隐形斗篷和广义相对论

发表于 2009年7月22日由李淼

最近几年，电磁隐形斗篷的研究很热门。

我完全是外行，去年早些时候，葛墨林老师给了我几篇文献，我看了一下。电磁隐形斗篷是利用介质的特殊介电常数（一般是各向异性的，所以介电常数是一个矩阵），使得电磁波通过该介质时，表面上看起来没有通常的反射、折射和衍射等效应，如同那里根本没有任何东西一样。这样，一个人或一个物体披上了这样的斗篷，就像穿上了隐身衣，看不见了。

电磁斗篷由于介质的关系，一般不会对所有频段都是隐形的。不但有很多理论研究，也有了不少相关的实验。

最近，葛老师及其学生写了一篇论文，指出可以用广义相对论的形式理论研究电磁隐形斗篷。我不知道他们的文章是否已经发表，所以就不贴出他们的文章了。他们的研究提出了一个有趣的问题，就是，宇宙间的介质的介电常数是否影响了光的传播，从而导致表面的宇宙学效应？例如宇宙加速膨胀。稍微思考一下，这种效应应该不会影响暗物质的确认，因为暗物质主要是靠引力透镜（可能被介质的介电常数影响）和星系及星系团的旋转曲线（不会被介电常数影响）决定的。

与葛老师等人研究呼应的是，最近Berkeley实验室的Xiang Zhang用光学材料来研究广义相对论效应，例如黑洞和引力透镜效应。

我抄录一个博客的部分内容如下：

Zhang, a principal investigator with Berkeley Lab’s Materials Sciences Division and director of UC Berkeley’s Nano-scale Science and Engineering Center, has been one of the pioneers in the creation of artificial optical materials. Last year, he and his research group made headlines when they fashioned unique metamaterials, composites of metals and dielectrics, that were able to bend light backwards (Invisibility Cloak Gets A Nanomaterial Boost), a property known as a negative refraction and unprecedented in nature.

More recently, he and his group fashioned a “carpet cloak” from nanostructured silicon that concealed the presence of objects placed under it from optical detection. These efforts not only suggested that true invisibility materials are within reach, Zhang said, but also represented a major step towards transformation optics that would “open the door to manipulating light at will.”

Now he and his research group have demonstrated that a new class of metamaterials called “continuous-index photon traps” or CIPTs can serve as broadband and radiation-free “perfect” optical cavities. As such, CIPTs can control, slow and trap light in a manner similar to such celestial phenomena as black holes, strange attractors and gravitational lenses. This equivalence between the motion of the stars in curved spacetime and propagation of the light in optical metamaterials engineered in a laboratory is referred to as the “optical-mechanical analogy.”

Zhang says that such specially designed metamaterials can be valuable tools for studying the motion of massive celestial bodies in gravitational potentials under a controlled laboratory environment. Observations of such celestial phenomena by astronomers can sometimes take a century of waiting.

“If we twist our optical metamaterial space into new coordinates, the light that travels in straight lines in real space will be curved in the twisted space of our transformational optics,” says Zhang. “This is very similar to what happens to starlight when it moves through a gravitational potential and experiences curved spacetime. This analogue between classic electromagnetism and general relativity, may enable us to use optical metamaterials to study relativity phenomena such as gravitational lens.”

Invisibility Cloak Gets A Nanomaterial Boost

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