I should go study again, so I didn't read everything yet. But it sounds logical to me that the shadow will travel at the speed of light and thus stay's behind. It's just like when you first see the airplane and then later hear the sound. In other words; you just get a dopler effect. I don't think that tachyons have much to do with the shadow, since the object consists of normal atoms and molecules. Maybe some tachyons (I don't really know much about those), but I don't think that would be significant next to the normal matter. Shadows travel at the speed of light, because they are simply the result of less photons hitting the surface.

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