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arXiv 0712.0085gr-qc
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关于拟局部量的四元组方法中的某些整体问题

On some global problems in the tetrad approach to quasi-local quantities

Laszlo B Szabados

摘要:

本文研究了用四元组方法寻找闭的、可定向的类空2-曲面S的准局部守恒量时可能遇到的全局拓扑障碍。首先,我们证明了如果S的开邻域是时空可定向的,则Lorentz标架丛在任何这样的S的开邻域U上总是全局可平凡化的,从而也可以在U上引入全局可平凡化的SL(2,C)自旋标架丛.然后证明了S上属于同一旋量结构的所有自旋标架总是具有相同的同伦类。另一方面,在亏格为g的2-曲面上,存在$2^{2g}$同伦不同的Lorentz框架场,并且这些同伦类与不同的SL(2,C)旋量结构之间存在自然的一一对应。

英文摘要:

The potential global topological obstructions to the tetrad approach to finding the quasi-local conserved quantities, associated with closed, orientable spacelike 2-surfaces S, are investigated. First we show that the Lorentz frame bundle is always globally trivializable over an open neighbourhood U of any such S if an open neighbourhood of S is space and time orientable, and hence a globally trivializable SL(2,C) spin frame bundle can also be introduced over U. Then it is shown that all the spin frames belonging to the same spinor structure on S have always the same homotopy class. On the other hand, on a 2-surface with genus g, there are $2^{2g}$ homotopically different Lorentz frame fields, and there is a natural one-to-one correspondence between these homotopy classes and the different SL(2,C) spinor structures.

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