在量子引力中对抗鬼现象的最新进展
Recent Progress in Fighting Ghosts in Quantum Gravity
摘要：
我们回顾了最近的一些结果，这些结果有助于更好地理解真空和高阶导数量子引力中的一般经典解的稳定性问题。在半经典和完全量子引力理论中，纯引力真空区的四阶导数项都是由可重整性要求的。然而，由于这些项的存在，该理论的谱中存在非物理的鬼态，从而危及经典解的稳定性。在量子层面上，鬼违反了么正性，因此鬼看起来与理论的一致性不相容。“主导”或“标准”方法是将高阶导数项视为低能量下的小扰动。这样一个有效的理论应该与高能极限的未知基本理论相结合。我们认为，至少可以说，这种情况的前景并不明确。另一方面，最近在了解可以使鬼不具有攻击性的物理条件方面取得了一定的进展。我们综述了这些结果，并讨论了满足这些条件的未知基本理论的性质。
英文摘要：
We review some of the recent results which can be useful for better understanding of the problem of stability of vacuum and in general classical solutions in higher derivative quantum gravity. The fourth derivative terms in the purely gravitational vacuum sector are requested by renormalizability already in both semiclassical and complete quantum gravity theories. However, because of these terms the spectrum of the theory has unphysical ghost states which jeopardize the stability of classical solutions. At the quantum level ghosts violate unitarity, and and thus ghosts look incompatible with the consistency of the theory. The `dominating' or `standard' approach is to treat higher derivative terms as small perturbations at low energies. Such an effective theory is supposed to glue with an unknown fundamental theory in the high energy limit. We argue that the perspectives for such a scenario are not clear, to say the least. On the other hand, recently there was certain progress in understanding physical conditions which can make ghosts not offensive. We survey these results and discuss the properties of the unknown fundamental theory which can provide these conditions satisfied.
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