CarlB
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Maybe this is the isomorphism that is needed to explain this better:
\left(\begin{array}{cc}a&b\\c&d\end{array}\right)<br /> \left(\begin{array}{cc}A&B\\C&D\end{array}\right) =<br /> \left(\begin{array}{cc}aA+bC&aB+bD\\cA+dC&cB+dD\end{array}\right).
then the isomorphism consists of multiplying the off diagonal elements by r and 1/r. Multiplication is preserved:
\left(\begin{array}{cc}a&br\\c/r&d\end{array}\right)<br /> \left(\begin{array}{cc}A&Br\\C/r&D\end{array}\right) =<br /> \left(\begin{array}{cc}aA+bC&(aB+bD)r\\(cA+dC)/r&cB+dD\end{array}\right).
and addition obviously is preserved too. Also note that the transformation preserves the trace as the diagonal elements are unchanged, and it is the trace that gives probabilities in the standard density matrix approach. But what is not preserved is the natural squared magnitude of matrices. That is, the squared magnitude of the untransformed matrix is |a|^2 + |b|^2 + |c|^2 + |d|^2, but the squared magnitude of the transformed matrix gets some r activity. The geometric squared magnitude is also not preserved. In particular, the coefficients for x and y are changed in magnitude by the r.
So this gives a way of rewriting QM where the traces (and therefore the usual way of calculating QM probabilities) is preserved, but the geometric method is not. This allows one to explore cases that are outside the standard model, but with a method that includes the standard model in that the geometric probability is identical to the trace in every standard representation of the standard model. It is only when we use these unstandard representations, (such as the su(2)/SU(2) modified by r) that the predictions can be made to differ, and only then in the geometric probabilities. The trace is unchanged.
I don't think that getting into the details of the "r" transformation advances understanding much. The same trick works in the Dirac matrices, but the number of degrees of freedom available to the representation is far far larger and you can't fit into a simple description like the above "r".
The better way of seeing everything is to just look at where the (matrix) diagonal primitive idempotents and the (matrix) democratic primitive idempotents map to (in the Clifford algebra), the rest follows automatically from that. If you want to know how many representations of the Dirac algebra exist that share the same diagonal representation (i.e. the same commuting operators) as the Weyl represenstation, for example, I think you should look at it geometrically. If you restrict yourself to representations where the spinor probabilities match the geometric probabilities, and you ignore the infinite number of cases you get by rotating x into y, you end up with 96 Weyl reps that share identical diagonal representations. The representations that give different geometric probabilites that I have found are all rotations, but are rotations that use cosh and sinh instead of cos and sin. That is, they rotate different signature elements of the canonical basis elements of the Clifford algebra.
For understanding all this it helps to understand how primitive idempotents appear in Clifford algebras. That is kind of complicated, but if you've heard of Radon-Hurwitz numbers you're probably already there. The paper I'm writing up on it is very very long and has lots and lots of examples and exercises so it should be easy, if boring, to understand. Give me a week to finish it off.
Carl
\left(\begin{array}{cc}a&b\\c&d\end{array}\right)<br /> \left(\begin{array}{cc}A&B\\C&D\end{array}\right) =<br /> \left(\begin{array}{cc}aA+bC&aB+bD\\cA+dC&cB+dD\end{array}\right).
then the isomorphism consists of multiplying the off diagonal elements by r and 1/r. Multiplication is preserved:
\left(\begin{array}{cc}a&br\\c/r&d\end{array}\right)<br /> \left(\begin{array}{cc}A&Br\\C/r&D\end{array}\right) =<br /> \left(\begin{array}{cc}aA+bC&(aB+bD)r\\(cA+dC)/r&cB+dD\end{array}\right).
and addition obviously is preserved too. Also note that the transformation preserves the trace as the diagonal elements are unchanged, and it is the trace that gives probabilities in the standard density matrix approach. But what is not preserved is the natural squared magnitude of matrices. That is, the squared magnitude of the untransformed matrix is |a|^2 + |b|^2 + |c|^2 + |d|^2, but the squared magnitude of the transformed matrix gets some r activity. The geometric squared magnitude is also not preserved. In particular, the coefficients for x and y are changed in magnitude by the r.
So this gives a way of rewriting QM where the traces (and therefore the usual way of calculating QM probabilities) is preserved, but the geometric method is not. This allows one to explore cases that are outside the standard model, but with a method that includes the standard model in that the geometric probability is identical to the trace in every standard representation of the standard model. It is only when we use these unstandard representations, (such as the su(2)/SU(2) modified by r) that the predictions can be made to differ, and only then in the geometric probabilities. The trace is unchanged.
I don't think that getting into the details of the "r" transformation advances understanding much. The same trick works in the Dirac matrices, but the number of degrees of freedom available to the representation is far far larger and you can't fit into a simple description like the above "r".
The better way of seeing everything is to just look at where the (matrix) diagonal primitive idempotents and the (matrix) democratic primitive idempotents map to (in the Clifford algebra), the rest follows automatically from that. If you want to know how many representations of the Dirac algebra exist that share the same diagonal representation (i.e. the same commuting operators) as the Weyl represenstation, for example, I think you should look at it geometrically. If you restrict yourself to representations where the spinor probabilities match the geometric probabilities, and you ignore the infinite number of cases you get by rotating x into y, you end up with 96 Weyl reps that share identical diagonal representations. The representations that give different geometric probabilites that I have found are all rotations, but are rotations that use cosh and sinh instead of cos and sin. That is, they rotate different signature elements of the canonical basis elements of the Clifford algebra.
For understanding all this it helps to understand how primitive idempotents appear in Clifford algebras. That is kind of complicated, but if you've heard of Radon-Hurwitz numbers you're probably already there. The paper I'm writing up on it is very very long and has lots and lots of examples and exercises so it should be easy, if boring, to understand. Give me a week to finish it off.
Carl
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{1 \over j^2} \to - {d \over dj } ({1 \over j}) \to {1 \over j-1} - {1 \over j } \to {1\over j(j-1)}<br />