |\psi \phi> doesn't make any sense as such. Of course we could define |\psi \phi> to mean for example a two-particle state where one particle is on state \psi and the other is on state \phi.
  5. Aug 2, 2011 #4
    I think you hit it on the nail.... I was confusing the label of the state and the vector quantity. thanks
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