In cosmology we come across terms like dark matter and dark energy. The dark matter is invisible. In several recent publications it is claimed theoretically as well as experimentally that an object can be made invisible using metamaterials or PBG materials. Can such 'invisible' objects be considered to be made of dark material? How one can detect such invisible object?
Thanks for your useful answers. Dear Jose Matias-Lopes, if and when existence of dark matter and dark energy is confirmed we have to clearly distinguish between dark matter , devoid of matter and invisibility of matter by cloaking!!
Correct me if I'm wrong but I think you should consider that "making an object invisible" by using meta-materials or photonic crystals strongly depends on the wavelength. Therefore the object only seems to disappear for a specific spectral range, if you look for it at some other wavelength you'll still be able to locate it.
I don't completely unstand what is dark matter. For me, I think, dark matter is invisible for all of light of wavelength, but the cloak made of metal material depends on the wavelength very much. So the causalility and Kramer-Krong relation can be applied to the metal material. But for case dark matter is completely invisble for any wavelength, there may be something breaking the causality and the Kramer-Krong relation. If it is true, in the dark matter world, it must be some new law we now don't know yet. In summary, this is very intriguing and inspiring question.
We should distinguish between 'invisible' and 'non-detectable'. Visibility usually refers with reference to human eye. An object may not be visible to human eye but can be detected by EMwave detectors. Light do not interact with dark matter,but dark matter may be detected by other types of wave detectors.
I suspect this question is more metaphysical than physical.
By analogy, if I cover an anvil with a cotton sheet, the anvil becomes "invisible" in that it cannot be detected using visual means (except when the sheet drapes on the anvil and reveals its shape, which is actually analogous to our current technology in making things "invisible".) It does not make the anvil into cotton, however. (From my days as an electronics Technician, one of the "perks" of the PC board repair school was playing with the gold-electroplating system: invariably, one of the techs would use it to electroplate a nickel coin with gold. It didn't become a gold coin.)
At the moment, dark matter and dark energy are synonyms for "we think we have evidence that something should be there but we can't detect it." Until we actually figure out what they are (by finding a way to detect them) it's very difficult to conjecture as to how to convert something else into them. And, to close the circle, even if we had some "magical" way to turn something into dark matter, we currently couldn't do better than verify that we once had been able to detect it, and now can't, so the "conversion" might as easily be conjectured to be teleportation (still in the realm of magic for complex objects), sleight-of-hand (hiding or misdirecting), or ablation, as that it had changed into dark matter. In all of those cases, the original would still be present, somewhere in some form other than dark matter, just more difficult to detect... and still less difficult than something we can't detect at all, like dark matter.
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