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I've been thinking about the probability interpretation of quantum states.
In the density matrix formalism, or in measurement algebra like Schwinger's measurement algebra, one makes the assumption that pure states can be factored into bras and kets, and that bras and kets can be multiplied together to produce complex numbers, and that the squared magnitudes of these complex numbers are the probabilities.
But when you try to write spinors in a geometric manner, one finds that one inevitably ends up with a choice of orientation, or gauge. For a paper dealing with the attempt by one physicist to avoid this orientation gauge, see Baylis:
http://www.arxiv.org/abs/quant-ph/0202060
One can eliminate the orientation gauge by converting back to the density matrix form. (In the above paper, this comes about because if [tex]R[/tex] is a rotor, then [tex]R^\dagR = 1[/tex].) Even when one doesn't deal with geometric interpretations of wave states, the fact that density matrices eliminate unphysical gauge freedoms should be clear: The arbitary (global) complex phase of a wave function obviously disappears in its density matrix, but the density matrix contains all the physical information of the wave function.
I suspect that one should attempt to avoid the factoring into spinors that led to the unphysical gauge freedom in the first place. That's all fine and good but doing this removes the complex numbers that gave the probabilities.
As a first step towards getting an alternative probability interpretation, I've found that one can replace the vector norm [tex]|\psi|^2 = < \psi_A | \psi_A>[/tex] with a matrix norm:
[tex]|A|^2_{N\times N} = \sum_j\sum_k |A_{jk}|^2[/tex].
If one assigns [tex]A = |\psi><\psi|[/tex], then the calculations work out the same (if [tex]\psi[/tex] is normed). That is, [tex]|A|^2_{N\times N} = |\psi|^2[/tex]. But this norm isn't a geometric norm.
To get it to a geometric norm, one can rewrite [tex]A[/tex] as a sum over Clifford algebra elements and then take the natural Clifford algebra norm (i.e. the Clifford algebra treated as a vector space):
[tex]|\alpha + \alpha_0\gamma_0 + ...+ \alpha_{0123}\gamma_0\gamma_1\gamma_2\gamma_3}|^2 = |\alpha|^2 + |\alpha_0|^2 + ...|\alpha_{0123}|^2[/tex]
Upon making this substitution, one finds, that for the cases of the Pauli matrices, or for the typical representations of the Dirac algebra (i.e. gamma matrices), one has that the geometic norm is proportional to the matrix norm with a constant of proportionality equal to the trace of the unit matrix.
It turns out that every representation of the Pauli algebra gives a norm which is proportional to the geometric norm. But this is not the case with the Dirac algebra. If one chooses a representation which diagonalizes algebraic elements that mix space and time, one finds that the geometric norm differs from the usual spinor norm. This is essentially what one would get if one boosted a representation.
Refusing to factor states into bras and kets amounts to rejecting Hilbert space (where norms are defined in terms of inner products of vectors) in favor of Banach space (where norms are defined in terms of a single state at a time) for the space of states. Yes, Hilber spaces have a lot of mathematical advantages over Banach spaces. But I suspect that there are Banach spaces that cannot be put into Hilbert form. What would be the physical interpretation of one of these?
Anyone else interested in this sort of thing or remember anything about it?
Carl
In the density matrix formalism, or in measurement algebra like Schwinger's measurement algebra, one makes the assumption that pure states can be factored into bras and kets, and that bras and kets can be multiplied together to produce complex numbers, and that the squared magnitudes of these complex numbers are the probabilities.
But when you try to write spinors in a geometric manner, one finds that one inevitably ends up with a choice of orientation, or gauge. For a paper dealing with the attempt by one physicist to avoid this orientation gauge, see Baylis:
http://www.arxiv.org/abs/quant-ph/0202060
One can eliminate the orientation gauge by converting back to the density matrix form. (In the above paper, this comes about because if [tex]R[/tex] is a rotor, then [tex]R^\dagR = 1[/tex].) Even when one doesn't deal with geometric interpretations of wave states, the fact that density matrices eliminate unphysical gauge freedoms should be clear: The arbitary (global) complex phase of a wave function obviously disappears in its density matrix, but the density matrix contains all the physical information of the wave function.
I suspect that one should attempt to avoid the factoring into spinors that led to the unphysical gauge freedom in the first place. That's all fine and good but doing this removes the complex numbers that gave the probabilities.
As a first step towards getting an alternative probability interpretation, I've found that one can replace the vector norm [tex]|\psi|^2 = < \psi_A | \psi_A>[/tex] with a matrix norm:
[tex]|A|^2_{N\times N} = \sum_j\sum_k |A_{jk}|^2[/tex].
If one assigns [tex]A = |\psi><\psi|[/tex], then the calculations work out the same (if [tex]\psi[/tex] is normed). That is, [tex]|A|^2_{N\times N} = |\psi|^2[/tex]. But this norm isn't a geometric norm.
To get it to a geometric norm, one can rewrite [tex]A[/tex] as a sum over Clifford algebra elements and then take the natural Clifford algebra norm (i.e. the Clifford algebra treated as a vector space):
[tex]|\alpha + \alpha_0\gamma_0 + ...+ \alpha_{0123}\gamma_0\gamma_1\gamma_2\gamma_3}|^2 = |\alpha|^2 + |\alpha_0|^2 + ...|\alpha_{0123}|^2[/tex]
Upon making this substitution, one finds, that for the cases of the Pauli matrices, or for the typical representations of the Dirac algebra (i.e. gamma matrices), one has that the geometic norm is proportional to the matrix norm with a constant of proportionality equal to the trace of the unit matrix.
It turns out that every representation of the Pauli algebra gives a norm which is proportional to the geometric norm. But this is not the case with the Dirac algebra. If one chooses a representation which diagonalizes algebraic elements that mix space and time, one finds that the geometric norm differs from the usual spinor norm. This is essentially what one would get if one boosted a representation.
Refusing to factor states into bras and kets amounts to rejecting Hilbert space (where norms are defined in terms of inner products of vectors) in favor of Banach space (where norms are defined in terms of a single state at a time) for the space of states. Yes, Hilber spaces have a lot of mathematical advantages over Banach spaces. But I suspect that there are Banach spaces that cannot be put into Hilbert form. What would be the physical interpretation of one of these?
Anyone else interested in this sort of thing or remember anything about it?
Carl
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