关于（超级）亮度的评论
Comments on (super)luminality
摘要：
最近，在一个有趣的工作arXiv：1106.3972中，讨论了大质量引力运动方程的解，并表明该解的波动之一是超光速的。也有人说，这排除了巨大的引力。在这里，我们发现解决方案本身是相当非物理的。对于这一点，我们表明，有另一种模式在同一背景下的增长，并克服了在任意短的时间内的背景，可以激发一个可以忽略不计的能量成本。这个解决方案是由控制超光度的参数触发的。此外，我们还表明，该解决方案，如果被视为一个完美的流体，没有休息框架，或者说，洛伦兹变换，需要提高到休息框架是超光速本身。这种流体的应力张量具有复杂的本征值，并且不能从任何物理上可感的物质中获得。此外，对于相同的设置，我们发现另一个背景的解决方案，其波动都是稳定的和亚光速。基于这些结果，我们得出结论，在arXiv：1106.3972中发现的超光度是使用不适当的背景的伪像，然而，这个解决方案代表了一个有指导意义的例子，了解大质量引力。例如，在这种背景下，博尔沃-德塞尔幽灵是不存在的，尽管这可能天真地看起来并非如此。
英文摘要：
Recently, in an interesting work arXiv:1106.3972 a solution of the equations of motion of massive gravity was discussed, and it was shown that one of the fluctuations on that solution is superluminal. It was also stated that this rules out massive gravity. Here we find that the solution itself is rather unphysical. For this we show that there is another mode on the same background which grows and overcomes the background in an arbitrarily short period of time, that can be excited by a negligible cost in energy. This solution is triggered by the parameter governing the superluminality. Furthermore, we also show that the solution, if viewed as a perfect fluid, has no rest frame, or that the Lorentz transformation that is needed to boost to the rest frame is superluminal itself. The stress-tensor of this fluid has complex eigenvalues, and could not be obtained from any physically sensible matter. Moreover, for the same setup we find another background solution, fluctuations of which are all stable and subluminal. Based on these results, we conclude that the superluminality found in arXiv:1106.3972 is an artifact of using an inappropriate background, nevertheless, this solution represents an instructive example for understanding massive gravity. For instance, on this background the Boulware-Deser ghost is absent, even though this may naively appear not to be the case.
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