CarlB
Science Advisor
Homework Helper
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Doug,
I have great sympathy for the way you're doing this. I think that tensors are icky because they make the assumption that there is only one sort of symmetry in an object. I'd much rather leave things in algebraic form.
Quantum physics is now done entirely by enforcing symmetry relations so tensors make a certain amount of sense, but in doing this, it becomes impossible to predict relations between different symmetries. Hence the neverending search for a unifying symmetry. What I'd prefer is a unifying symmetry that just happens to have the symmetries one needs, and this is exactly what you are doing.
Getting back to exp(A^u - A*^u) A^u/|A|. Can you write down what this if A happens to be infinitesimal? That is, for first order in the components of A, what is the above? I'd write down what I think it is, but since you're nearby, my tendency towards laziness is multiplied by my tendency towards bossiness and delegation, and probably squared by my tendency to not want to appear stupid in public (the small residual which has survived my being a physics crank), all these things conspire to induce me to ask you to compute the first order for the thing.
Oh, what the heck. We write
A = a_1 + a_j j + a_k k + a_l l
Then A-A^* = 2(a_jj+a_kk + a_ll)
and the exponential of this is
1 + 2(a_jj + a_kk + a_ll)
I'll leave the rest for you. Please feel free to change notation to something more readable.
[edit]Hmm. Sure looks like when we multiply this by A/|A| we're going to get back the whole algebra. In other words, we will get back four degrees of freedom, as advertised. And the commutation rules seem correct.[/edit]
Carl
I have great sympathy for the way you're doing this. I think that tensors are icky because they make the assumption that there is only one sort of symmetry in an object. I'd much rather leave things in algebraic form.
Quantum physics is now done entirely by enforcing symmetry relations so tensors make a certain amount of sense, but in doing this, it becomes impossible to predict relations between different symmetries. Hence the neverending search for a unifying symmetry. What I'd prefer is a unifying symmetry that just happens to have the symmetries one needs, and this is exactly what you are doing.
Getting back to exp(A^u - A*^u) A^u/|A|. Can you write down what this if A happens to be infinitesimal? That is, for first order in the components of A, what is the above? I'd write down what I think it is, but since you're nearby, my tendency towards laziness is multiplied by my tendency towards bossiness and delegation, and probably squared by my tendency to not want to appear stupid in public (the small residual which has survived my being a physics crank), all these things conspire to induce me to ask you to compute the first order for the thing.
Oh, what the heck. We write
A = a_1 + a_j j + a_k k + a_l l
Then A-A^* = 2(a_jj+a_kk + a_ll)
and the exponential of this is
1 + 2(a_jj + a_kk + a_ll)
I'll leave the rest for you. Please feel free to change notation to something more readable.
[edit]Hmm. Sure looks like when we multiply this by A/|A| we're going to get back the whole algebra. In other words, we will get back four degrees of freedom, as advertised. And the commutation rules seem correct.[/edit]
Carl
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