I am reading Griffiths and I am having trouble interpreting the results of measuring the x component of spin on a spin-up particle.(adsbygoogle = window.adsbygoogle || []).push({});

If you have a spin up particle, my understanding is that it is assumed to be up in the preferred axis, z. I would think that measuring its x component should give half probability of h-bar/2 and half of negative h-bar/2. But what I get is h-bar/2 times spin down (down in z axis right?):

[itex]Sx\uparrow=\frac{\hbar}{2}

\left( \begin{array}{ccc}

0 & 1 \\

1 & 0 \end{array} \right)

\left( \begin{array}{ccc}

1 \\

0 \end{array} \right)

=\frac{\hbar}{2}\downarrow[/itex]

I don't see why this would make any sense. You measure the x component of the spin of a spin-up particle and get h-bar over 2 times spin down? Does that mean the particle is now spin down on the z axis? Shouldn't it be on the x axis now that we measured it in respect to the x component?

Also, just to be clear: the generic spin-up spinor without a (z) superscript implies it is a spinor of the z axis right?

[itex]X=X^{(z)}[/itex] ?

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# Sx acting on up spin particle confusion

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